Coherency-based multimachine stability study

A new equivalent generator model to replace a group of coherent generators is discussed. The model is simple and can easily be implemented in transient stability programs. Derivation of the parameters of the proposed equivalent generator does not follow the common approach of calculating a new set of parameters for a new or hypothetical generator. The model is derived in such a way that its dynamic equations correspond to those of one of the generators it represents. The approach to the derivation of the equivalent generator equations is general. In the paper the model is derived for application in two different stability programs. In the first program a conventional method of stability analysis is used which incorporates classical generator models. The second program involves a noniterative algorithm in which generators could be modelled with any desired degree of accuracy. As the equivalent generator incorporates the equations of one of the machines in the system, it is possible to model all generators and controllers, including the equivalent generator, with any degree of detail desired. While the main objective of the paper is to introduce the equivalent model and its mathematical derivation, application of the new technique to reduce the order of a power system is also demonstrated.