Analysis of Current Sensing Discrepancy on HV Side Overcurrent Relay when Short Circuit Occurs

The occurrence of current sensing errors in relays is caused by several factors such as CT saturation, the wrong position of the core wiring on the CT, and the double grounded secondary connection. These factors can be known early by carrying out ratio testing, knee point and winding resistance. Detecting anomalies early to avoid system faults and non-system faults due to current sensing errors. On November 10, 2023, there was a fault in Bay Transformer #3 Pati substation caused by a short circuit in the ABSW jumper cable of phase S. The OCR relay on the feeder side gave a Trip indication, GFR HS 2 Phase B, with a fault current value R: 186 A, S: 13.199 kA, T: 158 A, N: 13,072 kA. Meanwhile, the OCR relay on the HV side felt an increase in the current of the R-S-T phase current of IL1: 3.973 kA, IL2: 2.395 kA, IL3: 4.347 kA, iE: 1.847 A and has passed the moment current setting limit, so that the fault should only need to initiate the LV CB side trip but also initiate the HV CB side trip with instant working time. From the results of the post-disturbance investigation analysis, it was found that the cause of the current sensing error on the OCR HV side relay is possible due to the occurrence of inrush current as a result of the appearance of the magnetization effect on the terminal and is supported by the measurement results of the winding resistance before the disturbance which is greater than the post-disturbance measurement results. Large measurements of winding resistance can be due to imperfect wiring conditions and there is a dissipation factor from the environment.