A Transformer-Less High-Power Converter for Large Permanent Magnet Wind Generator Systems

The power conversion systems for large wind turbines are facing a great challenge as today´s wind turbine power outputs approach 5 MW and above. The conventional low voltage power conversion system will suffer from a high transmission current, which significantly increases losses and cost of the cables as well as voltage drop. This paper proposes a modular, medium voltage, high-power converter topology for the large permanent magnet wind generator system, eliminating the grid-side step-up transformer, which is desirable for both onshore and offshore wind turbines. The converter modules are cascaded to achieve medium voltage output. Each converter module is fed by a pair of generator coils with 90 phase shift to get the stable dc-link power. The power factor correction (PFC) circuit enables the generator to achieve unity power factor operation and the generator armature inductance is used as ac-side PFC boost inductance. At the grid-side, H-bridge inverters are connected in series to generate multilevel medium voltage output and the voltage-oriented vector control scheme is adopted to regulate the converter active and reactive power transferred to the grid. Simulation results with a 2-MW wind turbine system and experimental results with a down-scaled 3-kW system validate the proposed topology and control methods. The proposed system can successfully deliver power from the wind generator to the grid.