Impact of Electromagnetic and Optical CTs on Transformer Differential Protection during Transformer Re-energization

Power protection system design addresses electromagnetic current transformer (EMCT) saturation by adjusting setpoints and time delays in relays. However, extreme saturation may distort the EMCT's secondary current, causing relay malfunctions, such as during power transformer re-energization. Optical current transformers (OCTs) have been developed to provide immunity to saturation and accurate primary current replication during transient events. This paper analyzes inrush current during power transformer re-energization using both EMCTs and OCTs, evaluating the impact of saturated EMCTs on inrush current measurement. Additionally, we compare 2nd harmonic and DC blocking methods in transformer differential protection across 81 cases with varying residual magnetization and re-energizing instances.