Protective Relay Settings of Tie Line Tripping and Load Shedding for an Integrated Steel-Making Cogeneration System

This paper presents the protective relay settings of tie line and design of load shedding scheme for an integrated steelmaking industrial power system to ensure the system stability after severe fault contingency on 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 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 the shutdown of critical loads due to external fault contingency, the under voltage and under/over frequency relays with proper settings are applied for the tripping of external and internal tie lines. To restore the power system stability for the islanding operation of the industrial cogeneration system after tie line tripping, the load shedding is designed to disconnect a proper amount of non-critical loads according to the governor response and protective constraints of cogeneration units. After performing the transient stability analysis of the industrial power system with the proposed protective relay setting and load shedding scheme, it is found that the voltage sag and oscillation problem caused by external Taipower system fault contingency can be mitigated by the tie line tripping in time to prevent the shutdown of critical loads and tripping of cogeneration units.