Stabilizing Droop Variation of Converter-Connected Generation in Autonomous Microgrids with Virtual Inertia Control

Droop scheduling plays an important role in the microgrid frequency regulation market, as the well-known proportional load sharing may be practically inapplicable in many cases of inverter-based microgrids. The drooping capability of each inverter unit in the microgrid is determined by the availability and response time of its associated energy storage. However, some renewable energy-based units may be chosen to operate at the maximum power point because of their zero fuel cost. This scenario demands some high capacity units to operate with a high or infinite droop coefficient, thereby restricting their contribution to the frequency regulation. In this work, we examine the influence of this droop variation on the small signal stability of the microgrid using the root locus technique. We then substitute some of the existing droop-controlled units with virtual inertia control at critical nodes to study the effect of coordinated droop and inertia control. Finally, the choice and size of energy storage necessary to emulate a certain level of inertia and droop are compared. The study shows that the stability of the system may be enhanced while saving on the economics of storage requirement if the inertia and droop control act in a coordinated manner.