Research On Multilevel Power Electronic Transformer Based On Modular Multilevel Converter

In recent years,the traditional transformer which lacks the functions of voltage sag compensation and power factor correction has been difficult to meet the requirements of the increasingly complex power system because of the rapid development of DC microgrid and distributed generation.Power electronic transformer(PET)arises at the historic moment in order to improve the controllability and compatibility of power system.At present,the power electronic transformer based on three-stage topology has been more widely studied because the three-stage structure of power electronic transformer topology has more flexible topology and a wider range of applications.Modular multilevel converter(MMC)has the advantages of low harmonic content,high transmission efficiency and good scalability.The combination of modular multilevel converter technology and three-stage PET makes it possible for PET to be used in high-voltage and large-capacity transmission and distribution systems.This study makes an in-depth study on the control strategy of MMCPET input stage and the topology of MMC-PET intermediate isolation stage under the condition of unbalanced grid voltage.First of all,it analyzes the MMC topology and working principle of the MMC-PET input stage and introduces the modulation mode and capacitor voltage equalization method adopted in the MMC converter of the MMC-PET input stage.In addition,it deduces the mathematical model of the MMC-PET input stage according to the MMC topology of the MMC-PET input stage.A double closed loop decoupling controller and a circulation suppression controller based on PI controller are designed,and a simulation model is built in Matlab/Simulink for simulation verification under the normal operation state of the power grid,for the MMC-PET input stage MMC converter.Secondly,the mathematical model of MMC-PET input stage in positive and negative sequence system is derived under the condition of unbalanced grid voltage.The positive and negative sequence inner loop current PI controller are designed respectively according to the mathematical model of positive and negative sequence system.This paper designs a positive and negative sequence current inner loop passivity-based control(PBC)based on Euler-Lagrange(E-L)model to simplify the complexity of the controller and speed up the response speed.After all,the MMC-PET input stage is a nonlinear system,and the traditional positive.In addition,negative sequence inner loop current PI controller can not eliminate the influence of nonlinearity on the control system performance.In addition,there are many PI parameters in the control system.The simulation model of MMC-PET input stage under the condition of unbalanced grid voltage is established in Matlab/Simulink.Besides,the PI control and passive control are compared and analyzed.In addition,the working principle of dual active bridge(DAB)converter is analyzed,and the mathematical model is deduced aiming at the MMC-PET isolation stage composed of dual active bridge input stage in series and output stage in parallel.The input voltage-sharing controller is designed to stabilize the input voltage and transmission power of each bi-directional active full-bridge converter.In view of the problems of low reliability,poor flexibility and small adaptive range of DAB used in the intermediate isolation stage of the existing three-stage MMC-PET.A new DC-DC converter topology of MMC-PET isolation stage based on modular multilevel technology is proposed to improve the flexibility of power supply and the ability of uninterruptible power supply.In the meanwhile,a model predictive control(MPC)control method based on independent value function is adopted for the single-arm modular multilevel converter on both sides of the DC-DC converter.It establishes independent value functions for multiple control objectives,which effectively avoids the weight allocation of the value function in the traditional MPC and reduces the amount of calculation of the MPC controller.Finally,the mathematical model of MMC-PET output stage is derived,and the double closed-loop controller based on PI controller is derived in view of the three-phase full-bridge voltage source inverter topology of MMC-PET output stage.The output level simulation model of MMC-PET is built in Matlab/Simulink to verify the effectiveness of the controller.