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High Voltage Laboratory: simulation, adjustment and test on

1. ZN Vout E lt 2 m o w IT E 0 503 uF o o A Q BRK to a E E 5 o YX co o LO co l o IT E 0 487 uF o E oO o 9 1 BRK o Z 4 E 19 E E S 2 a m a 9 a d J LT o I a E 0 489 uF o vW o se 75 9 ohm q BRK E 4 A 4 To E o E E o e 2 E 2 o o z 4 4 7 II E 0 482 uF o a 00 Q 1 BRK i E E o x o o 4 q II E 0 496 uF o o o D 1 BRK E lt 2 o co o I O Figure 4 9 Impulse generator circuit used in simulation of full lightning discharge using PSCAD Two waveforms are obtained the lightning impulse test waveform simulated by PSCAD is shown in figure 4 10 and the charge waveform of one of the stages in this case the fifth stage is shown in figure 4 11 The firing of the sphere gaps is produced after 0 52 seconds after starting the charge of the impulse generator 52 53 120 q 100 80 60 20 20 T gt gt E a O E TRE SS SS SST Ce EE ees 0 51998 0 52000 0 52003 0 52005 0 52008 0 52010 0 52013 0 52015 0 52018 Figure 4 10 Full lightning waveform for the impulse generator circuit simulated using PSCAD The peak voltage is about 100 5kV This value is similar to the other one obtained by PSpice simulation and shown in table 4 8 above 25105 20 0 4 EOS 10 04 5106 0 0 5 04 10 0 4 15 0 4 20 0 4 25 0 0 00 0 10 0 20 0 30 0 40 0 50
2. Figure 4 11 Charge waveform for the fifth stage of the impulse generator simulated using PSCAD This waveform shows the typical charge of a capacitor in this case the capacitor of the fifth stage of the impulse generator 54 4 2 2 Chopped tail Impulse The electric circuit of the impulse generator for simulating a chopped tail impulse using PSCAD is shown in figure 4 12 below The parameters used in the simulation are shown in the figure except the charging voltage which is 100kV It is used a load capacitance of 2pF that simulates the behaviour of an electrical insulator R x 3 Vout z 4 lt 2 m E o lt E 5 ma o LO N 0 5 uF E S E 8 m o Te E o Q N o Te S BRK 5 a E E es o x e o 75 9 ohm _ T E E o gt O Il E E PE o a oo Figure 4 12 Impulse generator circuit used in simulation of chopped tail impulse using PSCAD The firing of the sphere gap occurs after 50 ms after starting the charge of the generator and the time to chopping is of 3 5 us after firing The chopped tail waveform obtained in the simulation is shown in figure 4 13 It is necessary to emphasize that front and tail time decrease for the configuration of this circuit therefore if it wants to be adjusted to the international standard values for front and tail resistors must be calculated as defined by Hipotronics 11 54 55
3. HV arm 110 kV Oscilloscope 10 Mohm Figure 6 17 Representation of the voltage divider Thus figure 6 18 shows both causes of interferences the puncture in a capacitor and the bad contact in the low voltage arm TARR PEE T Capacitive Mixed Divider Impulse Generator HV arm Oscilloscope 10 Mohm Figure 6 18 Representation of the voltage divider with the explanation to the problems The puncture is shown with the number 3 in the high voltage arm of the voltage divider It causes that the divider ratio changes and thus the high amplitude of the interferences occurs The bad contact in the low voltage arm through a stray capacitance is shown with the number 1 It causes the slightly curved ramp of the negative voltage impulse on the wave tail as the waveform of the charge of a capacitor The number 2 shows the disruptive discharge passing the dielectric of this stray capacitor what causes the sudden return front 80 81 point a to b on the lightning impulse wave shape This shape changes because it is a random phenomenon and cannot be demonstrated clearly but this theory explains all the problems that affect the standard waveform and a correct display of the test waveform To prove that the problem is in the capacitive voltage divider a probe is used This is an electrical device for making contact with a circuit test point for test purposes and it permits to measure v
4. Thus the value of the stray capacitance per stage is C 0 489 pF 4 10 Figure 4 7 shows the electric circuit used in the simulation of the stray capacitances As the purpose is studying the possible problem which may affect the normal behaviour of the equipment at the laboratory it is decided to use parameters shown in table 5 6 which are usually used when a high voltage test is performed at the High Voltage Laboratory The results are compared to the simulation of the impulse generator with the same parameters shown in table 4 7 but without the stray capacitors Elements C8 to C12 represent the stray capacitors Table 4 7 Impulse test system parameters for simulation of stray capacitances Re 350 Re 200 0 Ren 18 kQ E 500 nF Uc 25kV 48 Rs5 350hm Rt5 cs 500nF 2000hm TCLOSE 0 1ms Rs4 350hm Rt4 2000hm Rs3 350hm Rt3 2000hm Rs2 350hm Rt2 2000hm Rs1 350hm Rt1 2000hm Figure 4 7 Impulse generator circuit used in order to simulate the influence of stray capacitances with PSpice The point V is where the maximum output voltage is measured The lightning impulse waveform obtained for this circuit with stray capacitances is shown in figure 4 8 The firing instant of the sphere gaps is a bit delayed until 0 1ms and the maximum step size used is 1 ns in order to pay attention to what occur before the rising edge of the waveform It is explained i
5. cece sees ccc e scence eee eeneeeneeeneeeeeeeneeeneeeneeeneeeeeeeeaes 72 Figure 6 9 Test waveform that shows electromagnetic interferences EMI Volts and seconds per division rate selected 500V div 100us div 100X probe attenuation It was captured using a Tektronix TDS 2012B digital storage oscilloscope 73 Figure 6 10 Test waveform that shows electromagnetic interferences EMI Volts and seconds per division rate selected 500V div 50us div 100X probe attenuation It was captured using a Tektronix TDS 2012B digital storage oscilloscope 73 Figure 6 11 Test waveform that shows electromagnetic interferences EMI Volts and seconds per division rate selected 200V div 100us div 100X probe attenuation It was captured using a Tektronix TDS 2012B digital storage oscilloscope 74 Figure 6 12 Experimentally generated impulse voltage with noise near the start of the impulse signal 28 cuss sescves cue de wes ens ce eeeebs aves cus Segoe cnecceeceus scasdekssieveees perca dees 75 xiii Figure 6 13 The signal noise due to electromagnetic coupling from the stack of capacitors to the LV arm of capacitive mixed divider Volts and seconds per division rate selected in both 200V div 1us div 100X probe attenuation It was captured using a Tektronix TDS 2012B digital storage oscilloscope ooooooccccocnccnccncnncnnanes 76 Figure 6 14 The signal noise due to e
6. 28 29 The typical triggering range for the 100 series generators is shown in figure 3 11 The impulse must be inside the highlighted area in order to ensure a successful firing of the gap o 100 SERIES TRIGGER RANGE Ny O 80 GAP POSITION kv 8 60 40 20 O 10 20 30 40 50 60 70 80 90 100 CHARTING VOLTAGE kV Figure 3 11 Typical triggering range 3 3 8 Motor drive mechanism As shown in figure 3 12 the design of the gap system consists of a fixed set of spheres that are connected to the waveshaping resistor provisions on the right side and a moving set of spheres that are connected to the stage capacitors on the left side The individual spheres are fixed to the gap support tubes and adjustment is provided to set the gap spacings The moving gap tube is connected to a DC motor drive and gear reducing package that provides remote adjustment of the gap spacing Since the gaps are all mounted to a common tube the spacing can be controlled with a high degree of linearity The motor drive system also includes a 10 turn potentiometer which is used to drive the panel indicator for gap spacing This unit does not have any automatic drive control 30 TRIGGER PIVOT SHAFT SPHERE SPHERE SUSPENSION ARM 1 RY FIXED SHAFT MOTOR BRACKET BEARING HOUSING CONNECTING ARM SWIVEL NUT PEDESTAL LIMIT SWITC POT gt T LOCK SHAFT CLAMP S A sas SIC LIMIT ADJUST SCREW PIV
7. Specifically an evaluation of the effects of such over voltages on insulators will permit to develop new electrical equipments insulator materials and in general to study high voltage phenomena The laboratory was only used to carry out tests on electrical indoor and outdoor post insulators but other electrical equipment such as high voltage cables can also be tested doing the appropriate changes established by international standards Impulse voltages are generated by an impulse generator based on Marx Impulse Generator that will be described in this chapter 3 1 Erwin Marx historical note Erwin Otto Marx 1893 1980 was a German electrical engineer who first described a system to generate impulse voltages It was in 1923 and nowadays the Marx Impulse Generator is widely used when high impulse voltage values are needed for laboratory testing purposes At the 3rd IEEE Pulsed Power Conference June 1981 the Erwin Marx Award was created and dedicated to the memory of Professor Marx and his concept of the Cascade Impulse Voltage Generator Nowadays the IEEE Erwin Marx Award established in 1997 recognizes outstanding technical achievements in pulsed power engineering science and technology by an individual over an extended period of time This prize acknowledges the importance of Professor Erwin Marx and is given biennially to individuals who have made outstanding technical contributions to pulsed power technology for at least te
8. cece eee ee eee e eee ee eee eeeees 44 Table 4 5 Obtained results values for front time cece sees eect scene eee eeeeenees 44 Table 4 6 Data for calculating the value of the stray capacitance 48 Table 4 7 Impulse test system parameters for simulation of stray capacitances 48 Table 4 8 Results of the simulation with stray capacitances 2 compared to the simulation without stray capacitances 1 ooooccocccccnccnncnncnncnncnncnccnnanoso 50 Table 4 9 Real parameters of the impulse generator measured and used in the simulation using PSCAD vascas soba a geadas Veggie iS 51 Table 6 1 Impulse test system paraMeters oooccccccccccccccncccccoccncconconccnncnncnnannans 66 Table 7 1 Example of measurement ssssssssssssessssssssesesssssssesesssssseesessssseeee 95 Table 7 2 Relation between peak value of disruptive discharge voltage in the sphere gap and peak voltage measured in the oscilloscope 96 Table 9 1 Indoor post insulators of organic material and with internal metal fittings 110 xvii xviii Abbreviations and symbols List of abbreviations AC ASINEL BRK CMD CAD DSO DC EMC EMI EHV HV HVL IEEE IEC LI LV MV NATO r m s SI UHV VHV Alternating Current Asociaci n de Investigaci n Industrial El ctrica Breaker Capacitive Mixed Divider Computer Aided Design Digi
9. Figure 3 13 Front panel of the control console The high voltage charging supply is measured by means of a resistive voltage divider located on the generator structure 3 3 11 Capacitive mixed divider The capacitive voltage divider shown in figure 3 14 measures the voltage of lightning impulse tests 32 Figure 3 14 Resistive voltage divider of the generator A voltage divider is a device that is intended to produce accurately a suitable fraction of the test voltage for measurement It usually has two impedances connected in series across which the voltage is applied One of them the high voltage arm takes the major fraction of the voltage The voltage across the other the low voltage arm is used for the measurement The components of the two arms are usually resistors or capacitors or combinations of these and the device is described by the type and arrangement of the components 26 The Laboratory of High Voltage has a Hipotronics Capacitive Mixed Divider Model CMD 500 The Hipotronics CMD Series dividers are general purpose series damped resistive capacitive dividers for measuring AC or impulse voltages They are capable of measuring voltages from 50 Hz to 599 Hz Lightning Impulses L I including Chopped Waves and Switching Impulses S I The divider is comprised of a single section HV arm a base assembly and HV electrode an LV arm and a coaxial measuring cable The following table 3 2 provides the main system s
10. O trabalho divido em tr s partes a primeira parte estabelece os fundamentos te ricos necess rios para a compreens o do mesmo imprescind vel conhecer como s o produzidas as descargas atmosf ricas e a categoria das sobretens es que s o objecto de estudo A segunda parte apresenta o trabalho de experimenta o feito introdu o ao equipamento do laborat rio dispon vel simula o do processo estudo das diferentes possibilidades e solu o dos problemas que surgiram avalia o de riscos para os utilizadores na instala o princ pios estabelecidos pelas normas internacionais calibra o do equipamento de medida e o ensaio de isoladores el ctricos Para concluir a terceira parte do trabalho mostra os resultados e conclus es obtidas Os passos a seguir em futuros projectos no Laborat rio s o tamb m descritos com a finalidade de alcan ar um conhecimento mais profundo das altas tens es N o pode ser esquecido que neste campo a experimenta o altamente importante iii Palavras chave acoplamento electromagn tico avalia o de riscos calibra o ensaios de alta tens o ao choque atmosf rico isoladores el ctricos Laborat rio de Alta Tens o simula o computacional Abstract The main purpose of this final project is to prepare the High Voltage Laboratory of the Faculty of Engineering of the University of Porto in order to carry out high voltage testing of electrical insulators specifically
11. The term cylindrical insulators is intended to cover insulators of the truncated conical form dielectric loss factor The factor by which the product of a sinusoidal alternating voltage applied to a dielectric and the component of the resulting current having the same period as the voltage have to be multiplied in order to obtain the power dissipated in the dielectric discharge The passage of electricity through gaseous liquid or solid insulation disruptive discharge A discharge that completely bridges the insulation under test reducing the voltage between the electrodes practically to zero Syn electrical breakdown disruptive discharge probability p The probability that one application of a prospective voltage of a given shape and type will cause a disruptive discharge disruptive discharge voltage The voltage causing the disruptive discharge for tests with direct voltage alternating voltage and impulse voltage chopped at or after the peak the 122 123 voltage at the instant when the disruptive discharge occurs for impulses chopped on the front dry lightning impulse withstand voltage Lightning impulse voltage which the dry post insulator withstands under the prescribed conditions of test 50 dry lightning impulse flashover voltage Value of the lightning impulse voltage which has a 50 probability of producing flashover on the dry post insulator under the prescribed conditions of test duration of the wav
12. de fixation inf rieure du du support Specs de industriente Hauteur cu la partie ERR de rupture superieur inf rieure filetage de isolant dd sec a ne bending SPENSE tarzude taraud la base E L Lightning Height of post Maximum Maximum n i a Painel impulse covet insulator nominal difference Tom Ein Potom inune Maximus E withstand hanna diameter of in deflection plat Am rs ba E BEAST voltage Bibs i insulating between 20 Capped upped bd voltage dry part and 50 f peat ome lower end E tios of bottom failing load thread h TA kV kV mm mm mm JO2 66 2 000 1300 JO4 60 75 4000 2 600 JO6 60 80 6 000 3 900 JO8 66 60 28 9541 85 8 000 200 15 JO 10 60 95 10 000 6 500 JO 16 60 125 16 000 10 500 JO25 60 145 25 000 16 400 JO2 75 60 2 000 1 450 JO4 75 75 4000 2 900 JO6 75 90 6 000 4 350 JO8 75 75 33 130 1 100 8 000 5 800 25 JO10 75 105 10 000 7 200 JO16 75 125 16 000 11 600 JO25 75 145 25 000 18 000 The insulator under test in the high voltage laboratory is JO4 125 where JO indoor post insulator of organic material 4 mechanical strength class 4000N 75 lightning impulse withstand voltage in kilovolts 125 kV Thus IEC post insulator Type JO4 125 indicates an indoor post insulator of organic material of strength Class 4 and with lightning impulse withstand voltage 125 kV 9 3 Test on indoor post insulators of organ
13. 20 The standard reference atmospheric conditions for tests shall be in accordance with IEC 60060 1 e temperature t 20 C e pressure bo 101 3 kPa 1013 mbar e absolute humidity ho 11 g m3 112 113 The correction factors shall be determined in accordance with IEC 60060 1 If the atmospheric conditions at the time of test differ from the standard reference atmosphere then the correction factors for air density k1 and humidity k2 shall be calculated and the product K k1 k2 determined The lightning impulse test voltages shall then be corrected as follows e withstand voltages applied test voltage K multiplied with the specified withstand voltage e flashover voltages recorded flashover voltage measured flashover voltage divided by K 9 4 Test waveforms A full lightning impulse test can have three possible results Firstly a high voltage insulator of organic material under test that withstands the voltage applied without damage Air around or along insulator s surface does not break down and is not able to conduct through it therefore a flashover arc along insulator s outside does not occur The test waveform is shown in figure 9 2 below ek TE O Acq Complete M Pos 40 00us CH2 R z Coupling BW Limit 100MHz Volts Div Coarse ji Probe 100 piraat Voltage Invert Off ICH1 100 CH2 5004 M100ys CH1 4 8 004 lt 10Hz Figure 9 2 Test waveform of an ins
14. ER NX Voltage Invert CH1 2004 M 25 0 us CH1 Z 0 00V CH1 vertical position 1 12 divs 2244 23 75us Figure 6 7 Enlarged test waveform which shows the run of negative voltage impulses on the wave tail Volts and seconds per division rate selected 200V div 25us div 100X probe attenuation It was captured using a Tektronix TDS 2012B digital storage oscilloscope When the parameter seconds per division of the oscilloscope is increased up to 25 ms div as shown in figure 6 8 it can be seen what really happens The standard waveform has a length of about 100 us thus it is too short time to be clearly shown with this time base The positive voltage impulse of 80 kV is the standard waveform and the negative voltage impulse of 110 kV is the run of negative voltage impulses Then there is a double exponential waveform which seems to represent the discharge of a capacitor through two different ways one produces the positive exponential wave and another one produces the negative exponential wave At the end the residual voltage is drained away and the signal stays on zero Tek ENE Acq Complete M Pos 45 00ms CH1 Coupling 25 ms BY Limit 100MHz Volts Div Probe 100 Voltage EF 80 kV 110 kV Invert CH1 100 M 25 0ms CH1 Z 20MmYy lt 10Hz Figure 6 8 Test waveform Volts and seconds per division rate selected 100V div 25ms div 100X probe attenuation It was captured using a Tektroni
15. m Vout 120 q 100 80 60 40 20 20 0 049994 0 049996 0 049998 0 050000 0 050002 0 050004 0 050006 0 050008 Figure 4 13 Chopped tail impulse waveform simulated using PSCAD 4 2 3 Chopped front Impulse The electric circuit and the parameters of the impulse generator for simulating a chopped front impulse using PSCAD is the same as shown in figure 4 12 above The firing of the sphere gap occurs after 50 ms after starting the charge of the generator and the time to chopping is of 0 1 us after firing The chopped front waveform obtained in the simulation is shown in figure 4 14 m Vout o E 70 60 50 40 30 20 10 LESS DUE RO ES ES AAA AAA 0 049994 0 049996 0 049998 0 050000 0 050002 0 050004 0 050006 0 050008 Figure 4 14 Chopped front impulse waveform simulated using PSCAD 56 This is a very fast waveform It would be more clearly shown if front and tail resistors are increased because front and tail times would also increase 4 3 Conclusion It is shown in this chapter how different computer simulators such as PSpice and PSCAD may be used in order to study the equipment of the High Voltage Laboratory Computer simulators attempt to simulate an abstract model of the impulse test system and help in adjustment of a part of the equipment when required For this reason it is a very popular tool used not only for engineering purposes 56 57 Chapter 5 Standard
16. oO d fe ANNA 0 50 150 250 1000 mm T Gap spacing d 500 Figure 7 4 Correlation between the breakdown strength and the gap distance 4 In impulse tests the sphere gap is usually protected by a resistance R lt 300 O Correction factors As indicated in IEC 60052 when the atmospheric conditions are not the standard as defined in this section small corrections are made for air pressure temperature and humidity Thus there are two correction factors for air density temperature and pressure and air humidity The correction factor for air humidity is usually small below 2 and as recommended by Kreuger 4 measurements shall not be made over 90 of relative humidity The results obtained may be unreliable due to condensation of water at the sphere surfaces Air density correction factor Disruptive discharge voltages corresponding to a given sphere gap spacing S under atmospheric conditions other than those specified as standard are obtained by multiplying the values in tables 2 and 3 of the Standard by a correction factor corresponding to the relative air density O The relative air density is defined by b A a X b 273 t 90 91 where e the atmospheric pressures b and bg are expressed in the same units kPa e tand toare the temperatures in degrees Celsius Standard atmospheric conditions for temperature and pressure are Temperature to 20 C Pressure Po 101
17. 2 242835455565 75 9 11 13 15 17 19 22 26 30 34 Sphere gap spacing S cm Figure 7 7 Peak values of disruptive discharge voltages in kV for full lightning impulse voltages of negative polarity depending on sphere gap spacing S in cm for a sphere diameter of D 75 cm 7 7 2 Measurement of peak value of full lightning impulse voltages The 50 disruptive discharge voltage Uso and the conventional deviation z shall be determined The value of the conventional deviation z shall be not more than 1 for full lightning impulse voltages This can be done by a multiple level test A minimum of 10 voltage applications at each of five voltage levels in approximately 1 steps of the expected disruptive discharge value is 94 95 needed to obtain Uso and to check the conventional deviation z for a fixed sphere gap spacing S Table 7 1 shows an example On the left the 50 peak value of disruptive discharge voltage after being corrected by air density and humidity correction factors is shown and on the right the process of determination the 50 disruptive discharge voltage and the conventional deviation is performed Table 7 1 Example of measurement 1 05 0 995 75 215 224 62 1 220 33 A Correction factors are applied according to the Standard When there is a peak voltage measured by the oscilloscope which deviates from the expected voltage value it mu
18. at chop time Time to chopping Tc The time to chopping T is the time interval between the virtual origin and the instant of chopping Characteristics related to the voltage collapse during chopping The characteristics of the voltage collapse during chopping are defined in terms of two points C and D at 70 and 10 of the voltage at the instant of chopping as shown in figure 5 2 During chopped lightning impulse tests the gap used for chopping shall be located as close as possible to the terminals of the test object without disrupting its electric field distribution The impedance of the chopping circuit shall be minimized by the use of the shortest possible leads to the chopping gap If the undershoot during chopping exceeds 50 of the voltage at the instant of chopping the distances can be increased but should not exceed a lead length greater than the height of the test object Standard chopped lightning impulse A standard chopped lightning impulse is a standard impulse that is chopped by an external gap after a time between 2 and 5 us Linearly rising front chopped impulse A voltage rising with approximately constant steepness until it is chopped by a disruptive discharge is described as a linearly rising front chopped impulse To define such an impulse the best fitting straight line is drawn through the part of the front of the impulse between 50 and 90 amplitudes designated E and F respectively in 5 3 The impu
19. be implemented is recommended In order to show and mind all the safety rules to potential users of the High Voltage Laboratory a poster can be designed printed and placed on a wall of the Laboratory in a visible location 8 2 5 Review the assessment and update if necessary The High Voltage Laboratory is a workplace that stays the same usually there are no significant changes therefore reviews or updates of this risk assessment are only necessary if new equipment is brought or layout is changed If there is a significant change in the High Voltage Laboratory the risk assessment here presented must then be checked and amended where necessary It is the best to think about the risk assessment when the change is being planned and it leaves more flexibility to solve other possible problems 8 3 Conclusions This equipment employs voltages which are substantially dangerous when contacted by operating personnel Therefore extreme caution shall be exercised when working with equipment While every practicable safety precaution has been incorporated the following rules must be strictly observed as stated by Hipotronics 11 e Operating and maintenance personnel must all times observe all safety regulations e The equipment must be kept away from live circuits e Do not change components or make adjustments inside equipment with voltage supply on Under certain conditions dangerous potentials may exist in circuits with power controls i
20. below 0 C or above 40 C may damage the electric and electronic circuits of the equipment The High Voltage Laboratory may be a very cold room for the equipment in winter 102 Control console and oscilloscope must be placed out of the Faraday cage and at least 2meters away of it Another suggested option for the future is to place the control console and measuring system in the room J002 nearby All the electrical cables between the Faraday cage and the control console must be protected by a metallic gutter as shown in figure 8 6 Moreover if this protection is connected to earth the metallic gutter placed around conductors works as a shield electromagnetic interferences EMI may be avoided in this way Thus inside it the noise voltage on conductors is reduced to zero as described by Ott 19 Figure 8 6 Metallic gutter which protects electrical cables Human error can happen for this reason these safety systems were implemented in the Laboratory Objects like paper or cardboard must not be left inside the Faraday cage because they may catch fire if touched by a flashover arc If a fire occurs there is available a fire extinguisher in room J002 as shown in figure 3 18 8 2 4 Decide further actions that must be implemented Two further actions are mainly suggested here a safety way and improvement of the safety system both in order to improve and insure a safe workplace The ground rod should always be employed after sh
21. electrical potential such as the top of the stack of capacitors of the impulse generator which can reach 500 kV and objects connected to earth in the vicinity It is recommended to avoid touching the Faraday cage when laboratory testing with extremely high voltages are being performed Ground potential can rises quickly if an electrical discharge to earth by the air occurs and peak values of very high voltage may appear in the cage and be fatal The current safety distance around the stack of capacitors is about 1 7 m and has been checked as an appropriate safety distance just up to 325 kV as maximum output voltage As recommended this safety distance is not enough if tests from 325 kV to 500 kV want to be performed The actual layout of the Laboratory is shown in figure 8 1 below 6000mm o ao a puise Generator sx R 1705mm 1 Voltage Divisor Insulator 5300mm ere yd Electric y Faraday Cage Panel Control Console and Oscilloscope Figure 8 1 Current layout of the Laboratory at room J003 of the Faculty of Engineering The Faraday cage is not big enough to fulfil the minimum safety distance thus if higher voltage values want to be used it is recommended to enlarge the cage in order to have a bigger safety area As shown in figure 8 2 it is necessary to increase the safety distance up to at least a distance equal to the height of the stack of capacitors of the impulse generator that
22. is 2 5 meters Once the new layout is mounted several voltage applications of increasing values between 325 kV and 500 kV must be carefully performed step by step in order to make sure that only expected electrical discharges occur This suggestion is an optimized 100 layout according to recommendations of Hipotronics 11 when tests with voltages up to 500 kV want to be performed 7800mm R 1500mm Voltage A Divisor R 2500mm 6650mm Fi R 2513mm a Faraday Cage aU Electrica Panel Control Console and Oscilloscope Figure 8 2 New suggested layout at room J003 of the Faculty of Engineering In figure 8 2 it is also included other two safety areas around the device under test insulator and the capacitive divider which makes the new layout safer As recommended by Hipotronics 12 the divider should be operated without any ground objects within 1 5 meters of the high voltage arm that is 1 5 meters Metallic objects and protrusions present within this distance may affect measurements ratio and response time and cause flashovers or incorrect partial discharge readings 8 2 2 Decide who might be harmed and how First of all it is assumed that the user has a basic understanding of electrical equipment and the functions to be performed by this laboratory equipment Only trained and qualified personnel should operate this equipment Special care must be taken whit groups
23. laboratory there are five steps that must be followed by the person in charge of the assessment for a correct plan They are explained below e Identify the hazards e Decide who might be harmed and how e Evaluate the risks and decide on precautions e Decide further actions that must be implemented e Review the assessment and update if necessary 8 2 1 Identify the hazards Hazards of the High Voltage Laboratory are basically divided in two e Charges retained by capacitors e Safety distances too short In this high voltage installation of the Laboratory people could be harmed mainly by an unexpected high voltage discharge if someone enters the zone delimited by the Faraday cage and touches a part of the equipment inside the Faraday cage that has a dangerous potential and it was unexpected for the user These dangerous potentials may exist in the circuit due to charges retained by capacitors Therefore special care must be taken especially when the discharge of the capacitors does not occur because a wrong adjustment of the sphere gap spacings The safety distance around the impulse generator as well as other points that reach dangerous potentials such as the high voltage arm of the capacitive divisor or the device under test insulator must be properly calculated before carrying out any lightning impulse tests 98 99 These safety distances make possible to avoid electrical discharges between two points of different
24. negative output equally requires a positive charging voltage When a selected voltage is reached charging process is halted by electromechanical means At this time an isolated electronic trigger see section 3 3 7 delivers a pulse to discharge the capacitor bank A simultaneous oscillographic recording is made of the impulse waveform 11 The discharge capacitance C should always be larger than the load C as otherwise the efficiency of the generator will be too low A factor 3 may be acceptable but higher values are preferred as described by Kreuger 4 The impulse test system is set with parameters shown in table 3 1 below Table 3 1 Generator parameters Rs 35 O Re 200 O Reh 18 KQ C 500 nF N 5 stages By means of changing front and tail resistors Rf and R it is possible to vary the discharging time and in this way to avoid the effect of stray inductances because the faster the velocity change of current in the circuit the bigger the effect of stray inductances as it can easily be demonstrated by basic principles of electromagnetism In addition the inductance of the circuit should be kept low If too much inductance is present a waveform as shown in figure 3 7a occurs With front times less than 1 us these oscillations cannot be prevented At about 1 us front time the oscillations have vanished but some overshoot still occurs as shown in figure 3 7b IEC standards allow therefore an ov
25. of the efficiency of the system as well Thus the simulation does not show any problem caused by stray capacitances as was thought to be the cause of the interferences before wave front that are explained in chapter 6 50 51 4 2 Simulation using PSCAD PSCAD is a computer simulator that permits to simulate more precisely the laboratory equipment Unlike PSpice it is possible to simulate not only full lightning discharges but also chopped tail and chopped front impulses A group of simple simulations are shown in this section in order to support future studies performed in the High Voltage Laboratory For full lightning discharges a five stages generator is simulated but for chopped tail and chopped front impulses a simplified generator is used It does not have any influence on the results obtained The values of the impulse generator are shown in table 3 1 but in order to be more precise the real values were measured by means of a digital multimeter and are used in the simulation They are shown in table 4 9 Table 4 9 Real parameters of the impulse generator measured and used in the simulation using PSCAD 1 36 3 199 9 496 18 2 34 6 238 482 18 3 35 0 203 489 18 4 35 8 203 487 18 5 35 3 202 503 18 4 2 1 Full Lightning Discharge The electric circuit of the impulse generator for simulating full lightning discharge by PSCAD is shown in figure 4 9 below 52
26. puncture voltage so that they will flashover before they puncture to avoid any damage The insulator designing requirement is that electrical discharge must take place through the air and not to puncture the insulator It is important an adequate geometrical design so that there will not be a big electrical field concentration which might break the insulator material Dirt pollution salt and particularly water on the surface of a high voltage insulator might create a conductive path across it and cause leakage currents and flashovers Flashover voltage can be lower than 50 when insulator s surface is wet High voltage outdoor insulators are shaped to maximize the length of the leakage path along the surface from one end to the other called the creepage length to minimize these leakage currents 9 2 Characteristics of indoor and outdoor post insulators Characteristics of indoor and outdoor post insulators for systems with nominal voltages greater than 1000 V are standardized by IEC 60273 This standard applies to post insulators of organic material intended for indoor service in electrical installations or equipment operating on alternating current systems with a nominal voltage greater than 1000V and a frequency not greater than 100Hz They are primarily intended for use in isolator switches disconnectors or as bus bar or fuse support The post insulator tested in the High Voltage Laboratory is an indoor post insulator of organic materia
27. storage oscilloscope DSO is a Tektronix TDS 340 A see figure 3 17 A detail of the connection of these components except the probe is shown in figure 3 16 Figure 3 16 Detail of the low voltage arm of the voltage divider the coaxial cable and the measuring device oscilloscope 34 Figure 3 17 Tektronix TDS 340 A 3 3 13 Layout of the High Voltage Laboratory The High Voltage Laboratory HVL of the Faculty of Engineering is located in the room J003 of the Department of Eletrical and Computer Engineering of the Faculty of Engineering In operation generator s parts are installed in a safe test area inside a Faraday cage or shield and controls are located nearby although the possibility of creating a control room nearby in room J002 is proposed In figure 3 18 the layout of the High Voltage Laboratory and location of the equipment are shown 6000mm Impulse Generator Voltage E Divisor E o o ise LO Electric Faraday Cage Panel E a N Control Console Fire Extinguisher P and Oscilloscope J 003 J 002 Figure 3 18 Layout of the Laboratory and earth connections This plan view shows the Faraday cage which is located in the upper right side of the room J003 Upper and left sides of this testing zone are made of metal sheet and right and down sides are made of metallic mesh
28. the insulating parts of the post insulator between those parts which normally have the operating voltage between them However to take account of the metal fittings attached to the post insulator the distance which in service conditions is covered by metal fittings is not included in the creepage distance corona discharge is the discharge with slight luminosity produced in the neighbourhood of a conductor without greatly heating it and limited to the region surrounding the conductor in which the electric field exceeds a certain value 14 cumulonimbus heavy masses of cloud with great vertical development the upper parts having a fibrous appearance and often spreading out in the shape of an anvil Associated with violent vertical currents and thundery conditions D design category Post insulators of organic materials are divided into two different design categories according to their construction The design categories covered by this standard are Design category A Cylindrical post insulators with internal metal fittings in which the length of the shortest puncture path through solid insulating material is equal to or greater than one third the external arcing distance between the metal fittings Design category B Cylindrical post insulators with internal metal fittings in which the length of the shortest puncture path through solid insulating material is less than one third the external arcing distance between the metal fittings
29. time T of a lightning impulse is the time interval between the origin of the wave shape and the instant on the tail when the voltage has decreased to half of the peak value 50 of the peak value And the efficiency of the process is as commonly defined Electrical _ Efficiency PSC pone SOUP IE 4 2 Total power input 40 41 or expressed in terms of the generator parameters max out 4 3 7 ai 4 3 where n Efficiency Umax out Maximum output voltage n Number of stages of the generator n 5 U Charging voltage per stage All these parameters are precisely defined in chapter 5 international standard for high voltage testing The circuit used in the simulation is shown in figure 4 1 below In the rest of the circuits used for the study only the values of front and tail resistors are changed and The table of results obtained is shown at the end of this section see table 4 5 42 Rs5 V 350hm oe 500nF Rt5 2000hm TCLOSE tus R1 1 2 U5 75ohm Rs4 350hm dl c4 18kohm C6 1491pF 500nF Rt4 AE TCLOSE tus 1 2 U4 Rs3 350hm rent C3 18kohm 500nF Rt3 2000hm TCLOSE tus 1 2 U3 Rs2 Rch3 R2 350hm c2 18kohm 75 90hm Rt2 500nF 2000hm TCLOSE tus 1 2 C7 R3 U2 371nF 10Mohm Rs1 Rch2 350hm C1 18kohm Rt 500nF 2000hm TCLOSE tus 1 2 U1 Rch1 18kohm Uc Figure 4 1 Impulse generator circuit used in simulation with PSpice The point V is where the maximum output voltage is measured T
30. voltage Uso or also called breakdown value 7 7 How to perform the calibration process The use of standard air gaps permits checking the approved measuring systems of the High Voltage Laboratory A measurement of voltage by means of sphere gap consists of establishing the relation between a voltage in the test circuit as measured by the standard air gap and the peak value of the voltage obtained from the measuring device of the Laboratory that is the digital storage oscilloscope DSO connected to the low voltage arm of the measuring system the Capacitive Mixed Divider If it is possible to access to low voltage unit of the Capacitive Divider by means of intentionally changing capacitance of the variable capacitor of this unit the divider output ratio can be changed and adjusted according to the voltage measured by the standard air gap in order to obtain a true voltage value in the oscilloscope As this operation cannot be made at the High Voltage Laboratory because it is not possible to access to low voltage arm of the Divider the calibration process must consist of establishing the relation between voltages by means of a function The peak value of the voltage obtained from the oscilloscope can be related to the true voltage in the test circuits by a mathematical function This function can be represented as a straight line a linear function Values of the calibration presented in this report are not real and can be used only as
31. with their experience 6 1 Preliminary When the work was planned at the beginning the main purpose of the Master s Thesis was to study how lightning impulse tests are made complying with international standards but when the equipment started using an unknown phenomenon appeared In that moment the main objective of this work changed its direction and was focused on discovering the origin of the problem and solving it but at the same time without forgetting the first objective of the master s thesis 66 The equipment of the High Voltage Laboratory is configured with front and tail resistors which are usually used and although it does not have any influence on the problem the device under test was a standard indoor post insulator of organic material and with internal metal fittings according to IEC 60273 All experimental waveforms that are shown in this chapter were obtained in the impulse test system configured according to parameters shown in table 6 1 and a charging voltage per stage of 25kV As the total number of stages n is five 125 kV positive are obtained as maximum output voltage theoretically but it must not be forgotten that this value is affected by the efficiency of the system as was shown in chapter 4 and therefore it is lower Table 6 1 Impulse test system parameters Rf 35 O Rt 200 O Rch 18 KQ Cg 500 nF n 5 stages The process is divided in parts and each part is studi
32. 3 kPa Humidity correction factor The disruptive discharge voltage of a sphere gap increases with absolute humidity at a rate of 0 2 per g m The average value of absolute humidity h under which values in tables 2 and 3 of the Standard were obtained is 8 5 g m These values shall be corrected for humidity by multiplying the values in those tables by the humidity correction factor k given by the following equation k 1 0 002 85 7 2 with the ambient absolute humidity h in g m 7 6 Advantages and disadvantages The sphere gap is widely used because of its advantages although it also presents some disadvantages Both are presented in this section based on the information depicted by Kreuger 4 The advantages are e It is a simple device and universally applicable e It measures the crest voltage which usually is decisive in dielectric testing e t measures all types of voltages lightning impulses and also DC AC and switching impulses e It has a large scope from a few kVs with small spheres of some centimetres diameter to MVs with spheres of some metres diameter The disadvantages are e lts accuracy is modest about 3 at impulses e t does not give a voltage reading but is used to calibrate the readings of the voltage divider in the case of the High Voltage Laboratory 92 e Time consuming in order to obtain full accuracy long test series are needed to determine the 50 disruptive discharge
33. 4 64 65 Chapter 6 Practical problems emerging in measurement Engineering is the discipline of applying technical scientific and mathematical knowledge in real life Sometimes the results of this real part differ from the theoretical and practical fundamentals studied at faculties of engineering because of the negative effect of certain element When it occurs this is the signal that a problem occurs and a meticulous search must begin For the engineer it is important to find where the problem is in order to understand it replace the damage component and recover the normal behaviour of the system This chapter reports on the practical problems emerging in measurement of the lightning impulse waveform of tests on insulators of organic material at the High Voltage Laboratory Such problems do not make possible to carry out lightning impulse tests on insulators of organic material as it is described in the relevant international standards and the solution must be founded They are therefore cause of multiple discussions To perform the study of the emerging problems in the equipment already mentioned it was possible to count on experienced engineers who presented their ideas and enriched the process a retired engineer with large experience in High Voltage Laboratories an engineer specialized in calibration of this kind of equipment and a group of professors of the Faculty of Engineering who in different moments helped and contributed
34. 4 4 Variation of the efficiency in percentage according to tail resistance in ohms It is considered a constant front resistance Of 750 c cc cceeece eee eeeceeeeeeeeeneeenees 46 Figure 4 5 Variation of the front time in microseconds according to front resistance in ohms It is considered a constant tail resistance of 4500 oooococcconcconcconnconnconncos 46 Figure 4 6 Variation of the tail time in microseconds according to tail resistance in ohms It is considered a constant front resistance Of 750 ooooconcccnnconnconnccnnconncos 47 Figure 4 7 Impulse generator circuit used in order to simulate the influence of stray capacitances With PSpiCe da sa 49 Figure 4 8 Lightning impulse waveform obtained in the PSpice simulation with stray cCapacitances secas sanear a ee ee aha ca oa AE es pees A Do Ni aaa 50 Figure 4 9 Impulse generator circuit used in simulation of full lightning discharge using PSCAD osas a esposa anna as saga e a aaa vada 52 Figure 4 10 Full lightning waveform for the impulse generator circuit simulated using PSCAD snes sa cuenta E idas aaa UT CS Sa Cas ra tea eas 53 Figure 4 11 Charge waveform for the fifth stage of the impulse generator simulated USING PSCAD osso quem Sagan LS sae A LO Ce USANDO pa mobs era CAS DAE TO Oo TS DA oa 53 Figure 4 12 Impulse generator circuit used in simulation of chopped tail impulse using PSCAD ia tert Les RARA Eai A aa SR Ae a Rd soa a a rea 54 Figure 4 13
35. 5 MHz t lt i Us 0 i 2 3 0 1 2 3 4 tas 4 tus Figure 5 5 Examples of lightning impulses with oscillations or overshoots 2 Mean curves shown as dotted lines If oscillations are present on the front points A and B should be taken on the mean curve drawn through these oscillations In the case of the waveform obtained at the High Voltage Laboratory there is a superimposed oscillation on the wave front 5 2 4 Tolerances The following differences are accepted between values for the standard impulse and those actually recorded a Peak value 3 b Virtual front time 30 c Virtual time to half value 20 It is emphasized that the tolerances on the peak value front time and time to half value constitute the permitted differences between specific values and those actually recorded by measurements These differences should be distinguished from measuring errors which are the differences between values actually recorded and true values Overshoot or oscillations in the neighbourhood of the peak are tolerated provided that their single peak amplitude is not larger than 5 of the peak value Its measurements shall be made by a system with specific properties but this problem was not found at the Laboratory and therefore not studied In commonly used impulse generator circuits oscillations on the wave front during which the voltage does not exceed 90 of the peak value have generally insignificant influence on test results 6
36. 6 21 Measurement of the test waveform made by the probe 1 and the capacitive divider 2 This experiment demonstrates that the problem is in the capacitive voltage divider and this part of the High Voltage Laboratory equipment must be replaced The waveform number 1 does not show any problem but the waveform number 2 has this problem mentioned before 6 5 Conclusions The problem does not affect the test because the problem is only in the measurement device therefore the device under test does not have to support higher voltage values As the parameters of the test waveform such front time tail time and peak voltage can still be read the equipment may be used for testing purposes but the capacitive voltage divider must be replaced if a measurement without interferences is required 82 83 Chapter 7 Calibration according to IEC 60052 This chapter presents a detailed description about how to perform the calibration of the Impulse Test System according to the international standard IEC 60052 15 This is a standardized procedure of voltage measurement by means of standard air gaps The experience in calibration acquired in another High Voltage Laboratory within the preparation of this work is reflected in this report Although verification of the control console was not possible to perform due to the need of a standard sphere gap that was not available this part intends to set the basis for a further work in calibratio
37. 9 00 294 50 450 00 Tail resistance ohm Figure 4 4 Variation of the efficiency in percentage according to tail resistance in ohms It is considered a constant front resistance of 750 Mainly front and tail time vary more according to front and tail resistors respectively Thus figure 4 5 shows the wave front time T front resistor Rf for a constant value of tail resistance of 4500 and figure 4 6 shows the wave tail time T2 tail resistor R for a constant value of front resistance of 750 It is seen that the higher front resistor the higher front time and the higher tail resistor the higher tail time 2 50 2 00 1 50 1 00 Front time us 0 50 0 00 23 86 49 47 75 00 Front resistor ohm Figure 4 5 Variation of the front time in microseconds according to front resistance in ohms It is considered a constant tail resistance of 4500 47 180 000 160 000 140 000 120 000 100 000 80 000 60 000 40 000 20 000 0 000 Tail time us 139 00 294 50 450 00 Tail resistor ohm Figure 4 6 Variation of the tail time in microseconds according to tail resistance in ohms It is considered a constant front resistance of 750 The impulse test system is designed to reach up to 500 kV but the efficiency of the system around 82 according to simulation decreases this value to about 409kV In the real test front and tail time peak volta
38. 96 97 Chapter 8 Risk assessment In last 10 or 15 years human safety and risk management in the workplace have become more and more important Nowadays workers and administrators are aware of the risks which are present at their workplace A risk assessment is an important step in protecting all the people who work or temporarily visit the workplace as well as complying with the law and protecting the institution image This chapter states necessary actions to prevent somebody might be harmed and those risks with the worst potential consequences at the High Voltage Laboratory of the Faculty of Engineering The Impulse Test System is equipment potentially dangerous thus safety must be priority in order to carry out safe lightning impulse tests The current importance of this field is shown by all the information which is possible to find about it In last time new journals on risk and safety have appeared longer established journals in the risk and safety field have gone from strength to strength and the large amount of recent research literature that has been generated in the risk and safety field is reflected in several completely new books which have been published 8 1 What is risk assessment A risk assessment is a careful examination of what in the workplace could cause harm to people Workers and others as visitors have a right to be protected from harm caused by a failure to take reasonable control measures The responsi
39. 997 Available on line http www agu org pubs crossref 1997 96JD01917 shtml C M Pinto Oliveira E S da Silva Ferraz H J de Sousa Teixeira T F Ferreira dos Santos Descargas Atmosf ricas T cnicas de Alta Tens o Faculdade de Engenharia Universidade do Porto Portugal 2006 Vasco Fernandes Flores Relat rio de Actividades do Novo Laborat rio de Alta Tens o da Faculdade de Engenharia da Universidade do Porto 2007 10 A J Neves de Sousa Simulac o do Gerador de Choque de Alta Tens o Projecto Semin rio Trabalho Final de Curso Faculdade de Engenharia Universidade do Porto Portugal 2006 07 11 Impulse Test System User s Manual Model IG 500 12 5 Serial Number 75 23474 Part Number DS 25 242 Hipotronics Inc 12 Capacitive Mixed Divider User s Guide Model CMD 500 Part Number AS45 326 Hipotronics Inc 13 Website of the Institute of Electrical and Electronics Engineers IEEE Available on line http www eee org 14 IATE The European Union s multilingual term base Available on line http iate europa eu 130 15 IEC 60052 Voltage measurement by means of standard air gaps International Electrotechnical Commission Switzerland 2002 16 100 Series Impulse Generators Instruction Manual Hipotronics 1992 17 A Glendon S G Clarke E F McKenna Human Safety and Risk Management Taylor amp Francis Group 2006 18 HSE Health and Safety E
40. Chopped tail impulse waveform simulated using PSCAD 55 Figure 4 14 Chopped front impulse waveform simulated using PSCAD 55 xii Figure 5 1 Full lightning impulse without oscillations or overshoots 59 Figure 5 2 Lightning impulse chopped on the tail cece ce cece cece cette eee eeeeeeeeees 60 Figure 5 3 Linearly rising front chopped impulse oooccccccccccccccnccnnccnccnccnncnccnnannns 62 Figure 5 4 Examples of lightning impulses with oscillations or overshoots 1 62 Figure 5 5 Examples of lightning impulses with oscillations or overshoots 2 Mean curves shown as dotted lines ssossssssssoosssssseooossssssosoossssssoooesssssssosessssseo 63 Figure 6 1 Waveform for a standard lightning voltage impulse test Volts and seconds per division rate selected 200V div 50us div 100X probe attenuation It was captured using a Tektronix TDS 2012B digital storage oscilloscope 67 Figure 6 2 Test wave with three interferences a positive voltage impulse before wave front and other two negative voltage impulses at wave tail Volts and seconds per division rate selected 2V div 2 5us div Captured using a Tektronix TDS 340A digital Storage Oscilloscope skries sia caes seine acy es sees teed evacuees dees asses 68 Figure 6 3 Test wave front with a fast positive voltage impulse and su
41. Faculdade de Engenharia da Universidade do Porto High Voltage Laboratory simulation adjustment and test on electrical insulators Manuel Angel Saboy Gabina Master s Thesis Report carried out within Master in Electrical and Computers Engineering Energy Director Prof Dr Antonio Carlos Sepulveda Machado e Moura June 2009 Manuel Angel Saboy Gabi a 2009 Resumo O principal objectivo desta disserta o a prepara o do Laborat rio de Alta Tens o da Faculdade de Engenharia da Universidade do Porto com a finalidade de permitir a realiza o de ensaios de alta tens o particularmente ensaios ao choque de descargas atmosf ricas de isoladores el ctricos de material org nico conforme a normativa internacional aplic vel e dar solu o aos problemas encontrados no equipamento que foram detectados no come o dos trabalhos no laborat rio As descargas de origem atmosf rica produzem sobretens es nas linhas el ctricas de transporte de energia que podem atingir centenas de milhares de volts o que causa esfor os diel ctricos sob os isoladores e pode p r em perigo uma opera o normal e com seguran a do equipamento el ctrico das esta es e subesta es Os isoladores el ctricos s o amplamente utilizados neste tipo de dispositivos como por exemplo seccionadores de tens o terminais de transformadores el ctricos linhas de transporte em alta tens o ou linhas el ctricas em infra estructuras ferrovi rias
42. Laboratory is a Marx Generator with five stages The analysis of this multistage system gives an introduction to point 3 3 that shows all the parts of the Laboratory in detail including the impulse generator Figure 3 4 shows the multistage impulse generator circuit Surprisingly high voltages can be generated with this circuit 20 21 _ HV output E Rech Rf Rt ES Rf C _ Rt Rech Rf E E Rt Rech HV DC charging supply Figure 3 4 Multistage impulse generator 11 As described by Kreuger 4 the charging resistors Rc are needed for charging the generator The front and tail discharge resistors are Ry and R respectively may be changed in order to adjust the front time and the time to half value in the generator according to the Standard see chapter 5 Sphere gaps are placed between stages The discharge capacitors C are charged in parallel and discharged in series what occurs when sphere gaps are fired When the first sphere gap is fired the others follow and a voltage n U is generated in the output terminal where n is the number of stages and U is the charging voltage and consequently the voltage between capacitor s terminals of any stage at the end of the charging process 3 3 Laboratory equipment In this section equipment and operating information of the High Voltage Laboratory is described in order to familiarize personnel with the set up and operation The main characteristics of th
43. OT SHAFT BEARING Figure 3 12 Motor drive mechanism 3 3 9 High voltage discharging relay The High voltage discharge relay is an electrically operated switch that opens when the charging supply is operating and closes when power is interrupted or if the generator is turned off The switch is connected in series with a discharge resistor that absorbs the stored energy in the generator The design of the high voltage discharge circuit is to rapidly discharge the high voltage capacitors to zero voltage under emergency conditions It is an external air insulated switch that is mounted on the base of the impulse generator 3 3 10 Impulse generator control system Hipotronics Impulse Generator Control System Model C 100M is an integrated system designated to charge and control the firing of the standard IG100 2 5 generator The control console is shown in figure 3 13 below The following items are found on the control console panel e Main power circuit breaker e Main power ON indicator lamp e High voltage ON pushbutton 30 31 e High voltage OFF pushbutton e High voltage ON indicator lamp e Pulse initiation pushbutton e Interlock Open indicator lamp e Three position charge rate control e Impulse READY indicator lamp e Dual range kilovoltmeter with high set point e Gap breakdown voltage meter e Open Close gap motor control switch e Meter calibration access hatches
44. Techniques for High Voltage Testing Principles of High Voltage Testing are standardized in order to make possible reproduction and comparison of the results obtained worldwide Organizations like the International Electrotechnical Commission IEC and the Institute of Electrical and Electronics Engineers IEEE prepare these documents which for the standard presented here establish standard methods of measurement of high voltage and basic testing techniques To perform laboratory testing it is also necessary the information described by the appropriate standard of the apparatus under test which states its electrical and mechanical characteristics prescribe methods of testing and acceptance criteria This Standard is for post insulators of organic material the IEC 60660 27 In this chapter a summary of the most important information depicted by the IEEE Standard 4 1995 is shown in order to explain the main characteristics of the standard lightning impulse waveform and test techniques used in the High Voltage Laboratory Whenever further information than presented here is required the Standard IEEE Std 4 1995 26 must be consulted 5 1 Introduction The IEEE Standard here presented defines terms of general applicability presents general requirements regarding test equipment objects and procedures and describes methods for evaluation of test results It is applicable to several kinds of high voltage tests but this report has as su
45. The high voltage conductor including any series resistor not in the shank itself shall be connected to a point on the shank away from the sparking point of the high voltage sphere This distance must be at least two times the diameter of the spheres 2 D Within the region where the distance to the sparking point of the high voltage sphere is less than B the high voltage conductor including the series resistor if any must not pass through the plane normal to the axis of the sphere gap This plane is situated at the connection point on the shank and shown in figure 7 1 7 3 3 Protective resistor for measurement of impulse voltages As depicted by the international standard 15 series resistance is needed with large diameter spheres to eliminate oscillations in the sphere gap circuit which may cause a higher voltage to occur between the spheres and if connected across the test object A series resistance may also be needed in order to reduce the steepness of the voltage collapse which might introduce undesirable stresses in the test object The resistor shall have a non inductive construction not more than 30 pH and its resistance should not exceed 500 Q as specified by the standard In the circuit the resistor is positioned at the high voltage conductor and shown in figure 7 1 7 4 Use of the sphere gap A sphere gap is an IEC standard measuring device when the conventional deviation z see note below at the time of use is for lig
46. Verification of the trigger system it was thought that the deterioration in this system might cause the positive voltage impulse before the wave front The electrical circuit was checked and the maximum output signal of the trigger system is 15kV e The choice of earthing system has implications for the safety and electromagnetic compatibility of the power supply therefore it was studied together with checking the conduction of all earth connections New earth connections were made e It was thought that the positive voltage impulse before the wave front was caused by the effect of stray capacitances because of the shape of this impulse which is shown in figure 6 15 below 78 Tek ERE SOOMS s 1 Acqs 337 5 kV Mo i ns CAT FEV 3 Apr 2009 14 45 14 Figure 6 15 Enlargement of the positive voltage impulse before the wave front Volts and seconds per division rate selected are for all 5V div 10ns div Captured using a Tektronix TDS 340A digital storage oscilloscope e The equipment layout was changed to minimize the influence of the LC circuit made of stray inductances and stray capacitances of the system But these stray capacitances have not influence on this problem as was shown in chapter 4 e The compensation of the probe of the oscilloscope was checked e All the electric circuit of the impulse generator was checked looking for any deterioration that might amplify any signal e Due to a potential difference bet
47. all of them make up the Faraday cage 34 35 The circle on the impulse generator shows the placement of the generator stack which is made up of capacitors charge resistors front and tail resistors and the sphere gaps as said above All elements in the Farday cage must be connected to earth using star connection The connection point must be as close as possible to the generator s structure This point is C in figure 3 18 The metallic structure of the generator where voltage transformer trigger system and generator stack are mounted is connected to earth by means of copper tape from point C to point A on the metal sheet Voltage divider and insulator s base are connected to earth at the point C on the metallic structure of the generator The control console and the oscilloscope located out of the Faraday shield are connected to earth at the point B All earth connection are made by means of copper tape with section 120 x 1 5 mm Figure 3 19 shows a photography of the Faraday cage of the Laboratory where connection points to earth A B and C are shown A gt A ia ca Figure 3 19 Detail of the earth connections of the equipment points A B and C This enclosure the mesh of conducting material protects electronic equipment people and other goods from lightning strikes and other electrostatic discharges Faraday cage shields the exterior from internal electromagnetic radiation dur
48. alues and tolerances for specific requirements on sphere shape and surface conditions The spheres shall be carefully made so that their surfaces are smooth and their curvature is as uniform as possible and the diameter of each sphere shall not differ by more than 2 from the nominal value As shown in figure 7 1 the spheres can be arranged in two different ways one of which is typical of sphere gaps with vertical axis and the other of sphere gaps with horizontal axis Both of them show the high voltage conductor with series resistor connected to the high voltage sphere and the other sphere is connected to ground 84 85 Figure 7 1 Vertical sphere gap left side and horizontal sphere gap right side 15 The points on the two spheres that are closer to each other are called the sparking points In figure 7 1 D is the diameter of the spheres S is the spacing between them A is the height of the sparking point and B is the distance from the sparking point of the high voltage sphere to any extraneous objects In order to reduce the influence of the shank of the high voltage sphere on the disruptive discharge voltage when the spheres are arranged vertically it shall be free from sharp edges or corners and the standard sets its dimensions It also specifies the dimensions of a stress distributor corona shield if it is necessary to be used at the end of the shank The earthed shank and the operating gear have a smaller effect
49. and mechanical cable holding They must bear cable s mechanical load which is transmitted through them to the tower and keep an electrical isolation between conductor and tower They must resist normal and abnormal voltages and over voltages as far as maximum ones planned Both insulating material and its surface and air surrounding must resist peak voltage values Insulators have to be tested in the same conditions as they will support during their service life They must comply with minimum standard requirements so that customers accept the quality of the product 9 1 Insulator parameters Insulator parameters are specified in every catalogue and they are defined below for a better understanding Puncture voltage is the minimum voltage that causes a portion of an insulator to become electrically conductive A partial or total break of the insulator can happen due to an electrical arc which goes through it It is the voltage across the insulator when installed in its normal manner which causes a breakdown and conduction through the interior of the insulator The heat resulting from the puncture arc usually damages the insulator irreparably Flashover voltage consists of an electric arc through the air between two points of the insulator which have normally nominal voltage This voltage causes that air around or along insulator s surface break down and be able to conduct through it A flashover arc along insulator s outside occurs bu
50. and the instant on the tail when the voltage has decreased to half of the peak value voltage at the instant of chopping The voltage at the instant of the initial discontinuity voltage ratio of a voltage divider The factor by which the output voltage is multiplied to determine the measured value of the input voltage WwW withstand voltage The prospective value of the test voltage that equipment is capable of withstanding when tested under specified conditions 128 128 129 References 1 2 3 4 5 6 7 8 9 A Almeida do Vale T cnica das Altas Tens es apontamentos Faculdade de Engenharia Universidade do Porto Portugal 1997 Ant nio C Sep lveda Machado e Moura T cnicas de Alta Tens o apontamentos Faculdade de Engenharia Universidade do Porto Portugal 2002 03 Martin A Uman Lightning McGraw Hill 1969 F H Kreuger Industrial High Voltage Volume 1 and 2 Delft University Press 1991 Fink Micah How lightning works Available on line http www pbs org wnet savageplanet 03deadlyskies 01lforms indexmid html Access on 30 May 2009 PBS Teachers Physics of Electrical Storms Available on line http library thinkquest org 030ct 00758 en disaster lightning physics html Access on 30 May 2009 Vernon Cooray Energy dissipation in lightning flashes Institute of High Voltage Research University of Uppsala Uppsala Sweden 1
51. and their dimensions are therefore less important As shown in figure 7 1 some clearance limits around the spheres are defined by this standard Thus the distance from the sparking point of the high voltage sphere to any extraneous objects such as ceiling walls and any energized or earthed equipment and also to the supporting frame work for the spheres if this is made of conducting material shall not be less than a distance B The value of this distance B depends on the sphere diameter D and the spacing between spheres S A spacing of 10 cm has been considered in figure 7 2 in order to represent how the minimum value of distance B changes according to their values It is possible to see that the smaller the sphere diameter D the bigger the minimum limit distance B These values were taken from table 1 of the Standard IEC 60052 86 E 2 a q oO S v 3 4 o w S E s E 5 625 10 125 15 25 50 75 100 150 200 Sphere diameter D cm Figure 7 2 Clearance limit minimum value of distance B depending on sphere diameter D both in cm for a spacing between spheres of S 10 cm Supporting frameworks for the spheres made of insulating material are exempted from this requirement provided that they are clean and dry and that the spheres are used for the measurement of alternating or impulse voltages only Other clearance limit showed in figure 7 1 and defined by the standard is the height A of the sparking point of the hi
52. ators ssssssssssssssssssessssssseeee 108 9 3 Test on indoor post insulators of organic material oooooocccoccconcccccnccccnnannnos 110 9 4 Test WAVETOMMS ess sve ceeds vole shite dies cues du Te esse cado Ae 113 Chapter 10 oncion as do drama de chee cede danada ima lira mada aa ade dana cd 115 RES scg A TOR RI GO as 115 Chapter TI uri dadas 117 Further WO Kia a Cesc as DR E ya ceed cha S DRI es GR SD DS GS Ca CS 117 Chapter 12 ARRAES RR 119 CONCLUSIONS sas sure est ana dano ss sacas ao aid ass A AA 119 Chapter 13 oo E AA TETEE TEES 121 Glossary Of terns ii ai 121 RETERENCES oasis nebidri cab pasado d pd da DS dd ds dd ba db nsei 129 List of figures Figure 2 1 Causes of lightning over voltages 4 oooocooccccccccncccccncccncnnconccnccnncnncnnanns 3 Figure 2 2 Comparison of various sizes of convective clouds that produce lightning discharges 3 ces scsecessscves cus Seve cen a a item Cuneo Dee onda Doe doca 5 Figure 2 3 Thunderhead Probable distribution in a thundercloud cumulonimbus mostly accepted Encyclopaedia Britannica Inc 1999 ooooooococccoccconcnonanonaronanonanono 5 Figure 2 4 Lighthins 4 ssa dors A dead oh ister ses 7 Figure 2 5 Shapes of stressing continuous voltages left and temporary over voltages MION sessions a ee eas A Gud reba IRA DADE a a deeeseneeeye 11 Figure 2 6 Shapes of stressing transient over voltages slow front up fast front middle and v
53. ators is justified For the first part PSpice is used because it provides quick simulations and accuracy what permit to create data tables in order to compare the results obtained But when chopped impulses want to be simulated PSpice is limited therefore it is necessary to count on more powerful computer simulators such as PSCAD for these purposes 4 1 Simulation using PSpice By means of computer simulation with PSpice it is possible to determinate peak voltage front time and time to half value of a test waveform and global efficiency of the system It allows finding the best solution to comply with international standards and to compare the results obtained Two simulations are made e Simulation of the Impulse Generator e Simulation of the Impulse Generator under the influence of stray capacitances between the generator stack and the metal sheet on the wall 40 4 1 1 Impulse generator simulation parameters As depicted in section 3 3 the impulse test system is set with the parameters shown there Capacitors of stage are fixed its capacitance is about 500 nF charging resistors are also fixed its resistance is about 18 kQ and the impulse generator has a fixed number of stages 5 stages Therefore there are two parameters R and R that may be changed in order to adjust the test waveform to requirements These requirements may be to obtain certain values of front time T and time to half value T to reach the maximum out
54. bject only dielectric tests with impulse voltages which are performed at the High Voltage Laboratory Requirements in the arrangement of the test object and definitions of the different discharges which can occur in high voltage test are shown below 58 5 1 1 Arrangement of the test object The arrangement of the test object is absolutely important and should be specified by the appropriate apparatus standard The electrical discharge characteristics of the test object may be affected by its general arrangement Its clearance from other energized or grounded structures its height above ground level and the arrangement of the high voltage lead among others may affect the flashover voltage As recommended by the IEEE standard 26 a clearance to nearby structures equal to or greater than 1 5 times the length of the shortest possible discharge path on the test object usually makes proximity effects insignificant 5 1 2 Interpretation of discharges in high voltage tests This section gives the interpretation of electrical discharges in high voltage test according to defined by the appropriate international standards Three different interpretations may be done and are explained below e Disruptive discharges e Nonsustained disruptive discharges e Nondisruptive discharges Disruptive discharge A disruptive discharge is a discharge that completely bridges the insulation under test reducing the voltage between the electrodes pract
55. ble engineer of the High Voltage Laboratory is legally required to assess the risks in the workplace so that it is put in place a plan to control the risks An accident can affect the institution image and a business When an update of the risk assessment of the Laboratory is necessary to do it should be made sure that the responsible engineer and all the users are involved in the process She or he will have useful information about how the work is done that will make the update of the assessment more thorough and effective what guarantees a successful process 98 The process is not complicated the risks are well known and the necessary control measures are in this case easy to apply As defined by Glendon Clarke and McKenna 17 the varieties of technical approach to risk as applied to safety health and environment issues have their origins in engineering An example of this approach is shown below Risk Probability x Magnitude It assumes rationality considering risk as being primarily about seeking safety benefits such that acceptable risk decisions are deemed to be matters of engineering judgement 8 2 How to assess the risks in the workplace A hazard is anything that may cause harm such as electricity at the High Voltage Laboratory The risk is the chance high or low that somebody could be harmed by these and other hazards together with an indication of how serious the harm could be To assess the risks in the
56. charge of capacitor bank should system power be interrupted The charging units are assembled in a steel tank and are vacuum filled with mineral based transformer oil The output connection is provided via a HV coaxial cable to allow the charging supply to be located away from the HV components of the impulse circuit 3 3 7 Trigger system In order to determine the exact moment of firing so that the time sweep of oscilloscopes and other measuring equipment can be tripped a triggered sphere gap according to figure 3 9 is used A trigger impulse of 15 kV is used to break down the annular gap This causes a distortion of the main field ultra violet radiation fills the gap and a source of charged particles occurs at the triggered electrode so that breakdown of the main gap is initiated P Ny X Figure 3 9 Triggered gap 4 The gaps should be in line and they should see each other Thus the ultra violet light of the first gap irradiates the others so that the breakdown is initiated If the radiation is blocked the firing of the generator will be inconsistent and often be incomplete Internal oscillations are prevented by dividing the front resistors R over the stages It would have been more practical to concentrate the front resistance Rp in series with the generator however stray inductance would cause oscillations within the stages which would spoil the wave shape and even cause breakdown of the discharge capacitors 28 Th
57. cles tend to gain a negative charge while lighter particles tend to gain a positive charge in collisions Then charged particles become separated due to differences in size and density moving to certain levels of the cloud system The amount of total charge and polarity is also affected by the temperature in the layer of the cloud the content of the water particles and several other conditions This theory is the most widely accepted although it is just one of many which attempt to explain the properties of the charge built up in electrical storms As it was mentioned above there is not an universal agreement that explains the phenomena To make possible a lightning discharge it is necessary that an electrical path is created and permits electric current to pass through Since opposite charges attract when two regions of strong and opposite charge are present they will attract and exchange electrons When they are separated and cannot exchange electrons through contact they must exchange charge through a medium In thunderhead systems the air serves as the medium between the two regions Since air is not conductive electric current cannot easily pass through it and in order for the regions to exchange electrons the air molecules must be arranged so that electrons can pass through it The process of ionization performs this procedure It is a physical process of converting an atom or molecule into an ion by adding or removing charged partic
58. d equipments in stations and substations behave under such over voltages Over voltage is any voltage between one phase conductor and earth or between phase conductors having a peak value exceeding the corresponding peak of the highest voltage for equipment 10 11 For any insulation configuration an over voltage is any voltage across its terminals higher than the peak of the power frequency voltage existing between them when all phase terminals of the equipments are energized with the highest voltage for equipment 2 3 2 Classification of voltages and over voltages According to their shape and duration voltages and over voltages are divided in the following classes as defined by the IEC 60071 1 34 a continuous voltage b temporary over voltage c transient over voltage d Combined over voltage A continuous voltage is a power frequency voltage A temporary over voltage is a power frequency over voltage of relatively long duration and a transient over voltage is a short duration over voltage of a few milliseconds or less oscillatory or non oscillatory and usually highly damped as defined by the international standard 34 In figure 2 5 below shapes of stressing continuous voltages and temporary over voltages are shown Both kinds of stresses are designed as low frequency stresses For temporary over voltage the over voltage may be undamped or weakly damped In some cases its frequency may be several times sma
59. ds When the hot air mingles comes into contact with colder air the moisture condenses into water droplets Clouds are created when these water droplets become visible The droplets increase in size as the cloud grows and eventually become so heavy that they fall as rain Thunderclouds are large anvil shaped masses that can stretch miles across at the base and reach 12 km or more into the atmosphere Such atmospheric conditions occur for example when cold polar air masses overrun regions of warmer air or when the earth is strongly heated by the Sun and transfers its heat to the air of the lower atmosphere A comparison of various sizes of convective clouds that produce lightning discharges is shown in figure 2 2 Thunderclouds range in size from small clouds which occur in the semitropics and in which the temperature may everywhere be above freezing to giant electrical storms which may have a vertical extent exceeding 20 km gor 2 1 16 Semitropical S 70 warm Giant thunderstorm 50 e 3 20F thundercloud E o 60 1 8 e 60 2 E 50 15 Typical 5 2 E ordinary thunderstorm v S 3 40r E 1 4 5 Ww A que E x 3 1 2 2 Bs 0 lt T CGN lt 0 0 1 20 Figure 2 2 Comparison of various sizes of convective clouds that produce lightning discharges 3 The height of a typical thundercloud is perhaps 8 to 12 km although strictly speaking typical values can only be presented for a given geographic location Wi
60. e equipment are described Further information is available in manuals of the equipment when required The Impulse Test System of the Laboratory is the Hipotronics Impulse Generator Model IG 500 12 5 and is designed for simulating lightning or transient voltage phenomena resulting from equipment association with power distribution systems as required by IEEE and IEC international standards This generator is designed to operate with a charging voltage of 100kV per stage and an energy rating of 12 5kJ per stage 22 The need for components which will reliably withstand short duration over voltage waveforms demands that simple and safe lightning simulators be available This test system includes a control console high voltage power supply capacitor bank with switching arrangement capacitive mixed divider and oscillographic monitoring system Note Equipment user s guides must be read carefully before starting This equipment employs voltages which are dangerous and may be fatal if contacted by operating personnel Extreme caution shall be exercised when working with equipment Never approach or touch a potentially live high voltage circuit without solidly connecting an appropriate ground conductor first 3 3 1 General description The multiplier circuit of Marx has a predefined number of stages Concretely the generator of the High Voltage Laboratory has 5 stages and is shown in figure 3 5 below Each stage is made up of a capac
61. e front Rise time The duration of the wave front of an impulse voltage is the total time occupied by the impulse voltage in rising from zero to the peak value For the sake of convenience of measurement the nominal value T1 of the duration of the wave front is defined as 1 25x the time interval between points on the wave front where the voltage is 10 and 90 of the peak value T1 is expressed in microseconds E error The difference between the measured value of a quantity and the true value of that quantity under specified conditions external insulation The air insulation and the exposed surface of the solid insulation of a piece of equipment which are subject to both electrical stress and the effects of atmospheric and other conditions such as contamination humidity vermin etc F fifty percent disruptive discharge voltage V50 The prospective value of the test voltage that has a 50 probability of producing a disruptive discharge flashover Disruptive discharge external to the insulator and over its surface connecting those parts which normally have the operating voltage between them The term flashover used in this standard includes flashover across the insulator surface as well as disruptive discharges by sparkover through air adjacent to the insulator full lightning impulse A lightning impulse not interrupted by any type of discharge G H 124 indoor post insulator A post insulator not intended to be expos
62. e is 1 2 ps It is different to internal over voltages which always happen with Alternating Current AC at power frequency 50 Hz so its rise time is much longer about ms Next information of the Impulse Generator is given below according to Hipotronics 11 e Total Number of Stages 5 e Maximum Voltage per Stage 100 kV e Total Energy Stored per Stage 2 5 kJ e Maximum Output Voltage 500 kV e Net Generator Capacitance 100 nF e Maximum Load 30000 pF Maximum load without drastically effecting efficiency e Number of Gaps Used 5 It is possible to calculate easily the capacitance value per stage The total energy stored per stage is 36 Therefore the capacitance per stage is 2 5kJ C 100kV gt C 500nF Nl Re 37 3 4 38 38 39 Chapter 4 Computer simulation Computer simulations have become a very useful part in engineering processes They allow to gain insight into the operation of a certain system or to observe its behaviour They are therefore widely used nowadays and make possible to study for example different configurations of the Impulse Test System of the High Voltage Laboratory In this chapter two computer simulators are used OrCAD PSpice 9 1 Student Version and PSCAD v4 2 1 The first one helps to carry out a study about how parameters change according to front and tail resistors and the second one simulates full lightning and chopped impulses The use of two simul
63. e relevant sample and routine tests not included in the type tests b Sample tests The sample tests are carried out to verify the characteristics of an insulator which can vary with the manufacturing process and the quality of the component materials of the insulator Sample tests are used as acceptance tests on a sample of post insulators taken at random from a lot which has met the requirements of the relevant routine tests c Routine tests The routine tests are intended to eliminate defective insulators and are carried out during the manufacturing process Routine tests are carried out on every insulator 9 3 4 General requirements for electrical tests International standards states the requirements listed below for lightning impulse tests a Lightning impulse test methods shall be in accordance with IEC 60060 1 b Lightning impulse voltages shall be expressed by their prospective peak values When the natural atmospheric conditions at the time of test differ from the standard values it is necessary to apply the appropriate correction factors c The post insulators shall be clean and dry before starting the electrical tests d Precautions shall be taken to avoid condensation on the surface of the post insulator especially when the relative humidity is high The standard 1 2 50 lightning impulse shall be used see IEC 60060 1 with the following tolerances e peak value 3 e front time 30 e time to half value
64. e system necessary to create this impulse of 15kV is shown in figure 3 10 PULSE O O E E A S o 28 O a O e COPE IMPU TRIGGER PRA E aoe GROUND our SCOPE Figure 3 10 Trigger system The basic operation is as follows When the pulse button on front panel of the control console see section 3 3 10 is depressed a short circuit across H E is caused And then next sequence occurs 1 Scr 1 fires and discharges capacitor C2 2 Capacitor C2 s energy is discharged into the primary of ignition coil on base 3 Ignition coil steps up voltage and causes ignition between stage 1 sphere gaps 4 Partial ionization triggers first gap and remaining stages fired 5 Capacitor C1 running to terminal A provides a trigger pulse for the Tektronix 507 scope After sparking in the first sphere gap a sequence of sparking in the next sphere gaps occurs This is the way how the capacitor s bank is triggered and impulse voltage for testing is obtained From the impulse out terminal to the trigger electrode in the first gap of the generator there is a delay cable which is fixed length This cable has a fixed delay equal to the transit time of the delay line which is approximately 200ns Terminal G impulse from scope allows remote firing from the scope main frame Power supply of trigger system has the function of supplying a controlled high voltage signal which will be used to ionize the electrode of the spark gap of the first stage
65. easily due to humidity and temperature conditions Local factors as trees some constructions chimneys etc also increase the incidence of lightning strikes And air conductivity can be higher in some areas due to air ion concentration In order to describe the lightning and thunder activity in a region usually keraunic level is used It is defined as the number of days during one year where thunder can be heard in a given area Keraunic level in some areas will vary a lot and it is a long term average which establish a statistical data base with information for different regions This parameter is very useful in order to design electrical installations and its protection system Keraunic level varies according to the part of the world reference values are given below 10 Table 2 2 Reference values for keraunic level in a year Temperate regions of the world 10 to 30 Alps and Pyrenees Europe 30 Florida the USA 100 African rainforest and Indonesia 180 Moreover there are several experimental formulas which try to establish a connection between certain parameters and keraunic level such as the annual number of lightning flashes hitting one square kilometre km of ground and the annual number of lightning flashes and the high of an object pole tower chimney etc but there is not any global accepted standard yet 2 3 Over voltages In some situations electrical equipment can suffer over voltage
66. ed conscientiously to lead to appropriate conclusions Apparently there are three different phenomena which affect the standard waveform and are explained in next section All waves shown in this report are captured and displayed using a Tektronix TDS 340A or Tektronix 2012B digital storage oscilloscope DSO Both DSOs are calibrated to national standards and use the same scope prove for X100 attenuation compensated in accordance with the guidance of IEC 60060 2 The DSO Tektronix 2012B does not belong to the High Voltage Laboratory for that reason it was not described in chapter 3 Two oscilloscopes were used because the Tektronix TDS 340A that belongs to the Laboratory did not have as many options as the Tektronix 2012B which were necessary to capture with more accuracy all the details of the problems In this way a possible influence of the measuring device on the problems was also ruled out As depicted in chapter 3 the measurement of impulse waveforms is performed using a capacitor divider approved and calibrated measuring device in accordance with IEC 60060 2 and 3 where the DSO input is connected to its low voltage arm The study of every theory proposed to solve the problem is shown in this report 6 2 Description of the problems Test waveform presents a shape different to the typical one described by the international standards see section 5 2 Something has effect on it but its origin is unknown They are interference
67. ed further works may be e Adjustment of front and tail times of the generator s waveform by means of simulation and implement the findings in the real equipment of the Laboratory according to international standards here depicted e As said electromagnetic interferences EMI affect the normal behaviour of the equipment A study about how they affect and how to prevent them is a very interesting future work e To perform the calibration of the laboratory equipment With the information showed in chapter 7 and the necessary equipment a table of equivalences may be obtained which will permit to test elements with more accuracy e Implementation of a safety system using an optical sensor Design of the control electronic circuit and the best place to locate it in order to avoid human errors that may be fatal in the High Voltage Laboratory 118 118 119 Chapter 12 Conclusions A wide knowledge of the High Voltage Laboratory is presented in this Master s Thesis from theoretical fundamentals to application of the international standards This work has studied most of the possible future projects which may be carried out and set important points such as the safety in a high voltage installation or the solution to the problems with the measuring device The practical part of this work has given a very useful experience in solving real problems in electrical equipment and it has been collected in this work so that it can be helpful
68. ed to outdoor atmospheric conditions For indoor installations subject to excessive condensation outdoor post insulators or special indoor post insulators may be used impulse An intentionally applied transient voltage or current that usually rises rapidly to a peak value and then falls more slowly to zero instant of chopping The instant when the initial discontinuity appears insulation coordination The selection of the dielectric strength of equipment in relation to the voltages which can appear on the system for which the equipment is intended and taking into account the service environment and the characteristics of the available protective devices By dielectric strength of the equipment is meant here its rated or its standard insulation level as defined below internal insulation Insulation comprising solid liquid or gaseous elements which are protected from the effects of atmospheric and other external conditions such as contamination humidity vermin etc lightning impulse An impulse with front duration up to a few tens of microseconds M N nondisruptive discharge A discharge between intermediate electrodes or conductors in which the voltage across the terminal electrodes is not reduced to practically zero nonself restoring insulation Insulation that loses its insulating properties or does not recover them completely after a disruptive discharge nonsustained disruptive discharge A momentary disruptive discha
69. eeeeeeees 17 3 1 Erwin Marx historical note cc rtirat neres eee eee eeeeene eens eens eeeeeenseeeseeee ees 17 3 2 Marx impulse GENELALOL cece cece nce e cece nee e erre err rare eee eeeeeeseeeseeeseeeseeeseeeee ees 18 3 3 Laboratory equipment s ssrrsrer srir errare rets AE eT TAREKA TAE EN PARENTA E ETE A EET Aa 21 3 4 Preparation of the test wis siccdaeserccernersds SEEE E EE E EET E E Ea 36 Chapter Ai ies 39 Computer Simular 39 4 1 Simulation using PSPICO vico aa TS A aa oeste a o 39 4 2 Simulation USING PSCAD cani add 51 4 31 Conclusion is 56 Chapter Darsana a oe gee ee sete LEE TEL eee eae ac aes a eee ae eee ee ee 57 Standard Techniques for High Voltage Testin3 ooocccccccnccncncccnononanoconccnccncnoncnncnnannss 57 Del Introduction iD AS so E va as aaa aaa 57 5 2 Tests with lightning impulse voltage cece cece cence ence eeeeneeeeeeeseeeseceseeeneees 59 Chapter 6 iscsi ssiccs disses ceca eee basa vector ces eae a Coed eos Won dada Coed ice desde a eee eecadwee 65 Practical problems emerging in measurement cece cece ence ences ec eeeceeneeeeseeneeeeeeeeeeees 65 SA ss heaves fel Soke were SSD A bei bd a aa 65 6 2 Description of the problems esssssessossssssssosssssssososssssssooosssessssoossssssesossses 66 6 3 Other things which were thought as cause of the problems Previous steps 77 6 4 Solution to the problem sa ccs ssensess cents csaaderecssceasonic
70. em shall be used for interpretation 0 ra A t us T l T Ti 1 67 T T 03T 05T Figure 5 1 Full lightning impulse without oscillations or overshoots 5 2 1 Terms used to characterize full lightning impulses Next terms listed below are used to characterize full lightning impulses as defined by the Standard Full lightning impulse A full lightning impulse is a lightning impulse not interrupted by any type of discharge as illustrated in figure 5 1 Value of the test voltage The value of the test voltage for a lightning impulse without overshoot or oscillations is its peak value Virtual front time T4 The virtual front time T of a lightning impulse is 1 67 times the time interval between the instants when the impulse is 30 and 90 of the peak value corresponding to points A and B in figure 5 1 T 1 67 Ty Typ 5 1 60 Virtual origin 04 The virtual origin 0 of a lightning impulse is the instant preceding that corresponding to point A in figure 5 1 by a time 0 3 T This is the intersection with the time axis of a straight line drawn through reference points A and B on the wave front Virtual time to half value T2 The virtual time to half value T of a lightning impulse is the time interval between the virtual origin O and the instant on the tail when the voltage has decreased to half of the peak value 50 of the peak value Standard lightning impulse The standard lig
71. en in tables 2 and 3 of the Standard IEC 60052 for the standard atmospheric conditions for temperature and pressure Temperature to 20 C Pressure bo 101 3 kPa The values were obtained under conditions of absolute humidity h between 5g m and 12g m with an average of 8 5g m In tables 2 and 3 of the Standard are given the values in impulse tests of the 50 disruptive discharge voltages Uso in kV for full lightning impulse voltages of negative and positive polarity respectively These tables are not valid for the measurement of impulse voltages below 10 kV For impulse voltages these values given have an estimated uncertainty of 3 for a level of confidence not less than 95 Note It is recommended that the sphere gap spacing should not be less than 0 05 D as it may be difficult to measure and adjust the gap with sufficient accuracy if the ratio of spacing to diameter is very small No level of confidence is assigned to those values in brackets Figure 7 4 is shown in order to give an idea about how the peak voltage U at which a sphere gap breaks down as a function of the sphere gap spacing and the sphere diameter D This relationship is used for calibrating impulse voltages This figure showed by Kreuger 4 is valid only for estimations therefore the exact values are well documented in tables of the IEC standard 90 aa 200 em D other voltages Sky D FA gt c D Y 1000 Za q 2
72. ershoot of up to 5 a b Figure 3 7 Effect of stray inductances 4 24 25 To reach this approved wave shape the stray inductance Ls of the generator circuit shall satisfy next condition as recommended by Kreuger 4 3 2 The efficiency n of the generator is n an o 3 3 U where U DC charge voltage per stage n number of stages in the generator Up Peak voltage of the output waveform Further information about lightning impulse front time time to half value and efficiency can be found in the instruction manual of the 100 Series Impulse Generator 3 3 2 Generator stack The stack of high voltage capacitors sphere gaps front and tail resistors and charging resistors are placed in this structure And also the trigger system and the motor drive mechanism are mounted on the base For various international standards different wave forms may be simulated by changing plug in resistors located on the capacitor bank Figure 3 8 Detail of the generator stack 3 3 3 High voltage capacitors All impulse capacitors used in Hipotronics Impulse Generators are designed for a minimum of 250000 full voltage shots and provide long trouble free operation They are single metal can dual bushing design to minimize circuit inductance and are mounted on the generator stack Their capacitances are about 500 nF per unit 26 Internal construction consists of Kraft paper Polyester film cast
73. ery fast front CAOWN ccceccceceeeccenceene eens eeeseeeseenseenseeeseeeeeees 12 Figure 2 7 High voltage waves caused by direct lightning stroke on a high voltage line Aire rasr ia as A a sa ote Vaan aa DOR RE RS SATO RS Hadas Da a be 14 Figure 2 8 Tower struck by lightning 4 ccecce cece eee eee eee eee tees eeeeeeeeeeeeeeeeeenees 15 Figure 2 9 Flashover from tower to power line due to lightning stroke in a tower 4 15 Figure 3 1 Simplified impulse generator 1 oococcccccccccncncncccncconccnccnccnncnccnnannss 18 Figure 3 2 Double exponential curve as generated by the impulse generator showed in USA seara E GRA AAG Haslet ae dade ee bobo a o a aes 19 Figure 3 3 Approximate wave shape of lightning impulse used in laboratory testing 1 20 Figure 3 4 Multistage impulse generator 11 oocooccocccccccccnncccccnnccncncnonccnncnccnnos 21 Figure 3 5 Impulse generator ssessossssssesoosssssssososssssssoosssssseooosssesseososssesseosos 22 Figure 3 6 Impulse generator circuit 11 cece ce se eec cece cee ee eee eee eee eee eee eeeeeeeeeeeees 23 Figure 3 7 Effect of stray inductances 4 oooocococcncccncnnconcncccccnccoccccccnccnccnncnnannos 24 Figure 3 8 Detail of the generator Stack ssssssssseseesssssseccessssseecesssssseeeessssseeee 25 Figure 3 9 Triggered gap 4 semen aisis asa vence snort cna aa aia Riad 27 xi Figure 3 10 Trigger syste
74. eter D both in cm for a spacing between spheres of S 10 CM ese eee eee ee eee eeeeeeeee 86 Figure 7 3 Clearance limit minimum and maximum values of height A depending on sphere diameter D both in cm ss sironnan sanna a E a RVE aR 86 Figure 7 4 Correlation between the breakdown strength and the gap distance 4 90 Figure 7 5 Layout of the Laboratory necessary to calibrate the measuring system according to ECO Zn ad KEERA Eaa 93 Figure 7 6 Clearance limit minimum value of distance B in cm depending on sphere gap spacing S in cm for a sphere diameter D 75 CM oooccccccccccccconcnncnnncnncncnnnancnnnons 94 Figure 7 7 Peak values of disruptive discharge voltages in kV for full lightning impulse voltages of negative polarity depending on sphere gap spacing S in cm for a sphere GiaMeterof D 75 Mi A A A AA aa RSRS GE aces 94 Figure 8 1 Current layout of the Laboratory at room J003 of the Faculty of Engineering 99 Figure 8 2 New suggested layout at room J003 of the Faculty of Engineering 100 Figure 8 3 Example of a limit switch OMRON Industrial Automation on the left and the safety system of the Faraday cage on the right oooooocccocccncccnnccncccncccnccanccannos 101 Figure 8 4 Light alarm signal of the High Voltage Laboratory oooooocccccocconcnncnncnnons 101 Figure 8 5 Emergency stop switch of the High Voltage Laboratory 101 xi
75. experimentally evaluate the proposed techniques under study These oscillations greater than the background noise due for example to the firing of spark gaps 29 are permissible as specified by IEC 60060 1 because it does not occur on the part of the waveform in excess of 90 of the peak voltage but at the High Voltage Laboratory the problem is worse In figure 6 12 this signal noise is only up to a 35 of the peak value but at the High Voltage Laboratory it can be up to hundreds of kilovolts kV values even two times higher than the peak value of the impulse voltage waveform Thus this idea is taken into account and the possible electromagnetic coupling from the firing of the test generator to the DSO input is studied The cable which joins the top of the stack of capacitors of the impulse generator to the capacitive mixed divider is removed this test permits to study the signal which is captured by the oscilloscope when the firing of spark gaps occurs and is just transmitted by the air by means of stray capacitances between components It shows that an electromagnetic coupling occurs between them and introduces disturbances to the lightning impulse voltage wave shape An exchange of electromagnetic energy in the form of radiated and absorbed power exists between them It is thought that electromagnetic coupling between circuits affects directly the LV arm of the capacitive mixed divider that is the DSO input because the electromagnetic
76. f megavolts MV Lightning is initiated by discharges within the cloud from which a leader reaches down to earth in steps of about 30 meters This leader is called stepped leader and it reaches the earth in about 10 ms Steps of the leader are directed each time in a different direction and thus the zigzag shape of the lightning is built up The path followed by the leader is often two to three times as long as the distance from cloud to earth From time to time the leader is forked so that the typical branched shape of lightning is formed And for this reason cloud to ground lightning is sometimes referred to as streaked or forked lightning The light of the leader is faint and can not be observed with the naked eye Its current is restricted in the order of 100 A When the leader approaches earth earth s electric field increases a lot and a positive leader travels upwards preferably from a pointed object After making contact a travelling wave moves upwards The front of this wave can travel with values as far as half the velocity of light This is accompanied by intense light so that the visible stage of lightning does not strike downwards as is commonly thought but upwards On its way upwards the branches are brightly illuminated This takes about 50 us and the current level is in the order of 20 to 100 kA But a cloud to ground lightning discharge is made up of one or more intermittent partial discharges Uman 3 calls the total di
77. fore the wave front so that similarity between figures 6 3 and 6 5 can be accepted what reveals that the theory of Kind and Feser 25 is completely fulfilled By means of a damping resistance between the impulse generator and the load capacitance that is the front resistance this oscillation can be appreciably reduced But it is a real limitation because the front damping resistance has to have a specific value in order to damp the oscillations as well as to obtain a time to front according to the standards 1 2 us More information this phenomenon can be found in the reference 25 already mentioned 6 2 2 The run of negative voltage impulses on the wave tail At this point the run of negative voltage impulses on the wave tail is studied As shown in figure 6 2 above the test waveform usually displays negative voltage impulses after the peak These impulses are triangular shaped fall as ramp function and rise very fast almost vertical as step function Such phenomenon is also random and cases like shown in figure 6 2 do not appear so usually Even when in some voltages applications negative voltage impulses appear after the peak it becomes evident when the wave tail voltage falls under 40 30 of the peak value At this time a run of negative voltage impulses triangular shaped with a no constant period no constant frequency is shown on the waveform captured by the DSO and just in a few voltage applications some negative voltage impu
78. gators are often not in good agreement These values may in fact depend on the particular environment in which the lightning discharge is generated The choice of some of the entries in the table is arbitrary Table 2 1 Data concerning cloud to ground lightning discharge 3 Minimum Representative Maximum Stepped leader Length of step m 50 200 Time interval between steps us 50 125 Average velocity of propagation of stepped leader mis 1 5x 105 2 6 x 108 Charge deposited on stepped leader channel C 5 Dart leader velocity of propagation mis 2 0x 108 Charge deposited on dart leader channel C y 1 20 Return stroke Velocity of propagation mis 5 0x 10 1 4x 108 Current rate of increase kKA Us 10 Time to peak current us 2 Peak current KA 10 to 20 Time to half of peak current Us 40 Charge transferred excluding continuing current C 25 Channel length km 5 Lightning flash Number of strokes per flash Time interval between strokes in absence of continuing current ms Time duration of flash s Charge transferred including continuing current C 9 Energy dissipation in lightning flashes was studied by Cooray 7 at the University of Uppsala Sweden The amounts of energy dissipation in different stages of ground flashes were estimated by electrostatic energy considerations Thus energy present in return strokes and ground flashes were measured and it can be described as follows A typical ste
79. ge and efficiency can be measured from the test waveform obtained by means of the measuring device that is in the High Voltage Laboratory the oscilloscope Tektronix TDS 340 A 4 1 3 Impulse generator with stray capacitances simulation parameters The metallic sheet on the wall inside the Faraday cage see figure 3 19 brings about the idea that stray capacitances between the stages and the metallic sheet may affect the normal behaviour of the system and some of the problems that are shown in chapter 6 might have their origin in this theory There are other stray capacitances that have influence on the normal behaviour of the generator system but only the ones between the stages of the generator stack and the metallic sheet are simulated in this section Thus the stray capacitance is calculated by means of equation 4 9 below 7 CHEE 4 9 Where the total area A of the stack of capacitors and spheres is shown in table 4 6 This value is divided by 5 because it is simulated with 5 stray capacitors one per stage The distance d from the generator stack to the metallic sheet on the wall the relative static permittivity of the air between e and the permittivity of free space ey are also shown in this table Values A and d are obtained by means of measuring of the parameters of the equipment 48 Table 4 6 Data for calculating the value of the stray capacitance A 0 8 m 2 90 m o 8 85 pF m Er air 1 0006
80. gh voltage sphere above the earth plane of the laboratory floor It only depends on the sphere diameter D and shall be within the limits shown in figure 7 3 It is seen that the bigger the sphere diameter D the bigger the minimum and maximum limit distances A These values are taken from table 1 of the Standard IEC 60052 900 E Minimum value of height A cm 800 700 E Maximum value of height A cm 600 500 400 300 200 100 2 5 625 10 125 15 25 50 75 100 150 200 Sphere diameter D cm Figure 7 3 Clearance limit minimum and maximum values of height A depending on sphere diameter D both in cm 86 87 The peak values of disruptive discharge voltages shown in the IEC 60052 tables 2 and 3 of the Standard are only valid for clearances around the spheres within the limits given in figures 7 2 and 7 3 It is important that the circuit is arranged so that at the test voltage there is e no disruptive discharge to other objects e no visible leader discharge from the high voltage lead or shank within the space defined by B e no visible discharge from other earthed objects extending into the space defined by B 7 3 Connections The sphere gap shall be connected in accordance with the next specified requirements which have been standardized by the international standard IEC 60060 2 7 3 1 Earthing Normally one sphere shall be connected directly to earth 7 3 2 High voltage conductor
81. han a measuring instrument Measuring high voltage with the aid of a sphere gap is based on the fact that air of know pressure and temperature always breaks down at the same field strength for example air of 1 atmosphere and 20 C needs about 3 kV mm to break down as defined by Kreuger 4 This element is used for calibration processes by determining the gap distance where breakdown takes place A high voltage circuit and its voltage divider can be calibrated with the aid of the international IEC 60052 tables and an inspector can check a test circuit in any laboratory anywhere in the world The method is not very accurate about 3 but it is easy and reliable The sphere gap is the simplest configuration where a uniform and predictable field occurs between electrodes and has been used as a simple and reliable method for measurement of peak voltage in many industrial test facilities for more than 75 years Moreover the mentioned standard provides values for laboratory testing which have been accepted as an International Consensus Standard of Measurements 7 2 Standard sphere gap The standard sphere gap is a peak voltage measuring device constructed and arranged in accordance with the standard IEC 60052 It consists of two metal spheres of the same diameter with their shanks operating gear insulating supports supporting frame and leads for connection to the point at which the voltage is to be measured The standard specifies reference v
82. he equipment but electrical insulators of the power lines are also subjected to the high voltage waves which can easily be as high as 1 MV Therefore it is important to know how these impulse voltages are to design insulators that withstand without deterioration not just its nominal voltage but also high over voltages of short duration A flashover arc can be formed between two metal points such as a power line and the metallic structure of its tower which may stress the insulator see figures 2 1 and 2 7 Lightning stroke Line E A SUB STATION Figure 2 7 High voltage waves caused by direct lightning stroke on a high voltage line 4 The shape of this voltage wave shown in figure 2 7 is determined by the shape of the lightning current This shape varies considerably from stroke to stroke but generally the current reaches its peak in 1 to 5us and falls down to a low value in about 50 to 100us This shape can be described as a e 2 1 The resulting voltage wave has the shame shape and produces the travelling waves as shown in figure 2 7 above u t 4 e e 2 2 Another mechanism that causes voltage waves is related to a lightning stroke in a tower or in the earth conductors which protect the line The lightning current has now to pass the impedance R j L of the tower as shown in figure 2 8 below 14 15 ReyWLl Figure 2 8 Tower struck by lightning 4 The self induction L is determined by the size of
83. he lightning impulse waveform obtained for this circuit is shown in figure 4 2 42 43 500 SKY 400 0KY 200 0K ov 119 2K Os 50us 100us 150us 200us 250us 300us o Yimax_out Time Figure 4 2 Typical lightning impulse waveform obtained in the simulation with PSpice The criterion chosen in order to study how parameters change according to front and tail resistors is to simulate with the highest the lowest and medium values for front and tail resistors The highest values for front and tail resistors are shown in table 4 2 Table 4 2 Highest values for front and tail resistors Remax 75 O Remax 450 O The lowest values for front and tail resistors are obtained with front resistors connected in parallel and tail resistors connected in parallel respectively They are shown in table 4 3 R pin 35Q 75Q 23 862 4 4 Rin 2002 450Q 1392 4 5 Table 4 3 Lowest values for front and tail resistors Remin 23 86 Q Remin 139 0 44 The medium values for front and tail resistors are calculated below and their values are shown in table 4 4 _ 23 8624 752 fmed T 49 470 4 6 1399 450Q R 294 50 4 7 tmed T Table 4 4 Medium values for front and tail resistors R med 49 47 Q Rtmed 294 5 0 4 1 2 Impulse generator obtained Results Obtained values in the simulation process are shown in tab
84. here gap spacing S must be equal to 12 cm therefore the minimum clearance limit B around the sparking point must be 96 cm according to tables 2 and 3 of the Standard 15 In figure 7 5 an area of about 1 meter is drawn around the sparking point of the sphere gap If the area of the Faraday cage would be increased as recommended in next chapter risk assessment higher values of voltage could be used and bigger clearance distances should be set In figure 7 6 for a sphere diameter D 75cm the minimum values of distance B depending on sphere gap spacing S are represented Data were taken from table 1 of the IEC 60052 15 94 o o o o o o N o o 176 208 ref N o o mo o o mo o o Minimum value of distance B cm oa o DE n ai HI sea IN 2 24 28 35 4 5 55 65 7 5 SaR spacing S no Figure 7 6 Clearance limit minimum value of distance B in cm depending on sphere gap spacing S in cm for a sphere diameter D 75 cm Peak values of disruptive voltages for full lightning impulse voltages only depend on the sphere gap spacing S and the sphere diameter D Figure 7 7 shows these voltage values of negative polarity for a sphere diameter D 75cm as established by the table 2 of the Standard IEC 60052 15 q 800 7 695750 oy 665 7254 gt 700 g gt 2 2 600 3 2 E 500 23 E E 400 5 gt E 300 222 gs 200 un 5 100 as s 0
85. htning impulse is a full lightning impulse having a virtual front time of 1 2 us and a virtual time to half value of 50 pus It is described as a 1 2 50 impulse and is internationally accepted 5 2 2 Terms used to characterize chopped lightning impulses However if a flashover arc occurs between both electrodes of the post insulator the waveform results in a chopped impulse It is characterized by an initial discontinuity decreasing the voltage which then falls toward zero with or without oscillations as shown in figure 5 2 Tc Figure 5 2 Lightning impulse chopped on the tail In figure 5 2 line a shows a chopped wave caused by a disruptive discharge and dotted line b shows a chopped wave caused by a nondisruptive discharge 60 61 Next terms listed below characterize chopped lightning impulses Chopped lightning impulse A chopped lightning impulse is a prospective full lightning impulse during which any type of discharge causes a rapid collapse of the voltage The collapse of the voltage can occur on the front at the peak or on the tail figure 5 2 A chopped lightning impulse may occur because of a discharge in the internal or external insulation of a test object Instant of chopping chop time for tail chopped impulses The intersection of the 10 70 line on the chop and the tail of the wave is shown in figure 5 2 Voltage at the instant of chopping The voltage at the instant of chopping is the voltage
86. htning impulse voltages less than 1 88 The conventional deviation z is affected by the condition of the sphere surfaces the availability of free electrons sufficient irradiation the dust contained in the air and the measurement procedures This requirement for the conventional deviation z ensures that the requirements for the surface conditions have been met Note 4 4 of the Standard IEC 60060 1 Disruptive discharge voltages are subject to random variations and usually a number of observations must be made in order to obtain a statistically significant value of the voltage The test procedures are generally based on statistical considerations The p disruptive discharge voltage of a test object is the prospective voltage value which has p probability of producing a disruptive discharge on the test object The conventional deviation z of the disruptive discharge voltage of a test object is the difference between its 50 and 16 disruptive discharge voltages It is often expressed in per unit or percentage value referred to the 50 disruptive discharge voltage 7 4 1 Conduction of the sphere surfaces An accurate field distribution is obtained by satisfying the following requirements in this section The curvature of the surface of the spheres shall be constant The surfaces in the neighbourhood of the sparking points shall be cleaned and dried but they do not need to be polished They must be smooth free of defects a
87. ic material The international standard IEC 60660 states the way to perform tests on indoor post insulators of organic material for systems with nominal voltages greater than 1000 V up to but not including 300 kV 110 111 This is applicable to post insulators of organic material for indoor service in electrical installations or equipment operating in air at atmospheric pressure on alternating current with a nominal voltage greater than 1000 V up to but not including 300 kV as defined by range of IEC 60071 1 and a frequency not greater than 100 Hz Composite insulators are not covered by this standard 9 3 1 Values which characterise a post insulator of organic material According to the Standard a post insulator of organic material is characterised by the following values where applicable the specified dry lightning impulse withstand voltage the specified dry power frequency withstand voltage the specified lightning impulse puncture voltage for post insulators of design category B only the specified mechanical failing loads the specified significant dimensions the maximum difference between the deflection at 20 and 50 of the specified mechanical failing load Service voltage is not considered as a characteristic of a post insulator The withstand voltages of post insulators under service conditions may differ from the voltages under standard testing conditions 9 3 2 Normal service conditions Normal
88. ically to zero These disruptive discharges are subject to random variation and usually a number of observations have to be made in order to obtain a statistically significant value of the disruptive discharge voltage Nonsustained disruptive discharge A nonsustained disruptive discharge is a discharge in which the test object is momentarily bridged by a spark or arc During these events the voltage across the test object is momentarily reduced to zero or to a very small value Depending on the characteristics of the test circuit and the test object a recovery of dielectric strength may occur and may even permit the test voltage to reach a higher value Such an event shall be interpreted as a disruptive discharge for post insulator tests as most of apparatus Nondisruptive discharge Nondisruptive discharges are those between intermediate electrodes or conductors They may also occur without reduction of the test voltage to zero but are not considered in post insulators tests because it is not possible a discharge between other points than high voltage and ground leads 58 59 5 2 Tests with lightning impulse voltage An impulse without oscillations or overshoot is shown in figure 5 1 Parameters defined in this section are strictly applied to impulses like this The waveform obtained by the equipment of the High Voltage Laboratory has oscillations on the wave front When this or overshoot occur the mean curve drawn through th
89. impulses after peak voltage of the standard wave shape 68 Tek SH 20MS s 1 Acqs 195 kV Voss Chi FBV Figure 6 2 Test wave with three interferences a positive voltage impulse before wave front and other two negative voltage impulses at wave tail Volts and seconds per division rate selected 2V div 2 5us div Captured using a Tektronix TDS 340A digital storage oscilloscope It is seen that the second negative voltage impulse is triangular shaped and falls as ramp function and rises very fast almost vertical as step function When the wave front is enlarged as shown in figure 6 3 a new phenomenon became relevant a superimposed oscillation on the rising edge The point A shows the origin of the lightning impulse waveform Tek SH 100MS s 1 Acqs 295 kV Asus gt W sons Chi 2 6V Figure 6 3 Test wave front with a fast positive voltage impulse and superimposed oscillation on the wave front Volts and seconds per division rate selected 2V div 500ns div Captured using a Tektronix TDS 340A digital storage oscilloscope And finally when the parameter seconds per division of the oscilloscope is increased the interferences on the wave tail are shown as a run of negative voltage impulses see figure 6 4 triangular shaped as well with a no constant period Its period increased with time and it goes from a few to tens of us This phenome
90. in the High Voltage Laboratory In short this report presents a collection of the most important information necessary for further works at the Laboratory what will allow reaching deeper points of knowledge in High Voltage Engineering 120 120 121 Chapter 13 Glossary of terms The most relevant terminology of this report is shown in this chapter In every engineering work it is necessary to define the most important terms applicable in a proper way A accuracy The degree of agreement between a measured value and the true value assured disruptive discharge voltage The prospective value of the test voltage that causes disruptive discharge under specified conditions B breakdown voltage is the voltage at which the insulation between two conductors breaks down It is the minimum voltage that causes a portion of an insulator to become electrically conductive The electrical breakdown of an insulator due to excessive voltage can occur in one of two ways puncture voltage or flashover voltage C chopped lightning impulse A prospective full lightning impulse during which any type of discharge causes a rapid collapse of the voltage chop time is the time interval between virtual origin and break down conventional deviation of the disruptive discharge voltage z The difference between the 50 and 16 disruptive discharge voltages 122 creepage distance Shortest distance along the contours of the external surfaces of
91. indicative values Thus this section aims to set the principles and give an example in order to make the correct calibration of the High Voltage Laboratory in the future 7 7 1 Layout with clearance limits The general arrangement of the sphere gap at the High Voltage Laboratory was considered vertical as shown in figure 7 1 left side The layout shown in figure 7 5 makes possible to perform the calibration of the system complying with clearance limits established 92 93 Electric Panel Control Console and Oscilloscope Figure 7 5 Layout of the Laboratory necessary to calibrate the measuring system according to IEC 60052 As it is explained in chapter 8 risk assessment it is not safe to use an output voltage higher than 325 kV with the current layout of the Laboratory because the Faraday cage is too small according to safety distances recommended by Hipotronics 12 between the block of capacitors and any other grounded object as well as the divider should be operated without any ground objects within 1 5 meters of the high voltage arm that is 1 5 meters For a sphere diameter D of 75 cm for example the voltage in the test circuit can be measured by the standard air gap up to 315 kV according to tables 2 and 3 of the Standard 15 complying with the safety requirement above mentioned This maximum peak value of disruptive discharge voltage for full lightning impulse may be of negative or positive polarity and the sp
92. ing testing Out of the Faraday cage the control console and the oscilloscope are located The electrical connection to earth is shown with a yellow line in figure 3 18 Earth connections are detailed below 36 e Faraday cage s earth connection e Earth connection of the metallic structure On this structure are placed the voltage transformer the stages of the impulse generator and other elements of the equipment as the trigger system e Earth connection common point on the metal sheet Metal sheets are on the building s wall and they are two sides of the Faraday cage square viewed from the top e insulator tested earth connection e Earth connection common point of the impulse generator the capacitive voltage divider and the insulator s base e Capacitive voltage divider earth connection The control console and the oscilloscope are connected to earth as well at point B in figure 3 19 3 4 Preparation of the test To make a test about certain electrical equipment at the Laboratory of high Voltage the steps which are shown below must be followed e To place the equipment which will be tested e To set the impulse generator by means of the suitable resistors e To calibrate the wave which is obtained e To execute the test based on international standards Lightning impulses are made up of high current values even higher than 150 kA in some cases and they have very short rise time The standard wave form rise tim
93. ion Switzerland 1999 28 P L Lewin T N Tran D J Swaffield J K Hallstrom Zero Phase Filtering for Lightning Impulse Evaluation a K factor Filter for the Revision of IEC60060 1 and 2 29 D J Swaffield P L Lewin N L Dao J K Hallstrom Lightning impulse wave shapes defining the true origin and its impact on parameter evaluation 30 D Kovac I Kov ov Electromagnetic Coupling of the Electrocal Drive EMC Part Il 31 AGARD Advisory Group for Aerospace Research amp Development Electromagnetic Interference and Electromagnetic Compatibility NATO 32 EMC PARTNER AG Company Brochure Designing and manufacturing portable test and measurement equipment for lightning tests 33 D Kovac Kov ov Electromagnetic Coupling of the Electrocal Drive EMC Part 34 IEC 60071 1 Insulation coordination Part 1 Definitions principles and rules International Electrotechnical Commission Switzerland 1993 35 G L Angel OrCAD PSpice Tutorial 2003 36 Gu a de Iniciaci n a la Herramienta OrCAD PSpice Departamento de Tecnologia Electr nica Universidade de Vigo 37 Orcad Capture User s Guide 2000 130
94. ion rate selected 200V div 5us div Prove attenuation 100X 114 Figure 9 4 Test waveform of an insulator of organic material 65kV per stage were 375kV applied and it is possible to see a positive peak and it is possible to see a positive peak and interferences and a damped oscillation after flashover arc occurs they are object of study Volts and second per division rate selected 200V div 5us div Prove attenuation 100X ooooococcconcconcnonanonaconaronaronanonanons 114 XV xvi List of tables Table 2 1 Data concerning cloud to ground lightning discharge 3 8 Table 2 2 Reference values for keraunic level in a year ccc eee eee scene eee eeeees 10 Table 2 3 Usual values for an impulse voltage cece eee ece eee ee eect ee eee eeeeeeeenees 15 Table 3 1 Generator parameters cceccecc cece sence eee eee eee eee eee eeeeeeeeeeeseeeeneeneees 24 Table 3 2 Main specifications of the capacitive voltage divider 32 Table 4 1 Resistors available for the study of the impulse generator by means of SIMULALION asas secas Aena 40 Table 4 2 Highest values for front and tail resistors cc s scene eee eeeees 43 Table 4 3 Lowest values for front and tail resistors oooooooccccccccnncnncnncnncnnans 43 Table 4 4 Medium values for front and tail resistors
95. is has been necessary to study all the equipment conscientiously in order to carry out tests in the High Voltage Laboratory Firstly the theoretical fundamentals and the equipment available in the High Voltage Laboratory have been presented in order to set a theoretical base and get to know the function of each part of the equipment what is necessary to understand how it works and may help in studying future problems in the equipment Secondly it has been shown two computer simulators that can be used in order to study the equipment On one hand Pspice is a quick simulator that allows obtaining results with accuracy in order to compare different configurations of the equipment On the other hand PSCAD is much better computer simulator but slower Chopped waveforms may only be simulated with this second one Both have their advantages and disadvantages therefore the appropriate one must be chosen according to the requirements and the information given in chapter 4 The main ideas in designing and simulating an impulse test system were shown There were some problems in the measuring device that emerged when tests on electrical insulators wanted to be performed Therefore the main objective of this Master s Thesis changed and focused on searching and solving the problem After a thorough study and conversations with engineers specialized in High Voltage Engineering it was discovered that the problems were caused by the measuring system Actua
96. it Off 60s 137 5s 100MHz ts Vol s Diw 237 5 kV Probe 100 Voltage Invert CH1 5004 M 50 0 us CH1 0 004 20 May 03 18 37 lt 10H2 Figure 6 10 Test waveform that shows electromagnetic interferences EMI Volts and seconds per division rate selected 500V div 50us div 100X probe attenuation It was captured using a Tektronix TDS 2012B digital storage oscilloscope 74 Tek Atle O Acq Complete M Pos 432 0 us CH1 y Coupling BY Limit sw osus sasas oe 50 kV Volts Div Probe 165 kV 100 Voltage Invert CH1 2004 M 100 us CH1 0 00 20 May 09 18 30 lt 10H2 Figure 6 11 Test waveform that shows electromagnetic interferences EMI Volts and seconds per division rate selected 200V div 100us div 100X probe attenuation It was captured using a Tektronix TDS 2012B digital storage oscilloscope The equipment cannot generate higher voltages than 125 kV and the stack of capacitors is absolutely discharged after 200 us therefore these interferences are just justified by a problem in the measuring device The solution of the problem is explained in section 6 4 Before the fast positive voltage impulse before the wave front is presented because both problems may be related 6 2 3 The fast positive voltage impulse before the wave front The fast high amplitude positive voltage impulse before the wave front is a random impulse that is its amplitude varies between voltage ap
97. itance a charging resistor front resistor and tail resistor Figure 3 5 Impulse generator Figure 3 6 shows the electrical circuit of the Impulse Test System based on the circuit shown in the previous section It is a generator with 5 stages and with the most important components such as the high voltage charging supply the trigger generator and the motor drive mechanism used to vary the sphere gap spacing 22 23 Motor Drive Charging Generator Mechanism Supply Figure 3 6 Impulse generator circuit 11 Aids As said in section 3 2 all stages are charged in parallel and discharged through sphere gaps when connected in series This means that after discharge the resulting voltage is E n E 3 1 where E Output Voltage Number of generator stages used Charge Voltage per stage k Losses due to system inductance 24 The rest of components of the circuit capacitors and resistors were already defined in section 3 2 2 Each stage is charged to voltage E When the system is triggered a voltage of 2 E appears across gap 2 This is due to the fact that one capacitor high voltage plate is pulled down to zero and a negative charge redistribution appears on the other plate The process is repeated yielding an output voltage of an opposite polarity from the charging supply In practice if a positive wave is desired the polarity of the power supply must be negative output The
98. l and with internal metal fittings as shown in figure 9 1 108 109 Figure 9 1 Post insulator under test This standard is intended to establish standard values of those electrical characteristics mechanical characteristics and dimensions which are essential for the interchangeability of post insulators and post insulator units of the same type Each post insulator is designated for a specific lightning impulse withstand voltage based on the standardized values given in IEC 60071 1 The minimum height to be chosen is determined by one of the electrical characteristics given in the standard i e dry lightning impulse withstand voltage wet power frequency withstand voltage and wet switching impulse withstand voltage as applicable and according to the relevant insulation coordination requirements The operating voltage is not specified because depending on service conditions especially contamination it cannot strictly be correlated with the height of the post insulator The composition of the post insulator i e the number the size and the positioning of insulator units is not specified For a given height of a post insulator however the composition together with insulator profile and size and shape of metal parts can all affect the electrical performance of the post insulator especially the wet switching impulse withstand voltage value 9 2 1 Mechanical characteristics Post insulators are standardized in mechanical strength c
99. l resistance the lower the tail time Both features can be described with the same reason Discharge time constant of the stack of capacitors is directly proportional to resistance as shown by equation 4 8 To R C 4 8 where Tp Discharge time constant in seconds R Resistance in ohms C Capacitance in Farads The main parameters of the impulse generator test can be depicted It easily allows seeing how they change according to values of front and tail resistors Figure 4 3 shows the change of the peak voltage when front resistance increases and for a constant tail resistance of 4500 The higher front resistor the lower peak voltage 409 000 408 000 407 000 406 000 405 000 404 000 Peak voltage kV 403 000 402 000 23 86 49 47 75 00 Front resistance ohm Figure 4 3 Variation of peak voltage in kV according to front resistance in ohms It is considered a constant tail resistance of 4500 For a constant value of tail resistance and varying front resistance the efficiency of the system decreases like peak voltage does that is the higher the front resistance the lower the efficiency Therefore figure 4 4 shows the variation of the efficiency according to tail resistance for a constant value of front resistance of 750 The higher tail resistor the higher efficiency 46 81 00 80 50 80 00 79 50 79 00 Efficiency 78 50 78 00 77 50 13
100. lasses based on values of the specified failing load in the bending test 9 2 2 Dimensional characteristics The following dimensional characteristics are specified e overall height e maximum nominal diameter of the insulating part e fixing arrangements e tolerances e minimum nominal creepage distance for outdoor post insulators only The composition of the post insulator is not specified The amount by which the creepage distance of an insulator may be increased within the specified dimensions varies according to the design and size of the insulator and where increased creepage distance is required it should be the subject of agreement between the 110 manufacturer and the purchaser in order to avoid designs which are unsuitable for service in polluted atmospheres 9 2 3 Table of characteristics Table Il of the Standard IEC 60273 shows the characteristics of indoor post insulators of organic material and with internal metal fittings A part of this table is shown in table 9 1 below Table 9 1 Indoor post insulators of organic material and with internal metal fittings i 2 3 4 5 e 7 8 10 T ht M i A Diff rence Distance maximale muximal entre fl che aniele Diametre a 20 pee ae Tension de fla e nominal Charge de rupture et 50 Tie ceraral Trou central infeneure et D signation tenue ata requence E an En il de la charge de fixation
101. le 4 5 below Maximum and minimum values for maximum peak voltage U front T and tail Tz times and efficiency n are highlighted Table 4 5 Obtained results values for front time 450 00 404 243 80 85 0 294 50 401 885 80 38 107 354 139 00 53 928 139 00 403 613 80 72 As a note it is possible to say that the impulse generator with R 49 470 and Rk 1390 complies with the international standard it will be detailed in chapter 5 The standard waveform has a front time of 1 2 us and a time to half value of 50 us therefore the waveform obtained in the simulation with times 1 34 us and 52 65 us respectively is within tolerance limits defined by the Standard 26 It is possible to list the main features of the impulse generator simulation extracted from data in table 4 5 e The maximum peak voltage and therefore the maximum efficiency are obtained with the minimum value of front resistor and the maximum value of tail resistor and vice versa It is because the lower the front resistance the lower the voltage drop of the discharging circuit And the higher the tail resistor the higher the discharging time and therefore the waveform reaches higher peak voltage 44 45 e The higher the value of front resistor the higher the rise time and vice versa the lower the front resistance the lower the rise time e The higher the value of the tail resistor the higher the tail time and vice versa the lower the tai
102. lectromagnetic coupling from the stack of capacitors to the LV arm of capacitive mixed divider Volts and seconds per division rate selected in both 200V div 250ns div 100X probe attenuation It was captured using a Tektronix TDS 2012B digital storage OscillOSCOPe sce eee eee ee eee eeeeeeeees 76 Figure 6 15 Enlargement of the positive voltage impulse before the wave front Volts and seconds per division rate selected are for all 5V div 10ns div Captured using a Tektronix TDS 340A digital storage OSCILLOSCOPE cee ece cece cece eee ee eee eeeeeeeneeeeee 78 Figure 6 16 Part of the equipment where there was a potential difference with provisional earth CONNECtION ceecceec cece cece eeec tence eect EA ASTANE NSAR 78 Figure 6 17 Representation of the voltage divider ccc ece sees eect sis rsrsr 80 Figure 6 18 Representation of the voltage divider with the explanation to the problems 80 Figure 6 19 Probe Tektronix P6015A cece eee cc cece eee ec cece eee ee eee eee re rara ren 81 Figure 6 20 Measurement of the test waveform made by the probe 81 Figure 6 21 Measurement of the test waveform made by the probe 1 and the capacitive divider 2 je cestos quis onda 82 Figure 7 1 Vertical sphere gap left side and horizontal sphere gap right side 15 85 Figure 7 2 Clearance limit minimum value of distance B depending on sphere diam
103. les such as electrons or other ions This happens when a large amount of charged particles attempts to pass through the neutral medium causing the electrons and protons of the medium to separate in order to create a path between the two regions on which charge can flow Then lightning occurs due to the extreme difference of charge between two regions When the difference of charge reaches a certain point air between both regions becomes ionized that is the air surround breaks down and lightning occurs When that phenomenon happens an extreme amount of energy is used up and is converted into light heat and sound which is seen as lightning and heard as thunder In conclusion lightning is a form of electrical breakdown of ambient air over extreme distances due to the difference of charge between them and as shown in figure 2 3 there are different kinds of lightning discharge that are listed below cloud to air cloud to cloud within cloud a b c d cloud to ground with negative and positive charge on earth s surface The most frequently occurring form of lightning is the intracloud discharge however this report only concerns of ground discharges because they are the ones that affect electric power lines and electrical equipment in stations and substations 2 1 3 The lightning stroke Most of thunderclouds are negatively charged as defined by Kreuger 4 The potential relative to earth may amount to several hundreds o
104. lightning over voltages tests attending to international standards and solving some practical problems which were found These over voltages can run up to hundreds of thousands of volts which cause dielectric stresses on insulators and could endanger normal and safe operation in electrical equipment Electrical insulators are widely used in power station and substation equipment for example insulators are used for disconnectors transformer bushings or condenser bushings in high voltage transmission lines and distribution lines or in traction current lines for railways among other things The work is divided into three parts the first part sets the theoretical fundamentals It is important to know how lightning occurs and the range of over voltages which is object of study The second part presents the experimental work made introduction to laboratory equipment available simulation of the process and study of the different variants and problems found assessment of possible risks of the equipment for the users principles established by the International Standards calibration of the measuring device and test of electrical insulators Finally the third part will show the results and conclusions obtained The steps for further work are given so that new projects can reach deeper points of knowledge and discover new aspects of high voltage engineering It should not be forgotten that in this field experimentation is highly important Key
105. ller or higher than power frequency f 50 Hz or 60 Hz 10 Hz lt f lt 500 Hz T 2 3600s 3600 s gt T gt 0 03 s Figure 2 5 Shapes of stressing continuous voltages left and temporary over voltages right Transient over voltages may be immediately followed by temporary over voltages In such cases the two over voltages are considered as separate events Transient over voltages are divided into e slow front over voltage Transient over voltage with time to peak 20 us lt Tp lt 20 us and tail duration T2 lt 20 ms e fast front over voltage Transient over voltage with time to peak 0 1 us lt T1 lt 20 us and tail duration T2 lt 300 us 12 e very fast front over voltage Transient over voltage with time to peak Tf lt 0 1 ps total duration lt 3 ms and with superimposed oscillations at frequency 30 kHz lt f lt 100 MHz In next figure 2 6 it is possible to see the shapes of these three classes of transient over voltages 1 0 0 5 5000 us T gt 20 us T gt lt 20 ms 1 0 0 9 0 5 0 3 20 us gt T gt 0 1 us T2 lt 300 us Tt 100 ns 2 T gt 3 ns 0 3 MHz lt fi lt 100 MHz 30 kHz lt f lt 300kHz T lt 3 ms Figure 2 6 Shapes of stressing transient over voltages slow front up fast front middle and very fast front down A combined over voltage consists of two voltage components simultaneously applied between each of the two phase terminals of a
106. lly there is not any problem because these interferences does not affect to the parameters calculated in the test but if the standard wave shape wants to be displayed it will be necessary to replace the damaged voltage divider with a new one Electromagnetic interferences also affect the low voltage arm of the voltage divider due to a bad contact therefore the digital storage oscilloscope DSO input may be affected by this effect 116 The international standard IEC 60052 has been studied in depth in order to shown the way how the calibration of the measuring system must be performed As shown it is possible to obtain a linear function that links the peak voltage in the sphere gap and the peak voltage measured by the oscilloscope It will assure accuracy in the results obtained In every High Voltage installation it is necessary to comply with the law assessing risks at the workplace This part has been studied in depth showing the safety systems that already exist in the Laboratory and suggesting new ones to be implemented in the future in order to make a safer workplace It is recommended to enlarge the zone inside the Faraday cage in order to permit tests using higher voltages up to 500 kV Finally a summary of the international standards applicable to electrical insulator tests has been presented what shows the first steps for future works in the High Voltage Laboratory 116 117 Chapter 11 Further work Suggest
107. lse is considered to be approximately linear if the front from 50 up to the instant of chopping is entirely enclosed between two lines parallel to the line E F but displaced from it in time by 0 05 T 62 Figure 5 3 Linearly rising front chopped impulse This impulse is defined by a The time to chopping T which is the time after point F where the slope of the voltage wave becomes and stays negative b The voltage at the instant of chopping c The rise time T which is the time interval between E and F multiplied by 2 5 d The virtual steepness S which is the slope of the straight line E F usually expressed in kilovolts per microsecond KV us 5 2 3 Terms used to characterize impulses with oscillations or overshoot In case of standard waveform shows overshoot or oscillations the determination of the peak value for a lightning impulse depends on the oscillation frequency or overshoot duration If the oscillation frequency is less than 0 5 MHz or exceeds 1 us the peak value is taken as the maximum value of the recorded trace as shown in figure 5 4 ow 0 1 2 3 4 tus 0 I 2 3 4 tus Figure 5 4 Examples of lightning impulses with oscillations or overshoots 1 62 63 If the oscillation frequency is greater than 0 5 MHz or less than 1 us the peak value is determined from the maximum value of the mean curve as shown in figure 5 5 or from the exponential fitting of the front and tail portions f gt 0
108. lses triangular shaped too are captured out of the series 70 71 In figure 6 4 four different test waveforms captured by the oscilloscope are shown They present four different random interferences on the wave tail that affect the standard waveform and are object of study They are captured in four different voltage applications These test waveforms seem to follow a kind of pattern which shows the way to discover the source of the interference on the wave tail Next figure 6 6 shows the test waveform with interferences on the wave tail As said above its period is variable it increases with time and goes from a few to tens of us and from one voltage application to another their amplitude change as well After the run of negative voltage impulses it seems that a residual voltage is kept but it is drained away after a certain time It is not clearly shown in this figure but usually it is in the order of milliseconds Tek ane O Acq Complete M Pos 420 0 us CH1 JW Coupling Em 130 kV BW Limit ida 67 5us 100MHz Volts Div 220 kV Invert CH1 200 M 100 us CH1 0 00 20 May 03 18 52 lt 10Hz Figure 6 6 Test waveform which shows the run of negative voltage impulses on the wave tail Volts and seconds per division rate selected 200V div 100us div 100X probe attenuation It was captured using a Tektronix TDS 2012B digital storage oscilloscope It must be emphasized that such high vol
109. ltage impulse which does not permit to make a right measurement consequently it is possible to state that both periods are equal To remind what is seen in chapter 5 of this report a full lightning impulse wave shape is specified in the international standards as having a front time T of 1 2us 30 which gives the next validity interval 0 84us lt Ty lt 1 56ps Finally it is possible to write as a conclusion that the fast positive voltage impulse before the wave front is due to an electromagnetic coupling on the low voltage arm of the voltage divisor but the positive impulse appears because of a problem in the measuring device that is explained in section 6 4 6 3 Other things which were thought as cause of the problems Previous steps During the search process several engineers gave their advices and some changes were made at the High Voltage Laboratory but they did not succeed They are listed in this section in order to present the information of the whole process and all the steps that were made e Shielding of the cables between the control console and the Faraday cage by means of a metal gutter connected to earth e Substitution of front and tail resistors in order to make sure that the problem was not caused by a deterioration of these elements or possible inductive components e Adjustment of the sphere gap spacings it was thought that it might affect the firing and cause the interferences on the wave front e
110. lue the flashover arc occurs during the wave front period see figure 9 4 However normally the flashover arc will occur during the wave tail period that is because insulator under test is able to withstand the peak lightning voltage without damage but air around it breaks down after some time under the effect of this high voltage value see figure 9 3 Tek AT O cg Complete M Pos 3 480us CH2 lar 7 Coupling BY Limit 100MHz Volts Div Coarse Probe 100 Voltage 4 Invert CH1 2004 CH2 5 004 M 1 00 us CH1 Z 8 004 lt 10Hz Figure 9 4 Test waveform of an insulator of organic material 65kV per stage were 375kV applied and it is possible to see a positive peak and it is possible to see a positive peak and interferences and a damped oscillation after flashover arc occurs they are object of study Volts and second per division rate selected 200V div 5us div Prove attenuation 100X These are the three possible results of the test on an electrical insulator After calibrating the equipment it will be possible to check the real lightning impulse withstand voltage which must be for the insulator under test JO4 125 of 125 kV and to study if this electrical insulator under test is accepted No conclusions has been presented in this chapter in order not to conclude wrong information about the device under test without the accuracy required 114 115 Chapter 10 Results In this Master s Thes
111. m eiir erei rerea EEEa inca abra nes ande ob vents busines bitebntis sealers 28 Figure 3 11 Typical triggering range essssssssosssssessoossssssososssssssooosssesssoosssseseoo 29 Figure 3 12 Motor drive mechanism cceeceee eee eee eee eeeeee eee cer eee eee rrenan rara rara 30 Figure 3 13 Front panel of the control console oooocccncccccccnconccnnoncnnncnncnncnncnnanos 31 Figure 3 14 Resistive voltage divider of the generator cece eee ee eee e scene eeeeenenes 32 Figure 3 15 Sketch of the capacitive voltage divider ccc ce eee eee eect srs rsrs 33 Figure 3 16 Detail of the low voltage arm of the voltage divider the coaxial cable and the measuring device oscilloSCOpe ooooooooroccroncnonanonaronanonaronaronanonanonanons 33 Figure 3 17 Tektronix TDS 340 Asmodeana iieri rn cna Eoo E RR ai 34 Figure 3 18 Layout of the Laboratory and earth CONNecti0nS ooocococccncccnccnncannccnn cos 34 Figure 3 19 Detail of the earth connections of the equipment points A B and C 35 Figure 4 1 Impulse generator circuit used in simulation with PSpice 42 Figure 4 2 Typical lightning impulse waveform obtained in the simulation with PSpice 43 Figure 4 3 Variation of peak voltage in kV according to front resistance in ohms It is considered a constant tail resistance Of 4500 ccc ccc ecce ec ceeceeeceeneeeneeeneeeneees 45 Figure
112. m of a shed or damage to the insulator due to the heat of the surface discharge is not considered as a puncture Q R random error Errors that have unknown magnitudes and directions and that vary with each measurement root mean square rms value of alternating voltage The square root of the mean value of the square of the voltage values during a complete cycle self restoring insulation Insulation that completely recovers its insulating properties after a disruptive discharge sparkover A disruptive discharge between electrodes in a gas or liquid 126 standard chopped lightning impulse A standard lightning impulse chopped by an external gap after2 5us standard lightning impulse A full lightning impulse having a virtual front time of 1 2 us and a virtual time to half value of 50 us surge A transient voltage or current which usually rises rapidly to a peak value and then falls more slowly to zero occurring in electrical equipment or networks in service switching impulse is a voltage impulse applied during dielectric tests complying with standards with a front duration of 0 1 to 0 3 ms and a time to half value of a few milliseconds 14 switching impulse time to crest is the time interval between real origin and peak value of the wave switching impulse half value is the time interval between real origin and 50 of peak value on the wave tail systematic error Errors where the magnitudes and directions are cons
113. minute after the previous voltage application e When resistors are placed in parallel it is necessary to make sure that each resistor is proportional to the energy which it will absorb e Every stage joined in series independently of the stages which are joined in parallel must have the same front and tail resistors 3 3 5 Charging resistors The charging resistors are of similar design to the waveshaping resistors but are typically of a higher ohmic value The charging resistors are of sufficiently high value to allow proper firing of the impulse generator without influencing the wave shape but not so high as to cause unequal charging voltage on the upper stages There are 5 units of about 18kQ mounted on the stack 26 27 3 3 6 Charging supply Each generator is supplied with a reversible polarity power supply that provides the DC voltage for charging the stage capacitors The charging supply provides a high voltage output up to 50mA and 100kV in DC The circuit design for the power supplies consists of a full wave bridge rectifier Since the load of the impulse generator is capacitive no internal capacitors are required within the power supply which simplifies the design All units have a simple polarity reversal mechanism that is motorized for use with C100 M controller as a standard feature Internally the supply contains its own current limiting resistor and system discharge resistor Heavy duty solenoids provide gravity dis
114. n at the High Voltage Laboratory of the Faculty of Engineering The international standard sets forth recommendations concerning the construction and use of standard air gaps for the measurement of peak values of the following four types of voltage e alternating voltages of power frequencies e full lightning impulse voltages e switching impulse voltages and e direct voltages It is necessary to calibrate the measuring system of the High Voltage Laboratory in order to ensure a reliable insulator testing Calibration permits adjustment of the voltage measurement at the control console so that the given voltage is the real one and imprecise values are avoided International Standard IEC 60052 has been prepared by the International Electrotechnical Commission IEC in order to carry out voltage measurement by means of standard air gaps As the purpose of this Master s Thesis is to study lightning impulse voltages this chapter only presents a summary about this part of the international standard which must be consulted when further information or details about the process are requested 84 7 1 Overview Calibration is the act of correlating the readings of an instrument with those of a standard in order to check the instruments accuracy which allows comparison with other experimental data The most widely known instrument for measuring high voltage is the sphere gap although its way of functioning makes it more a calibrating device t
115. n chapter 6 because it might be the reason of some interferences explained there 2 U5 C4 500nF TCLOSE 0 1ms 1 2 U4 C3 500nF TCLOSE 0 1ms 1 2 U3 C2 500nF TCLOSE 0 1ms 1 2 u2 C1 500nF TCLOSE 0 1ms U1 C12 0 489pF Rch5 18kohm C11 0 489pF Rch4 18kohm C10 0 489pF Rch3 18kohm C9 0 489pF Rch2 18kohm c8 0 489pF Rchi 18kohm 49 R1 75 C6 1491pF R2 75 8 C7 R3 371nF 1G 50 120K S0RV aoKv ov 40K 4 dd H La jo RE 1 1 H 00us 200us 300us 400us 500us 600us Time Os 1 o Vmax out Figure 4 8 Lightning impulse waveform obtained in the PSpice simulation with stray capacitances 4 1 4 Impulse generator with stray capacitances obtained results Obtained values in the simulation process with stray capacitances are shown in table 4 8 below Results of the simulation with stray capacitances 2 are compared to the results of the simulation without stray capacitances 1 Table 4 8 Results of the simulation with stray capacitances 2 compared to the simulation without stray capacitances 1 1 35 200 101 170 80 94 2 35 200 100 961 80 77 Comparing results obtained it is possible to say that the effect of stray capacitances between the generator stack and the metallic sheet on the wall causes a decrease of the peak voltage and therefore
116. n the off position due to charges retained by capacitors e To avoid accidents always remove power then discharge and ground by use of grounding rod prior to touching any parts e Do not tamper with interlocks e Do not depend upon door switches or interlocks for protection but always shut down high voltage rectifiers and other power equipment e Under no circumstances should any access gate door or safety interlock switch be removed short circuited or tampered with in any way except by authorized maintenance personnel when considered unavoidable nor should reliance be placed upon the interlock switches for removing voltages from the equipment e Never switch on the equipment while anybody is inside the Faraday cage As recommended before anyone begin using the equipment please read manuals and user s guides carefully in order to be aware of the risks 106 Nowadays a social pressure for greater personal safety in workplaces exists It is necessary a safety culture a leader and a supervisor for a safe performance of every work in this case laboratory testing Using safety rules and systems depicted in section 8 2 insures safety without a wrong use of the available space which will be therefore optimized 106 107 Chapter 9 Test on electrical insulators of organic material The function of electrical overhead transmission lines insulators is to keep the conductor isolated from ground and another conductor
117. n years 18 3 2 Marx impulse generator The first Impulse Generator was developed by Marx at the beginning of the 20 century This system permits to test elements of power lines under a range of over voltages which might occur during a product s service life due to lightning or transient voltage phenomena resulting from equipment association with power distribution systems Basically it consists of a stack of capacitors that are charged in parallel through charging resistors and discharged in series through discharging resistors front and tail resistors It is possible by means of sphere gaps at certain instant when the disruptive discharge voltage is achieved in the air between the sparking points of the spheres a flashover arc occurs and capacitors become connected in series through the short circuit This is probably the most common way of generating a high voltage impulse for laboratory testing because capacitors are charged in parallel and when they connect in series the addition of the voltages permits to reach very high voltage values that is the power supply used to charge all the capacitors is multiplied by means of the use of sphere gaps The operation principle of the Marx Impulse Generator is explained in this section first as a simplified generator and then as a multistage generator because the Impulse Test System of the High Voltage Laboratory is a Marx Generator with five stages 3 2 1 Simplified circuit In figu
118. nd free of dust In normal use the surfaces of the spheres become roughened and pitted When it occurs the surface should be treated If the spheres become excessively roughened or pitted in use they shall be repaired or replaced Moisture may condense on the surface of the sparking points in conditions of high relative humidity causing measurements to become erratic No air currents may be present in order to ensure accuracy however minor damage to the surface of the sphere beyond the region of sparking point is not likely to affect the use of the sphere as a measuring or calibrating device 7 4 2 Irradiation The disruptive discharge voltage of a sphere gap depends upon the availability of free electrons in the gap between the spheres at the moment of application of voltage Actions should be taken if the requirements for conventional deviation z are not met Irradiation is usually required for measurements below 50 kV peak for all sphere diameters and for measurement of voltages with spheres of 12 5 cm diameter and less for all voltage shapes For impulse voltage direct exposure of a sphere gap to the light from the impulse generator gaps may be sufficient otherwise when sufficient irradiation is not available the uncertainly associated with the values for disruptive discharge given in the standard should be increased 88 89 7 5 Reference values The disruptive discharge voltages for various spacing between spheres are giv
119. ndedenseadebaediancentensecdedvecsdecaoes 79 6 9 CONCLUSIONS serasa secede se AENEA EA TAG heen Eee che LEDs seek nee INCA dura 82 CAP 83 Calibration according to IEC 60052 ooooccconcnncnncnncnncnncnnanonoconoconcncconco cnc nn noncnncnnannss 83 DEL OVeIVIEW Guesa latin AS AS A AS AE A A AE AAA AA 84 7 2 Standard sphere Gap essssssesossssssessosssssssooosssssseoosssssseoossssesseosossssssososssee 84 AA A ON 87 7 4 Use of the sphere gap oooococcconcconcconoconoconoconoco nono nora E r nRa EEEREN A En EA EERE 87 Zid Reference VALUES coin A renan sentada aaa 89 7 6 Advantages and disadvantages ooooccoccncconcnncnncnccononcnononococoncnoncnnccncnonanccnnene 91 7 7 How to perform the calibration procesS cooccoocconcconcconcconccnncconccnocccoccnnncnnnss 92 Chapter Baieri iani en EERE IEEE TOE E cae ceucectevedeecuevedcecteuss 97 Risk assessment nsoni N EE EN NEE EEE ENEE even TNO Na a EEES 97 8 1 What is risk assessment eserse nsr eei i nEs i ESENE N EEE EEE EN EEE TES 97 8 2 How to assess the risks in the workplace sssssssssssssssssesssssossssssseeeseseseeeeeeo 98 8 3 CONCLUSIONS aa A NGS ESC T A Dag E Dea a 105 Chapter Oesiel e dec Ad 107 Test on electrical insulators of organic material oooococcconcroncconanonanonaronanonanonanons 107 9 1 Insulator parameters cous cove sive dove ceed oie ANERON AAEE dues Mould A EAEEREN O RASES 107 9 2 Characteristics of indoor and outdoor post insul
120. ng and laboratory testing tests to precisely determine and allow for high electric field effects Theoretical studies are carried out based on macroscopic or microscopic models Macroscopic modelling is used when voltage current and electric fields values in equipment must be tested Microscopic or molecular modelling is used in order to study how insulators behave under voltage and over voltage stresses and especially how ageing and dielectric breakdown mechanisms appear Experimental studies involve high voltage laboratory testing to measure certain parameters but outdoor tests are also carried out when it is necessary to check electric equipment in its final place of use In short it is essential to accompany theoretical studies with experimental testing in order to ensure efficient and safe installations But a second important feature cannot be forgotten frequency A right choice of mathematical models or laboratory tests must be taken into account due to the different kinds of phenomena produced by a wide range of frequencies from alternating voltages of power frequencies 50 60 Hz to full lightning impulse voltages in the order of hundreds of kHz or even MHz Each model is studied and just accepted for a certain range of frequencies considering the right hypothesis Satisfying the requirements on the validity domain insures reliability and quality of results obtained Chapter 2 Theoretical fundamentals Elec
121. noise should not affect the signal inside the measuring coaxial cable RG11 U it is a shielded cable surrounded by a conductive layer earth connected which provides protection of the signal from external electromagnetic interferences Electromagnetic interferences could affect the LV arm of the voltage divider because a bad contact in this element was found but it is not possible to explain as far as shown in next section 6 4 The signal noise due to electromagnetic coupling deliberately permitted by means of taking off the cable from the top of the stack of capacitors to the HV arm of the capacitive mixed divider is captured by the oscilloscope DSO and is shown in figure 6 13 76 Tek age O Acq Complete M Pos 2 680 us CH1 Coupling ld BW Limit 100MHz volts Div Loarse 150 kV 190 kV Probe 100 Voltage 55 kV Invert CH1 200 M 1 00 us CH1 400m lt 10Hz Figure 6 13 The signal noise due to electromagnetic coupling from the stack of capacitors to the LV arm of capacitive mixed divider Volts and seconds per division rate selected in both 200V div 1us div 100X probe attenuation lt was captured using a Tektronix TDS 2012B digital storage oscilloscope The captured signal is a damped oscillatory wave The impulse test system was configured to give rise to 125 kV as maximum output voltage and the signal noise reaches 190 kV peak to peak This value agrees with the measurement of
122. non is also random and usually starts when wave tail voltage value is under 40 30 of the peak value but in some voltage 68 69 applications single negative voltage impulses with the same shape are captured out of the series as shown in figure 6 2 Tek EEE 200kS s A 2 Acqs y Tek SIE 100kS s 1 Acqs e Ej el mery 125 ISS AE AAA J 60kV PIE LI er e 120 kv IN E O lgonv IN e am EV Lts M250 TES RT ay av MM NV Tek SED 200k5 s 1 Acs Fek EE 500kS s e 1 Acqs EH i r ay SE re f MARINS am 4 e ee 110 kV A 3018 _85ps q N os E mm ad INN IN 437 5 kV HY ss HAS KV UI UN yl weeps ne ic Ivo d Arroios TRIO Figure 6 4 Test waveforms that show the run of negative voltage impulses on the wave tail Volts and seconds per division rate selected are for all 2V div 100 250 and 500us div Captured using a Tektronix TDS 340A digital storage oscilloscope Thus as the origin of the interferences is not clearly known it is decided to divide the study in three parts for a better understanding The oscillations on the wave front The run of negative voltage impulses on the wave tail The fast positive voltage impulse before the wave front These three parts are explained below The oscillations on the wave front have an easy explanation therefore this phenomenon is explained firs
123. ns that occur in electrical equipment from stations and substations Thus the problem must be therefore studied in depth to understand how lightning works and the protections that may be designed in order to minimize its effects and electrical breakdown which might be still more costly than a proper high voltage insulation As defined by Uman 3 lightning is a transient high current electric discharge whose path length is generally measured in kilometres Lightning occurs when some region of the atmosphere attains an electric charge sufficiently large that the electric fields associated with the charge cause electrical breakdown of the air The most common producer of lightning is the thundercloud known as cumulonimbus However lightning also occurs in snowstorms sandstorms and in the clouds over erupting volcanoes It can take place entirely within a cloud intracloud or cloud discharges between two clouds cloud to cloud discharges between a cloud and the earth cloud to ground or ground discharges or between a cloud and the surrounding air air discharges 2 1 2 Thunderclouds definition and origin The thundercloud and its electric charges are the sources of lightning Thunderclouds are formed in an atmosphere containing cold dense air aloft and warm moist air at lower levels The warm air at low levels rises in strong updrafts when heated by the Sun carrying water steam into the sky to form clouds and the cold air aloft descen
124. nt Time to chopping Time to peak Total energy stored per stage Unit vector Virtual front time rise time Virtual origin Voltage peak Virtual time to half value in the tail Voltage xxi Chapter 1 Introduction Electrical systems are strongly limited by an important characteristic of electrical energy its storage is not possible on a large scale and it must be produced and transported to the places where it is required just at the moment Production and consumption points are usually far away from each other therefore it is necessary to resort to high voltage values in order to reduce losses in electrical lines and maximizes the efficiency of the electrical transport system Thus there is a wide range of values used in high voltage systems which are divided into five groups MV Medium Voltage HV High Voltage VHV Very High Voltage EHV Extremely High Voltage UHV Ultra High Voltage The high voltage values need an appropriate insulation level and the higher the voltage the higher the cost The process called Insulation Coordination determines the proper insulation levels of the components in a power system as well as their arrangements so that costs can be substantially reduced Insulation structure must withstand voltage and over voltage stresses to which the system or equipment will be subjected This area of knowledge requires simulation studies based on mathematical models scientific modelli
125. of students or other inexperienced visitors All the people must always bear in mind that extreme caution prudence care is required in order to be present at a lightning impulse test 100 101 8 2 3 Evaluate the risks and decide on precautions This section presents safety systems currently used at the Laboratory in order to prevent risks and shows precautions which must be taken by the users of the Laboratory To establish precautions necessary for working with the equipment the door of the Faraday cage has a safety system which can be compared with a limit switch see figure 8 3 When the door is open it cuts off the electrical supply of the control console of the impulse generator and prevents from charging the impulse test system of the Faraday cage on the right Whit voltage supply on a light alarm signal shown in figure 8 4 warns about extremely high voltages are being generated and therefore special care must be taken Figure 8 4 Light alarm signal of the High Voltage Laboratory In case of someone enter the safety area inside the cage without permission an emergency stop switch shown in figure 8 5 makes possible to stop the electrical supply of the system instantly Figure 8 5 Emergency stop switch of the High Voltage Laboratory The control console and measuring system oscilloscope must be placed in a clean room dust free preferably equipped with air conditioned Temperatures
126. oltage u t depends on the charging voltage U showed in the equation 3 3 as A and time constants t and tz that affect to rise and tail time respectively and only depend on values of R C and R2 C gt The international standard see chapter 5 defines the impulse voltage as a biexponential wave with 1 2us of front time and 50us of time to half value 20 The time constants t and T bear a certain relationship to the front and half value times which have to be computed for every wave shape For the 1 2 50us wave the relations are T 2 96 1 0 4 T 0 73 7 3 5 where Ts is the front time and T is the time to half value With values for the time constants t4 68 us and t2 0 4 us Ty and T satisfy the requirements established by the international standard and discharge resistors front and tail resistors have to be set accordingly Thus a waveform as shown in figure 3 3 is obtained U is the maximum output voltage i e the peak voltage and T is the time to peak voltage u t Up L gt Ups Peet toe SIRO rey tar a oe E Y A eer Tp T2 t Figure 3 3 Approximate wave shape of lightning impulse used in laboratory testing 1 A simplified impulse generator according to the circuit showed in figure 3 1 is used for impulse voltages up to 100 200kV For higher voltages the multistage Marx generator is used 3 2 2 Multistage generators As written above the Impulse Test System of the High Voltage
127. oltages up to 40 kV Thus the experiment is carried out using only the first stage of the impulse generator in order not to exceed the voltage limit of the probe The first stage is charged up to 25 kV and no trigger is used to avoid this extra 15kV of the trigger system Therefore the firing is made by reducing the sphere gap spacing to permit the disruptive discharge The probe is a Tektronix P6015A with an attenuation of 1000X and is shown in figure 6 19 Figure 6 19 Probe Tektronix P6015A Firstly it is checked the voltage measurement made by the probe The probe is used instead of the voltage divider and the result is shown in figure 6 20 Tek ma O Acq Complete M Pos 990 045 CH2 Coupling B Limit 100MHz Volts Di e Invert CHI 5 006 CH2 5 00 M 250us CHI 0 004 CH2 vertical position 8 04 divs 40 23 Figure 6 20 Measurement of the test waveform made by the probe The figure shows the typical lightning impulse waveform It is not shown any interference what confirms the theory explained above 82 Figure 6 21 shows two waveforms the waveform number 1 is obtained by the probe and the number 2 is obtained by the capacitive voltage divider which is damaged as is shown ek AE O Acq Complete M Pos 103 0 us CH2 Coupling BW Limit if j II e HE Wolts Diw IM Invert Off CH1 S 00kY CH2 5 00 M 25 0us CH1 0 00 lt 10Hz Figure
128. or oil dielectric with swedged low inductance sections They have an internal discharging resistor this bleeder resistor is connected in parallel in order to discharging the energy stored in the capacitor The internal discharge time constant is several minutes and these resistors should not be relied upon to safety ground the capacitors The capacitor terminals and the capacitor case should always be grounded with a shorting stick before touching any parts of the generator structure by hand 3 3 4 Waveshaping resistors Each stage has provision for up to 4 front and 4 tail resistors to be connected in parallel Depending on the calculated values of resistance required to test a particular load the appropriate combination of resistors may be plugged in accordingly All Hipotronics resistors are non inductively wound Front and tail resistors available in the High Voltage Laboratory are shown below e 2 units of 20 O R 6154 e 3units of 25 O R 6196 e 8 units of 35 O R 6201 e 3units of 60 O R 6197 e 5units of 75 O R 6199 e 6units of 200 O R 4966 e 6units of 450 O R 4967 Resistors selection must be done according to the rules below e To know the amount of energy necessary per stage and for the testing which will be carried out e To install necessary front and tail resistors in each stage according to the amount of energy per stage e Do not do any test with standard resistors and repetitions shall be done not before one
129. ork see chapter 11 The laboratory safety system depicted in section 8 2 3 becomes a more active system if as suggested it is included a system designed by using optical sensors which can be implemented to prevent risks When charges are retained by the stack of capacitors and someone enter the Faraday cage dangerous potentials exist in the circuit and may be fatal These points already shown highlighted in orange in figures 8 7a and 8 7b may have charges retained by capacitors which might be fatal for the users if an electrical discharge occurs Thus this suggested safety system would raise an alarm when someone tries to cross the dangerous area before discharging capacitors by means of the ground rod The optical sensor used in the design might be one as shown in figure 8 8 The placement of this sensor is shown in figure 8 9 Figure 8 8 Optical sensor Monarch Instrument which might be used in the design of the safety system 7800mm Voltage besar vr Generator Y Sula 6650mm Ground R Y 1 1 1 1 1 1 l 1 1 i l i By 1 El Im i 1 1 1 1 R 2553mm 1 Faraday Cage Control Console and Oscilloscope Figure 8 9 Suggested layout for the Laboratory It shows the placement of the optical sensor blue together with the potentially dangerous points orange dotted line and the safety way above purposed 104 105 As a conclusion another further action that must
130. pecifications Table 3 2 Main specifications of the capacitive voltage divider Working Voltage TIRAS OLORES High Voltage HV Arm Capacitive IS00pFES percet Low Voltage LV Arm saFNomina The HV electrode is a single toroid type A stress distributor corona shield is used at the end of the beam in order to avoid corona effect The HV arm is a series of custom made HV capacitor and series resistor elements The capacitor sections and resistors are mounted inside an insulating tube and filled with mineral based transformer oil The LV arm assembly consists of a custom made capacitor section This capacitor along with an impedance matching resistor is mounted in a cylindrical metal can The LV arm assembly connects to the output connector of the HV arm located on the front panel at the base of the Capacitive Mixed Divider 32 33 Figure 3 15 shows the capacitive voltage divider sketch provided by the manufacturer 12 SPECIFATIONS 500KV L l R1 175KV RMS 502 NOMINAL RATIO 250 1 CUSTOMER MEASUREMENT SYSTEM VIA RG11 U CABLE Figure 3 15 Sketch of the capacitive voltage divider 3 3 12 Oscilloscope used with voltage divider An oscilloscope is connected to the low voltage arm of the voltage divider by a coaxial cable This is a RG11 U cable for measuring purposes of 15 meters approximately The signal is attenuated by means of a high voltage passive probe of 100X The digital
131. perimposed oscillation on the wave front Volts and seconds per division rate selected 2V div 500ns div Captured using a Tektronix TDS 340A digital storage oscilloscope 68 Figure 6 4 Test waveforms that show the run of negative voltage impulses on the wave tail Volts and seconds per division rate selected are for all 2V div 100 250 and 500us div Captured using a Tektronix TDS 340A digital storage oscilloscope 69 Figure 6 5 Front oscillation of lightning impulse voltage IEC 601083 2 test data generator impulse waveform Case 11 sssssssssssesssssseeesessssssecesssssseesessssseeee 70 Figure 6 6 Test waveform which shows the run of negative voltage impulses on the wave tail Volts and seconds per division rate selected 200V div 100us div 100X probe attenuation It was captured using a Tektronix TDS 2012B digital storage OSCILLOSC PO EEN pendentes JEG Ye ee dous gaa he web Hoek Tees Race dont ea ea de ed 71 Figure 6 7 Enlarged test waveform which shows the run of negative voltage impulses on the wave tail Volts and seconds per division rate selected 200V div 25us div 100X probe attenuation It was captured using a Tektronix TDS 2012B digital storage OSCINLOSCOPE ii ua cay same rasa pad ARA aaa a Sacra 72 Figure 6 8 Test waveform Volts and seconds per division rate selected 100V div 25ms div 100X probe attenuation It was captured using a Tektronix TDS 2012B digital storage OSCILLOSCOPE
132. phase to phase or longitudinal insulation and earth It is classified by the component of higher peak value temporary slow front fast front or very fast front over voltage 12 13 In order to test the equipment of electric installations the following voltage shapes are standardized 34 a The standard short duration power frequency voltage It is a sinusoidal voltage with frequency between 48 Hz and 62 Hz and duration of 60 s b The standard switching impulse It is an impulse voltage having a time to peak of 250 us and a time to half value of 2500 us c The standard lightning impulse It is an impulse voltage having a front time of 1 2 us and a time to half value in the tail of 50 us d The standard combined switching impulse It is a combined impulse voltage having two components of equal peak value and opposite polarity positive and negative This is a general introduction about voltages and over voltages that may affect to electrical installations From now on this report only deals about lightning voltage impulses 2 3 3 Insulation Coordination Sometimes breakdown is inevitable so it should take place in a spark gap or a surge arrester instead of damage a more important network component As defined by the International Standard IEC 60071 1 the procedure for insulation coordination consists of the selection of a set of standard withstand voltages insulation level of the various components in the network
133. plications from a few to hundreds of kilovolts kV and it is even not captured in some few voltage applications As written before the maximum voltage reached in the equipment is 125 kV when the voltage at the sphere gaps reaches the disruptive voltage value a flashover arc occurs and all the stages become connected in series Due to this reason it is not possible that the wave captured using an oscilloscope is up to 295 kV at any moment see figure 6 3 and the right explanation to this interference must be discovered An impulse waveform generated experimentally in order to analyze a zero phase filter design for a revision of IEC 60060 1 and 2 High Voltage Test Techniques shows the way to find out the reason for this fast positive voltage impulse before the wave front The description made by Lewin Tran Swaffield and Hallstrom 28 in the revision mentioned above presents an unprocessed waveform which was generated from a two stage Marx generator with no load in circuit This waveform displays noise near the origin and may lead to an incorrect estimation of the true origin see figure 6 12 74 75 Voltage V 23 0 1 2 3 4 5 A i 6 Time s x10 Figure 6 12 Experimentally generated impulse voltage with noise near the start of the impulse signal 28 The noise is due to electromagnetic coupling from the test generator to the digital storage oscilloscope DSO input and was deliberately permitted in order to
134. pped leader return stroke process that neutralizes 5 C of charge dissipates about 5 5 x 10 J Of this energy about 3 5 x 10 J dissipates in the return stroke stage and 2 x 10 J in the leader stage A unit length of the first return stroke channel dissipates about 7 x 10 J m A typical dart leader return stroke process that neutralizes 1 C of charge dissipates about 12 x 10 J Of this energy 4 x 10 J dissipates in the return stroke stage and 8 x 10 J in the dart leader stage A unit length of the subsequent return stroke channel dissipates about 8 x 10 J m A typical ground flash with four strokes dissipates about 9 5 x 10 J Of this energy 4 5 x 10 J dissipates in the leader stages and 5 x 10 J dissipates in the return stroke stages In the analysis the following conclusions were also obtained e The charge that maximizes the energy dissipation during the leader stage depends on the charge density of the cloud For the values of cloud charge densities measured in experimental investigations this optimum charge is about 5 C e Fora given amount of charge neutralization a cloud flash dissipates more energy than a ground flash 2 2 Keraunic level There are specific areas in the world which are particularly affected by lightning Its origin is explained by means of three factors topological factors geological factors air ion concentration In some regions thunder clouds cumulonimbus can be formed more
135. put voltage that is maximum peak voltage of the waveform or to have the highest efficiency n of the system As the main objective of this Master s Thesis is to establish the principles in order to adjust the equipment of the High Voltage Laboratory in general it is not chosen any of the requirements said above but it is purposed as a further work to adjust front time and time to half value of the waveform to comply with the international standard As depicted in section 3 1 4 there are available waveshaping resistors with different resistances Then it is interesting to study how parameters of the waveform change according to resistors chosen The generator stack has 5 stages with two places for resistors per stage one place for a front resistor and another for a tail resistor therefore the study only takes into account those resistors which there are at least 5 units These resistors are shown in table 4 1 Table 4 1 Resistors available for the study of the impulse generator by means of simulation Re 330 7530 200 0 450 0 Output results are as said above front time T time to half value Tz peak voltage U and efficiency n The peak value U is the maximum value of the test voltage for a lightning impulse The front time T of a lightning impulse is 1 67 times the time interval between the instants when the impulse is 30 and 90 of the peak value that is T 1 67 Toy To 4 1 The time to half value or tail
136. re 3 1 the simplified circuit is depicted in order to explain the basic operation of an impulse generator The design of the Impulse Test System of the High Voltage Laboratory is based in this circuit Figure 3 1 Simplified impulse generator 1 Capacitors C and C2 are charged in parallel by the DC power supply U These capacitors are called discharge capacitances and they store the energy of the impulse generator The group Cy R and Rz is connected in parallel regarding to the capacitor C as far as the sphere gap is not triggered The voltages of C Cz and the sphere gap are zero at the beginning When the charging process starts the voltage at these elements start increasing and at a certain moment the air between both spheres breaks down and two new circuits appear Thus capacitors C and Cz are discharged through R and R respectively The capacitance C discharges itself through R with a time constant T 18 19 T R 3 1 And the capacitance C discharges itself through R with a time constant t3 In this process a double exponential wave is generated The output voltage u t results from the subtraction of both exponential curves due to the discharges of C and Cz as shown in figure 3 2 Figure 3 2 Double exponential curve as generated by the impulse generator showed in figure 3 1 1 These two charge displacements cause a voltage surge of the shape t t u A e e 3 3 The output v
137. rge o overshoot The value by which a lightning impulse exceeds the defined crest value 124 125 P partial discharge A discharge that does not completely bridge the insulation between electrodes peak value of impulse voltages The maximum value of impulses that are smooth double exponential waves without overshoot post insulator of organic material Post insulator intended to give a rigid support to a live part which is to be insulated from earth and from another live part The whole or part of the material composing the post insulator consists of organic materials i e of material pertaining to the chemistry of the compounds produced from carbon or to the chemistry of the compounds produced from carbon and silicon These organic materials may be used alone or in conjunction with other materials mineral or organic as fillers reinforcements etc p percent disruptive discharge voltage Vp The prospective value of the test voltage that has a p percent probability of producing a disruptive discharge precision The discrepancy among individual measurements prospective characteristics of a test voltage causing disruptive discharge The characteristics of a test voltage that would have been obtained if no disruptive discharge had occurred puncture A disruptive discharge passing through the solid insulating material of the insulator which produces a permanent loss of dielectric strength A fragment breaking away from the ri
138. s that appear on the wave shape and change from time to time Thus the equipment is being affected by a random phenomenon what makes the study longer and more difficult 66 67 As said it is a random problem therefore it does not appear in every voltage application and its shape and amplitude vary between them Thus in figure 6 1 it is possible to see a test waveform obtained in the Laboratory which is not affected by this unknown problem Its wave shape is the typical of a standard waveform as depicted in chapter 5 but front and tail times are not adjusted because it does not have any influence on the problem Tek oye O Acq Complete M Pos 200 0 us CH2 Coupling BY Limit Off SOR 100MHz 440 kV ha Volts Div Darse L Me EE 100 Voltage Invert Off CH1 2004 M 50 0 us CH1 8 004 Figure 6 1 Waveform for a standard lightning voltage impulse test Volts and seconds per division rate selected 200V div 50us div 100X probe attenuation It was captured using a Tektronix TDS 2012B digital storage oscilloscope But usually test waveforms are displayed with positive and negative voltage impulses on it Most of the times voltage amplitudes are even higher than the peak voltage of the standard wave shape In figure 6 2 below it is shown a test waveform as is normally displayed in the oscilloscope It is possible to see a very fast positive voltage impulse before wave front and two single negative voltage
139. s during its service life due to lightning or a failure in other elements of power distribution systems For example power systems which are not grounded are highly susceptible to over voltage during a phase to ground fault These over voltages may produce arcs or sparks which reduce the safe conditions Therefore it is necessary to properly define the voltages and over voltages which can appear in an electrical installation The selection of the dielectric strength of equipment on relation to the voltages which can appear on the system is called insulation coordination The equipment is intended for that voltage taking into account the service environment and the characteristics of the available protective devices The International Electrotechnical Commission IEC has established a number of standardized insulation levels Moreover IEC standards state a series of test requirements so that manufacturers and users of equipment can make standardized agreements on the characteristics of a network component 2 3 1 Definition of over voltage There are three types of over voltage depending on its origin that affect the design of insulation constructions and determine the requirements for over voltage tests 1 AC over voltages 2 Switching impulses 3 Lightning impulses They cause dielectric stresses which can be simulated with the suitable computational systems Then laboratory testing is made in order to know how electrical components an
140. scharge whose time duration is of the order of 0 2 seconds a flash and he calls each component discharge whose luminous phase is measured in tenths of milliseconds a stroke These discharges strokes are repeated at intervals of about 40 to 50 ms usually 3 to 4 times but 10 to 12 times are possible The repeated discharges follow the path of the first discharge This manifold repetition of lightning is too fast for the naked eye so that lightning is seen by us as one heavy flash stepped teader leader discharge in S0 tsec 3 amp repetitions a b c d Figure 2 4 Lightning 4 d Stepped leader the route of the steps determines the typical shape of lightning A positive leader meets the stepped leader A travelling wave moves upwards at about half the velocity of light the discharge thus takes place in about 50 us Several repetitions may take place at intervals of about 50 ms Because of the heavy currents in a narrow channel the temperature rises to about 20000 C and it is in the order of temperature of the surface of the Sun The air expands in a short time and an explosion occurs that it is known as the sound of thunder Data for a normal cloud to ground lightning discharge bringing negative charge to earth are given in table 2 1 The values listed are intended to convey a rough feeling for the various physical parameters of lightning No great accuracy is claimed since the results of different investi
141. st be removed so that it does not affect the mean value and the conventional deviation One of the values has been highlighted for this reason The criterion for the conventional deviation z shall be checked by applying 15 impulses at the voltage level of Uso 1 for lightning impulse voltages There shall be not more than two disruptive discharges The interval between voltage applications shall be not less than 30 s 96 As it is not possible to adjust the divider ratio of the voltage divider the calibration must be done by means of creating a table of equivalences between the peak voltage displayed in the oscilloscope and the peak value of disruptive discharge voltage in the sphere gap This is only an example because the necessary equipment was not available but it shall be obtained a table as shown in table 7 2 It shows a linear function that links both parameters With this relation it is very easy to know the real peak value of the standard waveform for any laboratory testing Table 7 2 Relation between peak value of disruptive discharge voltage in the sphere gap and peak voltage measured in the oscilloscope 350 wo Q o m al o e al o Q o Corrected peak value of disruptive discharge voltage kV No o o 180 1 220 33 289 5 322 3 V peak oscilloscope kV To obtain the table of equivalences of the equipment of the High Voltage Laboratory is purposed a further work
142. standing help and friendship during our work together with Eng Vasques de Carvalho Eng Orlando Tato Eng Neves Gomes and Prof Paulo Portugal All of them have contributed to my work with their experience and ideas Finally cannot forget Prof Dr Manuel P rez Donsi n and all the people in charge of Erasmus programme who gave me the opportunity to come to Oporto discover the neighbouring country its culture and people Vii viii Table of contents RESUMO cas risca crase A sus A cadence is iii ADA ii A A RT PR RA v Acknowledgements voce siccsocociio cn iii vii Table Of COMENTS ANOOO ON5 P sete vestesuedeswaue ix List OF TISUINES sasiss essa sansicesds aros dane bs eed Low o cia xi LISt Of table rr te xvii Abbreviations and symbolS ssessessessecsececcsecsecsecsecoecoecoecoecoecseecsecseceeeo xix Chapter iuris dia ad dica gado da 1 Introduction oi A ee etl eed eee eae eau qu baita 1 Chapter Zinc id ssa ada dona dorsal E A aaa RISOS Ra Rae aaa ad 3 Theoreticalfundamentals iii 3 2 1 Lightning and thunderclouds eeeeeee eee erre eee r rece re racer arara 4 2 2 Keraunie level ess raias sora EDS GR 9 2 3 4 Over picasa tanino paes rep a SAT ong LEA VN dia ANIE ITNI NEET TEA 10 24 impulse voltage orr nrk n renais paes ek Sek nese eb anak ad aud E PERIE EEN E Ea AEE es 13 Chapter 3uisccssoccesiericivosiei aaa 17 Generation of High Voltage impulses and Laboratory Equipment ceeeeeeeee
143. t they are designed to withstand this phenomenon usually without damage 108 Power frequency withstand voltage dry kV is the r m s root mean square value of sinusoidal power frequency voltage that the equipment can withstand during tests made under dry conditions and for a specified time Power frequency withstand voltage wet kV is the r m s root mean square value of sinusoidal power frequency voltage that the equipment can withstand during tests made under raining conditions and for a specified time Lightning impulse is a voltage impulse applied during dielectric tests complying with standards with a front duration in the order of one microsecond especially for standard lightning impulses 1 2 50 microseconds and a time to half value in the order of 50 us Lightning impulse withstand voltage kV peak value is the maximum lightning voltage which can be supported by an insulator without any damage in it Creepage distance mm is the shortest distance along the surface of the insulating material between two conductive parts It is also called leakage distance Clearance distance mm is the shortest air distance between conductors Maximum mechanical strengths such as tensile strength flexural strength compressive strength and impact strength are also insulator parameters but as they are not electrical ones they are not define in this chapter High voltage insulators are designed with a lower flashover voltage than
144. tage a voltage peak to peak of about 220 kV as shown in figure 6 6 is unconnected with the normal behaviour of the system therefore all the electric circuit was checked in order to find some damage component but it was not found any which could affect the normal behaviour of the equipment and cause such voltage amplitude These interferences both the positive voltage impulse and the run of negative voltage impulses show a high voltage value that does not exist in any point of the system it is not generated by the system Moreover if such negative voltage impulses showed in figure 6 2 were real it would cause a sudden fall of the waveform as a chopped waveform see chapter 5 but it does not occur On the contrary the waveform recovers and continues its fall as depicted by the international standard For this reason it is thought that a problem with the measuring system may be the origin of the interferences If the wave tail is enlarged the run of negative voltage impulses triangular shaped are shown more clearly see figure 6 7 Its period is variable and it is seen that in last triangles they do not fall as a straight ramp but as a slightly curved ramp what seems to be the typical charging waveform of a capacitor The residual voltage kept seems to be a charging waveform of a capacitor but with a substantially higher charge time constant 72 Tek Ana O Acq Complete M Pos 395 0us CH1 26 25 1s 37 5us 67 5us mnm
145. tage divider was also found In normal conditions the capacitance of this element is 371 nF but it was measured by means of a digital multimeter and a variable capacitance about 2 nF was discovered It is clearly shown that the problem is caused by a deterioration of components because after trying to repair the low voltage arm it worked for a few time but after some voltage applications the interferences appeared again This bad contact in the LV arm mentioned also causes that electromagnetic interferences affect the DSO input and the fast positive voltage impulse before the wave front occurs In short it is a random phenomenon thus it is not possible to demonstrate it using simulation because there are two causes e A puncture in a capacitor that changes the divider ratio e A bad contact in the low voltage arm that causes the wave tail falling as a slightly curved ramp and recovering quickly to the standard wave shape This bad contact is probably made through a stray capacitance because it seems to be the typical waveform of the charge of a capacitor with a sudden return to the standard wave shape caused probably by a disruptive discharge passing the dielectric of this stray capacitor This effect charge of stray capacitor disruptive discharge is repeated up to all the energy has been drained away The model used to explain the problem is shown in figure 6 17 80 EE aris Capacitive Mixed Divider Impulse Generator
146. tal Storage Oscilloscope Direct Current Electromagnetic Coupling Electromagnetic Interferences Extremely High Voltage High Voltage High Voltage Laboratory Institute of Electrical and Electronics Engineers International Electrotechnical Commission Lightning Impulse Low Voltage Medium Voltage North Atlantic Treaty Organisation Root Mean Square Switching Impulse Ultra High Voltage Very High Voltage xix List of symbols 5 ho e gt vxoamcoa Y c o N Do to Ambient absolute humidity Amplitude Area Angular frequency Atmospheric pressure Atmospheric temperature Capacitance Charge voltage Charge voltage per stage Charging resistance Correction factor Clearance limit around the sparking point Conventional deviation Correction factor for air density Correction factor for humidity Current Efficiency Discharge time constant Disruptive discharge probability 50 disruptive discharge voltage Distance Frequency Front resistance Generator s High Voltage capacitor Height of the sparking point Humidity correction factor Inductance Number of generator stages Output voltage Permittivity of free space P percent disruptive discharge voltage Relative air density Relative static permittivity Resistance Sphere diameter Sphere gap Sphere gap spacing Standard atmospheric pressure Standard atmospheric temperature XX Ta Te U u Stray inductance Tail resistance Time Consta
147. tant throughout the calibration process T time to half value of the wave tail Tail time of an impulse voltage is the total time occupied by the impulse voltage in rising to peak value and declining from that place to half the peak value of the impulse For the sake of convenience the nominal value T2 is measured between the nominal starting point virtual origin of the wave and the point on the wave tail where voltage is one half of the peak value T2 is expressed in microseconds us U uncertainty An estimated limit based on an evaluation of the various sources of error undershoot The peak value of an impulse voltage or current that passes through zero in the opposite polarity of the initial peak V value of the test voltage for lightning impulse voltage The peak value when the impulse is without overshoot or oscillations value of the test voltage for an impulse is the peak value which can be read in the oscilloscope 126 127 virtual front time T1 of an impulse is defined as time interval between 30 and 90 of the peak value multiply by 1 67 T1 1 67 T90 T30 13 1 virtual origin 01 The intersection with the time axis of a straight line drawn as a tangent to the steepest portion of the impulse or response curve virtual origin 01 is the point where the straight line traced on 30 and 90 of the wave front cut on X axis virtual time to half value T2 The time interval between the virtual origin
148. temperature and relative humidity service conditions are defined as follows by the Standard 9 3 3 the ambient air temperature does not exceed 40 C and its average value measured over a period of 24 h does not exceed 35 C the minimum ambient air temperature is 5 C 15 C or 25 C the altitude does not exceed 1000m the ambient air is not materially polluted by dust smoke corrosive or flammable gases and vapours or salt the average value of the relative humidity measured over a period of 24 h does not exceed 95 the average value of the relative humidity measured over a period of one month does not exceed 90 Classification of tests The tests are divided into three groups as follows a Type tests 112 The type tests are intended to verify the main characteristics of a post insulator of organic material which depend mainly on its design the material used and the manufacturing process They are usually carried out on one insulator and once only for a new design or manufacturing process and then subsequently repeated only when the design material or manufacturing process is changed When the change only affects certain characteristics only the test s relevant to those characteristics need to be repeated For this the type tests are divided into three sub groups according to their applicability Type tests shall be carried out only on insulators from a lot which meets the requirements of all th
149. the fast positive impulse made in figure 6 2 195 kV but as written before the amplitude of this interference varies between voltage applications but not its period and duration In any case if the electromagnetic interference affects directly the LV arm the divider ratio is not applicable therefore these voltages would really be lower amplitude ones Figure 6 14 shows the magnified signal noise from figure 6 13 Tek A y O Acq Complete M Pos 650 0ns CH1 Coupling Bim Limit 100MHz Volts Div 1 511s 155 kV Y Probe e 100 _ _ 190 kV 175 ns Voltage Invert CH1 2004 M 250ns CH1 400m 4 lt 10Hz Figure 6 14 The signal noise due to electromagnetic coupling from the stack of capacitors to the LV arm of capacitive mixed divider Volts and seconds per division rate selected in both 200V div 250ns div 100X probe attenuation lt was captured using a Tektronix TDS 2012B digital storage oscilloscope 76 77 In figure 6 14 the waveform presents second order harmonics into it during the first 250 ns after the origin A The period of the signal noise is 175 ns and it is equal to the period of the superimposed oscillation on the front edge as seen in figure 6 3 Its duration is 1 5 us and it is also approximately equal to the duration of the front edge of this wave shown in figure 6 3 which is 1 375 us It is necessary to remark on the fact that this waveform is affected by a negative vo
150. the tower and is in the order of 20uH and the resistance R is mainly determined by the earth resistance of the foundations 10 O is approximately a good value for it When it happens an over voltage U is caused by the lightning current passing R j L The voltage between the top of the tower and earth can be calculated by means of the next equation L 2 L U i R e 1 R ef 2 3 T T Inserting usual values like shown in table 2 3 the equation 2 3 gives a peak voltage of 1 MV Table 2 3 Usual values for an impulse voltage i 50 kA T 1 us T2 50 us The top of the tower reaches a higher potential than the line and flashover occurs from tower to line Electrical insulators must again withstand it without deterioration as described above This is a peculiar phenomenon because flashover takes place from the earthed electrical tower to the power line as shown in next figure 2 9 Figure 2 9 Flashover from tower to power line due to lightning stroke in a tower 4 Consequently a travelling wave enters the line in the way shown in figure 2 7 causing the same consequences 16 16 17 Chapter 3 Generation of High Voltage impulses and Laboratory Equipment The purpose of the High Voltage Laboratory of the Faculty of Engineering is to test electrical components and equipments which may be stressed by lightning impulse voltages that can reach hundreds of kilovolts or even more
151. thin a typical thundercloud there is a turmoil of wind water and ice in presence of a gravitational field and a temperature gradient Out of the interaction of these elements emerge the charged regions of the thundercloud There is not an agreement about the way or ways in that it occurs the exact arrangement of charge in the clouds has not been yet fully understood and there are different hypothesis One of the models hypothesizes that the upper part of the thundercloud carries a preponderance of strong positive charge while the lower part of the cloud carries a strong negative net charge Thus the main charge structure of the thundercloud is an electric dipole The charged regions of this dipole are of the order of kilometres in diameter In addition to the main cloud charges there may be a small pocket of positive charge at the base of the thundercloud that is a weak positive charge at the lower regions A representation of this theory is shown in figure 2 3 below air centre of ue e y positive charge want a Gt ud E cloud a Chad w ae 15C E contre of _ E a 15 C negative charge 8 Lo small centre of _ positive charge sC Figure 2 3 Thunderhead Probable distribution in a thundercloud cumulonimbus mostly accepted Encyclopaedia Britannica Inc 1999 This theory is based on the idea that heavier and larger parti
152. tly and then the other two phenomena are discussed separately 6 2 1 The oscillations on the wave front As shown in figure 6 3 the lightning impulse waveform generated by the equipment of the High Voltage Laboratory has significant noise on the rising edge This superimposed oscillation is clarified by Kind and Feser 25 the firing of the impulse generator causes the shape of the voltage for lightning impulse voltages deviates considerably from the theoretically calculated on the wave front 70 Figure 6 5 shows an example of the voltage waveform with superposed oscillations The noise on the rising edge may cause a wrong measurement of the value at which the signal is at 30 of the peak voltage The front oscillations result from the rapid firing of the upper stages of the multi stage 5 stages generator of the High Voltage Laboratory A voltage is suddenly coupled through longitudinal capacitance of the generator stages to the connecting lead of the load capacitance and it gets reflected at that end This conclusion has not been obtained when the simulation of stray capacitances between the generator stack and the metallic sheet of the Faraday cage was made see chapter 4 12 10 10 Voltage V o nN 1 IN 1 0 1 2 3 4 5 Time s x 108 Figure 6 5 Front oscillation of lightning impulse voltage IEC 601083 2 test data generator impulse waveform Case 11 It is necessary to obviate the fast positive voltage impulse be
153. trical insulators of organic material can be affected by lightning and must withstand high voltage values that are present in it An overview of this phenomenon and their consequences are explained in this chapter It aims to set the theoretical fundamentals and give an introduction in order to carry out other studies in this field If further information is needed books and articles shown in the references can be helpful Not a few industrial high voltage installations are placed in outdoor locations Thus electrical equipments are exposed to temperature and humidity changes wind rain and even occasionally lightning which can cause many problems in the equipments This work is focused on lightning impulses and they are tested at the High Voltage Laboratory They may be caused by the next three circumstances 1 A lightning stroke in the vicinity of a line or a substation A lightning stroke in the tower or in the ground wire of an overhead line 3 A direct lightning stroke in the line 1 Release of induced charge 2 Stroke to tower or earth wire 3 Direct stroke Figure 2 1 Causes of lightning over voltages 4 It is necessary to study how electrical equipments behave under this kind of atmospheric phenomenon in order to establish the necessary safety level so that a certain operational capacity do not be lost 2 1 Lightning and thunderclouds 2 1 1 Lightning Lightning over voltages are the main cause of many breakdow
154. ulator of organic material 25kV per stage were applied 125kV Volts and second per division rate selected 100V div 10us div Prove attenuation 100X If voltage applied on electrical insulator is higher than the maximum lightning voltage which can be supported then the air around breaks down and a flashover arc occurs along its outside Air around is then able to conduct electricity through it and capacitors discharge their stored energy through this way A short circuit is created This effect is shown in figures 9 3 and 9 4 the wave falls quickly almost instantaneous because the maximum lightning impulse withstand voltage for the electrical insulator tested here JO4 125 is 125 kV Figure 9 3 and 9 4 show a chopped tail and a chopped front waveforms respectively 114 Tek E O Acq Complete M Pos 3 480us CH2 2 y 7 Coupling BW Limit Off 100MHz Volts Div Coarse Probe 100 Voltage fc PAN Invert CH1 100 CH2 5 00 M 1 00 us CH1 4 8 004 lt 10Hz Figure 9 3 Test waveform of an insulator of organic material 35kV per stage were applied 175kV and it is possible to see a positive peak and interferences and a damped oscillation after flashover arc occurs they are object of study Volts and second per division rate selected 200V div 5us div Prove attenuation 100X If the maximum lightning voltage which can be supported by the insulator under test and air around is reached before the peak voltage va
155. utting off all power to ensure that circuits are electrically dead before touching any potentially dangerous point of the system as recommend by Hipotronics 11 It might be forgotten due to a human error at any time for this reason a safety way was designed and is ready to be implemented as shown in figures 8 7a for the current layout and 8 7b for the suggested layout above mentioned This design aims to highlight the way which must always be followed when operating and maintenance personnel enter the Faraday cage and remind them to discharge circuits by using of the ground rod It is an absolutely important safety rule because the points highlighted in orange in figure 8 7a and 8 7b impulse generator insulator and capacitive divisor may have charges retained by capacitors and it might be produced an electrical discharge fatal for the users when touched before discharging 102 103 6000mm Impulse Generator Ground Re Electric Voltage Divisor 6650mm Control Console and Oscilloscope b Figure 8 7a and b Layout of the Laboratory It shows a possible safety way that aims to avoid human error which may be fatal for the users if an electrical discharge occurs The design shown in figures a and b was made for the current and the suggested layouts of the Laboratory respectively 104 This section also introduces a new safety system which may be implemented and is proposed as a further w
156. v Figure 8 6 Metallic gutter which protects electrical Cables cece ese eee eee ee teenies 102 Figure 8 7a and b Layout of the Laboratory It shows a possible safety way that aims to avoid human error which may be fatal for the users if an electrical discharge occurs The design shown in figures a and b was made for the current and the suggested layouts of the Laboratory respectively cccecceeeceecceecceneceeeceseeeseeeseeeseeesees 103 Figure 8 8 Optical sensor Monarch Instrument which might be used in the design of the Safety System sir E RAD DS 104 Figure 8 9 Suggested layout for the Laboratory It shows the placement of the optical sensor blue together with the potentially dangerous points orange dotted line and the safety way above purposed cece ccc e eee e eee eec eee eeeceeeceeeeeeeeeseeeseenees 104 Figure 9 1 Post insulator under test cee cce cee ce eee eee eee eee eee crer eee rara rrenan 109 Figure 9 2 Test waveform of an insulator of organic material 25kV per stage were applied 125kV Volts and second per division rate selected 100V div 10us div Prove attenuation 100K onise e E E E eves 113 Figure 9 3 Test waveform of an insulator of organic material 35kV per stage were applied 175kV and it is possible to see a positive peak and interferences and a damped oscillation after flashover arc occurs they are object of study Volts and second per divis
157. ween two parts of the generator structure there was a flashover arc that caused electromagnetic interferences on the measuring equipment This part of the equipment and the new provisional connection to earth made in order to avoid this flashover arc on the metallic structure are shown in figure 6 16 Figure 6 16 Part of the equipment where there was a potential difference with provisional earth connection In short many different tests and changes were performed in order to understand the equipment behaviour and check the validity of all the recommendations 78 79 6 4 Solution to the problem Tests and studies show that the origin of the problem is in the measuring device A puncture in any capacitor of the high or low voltage arm of the capacitive voltage divider shown in figure 3 14 may be the cause of distortion of the standard waveform As it is a random phenomenon this loss of dielectric strength of the capacitor is not permanent but it got worse during the period of work at the High Voltage Laboratory The capacitive divider is made up of stacks of individual capacitors housed in oil filled cylinders of insulating material This puncture permits a disruptive discharge passing the dielectric of the capacitor Therefore the divider ratio change and these high amplitude impulses occur but they are never higher than the maximum output voltage of the generator A possible bad contact in the low voltage arm of the capacitive vol
158. which characterize the insulation of the equipment and also the proper choice of protection equipment such as spark gaps and surge arresters It is recommended that the selected withstand voltages should be associated with the highest voltage for equipment Anyhow this association is for insulation coordination purposes only The requirements for human safety are not covered by this Standard Lightning or switching impulse protective level is the maximum permissible peak voltage value on the terminals of a protective device subjected to lightning or switching impulses under specific conditions 34 2 4 Impulse voltage The difficulty to simulate lightning over voltages resides in the matter that its waveform and amplitude are highly changing Several different kinds of impulses were used in Europe or the United States of America for testing purposes before there was an international standardization More information about the current international standard for high voltage tests can be found in chapter 5 14 The origin of an impulse voltage may be due to two possible causes as defined by Kreuger 4 e direct stroke or e lightning stroke in a tower If a high voltage line is struck by lightning extremely high voltage waves appear in the line and move along it This situation is a direct stroke and can endanger the equipment in the substations Lightning arresters are applied at the entrance of the substation in order to protect t
159. words calibration computer simulation electrical insulator electromagnetic coupling High Voltage Laboratory High Voltage lightning impulse testing risk assessment vi Acknowledgements This Master s Thesis is the last step of my graduation and the first one of my professional career For this reason at this point of my life want to thank my family friends and mates for all their support help and encouragement during my life and especially during my time at the university They all are the most important and it would have been not possible to get here without them First of all thank you dad mum and grandparents for wishing me always a good luck We have shared experiences decisions stress and happiness during the last years Thank you because you all have never been missing whenever needed you Special thanks to my friends from university Pablo Yago el Turko Dani Juan Rubo and Julio cannot imagine better friends it was the best to spend these years with all of you We have had hard moments studying or working but we have also had very nice moments enjoying our resting time moments which hope we will never forget wherever we are To Lisa we have shared all our moments together with our friends in Portugal She has shown me all her support and patience and the most important always believed in me Secondly but not less important am very grateful to Prof Dr Antonio Machado e Moura for his under
160. x TDS 2012B digital storage oscilloscope 72 73 Another experiment is performed Electromagnetic interferences EMI are thought to be the cause of the problems Since front tail and charging resistors of the fifth stage of the generator stack are removed see circuit in figure 3 6 there is an impulse generator with 4 stages but disconnected to the measuring device therefore the discharging of the capacitors occurs through the tail resistors but surprisingly being no connected the generator stack to the voltage divisor the oscilloscope displays the waveforms shown in figures 6 9 to 6 11 The only connection that exists is a connection through a stray capacitance between the fourth stage of the generator and the top of the generator stack The measuring cable is connected to the top of the generator stack and the capacitor of the fifth stage is short circuited during this test as recommended by Hipotronics Tek al ye O Acq Complete M Pos 426 0 us CH1 Coupling BW Limit 225 kV TOUS 15045 100MHz cn Volts Div 237 5 kV Probe 100 Voltage Invert CH1 500 M 100 us CH1 J 0 00 20 May 03 18 34 lt 10Hz Figure 6 9 Test waveform that shows electromagnetic interferences EMI Volts and seconds per division rate selected 500V div 100us div 100X probe attenuation It was captured using a Tektronix TDS 2012B digital storage oscilloscope Tek Al yin O Acq Complete M Pos 186 0 us CH1 Coupling BW Lim
161. xecutive Five steps to risk assessment United Kingdom 2006 19 Henry W Ott Noise Reduction Techniques in Electronic Systems Wiley Interscience 1988 20 Francisco Crespo Sobretensiones en las redes de Alta Tension ASINEL 1975 21 IEC 60273 Characteristics of indoor and outdoor post insulators for systems with nominal voltages greater than 1000V International Electrotechnical Commission Switzerland 1990 22 IEC 60060 1 High Voltage Test Techniques Part 1 General Definitions and Test Requirements International Electrotechnical Commission Switzerland 1989 23 IEC 60060 2 High Voltage Test Techniques Part 2 Measuring Systems International Electrotechnical Commission Switzerland 1994 24 IEC 60060 3 High Voltage Test Techniques Part 3 Definitions and Requirements for on site testing International Electrotechnical Commission Switzerland 2006 25 D Kind K Feser High Voltage Test Techniques Available on http books google com books id nHqQnSM71_cC amp pg PP1 amp dq high voltage test techn iques kind feser amp client firefox a amp hl es Section 1 3 2 Access on 30 May 2009 26 IEEE Std 4 1995 Standard Techniques for High Voltage Testing Institute of Electrical and Electronics Engineers USA 1995 27 IEC 60660 Insulators Tests on indoor post insulators of organic material for systems with nominal voltages greater than 1000 V up to but not including 300 kV International Electrotechnical Commiss

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