Research On Control Strategy Of Three-level Cascaded Convert After DC-port Fault

With the goal of ‘carbon peak,carbon neutralization’ proposing,the program of ‘Aviation fuel-to-Electricity’ has been adopted.However,there are some problems in the airport ground power unit(GPU),such as low utilization rate,polluting gas emission,output voltage distortion and so on.Furthermore,civil aviation passenger traffic has been increased in decades.It also causes the increase of wide-body passenger aircraft demand.With the above demand increase,the problems of GPU are becoming more and more prominent.If GPU adopts multi-level cascaded converter instead of traditional AC-DC-AC converter,it can improve GPU utilization,power quality,even can realize centralized power supply.Therefore,this thesis researches the airport centralized GPU system based on 3LNPC-CC,and mainly researches the control strategy of 3LNPC-cascaded inverter(3LNPC-CI)when a power supply port fails.It can provide the theoretical support for the further engineering realization of GPU.The topology of 3LNPC-CC is analyzed in this thesis.Thus,it determines the different capacitor state of charging and discharging,which is controlled by the different switching states.Then,the mathematical model of 3LNPC-CC is analyzed.According to the model,the double closed-loop control strategy is adopted based on the CPS-SPWM modulation in this thesis.Finally,it is verified that the effective of CPS-SPWM and closed-loop control strategy by the simulation results.As the problem of unbalanced DC-port voltage between 3LNPC-CC modules and the inherent problem of unbalanced voltage between capacitors in module,the reasons are analyzed.According to the reasons,a control strategy of matrix generation modulation(MGM)is adopted to realize the normal operation once 3LNPC-CC suffer a DC-port fault.In addition,when there is a DC-port fault,3LNPC-CC can normally operation under the modulation degree under 0.82 by using the MGM according to the calculation results.When the DC-port of a module fails,the simulation verifies that the voltage keeps balance between modules and the capacitor voltage can achieve balance in the module.In addition,GPU can ensure stable and effective operation when there is a DC-port fault in 3LNPC-CC and the modulation is lower than 0.8.The CPS-SPWM modulation,the double closed-loop control strategy,the algorithm of capacitor voltage balance in module and MGM strategy are experimented by a 1kw step-down experimentally prototype based on the FPGA.Simultaneously,Verilog HDL is used in FPGA.Finally,when there is a DC-port fault,the whole strategies are verified by the experiment in this thesis,which includes modulation strategy,double closed-loop strategy,MGM strategy and capacitor voltage balance algorithm.According to the results of experiment,the whole control strategy adopted in this thesis can realize the voltage balance between modules.The results achieve the expected goal.