Solid-State, High-Speed Phase-Comparison Relay Based on Zero Crossings: I - Theory

A relay scheme has been developed based on the instantaneous phase current zero crossings. Transient and steady- state unbalances will produce shifts of the phase currents, therefore measurement of the deviation of successive zero crossings from 60 degrees leads to six possible fault indications per cycle. Comparison of the deviation magnitude at either end of a relay zone can be used to locate the fault. This scheme requires no additional delay due to analog filtering. It is shown that all types of faults with zero or finite impedance give at least two and a maximum of six fault indications per cycle.