A transformerless modular permanent magnet wind generator system with minimum generator coils

This paper has presented a modular converter for multi-coil direct-drive permanent magnet wind generator system. The converter modules are cascaded to achieve medium voltage output (6 kV~33 kV), thus eliminating the grid-side step-up transformer, which is desirable for both on-shore and offshore wind turbines. Each converter module is composed of a rectifier, dc-link and an inverter. The generator coils with 90 degree phase shift are rectified through the power factor correction (PFC) circuit and connected in series to get unity power factor, stable dc-link power and higher dc-link voltage. The generator armature inductance is used as the AC-side PFC boost inductor, thus reducing the system size and cost. The inverter adopts a neutral point clamped (NPC) five-level converter to match the dc-link voltage level and are cascaded to achieve multilevel medium voltage output. The vector control scheme is used to regulate the converter active and reactive power transferred to the grid. Simulation results with a 1.5 MW wind generator and converter system and experimental results with a scaled 3 kW system validate the proposed topology and control method.