Under-voltage relay settings for the tie-lines tripping in an upgrading cogeneration plant

This paper presents the modification of under-voltage relays to trip tie lines of an industrial customer when a new cogeneration unit and two main transformers are installed. First, system configuration and operation condition of the cogeneration facility are introduced. To examine the dynamic responses of the cogeneration system during the fault contingencies, the mathematical models of the cogeneration units, excitation systems, governor systems and loads are included in the computer simulation. The under-voltage relays are designed according to the critical clearing time of the cogeneration units. To verify the effectiveness of the proposed protective scheme, the transient stability analysis of the industrial power system under various fault contingencies are performed. It is concluded that the proposed under-voltage relay settings can be accurately used as a backup protective strategy and the dynamic response of the isolated cogeneration system can be significantly enhanced.