Comparison of Three-Level Grid-Forming Inverter Topologies for Unbalanced and Nonlinear Load Conditions in Microgrids

Energy production in microgrids is more and more driven by distributed energy sources like photovoltaic, wind or battery systems, replacing diesel generators. For this purpose, inverters are needed that can serve load balancing and reactive power provision. This paper investigates two three-level topologies for grid-forming inverters for such applications with regard to the DC link voltage dimensioning. For the dimensioning of the DC link capacitors, an analytical solution approach for operation at active power is presented. The dimensioning of the capacitors for reactive power operation is determined by a numerical solution in the form of a circuit simulation. The efficiency of the two topologies investigated is compared based on the simulated semiconductor losses. Finally the simulation results of the DC voltage link devi-ation are compared for one operating point with measurement results of an inverter prototype.