Study on initial slope characteristic quantity of return voltage

The dominate time constant obtained by return voltage method has a good effect on the moisture evaluation of the transformer insulation, but it is not accurate to diagnose insulation condition by means of the characteristic quantity. The initial slope of the return voltage is an important characteristic quantity which cans reflect insulation polarization characteristics effectively. Therefore two indexes are defined for analyzing the relationship between initial slope and the insulation condition in this paper. Two indexes are called S_r1 and S_r2 according to different the ratio of charging time and discharging time. And then the variation of S_r1 and S_r2 change with insulating state is discussed. The results show that S_r2 is mainly affected by thermal aging of the insulation paper; while the S_r1 can be used to analyze the moisture content of insulation. Two indexes can be used to distinguish the transformer insulation state of aging and moisture.