Continuous Operation And Converter Control Of Direct Drive Permanent Magnet Synchronous Wind Power Generation System Under Grid Faults

As a one of main energy sources to release the depletion of oils, climate warming and environment deteriorate in the world wind power generation has been increasingly emphasized by more and more countries. With the rapid increase of capacity of wind power generator set connected to grid, the mutual action between the wind power generator set and grid has been larger and larger. In one hand, the operation performance of generator set directly influences operation stability and energy quality of the grid connected to it. In the other hand the grid faults in real operation can cause the generator set to disconnect from the grid because of protection action and give rise to an enlarged fault resulting in a severely negative effect to restoring stabilized operation of grid. So from the grid safety it is necessary to design new grid code requirements for wind power which impose the wind turbines to stay connected to the grid under grid fault. That means the wind generator sets must have the capability to ride through low voltage. This paper is designed to research the operation and control of direct drive wind power generation system under asymmetrical grid fault conditions. This paper is organized as follows:In chapter1, we introduce research status and development trends of wind power generation technology in and out of domestic, and the main wind power generator set types at present. Then we also summarized research status of direct drive wind generation system in terms of low voltage ride-through technology.In chapter2, we introduce basic principles of wind power generation, power characteristic and control methods of output power of wind turbine, and discuss principles to capture maximum wind energy from changing wind and the control strategy to output maximum electrical power, then introduce three maximum power point tracking control algorithms to provide effective guides to optimum control strategy choice in direct drive wind power generation system under normal and grid fault conditions.In chapter3, a new converter control strategy of direct drive wind generation system is proposed based on conventional one on the which generator-side converter controls power transmitted into grid and grid-side converter controls DC bus voltage, while the new control strategy is the other way around. Then we simulate and compare two control strategies in the SIMULINK/MATLAB environment. On the new strategy because power regulation is realized in grid-side converter, the energy convertion efficiency is higher than that on the traditional one, response speed of system speeds up and dynamic performance is very good, various steady performance of parameters is also very good. In the meantime, the new control strategy can make this system to have definite ability to ride through low voltage without adopting any protection measures and offers an optimal choice for converter control of direct drive wind generation system under grid faults. However, the conventional strategy has advantages in terms of generator-side and DC bus voltage stability over the new one. Then the experimental platform of direct-driven wind generation system is constructed and software and hardware of it are designed to verify the performance of two kinds of control strategies proposed.In chapter4, the grid code requirments and grid incorporation process of wind power generator sets are dealed with, the fault types of grid are introduced. Five different measures to enhance low voltage ride-through capability in direct drive wind power generation system are proposed, and design of damping controller to restrain the oscillation is performed. Based on the control strategy of capacitor energy storage protection circuit, generator speed regulating (regulating blade pitch angle), DC bus voltage adjusting and grid reactive power support provided by grid-side converter, the control configuration and simulation model to fulfill low voltage ride-through of direct-driven wind generation system is offered and simulations are performed.In chapter5, function and configuration of phase-locked loop are dealed with and grid synchronization requirments of grid-side converter in the direct-driven wind generation system is clarified. Some present grid synchronization methods are researched and compared to seek out an optimal method suit for grid-side converter synchronization under asymmetrical grid fault conditions. This paper researches the work principles and performance of single synchronous reference frame software phase-locked loop, decoupled double synchronous reference frame PLL and second order general integrator-quadrature signals generator synchronization methods. Second order general integrator-quadrature signals generator synchronization method is researched by simulation and experiment to provide reliable information for continuous operation of direct-driven wind generation system under asymmetrical grid fault conditions.In chapter6, the control strategy and configuration of direct-driven wind generation system are given, the principle to keep DC bus voltage constant is discussed. Three different current controllers are provided and compared by simulating under SIMULINK/MATLAB environment. Finally based on the new control strategy and grid synchronization method offered in the chapter5and coupled with low voltage ride-through control strategy the continuous operation and control performance of direct-driven wind generation system under asymmetrical grid fault is verified.The conclusion summarizes research work of this paper and introduces the characteristic and originality in the paper, meanwhile gives the emphasis and direction of research work in the future.