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Retaining states of the original system in reduced order model

In order to get a controller with a small number of states, it is usually necessary to reduce the model of the plant. It is particularly important in decentralized power system control design because of the large number of nearly unstabilizable/undetectable modes from a specified control substation. A drawback of most model reduction techniques is that the state of the reduced order model has no physical significance. This paper presents a stepwise procedure for retaining certain specified state variables of the system in a reduced order model, thus allowing more detailed analysis, e.g., in simulation, of a particular part of the system using the reduced order model. The procedure can be used in conjunction with any model reduction technique. An example of the model reduction of a generator for the purpose of power system stabilizer design is presented to illustrate the procedure.