Positive–Negative Sequence Synchronverter for Unbalanced Voltage in AC Grids

In recent years, the global energy industry has been experiencing a remarkable growth of distributed generators, which are mostly based on power electronics grid interfaces. This phenomenon has led to the development of many different techniques to allow the parallel operation of these dispersed sources without communication links. Most of the proposed techniques aim at emulating the behavior of synchronous generators, as it is the case of the droop control, virtual synchronous generator and synchronverter techniques. The synchronverter algorithm has increased the interest of many researchers due to its ability to perform autonomous power sharing and the possibility of adjusting its virtual inertia to enhance the dynamic response. Several implementation approaches of synchronverters have been proposed in literature. However, in most cases, the synchronverter performance is only evaluated under normal grid voltage conditions. This paper proposes a positive–negative sequence synchronverter technique for three-phase voltage source converters connected to unbalanced grids. This method consists in separating the power control in positive and negative sequence components to improve the converter behavior under both unbalanced grid voltages and unbalanced voltage sags. Simulation and experimental results validate the proposed method highlighting its capability of limiting the converter output current, reducing power oscillations and avoiding undesired shutdowns.