Research On VSCF AC Excitation Doubly Fed Wind Energy Generation System And Its Control Technology

With the lack of general energy source,electric power and the worsening of power supply circumstance problems increasingly,it is provided with very important meanings to partially solve energy lack by using renewable energy source such as wind energy as the one of the most important clear substituted energy source in the future. Comparison with the general wind energy generation system,because of equipped transducer of VSCF AC excitation doubly fed wind energy generation system located at rotor loop,it not only possesses the characteristics of small cubage,light weight and low cost but also realizes flexible connection of mechanical-electric system,which only deals with slip power in double direction. VSCF AC excitation doubly fed wind energy generation system and its control technology is studied in this paper. In this thesis,the difference and relation between the general asynchronous and synchronous motor and DFIG is opened out. The equivalent-circuit model of DFIG is established , and the interrelations of key physical parameters such as power ,electromagnetic torque are discussed. For this basis , issues about operational characteristics,power floating,static operation performance and static state stability are qualitative analyzed. It concludes that the interrelations of control parameters under power control mode of DFIG are complicated,and the coupling is serious. Three kinds of complex-vector models suited for DFIG to analyze and control are proposed for the first time,and the interrelations and applications situation of it are analyzed. Mathematics model of DFIG in the synchronous rotary coordinate is established,and the internal unification and difference between grid-connected and stand-alone DFIG controlled with stator-flux-oriented scheme is analyzed. The key techniques and realization methods of control target,control mode,operational characteristic and system power floating of grid-connected DFIG are mainly studied,and the different disposals of system coupling variables by applying complex-vector model based on Internal Model Control (IMC) theory are also analyzed. It concludes that the essential reason is by using vector-control to realize high performance and obtain decoupling control of active power and reactive power at stator port of DFIG. Operational theory based on maximum wind energy capture of wind turbine is analyzed,and the essential reason for low utilization-ratio of wind energy by using the traditional CSCF generation mode is explained. Mathematics model of wind turbine is established,and grid-connected doubly fed generation system by using back-to-back voltage-source converter with wind turbine model is mainly studied. At the same time,to realize independent control of system active power,reactive power and rotor speed by adopting maximum wind energy capture control is pointed out. Then,how to use separate-excitation DC motor to simulate output characteristic curve of wind turbine in the laboratory condition is supplied. Realization theory,mathematics model and relative control scheme is also given. The influence on impact of grid-connection and how to obtain synchronization condition while the general generator voltage is different from grid voltage is mainly studied. Mathematics model of DFIG no-load grid-connection system is established,and indirect-field-oriented control scheme is studied. Doing excitation before grid-connection then switching control scheme for the sequence of the general grid-connection mode,this paper proposals a direct grid-connection method for the first time. The new sequence is direct grid-connection before excitation so that system can directly operate in nature generation mode without switching control algorithm. It reduces system cost and greatly simplifies the complex degree of system control. The interrelation and the influence on control system performance of position estimation and rotor speed estimation is studied. If we know rotor position we can know rotor speed by all means according to estimation. Changing another thought,this paper presents a rotor speed estimation method of Model Reference Adaptive Control (MRAC) theory rotor-flux-based applying to position estimation system for the first time,design thoughts,stability and improving methods of it are also analyzed. It concludes that rotor speed estimation and position estimation can track the practical value at last,and is provided with definite robust ability but influenced on rotor initialization position. At the same time,it provides a new ideal for such systems with rotor position,namely can get estimated position by estimated rotor speed without estimating rotor position directly. This paper studies several important loops of system experiment. During the course of obtaining rotor position to use gray-code absolute-photoelectrical-encoder,a gray-code data-reference table is presented to simplify programmer design. A new method for rotor initialization position oriented suited for engineering application is presented for the first time. It is easily realized without considering connection mode of stator and rotor windings. To realize function of voltage space vector in rotor side,a mathematics model of connecting inductance to rotor port in rotor-side converter in the synchronous rotary coordinate is established. The structure and simplified condition of control system based on current control mode or speed control mode is mainly analyzed. Operation theory and realization method of stator-flux-angle in engineering based on digital phase-locked-loop (PLL) orreal-time numeration are presented respectively,and applications situation of real-time numeration is analyzed. A small signal model for stability analysis of DIFG system is established,and the influence on system stability caused by rotor position and current loop bandwidth is pointed out. In experiments,several different operational status based on current loop,current control mode and speed control mode of grid-connection and stand-alone in DIFG system sub-synchronous operation are mainly investigated. It concludes that experimental analyses are in accord with analyses in theory,and anticipative purpose is given.