SASTRY AND VARAIYA : COHERENCY FOR INTERCONNECTED POWER SYSTEMS ASi

W E propose a model of some of the typical disturbances affecting an interconnected power system and derive preand post-disturbance models linearized around a base-case solution. These models are used to propose a method for identifying coherency. Security analysis is generally developed for a specified portion of an interconnected power system, called the internal or study system, and which is often coincident with the domain of a particular operating utihty, while details of the remainder or external portion of the system are considered only to the extent that they affect the study system. The external system is approximated by an “equivalent” of lower dimension; the aim is to realize valuable reduction in computational effort, and possibly in information requirements, without introducing an intolerable loss in accuracy. For purposes of transient stability analysis, one seeks a “dynamic equivalent,” that is, an equivalent system of lower order which nevertheless accurately reflects the short-term impact of the external system (see, for example, [4], [5], [71). The two approaches to the construction of dynamic equivalents which have been most developed are the modal reduction method [18] and the coherency method [lo], [13], [22]. We shall be concerned with the latter. Both approaches use a linearized model to approximate the behavior of a nonlinear power system model in the neighborhood of a base-case solution. (Such a linearized model is presented in the next section.) The coherency method is founded on the empirical observation that, following a disturbance (he or generating unit outage or sudden change in load), certain groups of generators “swing together,” that is, the generators in each group maintain nearly constant angular differences with each other. Each such coherent group can then be replaced by a single bus. The objective of the method is, therefore, to