Research On Maximum Power Point Tracking Technology Of Permanent Magnet Direct Drive Wind Turbine

New energy power generation technology has gradually become the focus of research in various countries.As a kind of green and renewable energy,wind energy is widely valued,and is gradually becoming one of the main forms of new energy utilization in the world.The rotating shaft of the direct drive permanent magnet synchronous generator(D-PMSG)is directly connected with the motor rotor,eliminating the intermediate gearbox,which can save some operation and maintenance costs compared with the doubly fed wind turbine.In the early days of D-PMSG wind power system,the rectifier of grid connected converter mostly used diode rectifier circuit with simple structure,which only had general discharge and start stop control functions,and could not implement maximum power point tracking(MPPT)control,resulting in low wind energy conversion rate.With the development of power electronic technology,D-PMSG grid connected converter gradually adopts full control rectifier,which provides conditions for MPPT control.In order to improve the wind energy utilization of D-PMSG,it is of great significance to study its MPPT control technology.Aiming at the problem that the output power of wind power system is difficult to reach the maximum power point(MPP)stably and rapidly,this paper studies the MPPT control method of D-PMSG.Firstly,it introduces the technical background of wind power generation and the research status of MPPT at home and abroad.The theoretical basis of wind power generation is discussed,and the mathematical model of D-PMSG is given through the analysis of its working principle.The topology of the common D-PMSG wind power system is studied.The principle and characteristics of various chopper circuits are analyzed in detail.Three phase uncontrollable diode bridge and boost circuit are selected as the rectifying link of grid connected converter.It lays a foundation for the research of MPPT control method and simulation.Secondly,the main methods of MPPT are studied.Based on the analysis of MPPT control principle and regulation mode,three MPPT methods,i.e.optimal tip speed ratio method,hill-climbing search algorithm and power feedback method,are compared and analyzed.The main advantage of mountain climbing search algorithm is that its MPPT implementation results will not change due to the sudden change of natural wind speed and the parameters of D-PMSG.However,the traditional fixed step climbing method can not take into account the steady-state and dynamic characteristics of MPPT control process.Then,in order to improve the MPPT tracking speed and stability of the traditional fixed step and variable step climbing search algorithm,a gradient variable step climbing method based on the optimal gradient is adopted.According to the relationship between the output power of D-PMSG and the duty cycle of boost circuit,the principle of realizing MPPT with this topology is deduced.Based on the theoretical analysis,the simulation analysis is carried out.The results show that compared with the traditional hill-climbing search algorithm,the gradient variable step size hill-climbing algorithm adopted in this paper has significant advantages in tracking speed and stability.Finally,in view of the shortcomings of gradient variable step climbing algorithm in MPP of D-PMSG wind power system,a fuzzy control method is adopted to realize MPPT of D-PMSG.In this paper,the fuzzy control strategy is used to realize the MPPT of D-PMSG.Based on the basic principle of MPPT and fuzzy control theory of wind power system,a fuzzy controller suitable for D-PMSG wind motor is designed according to the expected control effect reasoning fuzzy control rules.The fuzzy controller takes the difference between the duty cycle of the boost circuit in this cycle and the last cycle,the difference between the output power as the input,and the duty cycle of the boost circuit in the next cycle as the output.The MPPT control simulation platform of D-PMSG wind power system is built,and the MPPT control results using fuzzy control and gradient variable step climbing search algorithm are compared.It is found that the fuzzy control has better stability and tracking speed in MPPT.