Inrush Current Reduction of Three-Phase Transformers Using Optimal Energization Angle Control

When a transformer is energized, the inrush current can reach several tens of times higher than the steady-state magnetizing current, which may cause maloperation of protective relays, voltage dips, and deterioration of power quality. The inrush current occurs when the applied voltage phase at the instant of energization does not coincide with the residual flux in the transformer core, resulting in excessive flux that drives the core into saturation. This paper proposes a method to reduce such inrush currents by controlling the optimal energization angle of three-phase transformers to minimize residual flux asymmetry. The proposed approach utilizes a phase-locked loop (PLL) to track the source voltage phase and applies the mathematically derived optimal energization angle to suppress the inrush current. The validity of the method was preliminarily verified through simulations and further confirmed experimentally under both no-load and load conditions. Based on the measured voltage and current waveforms, ϕ– i hysteresis loop analysis, harmonic spectrum, and total harmonic distortion (THD) evaluation were performed to compare the results quantitatively. The objective of this study is to clarify the mechanism of inrush current generation and to experimentally verify that inrush current can be effectively reduced through energization angle control by mitigating residual flux asymmetry.