Research On Low Carrier Ratio Hybrid Modulation Strategy Of Direct-drive Permanent Magnet Wind Power Side Converter System

In recent years,the technology of wind power generation develops rapidly.Because of the limitation of power electronic devices,the switching frequency of converter should not be too high.Considering the uncertainties and fluctuations of wind energy,a low carrier ratio hybrid modulation strategy for the generator side converter system of direct drive permanent magnet wind turbine is studied in this thesis.Firstly,the basic structure and mathematical model of the generator side converter system of direct drive permanent magnet wind turbine are introduced.And the basic principles of space vector pulse width modulation and synchronous modulation are described.Secondly,in view of the serious harmonic distortion of the stator current of the Permanent Magnet Synchronous Generator(PMSG),a hybrid modulation strategy of full wind speed and low carrier ratio based on space vector synthesis is proposed based on the phase current harmonic distortion rate.The modulation strategy adopts asynchronous SVPWM at low wind speed and synchronous SVPWM at medium and high wind speed.From the view of stator flux trajectory continuity,aiming at no sudden change of three-phase current before and after switching,this thesis designs the switching principle of hybrid modulation strategy of full wind speed and low carrier ratio based on space vector synthesis.Simulation results show that the proposed method can effectively reduce current harmonic,torque ripple and achieve smooth switching.Then,aiming at the problem of fixed carrier ratio in the hybrid modulation strategy of full wind speed and low carrier ratio based on space vector synthesis,which results the carrier ratio to abruptly change during handover,a hybrid modulation strategy of full wind speed and low carrier wave ratio based on switching angle is proposed.Combining the switching angle concept of optimized synchronous modulation and the symmetry of three-phase PWM pulse,the switching angle of the hybrid modulation strategy is deduced.And the sector is re-divided according to the changing law of switching angle.Based on the continuity of the stator flux linkage trajectory,the switching method after re-dividing sector is deduced.Simulation results show the correctness of the proposed hybrid modulation strategy of full wind speed and low carrier wave ratio based on switching angle,continuous line voltage phase,no carrier ratio abrupt change and current impact.Finally,in order to expand the constant power range of PMSG and increase the power generation of wind turbine,the principle of overmodulation algorithm of two hybrid modulation strategies is analyzed and researched.Taking 15 pulse traditional space vector modulation as an example,the overmodulation algorithms of two hybrid modulation strategies are proposed.The simulation results show that the narrow pulses of the overmodulation algorithms of the two hybrid modulation strategies disappear in time sequence,and they can enter the overmodulation smoothly whithout switching,which ensures the symmetry of the output pulses.