Research On Multi-port Power Electronic Transformers For Distribution Network

At present,in order to promote the scientific utilization of clean energy and distributed energy,and to establish a more flexible energy network,power electronic transformer based on MMC have gained unprecedented attention.Power electronic transformer(PET)based on MMC not only cover the basic functions of traditional transformers,but also have the functions of power control,fault isolation,and its output waveform quality is good,and it’s easy to realize voltage level expansion and redundant control.But its high cost,huge volume,low power density,and sub-module voltage are not easy to be balanced,these defects hinder its wide application.In this paper,based on the three-phase four-arm MMC topology,which can significantly reduce the number of sub-modules,three new topologies are proposed,and the specific control methods of three new topologies are studied.Firstly,the structure characteristics and basic mathematical model of three-phase four-arm MMC are studied.On this basis,a four port PET topology based on two-level sub-module is proposed,and its control method is studied.For the control method of new topology,the first problem need to be solved is the symmetry of three-phase four-arm MMC.The three-phase four-arm MMC isn’t a three-phase symmetrical structure,one of its phases is composed of capacitors in series.Compared with the traditional three-phase six-arm MMC,its controllable degree is reduced.Therefore,a two-phase 60 ° control method is proposed to preliminarily ensure its symmetrical output.Next,the proportional integral current control based on dq coordinate system,deadbeat current control and isolation stage power control are studied.The simulation model is built in Matlab/Simulink,and control performance is good when the above control method is applied to it.Secondly,using the structural characteristics of three-phase four-arm MMC and three-phase six-arm MMC,on the basis of four-port PET topology based on two-level sub-modules,a five-port 10 k V and 20 k V contact PET topology is proposed,where it has commonality with four-port PET,corresponding control method is continue to be used.The sub-module capacitor voltage balance control and the improvement of the three-phase six-arm MMC DC voltage utilization control were studied in depth,and a five-port 10 k V and 20 k V contact PET simulation model was built in Matlab/Simulink and performance of the control method was verified.Then,based on the theory that multi-level sub-module can further reduce the number of sub-modules and reduce the cost and volume of the converter,a four port PET based on three level sub-module is proposed,in which the three-level double half bridge topology is used as the high-voltage stage sub-module,and the diode neutral point clamp topology is used to replace the original H-bridge topology of double active bridges used in the isolation stage.The controller design and modulation of the new topology are studied,and the corresponding Matlab / Simulink simulation of the new topology is built.Finally,the hardware experiment platform of MMC is designed,including the basic structure design of MMC hardware circuit,DSP main control circuit design,FPGA board circuit design,design of pulse width modulation program based on FPGA,etc.