Research On Spoke-Type Vernier Permanent Magnet Direct-Drive Wind Turbine Generator

Developing wind power generation technology is an important way to realize the strategy of "carbon peaking,carbon neutralization".At present,the annual growth rate of China’s onshore wind power installation tends to be flat.The abundant offshore wind energy resources make the offshore wind power development promissing.In order to improve the utilization of offshore wind energy and simplify the wind generator drives structure,the large-scale,direct-drive wind generator have become the main development trend.It prompts academia and industry to continue exploring the direct-drive wind generator with high torque density and high energy conversion efficiency.However,the torque density of conventional machine with single working magnetic field harmonic has approached the upper limit,which is difficult to meet the increasing demand for torque density in largescale direct-drive wind power generation applications.In recent years,a kind of vernier permanent magnet machine based on the principle of magnetic field modulation has attracted extensive attention because of its multi working magnetic field harmonics.According to the comparison of electromagnetic performances such as torque density and overload capacity of machine topologies,spoke-type vernier permanent magnet machine with high magnetic field modulation effect has strong application advantages and potential in large direct-drive wind generator.However,there is a flux barrier effect in spoke-type vernier permanent magnet machine,which greatly weakens the working magnetic field of the machine which limits its performance advantages in direct-drive wind power generation.On the one hand,special physical phenomena such as flux barrier effect are difficult to be mathematically characterized by analytical expressions,resulting in the lack of effective theoretical guidance for the improvement of machine topology.On the other hand,due to the complex electromagnetic structure of this kind of machine,the corresponding relationship between performance index and design parameters is not clear,and there is a lack of clear optimization design criteria for direct-drive wind generator.Under this background,this paper takes the spoke-type vernier permanent magnet wind generator as the research object,and deeply studies the working mechanism,electromagnetic performance analytical calculation,novel topology and optimization design method of this kind of machine.This paper is arranged as follows:Firstly,this paper reviews the state-of-art large-scale wind generator topology,and summarizes the application of magnetic field modulation technology in direct-drive wind generator.Shortcomings of existing research and technology developing trend are combed.According to the torque density and other basic performance comparison results of various machine topologies,the spoke-type vernier permanent magnet machine is selected as the research object of this paper.Then,the multi node magnetic circuit model of spoke-type vernier permanent magnet machine is constructed,and the analytical expressions of air gap magnetic field,no-load back EMF and electromagnetic torque are derived.The results show that the additional magnetic potential fluctuation in this kind of machine significantly weakens the working magnetic field harmonics of the machine.According to the harmonic characteristics of machine magnetomotive force and magnetic conductance,the "lumped magnetic conductance compensation" method of spoke-type vernier permanent magnet machine is proposed.Three new topologies of spoke-type vernier permanent magnet machine,including single bridge,double bridges and double stator,are derived,which weaken and eliminate the additional magnetic potential fluctuation respectively.Machine torque desity largely increases and its application advantages in direct-drive wind power generation is enhanced.Through the performance evaluation of no-load,rated load and overload working condition,the technical advantages of double stator spoke-type vernier permanent magnet wind generator are further excavated.Furthermore,according to the derived analytical expression of electromagnetic performance,the dimensionless size equation of double stator spoke-type vernier permanent magnet wind generator is obtained,the mapping relationship between key structural dimensions,slot pole matching and electromagnetic performance of the machine is established.And,the design method of MW-class double stator spoke-type vernier permanent magnet wind generator is proposed.Based on the method,the value law and optimal value range of key structural parameters are given.In addition,combined with dimensionless analysis and finite element calculation,taking 10 MW power level as an example,and with effective material weight and cost as optimization indexes,this paper carries out the comprehensive design of double stator spoke-type vernier permanent magnet wind generator.Based on the proposed general design method of machine,the electromagnetic design of 3-20 MW double stator spoke-type vernier permanent magnet wind generator is completed.Finally,this paper analyzes the electromagnetic performance of four types of 10 MW direct-drive wind generators: surface mounted permanent magnet synchronous machine,double stator spoke-type vernier permanent magnet machine,superconducting synchronous machine and double stator superconducting vernier reluctance machine.The key performance of the four wind generators,such as torque weight ratio,torque density,torque cost ratio,efficiency and fault operation parameters are compared.Based on the results,the application advantages and potential of double stator spoke-type vernier permanent magnet machine in MW direct-drive wind power generation are verified.