Fiber Optic Current Transformers for Transformer Differential Protection during Inrush Current: a Field Study

Saturation of electromagnetic current transformers (EMCTs) due to inrush current during re-energizing of a power transformer may lead to misoperation of differential protection. Fiber optic current transformers (FOCTs) can offer a solution to this problem by providing an accurate replica of the primary current during inrush conditions as they are free from saturation. This paper focuses on comparing EMCTs and FOCTs measurements of inrush current based on data obtained during a field test at a 500 kV substation. Waveform and harmonic analysis of the signals captured by a digital fault recorder from EMCTs and FOCTs reveal a significant difference in their RMS and harmonic content, thus demonstrating the influence of the saturation of the EMCTs on the current measurements. We also show that compared to EMCT, FOCTs measurements allow for better identification of the inrush current by means of DC component analysis.