Grid-Connected Technology For Direct-Drive Permanent Magnet Wind Power Generation Systems

With the rapid development of power electronics technology and permanent magnet materials, direct-drive permanent magnet wind power generation systems based on variable pitch control, variable speed operation have become a promising research direction in wind generation technology area due to their superiorities of high reliability, high efficiency, flexible power control ability, etc. This dissertation gives theoretical research on grid-connected technology for direct-drive permanent magnet wind power generation systems. Sensorless control of direct-drive permanent magnet wind generators, modeling and control of grid-connected converters and low voltage ride through (LVRT) are discussed comprehensively. Simulations and experiments demonstrate the correctness of the research results. The main research works are introduced as follows.Firstly, mathematical models of direct-drive permanent magnet wind generators are discussed, which provide theoretical basis for the subsequent chapters. In this section, mathematical models of permanent magnet synchronous machines (PMSMs) in different frames are given; structural and saturation saliencies of PMSMs are studied in detail; principles and characteristics of zero d-axis current control are briefly introduced.Secondly, sensorless related technologies rotor flux linkage position observation and initial position detection of PMSMs are discussed. As to rotor flux linkage position observation aspect, working principle and characteristics of saturation feedback double integrator based on coordinate transform (SFDICT), low pass filter compensation integrator (LPFCI) are analyzed in detail. For SFDICT, performance of SFDICT under different harmonic components and different amplitude limits is firstly presented; error expressions under different amplitude limits are proposed; based on them, small amplitude limit with angle compensation is proposed, which can get better accuracy and quality. For LPFCI, operation characteristics and error features under different harmonic components are presented. Furthermore, advantages and disadvantages of two sensorless methods used in direct-drive permanent magnet wind generators are analyzed and compared. The related simulation results are also given. As to initial position detection aspect, rotor initial position detection methods are studied. Given advantages and disadvantages of the existing methods and load characteristics of direct-drive permanent magnet wind generators, a low-cost easy-implementation rotor initial position detection method is proposed. Its principle and implementation procedures are introduced in detail. Simulation results indicate feasibility and simplicity of the proposed method.Thirdly, modeling and control of grid-connected converters are discussed. Many current regulators implemented in many frames exist in grid-connected converters. An evaluation method using complex vectors is proposed. Complex vector model of grid-side converter is given. Equivalent relationship of stationary PI regulators, stationary PR regulators and synchronous PI regulators among different frames are presented. Using complex vectors, stationary PI regulators, stationary PR regulators, synchronous PI regulators and decoupled synchronous PI regulators are comparative analyzed and evaluated from three aspects of decouple control, reference tracking performance, voltage disturbance rejection performance. The related simulation results are also given. To solve oscillation problem in the parallel of grid-connected converters, the model of n-paralleled grid-connected converters is built. Based on it, oscillation mechanism is studied using complex vectors. To suppress oscillation, a method using low order grid-voltage feed-forward is proposed, feasibility of which is demonstrated by experimental results in wind farm.Fourthly, modeling and control of LVRT are discussed. Based on instantaneous power theory, power relationship among main parts in direct-drive permanent magnet wind power generation systems are given in detail. Furthermore, their behavior during voltage sags is presented. Given the disadvantages of tradition un-loading branches and difficulties of modified algorithms, two LVRT control strategies are proposed. One is the combination of coordinate control of generator-side and grid-side converters and small un-loading branch. The other is the combination of coordinate control of generator-side and grid-side converters and energy storage capacitor. Their principle and implementation procedures are introduced in detail. As to control aspect of grid-side converters, the control method under un-balanced voltage sags is introduced in detail. As to control aspect of generator-side converters, system response properties at the beginning of voltage sags under different generator-side control goals are analyzed comprehensively. Based on the analysis, selection criteria of small un-loading branch and electrolytic capacitor are presented. The related simulation results are also given. Simulation results indicate feasibility of the proposed methods.As to experiment demonstration aspect, main experimental results in the25kW direct-drive permanent magnet wind generator experimental platform, the2MW direct-drive permanent magnet wind generator experimental platform, wind farm experiments are given, which demonstrate the correctness of theoretical research results including error expressions under different amplitude limits for SFDICT, the conclusion of two sensorless control strategies based on SFDICT and LPFCI, the proposed rotor initial position detection method, the proposed low order grid-voltage feed-forward.