New Stationary Frame Transformation for Control of a Three-Phase Power Converter Under Unbalanced Grid Voltage Sags

This article deals with a new nonlinear transformation providing representation of unbalanced three-phase signals (voltage and currents) as vectors that have constant components in a new rotating d'q' frame. Using the proposed transformations, the control system of a power electronic converter operating with an unbalanced grid can be equipped with proportional-integral (PI) controllers, without the use of resonant terms, for which antiwindup structures are more complicated and nonintuitive. This article presents the derivation of new direct and inverse transformations, features, a power converter model in a new frame, simulation results showing the waveforms of signals transformed to the new non-Cartesian frame, and simulation and experimental results of the voltage-oriented control for a grid-connected power converter.