Overcurrent (OC) protective relays use different methods to calculate the RMS value of the current signal, based on the CT data. One common technique uses the peak-to-peak value, which is called the Apparent RMS Value in this paper. But using this value, when there are noticeable harmonics in feeders current, may lead to OC relay malfunction. The ratio of maximum possible Apparent RMS value to True RMS value is introduced as the Apparent RMS Current Ratio Index (ACRI). Then maximum permissible current in a feeder is determined so that no relay malfunction is guaranteed. This malfunction occurs more often in capacitor banks feeders in MV/LV substations with big nonlinear loads such as big induction furnaces, especially in resonance situation. Results are investigated on a typical 63 kV/20 kV substation including a big nonlinear load compensated by four back-toback capacitor banks.
