Structural Coherency in Interconnected Power Systems

In this paper the notion of structural coherency is introduced as a generalized definition of coherency related to the order reduction that may occur in an external power system when forming its transfer function matrix. On the basis of a linearized model of the external system in polynomial matrix form, necessary and sufficient conditions, involving network and generator parameters, are given to ascertain the presence of structurally coherent generators. A practical coherency index is defined which allows one to evaluate the degree of structural coherency among the generating units. By using the proposed index, nearly-coherent generator groups can be easily identified and accurate dynamic equivalents are provided.