Design Of Power Conversion Unit Of Soft-Switching Solid State Transformer

The Solid State Transformer(SST)is the key component of renewable energy dominated power grid in the future.It plays an important role in realizing the flexible access of renewable energy and load and improving the controllability of power grid.Based on the literature review,this paper compares Dual Active Bridge(DAB)single-stage SST,LLC single-stage SST and Full Bridge(FB)+ Series Resonant Converter(SRC)two-stage SST.For the application of high voltage SiC MOSFET power devices,this paper believes that developing all soft-switching SST will be an important research direction in the future.Aiming at the SRC circuit which is commonly used in SST,this paper analyzes the fators which affect turn-off loss and the zero-voltage-switching turn-on condition of the circuit.On a 4.9kwIGBTSRC experimental prototype,the effects of phase angle,dead time,resonant frequency,and external paralleled capacitance on efficiency are studied.For the application of high-voltage SiC MOSFET power devices,a topology and control scheme of the soft-switching SST is studied in this paper.Based on AC and DC input,the soft-switching modulation method,switching process and zero-voltageswitching turn-on conditions of the front-end circuit are discussed.The working principle and zero-voltage-switching turn-on conditions of the back-end bidirectional LLC circuit are discussed.Based on the circuit parameter design,circuit layout design and PCB layout design,a prototype of a unit of soft-switching SST is built.Through experiments,the zerovoltage-switching turn-on for both front-end and back-end circuits is verified.The front-end and back-end circuits both achieve the efficiency of 99.3% at 10kW.The experimental results preliminarily prove the feasibility of the circuit scheme.