Resonance Mitigation of VSG Control under Weak-Grid Connected Microgrid

Power interfaces between the electrical generation sources and the microgrid can be classified as current-controlled inverters (CCIs) and voltage-controlled inverters (VCIs). When the microgrid connects to a weak grid with large grid impedance, resonances can be triggered in grid-connected inverters. This paper analyzes the resonance phenomenon in an autonomous microgird consisting of CCIs and VCIs. The output impedances of a LCL-filter CCI and a virtual synchronous generator (VSG) controlled VCI are modelled to analyze the grid-connected currents quality. Theoretical and simulation results show that the studied VSG control can mitigate the resonances between the CCI and weak grid. Inverters controlled as VSGs can be represented by a voltage source in series with its output impedance, which operates as paralleled impedance or admittance with the other subsystem. Then the impedance interaction is isolated by the studied VSG whose equivalent impedance is resistive in a wide frequency range.