The demands for massive renewable energy integration, passive network power supply, and global energy interconnection have all gradually increased, posing new challenges for high voltage direct current (HVDC) power transmission systems, including more complex topology and increased diversity of bipolar HVDC transmission. This study proposes that these two factors have led to new requirements for HVDC control strategies. Moreover, due to the divers. The demands for massive renewable energy integration, passive network power supply, and global energy interconnection have all gradually increased, posing new challenges for high voltage direct current (HVDC) power transmission systems, including more complex topology and increased diversity of bipolar HVDC transmission. This study proposes that these two factors have led to new requirements for HVDC control strategies. Moreover, due to the diverse applications of HVDC transmission technology, each station in the system has different requirements. Furthermore, the topology of the AC-DC converter is being continuously developed, revealing a trend towards hybrid converter stations.••Direct current transmission systemTopologyControl strategyAC-DC converterCurrently, the demand for both massive renewable energy integration and passive network power supply is steadily rising. In addition, global energy interconnection has become increasingly popular. As an important solution, high voltage direct current (HVDC) transmission systems can provide favorable access to distributed renewable energy and passive networks. It can also easily achieve asynchronous grid interconnection. Hence, the requirements for HVDC transmission systems are increasing, prompting the need for multi-terminal HVDC (MTDC) transmission systems,,,, whose main task is to design corresponding control strategies for power distribution according to the requirements of each terminal. Thus, in recent years, an increasing number of studies have researched the control strategies of these systems, such as master-slave control, DC voltage droop control, etc.,. However, due to the topological complexity of HVDC transmission systems and the different requirements for AC systems at each terminal, the control strategies are more complex and diverse,, ; therefore, further research is required. The line requirements of the HVDC transmission system are likewise experiencing ongoing development. Because of this, the transmission power of each line is required to have independent controllability. Each DC line of a true bipolar DC transmission system can operate independently, as opposed to the conventional pseudo bipolar DC transmission system. In addition, the AC-DC. 2.1. Pseudo bipolar HVDC transmission systemBoth VSC and LCC can be adopted in the construction of bipolar HVDC transmission systems, whereas bipolar MTDC systems are typically based on the development of bipolar VSC-HVDC technology. Thus, we predominantly discuss bipolar VSC-HVDC transmission systems. The two most common connection modes are pseudo bipolar connections and true bipolar connections. Specifically, the HVDC cables of a pseudo bipolar HVDC transmission system are connected to a converter. Especially, the traditional 2-level or 3-level VSC is grounded through a capacitor as shown in Fig.2, while the MMC VSC does not have to be grounded through a capacitor. In the pseudo bipolar HVDC transmission system, and the earth point is set between the positive and negative lines of the converter, with no direct current flowing through the ground point but returning through the negative cable (Fig. 2). When one of the DC cables fails, the two cables cannot operate normally,.2.2. True bipolar HVDC transmission systemIn the true bipolar HVDC transmission system, each direct current DC cable is connected to an independent converter. As shown in Fig. 3, the earth point of. 3.1. Development of MTDC transmission systemsCurrently, most DC systems are point-to-point terminal DC transmission systems. These systems are used to transport the energy produced by wind farms or traditional thermal power plants to the major AC network. In recent years, a few multi-terminal DC transmission systems have begun operating. By adopting the coordinated control strategy, the HVDC system integrates the requirements of each terminal, maintaining voltage stability, ensuring accurate distribution of power flow, and ultimately achieving flexible and controllable power transmission for the MTDC system. However, with the greater number of terminals and diversification of interconnected AC systems, the system-level control strategy of the DC systems has also become more complicated.3.2. Topologies of the MTDC transmission system(1) Radial-network MTDC systemIn terms of the MTDC transmission system, there are mainly three kinds of topology: radial-network HVDC system, circular-network HVDC systems, and meshed HVDC systems.The radial-network MTDC transmission system uses the power balance station as the radiation center to stabilize the voltage. This center controls the power to each radiatio.
