Improved calculations for no-load transformer switching surges

This paper deals with a modeling of components and calculation of transient overvoltages that build up on the transformer primary terminals after the transformer has been switched off by a vacuum circuit breaker (VCB). The tr ansformer is connected to the circuit breaker by a cable. The transient ove rvoltages are calculated for different cable lengths, and the cumulative pr obability of different are angles is investigated. In this study two cases are considered: transient overvoltages due to stead y-state magnetizing current switching, and transient overvoltages due to in rush current switching. It is shown that the case of inrush current switchi ng is worse, as virtual current chopping is possible, The cable is modeled by pi sections, whereas the transformer model is based on a terminal impeda nce. The VCB re-ignitions are modeled by means of withstand voltage charact eristics and high-frequency quenching capability, Due to shortage of field tests, this work uses only literature references to compare the results wit h actual measurements.