Study of a virtual-impedance of the capacitor-series inverter for wider accurate power control range

Abstract

A droop control method with virtual-impedance for capacitor-series inverter (CSI) is studied in this paper. Compared with the traditional inverters, CSI has more merits in low-cost reactive power compensation. Therefore, it may a promising solution in a microgrid (MG) to reduce the loss and improve the reactive power compensation, harmonics damping abilities. A control strategy based on the droop-curve is wide-spread in the MG system to achieve inverter’s regulation. However, the inherent flaw of droop control narrowed the accurate power output range to a very small scope, which will influence the application of CSI. The control loop which can emulate the impedance, called virtual impedance, is developed to realize wider precise power transfer. Additionally, the principles of the virtual-impedance are presented in the paper as well, by evaluating the margins of power control, transient performance, and stability of the inverter. Some tests on the experiment prototype are conducted, results indicated the proposed method is effective to extend accurate power control range, and power tracking error can be reduced to 0.5% from 17%.