Research On Power Quality Control Strategy Of Power Electronic Transformer Output Stage

Compared with the traditional transformer,the power electronic transformer(PET)can guarantee high quality power supply and facilitate the access of new energy,which is in line with the futurc development of smart grid.The PET is expected to be gradually applied to the distribution network in the future.However,the access of electric vehicles and the variety of loads all affect the performance of PET output stage inverter,thus affecting the stability and power quality of the power supply system.Therefore,this paper focuses on the power quality control strategy of the PET output stage,and mainly carries out the following work:Firstly,according to the topology of the five-stage PET,the mathematical models and basic control strategies of the PET input and isolation stages are analyzed.The overall simulation model is established,and the theoretical analysis and simulation results are compared and verified.Secondly,based on the four-leg main inverter of PET output stage,a dual quasi-proportional complex integral(DQPCI)controller is proposed to suppress the negative sequence component caused by unbalanced load.Taking advantage of the fourth leg,the zero sequence component can be effectively compensated.A hybrid control strategy is proposed based on the controller performance requirement analysis under harmonic current disturbance.The hybrid control strategy is realized by adding dual quasi-proportional complex integral and multi-resonance quasi-proportional resonant controllers in the main inverter voltage control loop,which can achieve effective suppression of negative sequence component and reduce the voltage drop of equivalent output impedance at each harmonic frequency.Finally,considering the access of electric vehicles(EVs).the PET output-stage topology with AC and DC capacity switchable power module is presented.The switchable power module can work in multiple modes:in DC/DC mode,it adopts the phase-shifting control strategy to provide fast charging interface for EVs;in DC/AC mode,it acts as the slave inverter in parallel with the main inverter,and the master-slave control strategy is adopted to supply AC load;in DC/AC mode,it provides the channel for harmonic current,which is beneficial to the safe and stable operation of PET without affecting the operation status of the main inverter.