The hybrid dc circuit breaker (HCB) has the advantages of fast action speed and low operating loss, which is an ideal method for fault isolation of multi-terminal dc grids. For multi-terminal dc grids that transmit power through overhead lines, HCBs are required to have reclosing capability due to the high fault probability and the fact that most of the faults are temporary faults. To avoid the secondary fault strike and equipment damage that may be caused by the reclosing of the HCB when the permanent fault occurs, an adaptive reclosing scheme based on traveling wave injection is proposed in this paper. The scheme injects traveling wave signal into the fault dc line through the additionally configured auxiliary discharge branch in the HCB, and then uses the reflection characteristic of the traveling wave signal on the dc line to identify temporary and permanent faults, to be able to realize fast reclosing when the temporary fault occurs and reliably avoid reclosing after the permanent fault occurs. The test results in the simulation model of the four-terminal dc grid show that the proposed adaptive reclosing scheme can quickly and reliably identify temporary and permanent faults, greatly shorten the power outage time of temporary faults. In addition, it has the advantages of easiness to implement, high reliability, robustness to high-resistance fault and no dead zone, etc.

The current-limiting reactor is currently the most used fault current limiting technique to ensure the fault ride-through of the multi-terminal dc grid. However, its configuration will significantly prolong the fault current clearance time and increase the dissipated energy of dc circuit breakers. Therefore, this paper proposes a cost-effective multiline fault current limiter for multiport application in multi-terminal dc grids, which can effectively limit the rising speed of fault current through the integrated currentlimiting reactors without time delay and significantly reduce their negative impacts on the performance of dc circuit breakers by bypassing the current-limiting reactors through the solid-state switch units during the fault current interruption process. The topology and operation process of the multiline fault current limiter are introduced at first. Then, the economic comparison of the proposed device with the existing solutions is presented. Finally, the feasibility and superiorities of the proposed device are verified by extensive simulations and scaled-down experiments.