Research On Key Technology Of Switched-Capacitor Power Electronic Transformer

With the continuous expansion of urban distribution networks,high-density access of distributed power sources and renewable energy,and vigorous development of information technology and power electronics technology,distribution network systems have new characteristics:distributed power generation technology is developing rapidly,involving a series issues of high-power,high-efficiency,high-quality energy conversion and control;the intelligence of power grid has received widespread attention and high demands on the controllability of power grid has been placed;the rapid growth of nonlinear loads has made the power quality problems of power system more prominent.This paper presents a switched-capacitor power electronic transformer,which aims to solve the difficulty of traditional power electronic transformers to meet the new development requirements of modern and future distribution systems such as miniaturization,light weight,high efficiency,and intelligence.Firstly,the concept of using switch-capacitor modules to achieve galvanic isolation is proposed.Unit-gain switched-capacitor converter is used as the structural basis for galvanic isolation,and its topology and control methods are analyzed.The switched-capacitor galvanic isolation module realizes 1:1 power transfer for differential-mode power,and for common-mode signals,it is desirable to suppress the magnitude of the common-mode current so as to achieve galvanic isolation and ensure safety of human body contact.This paper analyzes the common-mode current suppression method from the perspective of system and device parameters,and proves that the development of new material switching devices such as SiC has made such a power electronic transformer possible.At the same time,some other scheme ideas for reducing the common mode current are also given.Secondly,the research on the switched-capacitor DC-DC power electronic transformer was carried out,and bridge modular switched-capacitor DC-DC converter was proposed for voltage conversion.To reduce reverse power and system loss,a variety of improved phase-shift control methods are implemented.The topology of the bridge modular switched capacitor converter and the operating modes of the circuit under different phase-shift control method are analyzed,and the calculation results of the circuit characteristics are given.The advantages and disadvantages of the different phase-shift control modes are analyzed and compared.Additionally,the efficiency characteristics of the converter under different phase-shift control methods are compared and analyzed from the perspective of the magnitude of the reverse power.To guide the design of the converter control system,an improved state space averaging method is used to derive the small-signal model of bridge modular switched capacitor DC-DC converter.A 200W experimental prototype was built,and the correctness of the theoretical analysis was verified from the simulation and experimental results.Test results on operating efficiency were also given.Thirdly,the research on the switched-capacitor AC-AC power electronic transformer is carried out,and a new buck-boost matrix AC-AC converter is proposed for voltage conversion and frequency regulation.The topological structure,control method and operating modes are analyzed.Results of loss analysis and power density are given.Compared with other single-phase matrix AC-AC converters,it is proved that the proposed transformer structure has advantages on number of semiconductor devices,power density,and energy transmission efficiency.A 200W experimental prototype was built,and the correctness of the theoretical analysis was verified from the simulation and experimental results.The quality metric of voltage and current waveform of the prototype and the test results such as efficiency and loss were given.