Research On The Low Voltage Ride Through Capability Of Directly-driven PM Wind Generation System

Recent years, wind power has been developing rapidly, the rate of average annual growth is 28%. In addition, the penetration of wind power in grid has been rising. It will lead adverse effect to grid, if the wind turbine off-grid during the fault of grid. Wind power technology-leading countries have issued the quantitative standards of fault through which require on-line operation and providing reactive power supported during the fault until grid recovered. And In wind power market, direct-drive permanent magnet synchronous wind turbine have been developing rapidly for simple structure, low maintenance costs, high conversion efficiency, high reliability. This dissertation gives theoretical study mainly on the low voltage ride through capability of directly-driven PM wind generation system. Correlation techniques, for instance, DC over-voltage protection, reactive power control strategy of grid-side, are also discussed, and the accuracy of research results is demonstrated through simulation.First of all, the fundamental characteristics of wind turbine are discussed, and the transformation process from wind speed to the output-mechanical-torque of wind turbine is analyzed in detail. The model of wind turbine is constructed and simulated, which provides some part of theoretical support for the low voltage ride through.Secondly, the mathematical model of PWM converter is studied, and the part of the DC of Dual-PWM is described through analyzing the power transformation theory. The simulation models of the controller of grid-side converter which uses voltage-oriented vector control strategy and the controller of motor-side converter which uses the rotor flux-oriented vector id = 0 control strategy are built, and the simulation results show that the model and the control strategy are correct, which provides simulation platform support for the low voltage ride through.Thirdly, this paper analyses the operational aspect of the converter of directly-driven PM wind generation system when grid voltage dips in detail, and proves the accuracy of the analysis before by using Matlab/Simulink to simulate the running state when grid voltage dips, then describes the possible damages of the system if we don't take effective measures. After that, the power dissipative crowbar, the power storage crowbar and the auxiliary converter crowbar are analyzed amply, and the advantages and disadvantages of three different measures are compared through simulation. According to the demand of reactive power when grid voltage dips, a reactive power control strategy in grid-side converter is proposed, and its validity is confirmed.Then, the study puts forward a set of control logic for the based low voltage ride through. In the directly-driven permanent magnet wind power system simulation platform built with Matlab/Simulink, the low voltage ride through of directly-driven PM wind generation system is achieved by using the power storage crowbar and reactive power control strategy during the breakdown time.Finally, The fluctuation mechanism on DC voltage of directly-driven wind turbine with permanent magnet synchronous generator(D-PMSM) under asymmetrical faults is analyzed in detail. And a vector-oriented control strategy based on positive and negative voltage–oriented respectively (PN-VOC) is proposed, for restraining DC voltage fluctuation of D-PMSM when asymmetrical faults occurred, with the energy consumption crowbar for D-PMSM un-off-line operation on grid faults. The simulation results show that PN-VOC is practicable and correct and it has a good performance on inhibiting the DC voltage fluctuation, and the energy consumption crowbar has prohibited the over-voltage successful during the asymmetrical faults.