Partial Discharge Behavior and Ground Insulation Life Expectancy Under Different Voltage Frequencies

Power electronic transformers, namely solid state transformers, have aroused a widespread attention due to the integration issue of large scale renewable energy in modern power system. High frequency power transformer (HFPT) is one of its key components operating at several to tens of kHz. Under such high frequency voltage, accelerated insulation aging and premature insulation failure occur frequently. Partial discharge (PD) is considered as the main cause of electrical performance deterioration in HV systems or LV ones where overvoltage or surge may occur. Therefore, it is necessary to study the PD behavior and insulation life under different voltage frequencies, in order to have a better understanding of premature failure mechanism. In this paper frequency dependence of space charge and PD properties is firstly analyzed theoretically and mathematically, followed by the results of electrical aging tests conducted on polyimide (PI) films that were subjected to surface PDs under 5-30 kHz sinusoidal voltage. Discharge characteristics and lifetime of PI film under different frequencies have been discussed, along with the correlation between lifetime and PD. Both theoretical analyses and experimental results show that average PD amplitude and total PD number per cycle increase first and then decrease afterwards with voltage frequency, which means that the frequency induced inflection phenomenon exists in the tested frequency range. Synergistic action of high frequency induced thermal effect and reduced space charge dissipation time under high frequency is the reason for this novel phenomenon. Besides, strong linear correlation has been found between lifetime and PD characteristic total PD amplitude per second, which reveals the accelerated aging mechanism under high frequency electrical stress.