Towards large-scale direct drive wind turbines with permanent magnet generators and full converters

Large-scale permanent magnet generators with full power electronic converters are of increasing interest to many wind turbine manufacturers. These systems offer an efficient and reliable solution for offshore and low wind sites. However, the advantages of these generators necessitate a system design approach when selecting a power electronic drive and designing to meet the toughest grid code elements. The inability to control the generator´s rotor flux means that the necessary over speed margin for the turbine will affect the stator voltage and current seen by the drive. Given the inability to control machine flux, an optimised system with power electronic drive, for a particular permanent magnet generator, would incorporate some voltage and current margin combined with the minimal necessary dc chopper. This chopper ensures that over-speed during grid fault ride-through or frequency and inertial response does not cause excessive stator voltage or drive trips. In this paper, the key features of a PMG system, as distinct to other full converter wind turbines, are discussed. The operational conditions that effect significant speed changes on PMG systems are discussed and simulations are then used to demonstrate the effects of carefully selecting a chopper and brake resistor for the power electronic drive. These simulations demonstrate that a fully rated chopper is not necessary in all situations and that mechanical and electrical system interactions dominate.