Dielectric characteristics of oil-filled transformer insulation models under nonstandard lightning impulse voltages

Nonstandard lightning impulse voltages, such as waveforms with high-frequency oscillatory components superimposed on impulse waveforms due to reflections in the substation impinge on transformers in operation. This paper describes the results of an investigation of the dielectric characteristics of oil-filled transformers under such nonstandard lightning impulse voltages. The dielectric characteristics of oil gaps and turn-to-turn insulation models and section-to-section insulation models of oil-filled transformers were investigated for unidirectional damped oscillatory impulse voltages. This study discovered the following: the breakdown and partial discharge inception voltages of the turn-to-turn insulation model and the breakdown voltages of the oil gap and the section-to-section insulation model under the oscillatory impulse voltages were higher than those under a standard lightning impulse voltage. Besides, the breakdown and partial discharge inception voltages increased as the frequency or the oscillation damping factor increased. These results show V-t characteristics: the breakdown voltage decreases as the voltage hold time increases.