Grid-Forming Droop Control of a Modular Multilevel Converter in Laboratory

Electrical power systems are currently facing a steady increase of converter-based generation and a loss of conventional power plants. In order to compensate the decrease of inertia and to retain grid restoration capability, the converters have to operate in grid-forming mode and act as a voltage source. While the grid-forming concept itself is applicable to many converter types, the challenge for Modular Multilevel Converters (MMC), in which power input and power output are decoupled, is the voltage control of the submodule capacitors. This paper deals with the laboratory implementation of a grid-forming MMC with submodule-integrated as well as DC-bus-connected batteries. The objective was to enhance the established grid-feeding control, which decouples power input and power output of the converter, towards grid-forming control and operation. Therefore, the inner current controllers and outer energy balancing controllers were extended by voltage and PQ droop controllers and virtual impedances. The two main features shown in the test cases were instantaneous active power support and parallel operation of the grid-forming MMC with other voltage sources, i.e. with the grid and with another grid-forming converter. Load switching in islanded mode proved the ability to provide active power instantaneously while maintaining the voltage balance of the submodule capacitors. From these results, important functions of a grid-forming MMC can be derived for future power systems, e.g. instantaneous frequency support and inertia emulation or grid restoration capability.