Dielectric strength of oil-immersed transformer insulation with superimposed AC and lightning impulse voltage

The 50-Hz AC partial discharge (PD) triggered by a lightning impulse voltage of 1.2/50 μs and breakdown characteristics under superimposed voltage conditions are studied for several modes of oil-immersed transformer insulation, representing typical combinations of mineral oil and oil-impregnated paper. It is concluded that (1) no AC PD phenomena are triggered by lightning impulse voltage when the preapplied 50-Hz voltage is less than 50% of the RMS value of the AC PD inception voltage for all combinations of polarities, and (2) peak breakdown values of the superimposed voltage are reduced by 10 to 19% for a single applied lightning impulse with a low solid-to-liquid dielectric packing fraction (SLPF) for oil-impregnated paper in the intercoil model, but the difference in breakdown voltage between the superimposed voltage and a single lightning impulse is very small for a high SLPF for oil-impregnated paper, i.e. above about 30%