This paper focuses on the space charge behavior in oil-paper insulation systems used in power transformers. It begins with the importance of understanding the space charge behavior in oil-paper insulation systems, followed by the introduction of the pulsed electrostatic technique (PEA). After that, the research progress on the space charge behavior of oil-paper insulation during the recent twenty years is critically reviewed. Some important aspects such as the environmental conditions and the acoustic wave recovery need to be addressed to acquire more accurate space charge measurement results. Some breakthroughs on the space charge behavior of oil-paper insulation materials by the research team at the University of Southampton are presented. Finally, future work on space charge measurement of oil-paper insulation materials is proposed.

Progress of Space Charge Research on Oil-Paper Insulation Using Pulsed Electroacoustic Techniques

Transformer oil and oil-impregnated pressboard are major insulating materials in a HVDC (high voltage direct current) converter transformer which provides electrical power transformation between an AC and a DC system. However, the dc component on the valve side of a HVDC converter transformer gives rise to special conditions, regarding space charge for example, for the insulating materials. In this paper, the relationship between moisture and space charge in oil-impregnated pressboard was investigated under dc electrical fields at room temperature. Formation, accumulation and distribution of space charge were studied using a PWP (pressure wave propagation) technique with the moisture concentration varying between 0.3% and 1.7% by weight. Experiments have shown that moisture influences seriously space charge distribution and thus electric field distribution in oil-impregnated pressboard. The higher the moisture concentration, the faster the establishment of space charge and the deeper the charge penetration into the insulation. The influence of moisture on the distribution of space charge should be taken into consideration in the design of an insulating system under HVDC.

Moisture and space charge in oil-impregnated pressboard under HVDC

I Transformer oil a d oil-impregnated pressboard are major insulati g materials in a HVDC (high voltage direct cum nt) converter transformer which provides electrical power transformation between an AC and a DC system. However, the dc component on the alve side of a HVDC converter transformer gives rise to special conditions, regarding space charge for example, for the insulating materialr i .

And space charge in oil-impregnated pressboard under hvdc

It has been reported that the insulation design under DC stress is considered as one of the critical factors in determining the performance of high-voltage direct current (HVDC) superconducting cable. Therefore, it is fundamentally necessary to investigate the DC breakdown characteristics of the composite insulation system consisting of liquid nitrogen (LN2)/polypropylene-laminated-paper (PPLP). In particular, the insulation characteristics under DC polarity reversal condition should be verified to understand the polarity effect of the DC voltage considering the unexpected incidents taking place at line-commutated-converters (LCC) under service at a DC power grid. In this study, to examine the variation of DC electric field strength, the step voltage and polarity reversal breakdown tests are performed under DC stress. Also, we investigate the electric field distributions in a butt gap of the LN2/PPLP condition considering the DC polarity reversal by using simulation software.

http://iopscience.iop.org/article/10.7567/JJAP.53.08NL04/pdf

Experimental and analytical study of the DC breakdown characteristics of polypropylene laminated paper with a butt gap condition considering the insulation design of superconducting cable

In this study, the space charge dynamics in an oil and oil impregnated pressboard combined insulation system are investigated by the means of pulsed electroacoustic (PEA) technique under voltage polarity reversal conditions. To be closer to the real case in convertor transformers, a 1mm thick pressboard covered with a 0.5mm thick oil film were used as the testing sample. For comparison, three types of oil with different aging stages were used: fresh oil, medium aged oil and severely aged oil. The external electric field was firstly applied +10kV/mm for 1hour, then reversed to -10kV/mm in 1min period. After that, the -10kV/mm was applied for 1 hour. The results have clearly shown the impact of the oil film and the interface between oil and pressboard on charge dynamics under polarity reversal conditions. The electric field enhancement and the time constant of the 'mirror image effect' have been analyzed for different cases.

/pdf/space-charge-dynamics-in-oil-and-thick-pressboard-combined-4iy1pz2a87.pdf

Space charge dynamics in oil and thick pressboard combined system under polarity reversal voltage

Space charge plays an important role in the breakdown and conduction processes of a high voltage direct current (HVDC) insulation system. Electrical breakdown due to charge accumulation in transformer oil and pressboard can cause failures of power transformers, especially at DC polarity reversal. In this paper, we focus our attention on the charge accumulation and movement in an transformer-oil-impregnated pressboard under DC polarity reversal at 60/spl deg/C. Using a pressure wave propagation (PWP) technique, the distribution of space charge in the sample was measured both at stationary and transient conditions.

R. Liu

Papers

Charge storage and transport in oil-impregnated pressboard at polarity reversal under HVDC