基于谐振式推挽结构的三端口MMC-SST波动功率耦合方案研究

Research on Fluctuating Power Coupling Scheme of Three-port MMC-SST Based on Resonant Push-pull Structure

文中提出一种控制简单的基于模块化多电平变换器的固态变压器(modular multilevel converter based solid state transformer,MMC-SST)拓扑方案,利用谐振式推挽(resonant push-pull converter,RPPC)结构的高频链将MMC子模块(sub-module,SM)互联,形成一条低压直流母线。RPPC采用定频定占空比的开环控制使其工作在谐振频率,利用高频链的低阻抗特性实现三相对称的低频波动功率自然耦合,从而大幅降低SM电容的尺寸,明显提升了系统的功率密度,同时实现了桥臂2倍频循环电流的自然消除以及SM电压的自动均衡。文中首先对MMC-SST的波动特性进行介绍,进一步对高频链实现波动功率耦合的过程进行建模分析。最后,仿真与实验验证所提拓扑的优良性能。

A simple control modular multilevel converter based solid state transformer(MMC-SST)topology was proposed.The low voltage DC bus was formed by interconnecting all MMC sub-modules(SM)through the resonant push-pull converter(RPPC)high frequency link.The open loop operating mode with fixed duty cycle and fixed frequency was configured to RPPC,which working at its resonant frequency.The SM fluctuating power can achieve coupling naturally due to the low impedance characteristic of high frequency link,thus help to decrease the SM capacitor size and the SST power density is improved obviously.At the same time,the circulating current in bridge arms was eliminated and SM capacitor voltages realized automatic balancing.The SM fluctuating power was introduced firstly and then the fluctuating power coupling process was modeled and analyzed.Finally,the excellent performances of MMC-SST were verified by simulations and experiments.