Research On Key Technology Of High-Power-Density DC Trasnformer

In recent years,large-scale application of renewable energy and the emergence of new dc load promote the rapid development of dc distribution technology,and dc transformer(DCT)has also achieved great concern as the key equipment in dc distribution system.At present,modular parallel-series-connected DCTs are widely adopted in medium-and low-voltage dc distribution demonstration projects.However,limited by the withstand voltage of commercial semiconductor switches,the terminal voltage of single DCT submodule is low,which results in large number of submodules and low power density.This prevents the application of DCT in city or ship with constrained land.Hence,how to improve the power density of DCTs becomes an essential issue of promoting the urban dc distribution system.From the perspectives of topology,control strategy and engineering application,this paper makes researches on the key technology of high-power-density DCT,and the research in this paper can be divided into three parts.1)From the view of topology,in order to improve the voltage level of submodules in modular parallel-series-connected DCT,a novel series-connected three-phase bridge circuit is proposed by analyzing and summarizing the existing structures including half-bridge,full-bridge and three-level circuits.On this basis,three-phase triple-voltage dual-active-bridge(T~2-DAB)converter is presented,whose terminal voltage is triple of that of conventional full-bridge DAB converter.Hence,the number of submodules in DCT can be reduced a lot,so as the cost and volume of DCT equipment.In addition,zero-voltage-switching(ZVS)is realized for all switches and the turn-off currents is reduced,so that the efficiency is improved.In Chapter II,the operation principle,design procedure and control strategies are analyzed specifically.Comparison is conducted among DCT based on T~2-DABconverter and existing schemes,and the results show the advantages of DCT based on T~2-DAB converter in the aspect of cost,efficiency and volume.Further,aiming at reducing the volume of dc blocking capacitors in T~2-DAB converter,a three-phase triple-voltage LLC converter is proposed in Chapter IV,in which dc blocking capacitor operates in resonance to reduce the required capacitance and volume.Hence,the power density can be improved.Open-loop control with fixed switching frequency is adopted to simplify the control system,and high efficiency and fixed voltage conversion gain is ensured by optimizing the resonant parameters.In Chapter VI,the construction theory of three-phase triple-voltage topology is extended further.On the one hand,by transforming the series-connected three-phase bridge circuit,three-phase circuits with six-to-N times of voltage levels are proposed,along with the series-connected three-phase full-bridge circuit with dc blocking capacitors cancelled.On the other hand,with the three-phase triple-voltage topology introduced to modular multilevel converter,a three-phase three-level two-arm converter(TTC)is proposed.The series-connected three-phase bridge circuit operates as the director switch,and dc blocking capacitor is replaced by the cascaded half-bridge submodules to shape the ac output voltages.The number of half-bridge submodules is reduced dramatically,which helps improving the power density.In Chapter VI,the operation principle and control scheme are analyzed in detail,and experiment is conducted to validate its feasibility.2)For the perspective of control strategy,considering that ZVS turn-on is easily lost for DAB converter under terminal-voltage-mismatching and light-load situation,an improved control strategy and optimization method of control variables are proposed in Chapter III,on the basis of asymmetrical duty cycle modulation and phase-shifted control strategies.With such control strategy,ZVS turn-on is ensured for switches and the current stress of switch decreases.3)For the perspective of engineering application,intensive research is conducted on the design procedure of the key component of T~2-DAB converter,namely high-power three-phase high-frequency transformer,in Chapter V.Three nanocrystalline toroidal cores are assembled as the three-phase core,to constitute the centralized three-phase HFT and reduce the volume.The power loss and thermal models of HFT are established,based on which the structural parameters are optimized to minimize the volume and power loss.According to the optimization results,a 120k VA/10k Hz three-phase high-frequency transformer is manufactured,and relevant performance test is accomplished with 2000V/560V/120k W T~2-DAB converter platform employing all Si C-MOSFETs.