Research On Control Strategy Of Voltage Balance And Power Balance For Cascaded Power Electronic Transformers

Power Electronic Transformer(PET)is a new type of intelligent transformer that uses power electronics and high frequency converters.It can replace the traditional power transformer with large volume,realize the functions of voltage level conversion,electrical isolation and energy transmission.In addition,the power electronic transformer also has the characteristics of high power density,good controllability,strong compatibility and intelligence.It is expected to be widely used in the future for smart grid,locomotive traction,new energy power generation and other occasions.In this paper,the three-unit cascaded power electronic transformer is taken as the research object.The following research work is mainly done for the cascaded PET input cascaded H-bridge rectifier and the isolated-stage dual-active full-bridge DC-DC converter:Firstly,the structure and mathematical model of cascaded PET input stage cascaded Hbridge rectifier are introduced in detail.The power transfer expression of cascaded H-bridge rectifier is established,and the output voltage of cascaded PET input stage cascaded H-bridge rectifier is analyzed.For the reason of balance,the modulation methods of the two output side cascaded H bridges are introduced.Based on the sliding mode theory,this paper proposes a new double closed-loop cascade H-bridge rectifier based on sliding mode variable structure.The voltage balance control strategy is adopted.Compared with the traditional double closed-loop control,the simulation results show that: Control strategy When the voltage amplitude and phase of the grid side are abrupt,the DC-side voltage can maintain better stability and faster dynamic response under the new double-closed loop direct power control strategy based on sliding mode variable structure.The new double-closed loop direct power control strategy of the sliding mode variable structure is faster and more stable in the reference voltage,achieving voltage balance,and the system has strong anti-interference ability and good robustness.Secondly,the cascading PET isolation stage dual active full-bridge DC-DC converter is studied.The power transmission mode of DC-DC converter is analyzed.The principle of single phase shift control is introduced.Based on this,differential flatness is proposed.The theoretical single phase shift control strategy is compared with the single phase shift control.The simulation results show that the single phase shift control based on the differential flat theory has faster when the input voltage and load of the DC-DC converter are disturbed.Dynamic response performance ensures stable output voltage.Aiming at the transmission power balance of each unit DC-DC converter,a power equalization control strategy based on differential flattening theory is proposed.The simulation shows that under the condition that the inductance parameters of each unit are different,after the power balance control is input The output current of each unit of the parallel DC-DC converter can be ensured to achieve power balance.Finally,this paper proposes a cooperative control method for cascaded PET input and isolation stages.The input stage cascaded H-bridge rectifier does not perform voltage balance control.The modulation wave size of each rectifier unit is the same.The cascaded H-bridge rectifier is based on The model of the virtual flux linkage predicts the direct power control strategy.The isolated-stage dual-active full-bridge DC-DC converter controls the voltage balance of the rectifier stage and indirectly controls the output power balance of the DC-DC converter.The simulation results show that the model based on virtual flux linkage predicts the direct power control strategy to ensure the unit power factor operation of the network side.The cooperative control of the input stage and the isolation stage can realize the parallel output voltage stability of the isolated dual active-bridge DC-DC converter.The output capacitor voltage balance of each unit of the cascaded H-bridge rectifier and the transmission power balance of each unit of the DC-DC converter.