Start-up Strategy for a Sensorless Direct Drive PM Generator for Wind Turbines

Direct drives for wind turbines are a relevant and challenging issue, since they carry all of the advantages of gearless motion transmission. Nevertheless, the rotor and shaft diameters are sensibly greater, and they need customisation for the housing of the position sensor. This sensor is often forced to work in hard, even extreme, environmental conditions, and it actually represents the most critical item for the drive reliability. Statistics elaborated in collaboration with a manufacturer, on a plant with thirty wind turbines, have highlighted an MTBF, due to position encoders, of only one month. In this case, the cost of maintenance far exceeded the cost of the sensor itself, endorsing the choice of a sensorless solution. On the other hand, for the sake of efficiency, the front-end inverter, connected to the motor, is disabled until the rotor speed is greater than a threshold speed. Therefore, operations at low speed and standstill, which are the pitfall of most of sensorless techniques, are not requested at all. The working conditions of a wind synchronous generator perfectly match the intrinsic requirements of the Kalman algorithm, which is used in this work to get an accurate estimation of speed and position. The paper proposes a novel strategy to manage the delicate transition from the disabled to the enabled states of the inverter, with smooth mechanical and electrical transients.