Discrete wavelet transform for improving the accuracy of an unbalance current protection relay due to transient fault and inrush current signals

An unbalance current protection relay (60C, 51NC) cannot operate fast enough to avoid catastrophic failure against high system fault currents within capacitor units because traditional unbalance current protection relays are not capable of distinguishing and classifying transient fault current and switching inrush current. This paper proposes a new algorithm for the detection and classification of such current signals. The program used for the simulation was PSCAD/EMTDC. The current signals output was used as discrete wavelet transform (DWT) input. The wavelet-based fault and inrush current detection and classification unit received the various captured transient current signals, and used DWT to detect and analyse various cases studies. The actual inrush current signals from both the experimental setup and testing on an actual 115 kV HV shunt capacitor bank were used to verify the proposed algorithm. Results show that the proposed algorithm is effective at accurately identifying fault and inrush currents.