A dynamic equivalent using modal coherency and frequency response

A general method has been developed to determine a dynamic equivalent of a large external power system. The combination of modal coherency and frequency response is a new approach. The frequency response formula, used to determine the coherency, is obtained by linearizing the total system and diagonalizing the linearized system. The method is tested for classical generator models as well as for fifth-order generator models plus second-order exciter models. The Dutch power system, which is coupled with the West-European UCPTE power system, has been used to illustrate the method. A comparison of the time responses of the original model and the reduced model provides a reliable picture