Multitime Scale Short-Circuit Fault Control and Protection Cooperative Strategy for MMC-Type DC Distribution Networks

The development of multiterminal dc distribution network based on modular multilevel converters (MMCs) still faces the severe challenge of dc short-circuit fault. For dc pole-to-pole and pole-to-ground short-circuit faults, this article proposes a multitime scale short-circuit fault control and protection cooperative strategy. In terms of MMC fault control, the proposed fault control strategy combining hardware and software suppresses the fault inrush current at 1.5 p.u. and achieves multiple control objectives, including fault current clearance, nonfaulty pole voltage support, and continuous reactive power compensation. The fault control capability of MMC has been significantly expanded. In terms of relay protection, a selective protection scheme based on active harmonic injection of multiterminal MMCs is proposed. According to the orthogonality of different frequency signals and the proposed three-stage protection principle, the scheme is capable of effectively handling metallic and high-resistance faults, and the faulty line can be removed in as fast as 40 ms. The whole proposed strategy is experimentally verified on a 1-kVA MMC with the advantages of fast response time, low communication dependence, and high protection reliability.