A practical Evaluation Method for Inrush Currents and Voltage Dips during Transformer Energization

In this paper, we present a practical calculation tool for evaluating inrush currents and voltage dips during transformer energization. As power system configurations change more frequently with the increasing introduction of distributed and renewable energy sources, understanding these transient phenomena has become increasingly important. Excessive inrush current may trigger unintended relay operation, whereas voltage dips can disturb voltage-sensitive equipment. A simple and accurate evaluation method is therefore required. The tool developed can be used to compute inrush currents and voltage dips using analytical expressions derived from a simplified equivalent circuit, where the transformer magnetizing branch is represented by an ideal switch and a saturated inductance. This approach eliminates numerical integration and enables fast computation on web-based platforms. The tool supports several transformer winding configurations and accepts parameters in common engineering units. To validate its accuracy, sixteen analysis cases based on actual Japanese power systems were examined using both the developed tool and electromagnetic transient simulation. The results show that the developed tool reproduces the peak inrush current and maximum voltage dip with errors of less than 1%, demonstrating its adequate accuracy for engineering use. A detailed investigation was also performed to identify worst-case energization conditions in terms of the energization phase and residual flux. These results confirm that the tool developed provides a fast and convenient method to evaluate the impact of transformer energization on power quality, offering accuracy comparable to that of electromagnetic transient simulation and practical applicability for system planners and operators.