Control Method and Operational Impact of Decoupling AC and DC Powers during a Frequency Event in the Modular Multilevel Converter Using Internal Energy Storage

This paper presents a control scheme intended to manage the embedded energy storage capabilities of Modular Multilevel Converters (MMC) in order to facilitate frequency support. We expand upon existing research performed on Grid Forming algorithms for MMCs by allowing a tunable, temporary, disconnect between the AC and DC power through the use of energy stored within the Sub-Modules. This disconnect acts as a power buffer between the AC and DC networks to prevent propagation of faults and prepare a possible coordinated response from the other actors of the High Voltage Direct Current network. This buffer function causes the Sub-Module energy to temporarily drain in order to provide a rapid increase in power export from the converter, followed by a corresponding increase in power received across the DC link. In this study, we explore the limits of synchronising power that an MMC can provide for a given set of initial parameters by examining the stack current, stack voltage and stored energy; concluding that a standard MMC can act as a power buffer for a duration in the cycle time-range but potentially for longer, depending on embedded energy storage and frequency dips.