Half‐bridge modular multi‐level converter‐based multi‐terminal HVDC (HB‐MMC‐MTDC) technology is increasingly adopted as a promising option in the power transmission and distribution fields. However, how to cut off DC fault currents safely and ensure the continuous operation of the healthy parts of the HB‐MMC‐MTDC system under DC faults presents a huge challenge. This study proposes a generalised protection strategy on the use of a new RL‐fault current limiter (FCL) branch and hybrid DC circuit breakers (CBs), to deal with the bipolar DC faults for a given HB‐MMC‐MTDC system. It contributes to cut off the DC fault current before triggering MMC blocking, without influencing the continuous operation of the healthy parts of MTDC system. The operation characteristics of hybrid DC CBs and the equivalent circuit for the DC fault current is introduced, and the mathematical model of the DC fault current with the new RL‐FCL is discussed in detail. Theoretical and data analyses are performed to obtain the feasible parameters of the RL‐FCL. Simulation tests of an HB‐MMC‐MTDC system with different topologies are performed in PSCAD/EMTDC to validate the effectiveness of the generalised protection strategy.

With the development of power electronics technology, numerous studies have been conducted on voltage‐source converter‐based high‐voltage DC (VSC‐HVDC) transmission technology and low‐voltage DC micro‐grids. However, DC power distribution networks are more sophisticated than HVDC transmission networks with respect to the complex topology, changeable operation modes and use of diverse power electronic devices. Research on voltage‐source converter‐based medium‐voltage DC (VSC‐MVDC) distribution networks is still in its infancy stage, and whether or not some of the existing control and protection schemes used in VSC‐HVDC can be applicable to the distribution networks remains a question. In this study, a dynamic simulation platform with DC circuit breakers is initially established. This platform is based on the first dual‐terminal VSC‐MVDC distribution network constructed in China. Second, a complete overall control scheme is proposed for VSC‐MVDC distribution networks. The control scheme is proposed with an array of control strategies, including controls for system startup/shutdown, voltage coordination, operation mode transition, single‐station integration, system fault isolation, and recovery. Finally, the proposed scheme is validated through experiments using the dynamic simulation platform. Experimental results indicate the effectiveness of the proposed overall control scheme for VSC‐MVDC power distribution networks. Thus, the proposed scheme can be used in the design and operation of future VSC‐MVDC distribution networks.

The DC distribution technology brings challenges and opportunities to the penetration of large‐scale distributed renewable energy sources. In a medium‐voltage DC (MVDC) distribution network, operation mode switch occurs owing to some large disturbances. To manage these disturbances and achieve seamless switch of the operation mode, a hierarchical control strategy with two layers, namely local and global layers, is proposed in this work. The novel local layer controller called P–U–I controller is designed to enhance the voltage stability, improve system controllability and suppress overcurrent under large disturbances. This controller only needs local information and does not rely on fast communication. Furthermore, as the scale of the DC distribution network expands continuously, the number of system operation mode increases in geometric progression. Therefore, a global layer controller based on breadth‐first search algorithm is proposed. This controller can automatically identify the system topology and adjust the control mode of converters to optimise the system operating characteristics. Thus, this hierarchical control strategy can accurately control the system power in steady state, suppress overcurrent under large disturbances and suit large‐scale DC distribution networks. Finally, a three‐terminal MVDC distribution network simulation model established on power systems computer‐aided design and RT‐LAB validates the proposed strategy.

With the development and application of voltage source converter (VSC) high voltage direct current (HVDC) technology, DC distribution is gradually attracting the attention of researchers in recent years. However, many studies are still in the theoretical and exploratory stages. Compared with DC transmission networks, medium‐voltage DC (MVDC) distribution networks are more sophisticated with respect to the complex operation modes, high penetration of renewable energy resource, and use of diverse power electronic devices. Therefore, master–slave control strategy which has been widely used in HVDC transmission projects may not be the first choice for DC distribution networks. In this study, a three‐terminal DC distribution dynamic simulation platform is initially established and droop control strategy is adopted due to its flexible scalability. Secondly, plug and play solutions for key equipment such as VSCs and DC solid‐state transformers are proposed. Thirdly, the response characteristics of a droop control system under high renewable energy resource penetration and frequent transition of operation mode are studied. Fourthly, the system fault isolation and recovery strategy under DC pole‐to‐pole fault is proposed. Finally, the whole control scheme is validated through experiments using the dynamic simulation platform. In the future, this proposed scheme can be used in the control of VSC‐based MVDC distribution networks.

The bridge arms and DC voltage of China’s Four-Terminal Flexible DC Transmission Project exhibit persistent high-frequency harmonics over the medium to long term, causing issues such as overheating losses and electromagnetic interference within the converter stations. To address this issue, this paper first introduces the structure of the Four-Terminal Flexible DC Grid and the high-frequency harmonic characteristics on the DC side, clarifying the impact of control cycles on the harmonic distribution at converter stations. Through analysis of the modulating wave, it is demonstrated that the sideband harmonics originate from the coupling effect between the control cycle and the modulating wave, inducing high-frequency sideband harmonics on the bridge arm. A discrete switching equation for bridge arm voltage was established. Based on double Fourier decomposition, a mathematical model for sideband harmonics was derived, and the flow direction of these harmonics was analyzed. A four-terminal flexible DC system was constructed using PSCAD electromagnetic transient simulation, yielding harmonic distributions in the arm and DC-side sidebands. This validated the accuracy of theoretical analysis and ultimately identified the factors influencing sideband harmonics.

With the rapid deployment of flexible high-voltage direct-current (HVDC) systems, high-power press-pack insulated-gate bipolar transistors (PP-IGBTs) have become critical components. However, field data indicate premature overcurrent failures in PP-IGBTs under modular multilevel converter (MMC) operations. This study systematically investigates the short-term overcurrent failure mechanisms of PP-IGBTs under MMC-specific operational stresses, distinguishing them from conventional short-circuit or avalanche modes. A power cycling test platform emulating MMC submodule conditions was developed to capture the evolution of terminal characteristics. Furthermore, a double-pulse test platform was established to reproduce failure locations under service-relevant stresses. The study reveals a distinctive failure mode characterized by pronounced gate leakage and the eventual loss of normal gate control, leading to functional open-circuit failure—a phenomenon that has been rarely reported in the existing MMC literature. The experimental results indicate a decreasing trend in short-term overcurrent withstand capability with increasing clamping force and reveal a preliminary relationship between peak current and measured emitter-side temperature at failure. These findings provide insights into the failure mechanisms of PP-IGBTs and offer practical guidance for device selection, converter design, and reliability assessment.