Comparative Study of Wind Turbine Power Converters Based on Medium-Frequency AC-Link for Offshore DC-Grids

This paper considers an offshore wind turbine modular power converter (ac/dc) with permanent magnet synchronous generator based on three stages: an ac-ac converter, a medium-frequency transformer, and a full-bridge diode rectifier with LC output filter. Six converter topologies are compared; three of them are based on back-to-back (B2B) topology: the conventional B2B with three-phase sinusoidal waveform, the B2B with three-phase squared waveform, and the B2B with single-phase squared waveform. The other solutions are based on matrix topologies: the direct matrix converter, the indirect matrix converter, and the reduced matrix converter (RMC). A systematic methodology and main component models are introduced to evaluate the power loss, volume, and mass of the wind energy conversion system for all considered topologies as a function of the medium-frequency transformer operational frequency and the number of modules parallel connected. The comparison involves a tradeoff study between total power losses, size and weight of 10 MW modular converters. It was found that when Metglas alloy 2605SA1 is the transformer core material, the RMC topology leaves the best tradeoff between the considered performance indicators with an extra benefit in reducing the semiconductor devices cost of the modular converter.