The properties of space charge in oil-paper insulation during electrical-thermal aging

Valve windings of converter transformer withstand pulsating voltages. In order to reveal the relationship between the properties of space charge and electrical-thermal aging of oil-paper insulation in converter transformer, this paper did the electrical-thermal aging test at 70 °C under 7.6kV pulsating DC voltage and obtained the samples of different aging time. The internal space charge injection and accumulation properties of aged samples under 20kV/mm and 30kV/mm DC electric field was measured and analyzed by the Pulsed Electro-Acoustic (PEA) method. The results showed that homo-charges injection still occurs at both electrodes in the electrical-thermal aging samples. Aged oil-paper insulation was dominated by negative charge injection as the stressing time increases, which directly cause hetero-charge accumulation in the vicinity of the anode, and homo-charge accumulation near the cathode. It could help explain the reason of the electrical-thermal aging lifetime of oil-paper in sulation is obviously shorter than the electrical or thermal aging. In addition, the longer the aging time, the more the total charge accumulated in the samples.