Autonomous Control of Current- and Voltage-Controlled DG Interface Inverters for Reactive Power Sharing and Harmonics Compensation in Islanded Microgrids

In microgrids, voltage-source inverters interfacing distributed generation units can be operated in voltage- or current-controlled modes (VCMs/CCMs). In this paper, a coordinated control of CCM and VCM units for reactive power sharing and voltage harmonics compensation is proposed. This decentralized control scheme is based on the local measurement of signals. In this way, the need for communication links is removed which results in a simpler and more reliable structure compared to the communication-based control structures. To be more exact, the VCM units contribute to harmonics compensation by using capacitive virtual impedance which can fully compensate the effect of output inductance of the LCL filters. Furthermore, an adaptive virtual admittance regulated based on remaining capacity of the CCM units is implemented for the CCM units. For reactive power sharing, modified droop and reverse droop control methods are used for VCM and CCM units, respectively. The related droop coefficients are set by taking the limited capacity of the inverters and the distorted power into account. An experimental prototype is developed to evaluate the effectiveness of the proposed control scheme. Experimental and simulation studies show that the harmonics compensation is achieved by using only local measurements in the presence of virtual admittance/impedance schemes of CCM/VCM units. Furthermore, it is demonstrated that the reactive power sharing among the CCM and VCM units is obtained based on their remaining capacities.