Influence of space charges on the breakdown of oil-paper insulation

It is a well-known fact that when some insulants under dc voltage are subjected to a polarity reversal, the breakdown immediately after the reversal may appear at a substantially lower voltage than the breakdown without polarity reversal. Since complete discharging of the sample does not occur after the polarity reversal, the voltage due to the charges remaining in the sample will add to the reverse voltage applied and will, in effect, increase the value of maximum stress in the sample. References 1, 2, and 3 consider the transient-stress distribution at the instant of polarity reversal in a cylindrical sample of oil-paper insulation subjected to temperature gradients. In their considerations, the influence of polarization on the maximum stress occurring immediately after a rapid polarity reversal is not taken into account and, as will be indicated later, this leads to substantial errors. The purpose of this paper is to establish the electrical strength of oil-paper insulation under rapid polarity reversals, with all space charges present in the insulation taken into account.