Research On Improved Control Strategy Of Power Electronic Transformer Under Unbalanced Load

Power electronic transformer(PET)is a new type of intelligent transformer which is composed of power electronic devices and high-frequency transformer,it could achieve the functions of electrical isolation,power conversion and energy transmission.Due to its multi-port,flexible control,high compatibility and so on,power electronic transformer is widely applied to microgrid,and is becoming a very important power device in the development of smart grid in the future.Power electronic transformer is often faced with the problem of unbalanced load in microgrid,which has a serious impact on the stable operation of power electronic transformer and power quality.This thesis mainly studies the control strategy under the unbalanced load condition based on power electronic transformer,the main contents are as follows:(1)The topology of power electronic transformer studied in this thesis is three-stage cascade structure.The input-stage is modular multilevel converter,the isolation-stage is the dual active bridge and output-stage is three-phase four-leg converter.The mathematical model of power electronic transformer is established and its principle is expounded.The basic control strategy of each stage is studied to ensure the stable operation of power electronic transformer under normal condition.(2)Aiming at the unbalanced load condition,the mechanism of the secondary pulsating power in the system is studied,the power flow of the system and the influence of the secondary pulsating power on the dual active bridge are analyzed.Based on the relationship between the secondary pulsating power and the angle of phase-shift,the nonlinear part of the dual active bridge DC-DC converter is linearized and its small signal model is established.According to the small-signal model,the AC link is added to the DC link of the traditional control strategy,and the PR controller is used to control the phase-shift angle to realize the secondary pulsating power transmission in the dual active bridge DC-DC converter.(3)The linearized small signal model of the improved control strategy is established,and the transfer function is derived to verify the validity of the improved control strategy.Not only can the improved control strategy accurately allocate the proportion of the secondary pulsating power retained on the middle voltage DC bus and low voltage DC bus of power electronic transformer,but it can also suppress the voltage fluctuation of DC bus according to different requirements.Beside,it can control the influence range of the secondary pulsating power.(4)The characteristics of MMC under the condition of the secondary pulsating power are analyzed,including AC/DC power analysis and sub-module pulsating power analysis.By means of circulating current injection,the sub-module capacitor can store energy to carry on the the secondary pulsating power,so as to keep the mid-voltage DC bus stable.In addition,the influence of the current injection on the electric characteristics of MMC,such as the interphase current and the voltage of sub-module,is analyzed.Finally,through the coordinated control of all control strategies of power electronic transformer,the symmetrical output AC voltage,medium and low voltage DC bus voltage stability can be achieved under unbalanced load conditions and the power quality of power electronic transformer is improved.