Control Strategy Of Three-level Half-bridge DAB Converter Under Unbalanced Load

The three-level half-bridge DAB converter has the characteristics of high voltage and bipolar output.It is a kind of topology suitable for DC microgrid and other applications with positive and negative bipolar output requirements.This thesis takes the bipolar DC micro-grid as the research background and the three-level half-bridge DAB converter as the research object.In order to balancing capacitor voltage under unbalanced load,this thesis proposes a voltage banlance strategy by controlling modulation and a period sequence control strategy to realize capacitor voltage equalization during load fluctuation and switching.This thesis introduces from the following aspects.First of all,by dividing the mode under balanced load conditions and analyzing the power characteristics and soft switching characteristics in each mode,the optimal formula between the control variables under the TPS control strategy is derived to minimize root-meansquare(RMS)value of the inductor current.After optimized,the RMS value of the inductor current is the smallest,and all switches can realize soft switching in the full power range.In the next place,the unbalanced load current under unbalanced load conditions causes the unbalanced capacitor voltage by injecting DC offset into the inductor current and the transformer secondary current and causing the deadtime to have an asymmetrical effect on the transformer secondary voltage.To solve this problem,this thesis proposes a voltage banlance strategy by controlling modulation.This strategy achieves capacitor voltage banlance in the triple phase shift mode by controlling the specific edge of the transformer secondary voltage to balance its high-level time and low-level time.Afterwards,this thesis proposes a period sequence control strategy to solve the problems of the inductor current and the transformer secondary current DC offset and the unbalanced capacitor voltage under the unbalanced load conditions.By inserting the special voltage equalization period to traditional periods according to a certain rule to form the period sequence to control the converter,this strategy achieves zero DC offset in the traditional period and capacitor voltage balance.Eventually,on the basis of the above research,the feasibility of the two voltage balance strategies is verified by building a Matlab/Simulink simulation model.Finally,the experiment platform of the DAB converter based on the FPGA controller and the control program of FPGA chip based on the Verilog HDL language are completed.Several groups of low-voltage and low-power experiments under different unbalanced load conditions are carried out to verify the effectiveness of the two voltage banlance strategies.Whereafter,they are compared from complexity,voltage equalization ability,dynamic response speed,converter efficiency,and scope of application.