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 and application fields of these different topologies are analysed. This review facilitates the classification and understanding of extended MMCs and provides the necessary scaffold for the development of a general framework for new topologies and applications based on the MMC.

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.

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 low-loss 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 losses and efficiency of a modular multilevel converter (MMC) are strongly linked to the switching frequency, component selection, and the choice of internal control structures such as the circulating current control and sub-module (SM) capacitor voltage balancing algorithm. Equally, the structure of the SM impacts the overall converter operation and the efficiency of the conversion system. However, consideration for the impact of converter operating points on the overall system efficiency is required in order to derive a complete picture. Thus, this paper presents a comprehensive evaluation of the impact of inverter/rectifier operation and power factor on the losses of the different SMs in an MMC with a particular focus on SMs and operational limits for high-voltage DC transmission systems. Considering 27 different SM topologies, it is demonstrated that due to the voltage balancing requirements, the inverter or rectifier operation modes of the converter and the power factor, influence the average losses of SMs and also the maximum and minimum losses within a fundamental period.