Hybrid Single-Phase AC–AC Modular Multilevel DSCC Converters With Modulation and DC-Link Voltage Ripple Improvement

This paper proposes two hybrid topologies based on a single ac-ac modular multilevel cascade cascade converter (MMCC) with double-star chopper cells (DSCC), in each of which a voltage reference technique (VRT) is associated for generating the output modulating references. In doing so, either the grid-connected leg or the shared leg can be switched at the line frequency, thus reducing the switching losses. For proper operation of the VRTs in real-time, the input and output converter voltages must be in sync, hence a synchronization technique is proposed. Furthermore, when no dc-link capacitor is used in a DSCC converter, the dc-link is subjected to voltage ripples derived from the pulsewidth modulation (PWM) technique. In that case, high-frequency PWM techniques can produce high-frequency components across the arm inductors, which are reflected in the dc-link. Therefore, this paper focuses on the operability of the hybrid modular multilevel DSCC converters, but also on the investigation of the proposed VRTs in reducing the dc-link voltage ripple on the conventional DSCC converter using PWM with phase-shifted carriers (PSCs). Simulated and experimental results are presented to validate the proposed techniques and converters.