Power industry restructuring and competition in the deregulated electricity markets to provide increased demand causes operation of large power systems close to their stability boundaries. Although an interconnected power system may be stable against small disturbances, if there is no emergency corrective control to resynchronize all generators and prevent from fault spreading into the entire network, occurrence of large contingency may cause the system to lose stability and even lead to wide area blackout. Studies show that many wide area blackouts such as 2012 India blackout which led to massive economic losses may have been prevented by a fast feasible controlled splitting decision making (Wang, Shao, & Vittal, 2005). Hence, power system islanding is the last defense line to protect power grids from incidence of wide-area blackout. In other words, controlled system islanding also known as controlled system splitting provides the final remedial action against power system major incidence following a severe disturbance. If there is no proper remedial action in time, immediately after occurrence of large disturbance, it may lead to a catastrophic failures and power system blackout. If a system is encountered with severe instability problem, and emergency control fail to bring the faulted system back to the normal state, an islanding strategy executes by splitting the interconnected power network into several islands by disconnecting proper selected transmission lines. Achieving the proper islanding strategy which satisfies all steady-state and dynamic constraints within islands is a complicated scenario. Major efforts are needed to determine a splitting scheme with two following important characteristics (Ibrahim, 2011):
When to Split: Islanding starts exactly after separating detection. There are many different techniques to detect the interconnected power system islanding.
Where to Split: Many wide area blackouts may have been prevented and load-generation mismatch could have been reduced by fast, accurate, feasible controlled splitting strategies (Andersson et al, 2005; Yang, Vittal & Heydt, 2006).
Controlled intentional islanding separates a bulk power system into a number of stable islands by tripping selected transmission lines according to the minimum load-generation mismatch (L. Liu, W. Liu, Cartes & Chung, 2009). Hence, once separation is detected, the most important step is to find the optimal splitting points. In the literature, several approaches have been proposed to split a large power system into a number of stable sections following a wide area contingency. These procedures can be divided into two general categories. The first one is based on the coherent generators clustering and the second one is based on the network graph theory (Najafi, Hosseinian & Abedi, 2010).