Low-Capacitance CHB-Based SST Based on Resonant Push–Pull Decoupling Channel

Abstract

Cascaded H-bridge based solid state transformer (CHB-SST) has been the typical scheme in medium and low voltage AC/DC conversion applications, and large-size capacitance caused by 2 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">nd</sup> -order ripple of submodules (SMs) is the inherent problem of CHB-based systems. This paper proposes a novel low-capacitance CHB-SST (ZCHB-SST) based on zero-impedance resonant push-pull (ZRPP) decoupling channel for ripple power. ZRPP presents zero-impedance characteristic at its switching frequency equaling to series resonant frequency, which makes the equivalent parallel of CHB AC-link capacitors, and provides a decoupling channel for ripple-power. Therefore, the horizontal SMs ripple-power can achieve mutual cancellation based on their own three-phase symmetry, which significantly reduces the size of AC-link capacitance. In addition, the elimination of SMs ripple-voltage achieves the elimination of grid zero-sequence harmonics from root cause, and the mutual clamping of parallel AC-link capacitors also achieves the voltages self-balancing, which reduces the complex of system control. The CHB-SST configuration, operating principle, ripple-power decoupling, SMs voltage self-balancing, system design, and evaluation are discussed in this paper. Finally, the effectiveness of the proposed CHB-SST is verified by simulation and experiment.