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• We propose a multiport hybrid DC circuit breaker (MHCB) for fault isolation. • The MHCB is highly economical and capable of adaptive reclosing. • We describe the operating principle of the MHCB and design its parameters. • We design the reclosing criterion for the MHCB to avoid blind reclosing. • Simulations and comparisons with other schemes show the superiority and of the MHCB. In a high voltage direct current (HVDC) grid, hybrid DC circuit breakers (HCBs) are one of the effective solutions to interrupt fault currents. The conventional two-port HCB features high economic costs since the expensive insulated gate bipolar transistors (IGBTs) are needed. Each line in the multiterminal HVDC grid must be equipped with the HCB, which is extremely costly to implement. In addition, the likelihood of temporary faults is considerably raised by the fact that the HVDC grid transfers power across overhead lines (OHLs). By reclosing operations after temporary faults, the HVDC grid can resume normal operation. However, it is vital to discern between temporary faults and permanent faults before reclosing operation to prevent secondary shocks to the HVDC grid caused by blindly reclosing on permanent faults. For the problems of high cost and blind reclosing of HCBs, this article proposes a multiport HCB (MHCB) with high economy and adaptive reclosing capability. All ports of the MHCB share a single main breaker (MB) for fault isolation. The MHCB utilizes the capacitor voltage to lower the operating voltage of the metal oxide varistor (MOV), which reduces the number of IGBTs connected together at both ends of the MOV. With the above design, the expense of the MHCB is significantly decreased. Additionally, the MHCB achieves adaptive reclosing by using the difference of current amplitudes in the capacitor branch under permanent and temporary faults. [ABSTRACT FROM AUTHOR]

• A novel fast, and discriminative DC grid DC fault detection is proposed. • Unsymmetrical cable-end inductors employed for positive and negative poles. • Main detection index is defined as the difference between poles' DC voltages. • Negative detection index is obtained only for internal faults. • High accuracy using small inductors with high impedance fault detection. DC fault isolation is recognized as the major obstacle against widespread development of multi-terminal meshed DC grids. To fulfill DC grid protection requirements a fast, discriminative, and seamless DC fault detection and isolation mechanism is needed. This paper presents a DC grid protection system based on using asymmetrical pole inductors (APIs) for fault detection, and hybrid DC circuit breakers (HDCCBs) for fault isolation. The pole inductors are those normally used in series with cable-end HDCCBs to limit rate of fault current development. An intentional mismatch between positive and negative poles' inductors is proposed for autonomous fault detection using the difference between positive and negative poles' DC voltage magnitudes measured at all cable-ends. Better accuracy in discriminative symmetrical fault detection, and less sensitivity against fault impedance are achieved using lower series inductors compared to rate of change of voltage (ROCOV) technique as the most effective approach proposed so far. In order to investigate the proposed technique, a 2 GW symmetrical monopole DC grid equipped with proposed protection system is modeled in PSCAD. Simulation results are demonstrated to show the capabilities of the proposed protection system in fast and discriminative DC fault detection. [ABSTRACT FROM AUTHOR]