Research On Energy Coordination Control Strategy For Flexible HVDC System AC Side Fault Ride-through

High voltage direct current(HVDC)transmission technology is increasingly used in new energy grid-connected,large-capacity long-distance transmission and grid interconnection.Among them,flexible HVDC transmission based on modular multi-level converter(MMC-HVDC)has become a research hotspot due to its unique technical advantages.The paper takes the MMC-HVDC transmission system as the research object,and mainly studies the topology,basic working principle,control strategy and AC side fault ride-through.Firstly,based on the MMC topology,the working principle and operating characteristics of the modular multilevel converter are introduced in detail.The topologies of three typical submodules are respectively carried out.Secondly,from the three aspects of system control,converter station control and converter valve control,the control principle of MMC-HVDC system is expounded,and the direct current controller is designed according to the mathematical model of MMC-HVDC system;Aiming at the shortcomings of direct current controller applied in MMC-HVDC system with high control complexity and large computational complexity,this paper introduces an improved model predictive control and establishes a corresponding predictive model.Taking AC three-phase current symmetry,sub-module capacitor voltage equalization and circulation suppression as the objective function,the optimal switching state problem of MMC is transformed into the optimal solution problem of the objective funetion,and the multi-objective cooperative control of MMC-HVDC system can be realized.Finally,the operating characteristics of AC side unbalanced faults in MMC-HFVDC are analyzed,and the energy flow regularity of the converter during normal and fault periods is revealed.The coupling relationship between the internal power distribution of the converter and the components of the bridge arm current is established.At the same time,the AC side fault ride-through method for directly suppressing negative sequence current is introduced.Aiming at its defects,the paper proposes an AC side fault ride-through control method based on energy balance control.By optimizing and controlling the current components of each bridge arm to adjust the MMC power distribution,and the coordinated control of the AC side three-phase current and the components of the internal circulation in MMC is realized.The simulation model is built by MATLAB/SIMULINK platform.The simulation results show that the proposed control method is compared with the traditional method.The simulation results show that the MMC-HVDC AC side fault traversal method based on energy balance control and the traditional direct suppression of negative sequence current Compared with the method,the three-phase current symmetry can be controlled on the AC side,but the method of directly suppressing the negative sequence current eannot control the internal energy of the converter,and the method based on the energy balance control can realize the MMC AC three-phase current symmetry and internal Synergy of bridge arm energy balance.The research results of the thesis provide a solution for the selection of control methods for flexible direct current transmission under AC-voltage unbalanced conditions,and promote the development of HVDC transmission technology.