Contents I. Introduction
There are three control levels, commonly known as Hierarchical control, applied on the Microgrid (MG) to ensure its efficient operation. These control levels are called primary, secondary and tertiary [1] and have different response times and the need for communication. Droop control is mostly used at the primary control level that emulates the characteristics of interconnected synchronous generators and ensures desired power sharing between DG units without using communication. There are limitations of droop control particularly in power sharing under uncertain output impedances and dynamic load conditions. A combination of droop control with virtual impedance and modified virtual impedance control has been proposed in [2], [3] to achieve precise power sharing. Although virtual impedance control improves sharing of reactive power when feeder impedances are uneven, it is unable to support nonlinear and unbalanced loads in MG's islanded mode. The reactive power sharing has been improved under unequal feeder impedances in [3], which employs enhanced virtual impedance. But three phase converter encounters poor reactive power sharing with nonlinear loads. Using virtual resistance control [4], the reactive power sharing is improved for unequal feeder impedance, unbalanced and nonlinear loads. But if the load is disconnected, in any case, the MG would become unstable.
