Evaluation of steep-front overvoltage waveforms for oil-filled transformers

Recently, steep-front overvoltages have been impressed on transformers because of changes in the layout of substations and the high-frequency characteristics of transformers. In cable-system substations, which are free from external surges, the lightning impulse withstand voltage (LIWV) of the equipment is determined by internal overvoltages such as restriking surges. It is important to investigate the insulation characteristics of oil-filled transformers under steep-front overvoltage waveforms. In this paper, based on the insulation characteristics of element models of oil-filled transformers, a method of converting steep-front overvoltage waveforms into standard lightning impulse test voltages was attempted. This method was applied to the actual analytical waveforms of restriking surges and lightning surges, then obtained the results of 7 to 27 percent decreases in peak values. This makes reduction of LIWV expected.