A Virtual-Impedance Droop Control for Accurate Active Power Control and Reactive Power Sharing Using Capacitive-Coupling Inverters

Abstract

Capacitive-coupling inverters (CCIs) have low operation voltage and enhanced reactive power control capability. It is a promising alternative to provide reactive power and voltage regulation in microgrids (MGs). Droop control is one of the most widely used primary controllers under a hierarchical framework in an MG. However, conventional droop control performance needs improvement since the droop curve assumption is not valid in part of CCI's operational area. At the same time, reactive power sharing error due to the uncertainty factor of feeder impedance needs to be minimized. In this article, virtual-impedance droop control is proposed for CCIs to solve the two issues. A virtual impedance selection method is developed with consideration of CCI's second-order LC coupling branch. First, the virtual impedance is selected for reducing the coupling between the active and reactive power of CCIs. The stability and sensitivity of the system are also evaluated based on a small-signal model to provide guidelines for virtual impedance selection. The proposed control varies the equivalent impedance via a feedback loop in CCI's control for accurate power control and sharing in MGs under mismatched feeder impedance. The validity of the proposed virtual-impedance droop control is verified through simulation and experimental results.