Ultimate limits to the fully decentralized power inverter control in distribution grids

As low-voltage distribution grids and microgrids emerge, their decentralized control design is a major issue and needs rethinking, as several recent examples of oscillatory behavior in voltage and reactive power have been reported. This paper analyzes a few possible reasons for these oscillations. Oscillations can occur as a result of connecting distributed energy resources to a single phase only, which creates unbalanced conditions. In order to avoid these oscillations, a novel master controller for assigning the slow set points to the inverters is proposed, so that the interdependence between unbalanced phases is accounted for. Oscillations can also occur due to the lack of a master controller to ensure the existence of an equilibrium or even as a result of inaccurate power set points used by droop-based controllers. Finally, even the very fast power electronic dynamics, which are usually assumed to be instantaneous in literature, may cause high frequency instabilities because the power electronics are not an ideal power source and have stability and switch saturation limits.