Dual Control Structure of Modular Multilevel Converters for Power System Support

This paper presents an analysis of two philosophies to control the modular multilevel converter (MMC). Most MMC control strategies are based on the philosophy used in the traditional voltage-source converter, in which the dc-bus voltage is controlled through the ac-side power; this approach is referred to here as the classical control approach. Because the MMC has an additional degree of freedom, an alternative control philosophy has recently been proposed, in which the dc-bus voltage is controlled through the dc-side power. In this paper, a comparison of these control approaches is presented and their main differences are described. Considering the increasing requirements imposed by system operators to converter-interfaced systems, guidelines are also given to provide supplementary services such as virtual inertia provision, short-term frequency support, virtual capacitor emulation, and oscillation damping. The provided insights are useful for selecting the appropriate MMC control strategy and for developing new supplementary controls for facilities based on the MMC topology.