Research On Multi-port Converter And DC Transformer In Multi-voltage Grade DC Microgrid

The research content of this thesis is from the Shanxi Provincial Key Research and Development Program(International Cooperation Program)“Research on Bidirectional Power Converter Control Strategy in AC/DC Hybrid Microgrid(201603D421007)”and State Grid Shanxi Electric Power Company Science and Technology Project Research "Low-voltage DC Distribution Network Development Model and Key Technology Research(52053016000W)"In order to ensure that a variety of new energy sources are simultaneously connected to the power grid,multi-voltage grade DC microgrids have become one of the most important research contents at present.Multi-port converters and DC transformers act as the core devices in the DC microgrid,ensuring that multiple new energy sources are connected to the microgrid and play a vital role in maintaining the normal operation of the DC microgrid and the voltage stability of each voltage level bus.In this paper,the multi-voltage grade DC microgrid is taken as the research background,and the topology and control strategy of multi-port converter and DC transformer in multi-voltage grade DC microgrid are studied:(1)The structure and control of the basic components of multi-port converter and DC transformer in DC microgrid are studied,including photovoltaic module,photovoltaic boost converter and dual active full-bridge DC/DC converter.The photovoltaic battery was analyzed and modeled,and the influencing factors of photovoltaic power generation and the relationship among various influencing factors were studied.The topology and working principle of photovoltaic Boost converter are introduced,and different control strategies are analyzed.Finally,the different phase shift control modes of active full-bridge DC/DC converters are introduced,and the power transmission characteristics and working modes of active full-bridge DC/DC converters with single phase shift control are analyzed in detail.It laid the foundation for the study of multi-port converters and DC transformers.(2)A partially isolated four-port converter topology based on dual active full bridge DC/DC is proposed.The converter is composed of two boost converters and a dual active full bridge DC/DC converter.Not only the two-way transmission of power and the isolation between some ports,but also a variety of different new energy inputs can be realized.Then through the analysis and research of the four-port topology,a four-port operation mode is proposed.According to its operating mode,the control strategy of the four-port converter in different operating modes is designed.Finally,the feasibility of the four-port converter is verified by the experimental analysis of the various operating modes of the four-port converter and the transition between modes.(3)A DC transformer scheme suitable for medium and low voltage DC grids is proposed.By directly inverting the switching tube of the DC capacitor,the fault can be quickly removed and restored,and the operating efficiency of the system is improved.Based on this structure,two working states of the DC transformer are given.The equivalent model of the DC transformer in the series state of the input parallel output is derived,and the control strategy in this state is obtained,which ensures the reliable operation of the system.And based on the above DC transformer control strategy,a bandpass filter based on the second-order generalized integral link is introduced.Preventing the double-frequency pulsation component between the high and low voltage DC busbars affects transmission efficiency and system stability.Finally,the feasibility and working conditions of the transformer and the effectiveness of the filtering are verified by simulation and experiment.(4)A multi-port converter and DC transformer experimental platform based on multi-voltage grade DC microgrid is constructed.The hardware part is designed with the main circuit and the sampling circuit,and the switching tube and the high-frequency transformer in the circuit are selected.The software part uses DSP28335 as the control chip,designs the main program of control strategy and develops the switching tube phase shifting program.Finally,The feasibility of the multi-port converter and DC transformer structure and control strategy is verified by experiments.