A Calculation Method based on Analytical Formulas for Inrush Currents and Voltage Dips during Energization

If a voltage dip gets lower than a certain threshold at the moment when a transformer is energized, its mitigation is required. For this reason, when a transformer is energized, the voltage dip is calculated in advance and the need for its mitigation is considered. From the viewpoint of preventing unnecessary countermeasures, highly accurate calculations are required. However, electromagnetic transient analysis, which is known as a method with high accuracy, is not practical because its operation is rather complicated. In this paper, the analytical circuit is simplified by simulating the magnetizing inductance of the transformer with the inductance and the series connection of the ideal switch, and analytical formulas for the flux and current in the winding are derived for each switching mode. The calculated inrush currents and voltage dips during transformer energization using the derived analytical formulas agreed well with the results from electromagnetic transient analysis, which handled the nonlinear current-flux characteristics precisely.