Vector fitting algorithms for impedance modelling of converter based systems

This paper investigates Vector Fitting (VF) algorithm variants for frequency-domain impedance modelling of converters in different control modes. The VF variants are obtained by varying the error norm used for minimizing the error between approximated fitting function and the measurement data. Consequently, in addition to the L2 norm minimized in the standard VF, L1 and H2 norms are minimized and their fitting accuracy is compared. Furthermore, a comparison of accuracy is also carried out by minimizing these three norms in the modified standard VF known as Relaxed Vector Fitting (RVF). The results show that an additional degree of freedom introduced in RVF leads to higher accuracy fitting compared to standard VF. The accuracies of VF between the three norms differ only marginally. Minimizing the H2 norm results in high accuracy, but it requires pre-defined measurement frequency points, which can lead to an impractical definition of input data. L1 norm minimization results in large deviations at a few measurement points due to an additional weighting factor introduced in the error minimization function. Moreover, RVF minimizing L2 norm shows the highest VF accuracy.