Research On Optimal Control Strategy Of Variable Speed Constant Frequency Dfig Wind Power System Converter

For its small capacity,small size and low cost,the doubly-fed induction generator(DFIG)has become a popular wind power generator.The key factor to realize the stable operation of doubly-fed induction generator is the control of its converter,so the control strategy of doubly-fed induction generator converter is deeply studied in this paper.Firstly,the development trend and current situation of wind power industry are described,and the characteristics of converter,an important component of wind power system,and the current mainstream control strategy of wind power system are introduced.At the same time,the structure of doubly-fed fan is analyzed,the mathematical model of doubly-fed induction generator in three-phase static coordinate system and two-phase synchronous speed rotation coordinate system is built,and the vector control strategy is introduced,which provides the basis for the optimization and improvement of the converter control strategy in the future.Secondly,in order to solve the problems of vector control being too dependent on parameters,complicated debugging and poor dynamic performance,a model predictive current control is introduced into the rotor-converter of doubly-fed induction generator.However,the traditional model predictive control has the limitations of low control accuracy,large torque ripple and poor current waveform quality,so the two-vector model predictive current control strategy is studied.The three-vector model predictive control strategy has three voltage vectors in one control period,which improves the control accuracy to a certain extent.Its range can cover any direction and any amplitude.Meanwhile,it can control the current of d axis and q axis of the converter at the side of the doubly-fed fan without beat,effectively reducing the current pulsation of dq axis.Finally,the steady-state and dynamic performance of vector control and single,double and triple vector model predictive current control are compared and simulated.Since the stable operation of the grid-side converter is the basis of the control of the machine side converter on DFIG,the control strategy of the grid-side converter is also very important.The virtual synchronous generator is introduced into the grid-side converter.The virtual synchronous generator can increase the inertia support and voltage support for the doubly-fed wind power generation system,maintain the voltage stability on the DC side,and achieve the preset control objectives.Moreover,wind power systems are mostly located in remote areas such as mountains and sea,and the real environment will cause the unbalance of the power grid frequently.Therefore,an improved virtual synchronous generator under the unbalanced power grid is adopted,and the control objectives such as balancing current and inhibiting the active power twice the frequency of the power grid are set.The steadystate and transient performance of balanced and unbalanced power networks were analyzed on the MATLAB/Simulink simulation platform.It was proved that the improved virtual synchronous generator algorithm has obvious advantages in maintaining current balance,suppressing power fluctuation and improving system stability.Finally,the RT-LAB semi-physical platform is used to verify the overall control strategy of the converter in the doubly-fed induction generator.The results show that the control strategy of the DFIG rotor-converter and the grid-side converter can achieve good control effect,which verifies the correctness and effectiveness of the control strategy.