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.

The advantages of HVDC transmission in the current electricity grid environment have sparked an increased interest on HVDC system studies. A common challenge is the proper comparison and evaluation of different systems and control methods. Benchmark models facilitate this approach as they provide a common platform where such comparisons can be completed and independently validated. This paper provides a brief overview of key technologies for HVDC systems, such as converter, station and system configurations. This summary provides the necessary context for the review of current HVDC benchmark models available in the existing literature. The analysis and discussion based on the review identifies the current research gaps and opportunities for development of HVDC benchmark models, the importance of openly available data across all benchmark models covering a wide range of simulations and applications as well as the potential for development of extended models to facilitate large-scale network analysis and dc-grids.

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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.