Digital distance protection for composite circuit applications

An approach to numeric distance relaying is proposed that is suitable for EHV composite overhead line and underground cable circuit applications. The proposed relay incorporates a circuit shunt capacitance in addition to a series resistance and inductance. By modelling the protected circuit as a series of RLC sections within the relaying algorithm, greater reach point accuracy can be achieved. Unlike conventional distance relays that sense the fault loop impedance and compare this against a reach point value, the proposed relay uses a directional element that is based on the calculated reach point voltage and current values. Results are presented based on a composite 500 kV application study comprising two overhead line sections and two cable sections. Comparisons with conventional numeric distance relay algorithms show that overreach is virtually eliminated in the proposed relay. The simulation results have been verified with an implementation of the relay based on modern digital signal processing hardware.