Design of under voltage relay for an industrial plant with cogeneration units to enhance power quality of critical loads

This paper presents the design of under voltage relay settings for an industrial plant with cogeneration units to prevent critical loads from tripping due to voltage sags. The critical loads sensitive to voltage sags in the plant have been identified from actual system fault contingencies. The power quality of these loads is enhanced by quickly islanding the subsystem with cogeneration units before tripping of the tie line with external Taipower system. According to the transient stability analysis of the rest of the industrial power system nearby Taipower network, the critical loads non-sensitive to voltage sags are isolated from external system faults in time to restore the system stability with the execution of proper load shedding. To ensure the normal operation of critical voltage sensitive load by considering the voltage tolerance curves, the proper relay settings for tie line tripping of the internal subsystem has been determined. In this way, the critical loads and cogeneration units in the industrial power systems can be survived from system faults to mitigate the production loss of industrial customers.