Research On Balanced Control Of Modular Dual Active Full-bridge DC-DC Converte

With the rapid development of cities and industry,domestic demand for electricity has increased steeply.Power grid companies reduce losses by increasing transmission voltage levels.As a result,the requirements for all kinds of electrical equipment on the transmission network become higher.At the same time,with the concept of the smart grid and the energy internet,the grid needs to be able to accept new energy sources and other distributed power sources on the grid.Under the requirements of smart,compatibility,and clean,The Power-Electronic-Transformer based on power electronic transformer technology has gained attention and rapid development.Power Electronic Transformer(PET)is a cascaded power electronic transformer composed of a Cascaded H-bridge(CHB)rectifier and a Dual-Active-Bridge(DAB)DC-DC converter,which has the advantages of AC network side current waveform is good,power factor is adjustable,can provide AC/DC port,modular,easy to expand and other advantages.In order to meet the needs of high-voltage and high-power applications,power electronic transformers with multi-module cascade structure have become popular for research,but at the same time,the modular structure inevitably results in differences in parameters such as switching devices and circuit losses between each module,which leads to inconsistent transmission efficiency of each submodule,and its control strategy becomes more difficult to design.In this paper,we focus on the equalization control of a dual active full-bridge DC-DC converter in the middle stage of a power electronic transformer and design a control strategy for this modular circuit topology based on its operating principle and input-output characteristics.The main research work is done as follows:(1)In order to solve the problem of voltage equalization at the input of modular DAB converter,this paper converts its input problem into the output problem of front stage rectifier,explores the system stability and output voltage equalization of each module of cascaded H-bridge rectifier at the input of three-stage power electronic transformer,proposes a common duty cycle control method based on virtual dq vector conversion,and uses PI controller with infinite DC input gain Realize the control of AC current without static difference.Based on this,a hybrid pulse modulation method based on DC voltage sequencing is added to the mean-voltage loop solution to achieve stabilization of the output voltage of each H-bridge module around the reference voltage.(2)To solve the problem of current equalization at the output of the modular DAB converter,this paper proposes a distributed control strategy with the application background of DC solid state transformer,combining three operating modes of DC Solid-State Transformer(DCSST),adding five controllers,and nesting five controllers within each DAB sub-module based on the double-loop control of DAB to realize the distributed control of DAB converter.The proposed control strategy is able to correct the error of input voltage and output current imbalance caused by different parameters of submodules in different operating modes of DCSST,and achieve the purpose of balanced control.