What Are Modular Multilevel Converters?

Modular multilevel converters (MMCs) are a class of high-power, high-voltage power electronic converters in which the conversion function is distributed across a series stack of identical, switchable submodule circuits rather than concentrated in a single large power device. Each submodule contains a small capacitor bank and one or more semiconductor switches, typically insulated-gate bipolar transistors (IGBTs). By inserting or bypassing individual submodules in a controlled sequence, the converter synthesizes a staircase output voltage waveform that closely approximates a sinusoid, achieving low harmonic distortion without large passive filters. The MMC topology was first proposed by Rainer Marquardt in 2001 and entered industrial service in high-voltage direct-current (HVDC) systems around 2010, rapidly displacing earlier two-level and three-level voltage-source converter (VSC) designs.

The structural innovation of the MMC lies in dividing the DC bus voltage among many submodules in series rather than across a single switching stage. This allows each semiconductor device to block only a fraction of the full DC voltage, which means standard IGBT modules rated for a few kilovolts can be stacked to produce converters operating at hundreds of kilovolts. The redundant, modular architecture also provides natural fault tolerance: a failed submodule can be bypassed while the remaining stack continues operation, reducing the risk of full converter outages.

Submodule Topologies

The half-bridge submodule, consisting of two IGBTs and a DC capacitor, is the most widely deployed configuration. It requires the smallest component count per submodule but cannot block reverse DC polarity, which limits its fault-clearing capability in some HVDC applications. The full-bridge submodule adds two more IGBTs to create a four-quadrant cell that can generate negative voltages, enabling DC-side fault blocking without mechanical DC breakers. Hybrid topologies that mix half-bridge and full-bridge cells in a single converter arm offer a compromise between component cost and fault-handling capability. A comprehensive MDPI review of MMC submodule topologies and modulation techniques covers the full taxonomy of cell variants and their comparative performance.

Modulation and Control

Because an MMC arm may contain hundreds of series submodules, the control system must simultaneously regulate the output voltage waveform, the circulating current within each arm, and the capacitor voltage balance across all submodules in a given arm. Phase-disposition pulse-width modulation (PD-PWM) and nearest-level modulation (NLM) are the two dominant approaches. NLM is preferred for high-voltage systems because, at large submodule counts, the natural staircase produced by round-number switching already meets harmonic requirements without carrier-based PWM. Sorting-based capacitor voltage balancing, in which submodules are inserted in an order that drives all capacitor voltages toward a common target, is the standard industrial method. An arm energy-balancing outer control loop ensures that the average stored energy in each of the six converter arms remains equal, preventing slow drift toward saturation.

High-Voltage DC Transmission

MMCs have become the preferred VSC topology for HVDC links because of their low switching losses, low output harmonics, and modular scalability to arbitrary voltage levels. Commercial MMC-HVDC systems operate at bipolar DC voltages of ±320 kV to ±525 kV and power ratings up to several gigawatts. The technology enables power transfer across long submarine cables where AC transmission is impractical due to charging current, and it supports asynchronous grid interconnection because the DC link decouples the AC frequency on each side. IEEE Transactions on Power Electronics coverage of MMC achievements and challenges surveys recent progress in the field. An accessible technical overview of the converter's fundamental operating principles is provided by the MDPI Encyclopedia entry on Modular Multilevel Converters.

Applications

Modular multilevel converters have applications in a range of fields, including:

• Long-distance high-voltage direct-current (HVDC) power transmission
• Static synchronous compensators (STATCOMs) for reactive-power support in AC grids
• Variable-speed drives for large offshore wind turbine generators
• Medium-voltage motor drives in marine propulsion and industrial processes
• Battery energy storage system interfaces requiring bidirectional high-power conversion