| Nowadays, more and more countries support and develop their own wind power industry with great efforts, so there have been rapid advances in wind power technology, causing a considerable increase in the capacity of wind turbine installation. As the permanent magnet synchronous motor (PMSM) has such advantages as small mechanical loss, high efficiency, low maintenance cost and speedup-free gearbox, which match the special features of wind turbines well, it has become another important type of machine and gradually come into use subsequent to the application of a double fed induction wind turbine. However, the converter, according to the scheme of a directly driven wind turbine, is the only access to the power grid. Therefore, the converter must reach the high standard of performance and meet the requirements of its capacity, reliability, response speed and grid-connection. As far as the direct-drive wind power system is concerned, it is necessary to get over bottlenecks or difficult problems in the design, manufacture and test of the full-rate converter, which is of extreme importance to the stability of the entire system.This paper has made a deep research into the several key techniques related to the converter in the direct-drive wind generation system by use of theoretical analysis, simulation and experimental verification. The specific researches are described as following:To facilitate the analysis of modeling and control, the full power converter is divided into two parts:machine-side and grid-side converter. Firstly, for the machine-side converter, a novel control method has been proposed to avoid the phenomena of the motor driving over the speed limit and of SVPWM over-modulation that appear under strong-wind conditions. The method includes using two PI controllers to limit the power and voltage based on the original control strategy. Furthermore, the reliability and effectiveness of the algorithm has been tested in the wind field and simulation results. Secondly, for the grid-side converter, the analysis of performance of the LCL-style filter has led to the establishment of the mathematical model of VSC with LCL-style filter. The results of the experiments made in the wind field and simulation results show that the control technology is reliable and effective.With the further development of offshore wind power and a continuous increase both in the unit capacity of wind turbines and in the voltage of full-rate converters, there will certainly be a trend towards multi-level converters. In order to improve the waveform of output voltage of three-level neutral point clamped (NPC) voltage source converter (VSC) with unbalanced load, a delay signal cancelling (DSC) method has been used to distinguish and control the positive and negative sequence components. However, when the voltage waveform is modified, the imbalance of the current will be further raised. As a result, the neutral-point voltage will be oscillated seriously under the SVPWM. This paper deals with the impact of the negative sequence current on the neutral-point voltage oscillation, obtains the boundary condition in which to alter the unbalance by controlling small vectors of the redundant voltage, and presents the neutral-point control SVPWM method. Simulations demonstrate that the negative sequence voltage can be controlled while the negative sequence current intensifies the oscillation of the neutral-point voltage. But the oscillation can be suppressed by use of the integrated control strategy.To achieve the low-voltage fault ride-through (LVRT) capability for the permanent magnet direct-drive wind turbine, there is a great need for breakthroughs in three key techniques:the rapid and accurate extraction of three-phase power grid information, the energy management at the time of voltage drop, and the operation and control of grid-side converter under the asymmetric condition of power grid. To obtain the power grid information quickly and accurately, a DSC algorithm based on the software phase-locked loop (SPLL) technology is employed in this paper. To improve energy management, what needs to be done is to install a brake resistor in the DC-bus so as to consume the extra energy which cannot be transmitted to the power grid during the period of LVRT. To achieve the better performance under the condition of asymmetrical voltage drop of the power grid, this paper has worked out the double current loops method based on negative-sequence control strategy, the converter protection policy and the reactive power compensation strategy. The results of the experiments conducted in the wind field verify the feasibility and effectiveness of the LVRT technology.To improve the efficiency and matching of the designed wind power converter and simulate the real operation of the wind field, a M W-level wind power experimental platform needs to be built. First of all, there is need for establishing a mathematical model of the double three-phase PMSM with two sets of Y-connected three-phase symmetrical windings displaced in turn by30°. To obtain better control performance, a sensor-less control method based on non-singular high-order terminal sliding-mode observer is presented in the paper. The simulation and experiment results show that the system has good dynamic and static performances. Moreover, to solve the starting problem under the zero speed conditions, the fluctuating high frequency voltage signal injection method is employed for sensor-less control of dual three-phase PMSM, the simulation results show that the control technology is reliable and effective.A2MW-level directly-drive permanent magnet wind power converter system is designed successfully, and the basic compositions of the converter system are introduced, such as parameter selection, realization of control and protection, field testing and operation. |
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