Low order equivalents for overvoltage analysis of power transformers

This paper presents and tests two approaches to obtain low order equivalents circuits for transient analysis of a power transformer. The first one is based on a nonlinear refinement of a high order fitting of a frequency domain function, while the second one is a simple use of the vector fitting subroutine forcing a low order fitting. The high order system is also obtained via vector fitting. Both models are tested in the frequency domain and in the time domain. The time domain results were obtained via a numerical Laplace transform but they could easily be implemented in electromagnetic transients-(EMT) type of program. The results have shown that while the low order obtained with vector fitting tends to average some of the poles, the nonlinear refinement is capable of selecting a "minimum set" of the poles of the high order function.