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.

Chapter Contents:

• 19.1 Introduction
• 19.1.1 History
• 19.1.2 High-voltage laboratory testing
• 19.2 Pre-breakdown discharges
• 19.2.1 Electron avalanches
• 19.2.2 Streamer discharges
• 19.2.3 Leaders
• 19.3 Uniform fields
• 19.3.1 Electron avalanches in uniform fields
• 19.4 Non-uniform fields
• 19.4.1 Direct voltage breakdown
• 19.4.2 Alternating voltage breakdown
• 19.4.3 Impulse breakdown
• 19.4.4 Leaders
• 19.4.5 Sparkover, breakdown, disruptive discharge
• 19.5 The 'U-curve'
• 19.5.1 The critical time to breakdown
• 19.6 The gap factor
• 19.6.1 Test procedures
• 19.6.2 Air gaps of other shapes
• 19.6.3 Sparkover under alternating voltages
• 19.6.4 Sparkover under direct voltages
• 19.7 Flashover across insulator surfaces in air
• 19.8 Atmospheric effects
• 19.8.1 Introduction
• 19.8.2 Density effects
• 19.8.3 Humidity effects
• 19.8.4 Application of correction factors
• 19.8.5 Air density correction factor k 1
• 19.8.6 Humidity factor correction k 2
• 19.8.7 Other atmospheric effects
• 19.9 New developments
• 19.9.1 UHV at high altitudes
• 19.9.2 Testing transformers
• 19.9.3 Future work
• References

Inspec keywords: power transmission protection; surge protection; lightning protection; overvoltage protection; IEC standards; high-voltage engineering; power distribution protection; electric breakdown; air insulation

Other keywords: electricity distribution system; high voltage testing; high voltage engineering; switching operation; atmospheric air breakdown; air insulation; direct natural lightning strike; overvoltage; IEC standard; insulation failure; high voltage transmission system; switching surge; lightning surge; humidity condition; electricity transmission system; electrical stress

Subjects: Inorganic insulation; Gaseous insulation, breakdown and discharges; Distribution networks; Power system protection