Calculation of lightning overvoltage failure rates for a gas insulated substation

Lightning overvoltages can cause damage to the equipment in a substation, leading to poor utilisation of assets. In order to prevent this damage from occurring, an acceptable level of lightning protection should be installed. Designing this level of protection requires that the acceptable rate of failure for the substation due to lightning overvoltagest be defined. An acceptable failure rate is normally calculated based on experience and historic records of failure due to lightning overvoltages. However there is no method freely available to enable designers to calculate acceptable substation failure rates when this historical data is unavailable. The International Standard IEC-60071-2 “Insulation Coordination” Third Edition 1996 states the following; “For apparatus, acceptable failure rates Ra due to overvoltages are in the range of 0.001/year up to 0.004/year depending on the repair times.” This paper deals with an approach to calculate a failure rate for a substation based on a function of; lightning strike magnitude, lightning strike location, surge arrester location and substation topology. The substation, with 110kV unshielded overhead line connected to the substation via underground cable, is modelled using Alternative Transient Program Draw (ATP Draw). ATP Draw is graphical user interface for Electromagnetic Transient Program (EMTP), which is a software program used to simulate electrical transient phenomena.