Optimized Design And Reliability Evaluation Of DC Solid State Transformer Based On SiC MOSFET

DC distributed power source and the power load of the DC grid are increasing with the development of the smart grid.So it is necessary to promote the application the DC distributed grid.The DC solid state transformers(DC-SST),which can be used as the energy router,are the key equipment among DC distributed grid.The DC-SSTs can achieve high efficient DC-DC power conversion,reduce the power losses during the power conversion,control the bidirectional power flow,and insulate the power energy in the DC grid.In addition,based on the excellent thermomechanical performance of the silicon carbide(SiC)power devices,the efficient and reliability of the DC-SSTs can be improved effectively.This paper optimized the design of a DC-SST in the topology of dual active bridge(DAB)and established the experiment prototype.This paper also analyzed the advantages of the SiC MOSFET using in DAB based on the cooperation with Si IGBT devices.The main content of this paper is shown below:This paper established the small signal model of the DAB topology,controlled the voltage based on the single shift modulation,and validated the method on the test prototype.This paper derived the transfer function of a DAB through the analysis of operation condition,designed closed loop compensator,and compared the system frequent responsibility before and after compensated.This paper evaluated the system anti-interference capability in the terms of low frequent perturbation in the input voltage and step pulse of the output load referencing the interference of a practical DC distributed grid.In order to validate the anti-interference capability of DAB prototype and the effectivity of the voltage closed loop control strategy,this paper established a DAB test prototype and tested the power step pulse.This paper established a DAB DC-SST test prototype.This paper established a DAB DC-SST test prototype.In the part of magnetic elements design,the most suitable magnetic material and its size are effectively selected by AP rule,combined with ANSYS finite element model to analysis winding leakage magnetic field intensity and select the lowest winding leakage inductance interleaved winding structure.Meanwhile,the lower limit of the transmission power,the upper limit of the device current stress and the upper limit of the root mean square current value as the boundary condition,is used to determine the auxiliary inductance range at different DAB operating frequencies,which provides the basis for the selection of the working point of the DAB prototype.In the part of power device performance test,the static characteristics in the terms of threshold voltage,output characteristic and conduction resistance,and the dynamic characteristics in terms of turn-on/off speed and switching power losses under varies of gate resistance of SiC MOSFET are investigated based on the comparison with a Si IGBT with the same rating power level.Based on the DAB converter,a DC source and electric power load test prototype achieved the 1kW load,200 kHz frequency and 96.6% efficiency operation.In the part of power device performance test,the static characteristics in the terms of threshold voltage,output characteristic and conduction resistance,and the dynamic characteristics in terms of turn-on/off speed and switching power losses under varies of gate resistance of SiC MOSFET are investigated based on the comparison with a Si IGBT with the same rating power level.Based on the DAB converter,a DC source and electric power load test prototype achieved the 1kW load,200 kHz frequency and 96.6% efficiency operation.Based on the device lifetime model,this paper evaluated the reliability of the DAB converter considering the multi heating source thermal coupling.The fatigue indicators of the solder layer of SiC MOSFET and Si IGBT are calculated by Anand constitutive model in thermomechanical modeling.The lifetime of the power devices is calculated by Morrow physical model and then transferred to Coffin-Manson model in the relationship of lifetime and junction temperature swing.This lifetime model is validated by the results from an accelerated reliability test.Because the DAB converter has a higher power density based on the SiC power devices,the thermal coupling effectivity among the multi-devices is significant.In order to accurately evaluate the reliability of the DAB system,this paper established a thermal coupling model this converter system,and the coupling thermal network parameters were extracted.Considering the thermal coupling effectivity,the junction temperature profile of the DAB system during a daily load profile,which is used as a SST in DC distributed grid,is calculated and derived by rain flow algorithm to statistic the junction temperature swing.Based on the lifetime model mentioned above,the lifetime of SiC MOSFET in DAB system is 4.47 times compare with that of SiC IGBT.In conclusion,the reliability can be enhance effectively in the contribution of utilizing SiC devices into DAB converter as a DC SST in DC distributed grid.