Research On Control Strategy Of Power Electronics Transformers

Power electronic transformer is a device that combines power electronic devices and high-frequency transformers.In addition to the voltage conversion,electrical isolation and energy transfer of conventional transformers,it can also realize power flow control and power quality adjustment,effectively isolating voltage fluctuations and harmonic transfer.The research contents in this thesis mainly include:(1)The system structure of power electronic transformer is introduced.The topology and control strategies of each stage are described in detail separately.The control strategies of medium-voltage input stage mainly include the basic modulation strategy,the sub-module capacitor voltage control strategy and the arm circulating current control strategy.The control strategies of intermediate isolated stage mainly include the phase shift modulation strategy and the voltages and currents equalization control strategy.The control strategy of the low-voltage output stage mainly includes the space vector control strategy.(2)The basic control of power electronic transformers is introduced.According to the operating characteristics of distribution network in off-grid mode,a power coordination control strategy applied to off-grid mode is proposed.By establishing a globalized multi-port transmission model,the operation status of each port is unified,the standardization of the multi-port is realized,and the advantages of energy intercommunication and mutual support of each port of the power electronic transformer are fully utilized.The proposed power coordination control strategy is verified by MATLAB simulation system.(3)In a passive network,load imbalance is inevitable,even when the system is operating normally without faults.For the structural characteristics,when the load is unbalanced,the sub-module capacitor voltage ripples,the effective values and peak values of the arm currents will significantly affect the operating performance of the medium-voltage input stage.This thesis introduces and compares three circulating current control methods of medium-voltage input stage under unbalanced load conditions,and proposes a circulating current control method based on instantaneous information with over-current limitation to suppress potential arm over-current.The proposed method of circulating current injection based on instantaneous values and with overcurrent limitation offers a compromise between peak arm currents and capacitor voltage ripples.The mathematical expressions of capacitor voltage ripples of sub-modules under three circulating current control methods are derived.The correctness of theoretical derivation is verified by MATLAB and RIDS simulation system.