| In recent years,the development characteristics of large capacity,modular structure,lightweight and so on are shown in the offshore wind power generation system.With the improvement of wind turbine power,the traditional low-voltage two-level converter meets the power output by cascading to form a modular multi-level topology.However,modular multilevel topology of converter has the disadvantages of complex topology,high control complexity and low system reliability.Compared with above topology,the medium voltage multilevel converter system has simple topology and low control complexity,which can be widely used in high-power situations.Therefore,based on the analysis of the topology structure and control means of the high-power permanent magnet wind power generation system composed of the permanent magnet synchronous generator(PMSG)and the medium voltage converter,the following work has been done in the system model establishment and control strategy design.Firstly,the six phase PMSG stator winding structure is analyzed based on the wind power generation system of medium voltage six phase PMSG and distributed diode neutral point clamped(NPC)three level converter.The mathematical model and equivalent circuit of wind turbine,six phase PMSG and NPC three-level converter in dq synchronous rotating coordinate system are established.Secondly,aiming at the unbalance of neutral point potential in DC link of NPC three-level power converter,the space vector pulse width modulation(SVPWM)method and modulation flow are studied in this paper,and the mechanism of neutral point potential imbalance and potential balance control method is analyzed.A control strategy of neutral point potential balance which integrates modulation factor and vector selection is designed.The neutral point potential is controlled by dynamically adjusting the action time of redundant small vectors combined with the selection of composite vector group,and verified the effectiveness of this control method by simulation.Then,according to the design parameters of wind turbine,the power device selection calculation and power loss analysis are carried out,the power device loss calculation model is established.The power loss characteristics and temperature rise of the power device are analyzed through the power loss calculation in different application scenarios.In order to achieve the maximum wind energy capture of high-power permanent magnet wind turbine,a maximum power tracking control method for six phase PMSG and NPC three-level converter wind turbine is designed in this paper.The best torque setting value is calculated according to the measured wind speed,and the double d-q control current loop is established by the feedforward decoupling method.The maximum power tracking of six phase PMSG is realized by the double closed-loop PI control.The grid connected control strategy based on the grid voltage orientation is designed for the grid side converter.This method is used to maintain the stability of DC bus voltage and realize the unit power factor control of output power.Finally,the applicability and feasibility of the control strategy designed in this paper are verified from the overall structure based on the simulation analysis of wind power generation system based on six phase PMSG and distributed dual NPC three-level wind power converter.It provides a theoretical reference for the design and control of high-power permanent magnet wind turbine. |
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