Robust Control of Three-Phase Voltage Source Converters Under Unbalanced Grid Conditions

This paper proposes a new and robust control strategy for distributed energy resource (DER) units operating in parallel with a three-phase ac grid that could have unbalance voltages. It is well known that the grid imbalance imposes double-frequency ripples on the dc-side capacitor and low-order harmonics on the ac side of a voltage source converter widely used to interface the DERs with the grid. The proposed controller is able to overcome both problems. This not only improves the power quality and stability aspects of the DER and overall system, but will also help reduce the size of the dc-side capacitor. The proposed controller belongs to the family of current controllers and can in principle be added as an upgrade to the existing current controllers. Mathematical analysis and optimal control design aspects are presented and then the performance of the proposed controller is evaluated by various simulation scenarios and hardware-in-the-loop experiments.