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Identifying Faulty Insulator Strings by Detecting InsulatorCorona Pulse Cu rrent

Cheng Rong Li, Qian Shi, Yang Chun ChengNorth China E lectric Power UniversityQing he, B eijing, 100085,P.R.ChinaAbstract: The resolution to identify faulty insulator strings bydetecting insulator corona current pulse was investigated in ourlaboratory. The influen ces of some facts on the detectingresolution were also discussed, including the characteristics ofinsulators close to line, the resistance of faulty insulators, thelocations of the faulty insulators, and the amount of insu lators onthe string. The results showed that the corona pulse analysis couldbe used for identifying faulty insulator strings in the laboratory.Keywords: pulse current metho4 faulty insulators

I . INTRODUCTIONAt present, porcelain insulators are still widely used tomost transmission lines in China. There are severalmethods [1,2,3] available for inspecting faulty insukitorson-line. But by using them the operators have to climbtower, which will bring some troubles for operation.Detecting insulator current pulses could be a way to inspectfor the faulty insulator strings on ground. Some basicstudies [4 ] were carried out. We had expected that faultyinsulators in the string would discharge partially, but thetests in the laboratory indicated that it is not correct. Whenthe resistance of the faulty insulator on the string is lower

than 50MR, the faulty insulator can not produce partialdischarge due to the low voltage drop across it. Therefore,the detected current pulses from the string are produced bycorona dischw-es on good insulators. The aim of this paperis to try to find the measuring resolution of identifying thefaulty insulator string by detecting insulator corona currentand the some influences on the m easuring resolution.11. TEST SETUP

The tests were carried out in our high voltage laboratorywith the setup shown in Fig. 1.

0-78034754-4/9W$10.00 0 998 IEEE

Guo Lin, HuangXingquanHenan Electric Power CollegeZhen gzhou 450004,P.R.China

-Fig. 1 Corona pulse measun'ng circuit

A-Rogowski coil, 6-HV source, C-tested insulator stingD-DL1540L oscilloscope, E-phase angle reference signalF-coupling capacitanceThe HV source can supply 50Hz AC voltage up to

100kV.C is the tested insulator string. Stray capacitancewas used as the coupling capacitance. A high sensitive,wideband self-integrating Rogowski coil (sensor A) wasused to measure current pulses. The Rogowski coiI has afemte core. Its 3db cutoff frequency is from 200kHz to3MHz. The sensitivity of the coil is 20mV/mA. The pulsecurrent signals were changed into pulse voltage signals bythe integrating impedance and were displayed on aDL1540L digital oscilloscope screen. DL1540Loscilloscope produced by YOKOGAWA Co. has 200MS/ssampling rate. The oscilloscope can record pulsedistribution vs. phase angle in a cycle. When theoscilloscope was used to record the pulse distribution vs. acycle, the sam pling rate was se t to be SOMS/s.

We studied the insulator corona discharges. The resultsshowed [4] that the distribution of positive corona pulses ismainly in 20"-100" and that of negati ve pulses is mainly in220"-290". The amplitude and amount of current pulsesare increased with the applied voltage. The lower jitters arepresented when the negative pulses are chosen to analyzecorona discharges. Only the negative pulses whoseamplitude was larger than lOmv were considered as thecorona discharge pulses in the tests. The experiments weredivided as two groups: the resolutions of pulse currentmethod and the influences of some fac p on the resolution.

' D.DETECTING RESOLUTIONA. 35kV Lines

The tested strings, consisted of three pieces of XP-7

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porcelain insulators, were applied on 20kV AC voltage.The location of insulators in string was numbered from lineto ground. When the string was normal, none of thenegative corona pulses was detected in 200 cycles. Thelargest voltage drop across the insulator on the string ismeasured to be 7.8kV.There were two kind of faulty insulators (Sample#l,R=5MQ; Sample#2,R=25MQ) we tested. When a faultyinsulator was in a string, no matter where it was, theamount of detected pulse was more than 10. The largestvoltage drop across the good insulator was about IOkV.Table.1 is the number of detected corona pulses when asample was on the string. The data were achieved fromevery 25 cycles. The results showed that the maximumdetecting resolution for 35kV lines is a faulty insulator inthe string with three pieces of insulators on it. The resultsalso showed that even through the faulty insulator islocated in the same position in the same string, the num berof the detected pulses from the string is different due to thejitter of the corona characteristics of good insulators. Fi g2shows the typical pulse distribution vs. phase angle in acycle when Sample#l was on No 1 position in the string.Fig.3 shows the typical pulse distribution vs. phase angle ina cycle when Sample#2 was on No 1 position in the string.

TABLEITEST RESULTS OF A FAULTY INSULATOR IN STRING

the amount of pulses2 l O m v 2 2 0 m v 2 3 0 m v

faulty insulator(position)40 2744 37Sample#l(l) i:

~amp1 e#1(2) 71 12 214 12 0

j . . . . j . . . I . . .. / . . ..[. ..t . .;

I t IFig.2 the typical pulse distribution vs. phase angle in a cycle whenSample#l was on No 1 position on the string.

. . . . . . . .. . . *. . .......................................... .... . . .. . .. . -...................................... .... . . . I. . .. . : . *.-.. . : ;. . /_*.1 ; y : ; :* . . . .. .

. ......................................... ..............................................., , : : : IFig.3 the typical pulse distribution vs. phase angle in a cycle whenSample#2 was on No 1 position on the stnng.

B. I10 kV LinesThe strings consisted of seven pieces of XP-7 insulatorwere tested at 66.5kV. When a string was normal, the

number of negative corona pulses was about 78 in 800cycles. The largest voltage drop across the insulator in thestring was measured to be about 18.5kV.

There were two faulty insulators respectively with4Mn(Sample#3), 20MQ(Sample#4) resistance used to thetest. When Sam ple#3 or Sam ple#4 was put into the string,the number of corona pulses was detected to be about 24 in80 cycles.

When Sam pleW and Sample#4 were all put in the string,no matter where they are, detected corona pulses weremore than 30 in 80 cycles. Fig.4 shows the typical pulsedistribution vs. phase angle in a cycle when Sample#3 wason No 4 position and Sample#4 was on No 2 position onthe string. Fig.5 sho ws the typical pulse distributio n vs.phase angle in a cycle when Sample#3 was on No 7position and Sample#4 was on N o 6 position in the string.

............... .... ...... ....... ............................. : . : . \ . . . . . I

. . . . . . . .;.w;....... ........ . . ..... ....... ...... ; . . . . . . . .* .; + : : :+ : :. ..tiI -

Fig 4 he. typical pulse dlsmbution vs. phase angle in a cycle when Sample#3 was on No 4 position and Sample #4was on No 2 position on thestring.

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....... .............. .CHlrSV . CHinlOmV t ; j 2fnP/diVnc. 1:1 : Dc 1:1 j . i i ( 2mS~d iV). .. .. .................................. i ...... . . . . . . i. . . . . . . ....... ??"SO? .. . .. . . . . .: . . '.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. .1:..../.I...'....I ;.\. 1

TABLE2TEST RESULTS OF DIFFERENT CHARACTERISTICSOF GOOD

INSULATOR CLOSE TO LINETest faulty insulator insulators the amountofpulseNumber loosition) On No.1 3 l O m v 220mv 230mv

.................... : . . . . . . . I . . . . . . . . . . . . . . . . . . . . . . . .3 . . . ..-I* . .4 : :. . $ . . , .. ... . iFig.5 the typical puls e distribution vs. phase angle in a cycle whenSample#3 was on No 7 position and Sample#4 was on No 6 positionin the stringThe results showed that the detecting resolution for

1 lOkV lines is about two faulty insulators in a string withseven pieces of insulators.I V. INFLUENCES ON TH E RESOLUTION

A . the Amount of Insulators on the StringThe largest voltage drop in the normal string with sevenpieces of insulators was measured to be more than 18kVwhile the amount of detected pulses was less than 8. The

largest voltage drop in the string with two pieces of goodinsulators and one faulty insulator (Sample#l) was lessthan 11kV while more than 70 pulses could be detected.Compared the largest voltage drops and the amount ofpulses in the cases, we found that, although the voltagedrop across the insulator in the former is higher, the amountof the pulse detected is less. That can be explained that theamount of insulators in 11OkV line was more so that theimpedance in measuring circuit was larger and the pulsecurrent detected through the ground line was less. Itindicates that the more the amount o f insulators on thestring is, the less the detec ting resolution will be.B. the Corona Characteristics of the Insulators C lose to theLine

The good insulator with a large voltage drop across itcan produce corona discharges. When a string consisted offive pieces of good insulators and two of the faultyinsulators, the insulator on No.1 position in the stringplayed an important role in producing corona pulses. Forexample, sample#5 produced more current pulsescompared to sample#6 because the corona inceptionvoltage of Sample% was higher than that of Sample#5.Table.2 shows the test results.

In above cases, no matter where the faulty insulatorswere, the imp edance of the measuring circuit was same andthe difference of voltage drop across insulators could beignored. Therefore, only the difference of the coronainception voltage could be the reason the difference of theamount of the pulses. Our results indicated that the higherthe corona inception voltages of the insulators close to lineare, the lowe r the detecting resolution is.C. the Resistance of the Faulty Insulators

We found in the test that the voltage drops across faultyinsulators were dependent on insulator resistance. On thesame position, the voltage drop of the faulty insulator withlower resistance was smaller than that of the faultyinsulator with large resistance. So the voltage drops of thetwo cases were different. Compared to corona pulsedistribution, shown in Fig.2 and Fig.3, the am ount of pulsesin Fig.2 is more than that in Fig.3. It could be explainedthat the low resistance faulty insulator could cause a highervoltage drop across good insulators on the string,producing more active corona discharges and resulting inthe amount of detected pulses.D. he Location of the Faulty Insulator on the String

The voltage distribution in the string changed with theposition of the faulty insulators in the string. When thesame faulty insulator was on the different position, thevoltage drops across good insulators were different and theamount of pulses produced by them were different.Compared to corona pulse distribution produced by goodinsulators, shown in Fig.4 and Fig.5, the closer the faultyinsulators are to line, the larger voltage drop across thegood insulator is, the more the number of current pulse ismore.

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V .CONCLUSION1 . The detecting resolution for 35kV lines is a faultyinsulator in the string with three pieces of insulator, andthat resolution for 11OkV lines is two faulty insulators in astring with seven pieces of insulators.2 . The amount of insulators on the string influences thedetecting resolution. The more the am ount of insulators inthe string is, the low er the detecting resolution is.3. The corona characteristics of the insulators close to lineinfluences the detecting resolution. The higher the coronainception voltages of the insulators close to line are, thelower the detecting resolution is.4. The resistance of faulty insulators influences thedetecting resolution. The higher the resistance o f the faultyinsulators is, the lower the detecting resolution is.5 . The location of the faulty insulators influences thedetecting resolution. The closer the faulty insulators are toline, the higher the detecting resolution.

KACKNOW LEDGEThe authors gratefully acknowledge the Chinese PowerElectric Education Fo undation for financial support..W.REFERENCE

[ l] Bo Kehan, operation in service, Electric Power Press,1988.[2] Guo Zhongyao, introduction to detecting faulty insulators, FujianPower Information, No.1, pp.12-19, 1996.[3] H. Vaillancourt,J. P. Bellerive, M. St-Jean, and C. Jean, New LiveLine Tester for Porcelain insulators on High-Voltage Power Line, IEEETransac tions on Power Deliver y, vo1.9, Januaty 1994, pp.208-21 9.[4] C.R.Li, Q,Shi, and Y.C.Cheng, The Pulse Curren t from FaultySuspension Insulators and Some Types of Corona Discharges,Proceedings of 10 Intemationai Symposium of High Voltage Engineering,Volume3, outdoor insulation , pp125-128.

WI.BIOGRAPHIES

Institute in 1995 and received her M.S in electrical engineering fromNorth China Electric Power University in 1998. Her current researchinterests is detecting of faulty insulators on high-voltage transmitting lineYang Chun Cheng he was born on September 6 , 1974 He received thzB.S. and M.S. egrees in electrical engineering from Xian JiaotongUniversity and North China Elecurespectively. His current research imonitoring and diagnostics of insul

Cheng Rong Li received the Ph.D in electrical engineering from TsingHua Univers ity in 198 9. He is a professor in electrical engineerin g ofNorth China Electric Power University. His cuken t research interests aresurface flashover on dielectric materials, 2-pinch plasma for X-ray source,high pulse power sources, monitoring and diagnostics o f insulationmaterials and power equipments.Qian shi was graduated with a B.A. form GeZhouBa Electric Power

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