Research On Three-level DC/DC Power Module And Control Strategy Suitable For Dc Power Electronic Transformer Based On Three-level Converter

In traditional power systems,transformers are widely used to connect power grids of different voltage levels.However,transformer cannot actively control the voltage and current,so it is difficult to meet the future development needs of the smart grid.Power electronic transformer(PET)is a new type of transformer that combines power electronic technology and electromagnetic coupling technology,with high controllability of voltage and current.It is foreseeable that PET technology will become one of the key technologies of the smart grid in the future.In order to bear high voltage and high power,power electronic transformers often adopt a modular structure.The power module as the basic unit of power electronic transformers will determine the performance of the whole machine.This article focuses on the research of single power module.The article includes topology design of the power module,the full ZVZCS modulation strategy,switch voltage equalization control,and the power module with bipolar DC output capability..Firstly,this paper analyzes the characteristics of neutral point clamped(NPC)topology,and points out its advantages as the high-voltage side circuit for DC/DC power module.Then this paper analyzes the soft switching conditions of all the switch tubes in high-voltage side circuit and proposes a full ZVZCS soft switching modulation strategy.At the same time,the design of the magnetic inductance of the transformer is discussed.The topology of the three-level DC/DC power module topology,the full ZVZCS modulation strategy and the design of magnetic inductance are verified by simulation and experiment.Secondly,this paper points out the unique problem that the voltage on inner tube is higher than the outer tube in the three-level NPC topology when the power is reversed.Through the analysis of the switching process,the article clarifies the cause of the problem.Furthermore,this paper proposes active and passive methods to achieve the voltage balance of the high-voltage side switch.The effectiveness of the method is verified through simulation and experiments.Finally,through the optimization of the low-voltage side circuit of the power module,this paper proposes a DC/DC power module topology with bipolar DC output capability.The article analyzes the bipolar voltage balance principle,the source of the voltage difference between the bipolar,the soft switching of each switch,and the relationship between the current stress of the switch and the bipolar transmission power.In order to further compensate the voltage difference between the two poles,this paper proposes a strategy for balancing the two poles.Finally,the topology of the DC/DC power module with bipolar DC output capability proposed in this paper,the calculation of current stress and the strategy of bipolar voltage balance are verified by simulation and experiment.