Abstract: In this paper, two new concepts—“main out-of-step mode” and “minor out-of-step mode”—are proposed for power system reliability analysis. Large-scale power system studies found that out-of-step generator groups may have characteristics of the main out-of-step mode and the minor out-of-step mode. The generator groups with main out-of-step modes can determine the out-of-step interface of the large-scale power system, while generators with the minor out-of-step modes cannot play such a role. Therefore, the method of capturing the out-of-step interface by seeking the lowest voltage point (the out-of-step center) can only group the generators with the main out-of-step modes, and may fail to combine the generators with the minor out-of-step modes into proper coherent generator groups. Thus, it is necessary in engineering applications to equip the generators that are likely to have the characteristics of the minor out-of-step modes with separation devices based on off-line simulation studies in order to reduce the risk of further accidents caused by these generators after system separation.

Key words: Out-of-step center, Out-of-step interface, Separation device, Main out-of-step mode, Minor out-of-step mode

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