Improved PI algorithm for complex multi-area power system

PI algorithm is a method for predicting loss of synchronism. It was developed for power systems that can be represented by two-machine equivalent systems. This paper addresses the applicability of PI algorithm to complex multi-area power systems and its improvements to extend its application. Simulations have been performed on a multi-area power system model to evaluate the PI algorithm. Multiple misjudgments resulted from these tests. Therefore, the following countermeasures were proposed and added to the original PI algorithm to prevent such misjudgments: algorithm for detecting decelerating instability mode; θ_max method for preventing unwanted operation; and backup impedance relay to prevent operation failure. A simulation study has been conducted to verify the proposed countermeasures. As a result, all cases have been judged correctly which indicates that these countermeasures are very effective.