Converter And Control For Doubly Fed Induction Generator-Based Wind Power Generation

As a renewable energy technology already commercially available and with great potential to be further exploited, wind power generation experienced fast development in the last two decades. Variable-speed wind-energy conversion systems (WECS), especially the doubly fed induction generator (DFIG) -based WECS attract big attention for their exclusive advantages. With the support of "The National Eleventh Five-Year Research Programme of China" (2006BAA01A18,2006BAA01A20) and "The Tenth Five-Year Research Programme of Anhui Provience"(040120564), a number of the key topics, such as modeling of DFIG-based WECS, driving control strategies, engineering design of the back-to-back converters, and so on, are studied in this dissertation. It is through these studies that some innovative results are achieved.The main contents of the dissertation can be summarized as:1. Different mathematical models of DFIG are derived for different purposes in simulation and research, and the stress is put on the model with flux saturation and the Thevenin's equivalent model. In contrary to the complexity of the method of simulating the characteristics of the power grid by using synchronous generators, a method with less computation and higher efficiency is proposed to perform the simulation by controlled voltage source with frequency-droop and voltage-droop in accordance to its output active and reactive power respectively.2. The T-type equivalent circuit of DFIG is used to analyze its control mechanism, on which based the study on the stator-flux and the grid-flux-oriented vector control is carried out, and an adaptive resonant regulator-based control strategy, by which errorless control in steady state is achieved without any frame transformation for the rotor currents, is proposed.3. The performance of the vector control of DFIG is discussed, and comparison is made on the stability of the stator-flux and the grid-flux-oriented vector control strategies. To improve the disturbance-rejection from the back-EMF, a strategy of "virtual impedance control" for the DFIG controller is proposed. Analysis is done on the oscillation and its damping of DFIG's stator flux.4. The symmetrical component method is adopted to study the behavior of DFIG under unbalanced grid voltage, and the correlations among the pulsations of the stator-side active power, reactive power and the electromagnetic torque are investigated. The unbalance control strategies are studied based on either the dual synchronous reference frame (SRF) or the single SRF. As one of the single SRF-based control schemes, the mechanism of the direct rotor-voltage compensation control is proposed and fully analyzed in the dissertation. To overcome the influence of the coupling between the rotor-voltage compensation and the rotor-current control, a decoupled control scheme is put forward, by which the system performance is improved.5. Several typical sensorless control strategies of DFIG are studied. The speed observation based on either the stator excitation current or the stator voltage are both affected by the DFIG's operation modes, and the dynamic gains of both observers are also affected by the phase angles of the rotor currents. To overcome the above mentioned disadvantages, two speed observers, i.e. stator-current dual loop coordinated observer and stator-voltage dual loop coordinated observer, are proposed. And to overcome the nonlinearity between the dynamic gain of the speed observer and the deviation angle of the observed rotor current vector, for the rotor current -based model reference adaptive system (MRAS) speed observer, a deviation angle-based speed observer is proposed, consequently the system dynamic performance is improved.6. The mechanism of the no-load stator voltage control of the DFIG is analyzed, and two control approaches, one is based on a hybrid regulator composed of an adaptive resonant component and a PI component and another is based on a decoupled controller, are proposed.7. The electromagnetic transfer process of the DFIG during grid voltage dip is described, and the dynamic responses of the stator currents, rotor currents, stator flux and the electromagnetic torque to the grid voltage dip are quantitatively analyzed by modeling, meanwhile, the affection of rotor-current control and the types of the dip to the transfer process is discussed. Based on above study, several typical control strategies for improved LVRT performance of DFIG-based WT are further investigated. For the rotor-side crowbar-based LVRT control strategy, a set of control logic is put forward, with which the coordinated control between the WT and the power grid during grid fault and smoothly switching between different DFIG's operation states are achieved. For the transient flux compensation-based one, a virtual inductance control scheme is proposed to relieve the dependence on the leakage inductance of DFIG and reach better performance. For the current surge, which maybe happens at voltage recovery, the "Short Term Interruption" (STI)-based LVRT control strategy is researched.8. Several sets of test benches, with different power level, different types of machine sets, and different experimental purposes, are built, and a series of experimental results are achieved. The software calculation method of WT's dynamic behavior, such as wind turbine aero-dynamics, mechanical load moments, and gearbox and shafts dynamics, following which the prime motor is operated, is studied. Especially a test bench based on dual-MW-level DFIGs is set up, which is indispensable to simulating high power level DFIG-based WT. To meet the requirements of the start-up and low-speed operation of the drive system, rotor-side driving vector control of DFIG with its stator-side short circuit is researched, and good operation performance is obtained.9. The key parts engineering design of the back-to-back converters, driving the DFIG-based WT, is presented. For the DC-link capacitor design, the requirements to cope with load perturbation and the rotor-side active power pulsation during grid voltage dip are studied, according to the DC-link pulsation behavior. For the LCL filter design, the optimizing design schemes of the LCL filter are studied, according to the harmonic distribution in the AC side of the grid-side converter and the filtering requirements. For the long lead filter design, several converter-side du/dt filters and motor-side impedance matching networks are analyzed and designed, according to the wave reflection mechanism.10. A converter prototype, for 2MW DFIG-based WTs, is designed, with which the experiment has been taken on the dual-DFIGs-based test bench above mentioned. And the on-ground test on a dedicated test bench for WT has been accomplished.