Modeling and Control of a Hybrid Modular Multilevel Converter for High-AC/Low-DC Medium-Voltage Applications

Hybrid modular multilevel converters (HMMC) have emerged recently as an improvement over conventional MMCs due to their lower submodule capacitor sizing, smaller semiconductor count, and lower losses. HMMCs can achieve such advantages, while still maintaining modularity due to their intelligent use of line-frequency-switching medium-voltage switches in addition to the high-frequency low-voltage switch-based submodules. HMMC open-loop operation and performance are well elaborated in literature, but HMMC closed-loop submodule capacitor (SM-C) voltage control is not yet well demonstrated. Closed-loop balancing for HMMC SM-C is especially challenging as the operating mode changes every half-line cycle, making a unified continuous model and subsequent control development challenging. Furthermore, some HMMCs do not have a circulating current path to exchange energy among different arms/phases. This poses an additional challenge for energy transfer between HMMC SM-C. This article resolves these challenges by first proposing a converter model and then developing new control ideas. Novel methods to balance SM-C using midpoint-voltage and interphase circulating-currents are developed for HMMCs. Additionally, the model is utilized to explain power flows, elaborating on the fundamentals behind much lower SM-C size in HMMC compared to MMCs for the same operation. The developed model and controls are verified extensively on a medium-voltage 20 kW laboratory prototype.