A Procedure to Design Damping Virtual Impedance on Grid-Forming Voltage Source Converters with LCL Filters

Grid-forming (GFM) converter aims to enforce the voltage and frequency at its terminals in grid-connected and islanded microgrid scenarios. The generalized closed control (GCC) scheme is a well-known solution to achieve this objective, composed of a single-loop voltage control with embedded virtual impedance. Although originally proposed to operate in converters with LC filters at their output terminals, GFM converters that operate with LCL filters have been reported in the literature. The objective of this paper is to analyze the GCC scheme in GFM converters with LCL filters. To this end, the step-by-step design procedure for the damping virtual impedance is presented. In addition, the virtual impedance also allows the design of the voltage controller gains by approximating the plant to be controlled by a first-order system. Three well-known design methodologies for resonant controllers are also compared in the proposed scheme. The results obtained validate the virtual impedance design method and summarize the differences between the design methods for resonant controllers.