Research On Optimal Control Strategy Of Power Electronic Transformer

With the continuous development of smart grid and distributed energy,functional requirements of power grid are becoming more and more complex.Under this background,traditional power transformer is not suitable for future power grid.Therefore,power electronic transformer(PET)has been studied and developed.Compared with traditional power transformer,it can not only realize electric isolation and power transmission,but also provide harmonic filtering on the AC side.In addition,PET has advantages of flexible power flow control,high power density and redundancy design.To meet the needs of high voltage and large capacity applications,cascaded PET has been widely studied.An input-series output-parallel(ISOP)structure is adopted by this cascaded PET to reduce the voltage stress and current stress of semiconductors.However,the increase of PET component modules brings higher requirements for its control strategy.This paper mainly studies the key control strategies to improve the safety and stability of cascaded-H-bridge-based PET under frequent power fluctuation.The main work and innovations are as follows:1.Research on voltage and power balance control strategy of power electronic transformer.Firstly,the limitation of tradition voltage and power balance control strategy is analyzed.Then,impedance interaction between parallel DAB converters is analyzed,and an impedance-interaction-based power balance strategy is proposed to balance the power sharing.The proposed strategy matches the dynamic output impedances among parallel dual active bridge(DAB)converters,thereby the balance of PET can be achieved even the load changes.Consequently,the safety and stability of PET can be further improved.Finally,simulation experimental results verified the correctness and effectiveness of the proposed control strategy.2.Research on load current feedforward control strategy of PET.Based on the mathematical model of PET,the influence of load on the intermediate DC voltage and output DC voltage of PET is analyzed.Then,the application and limitation of feedforward control strategy is studied,and an improved feedforward control strategy is proposed.This strategy combines the virtual impedance control and feedforward control,which can further suppress the intermediate DC bus voltage.In addition,the proposed feedforward control strategy will not affect the balance performance of PET.Semi-physical experimental results verified the correctness and effectiveness of the proposed control strategy.3.Research on improved power balance control strategy of cascaded converter.For traditional power balance control strategy,the power compensation is inaccurate due to the power loss and sampling deviation of converters.In this paper,an improved power balance control strategy is proposed for cascaded converters.The cascaded converter analyzed this paper is comprised of a rectifier and a DAB converter.The proposed strategy generates the power balance coefficient by intermediate and output voltage loop.Then,control error caused by power loss and sampling deviation can be directly compensated in calculation equation by adjusting the power balance coefficient.Compared with traditional power balance control strategy,the proposed control strategy has higher power balance calculation accuracy,and the fault tolerance for the fault condition of missing load information or large sampling deviation.Semi-physical experimental results verified the correctness and effectiveness of improved power balance control strategy.