Method and apparatus for line power monitoring for uninterruptible
Contents
1. 1 which are commonly seen by a UPS An idealized form of third harmonic 10 15 20 25 30 35 40 45 50 35 60 65 2 distortion is shown by the waveform labeled 11 in FIG 1 Quite often this sort of distortion is caused by mag netic saturation of transformers and is usually more pronounced at the trailing side of the waveform A flat topped waveform 12 is also shown in FIG 1 which can be caused by a non linear load such as a switched mode power supply To avoid nuisance switching of the UPS where such distorted line voltage sources are pres ent special reference tables can be programmed into the controls for the UPS units so that they will not switch unnecessarily when such a waveform is present In effect the comparison between the input AC waveform and the reference has been desensitized so that a greater distortion of the waveform must occur before a fault will be found than ordinarily would be preferred since the system will now be less sensitive to actual power line failure conditions which may mimic the forms of distortion shown in FIG 1 For example the waveform 13 shown in FIG 2 shows the type of line failure which can occur in a network connected to rotating machinery or ferroresonant transformers In this case the line voltage rings down at a rate which is dependent on the load With a desensitized reference table the voltage on the power line would need to decay to a level substantiall2. The microprocessor is of conventional design which may selected from any of the various commercially available microprocessors or highly intergrated microcomputers and has associated random access memory 68 and pro grammabie read only memory 69 Other conventional UPS circuitry connected to the microprocessor not directly related to the monitoring of the AC input power is an AC output monitor 70 which provides its output to a multiplexer 71 The output of the multi plexer is provided through a rectifier 72 and an analog to digital converter 73 to the microprocessor The mi croprocesor communicates via the interface 42 with the operator or ancillary equipment The output from the microprocessor for controlling the operation of the inverter in the power module 28 is provided through gate control drive latches 75 The microprocessor and other control components are provided with power from a power supply not shown in a conventional manner The controller also includes a phase locked loop which includes a phase sensitive detector and voltage controlled oscillator circuit 80 The phase lock circuit 80 receives the output of a zero crossing detector 79 which is provided with 60 Hz line power at an appropri ate voltage level on a line 81 from the AC processing unit 61 The phase lock circuit provides an output signal on a line 82 to the microprocessor when the circuit 80 is synchronized with the phase of the line power Such phase locked loop
3. 860 149 8 1989 Johnston 361 79 FOREIGN PATENT DOCUMENTS 2025005 12 1971 Fed Rep of Germany 8501157 3 1985 Int l Appl US005229651A Patent Number 5 229 651 45 Date of Patent Jul 20 1993 OTHER PUBLICATIONS Trimble Charles R et al Hewlett Packard Journal Apr 1968 vol 19 No 8 Palo Alto California pp 1 16 Alexander McEachern Some Initial Results of Recent Developments in Graphic Power line Disturbance Monitoring IEEE Transactions on Power App and Systems Mar 1986 pp 571 577 Basic Measurement Instruments PowerScope Techni cal Sales Manual pp 8 9 Basic Measurement Instruments PowerScope User s Guide pp 348 349 Best Power Technology Inc Service Manual Apr 1989 pp 50 51 Primary Examiner A D Pellinen Assistant Examiner Fritz M Fleming Attorney Agent or Firm Foley amp Lardner 57 ABSTRACT AC line monitoring is provided for uninterruptible power supplies to detect AC power system line faults by comparing the presently received cycle of the AC power line signal with a reference waveform The refer ence waveform is formed of a composite of waveforms from prior cycles which adapts over time to the shape of the AC power line waveform so that waveforms other than pure sinusoids can be accepted without trig gering false fault conditions The composite reference waveform may be fo
4. AC line signal at times synchronized with the cycle of the AC line waveform b averaging together the sample values over a plu of cycles of the AC line signal to create a reference waveform given previous reference waveform values and a prior cycle wherein a refer ence waveform value at each sample time com prises a selectively weighted average of the value at that sample time in the prior cycle of the AC line signal and the previous reference waveform value at that sample time c comparing the AC line signal waveform with the 7 reference waveform at corresponding times in the reference waveform and the signal waveform to determine when a selected difference condition between the reference waveform and the signal waveform is exceeded and providing a fault signal when the difference condition is exceeded d providing the fault signal to an uninterruptible power supply to cause it to switch to provide power to a load from a battery in the uninterrupti ble power supply 39 The method of claim 38 wherein the step of aver aging is carried out to provide exponential averaging of the samples over the prior cycles with weights decreas ing exponentially going back in time 40 The method of claim 38 further including the step of phase locking the sample times at which the AC line signal is sampled with the period of the AC line signal so that sampling is carried out at equally spaced points in time over each cycle of the AC l
5. incorporated herein by reference A major factor in the ability of a UPS to switch smoothly from failing line power to battery power is sufficiently rapid detection of the line power fault Pref erably power can be switched to the battery backup within a small fraction of a cycle while nonetheless maintaining a system which is relatively insensitive to minor transient conditions and disturbances which are not indicative of a significant fault An overly sensitive switching scheme would result in false positives such that the UPS would be switched in at a time when it is not necessary One approach which has been used for line fault detection is to provide a table of reference values for each cycle of the power waveform e g the 60 Hz power waveform used in the United States The volt age on the AC power line should conform substantially to a sine wave at the line frequency whose phase does not change Each cycle of the waveform on the AC power line can then be sampled at specific points in time during the cycle and the values of the samples can be compared to the nominal table values with a default being indicated when the difference at each of the sam ple times exceeds a predetermined tolerance limit 7 Certain forms of waveform distortion are commonly seen which do not necessarily indicate a power line failure so that the UPS should not switch when such distortions occur Two examples of such waveform distortion are shown in FIG
6. ing times in the reference waveform and the signal waveform and providing the fault signal to the uninterruptible power supply so that it provides power to the load from the battery when a selected difference condition is exceeded 2 The apparatus of claim 1 wherein the means for producing a reference waveform does so in accordance with the expression 1 5 1 where a is choosen such that 0 lt lt 1 is the present value of the reference at the i th sample point in the th cycle 115 the value the reference at the i th sample point in the k 1 th cycle and SAM 1 is the value of the sample of the AC line input waveform at the th sample point the k 1 th cycle 3 The apparatus of claim 2 wherein a 0 5 4 The apparatus of claim 2 wherein the means for comparing compares to determine if the difference con dition is exceeded in accordance with the expression SAMP D where D is a chosen tolerance value for the sample point i 5 The apparatus of claim 4 wherein the value of choosen to be a constant for all of the sample points i 6 The apparatus of claim 1 wherein the apparatus is adapted for monitoring 60 Hz line power and wherein the means for sampling samples at a selected number of points in each cycle which are equally spaced in time 5 229 651 13 7 The apparatus of claim 6 wherein the means for sa
7. is the value of the sample of the input waveform at the i th sample point in the cycle and at the k 1 th cycle The comparison between the sample and the reference is preferably carried out in accordance with the expres sion SAMP k REF k D where is a choosen tolerance value which can be a constant or which can vary as a function of the cycle sample position i For example where a 60 Hz power line is being sampled sampling can be carried out at 64 times a cycle with the sampling times phase locked to the period of the input signal so that the samples from the subsequent waveforms are taken at the proper points on the cycles By phase locking the sampling of the present cycle to the cycle period samples will be taken at properly corresponding places on the input waveform which can be properly compared to the ref erence waveform even if the line frequency changes slightly To ensure that a slowly varying or ringing down type of line failure is detected the apparatus of the invention further estimates on a substantially real time basis the root mean square RMS value of the input power waveform Preferably this is carried out by an RMS computation on the stored reference waveform When the RMS voltage estimate falls below a threshold value a signal is provided to switch the UPS system to pro vide auxiliary power to the output lines from the bat tery The combination of the RMS estimate fault detec tion and the cont
8. supply of claim 25 wherein the means for comparing compares in accordance with the expres sion where 10 is a chosen tolerance value for the sample point i 28 The power supply of claim 27 wherein the value of D is choosen to be a constant for all of the sample points i 29 The power supply of claim 24 wherein the power supply receives 60 Hz line power and wherein the means for sampling samples at a selected number of points in each cycle which are equally spaced in time 30 The power supply of claim 29 wherein the means for sampling samples at a rate of 64 times a cycle 31 The power supply of claim 24 wherein the means for sampling includes an analog to digital converter for converting the sampled analog value to digital data 32 The power supply of claim 24 wherein the means for sampling includes means for phase locking the sam ple times to the period of the AC line signal so that sampling is carried out at equally spaced points in time over each cycle of the AC line waveform 33 The power supply of claim 24 further including means for providing an estimate of the RMS value of the AC line waveform and providing the fault signal to the power transfer means to switch to supply power from the battery if the RMS estimate value falls below a selected minimum value 34 The power supply of claim 33 wherein the means for estimating the RMS value carries out a root mean square calculation on the referenc
9. systems are well known and have been used in commercially available UPS systems The phase lock loop system also provides an output signal synchronized to the 60 Hz line power at 120 Hz line 83 to the microprocessor and a synchronized 3 84 kHz signal on a line 84 to the microprocessor The 3 84 kHz signal is at the sampling frequency required to sample the 60 Hz input waveform 64 times each cycle Under the control of the microprocessor the time of the sampling by the AC processing circuit 61 is controlled by signals on a line 86 and the operation of the analog to digital converter 64 is controlled so that samples from the present cycle of the AC input waveform are substantially 1 64th of the cycle width apart even if the frequency or phase of the AC input voltage changes slightly Thus each new sample as received by the microprocessor from the converter 64 can be placed by the microprocessor in a proper location in its RAM memory 68 added in a selected manner with the exist ing value for that sample in RAM e g by adding a properly weighted value of the present sample to a weighted value of the existing average to produce the value for the reference waveform in the memory 68 The computer controller can be implemented utiliz ing conventional connections of central processing unit CPU memory peripherals and signal conditioning devices For example the controller logic can be imple mented using a NEC UPD78C10 single chip m
10. 20 before the power outage both in mag nitude frequency and phase The transfer of the power supply from the AC input lines 20 to the battery 30 may be accomplished within a small fraction of a cycle of the 60 Hz power line frequency so that the consuming equipment connected to the AC output lines 22 sees very little disruption of the waveform of the power supplied to it The controller 38 must respond quickly enough to determine if a fault has occurred in less than a quarter cycle of the AC input power yet must not over react to transient conditions which do not indicate a fault or power failure or to long term operating con ditions which result in a distortion of the AC power on the input lines 20 from a pure sinusoid but which other wise do not indicate a power system failure The con troller 38 must also be able to detect a ring down type of line failure as illustrated in FIG 2 where the AC power declines gradually rather than abruptly In accordance with the present invention the con troller 38 constructs a reference waveform in an adapt ive manner which is a composite of many previous cycles of the AC waveform on the input lines 20 The composite is formed so that the reference waveform will adapt itself in shape to a systematically distorted waveform such as those shown in FIG 1 which are not indicative of a line fault The preferred manner of con structing the reference waveform is shown in FIG 3 The reference waveform c
11. IG 6 SAVE UPPER BANK A D NO RESULTS Sheet 4 of 9 5 229 651 GET LINE SAMPLE A D RESULT SAVE TEMPORARY REGISTER A SET SAMPLE HOLD BACK TO HOLD Sheet 5 of 9 5 229 651 July 20 1993 U S Patent 1 98 921 1053 2 1 AANI 434 2 1 X30N 434 1 1 02 finv4 INN INNOD 1 153 ON TANNVHO MAN 103135 INNOD U S Patent July 20 1993 Sheet 6 of 9 5 229 651 131 RESTORE REGISTERS U S Patent July 20 1993 SAVE REGISTERS START INVERTER PULSE WIDTH TIMER PERFORM MISCELLANEOUS METERING AND ALARM FUNCTIONS Sheet 7 of 9 5 229 651 Sheet 8 of 9 5 229 651 July 20 1993 U S Patent allXddV XddV OSWNS 318 1 SINL 8 1 1 5 LH9Ib 9Sz OSWNS SSWLL 9 141 5 1 p9 OSWNS OSWNS LX3N 438 OSWNS OSWNS 9 OL U S Patent July 20 1993 Sheet 9 of 9 5 229 651 RMS NEW APPX RESTORE REGISTERS FIG 11 5 229 651 1 METHOD AND APPARATUS FOR LINE POWER MONITORING FOR UNINTERRUPTIBLE POWER SUPPLIES FIELD OF THE INVENTION This invention pertains generally to the field of elec trical power systems to the monitoring of line power waveforms and particular
12. T 2 routine 140 the register values are saved 141 and an inverter pulse width timer is started 142 Various metering and alarm functions are per formed 143 and then the value of a counter variable CNTR is incremented 144 The resulting value is then checked to determine whether not is less than 4 145 and if it is the program exits from the routine 146 If the value of the variable CNTR1 is not less than 4 its value is reset to zero 148 and the pro gram continues 149 as shown in FIG 10 The value of a variable SUMSQ is then set equal to zero 150 and then a calculation of the sum of the squares of each of the 64 sample values in the reference waveform is performed 151 The resulting sum of the squares value SUMSQ is then divided by 64 152 which can be accomplished by a binary right shift of six places and then a value NDX is calculated as the value of the present SUMSQ divided by 256 block 153 A variable is then determined from a look up table using the calculated number NDX as the index for the look up table 154 This yields an approximation for the square root A calculation is then carried out to calculate a variable NEWAPPX as shown in the block 156 in FIG 10 according to a conventional Newton Method itera tion whereby a closer approximation to the square root of a number N is determined by the formula 7 4 where is the previous approximation The differenc
13. United States Patent Baxter Jr et al 54 75 73 21 22 51 52 58 56 METHOD AND APPARATUS FOR LINE POWER MONITORING FOR UNINTERRUPTIBLE POWER SUPPLIES Inventors Richard Baxter Jr Neenah Frederick A Stich Wisconsin Rapids both of Wis Assignee Best Power Technology Inc Necedah Wis Appl No 404 902 Filed Sep 8 1989 H02H 3 24 2 3 50 307 66 364 487 340 661 Field of Search 307 66 80 64 85 307 86 87 43 44 45 46 47 48 364 480 481 483 487 492 363 74 75 340 660 661 662 663 361 79 324 102 References Cited U S PATENT DOCUMENTS 3 662 182 5 1972 Ulmann et al 3 758 763 9 1973 Nohara et al 423 415 3 927 309 12 1975 Fujiwara et al 364 574 4 031 463 6 1977 Norberg 361 86 X 4 064 488 12 1977 Chapman 328 136 4 132 947 1 1979 Weischedel et 3 4 219 860 8 1980 DePuy 361 94 4 370 723 1 1983 Huffman et al 4 510 57 4 1985 Dagostino et al 4 692 854 9 1987 Baxter Jr et al 363 75 4 694 402 9 1987 McEachern et al 4 713 553 12 1987 Townsend et al 4 769 760 9 1988 Kroll et al ath 4 782 241 11 1988 Baker et 307 66 4 797 936 1 1989 Nakatsugawa et al 4 814 934 3 1989 Jones et al 4
14. _ is the value of the sample oft 10 20 25 30 40 45 50 65 14 he AC line input waveform at the th sample point in the k 1 th cycle c an uninterruptible power supply including an auxiliary power supply battery the uninterruptible power supply receiving the AC line power and normally providing the AC line power to a load and responsive to a fault signal switch to provide power to the load from the battery d means for estimating the RMS value of the AC line signal by carrying out a root mean square cal culation on the reference waveform and providing the fault signal to the uninterruptible power supply so that it provides power to the load from the bat tery if the RMS estimate value falls below a se lected minimum value 16 The apparatus of claim 15 wherein 0 5 17 The apparatus of claim 15 wherein the RMS esti mate Vrmsest is calculated in accordance with the expression 1 Vrms est L where is the number of samples per cycle and wherein a fault signal is provided if Vrms est is less than the selected minimum RMS value 18 The apparatus of claim 15 wherein the apparatus is adapted for monitoring 60 Hz line power and wherein the means for sampling samples at a selected number of points in each cycle which are equally spaced in time 19 The apparatus of claim 18 wherein the means for sampling samples at a rate of 64 times a cycle 20 The apparatus of claim 15 where
15. at a moderate change in the voltage waveform indicative of a fault will be detected while the reference can adapt and conform to systematic distortions of the power line from a pure sine wave which are not indicative of power line failure so that a unnecessary switching of the UPS is avoided In the line monitoring apparatus of the present inven tion a reference waveform is produced having values at selected sample times during the period of the input waveform such that each reference sample is the selec tively weighted average of samples occuring over sev eral prior cycles of the input waveform Preferably the reference waveform is formed as the weighted average 5 229 651 3 at each sample time of the prior cycle and the previous average value an exponential averaging in which the values of the samples in all prior cycles are averaged together with a weighting which decreases exponen tially moving backward in the time of the sample The current cycle is then compared to the reference cycle and a fault signal is provided if the present cycle devi ates from the reference cycle by more than a selected tolerance level A preferred procedure for determining the reference level is carried out in accordance with the expression 1 1 a SAMP where is choosen such that 0 lt a lt 1 REF x_1 is the value of the reference at the i th sample point in the cycle and at the k 1 th cycle and SAMP x_1
16. communications channels These functions are primarily for set up cali bration and operator interface and may be carried in any desired conventional manner and are not directly relevant to the present invention The main program loop is designed to be interrupted by time critical tasks and other asynchronous events Some of these tasks are for example metering and alarm scanning for the UPS or service of the serial communications channel and are not directly related to line fault detection In an exem plary implementation of the present invention line fault detection is controlled by two interrupt service rou tines These interrupt routines are invoked by operation of external hardware interrupt pins on the CPU and temporarily suspend the operation of the main program loop The operation of these interrupt routines desig nated as INT 1 and INT 2 are illustrated in the flow charts of FIGS 6 11 The INT 1 interrupt is executed nominally every 260 4 microseconds at a rate of 3 84 kHz where the invention is being utilized to monitor 60 Hz line power and is initiated by the 3 84 kHz signal provided from the phase locked loop The interrupt rate is de signed such that the line data samples are taken and processed in the fault detection algorithm 64 times per cycle The exact frequency and phase is controlled by the phase locked loop as described above This phase locked interrupt method insures the phase coherency of line data
17. d cause triggering with fixed ref erence waveforms but whose power delivering capabil ity is otherwise suitable for powering the protected load with proper filtering It is understood that the invention is not limited to the particular embodiments set forth herein as illustrative but embraces all such modified forms thereof as come within the scope of the following claims What is claimed is 1 AC line waveform monitoring apparatus for de tecting AC line power faults and providing a fault signal for control of an uninterruptibie power supply compris ing a means for sampling an AC line power signal at times synchronized with the cycle of the AC line waveform b means for producing a reference waveform at selected sample times during the period of the AC line signal given previous reference waveform val ues and a prior cycle wherein a reference wave form value at each sample time comprises a selec tively weighted average of value at that sample time in the prior cycle of the AC line signal and the previous reference waveform value at that sample time c an uninterruptible power supply including an auxiliary power supply battery the uninterruptible power supply receiving the AC line power and normally providing the AC line power to a load and responsive to a fault signal to switch to provide power to the load from the battery d means for comparing the AC line signal wave form with the reference waveform at correspond
18. e between the new approximation value and the old ap proximation value is checked to see whether it is less than or equal to one 158 If it is not the value of APPX is set to the value of NEWAPPX and the pro gram iterates back to block 156 to perform the calcula tion again This process continues until at the block 158 it is found that the difference between the variables NEWAPPX and is less than or equal to one at which point the program continues 160 as shown in FIG 11 At this point the RMS estimate is set equal to the value at that time of NEWAPPX 161 and then this RMS value is checked to see whether it is less than a selected MINIMUM value 163 If it is a line fault message is sent 164 which can be used by the UPS system to turn on the inverter In either case the pro gram continues to restore the registers 165 and exit 166 back to the main program The combination of an adaptive tracking reference waveform with an underlying true RMS estimate of the input power provides unique line fault detection in accordance with the present invention The line fault detection of the invention exhibits fast results good sensitivity and reliable triggering to true fault condi tions while reducing the occurrence of false triggering 20 25 30 35 45 50 55 65 12 due to noise Of particular significance UPS system operation can occur with an AC line input whose wave shape distortion woul
19. e samples is over the tolerance limit or when a selected pattern of samples is over the tolerance limit A preferred technique is to keep a running count which is incremented for each new sample that exceeds the toler ance limit and decremented for each new sample that is within the tolerance limit In addition multiple toler ance limits may be established such that the difference between the sample and the reference at each sample time may be found to be within various ranges of toler ance limits In addition to providing a fast comparison between the reference waveform and the present cycle criterion which will detect a fault occuring within a fraction of a cycle of the 60 Hz waveform the control ler 38 also preferably examines the incoming AC power to detect unacceptably low levels i e ring down condition or possible brownout where the line volt age drops to an unacceptably low level To monitor the actual line voltage a root mean square calculation is performed on the AC input power This may be carried out by accumulating one cycle of new samples and performing the RMS calculation over the cycle after all data has been gathered However such a calculation could be affected by short transients which would dis tort the apparent RMS value from the actual steady state RMS value To reduce the effect of such transients while still obtaining an essentially real time estimate of the RMS value of the incoming wavef
20. e waveform created by the means for producing a reference waveform 35 The power supply of claim 25 further including means for providing an estimate of the RMS value of the AC line waveform and providing a signal to the 25 30 35 45 50 55 60 65 16 power transfer means to switch to supply power from the battery if the RMS estimate values falls below a selected minimum value and wherein the RMS estimate V is calculated in accordance with the expres sion 1 Vrms est where n is the number of samples per cycle and wherein the signal is provided if Vrns est is less than the selected minimum RMS value 36 The power supply of claim 24 wherein the means for comparing compares the absolute value of the dif ference between the reference waveform and the AC line signal waveform at each sample point with a se lected difference level and provides a signal to incre ment a counter if the difference exceeds or is equal to the level and a signal to decrement the counter if the difference is less than the selected level and wherein the fault condition is determined if the count in the counter exceeds a selected value indicative of the fault 37 The power supply of claim 24 wherein the num ber of samples n per cycle is 64 38 A method of AC line waveform monitoring for detecting AC line power faults and controlling an unin terruptible power supply comprising the steps of a sampling an
21. form 5 229 651 15 2 means for producing a reference waveform at selected sample times during the period of the AC line signal given previous reference wave form values and a prior cycle wherein a refer ence waveform value at each sample time com prises a selectively weighted average of the value at that sample time in the prior cycle of the AC line signal and the previous reference wave form value at that sample time 3 means for comparing the AC line signal wave form with the reference waveform at corre sponding times in the reference waveform and the signal waveform and providing a fault signal when a selected difference condition is ex ceeded and for determining from the fault signal when a selected fault condition has occurred and then controlling the power transfer means to switch to supply power from the battery during the fault condition 25 The power supply of claim 24 wherein the means for producing a reference waveform does so in accor dance with the expression REF 1 a REF k 1 SAMP 4 1 where is choosen such that 0 lt lt 1 is the present value of the reference at the i th sample point in the th cycle 115 the value of the reference at the i th sample point in the k 1 th cycle and SAM 1 is the value of the sample of the AC line input waveform at the th sample point in the k 1 th cycle 26 The power supply of claim 25 wherein a 0 5 27 The power
22. icrocom puter as the computer 66 FIG 5 27C256 32K x8 15 20 25 30 35 40 45 50 55 60 65 8 EPROM 69 containing non volatile program instruc tions and fixed data and 2K 8 HY6116 68 for read write data memory A battery backup circuit is preferably provided for RAM data retention in the absence of primary logic power The UPD78C10 single chip microcomputer is a high integration device con taining an 8 bit CPU 256 bytes of local data memory 8 and 16 bit timer units a serial communications interface several digital I O ports and an 8 bit 8 channel analog to digital converter The A to D converters associated with the chip may perform the functions of the A to D converters 64 and 73 of FIG 5 An on chip interrupt controller manages interrupts from the integrated pe ripheral devices in addition to interrupts from external interrupt input pins two of which are preferably uti lized in the line fault detection procedure The CPU executes program instructions from the EPROM 69 with a stack being maintained in the 256 byte internal data memory to store return addresses and the machine context in the event of subroutine calls or interrupt processing In operation of the computer controller the CPU normally executes a series of instructions in an endless MAIN program loop This loop controls operation of communications with an external keyboard and display LED indicator devices and serial
23. in the means for sampling includes an analog to digital converter for converting the sampled analog value to digital data 21 The apparatus of claim 15 wherein the means for sampling includes means for phase locking the sample times to the period of the AC line signal so that sam pling is carried out at equally spaced points in time over each cycle of the AC line waveform 22 The apparatus of claim 15 including means for comparing the absolute value of the difference between the reference waveform and the AC line signal wave form at each sample point with a selected difference level and incrementing a counter if the difference ex ceeds or is equal to the level and decrementing the counter if the difference is less than the selected level and wherein the fault signal is provided if the count in the counter exceeds a selected value indicative of a fault 23 The apparatus of claim 17 wherein the number of samples n per cycle is 64 24 An uninterruptible power supply comprising a an auxiliary power supply battery power transfer means connected to the auxiliary power supply battery and to AC line power for responding to control signals to provide power to an AC output from either the AC line power or the battery c control means for providing control signals to the power transfer means the contro means compris ing 1 means for sampling the AC power line signal at times synchronized with the cycle of the AC line wave
24. ine waveform 41 The method of claim 38 further including the step of estimating the RMS value of the AC line waveform and comparing the value with a selected minimum value 42 The method of claim 41 wherein the step of esti mating the RMS values is performed by carrying out a root mean square calculation on the reference wave form UNITED STATES PATENT AND TRADEMARK OFFICE CERTIFICATE OF CORRECTION PATENTNO 5 229 651 DATED July 20 1993 INVENTOR S Baxter et al Page 1 of 2 It is certified that error appears in the above indentified patent and that said Letters Patent is hereby corrected as shown below In column 4 line 2 of the patent after of insert systematically In column 6 line 1 of the patent samp REF 2D should be samp REF gt D In column 13 line 67 of the patent th ei th should be the i th 1992 In column 13 line 68 and column 14 line 1 of the patent oft he should be of the In column 15 line 35 of the patent samp REF D should be samp REF 2D as written in the application on page 28 line 15 In column 16 line 54 of the patent back should be backward as written in the application on page 30 line 23 UNITED STATES PATENT AND TRADEMARK OFFICE CERTIFICATE OF CORRECTION PATENT NO 5 229 651 Page 2 of 2 DATED July 20 1993 INVENTOR S Baxter et al It is certified that error appear
25. inuous comparison of the present waveform cycle with the reference waveform provides a unique combination of sensitivity to rapidly occurring line failures non sensitivity to systematic non failure waveform distortions and voltage transients and accu rate detection of a slowly decaying power failure The line monitoring of the invention may be carried out under computer control with the samples from the AC input being digitized the reference waveform being calculated by a microprocessor to provide a reference waveform stored in memory and a digital comparison being made of the new cycle waveform with the refer ence cycle waveform Further objects features and advantages of the inven tion will be apparent from the following detailed de scription when taken in conjunction with the accompa nying drawings BRIEF DESCRIPTION OF THE DRAWINGS In the drawings 15 20 25 30 35 45 50 55 60 65 4 FIGS 1 are illustrative graphs of common examples of distorted power line waveforms FIGS 2 is an illustrative graph showing a ring down type of power line failure FIGS 3 is a graph illustrating sampling of the AC power line input waveform for comparison with a refer ence waveform in accordance with the invention FIGS 4 is a block diagram of an uninterruptible power supply system in accordance with the invention FIGS 5 is a block diagram of the line monitoring apparatus for utilization in a UPS syste
26. lue for the index point in the cycle block 115 The difference is then checked to see if the result 15 20 25 30 40 45 50 55 10 is less than an allowable difference value block 116 and if it is the running value of the counter is checked to see if the count equals zero 118 If not the value of COUNT is decremented by one block 119 before continuing and if the value of COUNT is zero the pro gram simply continues without changing the value of the COUNT If at 116 the result was found to be greater than or equal to the allowable difference the value COUNT is incremented by one block 120 and the value COUNT is checked to see if it is less than the TERMINAL COUNT block 121 If it is the program continues if it is not the program sends a LINE FAULT message which is outputted to the control components to turn on the inverter 122 The program then recalculates the reference utilizing the weighted average of the present reference at the present sample index value with the new line sample result to create the new reference value at the present sample index value block 125 and then continues 126 as shown in FIG 8 At this point in the program the value of the quantity INDEX is incremented by one block 128 and then the value of INDEX is checked to see whether it is greater than 63 block 129 If it is the value of INDEX is set to zero 131 In either event the program then continues and restores all
27. ly to the monitoring of line power for controlling the operation of uninterruptible power supplies BACKGROUND OF THE INVENTION Uninterruptible power supplies are in common use today particularly for supplying large computer sys tems where loss of line power can result in the interrup tion of programs and loss of valuable data Such unin terruptible power supplies may also provide a signal conditioning function to ensure that transient spikes low voltage conditions or distorted power waveforms on the AC power system do not disturb the operation of the computer to which the uninterruptible power sup ply UPS is connected Typically the UPS includes a battery which is interfaced through an inverter to the AC output power line One type of UPS operates in such a way that when a fault occurs in the input AC power the inverter is controlled to provide power from the battery to the AC output line which has the same frequency and substantially the same waveform as the normal input AC power Preferably the switching at the time of fault is accomplished as smoothly as possible so that substantial transient spikes or dips in the wave form supplied to the AC output lines does not occur The coupling of the inverter to the AC output may be through a ferroresonant transformer as illustrated in 0 5 Pat No 4 692 854 to Richard Baxter et al entitled METHOD AND APARATUS FOR MODU LATING INVERTER PULSE WIDTH the disclo sure of which is
28. m in accordance with the invention FIGS 6 11 are flow charts showing the program steps carried out by the computer controller for the power line monitoring system for a UPS in accordance with the invention DETAILED DESCRIPTION OF THE INVENTION With reference to the drawings an uninterruptible power supply UPS system utilizing the adaptive AC line monitoring of the present invention is shown gener ally in block diagram form in FIG 4 The UPS receives AC input power from the power system on lines 20 connected to a power interface 21 which provides AC output power on lines 22 to the equipment consuming this power such as computers and other electrical equipment which require reliable or conditioned power The power interface 21 is a conventional unit well known in the art which delivers power to the AC out put lines 22 from supply lines 24 and receives power from the AC input lines 20 For the UPS shown in FIG 4 the power on the lines 24 is provided through a ferro resonant transformer 26 although other conventional UPS structures may be utilized in accordance with the present invention The ferroresonant transformer 26 is driven with power on a line 27 from a power module 28 which receives line power on line 32 and the backup electrical power on lines 29 from an energy storage device such as the battery 30 shown in FIG 4 A con troller 38 receives electrical signals on lines 39 from the power module 28 for monitoring the AC inp
29. mpling samples at a rate of 64 times a cycle 8 The apparatus of claim 1 wherein the means for sampling includes an analog to digital converter for converting the sampled analog value to digital data 9 The apparatus of claim 1 wherein the means for sampling includes means for phase locking the sample times to the period of the AC line signal so that sam pling is carried out at equally spaced points in time over each cycle of the AC line waveform 10 The apparatus of claim 1 further including means for providing an estimate of the RMS value of the AC line waveform and providing the fault signal if the RMS estimate value falls below a selected minimum value 11 The apparatus of claim 10 wherein the means for estimating the RMS value carries out a root mean square calculation on the reference waveform created by the means for producing a reference waveform 12 The apparatus of claim 2 further including means for providing an estimate of the RMS value of the AC line waveform and providing the fault signal if the RMS estimate values falls below a selected minimum value and wherein the RMS estimate 5 5 is calculated in accordance with the expression 1 L 12 where n is the number of samples per cycle and wherein the fault signal is provided if Vimrs est is less than the selected minimum RMS value 13 The apparatus of claim 1 wherein the means for comparing compares the absolute value of the differ e
30. nce between the reference waveform and the AC line signal waveform at each sample point with a selected difference level and provides a signal to increment a counter if the difference exceeds or is equal to the level and a signal to decrement the counter if the difference is less than the selected level and wherein the fault signal is provided if the count in the counter exceeds a selected value indicative of the fault 14 The apparatus of claim 12 wherein the number of samples n per cycle is 64 15 AC line waveform monitoring apparatus for de tecting AC line power faults and providing a fault signal for control of an uninterruptible power supply compris ing a means for sampling an AC line power signal at times synchronized with the cycle of the AC line waveform b means for producing a reference waveform at selected sample times during the period of the AC line signal given previous reference waveform val ues and a prior cycle wherein a reference wave form value at each sample time comprises a selec tively weighted average of sample values over a plurality of prior cycles of the AC line signal in accordance with the expression 1 5 wherein is chosen such that 0 lt 1 xis the present value of the reference of the i th sample point in the th cycle _1 is the value of the reference at th th sample point in the k 1 th cycle SAMP
31. onsists of a series of samples which span the full cycle of the input waveform e g the 64 equally spaced samples shown in FIG 3 which cover the full period of one 60 Hz cycle 16 67 msec The reference waveform 50 is composed of a first half cycle 51 and a second half cycle 52 which as shown in FIG 3 may be rectified so that both have only positive sample values At each sample time i after the start of the cycle e g i extending from 0 to 63 to cover the full one sixtieth of a second cycle period the value of the reference waveform at that time is formed as an average of a sample at a corresponding point in time i from the last cycle of the input AC waveform and a weighted average of samples at corresponding points in time on many previous cycles For example the average may be formed as an exponential average of the last cycle sam ple with exponentially decaying weighted sums of all previous cycles Such an exponential averaging can bea calculated in accordance with the following expression REF x 1 REF k 1 SAMP 4 1 where 0 lt a lt 1 is the value of the reference waveform for the cycle k at the cycle sample point i 1 is the reference waveform which was previ ously calculated at the sample i for the k 1 cycle and x 1 is the value of the sample at the time i in the 1 cycle Although the value of a can range within the open set from 0 to 1 and obtain averaging it is pre fer
32. orm it is pre ferred that the RMS calculation be performed on the reference waveform as it is updated each cycle For example the RMS estimate may be performed in accor dance with the expression EA where is the number of samples each cycle e g 64 A brownout condition would be detected if the value of Vrmsest 15 less than a value Vrms min which is choosen as the minimum acceptable RMS voltage level A block diagram of a microprocessor implemented controller 38 for the UPS system of FIG 4 is shown in FIG 5 This controller in addition to its other duties in the UPS carries out the adaptive line monitoring of the present invention Among the input signals that the controller receives on the lines 39 is the AC input line voltage which is received by the controller on a line 5 229 651 7 indicated at 60 FIG 5 The AC voltage is received by a processing circuit 61 which filters the line voltage performs an absolute value function on the voltage and samples and holds the voltage at selected points in time The filter removes high frequency disturbances on the AC input line which can cause false line detection The filter is preferably a low Q band pass filter centered at 60 Hz which exhibits zero phase delay The samples from the sample and hold circuit are provided on a line 62 to an analog to digital converter 64 the digital out put of which is provided to a microprocessor 66
33. orm tracks the input samples but the use of power of 2 weights is preferred for code and execution time efficiency The interrupt routine then increments a reference table index value and compares it against the maximum allowable value of e g 63 If the index value exceeds this number it is reset to zero The context of the CPU is restored to its states prior to the INT 1 interrupt the primary register bank is se lected and processing returns to MAIN program loop The steps carried out by the INT 1 routine are set forth in more detail in the flow charts of FIGS 6 8 Upon entry into the interrupt at 100 the registers in the CPU are first saved 101 and the sample and hold pin of the CPU is set to the sample position block 102 Because there are 2 A D banks available in the CPU utilized the program first determines if the upper A D bank is selected block 103 and if so the upper bank A D results are saved block 104 If not the lower bank A D results are saved block 105 and the pro gram proceeds to get the line sample A D result block 106 This result is saved in the temporary register des ignated 107 and the sample hold pin is set back to hold 108 The program continues 110 as shown in FIG 7 to select a new channel group for the A D 112 and then restarts the A D converter 113 A computa tion is then made of the absolute value of the difference between the content of the register A and the REFER ENCE va
34. red that the value of a be substantially equal to 0 5 Various well known techniques may be utilized to compare the present cycle of the incoming AC wave form with the reference to determine if a line fault has occured One technique determines the difference in magnitude between the present sample at the sample time i and the reference for the sample time i and deter mines whether the difference exceeds a selected toler ance limit This fault condition may be expressed as 20 25 30 35 40 45 50 55 60 65 6 5 where 5 is the sample in present cycle at the sample time i and 1 5 a tolerance limit which has been choosen to yield the desired degree of sensitivity The value D may be a constant for all sample times i or can vary as a function of i The value of be obtained from a look up table of 64 values indexed by i The values in the look up table can be selected for example from experimental data obtained by testing the response of the system to various fault conditions The line de fault may be determined whenever for any one of the sample times i the difference between the sample and the reference is greater than or equal to the tolerance limit D The decision may also be made with various other criteria such as when a certain number of samples out of a selected number of consecutive samples exceed the tolerance limit or when a selected number of con secutiv
35. rmed of essentially all samples from prior cycles with exponentially decaying weighting An excessive deviation of the current waveform from the reference waveform results in a fault being detected which causes switching of the uninterrupible power supply to provide backup power The RMS value of the power line voltage is also estimated on a substantially real time basis preferably by calculating the RMS value of the reference waveform to provide a fault condition signal when the estimated RMS value falls below a selected level which also can be utilized to cause switching of the uninterruptible power supply 42 Claims 9 Drawing Sheets U S Patent July 20 1993 Sheet 1 of 9 5 229 651 FIG 1 FIG 2 U S Patent July 20 1993 Sheet 2 of 9 5 229 651 POWER INTERFACE FIG 4 Sheet 3 of 9 5 229 651 July 20 1993 U S Patent 4199 1391 HOLINON LAdLNO OV 5 4110 OV 18 01 Gl 31 9 5 SAG 29 INTA 31110589 Ad 09 9 913 AXA TALLINN 55300 N a901 JSVHd Lo LAdLNO OV Yd SNM 5 0 SLIOA OV ZH 09 NI SITOA OV 19 U S Patent July 20 101 REGISTERS SELECTED SAVE LOWER 105 BANK A D RESULTS F
36. s in the above indentified patent and that said Letters Patent is hereby corrected as shown below Column 16 line 62 comparing the value should read comparing that Signed and Sealed this Twenty eighth Day of June 1994 Attest Gig BRUCE LEHMAN Altesting Officer Commissioner of Patents and Trademarks
37. samples to be used in the detection algorithm regardless of input frequency and phase deviations When the interrupt routine INT1 is entered the rou tine first saves the context of the CPU registers by switching to an alternate register bank reserved for this interrupt In general terms the routine then sets up and transfers the result of the analog to digital conversion to the data memory At the same time that the capture and storage of the analog to digital conversion results is carried out the line data input sample and hold circuit is controlled The CPU directs the sample and hold circuit to sample the line input at a precise time and then hold the analog results for analog to digital conversion The held value is converted and then available for pro cessing in the subsequent INT 1 interrupt routine pro 5 229 651 9 cessing This precise control of the line input sampling in conjunction with the phase locked interrupt insures the phase coherency of samples for the fault detection algorithm The line sample digital data resulting from the analog to digital conversion is subtracted from a corresponding reference waveform data point stored in RAM mem ory and the absolute value of the result is compared against an allowable difference value The reference waveform represents a composite of previous corre sponding samples preferably formed by exponential averaging The allowable difference can be fixed pro grammable or selec
38. ted from an arbitrary table of differ ence values using the same index as the reference wave form table If the absolute value of the difference is less than the allowable difference value a software counter is decremented subject to a minimum allowable count of zero Conversely if the absolute value of the differ ence is greater than or equal to the allowable difference entry the same software counter is incremented The value of this counter is compared against a programma ble terminal count and the result of the comparison is used as the basis for determining if a line fault exists If the counter value is greater than or equal to the terminal count then a subroutine call is made to signal the line fault and the appropriate action is taken In the UPS embodiment the subroutine call turns on a power in verter providing backup power to the critical load despite the fault in the input line AC power The fore going method of determining a fault is desirable because it provides an easily adjustable sensitivity and immunity from false triggering due to noise The INT 1 interrupt routine then calculates and stores the new reference table value by exponentially smoothing the present sample with the previous value of the corresponding reference entry For example binary weights selected at 0 5 and 0 5 are applied to sample and the previous table value respectively Other weights can be used to control the rate at which the reference wavef
39. the registers 132 before exiting from the interrupt routine 133 A second CPU interrupt input is also connected to the phase locked loop and is connected to receive the 120 Hz output signal from the loop which is also phase locked to the AC line input waveform of interest This interrupt signals the start of each half cycle and occurs at the zero voltage crossings of the AC line input It is used to control the reference table RMS detection This interrupt routine can also be used to reset the reference table index used by the INT 1 interrupt routine if an arbitrary table of allowable difference values based on the position of the sample within a cycle is desired Such would be the case if it were desired to desensitize the line fault detection algorithm in portions of the waveform that are expected to exhibit a high rate of change with respect to time such as is the case near the zero crossings The INT 2 interrupt routine begins with a context saving operation followed by control functions incident to the operation of the inverter metering alarm and display functions The routine then increments the soft ware counter which upon reaching a terminal count invokes a routine to calculate the true RMS value of the exponentially smoothed reference waveform employ ing a fast table basis Newton method square root algo rithm and a numerical approximation to the mean of squares integral The 64 entries of the reference wave form are indi
40. ut lines 32 The power module is controllable to provide a charging current on lines 35 to the battery 30 under normal oper ating conditions drawing power from the AC power lines 20 to charge the battery The controller 38 re ceives signals on lines 39 from the power module 28 to monitor the operating conditions of the module and provides output control signals on lines 40 to the mod ule to control its operation The controller 38 is con nected to input output devices and ports 42 for commu nication with an operator or with other remote equip ment During normal operating conditions when the AC power on the AC input lines 20 is fault free the power module 28 connects the AC input lines 32 to the ferro resonant transformer 27 and also receives power from the lines 32 and delivers a charging current to the bat tery 30 The controller 38 also monitors the AC voltage provided on the line 32 to the power module to look for a fault condition As described above such fault condi tions may take the form of a sudden loss of AC power or a more gradual ring down of the AC power on the input lines 20 When the controller 38 detects such a fault it provides a control signal to the power module 28 to cause the power module to deliver power from the battery 30 to the ferro transformer 26 in such a way that 5 229 651 5 waveform the AC power now provided on the line 24 will substantially match the waveform on the AC input line
41. vidually squared and summed to a 24 bit sum of squares The result of the sum of squares is di vided by 64 using a repetitive right shift approach The upper 8 bits of the resulting mean value are used as an index to 256 byte table of square root approximations 60 65 The table value at the offset of the look up index is used as the first approximation to the input square root A conventional Newton method iteration is then applied to calculate successive approximations and the iteration stops when successive approximations are either zero or differ by an absolute value of only 1 The execution time efficiency of this method permits the calculation of the reference waveform RMS value as often as every cycle if desired although it is possible to calculate at a slower rate for example every other cycle 5 229 651 11 The resulting RMS calculation is compared against programmable minimum set point and the result of this comparison is OR ed with the line fault signal generated by the INT1 routine above so that a fault from either the RMS calculation or the INT1 routine fault opera tion will send a fault signal to the UPS system to turn on the power inverter The context of the machine prior to the interrupt routine is restored upon exit from the interrupt and entry back into the MAIN program loop The operation of the INT 2 routine is shown in greater detail in the flow charts of FIGS 9 11 Upon entry into the IN
42. y lower than desirable be fore detection occurs An undesirably large disturbance in the output voltage can result Another factor affecting detection of power line fail ure is the sensitivity of detection at various phase angles of each half cycle of the input waveform It has been found that in typical plots of cycle to cycle deviation of AC power deviations generally do not occur during the peak of the waveform but rather are more prevalent toward the zero crossings In particular with a ferro resonant transformer as the source a smal amount of phase perturbation can accentuate deviation around the zero crossings Thus a detection method which would be based on percentage change of the waveform is gen erally not an effective way of fault detection Generally it is better to have an absolute difference of equal value for all samples as the criterion or for severe cases larger programmable tolerances for the differences at the zero crossings It is further desirable to minimize unnecessary switching of the UPS due to power line surges so that switching does not occur unless the surge represents a persistent change in the power system voltage SUMMARY OF THE INVENTION In accordance with the present invention line power monitoring for uninterruptible power supplies is pro vided by adaptively producing a reference waveform to which the present waveform is compared The refer ence waveform changes at a relatively slow rate so th
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