Bearing currents in wind turbine generators: Fid-Bau Portal

Bearing currents in wind turbine generators

Whittle, M. / Trevelyan, J. / Tavner, P. J.

Increasing the availability of multi-megawatt wind turbines (WT) is necessary if the cost of energy generated by wind is to be reduced. It has been found that WT generator bearings have a surprisingly high failure rate, with failures happening too early to be due to classical rolling contact fatigue. One potentially important root cause of bearings failures, bearing currents, has been investigated in this paper. The use of pulse-width modulated power electronic converters in variable speed WTs results in the presence of a common-mode voltage which may drive stray currents through a parasitic circuit in the generator structure. In this paper it is shown that if appropriate mitigation strategies are not employed, the bearing lubricant may experience electric stress in excess of its dielectric strength resulting in electrostatic discharge machining (EDM) of the bearing. Moreover it is shown that rotor-fed machines are more susceptible than stator-fed machines due to the presence of a larger coupling capacitance in the stray circuit. This is significant due to the prevalence of doubly fed induction generator WTs. The need for further research in this area, particularly in order to quantify the damage caused by EDM and develop diagnostic and prognostic techniques for the application of condition monitoring to this phenomenon, has been demonstrated.