Doug Herr/Wind Products Manager
The inspection of numerous wind turbine gearboxes in operation and those returned for repair have shown the general progression of events that lead to the need for gearbox replacement. The bottoms two “whys” in Gearbox life problems – several known “whys” deserve further explanation. Bearing experts agree that the pitting of rollers starts with micropitting, also called grey staining or frosting. This consists of microscopic cracks only a few microns deep (about 0.0001 in.). Individually, these cracks are too small to be visible, but as they accumulate, they appear as grey stains on the roller surface. Eventually the bearing roller starts to shed its cracked and weakened surface losing a small bit of its precision tolerance. Furthermore, this contaminates the oil with microscopic, super-hard steel particles most of which are too small to be filtered out.
2. Why do rolling elements in bearings break through the oil film and contact the races, even though gearbox designers take great care to prevent this? Bearing and gear experts understand that even in a properly designed and lubricated gearbox, oil-film breakdown can occur during transient events with concentrated loading and skidding bearing rollers. The recent and extraordinary instrumentation of a wind-turbine gearbox in actual operation by JR Dynamics Ltd.2, measured the movement of gears, shafts, and even the rollers of the bearings to find the root cause. The data clearly showed that the gears and shafts shifted rapidly, and the bearing rollers skewed during transient-torsional reversals in the drive system. The load zone of the bearing shifts almost 180°, as shown in the illustration During torque reversals, bearing load directions change. Concentrated edge loading on the skewed rollers can break through the oil film. Slower unloaded rollers must accelerate rapidly as they suddenly become loaded, causing skidding that magnifies the surface stress on the skewed rollers. But this is still not the root cause.
3. Why are wind turbine drive systems subject to torsional reversals? The torsional load in a wind-turbine drivetrain is constantly fluctuating. There are many potential transient load events in their operation that can cause the drive system to rapidly go through a torsional reversal. Such events can include:
• Emergency stops
• Grid faults
• Generator short circuits
• Crowbar events
• Resonant vibration
• Wind gusts
• Control malfunctions
• High wind shutdowns
All wind turbines are subject to most of the listed torsional-reversal events. Although these torsional reversals are infrequent, they can be severe and damaging. Older, stall controlled, two-speed wind turbines with blade-tip braking are potentially subject to these same infrequent severe torque reversals in addition to reversals during normal blade tip braking, contactor engagements, and downshifting that are frequent but generally less severe. The later even occurs when controls shift load from one generator to the other.
Conventional torque limiters set at 150% to 180% of the wind turbine’s rated torque do provide some protection against forward torque spikes (where the bearing rollers are in position to take a 150% load). However, that same slip torque level in reverse is high enough to damage the skewed and skidding rollers during sudden torque reversals. The illustration Reversing torque despite a conventional torque limiter shows reversing torque and the torsional vibration present even when a standard torque limiter has clipped the maximum reverse torque at 150% of the wind turbine rated torque. A final “why” yields a simple retrofitable solution. Why can’t a torque limiter have different reverse and forward-torque settings? If the torque limiter mounted on the generator shaft could be set at a much lower reverse slip-torque setting than the forward setting, it could significantly reduce loads on the skewed and skidding rollers, minimizing and perhaps eliminating the start of micro-pitting. To do this, PT Tech has developed the WindTC Torsional Control product. Just as an electrical snubber circuit can selectively suppress positive or negative voltage spikes, a wind turbine can benefit from the snubbing of the reverse torque spikes. Torque reversals with and without a reverse-torque setting shows a partial power grid disconnect in two turbines. The red line shows and unprotected standard drivetrain, while the blue line shows a nearby turbine equipped with WindTC Torsional Control. By clipping off the first torque reversal it dampens the magnitude and has eliminated further torque reversals. This significantly reduces the stresses and potential damage to bearing rollers.
Fitting the torsion-control product described onto the generator shaft can prolong gearbox life and benefit the entire drive system, from rotor blades to generator. Controlling the levels of reverse-torque stress extends the fatigue life of all the driveline components.
• Wind-farm operators, by lowering operating costs. Furthermore, retrofits are made easy by simply replacing the coupling hub on the generator shaft.
• Wind-turbine designers and OEMs, as a low-cost method for ensuring predictable reverse-torque levels throughout the drive system. The device also reduces need to oversize bearings, gears, and numerous other components.
• Gearing and bearing designers, by reducing loads that compromise the oil film during torsional reversals.
For further reading:
1 “Improving Wind Turbine Gearbox Reliability,” Conference Paper, NREL/CP-500-51548, May 2007, from the National Renewable Energy Lab, authored by Walt Musial, Sandy Butterfield, and Brian McNiff
2 See article in February, 2010, issue of Gear Solutions.com entitled, “Trouble Shooting Wind Gearbox Problems”, by Jarek Rosinski and David Smurthwaite of JR Dynamics (www.jrdltd.co.uk)