Non-Intrusive Estimation of Single-Port Thevenin Equivalents in AC Grids

Abstract

This paper deals with the estimation of parameters of a Thevenin equivalent in an AC grid. We consider the practically relevant case of measurements coming from a grid-connected inverter. Our focus is on non-intrusive measurements, as we postulate and later experimentally observe transients that occur in normal operation as the primary source of information. We notice that even during quiet, after work hours there are typically enough variations in the grid to enable successful convergence of our procedure. For completeness, we also consider the artificial case of no external variations and show that injection of very small current perturbations suffices for convergence. Our procedure is based on real-time monitoring of the estimated determinant of the regression matrix. Regression matrix properties are also used to evaluate the integrity of the estimates and to initiate a brief sequence of perturbation in rare cases of the exceptionally restful grid. The estimation algorithm is simple enough to be implementable on standard industrial controllers, yet robust and reliable in terms of speed of convergence.