Cascaded H Bridge Medium Voltage Wind Power Converter

With the development of offshore wind power,the capacity of wind turbines tends to be large,and the outlet current of low-voltage 690 V wind turbines will increase drastically,bringing a series of challenges to the manufacture,operation and maintenance of the generating units.Permanent Magnetic Synchronous Generator(PMSG)has the advantages of no gear box,easy maintenance and wide speed range.It has great application value in offshore wind power generation.In order to reduce the outlet current of the unit,it is of practical significance to study the medium voltage wind power converter which is matched with the generator by using the permanent magnet direct drive medium voltage wind power generator.In this paper,by contrasting various medium voltage topologies,the research emphasis is selected as the cascaded H-bridge MV converter,and the selection and design of the wind power system including permanent magnet direct-drive generators and cascaded H-bridge converters,The converter steady state operation and fault control strategy and modulation methods are studied,the research content and main work include the following points:(1)Based on the detailed mathematical model of key aspects of PMSG wind power generation system,the machine-side converter and grid-side converter of cascaded H-bridge medium voltage wind power converter are studied respectively.The steady state operation under ideal grid condition The conventional vector control strategies and modulation methods of timing and grid-side converters are designed according to the typical type I and type II system parameter design methods,and the parameters of the PI regulator in the control loop are designed.The generator and converter in the paper are selected Type and design.(2)In order to improve the system performance,in view of the fact that the DC bus of a single power module has fluctuation voltage with large amplitude and variable frequency,in this paper,the influence of this fluctuation on the low frequency harmonics of grid-connected current is deduced.Based on the harmonic elimination effect and the dynamic performance of the system,the control strategy based on the frequency-variable notch in order to reduce the low frequency harmonics induced by fluctuation voltage of DC bus is proposed.In view of the traditional cascade H Bridge converter is connected to the grid via multi-winding phase-shifting transformer,which is complicated and cumbersome and the system volume and cost are increased.In this paper,an optimized modulation method based on carrier phase shift is proposed to improve the equivalent switching frequency on network side of the converter and with conventional multi-winding transformer instead of phase-shifting transformer.(3)In order to improve the generator fault-tolerant running capacity and output the optimal power in the fault condition of the converter,a control strategy combining the neutral point shifting with the optimized pitch control is proposed in this paper.Compared with the conventional same-level bypass method,this control strategy can make the wind turbine always output the maximum power in the state of inverter fault without damaging the motor.(4)The cascade H-bridge converter real-time hardware-in-loop simulation platform based on RT-LAB is built and the hardware and software of the system are accurately designed.The real-time performance and accuracy of the system are higher than that of offline simulation.