Research On The Key Techniques Of Multifunctional Hybrid Power Electronic Transformer

With the development of the power system,increasingly advanced techniques and equip-ment have been developed for the grid.Among which power transformers play a significant role as the power conversion interface.The single function of conventional transformer cannot meet the requirement of modern smart grid.However the power electronic technology has brought the possibility to optimize it.A hybrid transformer based on the conventional transformers and power electronic device has drawn a lot of attention.It has the abilities of voltage con-verter,power transmission,power quality control and distribution grid connection.Therefore,the research of hybrid transformer has important practical significance.This thesis takes hybrid transformer as the research object,sorting and analyzing the relevant key technologies of HT.The research focuses on the topology,modulation and control strategy of hybrid transformer,the design of multi-winding transformer as well as the hardware design of HT.The main contents and innovative points are as follows:1)The MHPET topology is presented considering the potential applications of hybrid transformers.The series and parallel converter topology is optimized.The parallel three level converters is adopted in the shunt converter which increase the compensation power.Three full bridge converters is used in the series converter.The series transformer is spared as a result.An interface module is connected to the common DC bus to connect the distribution grid.Thus,the MHPET proposed is capable of voltage converter,power transmission,power quality control and distribution grid connection which has a wide prospect.2)A SVPWM modulation strategy based on the correction factor is proposed to suppress the parallel zero-sequence current of the three-level converter,along with its principle analyzed.Based on the analysis of the mathematical model of the parallel converter and the zero sequence circulation model between the converters,the relationship between zero sequence circulation and zero sequence duty ratio is deduced.A correction factor is introduced into the SVPWM,to reducing the zero order duty cycle to zero.3)The control algorithm of the compensation module is elaborated in detail,with the comparison of different results of PI control,PI control with separated positive and negative sequence and PR controller.Furthermore,a two DOF current loop PR control strategy is proposed.Thirdly,the mathematical model of the switching function of the series converter is established for the analysis of feasible control strategy of its output voltage,which leads to a new strategy of voltage-detection calculation and control based on the all-pass filter and the PR controller.Finally,the control strategy of the compensation module proposed in this paper is verified by simulation and experiment.4)The topology of interface module is analyzed and the math model is built to achieve the off-grid function of multi-function hybrid power electronic transformer.The multi-link control strategy of interface module is proposed.This paper respectively analyzes the control strategy of outer loop based on VSG and inner loop based on voltage-current double-loop control.5)Based on the analysis of control strategy in MHPET,this thesis studies the design of MHPET prototype,which is thoroughly presented in the final part of this article,including the structure of MHPET prototype,multi-winding transformer,main circuit,control system,communication system and human-computer interaction system.Finally,the overall function of MHPET is proved by the application of prototype and related experiments.