Research On Modeling And Optimal Control Of Cascaded H-bridge Power Electronic Transformer

Under the background of contemporary energy internet,single function of traditional power frequency transformer cannot meet new requirements of power system.Therefore,with the development of power electronic devices and continuous advancement of distributed generation and smart grid,power electronic transformer has become a research hotspot at home and abroad.Power electronic transformer is a new equipment that combines power electronic technology with high-frequency voltage conversion technology,which has advantages of voltage conversion,electric isolation,connecting with distributed energy resources,controlling power quality and environmental friendliness.In recent years,it has been used in electric locomotive traction,new energy grid-connection,AC/DC hybrid distribution network and other fields.This paper takes cascaded H-bridge power electronic transformer as the research object,and thorough researchs on the power quality optimization of rectifier stage and isolation stage.Firstly,a composite current inner loop control strategy is proposed for the rectifier stage of cascaded H-bridge power electronic transformer—cascaded H-bridge rectifier,which is susceptible to disturbance and other factors.Primarily,the topology of cascaded H-bridge rectifier is introduced,the mathematical model is derived and the DQ transformation is used to decouple the grid-side current into direct current for controller design.Advantages and disadvantages of the two traditional controllers are analyzed,and a new control based on passive control combined with extended state observer as a disturbance observation is proposed.For the problem of unbalanced capacitor voltage on the DC side,the pulse compensation method is used to balance the voltage of each stage.Then,the proposed control strategy is compared with PI double closed-loop control and passive control in MATLAB/Simulink software.Results show that the system has fast dynamic response and good robustness under the proposed control strategy.Secondly,aiming at the problem of large backflow power in the isolated stage of cascaded H-bridge power electronic transformer—dual active bridge DC-DC converter,a control strategy combining artificial fish swarm algorithm with dynamic adjustment penalty function is proposed.Primarily,four phase-shift modulation strategies and the causes of backflow power are introduced.The system mode under triple-phase-shift modulation is analyzed in detail,and the transmission power and backflow power are derived.An appropriate mode is selected to optimize its backflow power by artificial fish swarm algorithm.Then,the proposed control strategy is compared with single-phase-shift and double-phase-shift modulation in simulation software.Results show that the proposed strategy can effectively improve the transmission efficiency of the system and reduce the backflow power.Thirdly,a virtual power balance control strategy is proposed to solve the problem of unbalanced power in the isolation stage of cascaded H-bridge power electronic transformer.Primarily,the causes of power imbalance of parallel output DC-DC converter are analyzed,and three common power balance control strategies are introduced.Then,the proposed control strategy is used to reduce imbalance of the system.The model is established in simulation software and compared with the system without balance control.Results show that the proposed strategy improves the reliability of multi-module operation.Finally,a three-module cascaded H-bridge power electronic transformer overall model is constructed and two cases of load mutation and grid side voltage fluctuation are set to further verify the effectiveness of the proposed strategy.Results show that when the system is disturbed,all variables can quickly reach stability with slight fluctuation.