Research On Voltage Sag Isolation Technology On Cascaded Power Electronic Transformer

Voltage sag is a key problem in the field of power quality,and the application of sensitive load and the development of distributed power generation make this problem more and more serious.Compared to traditional transformers,the power electronic transformer(PET)has the advantages of multi-port,multi-function,flexible and controllable,while not only provides AC/DC interfaces for renewable energy generation,distributed energy storage,electric vehicles and other equipments,also has a certain voltage sag isolation ability,which can provide a new means for the improvement of power quality.However,at present,the research on PET’s voltage sag isolation ability is still on the stage of qualitative analysis,and there is little research on its coordination with energy storage.In order to accurately measure PET’s isolation ability and provide the basis for energy storage configuration,this paper makes a detailed quantitative analysis around the ability of PET’s voltage sag isolation,and studies the optimal configuration of energy storage according to voltage sag and interrupt problems.Firstly,the input-part voltage sag isolation ability influenced by the PWM regulation,the DC ripple voltage and the current limiting of the power semiconductor devices is analyzed,as well as the isolation-part voltage sag isolation ability influenced by DAB power transmission capacity and the current limiting of the power semiconductor devices.Secondly,the voltage sag isolation capability of input-part and isolation-part are integrated,and the quantitative model of the voltage sag isolation capability of PET is established.The range of voltage sag amplitude and duration that PET can effectively isolate are determined,and the method of dividing the isolation capability into I and II levels is proposed.In addition,in order to improve the dynamic response ability of PET,a feedforward control method for isolation part is proposed,which can effectively reduce the control delay.Finally,the configuration of energy storage type and capacity is studied for the situation where PET,energy storage and electric vehicles are applied.In the light of voltage sag,a PET-ESS energy storage configuration method considering the PET’s isolation ability is proposed,which realizes the allocation of hybrid energy storage types and capacities.In the light of voltage interruption,based on the V2G response ability prediction model and the Reliability-at-Risk(RaR)model,a PET-ESS energy storage configuration method considering V2G response capability is established...