Out-of-step protection for synchronous motor

Synchronous motor has many merits, such as low temperature, stable operation, large output power, supporting the voltage of power grid and improving power factor in particular, In China, many synchronous motors are still equipped with the same out-of-step protection. Usually they use over current relay with the inverse time characteristics as short circuit and overload protection, and as out-of-step protection of synchronous motors. But this protection is not perfect. It cannot guarantee synchronous motors' working safety under all kinds of out-of-step situation, and cannot provide fundamental protection when non-simultaneous impact is caused by a power break out-of-step. Furthermore, on the premise of safety guarantee, it cannot keep the working continuity of synchronous motors. Through analyzing the harm of synchronous motors' loss-of-excitation, this paper suggests that synchronous motors' power factor angle φ and static-stable impedance can be used as the fixed setting of loss-of-excitation protection. Its validity has been proved by MATLAB simulating software.