A Review of Multilevel Converters With Parallel Connectivity

Abstract

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 lowvoltage components for high-voltage output becomes possible. Dually, submodule parallelization adds switched-capacitor behavior to CBCs/MMCs and has witnessed 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 article 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 article 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, this article introduces strategies for the simplification of submodules and creation of novel topologies yet maintaining parallel connectivity. We highlight the role of graph theory on creating new analytic and synthetic methodologies for multilevel converters. In addition, this article discloses the relationship between multilevel converters with parallel connectivity and switched capacitor converters.

Unified Graph Theory-Based Modeling and Control Methodology of Lattice Converters

“…This section provides the simulation and experimental results of lattice converters for verification purposes. control efforts [15]. Fortunately, lattice converters allow parallel connectivity, and hence hardware-based dynamic voltage balancing.…”

“…Notably, the set of edges (i.e., E x ) contain the node pairs, in which the source node and destination node appear complimentarily and sequentially. On top of this, we define a path as an ordered sequence of edges that links two nodes [15].…”

“…The balancing of capacitor voltage (or state-of-the charges for batteries) serves as one major control objective of multilevel converters [26,28]. For multilevel converters with parallel connectivity or switched-capacitor converters, hardware-based voltage balancing can be achieved by parallelization of submodules, leading to the great simplification of control efforts [15]. Fortunately, lattice converters allow parallel connectivity, and hence hardware-based dynamic voltage balancing.…”

“…Electronics 2021, 10, x 11 of 16 control efforts [15]. Fortunately, lattice converters allow parallel connectivity, and hence hardware-based dynamic voltage balancing.…”

“…Since modular multilevel converters (MMCs) utilize cascaded-bridge converters as their arms, MMC arms inherently enjoy modularity and scalability [14]. However, from a macro-level point of view, MMCs follow conventional two-level converters, thereby losing modularity and scalability when each MMC arm is treated as a simple component [15]. However, the serial connection of submodules enables only voltage sharing rather than current sharing, and thus each submodule has to handle the entire load current.…”