Vacuum circuit breaker and transformer interaction in a cable system

Vacuum circuit breakers can cause repetitive transient overvoltages with steep fronts in some cases, depending on the combination of the system configurations and type of circuit breaker operations. Laboratory testing has been performed to improve the understanding of the high frequency system interaction between vacuum circuit breaker, transformer, and cables. A dry type medium voltage transformer has been energized and disconnected in a 20 kV medium voltage laboratory cable system using a vacuum circuit breaker. The transformer is a standard transformer, equipped with additional voltage measuring taps along the disks of one of the high-voltage windings, enabling the measurement of the voltages in the winding when exposed to transient overvoltages. The results show that normal no-load energizing and disconnection of the transformer do not cause any high overvoltages. Disconnection of the transformer with an inductive load or a trip on inrush current during energizing can cause transient overvoltages. Surge arresters, both phases to ground and phase to phase, are recommended for a transformer exposed to these types of circuit breaker operations.