Research On Torque Ripple Reduction Strategy For Large Wind Turbine

With the large-scale development of wind turbine,ensuring its safe and stable operation under different working conditions is the most important work in the industry.Torque ripple is one of the important factors affecting the stable operation of wind turbine.Due to the frequent fluctuations in wind speed,the blades are forced unbalancedly to result in large aerodynamic torque ripples.In addition,a large number of harmonics exist in the stator current because of the non-uniform air gap magnetic field distribution or the non-linearity of the inverter during the operation of the generator.Large electromagnetic torque ripple produced by the generator running at low speed,acts on a load by means of the drive chain,which becomes a major hidden danger for the safe operation of the wind turbine.In this thesis,a 3MW direct-drive wind turbine in China is used as the research object,to study the collaborative control strategy of aerodynamic torque ripple and electromagnetic torque ripple.The main research contents are as follows:Firstly,the basic theories of aerodynamics and wind energy conversion were expounded,and the causes of two kinds of torque ripple were analyzed and calculated.At the same time,the mathematical model of wind turbine was established,to provide theoretical support for the further research of collaborative reduction strategy.Secondly,in view of the electromagnetic torque ripple,the harmonic voltage compensation method was adopted.Considering the coupling relationship between the electromagnetic torque and the input current,it is necessary to reduce the harmonic content in the current in order to reduce the electromagnetic torque ripple.Therefore,the harmonic current with higher content in the input current was extracted by low-pass filter,and the steady-state voltage corresponding to the harmonic current was calculated according to the relationship between the harmonic current and the harmonic voltage,and the calculated value was superimposed into the reference voltage by the coordinate axis transformation to counteract the harmonic voltage component produced by the harmonic current,which reduced electromagnetic torque ripple and speed fluctuation.For the aerodynamic torque reduction caused by the sudden change of wind speed and the fluctuation of speed,two control methods of synchronous pitch and individual pitch based on PID controller were provided.The deviation between the actual power and rated power of the wind turbine was taken as the input value of the PID controller,and the aerodynamic torque reduction caused by the two control methods were compared and analyzed under the same working condition.The results show that the aerodynamic torque ripple is smaller and the power is relatively stable under the individual pitch control method.Finally,in view of the collaborative reduction of torque ripple,considering the balance between aerodynamic torque and electromagnetic torque,based on the individual pitch control method and the harmonic voltage compensation strategy,the difference between the actual speed and the rated speed after eliminating the harmonic current was taken as the input value of the PID controller,and compared with the individual pitch control method.The results show that the collaborative reduction strategy can effectively reduce the aerodynamic torque ripple and electromagnetic torque ripple,but the power still fluctuates.In order to make the power more stable,the design of the controller was improved,the fuzzy controller was used instead of PID controller,the power was quantized by fuzzy control theory,and verified on Matlab platform.The results show that the collaborative reduction strategy of torque ripple based on fuzzy controller,has faster response speed and more stable output power,and the reduction of aerodynamic torque and electromagnetic torque is higher.