Statistical analysis of switching overvoltages in UHV transmission lines with a controlled switching

The objective of this work is to present a more precisely controlled switching strategy to reduce overvoltages due to ultra-high voltage (UHV) transmission line closing. Essentially the switching strategy is to find a suitable circuit breaker (CB) optimal instant of closing. The electric field variation in the arc-extinguish chamber has great effect on rate of decrease of dielectric strength (RDDS), when calculating optimal instant of closing with pre-breakdown between the contacts. The relative value k of a CB is defined as the ratio of RDDS to the rate of voltage approaching the zero cross. In this calculation, the electric field E should be as a variation due to the uneven medium of SF6. Consequently equations were derived from calculating the pre-breakdown instants. The ATP model of controlled closing UHV transmission line was built for switching overvoltage simulations considering RDDS and mechanical dispersion. The simulation results show that switching overvoltages are more accurate and closer to reality when regarding the electric field E as a variation. Contrarily, assuming E as a constant will increase the risk of failure via controlled switching. Furthermore, the simulation results demonstrate that k value can change the effect of the mechanical dispersion on switching overvoltages.