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 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.

This letter presents a comprehensive fault analysis for a modular multilevel converter high-voltage direct current (MMC-HVDC) system with hybrid half-bridge submodules (HBSMs) and full-bridge SMs (FBSMs) under valve-side single-phase-to-ground (SPG) faults. The analysis shows protection for upper arms is necessary due to the inevitable upper arm overvoltage. It further specifies that the proportion of FBSMs utilized in each arm should be at least 86.6% of the available voltage to safely clear valve-side SPG faults without overvoltage in the lower arms. The accuracy of the theoretical analysis is validated in the positive pole of a bipolar MMC-HVDC system with different ratios of FBSMs modelled in PSCAD/EMTDC.

Numerical methods in power flow (PF) studies for medium-voltage direct current (MVDC) distribution systems require repetitive computations, particularly in scenarios with time-variable facilities that often alter the system operation points. Conventional neural networks (NNs), though efficient in rapid PF calculations, face accuracy challenges with untrained data distributions and varied topology structures. This highlights the need for more robust approaches to improve reliability in diverse scenarios. This paper proposes a physics-informed fully convolutional network (PI-FCN) to address this issue. The architecture of the PI-FCN is enhanced with the inclusion of two additional layers: i) a channel combination layer and ii) a physics operation layer. The former channel combination layer strengthens the model feature extraction capability by converting all input channels constituted by initial PF data matrix into dc voltage, current and line conductance matrix channels. The latter physics operation layer reformulates the combined input channels by physical connections in MVDC systems. The new layers enhance the prediction accuracy and allow generalization of the model. Five multi-terminal MVDC (MT-MVDC) distribution networks with different dc voltage levels and network layouts are used to verify the superiority of proposed PI-FCN compared to other NNs in fixed and varied topology structures.