Electric power system dynamics- modes and coherent generators

The authors discuss the relationship between the oscillatory modes and coherent groups of an electric power system, and, in particular the effect of group aggregation on modes. It is shown that one typical aggregation method is equivalent to a similarity transformation on the linearised system equations. The analysis, and results obtained for a test system, suggest a hierarchical organisation of modes and coherent groups. The extent to which modes can be given a specific physical significance is clarified, as is the ability of different generators to affect particular modes