Modeling Single-Phase PQ Inverter for Unbalanced Power Dispatch in Islanded Microgrid

Abstract

For a three-phase islanded microgrid with non-stiff grid-supporting or grid-forming distributed generators (DGs), unbalance power dispatch from these DGs can negatively influence the voltage quality, stability, and lifespan of the DGs. One way to solve such unbalance is to have DGs that can compensate for the load unbalance by being able to dispatch unbalance power. In this work, single-phase controlled PQ inverter interfaced DGs are modeled for this purpose. A detailed control model of the single-phase PQ inverter is derived from which we approximate the power stage of the inverter to a voltage-controlled source. Simulation studies in PSCAD showed that the voltage-controlled model gives an accurate representation of the elaborate switching model of the single-phase PQ inverter. Coordination of unbalanced power dispatch is realized by stacking the single-phase inverter in a manner similar to the three-bridge four wire shunt active filter and dispatch setpoint are sent to the PQ inverters from the secondary and tertiary control level of the microgrid for load compensation purpose. Case studies of balanced, unbalanced and load compensated systems are presented and evaluated with two unbalance indices: phase voltage unbalance rate (PVUR) and true voltage unbalance factor (VUF). The studies showed that in an islanded microgrid with a non-stiff reference source, the primary source of voltage unbalance across three phase nodes is unbalanced loading and that by dispatching unbalanced power to the PQ inverters to compensate for load unbalance, a significant improvement in system balancing could be realized.