Weak Grid Impacts on the Design of Voltage Source Inverters —Virtual Inductance

In this paper, a virtual inductance control scheme is proposed to improve the stability of voltage source inverters (VSIs) in weak grids. The large grid impedance in weak grids can make grid-tied VSIs unstable. To keep VSIs stable in weak grids, control and circuit parameters can be designed to expand the stable operating region of VSIs. In this paper, a closed-loop state-space model is developed for PQ-controlled VSIs with the virtual inductance term in the controller. Using the root locus analysis, impacts of the LCL filter inductance, as well as the virtual inductance feedforward controller on the stability of VSIs are studied. The performance of the virtual inductance is then verified for both symmetrical and asymmetrical grid impedance setups through hardware tests carried out on a three-phase grid-tied VSI in the laboratory.