A Microprocessor-based Synchronous Motor Relay And Out-of-step Simulation

This paper analyzes the electrical characteristics of synchronous motors, with symmetrical component method, at the states of the symmetrical and asymmetrical faults. Based on the features and differences of positive sequence, negative sequence and zero sequence fault components when synchronous motors get involved in different faults, a reliable protection scheme is proposed for synchronous motors. Especially for the thermal overload, out-of-step protection and loss-of-excitation protection, the new methods are presented which are different from the traditional ones.In the state of light, half and fully loaded, synchronous motors' thermal characteristics are obviously different. But traditional thermal overload protection has not taken motors' thermal characteristics before the fault into consideration. Its trip time relates only to the current after faults. This paper introduces a scheme of motor thermal overload protection. It considers motors' working situation before the fault, and relatively factually reflects the thermal characteristics of motors before the fault. That can avoid motors' damages while fully exert overload capacity, and improve the reliability of electric dragging system and production continuity.In China, many synchronous motors are still equipped with the same out-of-step protection as 30 years ago. 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. Analysed the harm of synchronous motors' out-of-step and loss-of-excitation, this paper suggests that synchronous motors' power factor angle and static-stable impedance can be used as the fixed setting of loss-of-excitation protection. It's validity has been proved by MATLAB simulating software.