Inrush current phenomenon of a three-phase transformer is caused by mismatching polarities and absolute values of the initial magnetic flux induced by residual flux remnants in each phase core before the transformer is energized. Because the residual flux is the final value of magnetic flux transients that occur immediately after disconnection, the inrush current can be suppressed by considering the residual flux and controlling the timing of a circuit breaker. However, when a transformer is first energized in the field, the residual flux is unknown.

In this study, we apply lower frequency and lower voltage to a transformer system simulating a three-phase transformer and then integrates the transformer voltage by time to digitally calculate and capture the true residual flux. Next, when the transformers are energized, a phase angle control is applied to the timing of a switch to effectively suppress inrush current. As a result, the usefulness of the lower frequency and lower voltage application to suppress inrush current is indicated.