Wind Turbine Gearbox Reliability
London As a key failure point, gearbox reliability continues to dog the wind industry and as turbines become larger the push to improve performance becomes paramount. Alstom believes it has addressed the issue by ensuring that torque transmission is performed independently of rotor support. This improves the reliability of turbine drive train components, in particular the gearbox, which is better protected and subjected to lower loads. Jordi Puigcorbe and Alexis de-Beaumont explain.
One of the biggest concerns remaining in the wind industry is the reliability of the gearbox. ‘With our current wind turbine fleet currently going out of warranty period, we estimate that we are carrying a potential risk on gearboxes of about US$300 million. Failures are still relatively rare these days because our fleet is recent, but we expect this will change dramatically as our wind turbines approach their 5–7 years of operation. Our concern is such that we are even considering acquiring a company with gearbox servicing capabilities.’
This statement from a large US wind farm operator is far from being an isolated case in an industry that will see over 8000 MW of wind farm capacity go out of the warranty period every year in the US alone in the next few years.
Recently, Sandy Butterfield, a former chief wind turbine engineer at the National Renewable Energy Laboratory (NREL) in Colorado, was quoted as stating that the wind industry expects today’s gearboxes to last 7–11 years. This markedly contrasts with the 20-year design lifetime of the wind turbines. And the implications for the industry are huge, since changing a gearbox is typically a lengthy and extremely costly exercise.
The gearbox’s reputation for a high failure rate is linked to the extreme engineering challenge that gearbox technology faces in wind applications, and the difficulty in properly assessing the loads – and in particular the non-torsional loads that pass through the gearbox – and how these affect bearings and gears. Some manufacturers have chosen to move to direct drive to reduce the number of moving parts in the wind turbine more exposed to wear. But this has led to wind turbine specific generator designs that are usually more expensive and often come together with a long-term maintenance contract with the Original Equipment Manufacturer (OEM), which does not necessarily meet the operations and maintenance (O&M) concept of flexibility expected by customers.
Of course, much has been done in the last decade to design and manufacture gearboxes ensuring a high quality, often with associated with over-engineering and increased cost. Many efforts are also being put in to performing proper monitoring and maintenance to detect and prevent any avoidable damage. These efforts have limited gearbox breakdowns in infancy, and sometimes allowed some maintenance activities to be initiated earlier than before, but they have not helped resolving a key cause of the problem: the rotor support concept, and how it distributes loads among the wind turbine structure and the gearbox.
Loads affecting the gearbox are often underestimated because state of the art aero-elastic models do not consider complex non-linear phenomena produced during transients in the drive train like inner gearbox component dynamics coupled with bearings and support parts, and the flexibility influence of the rotor support on the gearbox that causes additional loads.
Establishing a Track Record
Alstom’s competitive availability figures are in part due to its rotor support concept, because less time is required for gearbox maintenance and repair. A study of more than 200 units of Alstom’s 750 kW wind turbines has shown a gearbox failure rate below 5% cumulated over the first nine years of operation. This number is remarkably low, and this statistic has the advantage of providing real life operation of the Alstom Pure Torque concept for longer periods than the megawatt class wind turbines.
Alstom also analysed the performance of its ECO80 platform, looking at the gearbox failure statistics of a representative sample of over 600 wind turbines of 1.67 MW in over 50 wind farms that have been operating for up to seven years, and performing endoscopic analysis of the wind turbines that accumulated the highest number of operating hours in the sample. Results proved comparably high reliability performance of the concept in the ECO80 platform. Based on these results, Alstom is confident that the majority of its wind turbines could operate with their original gearbox for their whole design lifetime.
Pep Prats, vice president of Advanced Technology, Wind, at Alstom, who was also one of the founders of Ecotecnia back in 1981, comments: ‘We have worked with this design for a long time; we actually introduced it already in our very first turbine, a 30 kW unit that we installed in 1984. We made a short attempt to use a more conventional design in the 150–225 kW turbines we sold in the 90s, but we then decided to come back to this original design with our 600–800 kW wind turbines in the late 90s, and have since then based all our wind turbines on this concept.’
Prats continues: ‘Another advantage of this concept is its scalability. Our new ECO 100 platform, with rotor swept areas of over 7800 m2, have to handle 20 years of very significant loads. This is being achieved by simply scaling up our rotor support concept, without major redesign of the shaft, support systems and the drive train to cope with the loads. It is a very unique design, with built-in reliability.’
The industry usually considers gearbox as ‘consumables’, since – as mentioned previously – it is anticipated to be changed at least once, if not twice during the lifetime of their wind turbine. The Alstom Pure Torque concept gets customers away from the idea that a gearbox is a consumable.
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