Modular Multilevel Converter with Stack-Parallel Cascaded H-Bridge Energy Storage Branch

Utility-scale Energy Storage Systems (ESS) provide ancillary services such as primary frequency regulation and load management. The integration of ESS in Modular Multilevel Converters (MMC) has been proposed in recent years, taking advantage of the modularity, reliability, and high efficiency of the topology. This paper discusses the design and control of a modified MMC topology with integrated partially-rated Energy Storage Cascaded H-Bridge branches in parallel with a portion of the MMC stacks, enabling the power flow between the ES branches and the ac or dc sides. The state-space dynamics of the system are detailed, with emphasis to the controller and balancing algorithms that arc required to achieve energy balancing between the split substacks. Power flow scenarios featuring 0.05pu ES contribution are presented, and a power step change is demonstrated. The energy deviation of the stacks and the overall impact on converter ratings are also discussed, mostly in the light of upgrades needed to increase the power capability.