Research On Soft-switching Characteristics And Multi-mode Control Strategies For DC Solid State Transformer Based On Modular Multilevel Converter

With the photovoltaics and wind power used as a representative of the distributed power largely access to the grid,the gradual increase of the DC load in industrial production and daily life,the transformation and optimization requirements for the urban distribution network system,as a result,DC distribution network has become a research hotspot in the field of electric power.DC solid state transformer is one of the key parts of the DC distribution network which can realize the voltage transformation and power transmission,especially the direct current solid state transformer based on modular multilevel converter(MMC-DCT)has been considered having broad application prospects because of the advantages of modular design,strong expansibility,high redundancy,strong fault handling ability and so on.Based on the background above,several key technologies of MMC-DCT system are studied and discussed in this thesis.Firstly,the topology and basic working principles of MMC-DCT have been analyzed,and the phasor-domain modeling of the system has been established.The thesis has introduced the the modulation strategies applied to MMC-DCT in detail,and the electrical characteristics of the system under quasi square wave modulation strategy,sine wave modulation strategy and triangular wave modulation strategy have been analyzed and compared.It is verified that quasi square wave modulation strategy has better engineering application value in the MMC-DCT system.Moreover,the thesis has discussed the submodule voltage balancing control stratiges.The correctness of the above modulation strategies and submodule voltage balancing control stratiges are validated in the simulations.Secondly,in order to reduce the switching loss and improve the working efficiency,the thesis has deduced and analyzed the power characteristics of the system and the working states of the switching devices under the quasi square wave modulation strategy.The cause of switching loss has been summarized and the half-bridge submodule with additional parallel capacitors has been introduced.The zero-voltage switching(ZVS)operation have been put forward for MMC-DCT system under quasi square wave modulation strategy and the system ZVS condition and region have been deduced.The proposed ZVS operation is validated in the simulations.At last,in view of the practical application scenarios of MMC-DCT system,the DC distribution voltage level sequence in China is preliminarily conceived.Combined with the system working characteristics,the working mode of MMC-DCT can be divided into low voltage control mode,medium voltage control mode and power control mode.According to the mathematical model of MMC-DCT,the control strategies and controller parameters under different working modes have been analyzed and designed.The effectiveness of the control strategies and the correctness of the controller parameters are verified in the simulations.