Understanding the availability of wind turbines (WT) is vital to maximize WT energy production and minimize the capital payback period. Previous work on this subject concentrated on reliability and the location of WT failure modes rather than root causes. This paper concentrates on the influence of weather and WT location on failure rate and downtime, to try to understand root causes and the consequences of failure. The paper goes further than a previous study, which used Windstats data from the whole of Denmark, by considering a limited population of identical WTs at three locations on the German Nordzee, Ostzee and in western Germany, using data from WMEP and local weather stations. This new study focuses more precisely than the previous study by using more reliable data. The data were analysed to find the WT failures and weather conditions and then cross-correlate them. To confirm their representativeness, the reliability characteristics of these smaller WT populatio ns followed the average trends of the overall WMEP survey. However, clear differences were observed in the failure behaviour of the WTs at the three locations. Annual periodicity was seen in the weather data, as expected, but not in individual WT population failure data. However, clear cross-correlations can be seen between WT failures and weather data, in particular wind speed, maximum temperature and humidity. These cross-correlations were more convincing than those found in the earlier, larger Danish study, vindicating the more focused approach. It is also clear from the analysis that Operation & Maintenance also has an impact on WT failure rates. These factors will be important for the operation of offshore WTs with the work indicating how weather conditions may affect offshore WT failure rates

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.

This is the peer reviewed version of the following article: Whittle, M., Trevelyan, J., Shin, W. and Tavner, P. (2013), Improving wind turbine drivetrain bearing reliability through pre-misalignment. Wind Energy, 17 (8): 12171230 which has been published in nal form at http://dx.doi.org/10.1002/we.1629. This article may be used for non-commercial purposes in accordance With Wiley Terms and Conditions for self-archiving.Additional information: Use policyThe full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-prot purposes provided that:• a full bibliographic reference is made to the original source • a link is made to the metadata record in DRO • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders.Please consult the full DRO policy for further details. AbstractImproving the reliability of wind turbines (WT) is an essential component in the bid to minimise the cost of energy, especially for offshore wind due to the difficulties associated with access for maintenance. Numerous studies have shown that WT gearbox and generator failure rates are unacceptably high, particularly given the long downtime incurred per failure. There is evidence that bearing failures of the gearbox high speed stage (HSS) and generator account for a significant proportion of these failures. However, the root causes of these failure data are not known and there is, therefore, a need for fundamental computational studies to support the valuable 'top down' reliability analyses. In this paper a real (proprietary) 2 MW geared WT was modelled in order to compute the gearbox-generator misalignment and predict the impact of this misalignment upon the gearbox HSS and generator bearings. At rated torque misalignment between the gearbox and generator of 8500 µm was seen. For the 2 MW WT analysed the computational data show that the L 10 fatigue lives of the gearbox HSS bearings were not significantly affected by this misalignment but that the L 10 fatigue lives of the generator bearings, particularly the drive-end bearing, could be significantly reduced. It is proposed to apply a nominal offset to the generator in order to reduce the misalignment under operation thereby reducing the loading on the gearbox HSS and generator bearings. The value of performing integrated systems analyses has been demonstrated and a robust methodology has been outlined.