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

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.