Regulation Method Of Abnormal DC Current For The Hybrid Cascaded Ultra-High Voltage Direct Current Transmission System

Line commutated converter based High Voltage Direct Current(LCC-HVDC)technology is mature.Modular multilevel converter based high voltage direct current(MMC-HVDC)using fully-controlled devices has no commutation failure(CF)problem and can control active and reactive power separately,of whose application is widespread as well.Compared LCC-HVDC system with the same voltage and capacity level,the investment costs and operating losses in MMC-HVDC are higher.Therefore,the Hybrid LCC/VSC Cascaded HVDC(HC-HVDC)with the advantages of LCC-HVDC and MMC-HVDC has favorable research and engineering application value,in which is the rectifier adopts the LCC,and the inverter adopts one LCC in series with three paralleled MMCs.There is still a lack of research on the analysis of transient phenomena in HC-HVDC systems.Therefore,this paper investigates the abnormal DC current when the AC fault occurs in the inverter station in the HC-HVDC system.Firstly,based on the±800kV Baihetan-Jiangsu hybrid cascaded Ultra-HVDC project under construction,a transient model of the HC-HVDC,which is the subject of this study,is established,and the basic control strategy of the system under steady state is introduced.A three-stage start-up strategy is designed to complete the start-up of the system.These lay the foundation for the following research.Then,when the faults causes the DC voltage of LCC at inverter side to decrease,an unbalanced current between MMCs can be generated.The long-term existence of this unbalanced current will affect the stable operation.More severely,it may generate overcurrent.This paper analyzes the mechanism of the unbalanced current,proposes a balancing control strategy by compensating the active power based on the unbalanced current,and verifies the its effectiveness under AC fault conditions.Finally,in view of the DC overcurrent caused by the AC fault at inverter side in the HC-HVDC system,it is that because the rectifier fails to reduce the DC voltage at the moment of the fault,resulting in the sending and receiving terminal voltage.This paper proposes a DC overcurrent suppression control based on fuzzy clustering and identification approach.The proposed approach can effectively reduce the DC overcurrent of the overall system and improve the fault recovery performance;simultaneously,the approach can reduce the DC overvoltage level of MMC and reduce the investment of arresters,which has good economic and practical value.