Modular Multilevel Converter Technologies for High Voltage Direct Current Applications
Summary
Modular multilevel converters (MMCs) have emerged as the preferred topology for voltage-source high-voltage direct current (HVDC) transmission, owing to their scalability, superior waveform quality and modular design. An MMC comprises strings of submodules, each containing energy-storage capacitors and semiconductor switches, arranged in series to synthesise high-voltage waveforms with low harmonic distortion. Common submodule types include half-bridge and full-bridge arrangements, as well as clamp-based designs, each offering trade-offs between fault ride-through capability, loss performance and complexity. Key control challenges centre on capacitor voltage balancing, suppression of internal circulating currents and high-performance modulation. Strategies such as nearest-level modulation and phase-shifted carrier schemes deliver multilevel outputs while enabling dynamic control of energy distribution. MMC-based HVDC links support multi-terminal networks, offshore wind integration, long-distance bulk power transfer and grid services such as frequency support and black-start capability. Recent advances have enhanced thermal management, refined loss-distribution modelling and developed hybrid submodule mixes to tolerate dc faults without interrupting power flow. These developments underscore the global significance of MMC-HVDC technology for decarbonised grids, interregional interconnections and the efficient integration of renewable generation.
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Recent studies have introduced an accurate loss-distribution methodology for large-scale MMC-HVDC systems, accounting for switching actions across half-bridge, full-bridge and clamp-based submodules during capacitor-voltage balancing. Benchmarking against a ±350 kV/1000 MW HVDC link demonstrated significant improvements in switching-loss prediction and cooling-system design. A comprehensive review of MMC developments has charted progress in modelling, control, reliability and new applications, highlighting solutions for circulating-current suppression, advanced modulation schemes and hybrid submodule configurations to enhance fault-ride-through capability. A systematic survey of submodule topologies and control strategies has compared half-bridge, full-bridge and bi-directional cells, proposing novel fault-diagnosis algorithms and tolerant-control approaches to maintain stable operation under component failure. Together, these works advance the practical deployment of MMC-HVDC technology by improving efficiency, reliability and adaptability to evolving grid requirements.
Modular Multilevel Converter Technologies for High Voltage Direct Current Applications publication trend
The graph below shows the total number of articles in modular multilevel converter technologies for high voltage direct current applications across all publications each year (not limited to Nature Index journals).
Technical terms
Modular Multilevel Converter (MMC): A multilevel power converter composed of series-connected submodules, each containing switches and storage capacitors, used to synthesise high-voltage waveforms.
Submodule (SM): A basic building block of an MMC, typically comprising semiconductor switches and a capacitor, which can be inserted or bypassed to generate voltage levels.
Half-Bridge Submodule (HBSM): A submodule with two switches and one capacitor, offering simple design and lower cost but limited dc fault tolerance.
Full-Bridge Submodule (FBSM): A submodule with four switches and one capacitor, capable of reversing voltage polarity and providing enhanced fault-ride-through.
Capacitor Voltage Balancing: Control methods ensuring equal voltage distribution among submodule capacitors to prevent over- or under-voltage conditions.
Circulating Current: Unwanted internal current circulating between converter arms due to voltage imbalances, requiring suppression to reduce losses and stress.
Nearest-Level Modulation (NLM): A PWM technique that selects the closest submodule voltage level to a reference waveform, reducing switching frequency and losses.
Voltage-Source Converter (VSC): A converter that controls the magnitude and phase of its output voltage to regulate active and reactive power flow.
References
- Loss distribution analysis and accurate calculation method for bulk-power MMC. Protection and Control of Modern Power Systems (2023).
- Modular Multilevel Converters: Recent Achievements and Challenges. IEEE Open Journal of the Industrial Electronics Society (2021).
- Overview on submodule topologies, modeling, modulation, control schemes, fault diagnosis, and tolerant control strategies of modular multilevel converters. Chinese Journal of Electrical Engineering (2020).
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