Computational Algorithms for Constructing Modal-Coherent Dynamic Equivalents

Recent research has shown that the most desirable features of conventional modal and coherent dynamic equivalents can be combined ina single equivalent when an rms coherency measure and a robust, random system disturbance are used to determine structurally coherent groups for coherency-based aggregation. In particular, a modal-coherent equivalent can be derived which preserves not only the coherent groups of the original system model, but also the modes of group to group oscillations. A modal-coherent equivalent represents a valuable tool for transient stability analysis since it is constructed only once for a given utility and can then be used in the transient stability study of any disturbance that might occur in that utility. Previous works have presented theoretical developments which explain the structural coherency mechanism on which the modal-coherent approach to dynamic equivalents is based, and have neglected the computational aspects of constructing modal-coherent equivalents. This paper extends the value of the modal-coherent approach by developing efficient computational algorithms for evaluating the rms coherency measure for the required random disturbance and for determining the structurally coherent groups using the computed values of the measure. These algorithms will allow modal-coherent equivalents to be constructed for large power systems at a reasonable cost.