Research On Control Technology Of Cascaded Brushless Doubly-fed Wind Power Generation System

With the fast development of wind power generation,the proportion of wind power will become larger and larger.The doubly-fed induction generator(DFIG)is the most widely used in the field of wind power,but due to the brushes,reliability of the DFIG is affected.Brushless doubly-fed induction generator(BDFIG)offers high stability and low maintenance consumption due to the elimination of brushes.It can realize variable-speed and constant-frequency power generation.It is especially suitable for wind power in high-power situations,showing broad application prospects in power generation.However,due to its complex structure and control method,the commercialization of BDFIG is limited to a certain extent.In order to advance the promotion of BDFIG,this thesis takes the cascaded brushless doubly-fed induction generator(CBDFIG)as the research object,and proposes control methods for BDFIG through the analysis of the mathematical model under vector control,maximum power point tracking(MPPT)and unbalanced power grids.The thesis mainly includes the following several parts:1.The research background of the thesis is summarized,different types of wind power systems and the application of BDFIG are introduced,and the current research status of the brushless doubly-fed machines and control strategies are clarified.2.Modeling and analysis of CBDFIG.The basic structure of CBDFIG is presented,and the dynamic mathematical model of CBDFIG is described.Based on the mathematical model,the steady-state equivalent circuit of CBDFIG is studied.At the same time,the internal power flow relationship and operation mode of CBDFIG are analyzed.Finally,the modeling method of CBDFIG is introduced and the dynamic characteristics of CBDFIG are simulated.Through the simulation study of speed adjustment characteristics of CBDFIG,it verifies the correctness of CBDFIG modeling,but also has a deeper understanding of CBDFIG characteristics.3.Vector control of brushless doubly-fed wind power generation system.The vector control is mainly realized by controlling back-to-back converters.The use of back-to-back converters can not only maintain the stability of the DC bus voltage,but also transmit the energy of DC bus fed back to the AC grid,ensuring the safe transmission of the converter’s active power.First,the dynamic modeling of the grid-side converter based on the space vector theory is analyzed,and the vector control technology based on voltage orientation is analyzed through the established mathematical model,which realizes the regulation of the DC bus voltage.Then according to the dynamic model of CBDFIG,the stator flux oriented vector control is analyzed,and the power control of CBDFIG is realized.Finally,simulation and experiments verify that the vector control of the CBDFIG system can realize variable-speed constant-frequency power generation and power control.4.Maximum power point tracking control of brushless doubly-fed wind power generation system with power signal feedback considering loss.The wind turbine model and the CBDFIG model considering the loss are studied,the copper loss of the stator and rotor of the CBDFIG is calculated,the accurate iron loss is obtained through the modified Steinmetz equation and the calculation method of the iron loss is given.The accurate reference power is determined by analyzing the power flow of the generator,and the power signal feedback MPPT control strategy considering the loss is proposed.Finally,simulation and experiments verify that the proposed control method can make the actual speed follow the reference speed,realize the maximum wind energy tracking control,and the power generation system has small output power pulsation and high stability.5.Modeling and control of CBDFIG in unbalanced power grids.The method of extracting power grid phase and the method of extracting positive and negative sequence components of voltage and current are studied.Under unbalanced grid voltage,CBDFIG power and torque will have pulsations,and current will be distorted.In order to alleviate these problems,four control targets are proposed: active,reactive power ripple suppression,balanced power winding current,and sinusoidal control winding current.In order to achieve the control targets,a dual synchronous rotating frame current control method is put forward.To ensure the dynamic performance,a main controller controls the current of the positive sequence synchronous coordinate system,while the auxiliary controller only controls the negative sequence current component.In order to speed up the dynamic response of the auxiliary controller,feedforward compensation is adopted for the negative sequence current control.Finally,simulations and experiments verify that the dual synchronous rotating frame current control strategy can not only suppress the power ripple and current distortion of the CBDFIG system under unbalanced grids,but also speed up the dynamic response of the current controller and quickly achieve the control targets.6.An experimental platform based on d SPACE 1103 is built,and the configration,software advantages and hardware circuits of the experimental platform are introduced.This experimental platform lays a solid foundation for the experiments of stator flux oriented vector control,power signal feedback MPPT control,and CBDFIG control under unbalanced power grids.