Field evaluation of automatic restart of essential motors using microprocessor-based protective relays

Industrial facilities that employ numerous electrical motors for vital functions require either continuous operation, ride through, and/or automatic restart operation for abnormal conditions, such as momentary voltage loss, voltage dips, or other system voltage transients. With large essential motors, it is impractical or not economical to provide an uninterruptible power source. Therefore, the next option to reduce the effects of transient interrupted service is to automatically sense the disturbance and initiate a sequenced restart of the essential motors based on process requirements, motor acceleration characteristics, and power system capabilities. A customized logic algorithm that has been programmed into microprocessor-based motor protection relays, eliminating additional undervoltage, control, timing relays, and wiring associated with traditional schemes, is described in this paper. The algorithm has been simulated during factory and site acceptance tests. This approach offers many user-configurable features that could be used in virtually any microprocessor-based relay protecting a medium-voltage induction motor. Also, the plant essential motors are prioritized into different categories, defining the order in which some essential motors are staggered and restarted together.