大规模风电经柔直送出系统交流汇集线路测距式纵联保护方案OA

To overcome the limitations of the existing power-frequency-based AC protection in a large-scale wind power transmission system via MMC-HVDC,where the negative-sequence current suppression strategy adopted by converters at both ends of the AC collection lines weakens the asymmetrical fault characteristics,a pilot protection scheme based on measured distance is proposed,which is suitable for AC collection lines in the large-scale wind power transmission system via MMC-HVDC.First,the misoperation mechanism of the existing distance protection is analyzed.By combin⁃ing the equivalent sequence network diagrams and boundary constraint equations under different fault scenarios,the two-end electrical quantity information and the real-imaginary part separation algorithm are utilized,and an analytical expression for the distance ratio coefficient from the protection installation location on the side of the sending-end con⁃verter station to the fault location is derived when an active negative-sequence current control strategy is adopted at the sending-end converter station.Based on this fault distance ratio coefficient,a highly sensitive protection initiation crite⁃rion is formulated to distinguish the internal and external faults.A simulation model of the two-end transmission system via MMC-HVDC is established on the PSCAD/EMTDC platform,and simulation results indicate that the proposed pro⁃tection scheme can accurately distinguish the internal and external faults.Meanwhile,this scheme is not affected by the amplitude of injected negative-sequence current,line length or transition resistance.