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Controlled islanding is an active and effective way of avoiding catastrophic wide area blackouts. It is usually considered as a constrained combinatorial optimization problem. However, the combinatorial explosion of the solution space that occurs for large power systems increases the complexity of solving it. This paper proposes a two-step controlled islanding algorithm that uses spectral clustering to find a suitable islanding solution for preventing the initiation of wide area blackouts by un-damped electromechanical oscillations. The objective function used in this controlled islanding algorithm is the minimal power-flow disruption. The sole constraint applied to this solution is related to generator coherency. In the first step of the algorithm, the generator nodes are grouped using normalized spectral clustering, based on their dynamic models, to produce groups of coherent generators. In the second step of the algorithm, the islanding solution that provides the minimum power-flow disruption while satisfying the constraint of coherent generator groups is determined by grouping all nodes using constrained spectral clustering. Simulation results, obtained using the IEEE 9-, 39-, and 118-bus test systems, show that the proposed algorithm is computationally efficient when solving the controlled islanding problem, particularly in the case of a large power system.

The method proposed in the procedure of the first step of the spectral clustering controlled islanding (SCCI) is actually equivalent to the application of slow coherency. The slow coherency method is very useful for ofiline analysis. However, the following two questions must be answered before slow coherency can be applied to identify suitable generator groups: 1) have the generators lost synchronism, or will they, i.e., is the separation of generator groups necessary? 2) How many generator groups should be formed? This means that there are distinct drawbacks when applying slow coherency online; however, the method can still be adapted to this purpose to a certain extent [1]. We think a better way is using an online algorithm to replace the slow coherency method [2], [3]. The drawbacks and limitations of the first step of the SCCI have been discussed in Section III-A.