Adaptive Negative Sequence Virtual Impedance-Based Voltage Unbalance Compensation in Microgrids

This paper addresses the challenges in compensating for unbalanced voltages in microgrids (MGs) dominated by inverter-based resources (IBRs). In the grid-connected mode of IBR, the grid can supply a negative sequence current to compensate for unbalanced voltages. In the absence of grid support, unbalanced loads and feeder impedances inherent to practical MGs complicate the design of voltage compensation strategies because of dependency on the inverter control strategy and the effective impedance between the inverter and point of common coupling (PCC). To address this issue, we propose an adaptive negative sequence virtual impedance (A-NQVI) scheme for the MGs dominated by high penetration of IBRs. The proposed approach is an easy add-on to the conventional voltage and current control with positive sequence impedance, enabling the injection of generated power into the grid and the compensation of unbalanced voltages at the PCC. The proposed scheme uses local measurements to extract sequence components by making use of a second-order generalized integrator (SOGI) and synchronous reference frame and is based on proportional-integral (PI) controllers. The performance of the proposed control scheme is validated on a modified IEEE 13-bus system in addition to a simple system with two distributed generators. The application of distributed control to inverters is also tested.