Traditionally, insulation coordination of power system apparatus relies on deterministic approaches where some conservative voltage stresses are defined and the insulation levels are then adjusted using protective ratios and margins. In the case of lightning stresses, this could easily lead to overinsulation or overdesign and subsequently to an increase in the cost of equipment. On the other hand, insufficient insulation could lead to power system failures with disastrous technical and economic consequences. In this paper we present a formalized method for predicting the lightning-related failure rate of power system equipment. In the probabilistic approach to insulation coordination of power system apparatus we quantify the expected risk of failure under lightning stresses to enable an economically optimized design.
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