The unique advantages of modular multilevel converters (MMCs) have led to wide adoption of the converter topology in high voltage dc (HVDC) transmission systems, with great potential for use in medium voltage motor drive and dc grid applications. Inspired by the structure of the MMC, many extended converter topologies have been developed in the literature. These include, in addition to conventional parallel multiphase connection, a variety of series-connected topologies. Extended MMCs integrate the modular nature of cascaded SMs of the MMC but with different topology structure leading to substantially different operating principles. Given the growing number of such topologies, this paper provides a critical review of MMC derived topologies and summarizes individual advantages and disadvantages. The control schemes * : Common characteristics and advantages/disadvantages of the whole category.

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Hybrid modular multilevel converters (HMMC) address the DC-fault blocking limitations of the half-bridge submodules (HB-SMs) of the standard MMC by combining HB-SMs with full-bridge submodules (FB-SMs). In order to improve the overall conversion efficiency, this article proposes two lowloss configurations for the HMMC. The main improvement in the proposed HMMCs is achieved by the combination of low-loss unipolar and low-loss bipolar SMs in the same arm of the HMMC. It is shown that the simplest structure (LLH1) can reduce the SM losses (switching and conduction losses in an SM) by 30% compared to the HMMC at the cost of limited fault blocking capabilities. The fully controlled structure (LLH2) reduces SM losses by 10% compared to the HMMC, and 30% compared to the FBSM-based MMC. These results represent an increase of 20% over the typical HBSM-based MMC for a converter (LLH2) that can provide fault-blocking capabilities. Assessment of efficiency in both HMMCs is provided over a range of operating conditions both in inverter and rectifier mode from an 800-MVA HVDC system model.

The circuit topology of a submodule (SM) in an modular multilevel converter (MMC) defines many of the functionalities of the complete power electronics conversion system and the specific applications that a specific MMC configuration can support. Most prominent among all applications for the MMC is its use in high-voltage direct current (HVDC) transmission systems and multiterminal dc grids. The aim of the paper is to provide a comprehensive review and classification of the many different SM circuit topologies that have been proposed for the MMC up to date. Using an 800-MVA, point-to-point MMC-based HVDC transmission system as a benchmark, the presented analysis identifies the limitations and drawbacks of certain SM configurations that limit their broader adoption as MMC SMs. A hybrid model of an MMC arm and appropriate implementations of voltage-balancing algorithms are used for detailed loss comparison of all SMs and to quantify differences among multiple SMs. The review also provides a comprehensive benchmark among all SM configurations, broad recommendations for the benefits and limitations of different SM topologies which can be further expanded based on the requirements of a specific application, and identifies future opportunities.

Low losses are a major advantage of the modular multilevel converter (MMC) in high-power applications. Losses on the MMC depend on a number of parameters such as switching frequency, voltage balancing algorithm, the topology of the sub-module (SM). This paper investigates the impact of different power factors in the losses of different MMC SMs with a focus on HVDC applications. Considering 27 different SM topologies, we demonstrate that the operation mode of the converter (inverter / rectifier) and the power factor affect not only the average losses but also the minimum and maximum losses within a period due to the voltage balancing requirements.