A Review of Multilevel Converters with Parallel Connectivity

Cascaded-bridge converters (CBCs) and modular multilevel converters (MMCs) enjoy growing popularity mostly due to modularity and scalability. Conventionally, their submodules allow only serial and bypass operation so that the use of low-voltage components for high-voltage treatment becomes possible. Dually, submodule parallelization adds switched-capacitor behaviors to CBCs/MMCs and witnesses an upward trend in recent years. Salient advantages of parallel operation comprise sensorless voltage balancing, capacitance saving, current sharing, and system efficiency optimization. To capture the advancement in the field, this paper reviews state-of-the-art multilevel converters with parallel connectivity, covering various submodules, macro-level circuit topologies, implementation challenges and solutions, as well as control and optimization schemes. In particular, this paper derives and classifies submodules as well as macro-level topologies according to basic H-bridge, asymmetrical half-bridge, and symmetrical half-bridge submodules. On top of that, we introduce strategies for simplification of submodules and create novel topologies yet maintaining parallel connectivity. Moreover, we highlight the relationship between multilevel converters with parallel connectivity and switched capacitor converters. Hybrids of switched-capacitor and CBCs/MMCs enable new applications that are not possible for either of them alone.