DIELECTRIC STRENGTH OF OIL-IMMERSED TRANSFORMER INSULATION WITH SUPERIMPOSED AC AND LIGHTNING IMPULSE VOLTAGE

The 50 Hz ac partial discharge (PD), triggered by lightning impulse voltage of 1.2/50 μs and breakdown characteristics under superimposed voltage conditions are studied for several models 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 pre-applied 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) of oil-impregnated paper in the inter-coil model, but the difference in breakdown voltage between the superimposed voltage and a single lightning impulse is very small for a high SLPF of oil impregnated paper, i.e. above about 30%. © 1990 IEEE