The analysis and calculation of the short-circuit fault current in the DC grid is of great significance to the design and configuration of the converter station and DC circuit breaker parameters. The existing flexible DC system not only includes the modular multilevel converter (MMC) converter, but also needs power equipment such as the fault current limiter and DC circuit breaker. Therefore, the system modeling and short circuit calculation of the multi-terminal DC system after adding the DC circuit breaker are also of great significance for the design of DC power system parameters and the grid troubleshooting ability. In this paper, firstly, the parameters of the four-terminal DC system of the modular multilevel converter (MMC) are simplified, and the analytical solution of the short circuit fault current of the multi-terminal DC system is given. Then, the external characteristics of the cascaded hybrid DC circuit breaker are modeled. Based on the equivalent circuit of the fault current in different stages, the short circuit calculation method of four-terminal MMC system with DC circuit breaker is obtained. This method can effectively describe the overall trend of fault current and provide the basis for the configuration of DC line protection settings and DC circuit breaker related parameters.

Multiport hybrid dc circuit breakers (M-HCBs) have been recognized as suitable devices for protecting dc grids. Most M-HCB topologies include load current switch (LCS), leads to inevitable conduction losses as load current may flow through LCS under no-fault conditions. Only a few multiport current-injection HCB (M-C-HCB) topologies have been proposed in previous articles to remove LCS from load current branch, however, additional power supply or complex additional circuit for repetitive capacitor precharging is required, resulting in increased system cost and reduced reliability. To overcome the main shortcomings mentioned previously, this article proposes a novel M-C-HCB topology, only one capacitor configured with a simple bridge arm circuit is used to realize the fault current communication process from load current branch to main breaker branch, and external precharging solution is required for only once. The LCSs composed of semiconductor devices is removed from the load current branch to achieve minimizing the conduction losses. The working principle is introduced in detail and the parameter designs are discussed. A comparison with some traditional M-HCBs and other M-C-HCB solutions is presented to show the efficiency and economic advantage. Finally, the proposed topology is validated with three-terminal HVdc grid simulations and a scale-down experimental prototype.

Since each branch of the multiterminal DC circuit system relies on the DC circuit breaker for breaking and fault isolation, the prohibitive cost and huge volume of the Hybrid DC Circuit Breaker (HCB) limit its development and broad application in multiterminal flexible DC systems. Multiport hybrid DC circuit breaker (MP-HDCCB) based on device and branch sharing reduces the configuration cost of the circuit breaker to a certain extent. In order to further reduce the cost of MP-HDCCB, a novel MP-HDCCB topology based on hybrid switching devices is proposed, adopting full controlled switching devices to achieve rapidity of breaking fault current, and using semi-controlled switching devices in series to withstand the transient interruption voltage (TIV), so as to reduce the construction cost and technical difficulty. In this paper, the working principle and fault breaking strategy of the topology are introduced in detail, then the parameters of the major circuit are analyzed theoretically, and the parameter design of each branch is given. In the end, the rationality and validity of the proposed topology is tested and verified by simulations and experimental tests.