DC circuit breakers (DCCBs) are vital for the safe and continuous operation of large high-voltage DC grids. Insulated gate bipolar transistor (IGBT)-based hybrid DCCBs (IHCBs) can interrupt currents quickly and have good operational characteristics. Thyristor-based hybrid DCCBs (THCBs) are superior to IHCBs in terms of interruption capacity because of the large surge current capacity of thyristors. Nevertheless, existing THCBs have limitations regarding one or more of the following characteristics: pre-activation, fast reclosing, quick interruption of small current and no requirement for an additional pre-charge source. This paper proposes a thyristor full-bridge-based DC circuit breaker (TFCB) with all favorable characteristics mentioned above. The TFCB contains a main conductor, a main breaker (MB), an auxiliary branch and an energy absorber. After an interruption, the capacitor in the TFCB can restore its initial voltage quickly to guarantee that the TFCB is available for the next interruption. Quick interruption of small currents is achieved using the additional charge current provided by the auxiliary branch. Pre-activation is achieved using the conduction path in the MB. The parameter designs are analyzed to ensure that the TFCB reliably interrupts currents without damaging itself. The proposed TFCB is validated using prototype experiments and PSCAD/EMTDC simulations.

Accurate fault calculation is vital for the parameter design and protection research of high-voltage DC (HVDC) grids. This paper proposes a general method to calculate short-circuit faults in HVDC grids, and the method is based on the premise that the modular multilevel converter (MMC) blocking and the current interruption have not occurred. The MMC is based on the average value model, and the MMC current source can reflect the impacts of the AC system and MMC control mode. The fault calculations based on variable and constant MMC current sources are presented. The numerical inversion of the Laplace transform is used to obtain the voltages and currents in the time domain. The proposed method has the following merits: 1) it can accurately calculate voltages and currents while considering the fault-generated traveling wave effect and impacts of the AC system and MMC control mode; 2) it is more time efficient than the electromagnetic transient simulation; and 3) it is applicable to HVDC grids with different configurations, such as symmetrical bipolar and asymmetrical monopolar. The proposed method is validated by comparisons with previous calculation methods and PSCAD/EMTDC simulation.

DC circuit breakers (DCBs) are expected to isolate faults in high-voltage DC grids. Multiple DCBs are required for a DC bus connected to multiple lines, and they form a DCB cluster. To reduce the construction cost, the DCB cluster can be integrated into a multiport DCB (MDCB). The integration of proactive-DCBs has been widely researched. However, in previous papers, only a few MDCBs have been proposed to integrate current-injection-DCBs (C-DCBs), and the interruption reliabilities of these MDCBs are reduced because multiple mechanical switches (MSs) should be opened during an interruption. This paper proposes a novel MDCB to integrate C-DCBs. In the proposed MDCB, all ports share one main breaker and one energy absorber. The interruption operations during a port fault and bus fault are detailed, and the backup protection for an MS failure is presented. The parameter designs of the proposed MDCB are discussed. The proposed MDCB is compared with the DCB cluster to show the economic advantage and is compared with the previous MDCB to show the reliability advantage. Finally, the proposed MDCB is validated using PSCAD/EMTDC simulations.