A new single-stage three-phase AC/DC medium voltage step-up transformer-less converter with ZVS for wind energy systems

The use of medium voltage DC grid in offshore wind farms power architecture is becoming a promising solution to eliminate the bulky line frequency transformers used in the conventional AC power transmission system. In this power architecture, several wind turbines are first connected to a collective medium voltage DC grid (20~60kV) through an AC/DC power conversion and a medium voltage step-up DC/DC converter. A central step-up DC/DC converter is then used to step-up the medium voltage to high voltage (> 600kV) for HVDC transmission. In order to reduce the number of power conversion stage, this paper proposes a new single-stage three-phase AC/DC medium voltage step-up transformer-less converter for high power wind turbines. The proposed converter is a constant frequency converter and it is controlled by varying the phase-shift between 2 three level circuits. A three-phase boost rectifier is then integrated with the transform-less step-up converter to become a single stage power converter. The converter´s characteristics and its operating waveforms will be discussed in this paper. Results will be provided to highlight the merits of the proposed converter.