A Per-Phase Power Controller for Smooth Transitions to Islanded Operation

This paper presents a droop-based controller for grid-tied three-phase inverters. The controller allows to regulate the inverter output power while operating grid-tied, to support the local grid voltage while operating islanded, and to seamlessly transition into this latter mode of operation. Remarkably, the use of the traditional droop control scheme for per-phase power control would lead to unequal frequencies among the phase voltages, which is not acceptable. Instead, the proposed controller allows independent power references tracking at each of the phases of a three-phase inverter while grid-tied and a proper transition into the islanded operation. Per-phase power control is crucial for several services in modern smart power networks, like demand-response and distributed unbalance compensation. The paper also reports the stability analysis of the whole control system and outlines a procedure for the design of the involved regulators. Simulation and experimental results considering a laboratory-scale prototype are reported and discussed to validate the proposed controller.