Research On Single Stage High Frequency SST Topology And Optimization Control

Solid State Transformer(SST),as a new type of electrical energy conversion device,has important research value in smart grids and distributed power sources.Limited by the withstand voltage level of conventional silicon devices,So far,scholars' researches on SST have focused on its modular cascaded topology structure and how to achieve voltage balance and power balance of each module in a cascading structure.However,a larger number of modules increase the volume and weight of the system.With the advancement of wide band gap semiconductor device silicon carbide(Si C)technology,the topology of SST can be greatly simplified,and the voltage and power ratings are expected to be pushed to higher levels.In this paper,a new single-stage high-frequency-link SST topology is proposed by multiplexing switches of the DAB converter and the interleaved parallel Buck circuit,which can be used in AC-DC power distribution networks.The SST reduces the number of power switches,and reduces the cost while achieving high power density and energy bidirectional transmission.In order to reduce the conduction loss of the switching device(IGBT)in SST,an optimized hybrid modulation strategy for rectified average current is proposed based on the extended phase shift control(EPS).By switching different control modes,multiple groups phase shift angles inside the bridge are selected for segmented modulation under different transmission power and voltage gains,and the average current value of the SST inductor is always kept to a minimum.Therefore,the IGBT power loss dominated by the average inductor current value is reduced,and the SST power transmission efficiency is improved.This paper first introduces the topology and equivalent circuit of the proposed SST in detail,and analyzes its modulation strategy,working mode and power transmission characteristics under Extended Phase Shift Control(EPS).Then,in order to reduce the conduction loss of the switches caused by the average value of the SST inductor current,the inductor current in different operating modes is analyzed in detail under the EPS control.A mathematical model between the rectified average current,voltage gain,and transmission power is established through a lot of formulas.The efficiency optimization curves under different operating conditions are obtained,and the rectified average current efficiency optimization modulation strategy is proposed.The reasons and solutions for the output voltage oscillation caused by the phase shift angle inside the bridge under the optimized modulation of the rectified average current are given.Finally,a 400 W experimental prototype is built using Infineon's IKW40N120T2(IGBT)switches,which verifies the feasibility of the SST topology and the rectified average current optimization modulation strategy.