Fundamental aspects of air breakdown

Abstract

Professor Norman Allen was the author of the original chapter entitled 'Fundamental Aspects of Air Breakdown' in editions 1 and 2 of High Voltage Engineering and Testing and the accompanying lectures for the IET International Vacation School series. The current author is pleased to acknowledge and incorporate much of his original work including 'Mechanisms of Air Breakdown'. Atmospheric air remains as the main insulant on electricity transmission and distribution systems even though transmission voltages have increased to over 1.2 MV AC and 800 kV DC [þve, -ve]. Such ultra high voltage (UHV) systems may be prone to more lightning strikes and system overvoltages than ever before. Some transmission systems are being developed that will operate at higher altitudes and in higher humidity conditions, under conditions not yet known, or adequately covered by IEC Standards. The electrical stresses that may lead to insulation failure are from two principal sources: lightning surge and switching surge. Direct natural lightning strikes, and also the overvoltages that may be induced from natural lightning, produce the conditions that build up to the breakdown of atmospheric air. Switching operations on high voltage transmission systems can also cause overvoltages (switching surges). These surges can also cause failure of the air insulation.