Robust V-I Droop Control of Grid-Forming Inverters in the Presence of Feeder Impedance Variations and Nonlinear Loads

In islanded microgrids, sharing load power and retaining voltage-frequency stability are two main tasks that should be guaranteed through grid-forming units. Accurate active and reactive power sharing can assure optimal and reliable operation of the microgrid. In practice, however, uncertain feeder line impedances and nonlinear loads could lead to harmonic voltages and consequently poor power sharing. In this article, a robust voltage-current (V-I) droop controller is proposed to address these issues. The main idea is to utilize a robust feeder impedance shaping technique to compensate the impedance uncertainties and the voltage deviations at the point of common coupling. Based on this strategy, an integrated scheme is proposed that consists of two main components: an inner-loop disturbance-observer-based controller for voltage and uncertainty control, and a novel V-I droop controller for power sharing. A joint analysis substantiates the stability and robustness of the integrated controller. Experimental results confirm the satisfactory performance of the developed technique as well.