Field observation and experimental investigation on breakdown of air gap of AC transmission line under forest fires

Many power transmission lines are across mountain areas and occurrences of fires under lines are responsible for a great number of line outages. Field observation indicates that insulation strength of gap may reduce to 19kV/m under forest fire condition. The influences of electrode geometry configuration, flame temperature, alkali metal salts KCl, solid particles and air gap filled with smoke&flame on the conductor-plane gap breakdown characteristics are studied by the fire of burning wooden cribs in this paper. AC breakdown voltage of conductor plane gaps under fire is reduced to one third of that under standard conditions. When particles are on the surface of conductor, the breakdown voltages reduce and are affected weakly by electrode configurations. The flame conductivity is increased after alkali metal salts are added into the flame and flashover voltage is 33% lower than that in the wood flame. Results indicated that flame temperature, solid particles and ion & electron of fire volume are the main cause of gap insulation strength reduction under forest fire conditions. Consequently, transmission line insulation failure under forest fire was is attributed to flame temperature, electrons and ions and solid particles which promote the discharge development.