Identification of a multiphase network equivalent for electromagnetic transient calculations using partitioned frequency response

This paper presents an algorithm for identifying a multiphase network equivalent for electromagnetic (EM) transient calculations. Conventional rational fitting methods intrinsically get into ill condition because the sⁿ terms in the rational function take a wide range of values which cannot be accurately treated within machine arithmetic accuracy. The proposed algorithm averts the ill conditioning by partitioning the given frequency response into sections along the frequency axis. Rational fitting is applied to each section of the frequency response to identify the poles, and then the corresponding residue matrices are obtained by a standard least-squares procedure using the entire frequency response. Adaptive weighting, column scaling, and iteration step adjustment are utilized to facilitate the rational fitting. The proposed identification algorithm is applied to obtain a reduced-order equivalent of a 500-kV power network, and the equivalent is used in a switching transient simulation to demonstrate its performance.