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 (VSIs) interfacing Distributed Generation (DG) units can be operated in Voltage or Current Controlled Modes (VCM/CCM). 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 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.