A simplified transformer model for the simulation of fast surges and adequate protection measures

Abstract

The paper describes a simplified transformer model to calculate the transient overvoltage caused by vacuum circuit breaker (VCB) switching surges. The transients produced by the VCB include various frequencies. Therefore we need a transformer model for a wide frequency range. When an unloaded transformer is switched off the voltage highly depends on whether reignition occurs or not. The transformer is modeled by means of series connected Foster sections which represent the two-port impedance. The impedance sequence to the transformer core contains a nonlinear inductance representing transformer saturation. Several simulations are performed both with and without protective devices such as R-C suppression branches and surge arresters. The analysis shows that surge arresters have to be installed at the terminals and between phases in order to provide adequate protection. The R-C branches provide suppression of reignitions if the values of R and C are optimally chosen.