Research On Bus Voltage Control Strategy Of Multi-Voltage Grades DC Microgrids

In order to adapt to renewable energy sources and multiple DC loads connected to the power grid,multi-voltage grade DC microgrids will become an important part of the future smart grid.As the key equipments in DC microgrids,the hybrid energy storage system and DC transformer play a very important role in maintaining the voltage stability of the DC bus and ensuring the normal operation of the DC microgrids.Therefore,the control strategy and topology of the hybrid energy storage system and the DC solid state transformer in the multi-voltage grade DC microgrids are proposed in this paper where the voltage stability of DC buses is taken as the goal.The main contents are as follows:(1)The structure and control mode of three typical DC converters in DC microgrids are studied.First,the topology of the PV Boost converter is introduced,and the output characteristics of the PV based on the MPPT control mode and the constant voltage control mode are analyzed.Then,the topology of Buck/boost bidirectional converter is introduced,and bidirectional control strategy is obtained though establishing its equivalent model.Finally,the topology of dual active bridge converter is introduced,and the power transmission characteristics under the single phase shifting control are analyzed in detail.Through the comparison and analysis of three typical converter application scenarios,it lays the foundation for the design of hybrid energy storage system and DC solid state transformer converters.(2)The coordinated control strategy and topological structure of cascade hybrid energy storage system is studied.The energy storage system structure and characteristics of converter of parallel and cascade are analyzed.Then,a hybrid energy storage system based on dual active bridge converter and Buck/boost bidirectional converter is designed,and its control strategy is put forward in combination with the operation characteristics of microgrids.The simulation results show that proposed structure and control strategy have many advantages.The frequency divis ion compensation of the DC bus voltage fluctuation is realized.Using supercapacitor as a buffer,the operation times of lithium batteries are reduced,and the life of hybrid energy storage system is extended.Meantime the hybrid energy storage system with low voltage and isolated access to DC microgrids is realized,which improves the security of system.(3)The coordinated control strategy and topological structure of DC solid state transformer based on dual active bridge converter are proposed.The topology of DC solid state transformer based on series parallel connection of dual active full bridge converter is designed.Then,the average equivalent circuit model of DC solid state transformer is established,and its control modes are analyzed.According to the actual operating characteristics of the multi-voltage grade DC microgrids,and based on the DC bus voltage stratification principle,a DC solid state transformer coordinated control strategy and working pattern selection scheme are put forward that aims at the fluctuation of DC bus on both sides of the DC solid state transformer.The proposed coordinated control strategy can accurately determine the running state of DC microgrids on both sides of the DC solid state transformer,effectively control the power transmission between DC buses,and maintain the stable and reliable operation of the whole DC microgrids.(4)A multi-voltage grade DC microgrids experimental platform based on DSP is developed.A multi-voltage grade DC microgrids structure is designed.The main circuit,sampling and conditioning circuit are constructed,and the system control program is developed.Thus,the DSP based multi-voltage grade DC microgrids experiment platform is constructed.Finally,the different operation modes of the microgrid are designed,and the effectiveness of the proposed control strategy is verified by experiments.