The relay measures the circuit voltage and current, as well as the angle between them. When the current lags the voltage by more than +/- 90 degrees, the real power flow is in the reverse direction - the reverse power relay is sensitive to the real component of the current (kW).
An early term for induction-disk relay operation was the “wattmetric” principle. A reverse-power relay develops torque in its disk, exactly like a watthour meter does from voltage, current and phase-angle quantities. In the relay, a set of contacts are linked to the disk instead of a counting mechanism. Applications for reverse power protection may be single or three phase—often used to protect the prime mover of a three-phase synchronous generator.
