A new aggregation method is developed that is similar to singular perturbation but can be used for systems that do not have a two time scale property and experience fault disturbances. The optimal modal coherent aggregation method is derived by formulating a parameter optimization problem that attempts to estimate the parameters of a reduced order model of fixed form. The performance index measures the squared error in all internal generator angle differances in the reduced and unreduced model for the study system where disturbances occur. The optimal modal coherent aggregation method is shown to have an identical form for both step and impulse disturbances which is similar to that for the singular perturbation aggregation method.