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Conference Record of the 2004 IEEE nternational Symposium on Electrical Insulation, Indianapolis, N USA. 19-22 September 2004

On-line Condition Assessm ent of High Voltage Current TransformersV.R. Garcia-Colon, R. Lifih-Garcia, M.A. Jacob0Gerencia de Equipos EltctricosInstituto de in vestigacio nes EltctricasRefon in 113, Edif.. 26 PBCuemavaca, Morelos, Mtxico

gcolon@ieee.org

Abstract This paper presents the developm ent of an ultra-wide-hand (UWB) PD technique that allows on-line diagnostic of 400kV CTs. The measurements carried out on 130 C T s werequalitative on-line measurements and included PD and thermalimaging inspection. PD me asu reme nb were carried o u t on CTsground connection using a cl am po n Rogowsld coil whose ou tputis centered at UWB requencies and then processed and fed into astandard PD detector using a frequency stretcher. Evaluationswere performed on CT s installed at seven transmissionsubstations of main Mexican Utility. The diagnostics were

performed establishing three levels of partial dischargesmagnitude (low, medium .and high)., Over 5% of CTs were. detected with high PD level. Those -%U be retired from semceand evaluated in laboratory through Power Factor, Capacitance,PD and DGA analysis.INTRODUCTIONHigh voltage Current Transformers (CTs) in service areexposed to electrical, thermal and mechanical stresses andenvironmental conditions that can cause deterioration ofinsulating system (oillpaper). D eterioration is associated withpartial discharges (PD) activity cau sing early failures that canbe catastrophic. Thos failures may result in damages toadjacent equipment, personnel injures and represents high

i replacement costs besides unexpected outages. Due to theabove factors, it is necessary to guarantee CT reliability by. , applying maintenance program s. However the cost, ncurred toevaluate each CT off-line when considering the large amountof CTs installed on each substation makes the ideaimpractical. Currently, visual inspection and porcelain cleaning is the main

maintenance that CTs receive, beside connection tightening when hot sp ots are detected. Dissolved ga s analysis (DGA) is. . not recommended due to the limited amount of oil contained: in CTs and the relatively high cost o f each test on a per CTbasis., To overcome this lack of information, the Instituto deInvestigaciones Eltctricas (IIE) appli es a n .on-line d etection.technique for incipient fault detection based on Ultra WideBand Partial Discharge (UWB-PD) detection techniquedescribed next.

Standard vs. UW B PD detection techniquesTo perform on-lme diagnosis on CT internal insulation, on avery expedite and reliable way, preferred methods were sough tbut no favorable results w ere obtained, except from the use ofa permanent monitoring system developed for,bushings andadapted to analyze instrument transformers [l]. This-lack ofon-line diagnostic techniques. is a consequence of theextremely high level of electromagnetic interference found inany substation, where it is impossible to apply traditional PDmeasurements according to IEC stand ards [2].A different way to solve this limitation is hy applying sensorslocated very close electrically to the source of PD and using afrequency measuring range ou tside the standard noisy section.Such sensors have to respond in the frequency range of PDand have to offer rapid attenuation for corona and otherelectromagnetic interference. Furthermore, . PD has adistinctive pulse shape that allows its classification whencompared to external noise.EXPERIMENTALOver 10 years ago PD detection using Ultra Wide Band(UWB) frequencies under lab conditions was demonstrated [3,4, 51 and in recent years UWB-PD detection has beendemonstrated for on-site application to diagnose cables,motors, tran sformers and generators. S uch a system isdepicted in Figure 1 and is reported elsew here [6:13].In this paper,. the same UWB system is applied to takeadvantage of spatial resolution of UW B measurements whenapplied to high voltage (400 kv) CTs while in normaloperation. However, considering that the earth connection of aCT contains 60Hz capacitive current plus high frequencysignals that include PD signal, corona, leakage current,comm unication sign als and influence -from other CTs, acareful analysis has to be performed to correlate PDmeasurements with phase occurrence and discriminateinterference from neighb oring CTs or corona. For this reasona 6OHz capacitive coupling sen sor was additiona lly employ edto allow PD synchronization to each phase voltage. With thisdevice corrected PD patterns were obtained and therefore,those patterns could be compared between phases and betweenCTs installed on both sides of a circuit breaker.

0-7803-8447-4/04/$20.00@2004 EEE. 182

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Figure 1 -Measuring Set upOne of the main elements in the system to perform on UWB isthe pulse stretcher. Xis amplifier is connected near theRogowski coil sensor. The stretcher depicted in Figure 2converts high frequency PD pulses (tens of MHz) into 40 to500kHz nipolar pulses that are fed into a co nventional digitalPD detector. Furthermore, an asynchronous standardcalibration pulse is fed into the Rogowski coil for continuousindirect calibration during all m easurements. This calibrationpulse appears as a continuous line on the N-Q-3 PD Patternsdepicted in all results obtained.Using the LWEi PD system, seven 400kV substations werevisited and 130 CTs were evaluated. The CTs populationunder study was two types: 108 of the same manufacturer andanother 22 CTs from different manufactures. All equipmentwas rated for 400kV and no previous conditioning, cleaning orconsiderations were taken prior to the m easurements.During all tests, fair weather with low winds and temperaturesof 20 to 30 C prevailed.

Figure 2 - Close up at conections UWB circuits and CT groundingleadRESULTSTrpical PD patterns obtained could be classified in threecategories; one under low PD activity; another for abovenormal PD activity and one with extremely high activity.Those PD Patterns are depicted in Figures 3, 4 and 5 ,respectively.DiscussionAAer performing all 130 measurements, PD levels wereplotted and Figure 6was obtained. High PD activity was thestandard in almost 70% of all measured CTs and interventioncriteria was developed. All readings over 300pC wereconsidered as abnormal and further maintenance work wasrecommended; those works include oil level verification,external cleaning and oil leakage detection. When readmgswere well over 1,OOOpC it was considered as extremely highand imm ediate actions was recomm ended, including dissolvedgas in oil analysis.Two CTs with the highest readings were removed fromservice and transported to a High Voltage lab, where a PDpattern with the same shape but 13% higher reading wasobtained, verifymg that indirect calibration and UWB systemcan be maintained with the same accuracy as those attainedunder lab conditions.During the development of experimentation, one CT with anabove normal PD reading suffered a failure and it is underinvestigation at present to correlate the cause of PD patternrecorded in the field.

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Figure 3 - PD Panem for C r s with low PD ctivity

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Figure 4 - PD Pattem for above normal PD activity

Figure 5 - PD Pattem for a CT with extremely high PDactivityCONCLUSIONSA very fast and portable system that allows PD detection forCT during normal operationwas presented and the cumu lativemeasurements allow classifying CT into three categories.Those categories allow focusing maintenance resources anddetect incipient faults in a larg e populatio n of equip ment likethose represented by CT in a ny traditional open substation.Calibration using indirect techniques may in troduce a 15 to30% error but PD readings with difference in one order ofmagnitude allow easy classification of that equipmentrequiring maintenance most urgently.ACKNOWLEDGEMENTSThis study was developed under the kind auspices of CTTfrom Comisi6n Federal de Electricida d and its su pport isrecognized here.

Figure 6 -All 130 PD levels from measnrements performed are summ arized in this plot184

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REFERENCES1. Lachman, M.F.; Walter, W.; van Guggenberg, P.A., On-line diagnostics of high-voltage bushings and currenttransformers using the sum current method ,Power Delivery,IEEE Transactions on,Volume: 15, Issue: 1, Jan. 2000,Pages155- 162.2. Intemational Electrotechnical Commissi6n, Standard IEC60270 High Voltage Testing - Partial Dischargemeasurements, Third Edition, Switzerland, 2000.3. J.T. Holbnrll, J.M. Braun, N. Fujimoto, G. Stone,Simultaneous Detection of Optical & Electrical Signals fkomPartial Discharge Phenomena in Voids in Epoxy, 7thInternational Symposium on High Voltag e Engineerin g, Paper72.09, Dresden, Germany, August 26-30, 1991, pp. 75-78.4. P.H.F. Morshuis, Partial Discharge Mechanisms, PhDThesis, Delft Univers ity Press, 1993.5 . V.R. Garcia-Colon, Evolufion of Partial DischargesWaveshapes in Spherical Vo id Using Ulfra Wide BandTechniques, PhD Thesis, University of Manch ester Institute ofScience and Technology, UMIST, August 1994.6. V. R. Garcia-Co lon, A. Nava-Guzman, L.A. Salgado-Salgado , R. C anales-Ruiz, Lo cation and assessment of partialdischarge on substation equipment using ultra wide bandtechnique s, EPRI Substation Equipment D iagnosticsConference VIII, New Orleans, February 2000.7. V.R. Garcia-Co lon, Powe r Cable On-line Diagnosis UsingPaaial Discharges Ultra Wide Band Techniques, IEEE LargeEngineering System Conference on Power Engineering,LESCOPEOI, Halifax, Canada, July 11 -13,2 001 .9. V.R. Garcia-C olon, Distribu ted PD measuring techniques@-PDM ) for installed power equipment diagnosis, 2002IEEE Conference on Electrical Insulation and DielectricPhenomena, Quinta Roo, M exico, October 2&24,2002.10. V.R. Garcia-Colon, Distributed PD Measurement onPower Underground Cables, Intemational Symposium onElectrical Insulation, ISEI200 2, Boston, Ma. April 200 2.11. C. G. Azcirraga and V. R. Garcia-Colon, PILC Cablesevaluation using on-line PD d iagnosis techniques, 2003 IEEEConference on Electrical Insulation and DielectricPhenomena, Albuquerque, New Mexico, USA, October 19-22,2003, CEIDP Paper 92.

12. Anton io Carv ajal and V. R Garcia-Colon, High CapacityMotors On-line Diagnosis Based on Ultra Wide Band PartialDischarge Detection, Sym posium on Diagnostics for ElectricMachines, Power Electronics and Drives, Atlanta, GA, USA,24-26 Augu st 2003.13. V.R. Garcia-Colon, J.M. Ramirez-Cruz, ReliabilityCentered Maintenance For Underground DistributionNetworks Using On-Line Distributed Partial DischargeMeasurements, XIIIth Intemational Symposium on HighVoltage Engineering, ISH 2003, D e l e the Netherlands, 25 -29August 2003.

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