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"ON-SITE PARTIAL DISCHARGE MEASUREMENT OFTRANSFORMERS

by Victor Sokolov,Vladimir Mayakov

ZTZ-Service Co

(Ukraine)

Georgy Kuchinsky,

Technical University,Sankt-Peterburg

(Russia)

Alexander Golubev

Cutler-Hammer

(USA)

INTRODUCTION

During recent years the technical policy of power utilities is changing under thepressure of economical considerations. There is a common tendency of moving fromthe time-based to the condition-based maintenance, to reduce maintenance costs, and,at a time, to prevent of sudden failure, particularly catastrophic one that typicallyfollows to insulation fault.A vital problem is also how to provide continuity of operation of the questionabletransformer especially if that shows symptoms of PD activity.Then, one may emphasize a clear tendency to perform maintenance procedures in-field condition, especially without de-energizing the equipment. Question arises howto verify quality of such works?Incipient failure of insulation attributes always to PD activity. PD measurement [1, 2,3, 4] is recognized as the main and the most reliable method of evaluation of theinsulation condition at the factories. Why it may be not duplicated in field,particularly to verify repair quality and to ensure that incipient fault suggested e.g. byDGA test will not progress to affect the insulation system?There have been several technical obstacles, especially high external noise and needsin special expensive facilities. Therefore, only system of acoustical PD detection hasbeen practically used, in some instances very effectively [5, 6].The recent progress in the PD measuring technique has opened really newopportunities in an effective rejection of external interference, and on-site diagnosis ofthe condition of transformer insulation, quite similarly to well established tests at thetransformer factories..Successful implementation of PD electrical test technique and its apparent advantageshas been reported in several papers [7, 8, 9, 10, 18].This paper discusses problems associated with PD activity and PD identification inPower Transformers, and effectiveness of diagnostic methods available. The casehistories with identification of problems in transformers using PD Analyzer arepresented.

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HISTORY

ZTZ-Service Co has utilized PD measurement technique since early 70th. At first, itwas an attempt to duplicate in the field of factory PD measurements. LargeElectrostatic shields placed on the bushings and separated source of energizing havebeen used to minimize external noise. However, feasible PD signal could have beenachieved on the level of 5,000-10,000 pC only [12].Special balance (bridge) circuits has allowed reducing PD signal to 600-900 pC (testsof shunt reactors [14]), and even to 50-100 pC using separate diesel-generator as asource of energizing [13]. Such tests due to high costs have been performedoccasionally, as a part of life extension program.Preferably, acoustic monitoring technique with piezoelectric transducers have beenutilized, basically on the units exhibiting PD or arcing activity through DGA test.Since 1998, ZTZ – Service has implemented Electric Partial Discharge Analyzer .Thepractical experience has confirmed that this test technique provides high enoughsensitivity to PD in field conditions and may be used as an effective diagnosticinstrument [11].The following transformers have been considered as candidates for PD monitoring:• Exhibiting PD or arcing activity through DGA test• After Repair and Refurbishment in-field• Ranking the units requiring in repair• Assessment of maintenance (processing) program• Assessment the condition of critical transformers (nuclear units, equipment of 750

kV transmission system, etc.)

Partial Discharge problems in Power Transformers. A View fromExperience

The Table 1 shows classification of PD generation sources in operating transformers,which has been defined on the basis of many years of experience. There have beenthree potential sources of PD generation in Power Transformers: “Core and CoilsAssembly”, Bushings, and LTC. On the other hand the sources of PD can beassociated with operative voltage, with voltage induced by main magnetic flux, andwith voltage induced by stray flux.

Statistics has shown that the problems associated with sparking and arcing within atransformer could be roughly distributed as the following:PD associated with main magnetic flux 31 %PD associated with stray flux 41 %PD associated with operative voltageConductor under floating potential, bad contacts 14 %Creeping discharge 14 %The most dangerous cases with PD activity in major insulation spaces make up lessthen 15 % of the total problems associated with arcing inside a transformer. In suchcase the unit must have been taken from operation immediately. On the other hand,failure analysis has shown [11] that about 15-20 % of annual sudden failures and mostof catastrophic events are caused by impairment of the conditions of main and minorinsulation due to a particle contamination or the ingress of moisture reducing the

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impulse withstands strength. These events definitely attributed to PD activityfrequently remain undetected.

MECHANISMS OF INSULATION DEGRADATIONINSULATIONDETERIORATION

Defect-free transformer insulation is characterized with apparent PD magnitude 10-50pC or lower. Increasing the PD level up to 100-300 pC is associated typically withpresence of particles and (especially just after filling the transformer with oil) withtrapped small air bubbles [16]. Ionization level 25-500 pC [17,19] in an oil barrierspace does not affect dielectric withstand strength and may be considered ascharacteristic of normal deterioration.

Defected condition.Defected but reversible change of insulation condition is associated with PD>1000pC,and may be caused by different degradation mechanisms:• Conductive mode particles, which bridge oil gap and result in discharges with

magnitude from 100 to 10,000pC[16].• Increasing the moisture content in a paper up to 3-4 % and relevant increasing

concentration of moisture in oil, which causes reduction of PD inception voltageby 20 % and occurrence of PD’s with the level up to 2000-4000 pC[20 ].

• Poor impregnation may cause discharges of about 1,000-2,000 pC [16].• Large (3-5 mm) air/gas bubble in oil may result in discharges ranging in

magnitude from 1,000 to 10,000pC [16].In general, PD level over 2500pC (in paper) and over 10,000 pC (in oil) may beconsidered as destructive ionization [1,19] in a long-term action.

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Table 1 Sources of PD generation

Components Source of PD Typical faults

Core and CoilsAssembly

oil-barrier-paperstructure and oil

ElectrostaticShields

Leeds

Operating voltage

PD attributed toreversible change ofinsulation conditionPD attributed toirreversible degradationof insulating material:

Oil/surface contamination withparticles, bubbles, staticelectrification, bad impregnation,high moisture,Partial breakdown in oil; surfacedischarge; creeping discharge

Sparking and arcing between badconnection“Conductor under floatingpotential” dischargesTracking in wooden blocks.

PD associated withvoltage induced by mainmagnetic flux

PD associated withvoltage induced by strayflux

closed loops between adjacentmembers linked y the main flux(insulated bolts of core, pressingbolts, pressing metal rings, etc);sparking due to floating potential closed loops between adjacentmembers linked by stray flux;floating potential (e.g.ungrounded magnetic shunts)

Bushings

Operating voltage Localized defect within the core:bad impregnation, high moisture,short-circuits between layers,sparking across the core surfaceBreakdown in oil;surface discharge across theporcelain

LTC

Operating voltagePD associated withoperating voltage at thefix tap position:PD associated withswitching process

Partial breakdown in the selectorand in the divertor switchcompartment

Poor or worn out contacts

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Mechanism of Incipient irreversible failure in oil-barrier insulation:

• Initiated by breakdown of oil gap which is registrated as apparent charge >10,000pC which rise rapidly to 100,000-1000,000pC[20,21].

• Progresses in surface discharge in oil across the barrier with PD magnitude over100,000 pC.

• Appearance of “white marks” on the surface due to forcing oil and water out ofthe pressboard pores followed with carbonized “black mark” on the barrier.

• PD 10,000-100,000 may cause irreversible damage during tens of hours.• Minimum energy required to cause incipient carbonizing the cellulose (heating

over 300C) is estimated as 0.1 J, which corresponds to several charge pulses of100,000-1000,000 pC [1],

• Stable charge in oil is associated with PD power P>0.4 W.• average rate of gas generation under effect of stable PD in oil- 50µl/J[22,23].• Further steps progress either in breakdown of the insulation space or in occurrence

of creeping discharge.

Creeping discharge

That is likely the most dangerous failure mode that typically results in catastrophicfailures at normal operating conditions. The phenomenon occurs in the compositeoil-barrier insulation and progresses in several steps:• Partial breakdown of oil gap.• Surface discharge in oil across a barrier (an appearance of black carbonized

mark on the barrier).• Microscopic sparking within the pressboard. The presence of some excessive

moisture stimulates this process.• Splitting oil molecules under effect of sparking. Formation of hydrocarbonsfollowed with the formation of carbonized traces in the pressboard. This process isresulted in lowering PD magnitude of apparent charge to 1,000-5,000 pC. Creepingprocess can continue from minutes to months or even years, until the treeingconductive path causes shunting of an essential part of transformer insulationresulting in a powerful arc.Cellulose destruction (creeping discharge) while progressing within the pressboardis associated with PD intensity q>1000 p C; PD power P=0.1-1 W, Average rate ofgas generation of 40-50 µl/Joule [23].

Failure of turn-to turn insulation

• Occurrence of sporadic PD pulses of magnitude over 400 –1000 pC duringseveral hours [24].

• Rapid rise PD activity up to q>100,000pC (duration up to tens of minutes).• Paper puncture, turn-to-turn short circuit (duration – several second).

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Diagnostic characteristics of a faulty transformer insulation

Faulty conditions can be characterized in terms of PD activity, fault gas generation,and changes in the conductance, capacitance and dielectric loss factor of a defectivearea. Typical scenario of initially defect-free insulation failure is the following:Contamination (particles, water, bubbles) � Occurrence of moderate PD �Occurrence of destructive PD � Gas generation � Progressing PD, accompaniedwith gas generation � Tracking/treeing, accompanied with gassing and changingdielectric characteristics, critical pre-failure PD � Breakdown

Relevant diagnostic characteristics are:• PD parameters: apparent charge magnitude, Pulse repetition rate, Discharge

Power, PD Signature.• Faulty gas content.• Gas generation rate as a rate of degradation of insulating materials, which is a

function of discharge power.• Change of Power Factor, Conductivity and Capacitance of the defective

insulation space [25].

Diagnostic technique shall advise how to distinguish between really dangerousproblems (e.g. destructive PD in the oil-barrier structure), and problems, which doesnot affect the functionality of a transformer to a danger extent, and an equipmentcould be kept in service at least for some while. Apparently, consideration aboutpossible critical transformer condition must be supported by design review,especially to assess “the sensitive points of the equipment.

Methods of PD registration

Three of most frequently used types of PD sensing are electric, acoustic andelectromagnetic. Con and Pro of the methods are presented in the Table 2. Ideally, thecombination of all three methods can be a powerful diagnostic tool:• Rough detection of the problem externally using electromagnetic sensor,• Identification of insulation condition with electric method,• Location of the PD source by an acoustic device.However, the only electric method can detect and identify defective condition thatassociated with PD activity ranged below 1,000 pC.Our experience has confirmed that acoustic detection of PD is a very effectivecomplementary tool to localize a source of gassing caused by very strong arcing inoil. Diagnostic method in this case can be: DGA-is a trigger and acoustic sensor-is alocating tool.However, the cases, when identification of arcing problems through “DGA +acoustic” technique is effective, are associated with the dissipation of high energy(typically, over 500-1000 kJ) and show clear only strong degradation process.Acoustic PD monitoring has not been effective in identifying of progressing creepingdischarge. Minimum PD signal effectively detecting by acoustic sensor in full-scaletransformer model is limited to the level of approximately 10,000 pC and above.

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Table 2 Methods of PD registration

TYPE OF SENSORS ADVANTAGES DISADVANTAGES

ELECTRICDirect connection to the testtap,or through high frequencyCT on the grounded wire,(“Rogovski coils”)Additional sensors in busduct, electrostatic shields,neural etc.

High sensitivityCan be calibrated in terms ofapparent chargeApproximate location of PDsourceAll capabilities to trend dataUse of PD pattern RecognitiontechnologySensors configuration canmatch for better noise rejection

De-energizing for sensorsinstallation

ELECTROMAGNETIC(ANTENNA)

Easy to usePossible assessing external PDproblems including PD in thebushingsServing as a noise (corona)channel

High disturbancesOnly discharges of extremelyhigh level can be detectedDifficult to distinguish anequipment having problemsfrom surrounding equipment

ACOUSTICPiezo-accelerometer placedon transformer tank

Easy to installCapability detecting acousticemission magnitude and trend,Pulse repetition rate and trendLocalizing a source of PDusing signal time of arrival todifferent locations

Low sensitivityMinimal detecting apparentcharge >10,000pCResponded to rain, sleet,electrical disturbances in thestationThe level of signal dependson coupling between thesensor and surfaceEffect of design (variablesinside the tank) on thepropagation of sound wave.

Experience with Partial Discharge identification using Universal PDAnalyzer

PD Technology

The portable Universal PD Analyzer UPDA “Twins” has been used for different in-field applications to detect PD activity in Power and Instrument transformers. TheUPDA monitors the wave forms for several power frequency cycles from several (upto 8) sensors simultaneously, and then identifies the PD pulses in the acquiredinformation using up to 5 independent noise deletion systems [26].Radio frequency pulses are measured at the output of the capacitance taps of thebushings (typically six sensors), neutral and tank/active part grounding wires. Someof measuring inputs is used for connection of acoustic sensors and electromagneticsensor (antenna).

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Reduction of noise is provided by the following measures:• Selection of frequency band. As a rule, the frequency band from 1 MHz to 20

MHz is selected. UPDA also has the option to choose full frequency band or touse an external filter.

• Filtering (sometimes, when radio transmission interference is very high).• Electronic processing of signals: phase and frequency selection, time delay

selection, signal comparison, etc.• Time window method (provided with the gate, which can be open and closed at

pre-selected moments).• Comparison of signals from different electromagnetic sensors (to reduce corona

disturbances, in CT’s specifically).• Balanced circuit- in the shunt reactors and CT’s.

High frequency band used in measurements creates some problems associated withattenuation of PD signals, while they travelling in a transformer to a sensor.Therefore, a calibration is important to evaluate sensitivity and to determine possibleimpact to a data by attenuation. High frequency band may also exert an effect onscreening PD impulses in the range of 100-1,000 kHz, which typically accompany ofpowerful PD 100,000-1000,000 pC. Tests procedure can be adjusted for the particulartransformer design.

Calibration is an important process, which is typically provided immediately afterthe monitoring system commissioning. Calibration allows for:• Recalculate pulse magnitude into an apparent charge.• Evaluate sensitivity of the each measuring channel.• Compose calibrating cross-matrix, which serve as an important diagnostic tool,

especially to distinguish between PD sources in the core, in the bushings or inwinding insulation, and to determine an approximate PD location.

Test Protocol includes cross-matrix, detected noise level, residual (white) noiselevel, maximum pulse magnitude, Pulse Repetition rate, and PD Power. Three-dimensional phase-resolved analysis is also available for PD pattern recognition.

Diagnostic method consists of several steps:• Evaluating PD characteristics.• Considering voltage/load/temperature effects on PD activity.• Considering accompanied factors (DGA, oil tests, and dielectric characteristics).• Design review.

Comparative tests at the factoriesThe effectiveness of noise deletion by UPDA Analyzer was evaluated by comparativetests in the factory. The tests using factory technique and UPDA were performed onthe transformer 500/220 , 100 MVA (Table 3) and on the Current Transformer 330kV (Table 4). Factory test setup uses all possible noise reduction means includingscreened room and disel-generator as a power source. The tests have shown a goodagreement between factory Lab’s and UPDA technique. It was found that UPDAAnalyzer achieves satisfactory signal to noise ratio, even without special procedures,which are critical for factory setup.

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Table 3 Comparative PD test on the 100 MVA, 500/220 transformer

Factory’s test technique UPDA AnalyzerPlace of sensorApplied voltage Special shied on

500kVbushingWithout shield Special shield on

500kV bushingWithout shield

HV ,neutral,Grounding476 kV,1.64 Ur

24pC 2800 pC 30.7pC 68.7pC

Table 4 PD test on the 330 kV current transformer

Place of sensorApplied voltage

Factory’s test techniqueBalanced circuit

UPDA AnalyzerDirect test

Measured tap,Grounding tape( «0» – electrod)275 �V (1.3 Ur.)

26 pC 11pC

In Field experience. Case of History.

Case 1 Verification the quality of in-field repair.

Auto-transformer 200MVA, 330/110 kV has failed after 13 years in operation due toexplosion of 330 kV bushing. The failure was accompanied with the tank rupture,damage to HV winding insulation and severe contamination of the core and coilsassembly with carbon, pieces of porcelain etc. ZTZ-Service has performed remedyrepair on the substation in-field conditions. Insulation condition after repair wasevaluated as satisfactory. (Water content in the pressboard 0.7 %, PF =0.2% at 45 C,low content of particles in oil, etc.). However, some doubt remained especiallyreferring to possible localized contamination in HV winding.PD measurement has been performed with UPDA Twins system at rated voltage andat 1.05 Ur. The unit was energized from 110kV side. Test results (Table 5) haveconfirmed a good condition of the insulation integrity.

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Table 5 PD Test ReportCustomer: ����, Date:12-18-1998Object: ����� 200.0/330/110, 122580, ��-2

Sensor DeletedNoise, [nC]

Max PD Magnitude RepetitionRate,[ppc]

PDPower,[mW]

[V] [pC]Bush_A_S 0,989 0,0266 53.2 1,11 0,531Bush_B_S 3,42 0,0376 75.2 1,88 0,904Bush_C_S 0,547 0,0298 59.7 2,07 0,838

Bush_Am_S 4,63 0,0266 47.9 1,01 0,294Bush_Bm_S 4,68 0,178 320 12,3 16,7Bush_Cm_S 0,66 0,075 135 11,1 9,06

Neutr_S 0,169 0,0133 40 10,4 1,11

Case 2 Assessing of the condition of 220 kV transformer after long service

The PD tests have been incorporated in the program of life assessment of the240MVA, 220/110 kV, core-form auto-transformer with the goal to make a decisionabout urgent necessity of insulation reconditioning after 28 years of service.Water Heat Run Test (WHRT) has revealed symptoms of significant insulation aging.The heating of the transformer up to 65C and holding for 24 hours has showndoubling of water-in-oil content, reduction in dielectric breakdown voltage by 20 %and increasing its variation coefficient to 15 % (symptom of presence of largeparticles). Particles in-oil count has shown the level of contamination denominated as“High”(NAS class 7). Water content in cellulose estimated through PF test andWHRT was about 2 %.

Table 6 Dissolve Gas Analysis results for the 240 MVA transformer

H2 CH4 C2H2 C2H4 C2H6 CO CO2 Σ C3 1-C4H8 �2 N2

4.7 ���. ���. 52 ���. 162 1388 67 2039 0.60 2.21

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Table 7 Calibrating cross-matrix

Calibratingimpulse

Sensors inputs

���� A_220 B_220 C_220 Am_110 Bm-110 Cm_110 Neutral GroundA_220 -Tank 1.000 0.054 0.076 0.120 0.033 0.065 0.022 0.022B_220 – Tank 0.231 1.000 0.212 0.077 0.269 0.096 0.038 0.038C_220 – Tank 0.081 0.135 1.000 0.054 0.095 0.135 0.027 0.027

Am_110 –Tank

0.054 0.076 0.043 1.000 0.207 0.065 0.022 0.022

Bm_110 –Tank

0.022 0.044 0.061 0.050 1.000 0.033 0.011 0.011

Cm_110 –Tank

0.073 0.164 0.145 0.145 0.255 1.000 0.036 0.036

Neutral – Tank 0.058 0.050 0.058 0.033 0.075 0.067 1.000 0.017Tank - Ground 0.143 0.143 0.214 0.071 0.107 0.143 0.393 1.000�_220 –Tap 1.000 0.069 0.069 0.139 0.056 0.069 0.028 0.028�_220 – Tap 0.290 1.000 0.290 0.161 0.516 0.194 0.065 0.065�_220 – Tap 0.091 0.109 1.000 0.073 0.127 0.164 0.036 0.036

Am_110 – Tap 0.046 0.062 0.031 1.000 0.115 0.069 0.015 0.015Bm_110 – Tap 0.022 0.033 0.033 0.050 1.000 0.044 0.011 0.011�m_110 - Tap 0.105 0.063 0.053 0.074 0.126 1.000 0.021 0.021

Table 8 PD Test ReportObject:TDCTGA-240000/220, 79463, 2, data17.09.1999Location:SB Baltijskaya

SensorPD Power

[mW]Residual

noise[nC]

Max. PulseMagnitude

[nC]

RepetitionRate [ppc]

A_220_S 84.5 0.074 0.298 150.0B_220_S 53.1 0.128 0.355 128.0C_220_S 88.3 0.078 0.389 132.0

Am_110_S 20.5 0.068 0.127 95.1Bm_110_S 9.39 0.036 0.160 31.7Cm_110_S 1.38 0.115 0.066 7.35Neutral_S 20.1 0.057 0.303 32.4Ground_S 0.14 0.031 0.0426 0.466

PD tests data are shown in the Tables 6,7,8, including calibrating cross-matrix, thelevel of residual (white) noise and parameters of PD. The results have shown amoderate deterioration of the insulation integrity. Maximum PD magnitude in theinsulating spaces having minimal safety margin (insulation of the series winding220kV) was below 400pC. Correspondingly, it was advised postponing a repair of theunit and prolong operation of transformer without oil and insulation processing.

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Case 3. Assessment of seriousness of gas generation in 300 MVA ,500 kVtransformerOne of two sisters –type shell-form autotransformers shown clear generation of faultygases, likely associated with combination of PD activity , localized oil heating ,andcellulose decomposition Continuing of operation of the both units was very critical.The problem arose how serious is the symptom of abnormality? Will it progress toaffect the insulation system?

Table9 DGA in oil data for the 300 MVA transformer

H2 CH4 C2H6 C2H4 C2H2 CO CO2Object Data

[ppm]

T1RS 27.09.97 1 1 4 1 1 4 49

T1RS 17.10.97 53 2 1 1 2 46 235

T1RS 12.11.97 154 6 2 3 2 105 684

T1RS 14.05.98 447 32 11 23 1 580 1920

T1RS 30.06.98 563 151 50 148 1 605 1757

T1RS 23.07.98 557 155 45 156 <1 588 1720

T1RS 02.09.98 674 204 70 217 <1 722 2174

Table 10 PD Test reportCustomer: E.T.Resistencia,T1RS,T2RS,Date 07.22.1998

Object Sensor noise,[pC]

MaximumPulse

Magnitude[pC]

RepetitionRate[ppc]

PDPower[mW]

Bush U 127 1,270 100 107

Bush V 127 225 1000 471

Bush W 142 252 1800 806

T1RS,Gassing;

Neutral 37.8 0 0.759 0.00046

Analysis of PD characteristics has pointed out at a defective condition of thetransformer. Particularly presence of the source of PD generation on the phase U closeto the HV terminal has been admitted. However, the level of ionization could notcause destruction of insulating material and explain the observed rate of gasgeneration. It was suggested that the problem is not attributed to PD in the majorinsulation and the unit may be left in operation for a certain time (at least one-year).Unfortunately, it was not possible to perform PD test at the fully loaded transformerin order to establish a relation between partial discharges and stray flux.

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Case 4 Verifying insulation health of transformers at Hydraulic Power Plantearly in their service life

PD test has been carried out on the 21 sister units rated 172.5 MVA, 500 kV to verifytheir condition after 2-4 years of service and specifically to assess condition of theSF6 500 kV bushings. All of tested step up transformers was installed on the sameHydro plant.PD test results are shown in the Figures 1 and 2. We want to highlight the following:• The achieved level of residual noise was less then 20 pC.• The PD parameters measured on HV side comply with defect-free condition

(average value of PD magnitude 28 pC, average PD Power 0.14 mW).Only in the neutral of one unit PD level of 670 pC and PD Power 7.2 mW have beenobserved. This not dangerous ionization was likely attributed to insulation of no-loadtap changer.Several significant PD sources were observed in bus ducts on 13.8 kV side of a GSUtransformer. An additional investigation was not done since bus ducts were out of thegoal of the describing test.

Case 5 Identification of critical PD source in the 750kV auto-transformer

The bank of three single-phase auto-transformers rated 333MVA, 750/330kV wasenergized after storing during 9 months. Sporadic crackling sound has appeared fromthe tank of the phase “A.” PD test was advised to identify the problem.The tests have been performed at three voltage steps of 0.5Ur, 1.0Ur and 1.05Ur usingphase “B” as a supplementary transformer. Residual noise was achieved at the level of50 pC.Intensive ionization has occurred after increasing the voltage up to 1.05Ur.Destructive PD with apparent charge over 12,000 pC had shown destructive problemon 750 kV side. Visual inspection has revealed traces of progressing creepingdischarge across the 750 kV bushing insulation (see Fig.) The problem was associatedwith free water penetration through loosed top sealing of the 750 kV lead.

Table 11 PD Test Report

Sensor Voltageon HVSide, [kV]

Maximumapparent charge,

[pC]

Comments

A750 S 72A330 S

230100

No symptoms ofabnormality

A 750 S 72A 330 S

750100

No symptoms ofabnormality

A 750 S 787 11400A 330 S 2000

Occurrence of intensiveionization

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CONCLUSION

1). The practical experience has confirmed that the modern PD electric measuringtechnique can provide high sensitivity to PD’s in field conditions and comparable toone that can be achieved in laboratory tests at the transformer factories. Such atechnique may be effectively used as an diagnostic instrument.

2). Only 15% of insulation problems associated with arcing or sparking are related todischarges in major insulation and provide an imminent danger for transformeroperation. PD technique should advise how to distinguish between dangerousdischarges (e.g. destructive PD in the oil-barrier structure), and problems, which donot affect equipment functionality to a critical extent.

3). Our experience shows that PD technique can be effectively used in field conditionsfor:• monitoring of the units suspecting in PD or arcing activity by DGA test,• assessment after Repair and Refurbishment in field conditions,• ranking units requiring repair,• assessment of maintenance (processing) programs,• assessment of condition of critical transformers.

4). The advantages of the PD measuring technique offer ample scope for on-linecondition monitoring of power and metering transformers.

REFERENCES

[1] Kuchinsky,G.S.” Partial Discharge in High voltage Apparatus”, Published“Energia”1979,Leningrad,(in Russian)[2] International Standard IEC 60270 “High –voltage testing-Partial dischargemeasurement”,1999[3] Viale,F.,et.al.,”Partial Discharge measurement in transformers and theirInterpretation”, CIGRE Papers,1968,12-12[4] Haraldsen,S.,Winberg,K.,”Investigation on different partial discharge locationmethods on Power Transformers”, CIGRE Papers,1968,12-09[5] Berent, Dennis”Acoustic monitoring and gas-in-oil analysis fortransformers”,Proceedings of the Sixty –Second Annual International Conference ofDoble Clients”,1995,Sec.8-3[6] Shertukde, H. M., ”Detection of Partial Discharges using a Fault DiagnosticDevice for Electrical Power Transformers”, Proceedings of the Sixty –Fifth AnnualInternational Conference of Doble Clients”,1998,Sec.8-13[7] Moore, H..,et.al.,”Application of In Service Transformer Monitoring Systemsduring Tests at Consolidated Edison’s Station”, Proceedings of the EPRI SubstationEquipment Diagnostics Conference V,1997,[8] Cooke,C.M.,”Detection of Internal Surface Discharges via Electrical PD-PWAMethods, Proceedings of the EPRI Substation Equipment Diagnostics ConferenceVI,1998,New Orleans[9] Moore,H.R.,Cnu,D.,Savio,L.,” Application of In Service TransformerMonitoring Systems during Tests at Consolidated Edison’s Station”, Proceedings ofthe EPRI Substation Equipment Diagnostics Conference VI,1998,New Orleans

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[10] Sundermann,U.,Fruth,B.,Gross,D.,”On-Line Partial Discharge Testing ofPower Transformers, Proceedings of the EPRI Substation Equipment DiagnosticsConference VI,1998,New Orleans[11] Sokolov,V.V., Berler, Z.,Rashkes,V., Effective Methods of the Assessment ofthe Insulation System Condition in Power transformers based on practicalExperience”, Proceedings of the EIC/EMCWE’99 Conference, October 26-28,1999,Cincinnati,OH[12] Gurin,V.V.,Swie,P.M.”Measurements of Partial Discharges on Transformersin-field condition”, Electrical Station, 1975, #5[13] Gurin,V.V.,Sokolov,V.V.,Kuchera ,V.,Valenta,L.,”In-Field Diagnostic of theAutotransformer by means of measurements and localization of PartialDischarges”,Electrical Stations,1993,#10, (in Russian)[14] Sokolov,V.,Shkrum,V.,”Experience with Life Assessment and Refurbishmentof 400 kV Shunt reactors”, Proceedings of the Sixty –Seconde Annual InternationalConference of Doble Clients”,1995,Sec.[15] Sokolov,V.V., Vanin,B.V.”Evaluation and Identification of typical Defectsand Failure Modes of 110-750 kV Bushings” , Proceedings of the Sixty –AnnualInternational Conference of Doble Clients”,1997,Sec.[16] Harrold,R.T.,”The Influence of Partial Discharges and related Phenomena onthe Operation of Oil Insulation Systems at very high Electrical Stresses ,“ IEEETransaction on Electrical Insulation,Vol.EI-11,#1,1976[17] �orozova,T.I.,”Dielectric withstands Strength of internal insulation oftransformers under prolong effect of operative voltage”,”Elecrtotechnika”,19975,#4(in Russian)[18] Bengtsson,C.,”Procedure and Field Measurement for Diagnosis of InternalArcing in a GSU Transformer ’Proceedings of the 1997 International Colloquium ofCIGRE SC12 Transformers”, Sydney, October 1997[19] Yakov,S.,et.al./”Corona in Power Transformers”,CIGRE Papers,1968,12-06[20] Kalentyev, Yu.,”Investigation of Short –Term and HV Power Long-TermBehavior of Oil-Barrier Insulation of transformers in Real Operation Conditions”,Dissertation, Sankt-Peterburg, Russia, 1985.[21] Marutchenko,P. Morozova,T., ”Voltage versus Time Characteristics ofSurface Discharge in Transformer Oil under Long –Time Voltage Action”,Elektrotechnika,1978, 4, pp. 25-28 (in Russian).[22] Arakelyan, B., E. Senkevich, E., ””Early Diagnostics of Oil-Immersed HVEquipment “, Electrical Stantions, , 1985, #6 (in Russian).[23] Senkevich,E.D.,Shtern,E.N.,”Diagnostic of Partial Discharges in the models oftransformer insulation”,Elektrotechnika,1974,#4,(in Russian)[24] Ryzhenko,V.,Sokolov,V.,”Effect of Moisture on dielectric withstand strengthof winding insulation of Power Transformers”, “Electrical Stations”,1981,#9 (inRussian)[25] Sokolov,V.V.,Vanin,B.V.,. “Evaluation of Power Transformer Insulationthrough Measurement of Dielectric Characteristics”, Proceedings of the 63rd AnnualInternational Conference of Doble Clients, 1996, sec. 8-7.[26] Z. Berler, A. Golubev, A. Romashkov, I. Blokhintsev “A New Method ofPartial Discharge Measurements”, CEIDP-98 Conference, Atlanta, GA, October 25-28, 1998.

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Biography

Dr. Victor V. Sokolov, a CIGRE Member and the Convenor of CIGRE WG 12-18“Transforrmers Life Management”, obtained his MSEE in 1962 from the KharkovPolytechnic University, and Ph. D. in High Voltage Technology in 1982 from theKiev Polytechic University. Dr. Sokolov possess a world level expertise in allquestions of designing, manufacturing, testing and maintaining power transformers.He is an author of numerous publications. Now he is the Technical Director of theScientific-Engineering Center “ZTZ-Service” (Ukraine).

Vladimir P.Mayakov, a member of CIGRE ,is the head of Field Test Laboratory ofthe “ZTZ-Service”Co. In 1968 he got his MSEE from the Technical University inZaporozhye. Prior “ZTZ-Service” he worked for Zaporozhye’s High VoltageApparatus Plant as the head of the PD Test Laboratory, providing quality control forcurrent transformers and then for ZTR (Transformer Plant).His field of activity coversIn-Field methods for evaluating the condition of transformer equipment, particularlyIn-Service and On-Line Tests. He has 12 publications.

Prof. George S. Kuchinsky got his MSEE (major- High voltage Technology) andthen Ph.D. from the Leningrad Polytechnic Institute (now St. Petersburg TechnicalUniversity), Russia. Since 1947 he is lecturing and providing R&D activity in hisalma mater being state certified as a Professor and Doctor of Technical Sciences. Heis a Member of the International Power Academy, Honorary Academician of theRussian Academy of Electrotechnical Sciences, Distinguished Scientist of RussianFederation, Member of the Russian Association of Electrical Engineers. Hisprofessional interests concentrate on electrical-physics processes in high voltageequipment insulation, electrical discharges and permissible operating voltagegradients in transformer, cable and capacitor insulation. He is an author of 14 booksand 200+ papers.

Dr. Alexander Golubev got his MS in Experimental Physics and Ph.D. in Physicsand Mathematics from the Moscow Physical Technical Institute (Russia) in 1978 and1985, respectively. He has an extensive experience in research and design in Laserand Electron Beam Generation, Plasma Coatings, High Frequency Measurements.Since 1995 he is a Manager of R&D of the Predictive Diagnostics Division of Cutler-Hammer Engineering Services. He develops new technologies for on-line monitoringand diagnostics of high voltage electrical equipment, produces monitoring equipmentand provides on-site expert evaluations of equipment state for electric utilities andindustrial customers.

17

No

1510

64

No

1510

76

No

1510

74

No

1510

73

No

1510

75

No

1523

99

No

1523

97

No

1550

19

No

1525

76

No

1525

73

No

1525

74

No

1525

75

No

1540

68

No

1540

66

No

1540

67

No

1540

69

No

1540

70

No

1550

21

No

1550

20

No

1550

22

U

V

W

Neut

0

100

200

300

400

500

600

700

PD

, pC

Nam eplate num ber

Phase

M ax. Pulse m agnitude, pC

UVWNeut

Fig1 Distribution of PD magnitudes in the 20 GSU Transformers installed at HPP

18

No

1510

64

No

1510

76

No

1510

74

No

1510

73

No

1510

75

No

1523

99

No

1523

97

No

1550

19

No

1525

76

No

1525

73

No

1525

74

No

1525

75

No

1540

68

No

1540

66

No

1540

67

No

1540

69

No

1540

70

No

1550

21

No

1550

20

No

1550

22

U

V

W

Neut

0.00000

1.00000

2.00000

3.00000

4.00000

5.00000

6.00000

7.00000

8.00000

PD

Pow

er, m

W

Nameplate number Phase

PD Power

UVWNeut

Fig.2 Distribution of PD Power measured on the Transformers at HPP

19

Fig. 4Graphic presentation of calibrating cross-matrix, based on different propagation of impulse wave. 240 MVA, 220/110 kV,

autotransformer, phase “A”.

0 , 0 0 1

0 , 0 1 0

0 , 1 0 0

1 , 0 0 0

1 0 , 0 0 0A _ 2 2 0 A m _ 1 1 0 N e u t r a l G r o u n d

A _ 2 2 0 - T a n k A m _ 1 1 0 - T a n k N e u t r a l - T a n k

GroundNeutralAm A