Abstract
On-line critical contingency selection and ranking based on a composite security index is presented in this article. A new method for obtaining a composite scalar index, in terms of both line flow and bus voltage limit violations, has been developed based on the concept of hyper-ellipse inscribed within a hyper-box. The proposed index provides an efficient method for filtering and ranking the credible contingencies and is defined in such a way that a proper differentiation between the secure and insecure states are possible. The composite security index can completely eliminate the masking problem in contingency selection and ranking. In addition, it is seen that if the system is insecure, the exact location of limit violations in buses and branches can be determined by monitoring the violation vector so that proper preventive and corrective control actions can be taken. Extensive simulations have been done using IEEE 30-bus and IEEE 118-bus systems, and the results show that the proposed approach is effective in security assessment.