Multiscenario cascading failure analysis using TRELSS

This paper summarizes the southern company methodology and experience in advanced analysis of cascading failures. Cascading failures may occur due to loss of generating units at a station, breaker failures, common tower and common right-of-way circuit outages, etc. Once an initiating event occurs, the protection system opens the surrounding breakers so that several protection control groups (PCGs, a set of components which go out of service due to action of breakers) are forced out of service. In its turn, these initial outages could cause severe overloads and voltage deviations on the operational transmission facilities. This may trigger further action by the protection system isolating other PCGs, and so on. Cascading outages can propagate through the interconnection incurring significant loss of load potentially leading to system collapse. Southern company has developed and implemented a unique methodology that allows screening multiple initiating contingencies, simulating the cascading process, evaluating the system impacts, ranking the cascading scenarios based on their severity and likelihood, and identifying the top contingencies that require primarily attention. The methodology has been successfully used for several years. It has already influenced real-life decision-making in transmission system enhancement projects´ prioritization and selection. This paper contains a brief description of the southern company´s approach as well as examples of its practical application.