Faulted phase selection function based upon impedance comparison in a distance protection relay

The distance protection relay is widely used around the world for the protection of transmission and distribution networks and it is especially essential in its application within bulk power systems. The faulted phase and the fault point on the power line must be determined by the relay with great certainty. In this paper, we propose a unique method to enhance the faulted phase selection function that uses impedance. In this method, an impedance comparison technique, the faulted phase is determined by identifying the minimum of the phase-to-phase impedances based upon the rule that a phase-to-phase fault impedance becomes a minimum for two-phase faults. In order to demonstrate the validity of the new technique, both the conventional method (voltage comparison method) and the proposed method are evaluated in detail for various test cases utilizing Toshiba distance protection relays, GRZ100 and a Real Time Digital Simulator (RTDS). These test cases include evolving faults to other phases and cross-country faults involving parallel double circuit transmission lines. The results show that the conventional method does not always have sufficient capability to discriminate the faulted phase correctly for the cross-country fault cases which have no healthy phases, on the other hand, the impedance comparison method demonstrates superior performance for all of the test cases. The proposed impedance comparison method is shown to be a promising method to enhance the faulted phase selection function for the distance protection relay and has been put to practical use successfully.