The impact of oil-paper insulation moisture and aging on the initial slope of the voltage

The polarization spectrum method based on the recovery voltage is a non-destructive method for insulation diagnosis and lifetime evaluation. It has been used to evaluate the insulation condition of the transformer. The main time constant is often used for diagnosis of oil paper insulation moisture state, and the initial slope of the voltage is used to diagnose the thermal aging state of oil-paper insulation by return voltage measurement method, but the results are not accurate. Since the initial slope is closely related to the polarization conductivity, based on the basic method of return voltage, the paper analyze the effect of oil paper insulation with different moisture or thermal aging state on the voltage initial slope by changing charging and discharging time. Then the examples are analyzed when the moisture content in the transformer increase, the initial slope of the decay voltage significantly increased; the initial slope of the recovery voltage also increases obviously when the thermal aging degree of the transformer increased. The results show that it really has some effect on the initial slope of decay voltage when oil paper insulation damp, so as the initial slope of the return voltage when oil paper insulation thermal aging.