An AC–DC Isolated MMC-Based Structure Suitable for MV SST Traction Applications

This paper presents the theoretical study and experimental validation of a single-stage bidirectional ac-dc topology for solid-state transformer (SST) applications. This topology is based on the modular multilevel converter (MMC) and employs the concepts of interleaving converters and integrated power stages. A single magnetic element operating at medium frequency is employed to perform galvanic isolation and coupling between the grid-side and load-side converters. The main characteristics of the structure are derived from the aforementioned characteristics, such as passive elements operating with an effective frequency higher than the switching frequency, minimized current ripples at high frequency, as well as reduced volume and weight. Experimental results of the converter are obtained in a small-scale prototype, operating with a switching frequency of 5 kHz, nominal power of 1.5 kW, output voltage of 300 V, and the supply voltage of 165 V at a frequency of 60 Hz. High-power factor of 0.99, low-harmonic distortion of the input current with 3.6%, and efficiency in the 87% were achieved.