Research On Control Of Dual Stator-brushless Boubly Fed Wind Energy Conversion System

From the global perspective,the decline of wind power price is a general trend in the development of the energy industry.Therefore,innovative technologies need to be continuously explored to reduce the initial and operational costs of wind power systems and provide more competitive products.The conventional doubly fed induction machine(DFIM)is one of the most effective solutions for wind energy conversion system(WECS).However,the existing brushes and slip rings seriously impact the reliability of the system.The novel brushless doubly WECS can realize brushless operation while retaining the merits of the conventional doubly fed system.It is an effective solution to the cost pressure and reliability requirement of the wind energy market and has gradually becoming a research hotspot during recent years.However,the structure and working principle of the brushless doubly fed induction machine(BDFIM)are complex,and the corresponding control method is not mature,which is one of the key problems that restrict its development.The existing research on the control technology of the BDFIM is based on the simplification of the BDFIM,in which the BDFIM is simplified as a DFIM.The BDFIM realizes brushless operation and dual-port feeding through special design of stator and rotor.Its structure and electromagnetic characteristics are more complicated than the DFIM,and the simplification in control will lead to potential problems.This dissertation aims to derive and analyze the output-voltage model,current-loop model,power model and virtual power model of the BDFIM,and propose control methods that are suitable for the BDFIM.The research results of this dissertation are as follows:1.The background of this thesis have been explained.The state-of-the-art of WECS in terms of development history,latest development,and development trend have been reviewed.Besides,the development of BDFIMs and the corresponding control strategies have been comprehensively overviewed.Moreover,the potential application prospects of BDFIMs have been discussed.2.The topology and the working principle of the dual stator-brushless doubly fed induction machine(DS-BDFIM)are introduced.The dynamic model considering saturation effect is established for the first time,and a numerical simulation method for the DS-BDFIM is proposed.The main electrical parameters of the DS-BDFIM is experimentally measured through the equivalent-circuit method.The operation mode,cross-coupling characteristic,and magnetizing characteristic of the DS-BDFIM are analyzed.3.The DS-BDFIM based stand-alone WECS is studied,in which the system topology and operation principle are introduced.The output-voltage model is derived for the first time,and the control mechanism is revealed.A controller based on control of the magnetizing current is proposed,which can not only simplify the design of voltage control loop,but also increase the stiffness of the stand-alone control system and reduce the impact of load changes on the output voltage.In addition,the performance of the DS-BDFIM and the effectiveness of the proposed controller are verified by both simulation and experiment.4.The current-loop model of the BDFIM is derived and analyzed,and it is pointed out that the conventional simplified model cannot reveal the characteristic of the real current loop.An improved feedforward current control method based on the complete model of current loop is proposed,which can equivalently cut off the complex feedback branch and simplify the corresponding controller design.The steady-state and dynamic performance of the conventional and the proposed current controllers are comparatively analyzed,which reveals that the improved feedforward current controller has better steady-state and dynamic performance.Experimental tests are carried out which demonstrate the superiority of the proposed current controller.5.The power model of the BDFIM in general reference frame is derived for the first time,based on which a model predictive power control(MPPC)method is proposed.The developed MPPC controller is not restricted to a specific reference frame,which contributes to high flexibility and universality.The influence of rotor circuit is embedded within the predictive power model and being inherently considered in the developed MPPC controller,which contributes to accurate and decoupled power control of the BDFIM.Numerical simulation and experimental tests considering different operation condition are carried out,which demonstrate the effectiveness and superiority of the proposed MPPC controller.6.The virtual power model of the BDFIM is derived for the first time,and the control characteristics of virtual power are analyzed in a specific view.Based on which a model predictive virtual power control(MPVPC)method is proposed for the BDFIM based WECS.The proposed MPVPC controller can unify the control structure between the grid synchronization mode and the grid connected mode,and only the switching of a flux feedback is required to realize the transition between two modes.In addition,fast and smooth grid connection and decoupled control of active power and reactive power can be realized.Numerical simulation and experimental tests are carried out which demonstrate the effectiveness of the proposed MPVPC controller.7.Based on the real-time dSPACE DS1103,the experimental platform of the BDFIM system is built,which lays a solid foundation for verifying the effectiveness of the developed control system.