Research On Control Strategy Of Medium Voltage Three Level Network Side Converter For High Power Permanent Magnet Wind Turbine

At present,the wind power industry is developing towards the trend of large capacity,modular structure and lightweight.In the offshore wind power generation system,high-power direct-drive wind turbines have gradually become the mainstream model.Among them,the medium-voltage multi-level converter system has gradually replaced the traditional low-voltage two-level converter as the best choice of high-power wind power converter system because of its advantages of low loss,high efficiency and easy installation and control.When grid voltage sag/surge fault occurs to large capacity wind turbines,it will be more difficult to restore the grid fault if the traditional natural shedding control mode is still adopted.Therefore,based on the analysis of the control strategy of high-power permanent magnet wind power system,this paper carries out related research on the low/high voltage crossing technology of medium-voltage three-level wind power system:Firstly,the topological structure of Neutral Point Clamped(NPC)three-level converters is analyzed,and mathematical models of wind turbine,PMSG and medium-voltage three-level NPC converters are established.On this basis,the zero-D-axis current control strategy on the machine side and the grid voltage directional vector control strategy on the grid side are designed,and the stability of the proposed control strategy under normal operating conditions is verified by simulation.Secondly,according to the equivalent circuit of the NPC three-level converter mathematical model,Space Vector Pulse Width Modulation(SVPWM)method is used as the Modulation algorithm of the wind power converter in this paper.On this basis,the analysis of NPC three-level inverter reason and harm of the neutral voltage imbalance,and put forward a kind of neutral voltage control method based on redundant small vector,namely through improved redundancy small vector effect time to achieve the purpose of inhibiting neutral voltage,then the control method is validated by computer simulation under different modulation conditions is effective.Then,the grid-connected fault crossing problem of the medium-voltage three-level wind power system is studied,and the mechanism and characteristics of the voltage instability on the DC side of the full-power converter under fault crossing are analyzed.In order to solve the problem of voltage drop,the corresponding solutions were proposed from three aspects:Crowbar circuit on the DC side,power limit of the converter on the machine side and reactive compensation of the converter on the network side.Finally,a joint control method based on the unloading circuit and improved reactive compensation was designed to realize the safe and stable low voltage crossing of the wind turbine.In this paper,the controllability of the grid-side converter is analyzed,and a high voltage crossing strategy of active control of the DC voltage and dynamic reactive power output is proposed to ensure the safety of the DC side and the control requirements of the grid-side converter during the fault period,so that the wind turbine can achieve high voltage crossing smoothly.Finally,combined with the midpoint potential control strategy,the low/high voltage crossing technology method designed in this paper is simulated and studied,which verifies that the proposed scheme can realize the safe and stable operation of wind turbine during grid-connection fault.