Current status and challenges of relay protection technologies in MVDC distribution networks

The medium voltage DC (MVDC) distribution networks have the disadvantages of low inertia and low fault current withstand capability, removing faults through converter blocking and DC breaker operation. Current protection technologies of the MVDC distribution networks are difficult to meet the requirements of both rapidity and selectivity, and facing the challenge of converter blocking and power failure in the event of bipolar short circuit fault: Most of protection technologies cannot remove fault areas before the blocking, and the topology without circuit reactors causes new boundary characteristic, making traveling wave protection technology, which meets the speed requirements in VSC-HVDC, hardly to be applied directly. In this paper, the topology and protection scheme of existing MVDC distribution network are analysed, and the fault characteristics in MVDC distribution networks are pointed out. Finally, the feasibility and limitations of existing protection technologies with respect to rapidity and selectivity are assessed and the key challenges of the MVDC distribution networks are concluded.