Research On Protection Method Of LCC-MMC Hybrid Three Terminal HVDC Transmission Line

HVDC transmission has more obvious advantages over AC transmission.In order to further improve the technical level of HVDC and solve the problem of commutation failure,LCC-MMC Hybrid Based High Voltage Direct Current transmission system has gradually become a research hotspot.Furthermore,the relay protection of HVDC has become one of the research hotspots.In this paper,LCC-MMC hybrid three-terminal HVDC transmission system is used for research.In order to ensure the safe and reliable operation of the system,the protection method of HVDC transmission line is studied.The specific work is as follows:(1)The working principle and control strategy of LCC-HVDC(Line Commutated Converter)and MMC-HVDC(Modular Multilevel Converter)are thoroughly studied.A LCC-MMC hybrid three-terminal HVDC transmission system model is established in PSCAD.And the simulation proves that the designed model is correct.(2)Based on this model,The differential protection of hybrid three-terminal bipolar HVDC transmission lines based on Hausdorff distance algorithm is studied.The fault component value on both sides of T-zone is used to identify fault direction and the similarity difference between the current waveforms on both sides of the line(quantified by the difference of real-time short window data)is used as the protection criterion to identify internal and external fault as well as unipolar fault and bipolar fault.In addition,the difference between the current sum of the three ends is taken as the criterion of the T-zone bus fault identification.Thus a new protection principle for hybrid three-terminal DC transmission lines is formed.(3)Based on this model,using Hausdorff distance algorithm,the relay protection of hybrid three-terminal bipolar DC transmission line based on longitudinal impedance is studied.The protection method uses the magnitude of current fault components on both sides of T-zone to identify fault direction.Then the magnitude of line impedance value processed by Hausdorff distance algorithm as the criterion to identify internal and external fault as well as unipolar fault and bipolar fault.In addition,the difference of longitudinal impedance on three ends is used as the criterion to identify T-zone bus fault.Therefore,a new protection principle for hybrid three-terminal DC transmission lines is formed.