The Influence of Electro-Thermal Aging on Space Charge Distribution in Polyimide Under Square Wave Voltage

As the insulation of inverter-fed motor, polyimide (PI) works under square wave voltage, with high frequency and steep rising/falling edge. In order to explore the insulation performance of PI, the samples were subjected to the accelerated aging treatment under the square wave electric field at 200°C. By observing the surface morphology of the samples, it is found that there are obvious partitions in the samples. The breakdown point of the sample is distributed in the region with the combined effect of electrical and thermal aging. Conduction current test and DC space charge test were carried out on the electro-thermal aging region of PI samples with various aging degrees. The results show that electro-thermal aging promotes the increase of conduction current and leads to a slight increase of the threshold electric field. As for the charge distribution in the polarization, heterocharges accumulate near the interface of the sample after aging, which is likely to do with the destruction of the polymer insulation structure. By observing the amount and rate of charge annihilation in the relaxation process, aging increases the density of deep traps in the insulation and introduces deeper traps. Moreover, during the aging process, there is an obvious corona discharge phenomenon in the air gap between the electrode edge and the sample, it is thus worth further exploring the insulation performance of the region where corona discharge and thermal aging act simultaneously.