EE509 Partial Discharge

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2004 In tem tio ml Conferenceon SolidDielec aics, Toulouse, France, July 5-9, 2004

PartialDischarge M easurement in XLPE Cable Joint byUsing VHF SensorXin Li, Chengrong Li, Wei Wang, Bin Wei, Weijiang WanSchoo l of Electrical EngineeringNorth China Electrical Power U niversityBeijing, 10220 6, P. R. China

Ab stract: Based on the electromagnetic couplingmethod, a new kind of clamp current sensor wasdesigned to detect partial discharges (PD) in XLPEcable joint. The sensor is a Rogowski coil with anon-crystal core, its bandwidth is located in very highfrequency band (VHF) from lMHz o 60MHz. Ourtest results both in our laboratory and on-sitemeasurement indicated that the sensor is suitable forPD on-site testing and on-line monitoring in XLPEcable oint.Key word VHF sensor; clamp current sensor; XLPEcables oint; partial discharge measuringINTRODUTIONIn recent years, XLPE po wer cables have been used ontransmission lines and distribution net. According tostatistics, more than500,000 km long XLPE cable hasbeen used in the world. Hence, it is important to k eeppower cables and accessories in safe for reliablepower.Partial discharge in XLPE cables not only induceselectrical tree in cable insulation, degrades insulation,but also causes a failure in cable joint. T herefore, PDon-site detection is very necessary for finding outhidden faults on time.Generally, the conventional PD measurement is in lowfrequency, and has not an effectiveability of anti-noisein the field. As a result, the conventional PDmeasurement is difficult to be used for on-linedetection. As a result, the width of PD pulse in XLPEcables is found to be about Ins - Ons[l], which has awide frequency spectrum up to several hundreds MHz.Therefore, it becomes possible to prevent it komexternal interference with very high freque ncy (VHF).At present, PD detecting methods used in the XLPEcable joint mainly include difference [2], directioncoupling [3], distributing voltage capacitance [4] andhigh-frequency capacitance IS]. Most of them requestto put measuring units or sensors into accessories ofcables, or to change their configuration when layingcables. However, it is not c onven ient for the cablesthathave been in operation.Compared with mentioned above, electromagneticcoupling method using clamp-form VHF sensor isvery suitable for on-line PD detection in XLPE cable0-7803-8348-6/04/S20.00 02004 IEEE.

joint w ithout changing the configurationof accessories.Moreover, it is very simple to install clamp-formsensors on the two sides of the cable joint or on thecable terminals, or on the grounding wires of cables.The m easuring circuit used in this technology has nodirect electric connection with the power cables, bywhich pow er cables not only op erate in safety, but alsoproduce a low noise level to the measuring system.Therefore, the VHF metbod has advantages of portable,convenience and d e t yDEVELOPMENT OF VHF CLAMP-FORMSENSORAiming at on-line detection of llOkV XLPE cablesand cable joint, VHF band ranging from lMHz to60MHz is adopted to improve the anti-noise abilityand the signal-to-noise ratio. Our laboratory resultsindicate that it is feasib le to d etect XLPE-cable PD byuse ofVHF band.

Fig. 1 The configuration of the current sensorThe magnetic core of the sensor adopted on highfrequency band is made of nickel steel amorphousbody material with high ratio of magnetic inductivity,which is able to bear grind and erosion. The coreconsists of two half rings twisted by winding in serieswith one integral resistance, which is put intotwo-half-loop metal shielding box. With one rotatingaxis and one fixed bolt on the end, the two half ringboxes cou ld be integrated or separated freely. Figu re 1shows the true configuration of the sensor.According to the external diameter size of the 1 OkVXLPE cables, the m agnetic core with 114-mm internaldiameter is suitable. This makes it very convenient toinstall the sens or on the cable, shown on fig. 2.

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Fig. 2 the V H F current sensor on the cableOnce PD happens in insulation and cable joint, thecurrent pulses would flow along the cable shieldinglayer. While current passes through the sensor, thesecondary winding of the sensor can induce currentpulses so that the senso r finally can detect PD signals.Fig. 3 show s the measuring principal of the sensor, inwhich M is a mutual inductance of the winding, is aself inductance of the w inding, and RS is an equivalentresistance, Cs is an equivalent scattering capacitance,and R is an integral resistance.

crke Fig. 3 sensor configure & equivalent circuitThe amplitude -frequency response characteristic ofthe sensor is shown in figure 4, indicating that thefrequency band is between lMHZ and 60MHz.

.6- Fig. 4 amplitude-frequency characteristic of the s ensor

LABORATORYEXPERIMENTIn order to compare with 1EC60270 method, we didour PD tests by using the VHF cla m pf om sensor andthe conventional sensor in our laboratory. Theexperimental circuit is show n in figure 5 . The sensors were installed on the anti-corona IlOkVXLPE-cable with stress cone on the two ends. TheVHF sensor was located in the middle of our 2 m longcable. The VHF detecting system comprises one VHFclamp-form current sensor, one preamplifier with thebandwidth kom lMHz to lOOMHz (35dB) and oneDL1540L d igital oscilloscope with four input channels.The con ventional sensor was located in the end of the

cable through a high voltage capacitor. Theconventional PD measuring system contains onelOOOpF capacitor, one impedance box and one DST-4PD detector. The PD source used in the tests was astandard needle-plate model. To make a bettercomparison, the PD data were put into the oscilloscopeso that two measurement data could be shown at them e ime. The typical PD s ignals are shown n figure6.From Fig. 6, it is clear that the signals b m VHFsensor are a consistent with the signals from theconventional sensor acco rding to IEC60270.

-Fig. 5 the conn ection of the experim ent

TimeddkFig. 6 (A) signal detected by conventional PD detectoi

3~ .;:L.LL . $4- i z ia i. a+: ,. .i:giFig. 6(B) signal detected by the VHF current sensor

me . d& - ..

ON-SITE TESTINGMeasurementof Partial DischargeIn order to confirm ou r VHF clamp-form senso r abilityin detecting PD in cab le joint, the on-site measurem entwas carried on at one underground llOkV cable power



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line, as shown in Fig. 7. We installed our WOVHFsensors on the two sides of the oil insulated andextruded 110 kV cablejoints for PD signal acquisition.The measuring system included the VHF clampformcurrent sensor, one preamplifier and one DL1540Ldigital oscilloscope and one laptop computer. The dataacquired by the oscilloscope were put into the laptopto be stored through GPIB card. Thus it is convenientto deal with data processing later. We tested 21 cablejoints and found that PD only could be detected inthree of the cable joints. T he typical on-site PD pulsesfrom VHF sensors are shown in Fig. 8. These signalsare all in the range O-900r 180-2700f powerfrequency. Therefore, we believe that the VHFclampform sensor is suitable for the on-site PDmeasurement in cable joints.

Fig. 7 1 OkV XLPE -cable measuring setup for on-site

. .odm ' oma . a i m ' adla 0.mTime sldivFig. 8 VHF PD signals detectedon iteAcquisition ofPower FrequencyIn addition to VHF signal, the phase information ofpower-frequency signal should be acquired foranti-noise processing and data processing, such ascharacteristic s tatistics and pattern identification.In order to get the AC signals from the power cables,we put a small sensor into the VHF box. Th efrequency band of this small AC sensor was focusedon 50 Hz. By this sensor, we could obtain the phase

information of the P D signals we got from the field. Itis very convenient to get P D and A C phase signals atthe same time without any electrical connection withthe detected power cable on-site by using our on-sitePD detecting system in cable joints.CONCLUSIONS(1) Our developed VHF clampform sensors are veryconvenient to be installed on-site testing for cablejoints. VHF sensors with clamp form have no need ofchanging the structure of cable and its accessories.(2 ) Our tested results from th e Laboratory and the fieldindicate that VHF sensors with clampform couldmeasured PD occurred in power cable joints. Themethod has advantages of portable, safety andanti-noise.(3) It is also indicated from th e on-site testing that thebackground noise level in the field is very high, whichdecreases themeasuring sensitivity of the VHF sensor.REFERENCES[l ] Shim, et al. Digital signal processing applied to thedetection of partial discharge. An overview [J].IEEE Electrical Insulation Magazine, 2000,16(3)[2] Ginzo Katsuta, et al. Development of a methodpartial discharge detection in extra-higb voltagecross-linked polyethylene insulated cable lines.IEEE Transactions on Power Delivery, Vol.7,No.3,1992131 D. Wenzel, et al. Recognition of partial dischargeson Power Units by Directional coupling. T he 9"ISH, 1995[4] Th. Heizmann, et al. On-site partial dischargemeasurements on premo ulded cross-bonding jointsof 170kV XLPE and EPR cables. IEEETransaction s on P ower D elivery, Vol. 13, No.2,1998[5 ] Xu Yang, et al. UHF partial discharge detection incable and cable accessories, Advanced TechnologyofElec trical Engineering and Energy, Vol. 21, N o.1,2002[6] H. Borsi, et al. Monitoring of Partial discharges(PD) in high voltage cables. The 8" ISH, 1993[7l N.H.Ahmed, et al. On-line partial dischargedetection in cables. IEEE Transactions onDielectrics and Electrical Insulation, 1998, Vol. 5,No.2Author mail address:Dr.Cheng Rong LiHigh V oltage & EM C LaboratorySchool of Electrical EngineeringNortb China E lectric Power UniversityDewai, Zhuxinzhuang, Beijing, I0220 6P.R.China


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EE509 Partial Discharge