Protection scheme for MMC-HVDC DC lines based on boundary current signals

The application of modular multilevel converter (MMC) is attracting more and more attention in flexible DC systems, particularly with the increasing penetration of distributed generators. The development of MMC-based high voltage DC (MMC-HVDC) technology is constrained by the lack of operational experience and appropriate protection schemes. It is necessary to investigate a reliable protection scheme for MMC-HVDC DC line. In this paper, the difference of boundary current signals under internal faults and external faults is firstly analyzed in detail. A definition is introduced to digitize this difference. Furthermore, based on the difference of boundary current signals, a protection strategy for MMC-HVDC DC lines is proposed. Various simulations are carried out in PSCAD/EMTDC on different fault conditions, such as different fault resistances and distances. It is demonstrated that the proposed scheme is effective and is capable of distinguishing faults quickly and accurately.