Investigation of Flashover Performance of Snow-Covered Breakers

When snow is unevenly distributed on horizontal insulators there is a higher risk for flashover. This was investigated for a 400 kV circuit breaker with horizontal breaking chamber insulators. The highest risk occurs when connecting a line to a generator. For a short moment during the synchronization the voltage can reach 2.5 times the normal phase-to-earth voltage. A test method that simulates worst snow conditions in Norway during the synchronization was developed. When simulating these conditions it was possible to construct flashover curves as function of time under voltage. A comparison with the synchronizing voltage made it possible to estimate the risk for flashover during the synchronizing time. There is a very low flashover risk at nominal phase-to-earth voltage. At 25% overvoltage one can expect 50% flashover probability during critical snow condition. A mechanism that shows the interaction of the important flashover factors is presented.