Dynamic Differential Protection of Transformer Considering CT Saturation Based on Waveform Distortion Estimation

The protection relay performs a significant role in maintaining the power system’s reliability and stability. However, Current Transformer (CT) saturation affects the overcurrent, differential and distance protection under various conditions. Under the CT saturation condition, the secondary current waveform is distorted depending upon the level of saturation. This paper describes an adaptive differential protection scheme for a transformer considering the effect of magnetizing inrush and the CT saturation. In the proposed methodology, the inrush conditions and CT saturation conditions are detected using the waveform distortion estimation and current polarity index technique by utilizing the current signal parameters. The power system is simulated in PSCAD software to create various fault conditions in the transformer. Different transformer internal and external faults have been generated, considering mild to heavy CT saturation phenomenon. The proposed dynamic differential relaying method is implemented in MATLAB software for CT saturation detection and transformer fault zone identification. A hardware prototype has been developed to simulate practical inrush and fault scenarios on a transformer to validate the proposed relaying technique. The proposed algorithm is implemented in the microcontroller, which efficiently detects the occurrence of different levels of CT saturation and successfully operates for all internal faults. On the other hand, the proposed relaying scheme remains inoperative for all inrush conditions and out-of-zone transformer faults by effectively detecting CT saturation.