High-voltage direct current (HVDC) transmission systems based on modular multilevel converters (MMCs) are a promising solution for efficient bulk power transmission over long distances. As grid-connected converters, the occurrence of grid-side faults is rather common, leading to imbalances and distortions of the ac-side voltages. Therefore, operation and control of MMC-HVDC systems under grid imbalances becomes of great significance in order to satisfy grid codes and reliability requirements of the system. The aim of this paper is to provide a comprehensive review of operation and control methods applied to MMC-HVDC systems with a specific focus to unbalanced ac grid conditions. The methods are classified based on their main control targets that include ac-side power control, control of the circulating current, and dc-side power ripple suppression control. Special attention is given to the comparison of the different control methods and specific requirements under certain operating conditions that include grid, load or internal imbalances.

High-voltage direct current (HVDC) grids will require the development of dc protections that provide fast fault isolation and minimize the disturbance caused to the existing ac power networks. This paper investigates how the dc fault recovery performance of a half-bridge modular multilevel converter (HB-MMC) is impacted by different dc protection design choices. An HB-MMC point-to-point HVDC system that is protected with dc circuit breakers (CBs) is simulated on a real-time digital simulator using detailed switch models of the converters and switch gear. A dc CB controller has been developed and implemented in a software-in-the-loop fashion, and has been made available free for download. A novel blocking scheme for the HB-MMC is proposed, which limits the prospective dc-side fault current, benefiting dc switch gear. A comparison of circulating current controllers shows that the standard d -q controller is likely to be unsuitable for fault studies. Finally, benchmarking shows that a 48% reduction in power-flow recovery time and a 90% reduction in the energy dissipated in the circuit breaker can be achieved, along with other benefits, depending on the protection design.

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.

This paper develops an alternate arm converter (AAC)-based point-to-point high-voltage direct current (HVDC) transmission system compatible with the existing benchmark models of modular multilevel converter (MMC)-based HVDC and multiterminal dc (MTDC) grids. The developed AAC-based HVDC system expedites the future development of multi-converter, multiterminal benchmark models that enable the system level studies with detailed models of multiple topologies. The configuration and design parameters of the topology, are derived in alignment with the existing MMC-based benchmark systems. Detailed analysis and simulation studies based on real-time digital simulations verify the operation and performance of the developed AAC-based HVDC system and the associated control functions. The model developed in RTDS is also provided as supplementary material in order to enable further research and development on multiple topologies-based dc grids.

The negative-sequence current of a modular multilevel converter (MMC) under unbalanced ac grid conditions increases the voltage ripples of the submodule capacitors. This paper investigates the impact of circulating current control in capacitor voltage ripples under different unbalanced ac grid conditions, while considering multiple control objectives and independent control of active and reactive power. It is demonstrated that higher order harmonic injection in the circulating current through a reference calculation based on instantaneous information improves operation of the MMC reducing capacitor voltage ripples at the cost of increased peak arm current and dc current oscillations. The efficacy of the circulating current control is validated through real-time digital simulations of an 800 MVA benchmark MMC high-voltage direct current transmission system.

The alternate arm converter (AAC) is an emerging fault tolerant multilevel converter topology from the same family of multilevel converters as the modular multilevel converter (MMC). Due to the alternate operation of the converter arms, energy balancing in the AAC is not continuous, but restricted to small time intervals. This paper develops a gradient-based current control and energy balancing method for the AAC. The proposed strategy enforces the dynamic limits on the redundant SMs during the overlap period and allocates effectively the maximum available number of redundant SMs to control the circulating current. The choice of the gradient as the circulating current control parameter improves the energy regulation capability of the AAC and the enforcement of dynamic limitations avoids distortions of the output voltage. Results from an AAC-based HVDC converter model derived from the CIGRE benchmark MMC system demonstrate that the proposed strategy delivers improved energy control and balancing with good harmonic performance compared to existing current control methods for the AAC while also maintaining zero-current switching of the director switches of the AAC arms.