Control of a Three-Phase Power Converter Connected to Unbalanced Power Grid in a Non-Cartesian Oblique Frame

The article presents a new approach to positive and negative sequence current vector control of a grid-connected three-phase three-wire power electronic converter operating under grid voltage imbalance conditions. The concept utilizes the representation of unbalanced converter current in the new coordinates frame in which the current vector components are constant. The nonlinear trigonometric transformation of two-dimensional current vector components from the stationary frame to the new frame is found online depending on the reference current asymmetry. The presented concept of new coordinates utilization allows implementation of proportional-integral terms as current regulators without the use of resonant terms and without the use of the measured current symmetrical sequences decomposition. The article presents the theoretical approach, simulation results, as well as laboratory tests results.