2. Coherency in Power Systems

There has been a continuing need over the past several decades to model larger and larger interconnection wide models. Models of the complete interconnections with up to 50,000 buses are regularly used for system planning studies. These models typically go down to 115 and 69 kV levels, but ignore underlying 35 kV sub-transmission networks. With the growing deployment of plug-in vehicles, distributed generation and smart load controls, along with the need to perform realistic system restoration drills there is a need to model interconnections down to the feeder breaker level. Restoration drills also require modeling of power plant auxiliaries and emergency generator systems, especially for nuclear units. It is conceivable that the size of interconnection wide models could grow by another order of magnitude. The EPRI DYNRED (Dynamic Reduction) computer program reduces a large-scale system model into a smaller equivalent model for use in transient stability studies. The program has been used since the 1970s to build equivalent models of the Eastern U.S. and Western U.S. interconnected power systems. The DYNRED program accepts a normal transient stability database as input, and develops an equivalent that is a fraction of the size of the full power system representation, while adequately retaining the dynamic characteristics of the full system. The reduction process requires only a fraction of the time needed for a transient stability simulation.