Research On Control Strategy Of Cascaded Power Electronic Transformers

With the continuous in-depth research on the concept of smart grid and energy Internet,traditional power transformers have exposed many shortcomings.Compared with traditional transformers,power power transformers not only have the functions of traditional transformers,but also have small size and can achieve grid-side voltage.The advantages of current power factor operation and stable grid-connected interface have gradually become the core equipment of the new generation of power grids.This paper takes the single-phase cascaded power electronic transformer as the research object,and focuses on the following research work for the cascaded input stage CHB and isolation stage DAB:(1)Firstly,the topology and working principle of the input stage CHB of the cascaded power electronic transformer and the isolation stage DAB are introduced,and the current commonly used modulation methods of the cascaded H-bridge rectifier are introduced.At the same time,the current commonly used isolation stage DAB is also introduced.Three kinds of phase-shifting control methods,and the expressions of transmission power and return power of DAB under single-phase-shift and doublephase-shift control are analyzed and deduced respectively.(2)Analyze the control purpose of the cascaded input stage.In order to ensure that the grid-side voltage and grid-side current are unit power factors,this chapter proposes to use a common duty cycle control based on the virtual dq vector transformation,and establishes such a control to place an order The equivalent mathematical model and small-signal model of the cascaded H-bridge rectifier;then the reasons for the unbalanced output capacitor voltage of the cascaded H-bridge are analyzed.In order to solve the problem of capacitor voltage balance,two solutions are proposed: one is in the CPS-Under the SPWM modulation strategy,a voltage equalization loop is added to the virtual dq vector control link to balance the output capacitor voltage balance;the other is under the HPWM modulation strategy,a voltage balance control method based on capacitor voltage sequencing.Then,the output voltage harmonic content under the two methods are compared and analyzed,and the simulation model of CHB power electronic transformer is established in MATLAB/simulink to verify the correctness and effectiveness of the proposed control strategy.(3)Introduced the working principle of DAB synchronous triple-phase-shift control,and then established the mathematical model of transmission power and return power for DAB converters under the synchronous triple-phase-shift control method.Then the soft switching conditions of DAB in the two working conditions of voltage conversion ratio k?1 step-down state and k<1 step-up state are analyzed,and the two working conditions are obtained respectively,with soft switching as the constraint condition.The mathematical expression of the optimal phase shift angle,and then the optimal return power control of the synchronous triple phase shift control under all working conditions is proposed.Finally,a DAB simulation model based on synchronous triple phase-shifting control is established in MATLAB/Simulink,and the simulation results verify the correctness and effectiveness of the proposed control strategy.