Research On DC Short-Circuit Fault Protection Strategy Of MMC-HVDC System Based On Superconducting Fault Current Limiter

Voltage source converter(VSC)based high voltage direct current(HVDC)transmission is playing an increasingly important role in the large-scale AC-DC hybrid power grid due to its flexible and convenient control characteristics.At present,the modular multilevel converter(MMC)composed of a number of modules has been widely used in the HVDC application due to its advantages of high efficiency and easy expansion.The DC short-circuit fault of the HVDC system is a kind of common and serious fault,which will affect the safe operation of the converter,DC power grid and AC power grid.Therefore,under the DC short-circuit fault of the HVDC system,major and urgent national needs and important issues that need urgent research is that how to protect the MMC,clear the short-circuit fault of the DC grid,suppress the impact of the fault on the AC grid,and avoid the local fault from turning into the global fault,and ensure the safe operation of the large-scale AC-DC hybrid grid.Aiming at the characteristics of DC short-circuit fault in HVDC system,a protection strategy of the DC short-circuit fault based on resistive superconducting fault current limiter,double thyristor switch and MMC(RSFCL-DTS-MMC)is proposed,and a new solution of the research on the DC short-circuit fault protection of HVDC system is provided in this paper.The research contents and primary contributions are listed as follows:(1)The topology structure and control method of the HVDC system,and the topology structure,working principle and mathematical model of the MMC are analyzed in detail.In addition,the control strategies of the MMC including the capacitor voltage balancing control,the circulating current suppressing control,and the overall system control are introduced.On this basis,the DC short-circuit fault characteristics of the HVDC system are analyzed in detail,including the DC short-circuit fault characteristics before the MMC is blocked and the DC short-circuit fault characteristics after the MMC is blocked.(2)The typical structure,equivalent circuit and working principle of the RSFCL are deeply studied,and on this basis,an accurate mathematical model of the RSFCL is established,including its thermal model and mathematical model.The coupling relationship between its thermal model and mathematical model is revealed.Aiming at the accurate mathematical model of the RSFCL,the simulation system is built in MATLAB and the accurate mathematical model of RSFCL is simulated to verify the validity and accuracy of this model.The quenching characteristics of the RSFCL under DC current impulse and AC current impulse are respectively studied and revealed.Based on the consistency of the R-Q curve of the RSFCL,the high precision and fast simulation model of the RSFCL is improved and its accuracy and rapidity are verified in MATLAB.The advantages of this improved model are small amount of calculation data and low complexity.Besides,this improved model is highly suitable for power system simulation software PSCAD/EMTDC,which not only ensures the simulation accuracy,but also improves the simulation speed.(3)According to the DC short-circuit fault characteristics of the HVDC system and the quenching characteristics of the RSFCL,the topology structure of the RSFCL-DTS-MMC is proposed,and its working principle is analyzed.On this basis,the protection strategy of the DC short-circuit fault of the RSFCL-DTS-MMC based HVDC system is studied,including its working principle and operation process.The proposed strategy has the advantages of low loss,fast current limiting response,and low cost.In addition,when the strategy clears the DC short-circuit fault,it also effectively limits the fault current of the AC power grid and effectively protects the AC power grid.Aiming at the RSFCL module in the system,the influence of the number of parallel branches n and the length of the superconducting tape of a single branch l on the system is deeply studied,and the optimal configuration strategy of RSFCL parameters is designed.Under this strategy,the objective function of the RSFCL parameter optimization is established,and the RSFCL is optimally configured from three aspects: current limiting effect,safety performance and economic index.Finally,the optimized system is simulated in PSCAD/EMTDC to verify the effectiveness of the proposed RSFCL-DTS-MMC based DC short-circuit fault protection strategy for HVDC systems.