Bearing Currents in Wind Turbine Generators Matthew Whittle, Jon Trevelyan, Li Ran, Junjie Wu 16th-19th April, 2012 EWEA 2012 Copenhagen www.reliable-renewables.com 1

Contents Wind turbine generator failure Types of bearing currents The stray circuit Calculating the stray capacitances Simulation results Discussion and conclusions Offshore wind farm, near Utgrunden, Sweden GE Energy [www.ecomagination.com]

Wind Turbine Generator Failures A survey of over 1000 failed wind turbine generators showed that bearing failure is the dominant cause of wind turbine generator failure Field data from other industries has shown that in low voltage electrical machines (such as those used in wind turbines) bearing failures dominate, accounting for approximately 50 % of failures. In 2010 Chen and Alewine published the findings of a survey of 800 failed wind turbine generators which showed a similar picture. Source: Alewine, K., Chen, W., “Wind Turbine Generator Failure Modes Analysis and Occurrence”, Windpower 2010, Dallas, Texas, May 24-26, 2010. 3

Types of Bearing Currents Classical low frequency bearing currents caused by magnetic asymmetry (strong 50 Hz component). Bearing behaves like a resistor (fluid film separation not maintained, metal-to-metal contact). Low speed, lubrication starvation. But bearing currents are the least of your worries in this case. dV/dt currents. Because of the high slew rate of the common-mode signal significant capacitive currents may flow in the bearing. Electrostatic Discharge Machining (EDM). Bearing behaves as capacitor – lubricant is dielectric. So if electric field strength exceeds lubricant dielectric strength, it is broken down and discharge occurs. If rotor earth via gearbox is lower impedance route than via stator the ground current can be forced through the bearings. Circulating currents. Winding-frame capacitive coupling results in unbalanced currents in the winding. This induces an emf along the shaft which can drive currents around the frame-bearing-shaft-bearing loop.

The Stray Circuit Source: Zika et al (2009)e Field data from other industries has shown that in low voltage electrical machines (such as those used in wind turbines) bearing failures dominate, accounting for approximately 50 % of failures. In 2010 Chen and Alewine published the findings of a survey of 800 failed wind turbine generators which showed a similar picture. Source: Zika et al (2009)e 5

The Common-Mode Signal

Computational Case Study 2 MW DFIG Rotor mass = 1700 kg 6330 bearing Deep groove ball bearings Single row

Stray Capacitive Circuit

Calculating the Stray Capacitances I The windings-rotor capacitance, Cwr, and the rotor-frame capacitance, Crf, were relatively easy to calculate from the machine geometry The bearing capacitance is difficult to calculate because of the complex geometry. However, by assuming that it is the contact area that dominates (because here the film thickness is much smaller than elsewhere) we can treat the bearing as N flat plate capacitors (where N is the number of balls in the load zone) …

Calculating the Stray Capacitances II Therefore the plate area may be computed using Hertzian contact mechanics, and the plate separation using the Hamrock Dowson film thickness equation Thus the bearing capacitance is given by 𝐶 𝑏 = 𝜀 0 𝜀 𝑟 𝑑 𝑖 𝑛 𝐴 𝐻𝑧𝑖 where 𝐴 𝐻𝑧𝑖 is the Hertzian contact area of ball 𝑖, 𝑑 is the Hamrock-Dowson minimum film thickness, 𝜀 0 is the permittivity of free air and 𝜀 𝑟 is the relative permittivity of the grease

Results show EDM Not sure that these results are correctly plotted. Why are outputs so heavily damped? CHECK THIS SLIDE Each spike corresponds to a discharge of energy to the bearing raceway 11

Comparing Rotor-fed with Stator-fed I

Comparing Rotor-fed with Stator-fed II Topology Peak Current density (A/mm2) Rotor-fed 0.95 Stator-fed 0.43 According to Kerkman et al. (1997) Jb > 0.8 A/mm2 can significantly endanger bearings.

Conclusions The use of PWM switched PECs can cause premature bearing failure in WT generators Where the rotor is fed by the PEC the bearings are much more vulnerable to EDM than when the stator is fed Appropriate mitigation, for example shaft grounding and bearing insulation should be employed Because of the potential for significant reduction in bearing fatigue life it is recommended that the possibility of incorporating the shaft grounding arrangement into the drivetrain condition monitoring system be considered

Thanks for listening Any Questions? This work was funded by the EPSRC through the FRENS joint UK-China project (www.reliable-renewables.com). Matthew Whittle E: m.w.g.whittle@durham.ac.uk T: +44 (0)7792679431