Residual flux estimation method for three-phase transformers without zero-sequence circuit on the switch-off side

Part of the magnetic flux will remain in the transformer core after it is switched off, referred to as the residual flux. When an unload transformer being energized, inrush current may flow in the voltage jump side, causing relay malfunction and many other problems. This paper presents a method to estimate the residual flux for a three-phase transformer without zero-sequence circuit on the switch-off side. The estimating method is based on the analysis of the switching-off transient process and mechanical characteristics of the vacuum circuit breaker. With this method, we could choose appropriate voltage angles to close the transformer for avoiding inrush current. The method has been tested in our laboratory. The results shows that inrush current has been restrained to a lower level after estimating the residual flux.