Error Reduction in Contingency Ranking of Power Systems Using Combined Indexes

Complexity of calculations prevents utilities from doing online security studies, and that makes ranking and selection of events an essential part in the field of dynamic security assessment. In this paper, after a short review on security assessment, some indexes such as generators rotor angle, generators speed, rotor acceleration, the difference between kinetic and potential energy etc. have been introduced for ranking of probable contingencies. These indexes have been calculated and compared for different events. In addition to description and investigation of presented indexes, some probable events have been ranked in a case study network. Calculated indexes were finally compared with transient energy margin in order to determine their accuracy. Evaluation of results showed that some errors are inevitable and an individual index is not capable of describing the severity of an event completely. In order to reduce the error of ranking, it is suggested to use a combination of indexes. Several simulations were done in a case study system to find proper combinations of individual indexes, which leaded to finding new combined indexes which are able to rank the contingencies with minimum errors.