High-Frequency-Link Current Stress Optimization of Cascaded H-Bridge-Based Solid-State Transformer With Third-Order Harmonic Voltage Injection

Cascaded H-bridge (CHB) -based solid-state transformer (SST) consists of CHBs and dual active bridge (DAB) converters. Series-resonant DAB converters (SRCs) are widely used for its advantages of soft switching characteristic and simple control. However, the high-frequency-link (HFL) currents of SRCs contain double-line-frequency harmonic fluctuation in three-phase CHB-based SST, resulting in high current stress. In this article, the SRC is substituted with dc terminal dynamic equivalent circuit, and the analytical expression of the HFL current fluctuation is deduced. With consideration of third-order harmonic voltage injection (THVI), the HFL current fluctuation characteristics with this method are analyzed and the optimal amplitude and phase of the injected third-order harmonic voltage are derived, with which the HFL current stress can be minimized. Both the simulation and experiment results indicate that the HFL current stress of SRCs can achieve a reduction of about 20% with the optimized THVI method.

I. Introduction

Over the past decades, significant progress has been made on solid-state transformer application in railway traction, smart grid, and distributed renewable energy generation grid-connected system [1]–[4]. With the function including voltage step up/down and galvanic insulation of traditional line-frequency transformer, solid state transformers are characterized with the reduction of copper and iron consumption, output voltage regulation, power quality conditioning, dc interface with renewable energy, and storage systems [5], [6].