Computer Code for Rational Approximation of Frequency Dependent Admittance Matrices

This paper deals with the problem of approximating with rational functions a matrix whose frequency-dependent elements have been obtained from calculations or from measurements. Based on a previously developed technique (vector fitting), a set of callable routines have been written in the MATLAB language. These routines allow for rational approximation with a common set of stable poles, automatic selection of initial poles, passivity enforcement, and creation of an equivalent electrical network which can be imported into ATP-EMTP. Usage of sparse arithmetic permits the computer code to handle large systems. The methodology is demonstrated by application to a frequency-dependent network equivalent of a radial distribution network (phase domain), for which the accuracy is validated in both the frequency domain and the time domain. The computer code is in the public domain and is available from the author.