Research On DC Transformer Based On Cross Dual Connection Type Energy Storage Module

In recent years,with the increasing demand for renewable energy access and various power load plug-and-play,high-quality power quality is becoming the pursuit of the users.As far as power quality is concerned,frequency,voltage,waveform and power are far from meeting users' requirements.With the development of power electronic technology,power electronic transformer innovate power transformer and improves power quality of distribution network.Among them,DC transformer(DCT) is a key technology in DC distribution network.However,DCT is only a device for energy conversion and transmission.At any time,the sum of transmitted power of two ports of DCT is zero,and the ports are interrelated,closely coordinated and mutually affected,which hinder the improvement of power quality of dc distribution network.At the same time,it greatly restricts the application of dc transformer in AC/DC hybrid and distributed power supply access.In order to realize the independent control of the energy of different ports of DCT and the power between ports can be decoupled from each other,this paper combines the energy storage component with DCT to study the DC transformer structure based on cross-dual energy storage module.That is,an energy storage system-DC transformer(ESS-DCT).This paper takes this transformer as the research object,and studies the power decoupling of DCT port and the improvement of power quality of DC distribution network from the following aspects:First,the system topology and working principle of the ESS-DCT studied are studied.The system topology consists of a three-layer structure,namely:a system-level input-series and output-parallel(ISOP) structure,and a module-level input-series output-series(ISOS) structure.Basic-level bidirectional isolated DC/DC converter;and comparative analysis of the DC transformer without energy storage and the power characteristics of the DC transformer port with energy storage described in this paper,the steady state mathematical model of the energy storage module is established;and according to ESS-DCT circuit topology,starting from the control strategy of the bidirectional isolated DC/DC converter at the bottom of the system,then analyzing the control strategy of the energy storage module ISOS,and finally analyzing the system-level ISOP control strategy.Secondly,the power decoupling of ports,power distribution within modules and power distribution between modules of ESS-DCT system are analyzed.According to the corresponding control strategy,the power characteristics of the ports of the system and the power distribution of the energy storage unit of ESS-DCT are compared and analyzed under three working conditions of constant single-terminal voltage,constant double-terminal voltage and constant double-terminal current.In addition,MATLAB/Simulink simulation and experimental verification are carried out for the ISOP system composed of a single energy storage module and multiple energy storage modules.Finally,under the premise of realizing the power decoupling of the system port and the power distribution of the energy storage unit,the topology of a hybrid energy storage DC transformer is studied for the problem that the sudden change of the pulsating load power has a severe impact on the normal operation of the energy storage unit.Firstly,the topology structure is introduced,and the corresponding power allocation control strategy is designed to realize the instantaneous power variation by the power storage unit,and the energy storage unit compensates for the gentle power difference,thereby fully utilizing the advantages of hybrid energy storage and extending its service life.Lastly,the feasibility of the topology and control strategy is verified by MATLAB/Simulink simulation and physics experiment.