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AC-excited Doubly-fed Induction Generator Wind Power Generation For Grid Connection

Posted on:2009-11-14Degree:DoctorType:Dissertation
Country:ChinaCandidate:Y F RenFull Text:PDF
GTID:1102360275471253Subject:Power Machinery and Engineering
Abstract/Summary:PDF Full Text Request
With the gradual exhaustion of the fossil fuel and serious environment pollution, sustainable energy plays an important role in distributed generation, and its percentage in the energy supply system will be ever increasing. The exploitation and utilization of renewable energy is inevitable choice for solving energy and environment issue. Wind energy, as one kind of clean, renewable energy, has a promising future of being developed and used in large scale. Grid connected wind power generation has become main technology because of compensation and sustentation from large power grid. The AC-excited doubly-fed induction generator (DFIG) has recently become the most widely used wind turbines for variable speed constant frequency (VSCF) wind power generation systems in industry, since it presents noticeable advantages such as: the variable speed generation, the decoupled control of active and reactive power, the reduction of mechanical stresses and acoustic noise, the improvement of power quality, and the use of a power converter with a rated power of about 30% of total system power.The key technology of AC-excited VSCF DFIG wind power generation system has been deeply studied in this dissertation. This thesis covers the operation principle analysis, control strategy realization, dynamic modeling and simulation of wind farm connected to grid for DFIG driven by a variable speed wind turbine. Main contents include as follows:1. The operation principle of AC-excited VSCF DFIG is introduced. Using the coordinate transformation theory, the application of power and turns invariance are discussed respectively. The dynamic machine model is derived simply in three-phase stationary frame, and the mathematical model of DFIG is derived according to generator and electromotor convention in d-q synchronous rotating reference frame in details. The work is important for other researchers, particularly for those who do computer simulation study of DFIG.2. Wind turbine drive train model is built based on mechanical power extracted from the wind, transfer and maximum power point tracking (MPPT) control algorithm. The response characteristics of an 850kw VSCF DFIG wind power generation system are simulated during variations of rotating speed. The MPPT control and excellent dynamic response is demonstrated by the simulation results. 3. There are different operation states and control strategies of AC-excited VSCF DFIG wind power generation system before and after grid connected. The rotor converter adopts no-load connected grid control strategy and no-load run before cutting-in, maximum wind energy tracking control and generator run after cutting-in. The VSCF DFIG wind power generation is transient response proceeding, which is influenced by operation characteristics of generator before and after grid connected. A novel S-Function is adopted to describe the mathematic model of DFIG based on state equation. A complete simulation model of VSCF DFIG wind power generation system based on MATLAB/Simulink and SimPowerSystems is established, which can be used to not only simulate subsystems of no-load operation and generation operation, as well as can be used to but also accurate control parameters of DFIG in simulation. The simulation model have excellent transportability, and can be used DFIG of different power rated if only design relevant controller. At the same time, simulation confirms the effectiveness of the proposed dynamic model during variations of rotating speed.4. The current inner loop and the speed outer loop controllers was designed based on typical PI control and internal model control (IMC) controllers, as well as fuzzy controller adopted in speed outer loop. Simulation result comparison was conducted on using the traditional PI, IMC and fuzzy control for rotor side converter. Simulation waveforms in variation of control strategies demonstrate that rotor side converter can operate in sub-synchronous rectify and super-synchronous inverter with unit power factor respectively. In addition, rotor side converter can also operate non-unit power factor when reactive current reference is changing. The simulation results demonstrate decoupling control ability of active power and reactive power, and AC-excited VSCF operation under three control strategies. The fuzzy controller has better performance than other two controllers.5. The main circuit topology, system function, energy flow and control approach of grid side converter and parallel power quality controller (PPQC) were comparatively studied. A novel combined control strategy for both wind power grid connected generation and PPQC is proposed under circuit topology invariant. A 7.5kw system configuration and control strategy are analyzed and verified by simulation. The results of simulation prove the feasibility presented in this thesis. 6. With the increasing capacity of wind farm, the current impact on connected to grid could not be ignored, thus wind farm grid connection should be investigated in depth. In power system analysis, it is usually convenient to use a per unit system to normalize system variables. A well-chosen per unit system can minimize computational effort, simplify evaluation, and facilitate understanding of system characteristics. A full per unit equation of DFIG is derived according to electromotor convention. Taking the parameters of wind farm with 5 DFIGs integrated into the weak grid for example, the simulation model is developed using MATLAB/SimPowerSystems. This thesis analyses in depth the impacts of grid connected wind farm, including wind speed varieties, voltage sags, single-phase short circuit and three-phase faults.The research work of dissertation has realistic guiding significance for analysis, modeling and control of grid connected AC-excited VSCF DFIG wind power generation system.
Keywords/Search Tags:Doubly-fed induction generator (DFIG), Dynamic model, Control Strategy, Flexible grid connection, System simulation
PDF Full Text Request
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