A Decentralized Soft-start Method for a Cascaded H-Bridge and Quad Active Bridge-based Medium Voltage AC-DC Converter

AC-DC converters with cascaded units are gaining popularity for medium-voltage (MV) level grid connection; however, they can get damaged due to high inrush currents during the start-up sequence if there is no soft-start method implemented in each stage. Moreover, in MV systems, measurement of grid voltage is challenging as it is inaccessible at the cascaded modules and hence needs to be communicated. This becomes cumbersome and unreliable when the number of units is high, due to wiring and isolation requirements, communication bus failure, expensive optical fibers, etc. In this article, we address these issues and challenges by proposing a decentralized soft-start-up procedure for a cascaded ac-dc system composed of Cascaded H-Bridge (CHB) and Quad-Active Bridge (QAB) converters. The proposed soft start-up sequence charges up the QAB output dc-link by limiting the inrush current and achieves grid-synchronization of the CHB stage in a decentralized fashion, which does not require any grid voltage measurement or phase-locked loops (PLL) as used in conventional methods. The proposed method relies on virtual resistance emulation and reactive power versus frequency droop dynamics to achieve grid synchronization and limit inrush current during the process. The stability analysis, simulation and experimental results of the proposed start-up method are presented for a system of 5 series-connected ac-dc units.