Research On Key Issues Of Multi-phase Direct-drive Permanent Magnet Wind Power Conversion System Based On MMC

With the rapid growth of the world's energy demand,fossil energy will eventually be exhausted.The development of renewable energy such as wind power is the only choice for all countries in the world to cope with the energy crisis.Wind power is a strategic emerging industry in China.It is of great research value and practical significance to study the theory and technology of wind power generation for improving the level of wind power technology in China.In recent years,the standalone power of wind generation system is more and more large.If the traditional 690 V wind power conversion system is still adopted,it will cause the system to be large in volume and heavy in weight.It is not only difficult to manufacture,transport and install,but also high in operation and maintenance cost.In addition,with the increasing proportion of wind power generation in the power grid year by year,whether the wind power system can operate safely and reliably directly affects the security and stability of the power grid,and users' requirements for the reliability of the wind power system are higher and higher.In this paper,supported by the key program of National Natural Science Foundation of China "Research on the key basic problems of the integrated system of multi-phase direct-drive permanent magnet wind power conversion(51737004)" and general program of National Natural Science Foundation of China "Structure and control technology of direct AC/AC grid-connected system of medium voltage high power wind turbine(51977070)",a multi-phase direct-drive permanent magnet wind power conversion system based on MMC is proposed by combining the modular multilevel converter(MMC)with the multi-phase permanent magnet generator.This paper focuses on the topology,working mechanism,operation characteristics,control strategy,fault-tolerant operation method and other key issues of the converter system.The main research contents are as follows:1)A new topology structure of multi-phase direct-drive permanent magnet wind power conversion system based on MMC is proposed.This paper analyzes the topological structure characteristics of multi-phase directdrive permanent magnet wind power conversion system based on MMC,expounds the working principle of the system,and builds the circuit model of the system.This paper analyzes the working characteristics of MMC and multi-phase permanent magnet generator,studies the influence of multi-phase generator parameters on the control performance of convertion system,and establishes the system control model.2)A new control strategy of multi-phase direct-drive permanent magnet wind power conversion system based on MMC is proposed.This paper analyzes the energy transformation process of the converter system,studies the working characteristics of the machine-side control strategy and the gridside control strategy,and analyzes the relationship between the control strategy and the control performance.Aiming at the machine-side converter,the relationship between the characteristics of the controller and the current,power and torque distribution of the system,as well as the influence on the system design,a voltage balancing control strategy is proposed,which is good for the balance of the mechanical stress of the system.Aiming at the grid-side converter,the application of model predictive control strategy in the system is studied.A reverse MPC strategy is proposed,which can improve the stability and transient performance of the system and greatly reduce the computational complexity of the control strategy.3)A new method of suppressing the voltage ripple of MMC submodule based on the flying energy channel is proposedThis paper analyzes the generation mechanism and fl uctuation characteristics of the capacitance voltage ripple of MMC submodule,and realizes the suppression of the capacitance voltage ripple of MMC submodule by combining the bridge coupling inductor with the flying LC circuit and applying "distributed high frequency voltage + high frequency current",which can reduce the volume,weight and cost of the capacitors and bridge inductors of the MMC.Compared with the traditional method based on "high frequency common mode voltage + high frequency current",this method does not generate common mode voltage and has no EMI problem.4)A new fault-tolerant control method of multi-phase direct-drive permanent magnet wind power conversion system based on MMC is proposed.This paper analyzes the energy transmission characteristics of the converter system when a phase winding or a converter module of the multi-phase generator fails,and studies the control theory and fault-tolerant control method of the continuous operation under the local fault of the conversion system.In view of the fault on the machine-side,a fault tolerance hierarchical control strategy is proposed to bypass or deload the module according to the fault severity level,so as to maximize the power output capacity of the system and improve the power gen eration efficiency of the system.According to the exhaustion of redundant modules,the reference voltage vector is modified according to the principle of phase voltage constant or line voltage constant to ensure the system continues to operate with faults.To sum up,this paper combines MMC with multi-phase generator,and proposes a new topology of multi-phase direct-drive permanent magnet wind power conversion system based on MMC.Compared with the conventional low-voltage direct-drive permanent magnet wind power conversion system,the system can effectively reduce the stator winding current,increase the power of the system,and improve the grid voltage level by using the modular and fault-tolerant characteristics of MMC and multiphase generator.The system can be directly connected to the medium and high voltage grid without the need for a grid-connected transformer,which can improve the reliability and energy conversion efficiency of the wind power conversion system,and meet the development requirements of the wind power system to single machine with large capacity,high voltage level and high reliability,which has certain theoretical and practical significance.