Improved Transmission Line Protection during Cascading Events

Relay operations during cascading events play an important role for the power system stability. Without knowing the whole system information, distance relay may make wrong decision from the view of system reliability since it operates only according to local measurements. This paper introduces an improved transmission line protection scheme during cascading events. It utilizes the interactive method of system-wide and local monitoring and analysis tools for on-line automated analysis of protective relay operation. The new systemwide analysis tools based on Vulnerability Index (VI), Margin Index (MI), and Equivalent Parallel Path Approximation (EPPA) allow the vulnerable transmission lines of the system and vulnerable conditions to be identified. Then new local on-line monitoring and control tools are capable of analyzing performance of relay operation and informing the centralized tool about the outcomes that will be invoked to mitigate incorrect or undesirable relay action. A case study is presented to demonstrate the improved transmission line protection scheme using the new tools.