A method to construct equivalent circuit model from frequency responses with guaranteed passivity

Converting the frequency response of a network into an equivalent time-domain circuit is a common task in several fields, such as power system simulations. A few good methods, such as the vector fitting method, have been proposed to do the conversion. Unfortunately, these methods sometimes produce non-passive equivalent circuits which are hard to realize in time domain or can lead to unstable simulations. In order to address the concern, this paper proposes a new conversion method for single-input single-output systems with guaranteed passivity for the resulting circuit. The basic idea of the proposed method is to represent the equivalent circuit as a matrix with varying dimension and unknown values. Genetic algorithm is then applied to find the values and dimension by minimizing the errors between the desired frequency response and that produced by the equivalent circuit. Since the equivalent circuit consists of only passive elements, the circuit passivity is always guaranteed. Details of the problem formulation and the solution algorithms are presented in this paper. Performance of the proposed method has been confirmed by several case studies.