Research On Gride Side Nine Switch Converter Based On Doubly Fed Wind Turbine System

As China’s installed capacity of wind power increases year by year,double fed induction generator wind turbines account for a major share of the current wind power market in China due to small capacity of converter,low price,and low harmonics of generator.However,the special structure of the direct grid connected stator of the double fed induction generator makes it very sensitive to voltage distortion.When the power grid fails,it will affect the normal operation of the doubly fed induction generator.In order to eliminate the influence on doubly fed induction generator under fault conditions of the power grid,the fault ride through has become a key problem to be solved urgently.In this paper,a new topological structure of nine switch converter is used to replace traditional double fed wind turbine grid side converter.With only three IGBTs added,grid connected control and voltage series compensation can be integrated,thus solving the problem of double fed wind turbine system can not fault ride through,which causes double fed wind turbines with grid disconnection,waste of resources and other phenomena.Compared with some other fault ride through schemes,converter volume is reduced to a certain extent,which has certain theoretical research value and engineering application prospects.Firstly,the working state,mathematical modeling and SPWM modulation method of the nine switch converter in the new topology is analyzed.Through simulation,the correctness of the SPWM modulation method is verified,the coupling relationship of the common switches in the nine switch converter is eliminated,and the independent control of the upper and lower channels is realized.Secondly,the operating principle of the system topology is analyzed;Furthermore,the system implements fault ride,requires a stable double fed wind turbine system as a precondition,and performing mathematical modeling of rotor side and grid side converters under ideal grid conditions,through mathematical modeling forms control strategy.In the Matlab/Simlink simulation platform,a simulation model of VSCF wind turbine system under steady state condition is established.The simulation verifies the stability of the system operation under different wind speed conditions and the correctness of the control strategy.Finally,research and analysis on control strategy of grid side nine switching converter for system fault conditions.Dynamic voltage restorer voltage detection method,phase locked loop technology,dynamic voltage restorer compensation strategy,dynamic voltage restorer control strategy,grid side nine switch converter control are studied,the voltage distribution problem of the DC bus of the nine switch converter and the selection of the modulation method of grid side nine switch converter.The simulation results show that the nine switch converter operates during voltage fault of the power grid,through its dynamic voltage restorer function,it can compensate for the short term distortion of the power gridvoltage.The nine switch converter can maintain the voltage stability of the DFIG terminal under various voltage short circuit fault conditions,and the auxiliary double fed wind turbine achieves flexible fault ride through.