The Effect of Humidity on Partial Discharge Measurement

Supawat Naprasert, Somboon Chongchaikit, Surapol PuthwattanaDepartment of Electrical Engineering, Faculty of Engineering

Chulalongkorn University254 Phyathai Road, Patumwan, Bangkok Thailand, 10330

E-mail: supawat9ghotmail.com

Abstract

This paper is a study of the effect of humidity onpartial discharge measurement in rotating machine. Aswe know, partial discharge measurement is effected byseveral factors and the humidity is one of the mostimportant factors. To study the effect of humidity onpartial discharge test, slot defect stator bar modelswere manufactured for data acquisition at 40°0, 60%and 70°0 relative humidity respectively. The data werethen analyzed by visual inspection to see the differenceof three dimension distribution of Hj1( ,q) and two

dimension distribution of Hqmax( Si5), Hqn( Si5), Hn( i ),H(q) and H(p). Statistical values were also calculatedin order to compare the trend of those parameters ateach level of relative humidity. The test result showsthat visual inspection in the same stator bar model ateach level of relative humidity has different patterns.In addition, the statistical parameters change upon thevariation of relative humidity. Therefore, partialdischarge measurement without consideration onhumidity can lead to misinterpretation.

INTRODUCTION

Generator is one of the critical components in powerplant and power generating system. Therefore, anappropriate maintenance program should be applied tothe generator in order to make power system morereliable and to prevent generator at early stage beforefailure. Most of the troubles concerning the generatoroccurred during an operation comes from statorwinding deterioration and continuous deterioration ofstator winding can ultimately result in generatorfailure. Many tests can be performed to assess thestator winding condition that help to better amaintenance plan and one of those tests is partialdischarge measurement. Since the partial discharge hassignificant effect on the insulation life, a new partialdischarge detector has been developed for monitoringthe condition of electrical insulation. Partial dischargemeasurement using digital techniques which makesmore complex functions can be calculated easily andyield many different parameters [1],[2]. Eachparameter gives specific information which leads tobetter evaluation and possesses more advantages thanconventional techniques. Although partial dischargetest is one of the most effective tools used to assess thecondition of insulation, it is influenced by several

factors as mentioned above especially the humidity.For this reason, understanding the effect of humidity isuseful for partial discharge analysis.

EXPERIMENTAL DESCRIPTION

Stator bar model without artificial defect is calledperfect bar model which is shown in figure 1 Itconsists of copper strand, insulation, low-resistancesemiconductor, high-resistance semiconductor andaluminum bar. Three perfect stator bars models (namedS1, S2 and S3) were formed to study the effect ofhumidity. Perfect stator bar (S1) was tested at 60%relative humidity, perfect stator bar (S2) was tested at60% relative humidity whereas perfect stator bar (S3)was tested at 40°0o, 60% and 70°0 relative humiditysuccessively. Twenty measurements from each barhave been performed at 6.6 kV and then werescratched to simulate slot defect stator bar models.

Low resistance Aluminum barsemi condulctor

Copp

inHigh resistancesemiconductor

?er strand

isulation

Fig. 1 Perfect stator bar model

Artificial defectLow resistancesemiconductor Aluminum bar

\ / eCopper strand

InsulationHigh resistancesemiconductor

Fig.2 Slot defect stator bar model

To simulate the progress of slot degradation, thesemiconductor layer covering stator surface of thesame bar was scratched at different defect sizes andwas tested again. Firstly, slot defect stator bar model

1-4244-0189-5/06/$20.00 ©2006 IEEE.

Nvl...UU..UUUL

L---L-

r-

57

(SI) was scratched at three different defect sizes whichare 2x5 cm2, 2x10 cm2 and 2x15 cm2 to simulate alower degradation level comparing to slot defect statorbar model (S2) and each defect size was tested at 60%relative humidity while slot defect stator bar model(S2) was scratched at three different defect sizes of2x10 cm2, 2x20 cm2 and 2x30 cm2 to simulate a higherdegradation level comparing to slot defect stator barmodel (SI) and each defect size was also tested at 60%relative humidity. The last slot defect stator bar model(S3) was scratched at four different defect sizes (2x5cm2, 2x7.5 cm2, 2x10 cm2 and 2x15 cm2) and eachdefect size was tested at 4000 relative humidity. Inaddition, slot defect stator bar model (S3) with thedefect size of 2x15 cm2 was tested at 60% and 70°Orelative humidity as well to study the effect ofhumidity on partial activity at the same defect size.

PARTIAL DISCHARGE ANALYSIS

The data from each stator bar was analyzed by visualinspection and compared the difference of the threedimension distribution, H1( S7 ,q) which displays therelationship between discharge magnitude (q),discharge intensity (n) and phase angle( i ). Thefollowing two dimension distributions were alsoprocessed: the maximum pulse height Hqmax( $7) themean pulse height Hqn( S ), the pulse count Hj( z) , thenumber of discharge vs. discharge magnitude H(q) andthe number of discharge vs. discharge energy H(p).Alldistributions of each stator bars which were testes at4000, 60% and 7000 relative humidity were comparedto study the effect of humidity on partial dischargemeasurement. Although the distributions havecharacteristic shapes which vary upon the size of thedefect, it's quite difficult to observe the change ofthese distributions, Thus the following statisticalparameters are also calculated to observe the change ofparameters when the degradation progresses.

The first parameter is skewness which represents theasymmetry of the distribution. If the distribution issymmetric, Sk = 0, if it is asymmetry to the left, Sk >0, if it is asymmetry to the right, Sk < 0. The skewnessis defined as equation 1[3].

N (qi-p) 3PiSk Y (1

i=1 I

Where qj is the record value and p1 is the probability offrequency of appearance for that value qj in timewindow i, Vt is the mean value E qi. pi, and ca is thevariance CG2 = ( qij )2. pi. The Sk+ and Sk- are theskewness of distribution in the positive and negativehalf of the voltage cycle respectively.

The second parameter is kurtosis representing thesharpness of the distribution. If the distribution has thesame sharpness as a normal distribution, Ku = 0. If it issharper than normal, Ku > 0, and if it is flatter, Ku < 0.The kurtosis is defined as equation 2[3].

N (qi -A)4PiKu =

itl(2)

The Ku+ and Ku - are the kurtosis of distribution inthe positive and negative half of the voltage cyclerespectively.

The third is fractal analysis[4]. In particular, the fractalmethod processes two parameters: the fractaldimension and the lacunarity. The fractal dimensioncorresponds to the roughness of a surface of threedimension distribution. The fractal dimension (D) isderived from equation 3.

N(L) = KL-D (3)

The lacunarity could be compared to the density ofsurface of three dimension distribution. A low valuecorresponds to an empty surface; a high valuecorresponds to a dense surface. The lacunarity (A) isdefined as equation 4.

A=M2(L)-[M(L)2][M(L)2 ]

(4)

where M(L) is the mean and M2(L) is the variance ofthe number of sites per box. N(L) is the mean of boxesper site, K is a constant and L is the length of box.

TEST RESULTS AND DISCUSSION

Figure 3,4 and 5 show the results of partial dischargemeasurement of slot defect stator bar model (S3) testedat 4000, 60% and 7000 relative humidity respectively. Itis clearly visible that partial discharge activity at 4000relative humidity is quite differ from partial dischargeactivity at 60% and 7000 relative humidity noticingfrom the higher discharge magnitude and the numberof the pulse count. Visual comparison of the testresults at 60% and 7000 relative humidity show somedifferences, but it is quite difficult to quantify thesedifferences. Figure 15,16 and 17 show skewness,kurtosis and fractal analysis of slot defect bar model(S3). They show the change of statistical parametersupon the variation of humidity. Figure 6 to 14 show allstatistical parameters of slot defect stator bar models(S1, S2 and S3) tested at 60%, 60% and 40% relativehumidity respectively. The results show that bothskewness and kurtosis of a lower degradation model(S1) cannot be used as tools to show the trend ofinsulation degradation while in a higher degradation

1-4244-0189-5/06/$20.00 ©2006 IEEE. 58

model (S2) tested at the same humidity a better trendcould be shown. Nevertheless, fractal analysis of allmodels (S 1, S2 and S3) shows the best trend ofinsulation degradation, even though the partialdischarge activity is low. Figure 8,11 and 14 of modelS3 show that all statistical parameters can show thetrend of insulation degradation, even though bothskewness and kurtosis cannot show the trend ofinsulation degradation at high relative humidity.

Fig.3 S3 model at 4000 relative humidity

Fig.4 S2 model at 60% relative humidity

Sk+ vs. Sk- Plot of Hqn at Different Defect Sizes

1.7

1.6

1.5

Sk- 1.41.3

1.21.1

* PerfectBar#Sl

M DefectBar #S(2x5cm')A Defect Bar #Sl(2xlOcm')|X Defect Bar #Sl(2xl5cm')

1 1.1 1.2 1.3 1.4 1.51.6 1.7 1.81.9 2Sk+

Fig.6 Sk plot of SI model at different defect sizes

Sk+ vs. Sk- Plot of Hqn at Different Defect Sizes

1.71.67 * Perfect Bar #S2

k 114Defect Bar #S2(2xlOcr)

1.3 |A Defect Bar #S2(2x20cm')1.2 x X Defect Bar #S2(2x3Ocm')1.1

1 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2Sk+

Fig.7 Sk plot of S2 model at different defect sizes

Sk+ vs. Sk- Plot of Hqn at Different Defect Sizes

1.71.6 * Perfect Bar #S31.5 Defect Bar #S3(2x5cm')

Sk 14A Defect Bar #S3(2x7.5cm')

1.3 ! *!x Defect Bar#S3(2xlOcm')1.

Defect Bar #S3(2xl5cm')13

1 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2Sk+

Fig.8 Sk plot of S3 model at different defect sizes

Ku+ vs. Ku- Plot of Hqn at Different Defect Sizes

-1.5 -0.5 0.5 1.5

Ku-

-0.2# Perfect Bar #SI1

04Defect Bar #S1(2x5cm')

06 A Defect Bar #S1(2x7.5cm')-0.8X Defect Bar #Sl(2xlOcm')-1-1.2-1.4

Ku+

Fig.9 Ku plot of SI model at different defect sizes

Ku+ vs. Ku- Plot of Hqn at Different Defect Sizes

-1.5 -0.5 0.5 1.5

-0.2I0I 4 * Perfect Bar #S2

Ku- -0.6 * Defect Bar #S2(2xlOcm')I* * I~~~~-0.8A,DefectBar#S2(2x20cm')

K1 X Defect Bar #S2(2x3Ocm')

1u4Ku+4

Fig.5 SI model at 60% relative humidity

Fig.10 Ku plot of S2 model at different defect sizes

1-4244-0189-5/06/$20.00 ©2006 IEEE.

.,A

59

Ku+ vs. Ku- Plot of Hqn at Different Defect Sizes

-1.5 -0.5 0.5 1.5

Ku-

-0.2-0.4 * Perfect Bar #S3

-0.6 * Defect Bar #S3(2x5cm')*0 Defect Bar #S3(2x7.5cm')

XDefect Bar #S3(2xlOcm)O12 Defect Bar #S3(2xl5cm')

-1.2-1.4

Ku+

Fig.11 Ku plot of S3 model at different defect sizes

Ku+ vs. Ku- Plot of Hqn at 40%, 60% and 70% Relative Humidity

-2 -1 0 1 2

Ku-0 .2 * Defect Bar #S3 (2xI 5cm2)-0. 4 40% Relative Humidity-0.6 N DefectBar#S3 (2x1cm)-0.8 60% Relative Humidity

A DefectBar#S3 (2x1cm)-1 70% Relative Humidity-1.2

-1.4

Ku+

Fig. 16 Ku plot of S3 model at different humidity

Fractal Analysis at Different Defect Sizes2.4

22352.3

2 2.25 -Perfect Bar #SI2.2 E Defect Bar #SI(2xScm)

2.15 *ADefect Bar #SI2x7.5cm')2.1 XDefect Bar #S1(2xIOcm)

2.052°50 0.01 0.02 0.03 0.04

Lacunarity

Fig. 12 Fractal analysis of S1 modelat different defect sizes

Fractal Analysis at Different Defect Sizes

2.42235223 * PerfectBar#S22.2 * Defect Bar# S2 (2xlOcm1)

2.15 Defect Bar # S2 (2x20 cm)2.1 4 X Defect Bar # S2 (2x30cm)

2.05

0 0.01 0.02 0.03 0.04 0.05

Lacunarity

Fig. 13 Fractal analysis of S2 modelat different defect sizes

Fractal Analysis at Different Defect Sizes

2242.3 x4O* Perfect Bar #S3

2=253 - Defect Bar #S3(2x5cm')2.2 - Defect Bar #S3(2x7.5cm')

a215 X Defect Bar #S3(2xlOcm1 )

2.05 - Defect Bar #S3(2xl5cm')2

0 0.01 0.02 0.03 0.04 0.05

Lacunarity

Fig. 14 Fractal analysis of S3 modelat different defect sizes

Sk+ vs. Sk- Plot of Hqn at 40%, 60% and 70% Relative Humidity

2

* * DefectBar#S3 (2x1cm')40% Relative Humidity

* Defect Bar #S3 (2xI1cm')60% Relative Humidity

* Defect Bar #S3 (2xI1cm')70% Relative Humidity

0 0.5 1 1.5 2

Sk+

Fig. 15 Sk plot of S3 model at different humidity

2.42.362.32

= 2.28,72.24-

5 2.16a2.12-2 082.04

0 0.01 0.02 0.03 0.04 0.05

Lacunarity

Fig. 17 Fractal analysis of S3 modelat different humidity

CONCLUSION

The test results show that the lower relative humidity,the higher partial discharge activity, so we canconclude that partial discharge phenomenon isinfluenced by the humidity. In addition, it wasconfirmed that the change of skewness and kurtosis atlow relative humidity can be directly correlated withthe insulation degradation, but at high relativehumidity the correlation could not be shown. However,the fractal analysis is the best effective tool used toassess the condition of insulation since it shows a goodcorrelation at both low and high relative humidity.

REFERENCES

[1] E. Gulski., F.H. Kreuger, "Computer-aidedrecognition of Discharge Sources," IEEE Transactionson Electrical Insulation, Vol. 27 No 1, pp.82-92February 1992.[2] A. Krivada, "Automated Recognition of Partialdischarges," IEEE Transactions on ElectricalInsulation, Vol. 2 No 5, pp.796-821, October 1995.[3] Yue Bo., Li Jian., Cheng Yonghong., HengkunXie, "Study on the Multi-Stress Aging of StatorInsulation Based on Fingerprint Parameters," Proc. Of2001 International Symposium Electrical InsulatingMaterials (ISEIM 2001), pp. 729-732, 19-22 Nov.2001.[4] Satish., L. and Zaengl, W.S. "Can Fractal be usedfor Recognizing 3-D Partial Discharge Patterns?,"IEEE Transaction of Dielectrics and ElectricalInsulation, Vol. 2, pp. 352-359, 1995.

1-4244-0189-5/06/$20.00 ©2006 IEEE.

Fractal Analysis at 40%, 60% and 70% Relative Humidity

* Defect Bar #S3 (2xI cm)40% Relative Humidity

* Defect Bar #S3 (2xI1cm)60% Relative Humidity

A Defect Bar #S3 (2xI1cm)70% Relative Humidity

1.5Sk-

0.5

60