Vacuum Circuit Breaker Transients during Switching of an LMF Transformer

Switching transients associated with circuit breakers have been observed for many years. With the wide-spread application of vacuum breakers for transformer switching, recently this phenomenon has been attributed to a significant number of transformer failures. Vacuum circuit breaker switching of electric arc furnace and ladle melt furnace transformers raises concern because of their inductive currents. High frequency transients and overvoltages result when the vacuum breaker exhibits virtual current chop and multiple re-ignitions. This paper will present a detailed case study of vacuum breaker switching of a new ladle melt furnace transformer involving current chopping and re-strike simulations using the electromagnetic transients program. A technique that involves a combination of surge arresters and snubbers will be applied to the ladle melt furnace to show the switching transients can be successfully mitigated. Additionally, some practical aspects of the physical design and installation of the snubber will be discussed.