Effect of space charge on breakdown characteristics of aged oil-paper insulation under DC voltage

Due to its excellent performance, oil-paper insulation has been widely used in many power apparatuses including converter transformer of high voltage direct current (HVDC) system. Space charge accumulation phenomena may significantly distort local electric field inside transformer and cause oil-paper insulation overstressing. As a result of long term operation, oil-paper gradually degrades. In most severe cases, the breakdown of insulation system may occur. In this work, the effect of space charge on breakdown characteristics of aged oil-paper insulation is investigated. Oil-paper samples with different aging states are obtained by thermally accelerated aging for various periods of time, categorized by the degree of polymerization (DP) of 1107 (new), 725, 504, 376 and 241, respectively. Experimental results show that the breakdown voltage of oil-paper samples without pre-stressing remains almost constant with thermal degradation. After the homo-polarity pre-stress, when the DP value is higher than ~300 (from 1107 to 376), the breakdown voltage has an increasing tendency with an increase in thermal degradation. On the contrary, the breakdown voltage presents a decreasing tendency after the hetero-polarity pre-stress. When the DP is lower than ~300 (DP241), the changes of breakdown voltage does not remain the same trend as previous samples after pre-stressing. Meanwhile, space charge distributions in different aged samples are measured by the pulsed electro-acoustic (PEA) method, and the trap parameters are correspondingly evaluated. It has been experimentally demonstrated that special attention should be paid to space charge accumulation and aging state of oil-paper insulation, which may lead to severe insulation accidents.

[1]  D. Linhjell,et al.  Dielectric response of mineral oil impregnated cellulose and the impact of aging , 2007, IEEE Transactions on Dielectrics and Electrical Insulation.

[2]  G. C. Stevens,et al.  Review of chemical indicators of degradation of cellulosic electrical paper insulation in oil-filled transformers , 1994 .

[3]  Hai-Bao Mu,et al.  Effects of paper-aged state on space charge characteristics in oil-impregnated paper insulation , 2012, IEEE Transactions on Dielectrics and Electrical Insulation.

[4]  G. Chen,et al.  Space charge in polymeric insulation under ac electric fields — A schematic model , 2013, 2013 Annual Report Conference on Electrical Insulation and Dielectric Phenomena.

[5]  Farouk A M Rizk,et al.  Analysis of Space Charge Generating Devices for Lightning Protection: Performance in Slow Varying Fields , 2010, IEEE Transactions on Power Delivery.

[6]  D. Linhjell,et al.  Aging of oil-impregnated paper in power transformers , 2004, IEEE Transactions on Power Delivery.

[7]  Wen-Wei Shen,et al.  Identification of electron and hole trap based on isothermal surface potential decay model , 2013 .

[8]  Jacques Lewiner,et al.  Evidence of strong correlation between space-charge buildup and breakdown in cable insulation , 1996 .

[9]  G. C. Stevens,et al.  Kinetics and mechanisms of the low-temperature degradation of cellulose , 1994 .

[10]  A. Otto,et al.  HVDC Corona Space Charge Modeling and Measurement , 2011, IEEE Transactions on Power Delivery.

[11]  Ming Dong,et al.  Novel characteristic parameters for oil-paper insulation assessment from differential time-domain spectroscopy based on polarization and depolarization current measurement , 2011, IEEE Transactions on Dielectrics and Electrical Insulation.

[12]  T. Maeno,et al.  Space charge dynamics in LDPE films immediately before breakdown , 2001 .

[13]  Yuzhen Lv,et al.  Effect of nanoparticles on charge transport in nanofluid-impregnated pressboard , 2012 .

[14]  Gian Carlo Montanari,et al.  The role of trapped space charges in the electrical aging of insulating materials , 1997 .

[15]  P. Purkait,et al.  Investigation of polarization and depolarization current measurements for the assessment of oil-paper insulation of aged transformers , 2004, IEEE Transactions on Dielectrics and Electrical Insulation.

[16]  Teruyoshi Mizutani,et al.  Space charge measurement techniques and space charge in polyethylene , 1994 .

[17]  P.H.F. Morshuis,et al.  Space charge measurements on impregnated paper: a review of the PEA method and a discussion of results , 1997 .

[18]  R. Liao,et al.  Space charge behavior in multi-layer oil-paper insulation under different DC voltages and temperatures , 2010, IEEE Transactions on Dielectrics and Electrical Insulation.

[19]  E. Gulski,et al.  Time-to-breakdown and breakdown voltage for oil-impregnated insulation subjected to thermal aging , 2012, 2012 International Conference on High Voltage Engineering and Application.

[20]  Xiao Chun,et al.  Relationship between Breakdown in Polymer Dielectrics and Space Charge , 2006 .

[21]  Yasuo Suzuoki,et al.  Study of space-charge effects on dielectric breakdown of polymers by direct probing , 1992 .

[22]  T. Maeno,et al.  Space charge behavior in low density polyethylene at pre-breakdown , 2005, IEEE Transactions on Dielectrics and Electrical Insulation.

[23]  Rong Zeng,et al.  Study on the Influence of Space Charge on the Upward Leader Emerging From the Conductors of UHVDC Transmission Lines , 2015, IEEE Transactions on Power Delivery.

[24]  M. Ali,et al.  Degradation of cellulosic insulation in power transformers .4. Effects of ageing on the tensile strength of paper , 2000 .