Design Of Three-level Converters For A Dual Three-phase Permanent Magnet Synchronous Generators

With the increased power capacity of offshore wind turbine,the low-voltage system is hard to improve since it is limited by the number of power electronic components in parallel.The application of medium-voltage three-level full power converter has become a hot topic in domestic and overseas research.In this dissertation,the three-level converters controlling dual winding generator is analyzed.The vector control method and modulation strategy of three-level full power converter are designed.The converter parameter has been designed and the hardware in loop(HIL)test has been completed.The main contents and work of this dissertation are as follows:Firstly,the structure and working principle of three-level full power converter are analyzed,including generator-side converter,grid-side converter and chopper module which completes low voltage ride-through.Based on the advantages of dual three-phase generator to increase the power level of the system and suppress the corresponding harmonics,two three-level full power converters are used to control the wind turbine with dual y-phase-shifting 30°permanent magnet synchronous generator.Secondly,the vector space decoupled(VSD)transformation is used to model the dual y-phase-shifting 30 degree PMSG.The voltage,flux equation and coordinate transformation matrix of the generator are derived.After VSD transformation,the d-q subspace contains the DC quantity and the 12 th AC quantity,which represent the fundamental wave and the 11 th and 13 th harmonic components in the six phase static coordinate system respectively.Therefore,a parallel resonance controller is proposed to suppress the 11 th and 13 th harmonics in the d-q subspace.The z1-z2 subspace contains the 5th and 7th harmonic components of the generator.After the corresponding rotation matrix is designed in z1-z2 subspace to transform them to the 6th AC quantity,PIR controller is used to suppress the 5th and 7th harmonic components in this subspace.The control method of current loop reduces the harmonic current of the generator and prolongs the life of the generator.Then,the three-level modulation strategy is analyzed.The reason of three-level neutral point potential imbalance is analyzed from the connection mode of differentvoltage vectors of three-level converter.The method of adjusting the action time of positive and negative small vectors in each seven segment period to reduce the voltage difference is derived.The parameter design of three-level full power converter is analyzed,and the parameters of du/dt inductor are designed for limiting over-voltage of generator.LCL filter is designed for increasing the dynamic response speed of current and improving the effect of filter.Parameter of absorption capacitor is designed for suppression of IGBT switching off overvoltage.The capacitance of DC bus is designed according to the voltage ripple of DC bus.The design of the key components makes the converter meet the requirements of grid connection with low cost,and provides the actual parameters for the prototype development.Finally,a hardware in the loop simulation system based on RT-LAB is built to verify the correctness of the control method and parameter design of the converter.The program code that can run in DSP is written.It provides theoretical basis and engineering experience for the 4.5 MW three-level converter of Mingyang smart energy and its application in dual three-phase generator.