This paper reviews the slow coherency method for decomposing multi-machine power systems into coherent areas. The slow coherency method identifies weakly coupled groups of generators with the slowest inter-area modes of oscillation. This decomposition method has a sound mathematical basis. A case study is described in which the decomposition technique is applied to a power system (the Yemeni electrical network), and its dynamic equivalency is determined. This selection was based on choosing an already developed network. Results obtained indicate the necessity for performing such an analysis to improve the performance of developed networks.
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