New Voltage Control Strategies for VSC-Based DG Units in an Unbalanced Microgrid

This paper presents the positive-sequence, negative-sequence, and zero-sequence voltage and current control schemes in the dq frame for the voltage-source converter (VSC)-based distributed generation (DG) units in order to compensate for voltage unbalance in a microgrid. The objective of these schemes is to control the positive-, negative-, and zero-sequence components (separately and independently) of the voltage at the point of common coupling and the VSC currents to their respective reference commands. Dynamically varying limits have been proposed for the positive- and negative-sequence references for the current control schemes in order to protect the VSC from overloading (under unbalanced conditions) and unsymmetrical faults. The active power control, frequency control, and the reactive power-voltage droop control schemes decide the references of the positive-sequence voltage control scheme in order to fulfill the objective of using the same control schemes for the grid-connected and the islanded modes of operation of the microgrid, thereby eliminating the need for islanding detection. The performance of the various control schemes employed for controlling the VSC-based DG unit has been tested on two identical VSC-based DG units feeding power to the IEEE 34 node distribution network implemented in PSCAD/EMTDC.