Impact of Electromagnetic and Optical Cts On Transformer Differential Protection During Transformer Re-Energization

The design of power protection systems is based on the potential for saturation of electromagnetic current transformers (EMCTs) to occur at certain points during power system operation. To address this issue, compensation in the form of adjusting setpoints and time delays in relays is commonly employed. However, in some cases, the magnitude of saturation may be so high that the EMCT's secondary current becomes highly distorted, leading to malfunction of the relay. One of these cases is re-energization of a power transformers that results in surge of inrush current. New sensing technologies suitable for power system applications, including optical current transformers (OCTs), which are immune to saturation and able to accurately replicate primary current even during severe transient events, have been developed. This paper provides an analysis of inrush current during the re-energization of a power transformer as recorded by both EMCTs and OCTs. We present a model that aims to evaluate the impact of saturated EMCTs on the measurement of inrush current. In addtion, we provide a comparative analysis of 2nd harmonic and DC blocking methods in transformer differential protection for 81 cases with different residual magnetization and re-energizing instances