Modification of under-frequency relay settings for the upgrading of a cogeneration plant

This paper presents the modification of under-frequency relays to trip tie lines of an industrial customer when a new cogeneration unit and two main transformers have been installed. First, system configuration and operation condition of the cogeneration plant are introduced. To examine the dynamic responses of the isolated cogeneration system, the mathematical models of the cogeneration units, excitation systems, governor systems and loads are considered. The external and internal under-frequency relay settings are modified according to the simulation results of the transient stability analysis. To verify the effectiveness of the proposed protective scheme, the computer simulation of the industrial power system under various operation conditions and fault contingencies will be performed. It is concluded that the proposed under-frequency relay settings can enhance the dynamic response of the isolated cogeneration system and reduce the abnormal operation of protective strategy.