In this paper the multi-machine Heffron-Phillips model is generalized to handle multi-machine power systems with frequency and voltage-dependent loads. A systematic method for extracting state equations is proposed which preserves structure and is able to account for infinite bus directly. The generalized model's block diagram is similar to that of Heffron-Phillips and is suitable for generalizing dynamic studies of networks with impedance loads to networks with frequency- and voltage-dependent loads. Two applications of this model are presented: (1) studying effects of load model parameters on dynamic stability of a five-bus network, and (2) designing a PSS for a one-machine infinite bus with frequency- and voltage-dependent local load and comparing it with a PSS designed based on impedance load.

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