| In recent years,offshore wind power generation has developed rapidly.With the increase of installed capacity and offshore distance of offshore wind farms,the adoption of AC grid-connection and traditional HVDC grid-connection has resulted in high power loss and high cost.The High Voltage Direct Current(HVDC)transmission technology based on the Multilevel Modular Converter(MMC)has the advantages of low harmonic content,no reactive power compensation,no commutation failure,and high controllability.It has gradually become an ideal scheme for large-scale and long-distance offshore wind farms to be connected to the grid.However,MMC converter stations and wind farms have low inertia.Once a fault occurs in the offshore AC system,the fault process will develop rapidly,which seriously threatens the stable operation of MMC-HVDC connected with offshore wind farm.In view of this,this paper studies the fault characteristics of MMC-HVDC connected with offshore wind farm based on the fault of offshore AC system,analyzes and calculates the short-circuit current of the offshore converter station and the offshore wind farm,and designs the fault coordination control strategy for the system to reduce the adverse impact of the fault and enhance the fault ride-through ability of the system.Firstly,this paper analyzes the basic structure and operation control principle of MMC-HVDC connected with offshore wind farm,deduces and designs the operation control strategies of the offshore converter station and the offshore wind farm.Based on the control strategies described above,a simulation model of MMC-HVDC connected with offshore wind farm is built on the PSCAD/EMTDC platform,and the simulation model is tested,analyzed and verified.Then,in view of the three-phase grounding fault of the offshore AC transmission line,this paper carries out transient analysis and short-circuit current calculation of the wind farm side MMC converter station(WFMMC)and the PMSG wind farm respectively.For WFMMC,the generation mechanism and development law of short-circuit current are analyzed firstly,and the bridge arm transient equivalent model of the grid side MMC converter station(GSMMC)is established based on the fault transient characteristics of GSMMC.Then,based on the RLC equivalent circuit model of the fault loop,the calculation method of short-circuit current of WFMMC is proposed,which takes into account the dynamic switching process of the bridge arm submodules.Finally,the correctness of the transient analysis and calculation method is verified by simulation.For the PMSG wind farm,firstly,based on the control strategy of PMSG,the analytical expression of short-circuit current of PMSG wind farm is obtained through theoretical derivation.Then,combined with the simulation results,the transient characteristics of PMSG wind farm are analyzed and the correctness of the analytical expression of short-circuit current is verified.In addition,the transient characteristics of the system under different operating conditions and parameters are compared and analyzed,and the key factors affecting the system transient are summarized.Finally,this paper analyzes the operating characteristics of PMSG and WFMMC under the offshore AC fault.Based on this,the fault coordination control strategy is designed for the system.The strategy is mainly characterized by the constant current control of WFMMC and the machine side load reduction control of PMSG.In order to make the machine side load reduction more quickly,the control strategy of machine side load reduction is improved based on current compensation.The improved strategy can not only suppress the DC bus overvoltage,but also quickly adjust the machine side q-axis current,so as to realize the rapid load reduction of the wind turbine.The simulation results show that the proposed fault coordination control strategy can effectively improve the fault operation characteristics of the system and enhance the fault ride-through ability of the system. |
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