Research On Control Strategy Of Cascaded Power Electronic Transformer

Power electronic transformer,also known as solid-state transformer,is a new type of power transformer.It combines power electronic conversion technology with traditional power frequency transformer,overcomes the shortcomings of traditional power frequency transformer,and has the advantages of high efficiency,adjustable power factor and easy control.It also has the advantages of improving power quality and controlling the tide.and providing grid-connected interface for distributed energy,it have broad application prospects and high research value.This paper takes cascaded power electronic transformer as the research object,its topological structure includes rectifier stage,isolation stage and inverter stage,it belongs to three-pole structure.The structure has excellent comprehensive performance and complete functions.It is the main trend of power electronic transformer topological structure development.The main contents of this paper include:(1)The topology and working principle of the rectifier stage are introduced,and the carrier phase-shifted SPWM modulation mode is analyzed by Fourier transform.The results show that the carrier phase-shifted SPWM modulation can eliminate the harmonic content and improve the harmonic characteristics of the output waveform.At the same time,the topology of isolation stage double active bridge DC-DC converter and its working state under double phase-shifting control are analyzed.(2)The mathematical model and small signal model of the rectifier stage are established.In order to solve the problems of high switching frequency and high loss of cascaded H-bridge DC capacitors under capacitor voltage balance control strategy based on sequencing method,an optimized capacitor voltage balance control strategy is proposed.The strategy sets a maximum allowable difference between the capacitor voltage and the capacitor voltage mean,changing only the switching state of the device beyond the difference,while the other devices maintain the current switching state unchanged.The simulation results show that the strategy can reduce switching frequency and switching loss,and the fluctuation of output capacitor voltage is slightly increased,but within the allowable range of fluctuation,the influence on the overall waveform is also small.(3)The mathematical model and small signal model of isolated stage dual active bridge DC-DC converter are established,and the power characteristics of the converter under double phase-shifting control are.For the converter with large backflow power under double phase shift control,this paper uses a minimum backflow power control.The simulation results show that the algorithm can reduce the backflow power while restraining the secondary ripple and reducing the loss.In order to validate the validity of rectifier and isolation control,the simulation model of cascaded power electronic transformer system is built by using MATLAB/Simulink software,the simulation results of full load,unbalanced input voltage,unbalanced load and harmonic input voltage are given respectively.The results show that the cascaded power electronic transformer system is effective,it operates well under different conditions and can respond quickly to maintain the stability of output voltage.(4)The model and output power characteristics of wind power grid-connected system based on cascaded power electronic transformer are studied.For the rectifier stage and inverter stage of the system,the decoupling control based on hill-climbing search method and double closed-loop decoupling control based on voltage outer loop and current inner loop are adopted respectively.Aiming at the problem that the search step is difficult to determine in the hill-climbing search method,an optimized hill-climbing search method is proposed and simulated.The simulation results show that the optimized hill-climbing search method has fast response speed,can quickly track the optimal speed of the system,and reduces the disturbance in the control process,and improves the system performance.Stability.