| DC distribution network has attracted much attention due to its advantages of more efficient and reliable reception of new energy distributed generation systems such as wind and solar,and more adaptable to the changing trend of load characteristics on the user side.It is also an important part of the future intelligent distribution system.As the interface of electric energy conversion in DC microgrid,the stable and efficient operation of isolated DC-DC converter is of great significance for the development of DC microgrid,especially the IBDC(isolated bidirectional DC-DC)converter with energy bidirectional flow capbility,which is more conducive to promoting energy saving,emission reduction,and achieving sustainable energy development.Compared with two-level IBDC converter,three-level IBDC converter has the advantages of lower voltage stress of the switching device and smaller passive components,which can improve the performance of the converter while reducing the cost.For common DC interface applications with voltage levels between 600 V and 800 V,the power loss comparison analysis of two-level DAB converter using SiC MOSFET device and three-level TL-DAHB converter using Si MOSFET device is carried out in this paper.Based on this,the efficiency optimization control method of the TL-DAHB converter is further designed.In addition,the design of the TL-DAHB converter prototype experimental platform and the experimental verification of the proposed optimization control method are also completed in this paper.Firstly,the paper analyzes the working principle of TL-DAHB converter under SPS(single-phase-phift)control and EPS(extended-phase-shift)control,and all operating modes of the circuit in one switching cycle are analyzed in detail.Besides,relevant characteristics under this two control strategies are also analyzed and compared,such as power transmission,soft switching,peak current,RMS current,reflux power,and reactive power.It is pointed out that EPS control is more flexible than SPS control and has the advantage of optimizing converter performance.Secondly,the power loss models of the DAB converter and the parallel TL-DAHB converter which transmit the same power in the DC interface application with bus voltage between 600 V and 800 V are established respectively.In the power loss model,the conduction loss and switching loss of the active devices in each converter are quantitatively analyzed and calculated.By comparing the losses,it is found that the power loss of the parallel TL-DAHB converter based on Si MOSFET device is smaller than that of the DAB converter based on SiC MOSFET device,which means it can achieve higher efficiency of energy conversion.Then,based on the comparison of the characteristics of the two control strategies and the power loss of the two converter topologies,this paper designs a further efficiency optimization control method for the TL-DAHB converter controlled by EPS.The loss analysis results show that the conduction loss of the internal anti-parallel diode in the switch tube is the main component of the losses in the TL-DAHB converter.Therefore,in order to improve the overall efficiency of the converter,the peak current is indirectly taken as the optimization target,and the Lagrange multiplier method is used to solve the global optimal phase-shifting combination which minimizes the peak current under EPS control with different phase-shifting relations.Under the same transmission conditions,the peak current characteristic under the peak current global optimization EPS control designed in this paper is compared with those under SPS control and traditional EPS optimization control.The simulation results verify the effectiveness of the proposed optimal control method.In order to realize the closed-loop control of TL-DAHB converter under peak current global optimization EPS control strategy,this paper designs the optimized controller and applies the state space averaging method to establish the small signal model of TL-DAHB converter under EPS control.According to the mathematical model of the converter,the transfer functions from phase shift ratio to output voltage and input voltage to output voltage are simulated and verified.Based on this,the parameters of the closed-loop controller are designed,and the stability of the closed-loop control system of peak current optimization EPS control strategy is verified by simulation.Finally,this paper introduces the experimental platform of TL-DAHB converter prototype,and describes the selection of active devices,inductor and transformer design,driving circuit and sampling and conditioning circuit design.Based on the prototype experimental platform,the working modes of TL-DAHB converter under SPS control and EPS control are verified.Besides,the characteristics of peak current and efficiency are also compared among SPS control,traditional EPS optimization control and peak current global optimization EPS control.The experimental results verify the correctness of theoretical analysis and the effectiveness of the designed optimization control method. |
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