Controlled switching for energizing 3-phase transformers in isolated neutral system

When an unloaded transformer is being energized, a large inrush current may flow depending upon the residual flux in the core of the transformer and the closing phase of the circuit breaker. A large inrush current may cause voltage fluctuations in the transmission system. One method of suppressing the inrush current is to perform controlled switching of the circuit breaker. In this paper, a new method of controlled switching of a 3-phase transformer in an isolated neutral system was examined. A method employing 3-phase circuit breakers with one operating mechanism and also single phase circuit breakers was used to devise a method of control switching for suppressing inrush current when the transformer is energized. In experiment using a 3 kV transformer, actual inrush currents were greatly suppressed.