Design of virtual impedance controller for parallel-connected converters in a microgrid

Abstract

This paper addresses the significant challenge of inaccurate power sharing among Distributed Generators (DGs) in islanded microgrids, which is primarily caused by mismatched feeder and line impedances. Conventional decentralized control solutions often fail to ensure accurate power sharing, especially when line impedances are resistive. To overcome this, the paper proposes a robust, coordinated Virtual Impedance Control (VIC) strategy for DGs. This method implements fixed virtual resistance and virtual inductance to standardize the output impedance characteristics of parallel-connected inverters, thereby minimizing impedance discrepancies and enhancing system stability through increased damping. The theoretical analysis and design of the VIC were validated through MATLAB/Simulink simulations under a resistive line condition. The results demonstrate a substantial improvement over the traditional control method. During a significant load disturbance (active power changing from 83 kW to 130 kW and reactive power from 6 kVAR to 12 kVAR), the VIC successfully achieved equal reactive power sharing (6 kVAR vs 6 kVAR) and near-perfect active power sharing (65 kW vs 65 kW) between the two inverters. The proposed approach improved transient response comparative analysis, showed a significant reduction in settling time, dropping from 150 ms without VIC to 80 ms with VIC. Critically, the VIC eliminated the frequency variations and voltage drop oscillations observed without the controller, confirming its effectiveness in stabilizing the microgrid and ensuring accurate load distribution