Research On Voltage Balance Coordination Control Strategy Of Modular Cascaded Power Electronic Transformer

The status of clean energy in the world continues to rise,making the power grid gradually develop towards the direction of security,control,extensive interconnection and intelligent interaction.Transformer is the core equipment of voltage change and frequency conversion in Power grid.In recent years,Power Electronic Transformer has gained wide attention.In order to cope with the high voltage and high power application scenarios and the higher demand of transmission and distribution level and capacity,the input series topological structure of power electronic transformers has attracted more attention.Therefore,it has important theoretical value and engineering application prospect to study the power electronic transformer with series topological structure.This thesis takes single-phase two-unit cascading power electronic transformer as the research object,and studies the voltage imbalance of each module in the input stage,the current imbalance of each module in the intermediate stage,and the frequency conversion speed regulation of the output stage.The specific work is summarized as follows:For the input stage of single-phase cascaded power electronic transformer,the requirement is to obtain the capacitor voltage with fast balance and strong antiinterference ability.In this thesis,the mathematical model of the rectifier stage is constructed by the SOGI method,and the voltage and current double closed-loop control strategy is adopted for the rectifier stage.The PI controller is used for the outer voltage loop,and the non-inductance decoupling control strategy is used for the inner current loop.The voltage balance rate of input stage is further accelerated by using two-dimensional modulation.The simulation results of Matlab/Simulink on the input stage show that the control strategy and modulation strategy.The requirements of the intermediate stage of cascaded power electronic transformer are: bidirectional energy flow.Since the voltage balance of each module unit has been completed in the input stage,the single phase shift control is realized in the intermediate stage,which can meet the requirement of bidirectional energy flow while realizing electrical isolation.The feasibility of the control strategy adopted for the intermediate stage is verified in Matlab/Simulink.For the output stage of singlephase cascade power electronic transformer,its requirement is to realize the function of frequency conversion.After comparing SPWM algorithm with SVPWM algorithm,the SVPWM algorithm is used for frequency control.The frequency conversion effect of SVPWM algorithm is verified by simulation in Matlab/Simulink.The results show that the output waveform of SVPWM algorithm is symmetric in phase and has good sinusoidal degree.Finally,the low-power experimental platform of the modular cascade power electronic transformer is build adopted in this thesis for the single-phase two-unit cascade power electronic transformer.