Comparative Study on Conductivity and Moisture Content Using Polarization and Depolarization Current (PDC) Test for HV Insulation

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1 TRANSACTIONS ON ELECTRICAL AND ELECTRONIC MATERIALS Vol. 15, No. 1, pp , February 25,201 4 Regular Paper Comparative Study on Conductivity and Moisture Content Using Polarization and Depolarization Current (PDC) Test for HV Insulation N. A. M. jamail' Faculty of Electrical and Electronic Engineering, Universiti fun flussin Onn Malaysia IUTPIMI, Batu Pahat and Faculty of Electrical Engineering Universiti Eknologi Malaysia (UTM), 8131 QJohor Bahru, Johar, Malaysia M. A. M. Piah and N. A. Muhamad Faculty of Electrical Engineering Unive~iti Teknologi Malaysia (UTM), Jahor Bahru, Johar, Malaysia Q. E. Kamarudin Faculty of Mechanical and Manufacturing Engineering, Universiti Tun Hussein Onn Malaysia (UTHM), Batu Pahat, Jahor, Malaysia Received February 3,2012; Revised November 25,2013; Accepted December 11,2013 The Polarization and Depolarization Current (PDC) measurement is an efficient and effective diagnostic technique based on time domain measurement, for evaluating the high voltage hsulation condition. This paper presents a review and comparison results from several published papers on the application of the PDC method to ding the conductivity and moisture content of various types of insulators. For solid insulation, the study was focused on cable insulation, electric machine stator insulation, and paper insulator in transformer insulation with different conditions. For liquid insulation, the review and comparison was done on biodegradable and mineral transformer oils, with fresh oil condition, and aged condition. The results from previous researchers tests were complied, analyzed and discussed, to evaluate the application of the PBC method to monitor the conductivity and moisture of HV equipment insulation systems. From the review results, the PDC technique successfully gives an indication of the conductivity and moisture level of high voltage insulation. Keywords: Polarization and depelarization current, Conductivity8 Moisture content, I-fVinsulations 1. INTRODUCTION The Polarization and Depolmtion Cment test had been applied t~ many items of electrical apparatus to monitor their condition, such as machine stator, transformer and power cables. The PDC test was used for maintenance and diagnastic tests '~uthor to whom all correspondence should be addressed: norakmal@urhm.edu.my copyright KIEEME. All rights reserved. Th~a 13 aprrrrrrir amda dmiurd under the & allhe CoaIivm Cornmanr Amibul#on hdornmwc#j Ltcense Ihnp.//creaf~vccommonr orglltcenrewbrnsa 01 wh~en permits unresmood mnrornrnorefal use. dinnbunon, and rrpmoudon In any medium, provlded theoriglnal rork~r pmpnly dled. periodically conducted on machine stator insulation systems, to continuously monitor both the charge and discharge current, during a step voltage test [2,31. Researcher [41 has applied fhe Polarization IRepolarization Current (PDQ analyzer for the insulation assessment of power cables since Researches [5-71 have applied the PDC technique for XLPE insulation that is subjected to wet ageing. This technique has also already been applied as a quality assurance tool for the assessment of power transformers by researchers [a-151, Researches [ have been done to investigate the moisture content and conductivity of the oil insulation, focused on transformer oil. Research has been carried out to find the dielectric responsive function and maximum conductivities of biodegradable

2 8 Trans, Electr. Electron. Mater. 15(1) 7 (2014): N. A. M. jamail eta/..b'% and minerd msformr oils with different moisture levels (dried normal, or wet) for comparative analyais, by researcher [17J. 2. EXPERIMENTS 2.1 The insulation conductivity concept By examining the PDC curves, parameters such as conductivity and moisture content in the insulation can be estimated. Figure 1 shows an example of PDC curves in a db plot. The figure shorn the oil conductiviw oil properties, geometry, ageing and water content influence on the PDC-Curves [18]. Based on the figure, the conductivity of the insulation can be measured from the front tail of the PDC curve. The value of conductivity affects the polarization current, mainly in he he mge telo0 s. Higher conductivity leads to a higher current value. Moisture content can be measured from the end tail of the I)DG curve. From the measurements of galarization and depolarization currents, it is possible to estimate the dc conductivity a, of the test object. If the test abject is charged for a sufficiently long time, so that f(t+t,)eo, the dc conductivity of the composite dielectric can be expressed as [1,16,17,19,20]: g 0s---, ,-,---- -, Z P Tlms inswe Fig. 1. Oil conductivity, oii properties, geometry, ageing and water content influence on the PDC-curves '2 PDC measurement technique Fig, 2. Principle of test arrangement for PDC measurement. The polarkation currents measurement is performed by applying a dc voltage step on the dielectric materials, and the depolarization current is measured by removing the dc voltage source, incorporating a switch, which turns an to shart circuit the tested objects. The dc voltage applied was 1,000 X for about 19,000 seconds for the polarization and depoladzation time. The principle of FDC measurement is shown in Fig MEASUREMENT RESULTS AND ANALYSIS I '\&%w I WA 1 AW Effect of moisture tent and temperature On pig. 3. ~ t$ct of papa moisture content on ibe polarization current, transformer insulation measured at 15 C and 10 C [131. Ia order to access the low temperature effect an BBC measurements, an adiabatic climate chamber with +O.I "C accuracy was used, to vary the temperature at 10 and 15C. Figure 3 and Fig. 4 show the effect d moisture content on the polarization and depolarkation current measured at 10% and 1st. From these figures, it can be seen that the arnplimde ofthe long term dc polarization/ 1 Q* i depolmkation current & very sensitive to the moisture content 2 1 j in paper, Any increase in moisture content shifts upward the long term range of the dc polarizatian/depolarization current, For the temperature effect, it can be seen that the PI36 current magnibde increases WMI increasing temperature from 10 C to 15 T2. for both initial and long tenn PDC values. The temperature of insulation has great influence on the results, and the interpreeation of PDC measurements [I$]. After 100 s, the current shows an ohbus vend, because at this b e m0kxules klside the material get enough energy to move inside the material, from rent, measured at 20 c and -10 t [13]. positive to negative electrode. 1 ty> I YLr tm:o 13 tlrnvc Fig. 4. Effect of paper moisture content on the depolarization cur- 3.2 Conductivity of transformer insulation Pipre 5 shows rhe Pariation of polarization current with paper conductivity. It is observed that the change in paper conductivity tends to affect the tail of the polarization currents [161. Whereas, Fig. 6 shows that the initial part of the curve is primarily controlled by the parameters of the liquid dielectric. Higher liquid

3 Trans. Elecb. Elearon. Mater. 15(1) 7 (2014): N. A. M. Jamail era/. 19Eta QEt loa Time h csc O~OE*LI 4 3,.,,,,,,, 1 Tiwin 566 1!X Pig, 7. Charge currents for epoxy-mica bars and a polyester-mica coil, before (dry) and after (humid), one weekunder a humid atmosphere [el. Fig. 5.Variation of polarization currents with paper conduceivity (a) a, ~1.5 pslm; ap,+=0.15 pslm; ow,=0.015 gslm, and (b) c ~~~s4.2 R/m; a,,,,, =042 Blm; apap,=420 fslm [16,201. a w.-ro C I 1 rlmn In see 106 Fig. 8. Discharge currents for epoxy-mica bars and a polyester-mica coil, before (dry) and after (humid), one week under a humid atmosphere [6]. Table 1. Machine insulation current for different condition,5e+02, M -3pSC t ID 100 1m Fig. 6. Variation of polarization currents wlth different oil conductivity [17,22j. insulation moisture or conductivity tends to increase the magnitude of rhe polarizatian currents during the head of the PDC curve. A bigher value of conductivity tends to increase the magnitude of the BDC. These resula show that the initial amplitudes of polarization current can be used to estimate the oil conductivity of a eansformer, without performing direct conductivity measurement. 3.3 Machine insulation results The analysis for PDC measurement for machine insulatian was done, based on the results in a paper published by researchers [el. Figure 7 and Fig. 8 show that the palarkation and depolmiuization current for polyester-mica coil is higher than far epoxy-mica bar, in both conditions. Their conductivity increases sharply after moisture absorption, which caused a significant change in the dielectric response of the insulation system, resulting in the occurrence of an interfacial golarkation peak within the b e frame of observation. Table 1 shows the maximum and minimum current of them two insulations. The minimum value Insulation Miudmum Mlnlmurn Maximum Minimum Condition Ipol IuA) Ipol (ua) Idepol (ua) Idepol (ua) DIyePmY- 78,- mica bar Humid mica bar Dry polyes- ~ B T T coil ~ C ~ Humid polyester mica mil interprets the moisture absorption of the insulation, which indicated higher moishue absorption for polyester-mica coil, The result was compiled, and is shown in Pig. 7 and Fig. 8. Higher solid insulation moisture QT conductivity tends to increase the magnitude of the polarizatian and depolarization currents, for longer time. 3.4 Effect of moisture content and temperature on cable insulation The analysis of PDC measurement for cables was done by researcher 141. figure 9 shows the polarization and depolarization current of 2 lan XLPB cable, after 4 years in service at temgerature ZIT, and 5 years later, after supplying fault current at temperawe 3OC. It is observed that this ageing process leads to a very high polarization and depolarization current at temperature 30 C. This is a case of severe thermal aging. The high temperature

4 10 Trans. Electr. Electron. Mater. 15(1) 7 (2 014): N. A. M. jamail eta!. with porn joint insulation quality Cables in Group B appear to share the characteristics of cables in Groups PI and C. The above table shows that cables with a, > 10 x 10." Slm, and degree of nonlinearity BONL > 1.2 but < 2, could have joint andlor water tree problems. Cables with a, 10 x 10.' Slm and DONL >1.2 but < 2, have high water tree density, where no tree bridges the insulation. Cables with sap, < 10 x 1a'16 Slm and DON < 1.2, are in good condition. As the temperature increases, the apparent conductivity in- 1 IP trkar im I KW creases. This is due to insulation degradation with temperature, which refers 0 the ageing process. Fig. 9. Polarization and depolariz~tion current of 2 hxlpe cable, after 4 years in service (T=Zl C), and 5 years later, after supplying fault current (T=30 C) [4]. 4. CONCLUSIONS 1 t 0 1 ib3x ih* Eul Fig. 10. Polarization and depolarizadon current of aged XLPB cable, before and after repair due to free water [Ql. Table 2. Cable conductivity for different conditions. Cluster Gable joint Fault New cable a, DQNL A X X v <lox 10''~ ~1.0 PDC measurements can be used to determine the condition of HV insulation. The PBC measurement results presented suggest that higher moisture contents and other ageing conditions due to tc?mgerahne dependence show higher conductivities and moisture content, of both solid andliquid insulations. This paper reviewed and compared the results of PDC conductivity and m oim content, for ttansformer, cable and machine. From the comparison results, it concluded that the polarization and depolarization current results could be used to determine the conductivity and moisture content sf solid and liquid insulation. Higher values of palarbadon and depolarization currents can contribute to higher conductivity and moisture content of insulation. ACKNOWLEDGMENTS The authors gratefully aclmowledge the Malaysia Ministry of Higher Education NOHE), Universiti Tun Hussein Bnn Malaysia (UTHM), Universiti Teknologi Malaysia (Vot 4L133,05182,4L607, 4F097 andvat 4ID141, for financial support. REFERENCES indicates that the cable was ageing, and had a fault candition. Ewe. 10 shows the polarization and depalarization current of aged XLPE cable, before and after repair due to free water, It can be seen that conductive contaminants, especially free water, cause high conduction current. An improvement after repair is clearly seen &om the result when the polarization and &polarization current is lower than before repair. The straight shape sf f depol means either good cables or cables having problems with water, and one prominent rook of I depol refers to thermal aging (due to highload, fault, etc.) [ Conductivity of cable insulation The PDC application to cable insulation was done by researchers [6,7]. Based on their papers, the tests were dassifiedinto several cable classifications, as listed in Table 2. Cluster A contains the newest cables with no joints, and some intermediate aged cables. Cables in Cluster B had failed, and had been repaired many rimes, but there was widespread degradation of the cable insulation. Cables in Cluster 6 contain one of the newest cables, I11 C. Elcanayake, T. K Saha, H. Ma, and D. Wan, Bower and Energy Society Gend Meeting, 2010 IEm, gp [2] B. David, R Soltani, and L. Lamarre, Dielectrics and Electrical [a] Insulation, JBEERansactions on, vel 17, pp C. Sumereder, R Woschitz, and M. Muhr, Electrical insulating Materials, 2005, (ISBIM 2005). Proceedings of 2005 International Symposium on, 2005, pp Val. 3. [4] S. A. Bhumiwaf Electrical Insulation (ISEI), Conference Record of the 2010 IBEB International Symposium on, pp [5] M. Abou Dakka, 9. S. Barnji, andd T. BWM, Blectrical Insulation and Rielecteic Phenomena, 2001 Annual Report. Conference on, 2001, pp [6] B. Byegoke, B. Birnvhistle, and J. LyaU, Solid Dielectrics, ICSKJ '07. IEW International Conference on, 2007, pp [7] B. Oyegoke, D. Bitwhistle, and J. Lyall, Science, Measurement & Technology, ET, vol. 2, pp , 2008 [DOI: 0~ S/iet-smt ~. [8] S. A. Bhumiwat, Electtical Insulation, ISH Conference Record of the 2008 IEEE International Symposium on, 2008, pp (91 S. A. Bhumiwat and P. Statmann, "Quality assurance after mnsfower refurbishment by mew of polarisation depoladsation currents analysis," in Power Tech Conference Roceedings, 2003 llw3 Bologna, 2003, p. 6 pp. Vo1.2. [lo] S. A. Bhumimt, Electricd Insulation, Conference Record of the 2004 WEE International Symposium on, 2004, pp. 500-

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