| Distributed photovoltaic is an important measure to promote the development and utilization of renewable energy in China.The Triple Active Bridge(TAB)converter can realize power transmission and conversion between each two ports.Compared with Dual Active Bridge(DAB)converter,it has higher control flexibility and better power supply reliability.Among them,the application of high-frequency transformer can not only realize the complete isolation and voltage matching between ports,but also greatly reduce the volume and weight of the device and improve the overall conversion efficiency of the device.Therefore,TAB,as an active exploration and attempt of distributed photovoltaic access to DC distribution system,has become one of the key research hotspots of renewable energy.At present,the research on TAB is still in the preliminary theoretical research and discussion stage,and there are still a large number of theoretical and common key technical problems to be solved.Based on the national key research project "Key technologies and equipment of distributed photovoltaic multi-port access DC distribution system(2018YFB0904100)" and "Key Technology and Application of Medium and Low Voltage DC Power Distribution System(2018YFB0904700)",loss optimization and transient characteristics optimization of the TAB are studied.The simplified analysis model of the high-frequency-link ac current,the TAB loss optimization method,the dc bias suppression method,optimization of output voltage transient performance under port energy fluctuation are all analyzed and studied.It lays a theoretical foundation for the common problems in the research process of TAB and its promotion and application.The main innovation points are as follows:(1)Based on the existing modal analysis method,the switching process of each mode of TAB is analyzed for a specific working mode,and the power characteristics and soft characteristics of TAB are analyzed.By comparing the current research status of DAB,the key problems to be solved in TAB are put forward.Considering the symmetric topology of each port of TAB,the feasibility of superposition theorem in TAB analysis is demonstrated in detail,which lays a foundation for the feasibility of subsequent simplified analysis methods.(2)A simplified analysis method is proposed for the calculation of high-frequency chain current of TAB.Considering the challenges posed by the diversified combinations of phase shift angle and duty ratio to the existing current analysis methods,a new high-frequency chain current calculation method based on the superposition theorem is proposed in this paper.TAB is decomposed into several sub-circuits with double voltage sources,and the general expression of inductor current in the sub-circuit model is derived.Then,the expressions of current vertices,peak current and RMS current of high-frequency chain current at each port are obtained.The simplified analysis method presented in this paper is easy to operate and has high accuracy,and is not affected by the working modes and phase-shifting control methods.It does not need to classify the working modes and calculate the current throuh piecewise linear method.It is not easy to miscalculate under the condition of many freedoms and various combination modes.(3)Based on the simplified analysis model of high-frequency chain current of TAB,the loss optimization method of TAB is proposed.One of the key advantages of TAB application is the soft switching ability of the switch.For high voltage switching devices,realizing ZVS of all switches is an effective way to reduce the loss,and also an effective means to improve the conversion efficiency.Based on the simplified analysis model of high-frequency chain current above and the ZVS constraint conditions of the switch,the ZVS region of the full switch and the global current stress of the converter are described.The duty ratio of port 1 square wave voltage and the phase-shift angel between the ports are selected as the control variables to realize multi-objective optimization of switching loss and on-state loss in full power range.(4)In view of the dc bias current in the inductor and the transformer in the transient regulation process,a simplified dc bias analysis and suppression method is proposed.The dynamic change of phase-shifting angle will break the volt-second balance of the inductor,and then generate dc bias current,which will affect the transient response characteristics of the converter.In order to improve the dynamic characteristics of the system and the overall conversion efficiency,the mechanism of TAB dc bias is analyzed in this paper.A simplified circuit analysis model of dc bias is proposed,and a mathematical model of dc component elimination method is derived based on this model.A variety of suppression methods based on the combination of driving signals are obtained.The proposed method can eliminate the dc component of the high frequency chain current and the transformer excitation current at each port simultaneously in a unit cycle,without adding additional hardware facilities,and has the advantages of simple operation and good generalization and portability.(5)The energy fluctuation of the port is divided into input port voltage fluctuation and output port power fluctuation,and the small signal modeling analysis of the disturbance variable and output voltage is carried out respectively.Then,aiming at the energy fluctuation of the input port,the direct power control method is proposed in this paper.It can effectively improve the transient response characteristics of the output voltage when the input port voltage surges.In view of the power sudden change of the output port,this paper proposes the load current feedforward control method.It can effectively improve the transient response characteristics of the output voltage and reduce the transient response time.Finally,based on the simulation results,the effectiveness and feasibility of the proposed method are verified.In this paper,based on the construction of "smart grid" demand for multi-port dc converter fact,TAB is selected as the research object.The high frequency chain current solution method,switch loss,on-state loss,dc bias suppression method and transient response of the port volatge have been carried out in-depth study.TAB has broad application prospects in grid-connected systems,energy routers,DC transformers and other scenarios,with advantages of high control flexibility and good power supply reliability.The simplified analysis and optimization method of TAB loss and transient response proposed in this paper has strong portability,which can provide a reference for the analysis of multi-port DC converters and provide a useful reference for the challenges encountered in the practical process of power electronic equipment. |
