Avoiding AC/DC grid interaction in MMC based MTDC systems

In this paper, the interaction between ac and dc grids is studied for two types of MMC control structures; conventional and non-conventional control structures. Linear analysis methods that are based mode shapes and participation factors are used to identify dynamic interaction in a hybrid ac-dc power system. The analysis uses as a test system a three terminal Modular Multilevel Converter (MMC) based Multi-terminal High Voltage DC (MTDC) grid connecting three multi-machine asynchronous ac grids. The results show that there is some dynamic interaction between ac and dc grids under conventional MMC converter control structure, while the non-conventional MMC converter control structure decouples the ac and dc side of the MMC converter, thus avoiding interaction between the ac and dc grids.