Pre-Commissioning Tests on Dry Type Transformer
Contents
1. 500 Voltage ratio 3300 433 Vector Group Dyn1 Type of tap changer Off load tap changer E Polarity amp Vector group Verification TABLE V POLARITY amp VECTOR GROUP CHECK The Calculations of various tests performed on this CRT are as follows A Turn Ratio Test Vector Group Checked Dyn1 O K RACH OOR NNT eta EAEE F Separate Source Withstand Voltage Test a Applied 10KV between HV winding to Earth for 1 m b Applied 3 KV between L V winding amp Earth for 1 min 4 12859 12 859 12 858 12 526 12 525 12 526 B Magnetic Balance Test G Induced AC over voltage withstand test Applied 2 x 433 Volts at 124 8 Hz for 50 Secs H Open circuit test TABLE II PT ratio P 660 110 6 T Tan IN VOLTAGE T CT ratio C 20 1 0 am i he i 125V 25 6 V 12 08 V 7 99 V 17 14 V 25 34 V 13 14 May 2011 Total MF C P WMSM 24 B V M Engineering College V V Nagar Guyjarat India National Conference on Recent Trends in Engineering amp Technology TABLE VI O C TEST MEASUREMENT Voltage 100 110 7233 LE ec No Load Loss in Watts 90 W1 W2 W3 Total M F 41 5 24 996 No Load Loss in Watts 100 52 24 1248 No Load Loss in Watts 110 70 24 1 680 D Short circuit test PT ratio P 440 110 4 CT ratio C 120 1 120 WMSM 0 2 Total MF C P WMSM 96 Temp 28 C TABLE VII S C TEST MEASUREMENT Load Loss in Watts Tap 1 W1 W2 W3 Total M F 50 92. connected to the ground and to the other terminal of the testing transformer The test shall be commenced at a voltage not greater than one third of the full value and be brought up gradually to full value in not more than 15 s After being held for the specified time of 60 seconds it should be reduced to one third or less of the maximum value and the circuit open This test is successful if no collapse of the voltage occurs Careful attention should be given for evidence of possible failure that could include an audible sound such as a thump or a sudden increase in test circuit current Any such indication should be carefully investigated by observations by repeating the test or by other test or determine if a failure has occurred G Induced AC over voltage withstand test In this test twice of the primary voltage and frequency given on LV side to verify the AC withstand strength of each line terminal and its connected windings to earth and other windings the withstand strength between phases and along the windings under test AC Short duration ACSD test is intended to verify the AC withstand strength of each line terminal and its connected windings to earth and other windings the withstands strength between phase and along the windings under test For Um lt 72 5 KV ASCD test is carried out without partial discharge measurements for 60 secs For Um gt 72 5 KV the test is normally performed with partial discharge free operation
3. of the transformer under operating condition However the requirements for partial discharge measurement during the ACSD test may be omitted ACSD test is always performed with the measurement of partial discharge during the whole application of test This test is not a design proving test but a quality control test and is intended to cover temporary over voltages and continuous service stress It verifies PD free operation of the transformers under operating conditions Applied AC voltage shall be as nearly as possible sinusoidal and its frequency is sufficiently above the rated frequency to avoid excessive magnetizing current 13 14 May 2011 during test The test voltage is the peak value of voltage divided by root V2 The test time at full voltage shall be 60 seconds for test frequency up to and including twice the rated frequency For frequency above twice the rated frequency the time duration of test shall be 120 X Rated frequency Test frequency H Open circuit test No load losses are those losses that are incident to the excitation of the transformer No load losses include core losses dielectric losses and conductor losses in the winding due to excitation current These losses change with change in excitation voltage No load current is the current that flows in any winding used to excite the transformer when all other windings are open circuited Fig 4 Open Circuit Test The purpose of the No load test is to measure no lo
4. type transformer Fig 1 Cast Resin type Transformer Pre Commissioning Test Prior to energizing the transformer several pre commissioning tests are done The objective of these tests is to confirm that the transformer has not suffered damage during transit and also to check any inadvertent slips in the factory tests or supply II PRE COMMISIONING TESTS Following tests are done on the transformer to pre determined standards A Turn Ratio Test Ratio between all the three corresponding H V amp L V phases is to be measured on all 13 14 May 2011 B V M Engineering College V V Nagar Gujarat India National Conference on Recent Trends in Engineering amp Technology taps It is desirable to do this test by a Ratio meter But if it is not available a simple test of measuring voltages can also serve the purpose Fig 2 Megger Referring to name plate find out which terminals of H V amp L V correspond to one phase e g for a vector Group of Dyn 11 H V Terminals 1U 1V amp L V terminals 2U 2N correspond to U phase Apply single phase 415V or 240V AC 50 Hz to H V side and measure voltage on the L V side Measure these voltages on all taps and note them in Commissioning Report Repeat for the other two phases These observations should indicate a consistent trend of variation in line with the details given in name plates Numerical values should approximately check with the voltage ratio B
5. 6 4800 Load Loss in Watts Tap 3 53 96 5088 Load Loss in Watts Tap 5 55 96 5280 IV CONCLUSIONS Dry type Resin cast Transformers requires very little maintenance from time to time However inspection should be made at regular intervals amp corrective measures shall be taken when necessary to ensure the most satisfactory service from this equipment Evidence of rusting corrosion on enclosure and deterioration of the insulation and paint should be checked and corrective measures shall be taken whenever necessary 1 2 3 4 5 6 7 REFERENCES Service Manual for installation commissioning operation and maintenance of transformers VOLTAMP TRANSFORMER LTD Testing of Three Phase Power transformer by Brahambhatt Sohil H SVIT VASAD Operation Installation Commissioning amp Maintenance Manual by TRANSDELTA TRANSFORMERS PVT LTD Electrical Testing amp Validation O7 HARA ENGINEERING SERVICES LTD PRESENTATION ON CAST RESIN DRY TYPE TRANSFORMERS by VIJAI ELECTRICALS LTD www eaton com Web Source on Dry type Transformer 13 14 May 2011 B V M Engineering College V V Nagar Gujarat India
6. Magnetic Balance Test This is a simple test to detect shorted turns in a winding Its principle is that shorted turns oppose establishments of flux in that limb because of the current that circulates through the shorted turns Apply single phase 415 or 240V AC To such H V line terminals which would energies U phase Outer limb Use an averaging instrument like a Multimeter Measure the voltage induced in the V phase Center limb and W phase other outer limb Measure also the current drawn by the energized phase This test can then be carried out on L V Side Centre limb being nearer to the energized limb more flux passes through it and less flux in the outer limb The division is around 70 30 amp hence the measured voltages will be having approx this proportion Next energize W phase and measure the other two voltages as before Results should be similar to previous ones Then energize V phase Centre limb As both the outer limbs are symmetrically located w r t centre limb flux will divide equally between them Hence the voltages measured on outer phases will be equal Also for reasons explained earlier currents drawn when outer phases are excited will be equal that for centre phase will be less approx 70 Thus these observations will confirm the healthiness of the windings 13 14 May 2011 In case one of the phases has shorted turns it will draw a comparatively large current when it is energized When other phases are
7. National Conference on Recent Trends in Engineering amp Technology Pre Commissioning Tests on Dry Type Transformer Hardik V Rupareliya Nidhish G Mishra PG Student Dept of Electrical Engineering Gujarat Technological University Gujarat India Associate Professor BVM Engg College Gujarat India hardik thakkar_321 yahoo co in nidhish57 yahoo co in Abstract The purpose of commissioning tests on transformer is to satisfy to pre determined standards that all the equipment erection is correct and that all the equipment connections cables have been installed in accordance with the approved erection drawings and diagrams Furthermore to demonstrate to the satisfaction of the client that the foregoing work has been done and that the equipment functions as designed This paper discussed procedures and testing results of transformer Index Terms Commissioning ACSD CRT I INTRODUCTION Transformer is a device that transfers electric energy from one circuit to another usually with a change in voltage Transformers work only with a varying electric current such as alternating current Transformers are important in the distribution of electric power They raise the voltage of the electricity generated at a power plant to the high levels needed to transmit the electricity efficiently Other transformers reduce the voltage at the locations where the electricity is used Many household devices contain transformers to
8. ad losses at a specified excitation voltage and a specified frequency The no load loss determination shall be based on a sine wave voltage The average voltage voltmeter method is the most accurate method for correcting the measured no load losses to a sine wave basis and is recommended This method employs two parallel connected voltmeters one is an average responding voltmeter the other is a true rms responding voltmeter The readings of both voltmeters are employed to correct the no load losses to sine wave basis For the determination of the no load losses of a single phase transformer or a three phase transformer the frequency of the test source should be within 0 5 of the rated frequency of the transformer under test If the excitation frequency is beyond the specified tolerance then the test voltage shall be adjusted to maintain the V F ratio corresponding to the ratio of rated voltage and rated frequency The voltage is adjusted to the specified value as indicated by voltmeter Simultaneous values of rms voltage rms current power and the average voltmeter reading shall be recorded D Short circuit test For the H V side voltage and the impedance it would be possible to calculate the current which would flow in the H V side with 415V applied to it while keeping L V side shorted If the 415V source can feed that current a short circuit test can be carried out This test would confirm proper contact engagement at all ta
9. energized flux and hence voltage in the shorted phase will be significantly reduced Hence all three observations will indicate the shorted phase Note the observations in the Commissioning Report C Measurement of Insulation Resistance Measure Insulation Resistance IR between windings and between windings and earth with a 2500 1000V Megger preferably motor driven otherwise hand driven Before measuring I R thoroughly clean all the bushings with clean cotton cloth if required using Carbon Tetrachloride Also there should be no external connections to the transformer terminals Check and adjust if required the infinity setting of the Megger Lead wires from the Megger to the transformer should run independently and be permanently clamped They should not have any joints It is known that the I R reading continues to increase initially and for comparison purpose reading is to be taken at 1 minute of energizing It is also known that I R value is dependent on temperature Hence temperature at the time of measurement should be noted Compare the I R values measured with the Factory results keeping in view the temperature at the time of measurements Note the observations in commissioning Report Also measure and note the I R values of the Power Cables D Measurement of Winding Resistance To measure the calculation of I R losses to Calculate winding temperature at the end of Temperature rise test and as a bench mark
10. for assessing possible damage in the field If the winding has tapping then resistance shall be measured at all taps otherwise the resistance of each winding shall be measured by following methods i Voltmeter Ammeter Method 41 Bridge Method Fig 3 Measurement of Winding Resistance The resistance is measured at ambient temperature and then converted to resistance at 75 C B V M Engineering College V V Nagar Gujarat India National Conference on Recent Trends in Engineering amp Technology E Polarity amp Vector group Verification Polarity and phase relation tests are of interest primarily because of their bearing on paralleling two or more transformers Phase relation tests are made to determined angular displacement and relative phase sequence Phase relation or vector group verification test is performed on a three phase transformer The phasor diagram of any three phase transformer that defines the angular displacement and phase sequence can be verified by connecting the HV amp LV leads together to excite the unit at a suitably low three phase voltage taking voltage measurements between the various pairs of leads and then either plotting these values or comparing them for their relative order of magnitude F Separate Source Withstand Voltage Test A normal power frequency such as 50Hz shall be used and the duration of shall be one minute The winding being tested shall have all its parts joined together and
11. p positions Apply 3 Ph 415V 50 Hz to H V side keeping L V side shorted Measure the 3 line currents at all tap positions If the switch is an OFF CIRCUIT switch supply B V M Engineering College V V Nagar Guyjarat India National Conference on Recent Trends in Engineering amp Technology has to be disconnected before changing tap Note the observations in the Commissioning Report Short Circuit Test Current 1 R i i C Measurement of Insulation Resistance at 28 C i H V winding to Earth 38 8 GQ ii L V winding to Earth 680 MQ ui H V winding to LV winding 59 4 GQ D Measurement of Winding Resistance a HV winding Resistance in mQ TABLE II MEASUREMENT OF HV WINDING RESISTANCE 1ULV 1VIW 1W1U 1 2268 226 8 227 0 2 2216 221 17 217 Circuit 2159 215 1 215 5 209 9 209 9 209 9 Fig 5 Short Circuit Test A Consistent trend indicates healthiness If short circuit test is not possible due to limitation of source carry out one tap changing operation over the entire range increasing as well as decreasing b LV winding Resistance in mQ TABLE IV MEASUREMENT OF LV WINDING RESISTANCE 1 404 1 406 2W2N 1 408 HI CASE STUDY This is the study of tests conducted on 11 Oct 2010 of 500 KVA Cast Resin Transformer CRT at VOLTAMP TRANSFOMERS LTD Under guidance of Mr Rajendra Chaudhri and Mr Pankit patel The specifications of the CRT are as follows Job No 18530 1 KVA
12. raise or lower house current voltage as needed For example Television sets and stereo equipment require high voltage where the doorbells and thermostats require low voltages A simple transformer consists essentially of two coils of insulated wire In most transformers the wires are wound around an iron containing structure called the core One coil called the primary is connected to a source of alternating current that produces an alternating current in the other coil This coil called the secondary is connected to a separate electric circuit Transformers are classified into mainly two categories 1 Distribution transformers are used to step down the distribution voltage to a standard service voltage or from transmission voltage to distribution voltage Up to size SOOKVA transformer is called Distribution transformer We can keep this transformer in operation all the 24 hours a day whether they are carrying any load or not 2 Power transformer is used in generating stations and sub stations at each of a power transmission line for stepping up or stepping down the voltage Above size SOOKVA transformer is called Power transformer This transformer is disconnected during light load period A Dry type transformer is defined as a transformer in which the core and windings are not immersed in an insulating liquid A Dry type transformer having one or more windings encapsulated in solid insulation is called an encapsulated winding dry
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