CONDITION MONITORING OF COMPOSITE INSULATORS USING PARTIALDISCHARGE ANALYSIS

I.A.D. GiriantariDepartment of Electrical Engineering Udayana University

Kampus Bukit Jimbaran Bali- IndonesiaEmail: dayu antarighotmail.com

Abstract: Composite insulator are widely used as anoutdoor insulator now days. Deterioation due to severeweather and vandalism are usually undetactable.There are many techniques have been introduced inorder to detect the insulator condition, one of thetechnique is introduce here by using Partial Dischargeanalysis. The work has been done by examining thePD patterns and behaviour of three different insulatormaterials that are subjected to high humidity andpollution in the small chamber. This work hasexhibited a condition monitoring of compositeinsulator using PD analysis. It showed an assosiationbetween the PDs pattern and behaviour and theinsulator condition. In addition the method used hasgave an oppurtunity to do on-line monitoring.

INTRODUCTION

Outdoor insulators are subject to severe weatherconditions, pollution, and vandalism, which result indeterioration. The effects of deterioration are differdepend on the insulators material. Compositematerials are widely used for outdoor insulator inpresent. It is known that composite outdoor insulatorsare sensitive to weather, pollution, and vandalism, butthey are light weight, and handy. Deterioration oncomposite insulators have reduced their performance,but it is unavoidable. However, regular monitoring onthe insulator condition can avoid the total breakdown.

There are various work have been done tomonitor the condition of the outdoor insulator,however the use of partial discharge analysis have notbeen used yet. The used ofPDs analysis to monitor theinsulator condition can give an early identification ofthe insulator condition, and it has the possibility to doon-line monitoring.

The aim of this work is to develop a conditionassesment method for composite insulator using PDmonitoring techniques and methods. On-linemonitoring of insulators which are based onmeasurement of overall leakage current also beenconducted. However this is a relatively insensitivetechnique. Detail analysis of the PDs associated withdischarges that are occurred on the insulator have thepotential to provide a more sensitive analysis ofinsulation condition.

EXPERIMENT

Experiments were done at High Voltage Lab.UNSW Australia. There were 6 samples from threedifferent materials: EPDM insulator, SiR Insulator,and SiR Arrester Housing with alumina filler. Threesamples (1 sample each material) were polute bypollution slurry made of kaolin, salt and water basedon International Standard IEC-507 [1], and the other 3samples were left in clean condition (uncontaminated).

Test were performed in a small chambe made ofperpex. Only one object put in the chamber at a time.Sample was subjected to high humidity by injectingwater vapour with input rate l100mg/hr/m3 for 8hours/day, followed by UV radiation from an UVlamp (Ultravitalux 300W) for 16 hours/day. Thetemperature in the chamber was keep between 250-360C. Nominal Voltage were applied continually for 7days. The uncontaminated samples were tested in theways as the contaminated one.

The circuit for PD measurement and calibrationprocedure is based on the IEC-270 Standard [2].Discharges were measured and recorded every 5minutes by utilizing a Computer Discharge Analyzer(CDA3) system developed by the High VoltageLaboratory at UNSW. PD maximum and PD countsversus phase of discharge patterns are displayed foreach half cycle of voltage. Calculated statisticaldistributions of discharges are also displayed side byside with the discharge pattern. The IEC integrateddischarge quantities, maximum discharge values,average discharge value, discharge power, anddischarge current are also provided by the CDA3display. Moreover, CDA3 can also disply the scatterplots of all measured PDs in magnitude versus phaseform [3].

The discharge characterization used somestatistical distribution parameters: Mean, Deviation,Skewness and Kurtosis. The mean, the first moment,gives an estimate of the central value in degrees of thedistribution for each half-cycle. The standarddeviation or the second moment indicates the width ofthe distribution in each half-cycle in degrees.Skewness or third moment, characterises the degree ofasymmetry of the distribution around its mean.Kurtosis or fourth moment is a non-dimensionalquantity which indicates the degree of sharpness orflatness of the distribution in each half-cycle.

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RESULTS AND DISCUSSION

PD phase-resolved.

Clean SiR Arrester Housing did not showed any largedischarge activity after 8 days of test. It started fromvery small number and magnitude of discharge thentended to increase over time. Discharge mostlyoccurred around 900 (positive peak voltage) and 2700(negative peak voltage) as shown in figure l(a).Although, there is a small increase on magnitude andnumber of PDs during the 8 days of test, surfacedamage around the skirt was observed. The dischargesoccurred due to water droplets accumulating at theedge of the skirt. This produced more damage at thisarea than at any other spot.

iNi ,.......................... .....

a. Uncontaminated b. ContaminatedFigure 1. PDs phase-resolved of uncontaminated and

contaminated SiR Arrester

Contaminated SiR arrester housing showedsimilar discharge pattern to the uncontaminated one,peak mostly occurred on the positive and negativepeak voltages. Large spikes with magnitude around1200pC occurred after 3 days test as shown in figurel(b). The large spike indicated the treeing process hasstarted.

The average PDs and the IEC current of bothcontaminated and uncontaminated SiR AresterHousing were increased by the time as shown in figure2 and figure 3. Furthermore, the PDs magnitude ofcontaminated SiR Arrester Housing has increasedsignificantly by the time compare to the theuncontaminated one as shown in figure 4.

EP.11t~ DA-ty 3C-, A-t,S

Figure 3. PDs average versus time of both contaminated anduncontaminated SiR arrester housing

pC

Figure 4. PD maximum versus time of contaminated anduncontaminated SiR arrester housing

The clean SiR insulator did not show any largedischarge activity. Maximum discharge level on day 8was only about 100 pC and appeared randomly inphase, whereas discharges of the polluted insulatoroccurred mostly on the peak of the positive andnegative half-cycles shown in figure 5(a) and 5(b).The maximum discharge magnitude of the pollutedSiR insulator showed a significant changed on day6and then stayed at a similar level as shown in figure 6.

The IEC current, and the average dischargemagnitude over time show similar patterns to themaximum discharge (figure 7 and figure 8). Thisinsulator material exhibited a superior withstand-ability to the electrical discharge compared to the SiRof the arrester housing. It is confirmed by there beingan unaffected surface after exposure to extremely highlevel of discharge magnitude.

Figure2. PDs IEC Current versus time of both contaminatedand uncontaminated SiR arrester housing

a. uncontaminated b.contaminatedFigure 5. PD phase-resolved of uncontaminated and

contaminated SiR insulator.

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Average PDs; Magnitude versus time

Maximum PD Magnitude versus Time

El Polluted Arrester El Clean Arrester

IEC Current vessTime

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Maxi PDs magnitude of SiR IIn - --rsu Time

E*Polluted SiR -insltor *Non Polluted SiR -Insltor

Figure 6. PD maximum versus time of both contaminatedand uncontaminated SiR insulator

IEC C:=PD ~fSiRh1

Figure 7. PDs IEC Current versus time of both contaminatedand uncontaminated SiR insulator

Ag~~PDg~~d~~fS ~ ~ Ti-

cycles, but similar shapes on the PD numbers plot.Pollution has not changed the overall number of PDsbut the magnitude has changed. Furthermore,pollution has significantly reduced the EPDMinsulator breakdown voltage.

The clean EPDM insulator shows very littlechange of maximum PD magnitude during test.However, the polluted insulator shows a significantincrease of maximum PD magnitude with time asshow in figure 10. It started with a much highermagnitude on the first day than the clean insulator,although the much lower voltage only was applied.The EPDM insulator shed material has very goodwithstand-ability to the electrical discharge, as there isno damage visible on the insulator surface. However,It is not recommended for this particular insulator tobe used in polluted condition due to low inceptionvoltage of discharge and breakdown.

The IEC current of the clean EPDM insulator isalmost constant at 19OtA during test. It is in contrastto the polluted insulator, which shows substantialincrease with time as shown in figure 11. The averagePDs magnitude of the polluted EPDM insulatorincreased significantly from the first day of testwhereas there was no change shown by the cleanEPDM as shown in figure 12.

Figure8. Average PD magnitude versus time of bothcontaminated and uncontaminated SiR insulator

The PDs behaviour of the clean SiR insulatordiffers to the SiR arrester, but the polluted behaviouris similar for the SiR insulator and arrester. It appearsto be affected by the filler material and concentration,and possibly inappropriate manufacturing. Althoughthe cause of the different reaction of the materials totest is not able to be precisely determined, thedifferences in the PD pattern exhibit the sensitivity ofPD characteristics to different material properties. It isthis diagnostic sensitivity which may be usefullyutilised in insulator monitoring.

The clean Ethylene Propylene Diene Monomer(EPDM) insulators show discharge activities from thefirst day of test. The phase-resolved discharges showsymmetrical discharge patterns as shown in figure 9(a).There is little difference in PD magnitude anddistribution for positive and negative half-cycles.However, note that the numbers are different.

The polluted EPDM insulator shows a muchhigher PD magnitude than the clean case as shown infigure 9(a) and figure 9(b) (note that phase angle ofinception and extinction have been masked by thescale change). However, it is of interest that withpollution there is a substantial difference in PDmagnitude between the positive and negative half-

a. uncontaminated b. contaminatedFigure 9. PD phase resolved of both contaminated and

uncontaminated EPDM insulator on dayl

MAXIMUM PDs OF POLLUTED AND CLEAN EPDMINSULATORS VERSUS TIME

35000 ==

30000

25000 C_ _i_X_-X/

20000DPC 1501z

100 /

day

*Polluted EPDM 5CI EPDM|

Figure 10. Maximum PD versus time of contaminated anduncontaminbated EPDM insulator

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* polluted Elclean

Figure 11. PDs IEC Current versus time of contaminatedand uncontaminated EPDM insulator

A-erage PD smgnitude of EPDM Insl- v Tim

0PPI11t.d EPDM OCI... EPDMI

Figure 12. Average PD versus time of contaminated anduncontaminated EPDM insulator

Statistical Distribution

The phase-resolved plots of the PDs mostly showalmost normal Gaussian distribution, where they tendto cluster at a certain phase point. This makesstatistical analysis possible [4] in order to get moreinformation about the distribution. Statistical analysisof PD data characterises an univariate distributionrelated to its moments. By considering a discretedistribution function yi f(xi) where i = 1,2,..., n I

moments that represent the distribution on bothpositive and negative half-cycle can be calculated [5,6].

The first moment is the Mean (p), which is adimensional quantity that gives the central value of theclustered distribution [f(x)] at each half-cycle. It isdefined as [4]:

'L

n

E Xi.f (xi)i=lfE f(xi)i=l

The mean value of the phase-resolved pattern has thesame unit as xi, which is degrees (0 ). In the CDA3,the mean is calculated separately for each half-cycle.A PD distribution that is clustered at positive peakvoltage will have a mean of 900.

The second moment is the Standard deviation (a)of the PD distribution [f(xd] that characterises thewidth of the distribution around its mean value, (°). Itsunit is the same as the unit of the mean. Standard

deviation is defined as:n

Z (xi -i4,.f(xi)7= i=l

n

Zf(xi)The third moment is the Skewness Sk off(xd that

characterises the degree of asymmetry of thedistribution. It is a non-dimensional quantity that isdefined as:

n

E (Xi - /a)3 . Xi )Sk = '= n

C7 f(xi)

A symmetrical distribution is indicated by a zero valueof skewness. A positive value of skewness indicatesan asymmetric distribution with tail extending to theright, towards larger (p. A distribution with tailextending to the left, towards smaller (p, is indicatedby a negative value of skewness.

The fourth moment is the Kurtosis Ku of f(xdthat is a non-dimensional quantity which characterisesthe degree of the flatness or sharpness of thedistribution relative to a normal Gaussian distribution.It defines as:

n

Y,(Xi - ,U)4 AXi )Ku= i='

n

a74.f(Xi)i=l

-3.0

Kurtosis value is zero for a normal distribution. Apositive value of kurtosis indicates a sharperdistribution than normal distribution. Similarly, anegative value of kurtosis indicates a flatter thannormal distribution.

The statistical distribution gives kwantitative dataof the discharge which assosiated to the insulatorcondition. The change of the statistical distributioncan be assosiated to the change of the insulatorcondition.

The mean value that determines the PD peakposition was similar for both polluted and clean SiRarrester and EPDM insulators at about both positiveand negative voltage peaks. However for the SiRinsulator, mean values moved from before phasevoltage peaks to around peak voltages. Skewnessvalues of all polluted materials were more negativethan for the clean materials, in particular on thenegative half-cycle.

An interesting pattern was shown by the kurtosisat negative half-cycle of all materials on day 4. TheSiR arrester kurtosis changed from a negative topositive value on day 4 and then stayed positive.However, the SiR insulator kurtosis changed to bemuch more positive on day 4 but then changed to anegative value again on the next day as shown infigure 14. This change may be correlated to the self-healing or hydrophobicity recovery of the SiRinsulator, whereas the arrester continued to deteriorate.An electrical tree was observed on the arrester surfaceand its appearance correlated with this change.

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IEC CURRENT VERSUS TIME OF EPDM INSULATOR

PC 3

55

Figure 13. Max PDs Kurtosis of SiR arrester, SiR insulator,and EPDM Insulator for both contaminated and

uncontaminated

Scatter PlotAn alternative form of visualisation of PD

patterns is to use a scatter plot display of PDs. Thescatter plot is the 2D image of n-q-y presentationwhich is based on the q-y polar plot that was firstlyproposed in 1968. The scatter plots provided by theCDA3 system is a 2D plot of the discharge phasevoltage position with each PD shown as a dot. Eachdot represents a discharge. The density of dotscorresponds to the discharge frequency. Although theinformation that is provided by the scatter plot ismostly qualitative information, it is a very effectivepresentation of PD patterns and is very sensitive todifferences in PD activity

The discharge scatter plots gave a better visualindication of the type of discharge that had beenoccurring. Surface discharge was indicated bysymmetrical patterns clustered around both positiveand negative voltage peak which are found on theuncontaminated case as shown in figue 14 (a). Thescatter plots the void defect are showed by a sandwichshape. Refer to the scatter plot identification by Phung[3], the top cluster is identical to the scatter plotpattern of a spherical void defect at inception, and thebottom part is identical to the scatter plot of theelectrode bounded void at inception as hown in figure14 (b).

Figure 14. PDs Scatter plots pattern

CONCLUSION

Phase-resolved PD of all materials showed a

typical pattern of maximum and average PDmagnitude which provided some characterisation ofthe pollution condition. The statistical analysis of thePD distributions and, in particular the kurtosis, givequantitative results of identification of the surfacecondition. It is of interest to note that the result showthat the increase of PD magnitude and IEC current donot reflect the surface deterioration.

The identifications are that scatter plot analysisgive a very good qualitative result of indication ofsurface condition with different types of dischargesoccurring as degradation proceeds

ACKNOWLEDGMENT

The author wish to thank to A/Prof. TRBlackburn and Dr. Toan Phung for their support forthe whole test that have been done at High VoltageLab. UNSW Australia.

REFERENCES

1. International Standard IEC507," ArtificialPollution test on high voltage insulators to beused on ac system", second edition 1991

2. High- Voltage test techniques - Partial DischargeMeasurement. International ElectrotechnicalCommisions Standard IEC 60270, Publication2000.

3. Phung, B.T., Computer-based Partial DischargeDetection and Characterisation, in School ofElectrical Engineering. 1997, The University ofNew South Wales: Sydney.

4. Giriantari, I.A.D., Blackburn,T.R., Surfacedischarge on composite polymer insulator,CIGRE symposium, Cairns Australia, 2001.paperno. 200-08.

5. Giriantari, I.A.D., Blackburn,T.R,Characterisation ofSurface Partial discharges on

SiR insulators under high humidity andpollutionconditions, International Symposium on HighVoltage, Bangalore 2001.

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