Protective Relay Settings of Tie Line Tripping and Load Shedding for an Integrated Steelmaking Cogeneration System

This paper presents the protective relay settings of tie line tripping and design of load-shedding scheme for an integrated steelmaking industrial power system to ensure system stability after a severe fault contingency on an external utility power network. The transient stability analysis of the industrial power system with the equivalent external Taipower system has been executed for an actual fault contingency in 2005. The mathematical models of 13 cogeneration units in the plant and several large generators of a nearby Taipower thermal plant are included in the computer simulation to solve the system transient response for the fault contingency to verify the accuracy of the models used. To prevent tripping of critical loads due to an external fault contingency, the undervoltage and under/overfrequency relays with proper settings are applied for the tripping of external and internal tie lines. To restore system stability of the industrial power system for the islanding operation after tie line tripping, load shedding is designed to disconnect a proper amount of noncritical loads by considering the governor response and protective constraints of cogeneration units. According to the transient stability analysis of the industrial power system with the proposed protective relay settings and load-shedding scheme, it is found that the voltage sag and oscillation problem caused by fault contingency in the external Taipower system can be mitigated if the tie line tripping is executed in time to prevent the shutdown of critical loads and tripping of cogeneration units.