The voltage balancing of the floating capacitors in modular multilevel converter (MMC) is of great importance for the safe operation. However, the existing voltage balancing algorithms usually suffer from high switching frequency, leading to unnecessary switching losses and device costs. This paper proposes an adaptive capacitor voltage balancing method for high-power MMCs with reduced switching frequency. The operational characteristics and switching process of the sub-modules (SMs) are analyzed in detail. And then the relationship between the switching frequency and the capacitor voltage deviation is derived. Based on the analysis, an adaptive method is proposed to make a trade-off between the switching losses and the balancing effect through a closed-loop control of the alternating number of the SMs. The calculation formulae of the switching frequency are also presented. PSCAD/EMTDC simulation and RT-LAB based real-time control hardware in the loop (HIL) test results demonstrate the effectiveness of the proposed method under different operating conditions.

Compared with conventional offshore wind farm with ac collection and high-voltage dc (HVDC) transmission system, the offshore wind farm with all dc system shows advantages with characteristics of higher power density, lower costs, and higher efficiency in many analyses. This letter proposed a novel all dc system for the offshore wind farm collection and integration. The presented dc system is composed of several medium-voltage (MV) dc collection systems and a dc transmission system. A novel modular isolated dc/dc converter (MIDC) topology is proposed for the MV dc collection system to raise the medium dc voltage to a higher level, while realizing the maximum power point tracking (MPPT) of the connected wind generators. Thorough comparisons of three dc/dc converter candidates are carried out, and the LLC resonant converter is found to be the optimal submodule (SM) of the MIDC. Furthermore, a two-layer control system for the MIDC is developed to obtain current and voltage sharing while achieving the MPPT of the wind generators in the meantime. A time-domain simulation model consisting of 160 SMs is developed using PSCAD, and a prototype with nine SMs is built for the experiments. The feasibility of the proposed pure dc system is verified by both the simulation and experimental results.

The missing capability of DC fault current limitation is an inherent disadvantage in the typical half-bridge modular multilevel converter (MMC), which restricts its application in long-distance high voltage direct current (HVDC) systems. The full-bridge submodule (FBSM) and clamped double submodule (CDSM) topologies allow for fault current self-clearing, but also suffer from additional hardware cost and power losses. In this paper, an improved submodule topology utilizing the new reverse-blocking insulated gate bipolar transistor (RB-IGBT) is proposed to improve system economy without compromising its performance. The operating principles and DC fault characteristics of the proposed topology are analyzed in details, including comparison with various topologies in terms of cost, losses, fault current clearing speed and thermal. Compared with the traditional CDSM topology, the proposed RBSM can clear DC faults faster with reduced cost of switching devices. A DC fault protection strategy for MMC-HVDC system is also presented. Simulation results under normal operating, permanent and transient fault conditions are carried out to verify the effectiveness of the proposed topology.

With the expansion of offshore wind farms (WFs), the impedance of WFs exhibits increased negative damping and stronger impedance coupling with the modular multilevel converter (MMC). These effects result in sharper resonance peaks in the impedance of WFs, exacerbating oscillation problems. Existing oscillation suppression methods to improve the damping of individual converters are limited in effectiveness for systems with deep interaction. To address this issue, this article reveals that the negative-sequence impedance of the MMC is a crucial factor in its interaction with WFs. Based on this insight, a control method is proposed to suppress impedance coupling by blocking interaction paths. This method enables more targeted and effective improvements in system stability, especially within strong interaction frequency bands. The effectiveness of the proposed control method is validated through comprehensive simulations and experiments.

This study develops a method to decompose and quantify the fault current limiting ability of fault current limiting devices (FCLDs) and calculate the amplitude and the energy limiting contribution of FCLDs in dc grids. The coupling of multi-type FCLDs and the mechanism of fault current limiting of modular multilevel converter (MMC) are studied by analyzing the $\lambda $ – $i$ flux circuit in a single-terminal system. Based on this analysis, the fault current limiting contributions of FCLDs are obtained by decomposing the fault current limiting ability. Furthermore, a method that can transform dc grids into the superposition of several single-terminal systems is proposed. The decoupling between FCLDs and the decoupling between MMCs are realized in the dc grid, and the fault current limiting contribution of FCLDs on any MMCs’ fault current in the dc grid can be calculated. The proposed method is validated on a scaled-down single-terminal system in the laboratory and on a simulation model based on Zhangbei dc grid in PSCAD/EMTDC. An application that optimizes FCLD parameters economically through contribution analysis is proposed to illustrate the significance of this study.