Topology Of High Voltage And Large Power DC-DC Converter Based On Modular Multi-level Structure

With the introduction of the DC grid concept,the large capacity DC power transmission equipment has become a research hotspot.The DC-DC converter is an important part of DC power.Generally the voltage level of the converter used in DC grid is high,and existing HVDC converter technology is not mature enough.Therefore the high voltage and large power DC-DC converter is urgent needed.The article proposed the multi-level modular DC-DC converters which can be applied in the DC grid of the future.First,the paper introduced the high voltage and high power DC-DC converters which proposed by the scholars in the past few decades,and summarized the ideas and methods to achieve a settlement of high-voltage power conversion,including the series-parallel power devices technology,multi-level technology,and inverter sub-modules series-parallel technologies.The article respectively introduced the high voltage and large power DC-DC converter topologies as well as the advantages and disadvantages of the different topologies based on these three technologies.Then the article presented a unidirectional step-down DC-DC converter,and described the topology,working principle and the advantages of the new DC-DC converter.The simulation in PSCAD/EMTDC environment showed the new DC-DC converter could realize the function of the DC transformer.Low switching frequency,low loss and the ideal DC voltage and current waveforms indicated a good performance of the converter.The thesis also proposed a bi-directional DC-DC converter,and the converter could achieve the bi-directional flow of power.The paper introduced the topology of the bi-directional converter,as well as the working principle.For the power transmission capacity of the converter,the thesis put forward the two-phase and the multi-phase DC-DC converters to adapt to the high voltage and large power occasions.The paper also respectively described the control strategies of un-idirectional DC-DC converter and bi-directional DC-DC converter.The simulation verified the feasibility and reliability of the control strategy.Based on the proposed topologies,the article analyzed the prospects of the DC-DC converter.Including DC connection of offshore windfarms,VSC and LCC interconnection,and remote load interconnection via multi-terminal HVC.These prospects indicated that the DC-DC converters will play a pivotal role in the DC grid construction of the future.