Influence of switching impulses on development of partial discharges in oil-impregnated paper insulation

An examination is made of the effects appearing in oil-impregnated paper insulation exposed to electrical stress composed of AC working voltage and switching impulses. Special test arrangements to generate switching impulses superimposed on AC voltage have been adopted. The tests were carried out on oil-impregnated transformer board specimens with various moisture contents, at temperatures up to +70°C. It has been shown that the partial discharges initiated by switching impulses can be intercepted and sustained by AC working voltage at a magnitude much lower than that of partial discharges inception voltage. The effect of interception becomes more pronounced at elevated temperatures and higher moisture contents of the insulation tested. At combined electric stress the deterioration of insulation seems to be more progressive than that at separated exposures