A simplified approach to model grid-forming controlled MMCs in power system stability studies

This paper validates a phasor model of a grid-forming controlled Modular Multilevel Converter (MMC) suitable for large AC grid dynamic studies. The adopted MMC model represent with accuracy the energetic exchanges between the AC side, the DC side and the equivalent internal energy inside the MMC (represented by a single state variable). After description of the converter model, the adopted control strategy is presented and discussed. The simplified model of the converter with its control is validated against a detailed electromagnetic transient model in different control modes (active power control and DC voltage control). Finally, the compatibility of the simplified model with large scale AC dynamic simulations under phasor approximations is validated through the simulation of the Nordic 44-bus test system including grid-forming MMCs.