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Wind turbine gearboxes and the effort to improve their reliability

Geared wind turbines are at the heart of today's industry, yet have earned a negative reputation for poor reliability and early failure, with gearboxes marked as the troublemaker. Eize de Vries looks at their design history and the efforts to improve reliability and service life.

Fast-speed geared drive turbine components are relatively easy to access
Fast-speed geared drive turbine components are relatively easy to access

Early "Danish-concept" wind turbine models played a key role during the pioneering days of the wind industry, and their heritage is still evident in current non-integrated drive systems. They feature a main bearing, main shaft, gearbox, intermediate shaft and generator all fitted in line. And it is from these models that many other alternative geared drive systems have since been developed.

In the early days, the gearboxes and generators tended to be off-the-shelf products, which came from either an agricultural or industrial application - quite different from today's dedicated custom-developed components.
  
Yet, despite the initial scientific inexperience and inadequate knowledge base behind these designs, the track record of these wind turbines, particularly the Danish and German models, is good. This is often attributed to substantial safety margins that compensated for the gaps that existed in wind technology and in the understanding of wind-induced loads on turbines.
  
Siemens Wind Power, formerly Bonus, has built geared wind turbines since 1980 and, following a recent survey conducted on a substantial number of its turbines installed during the mid-1980s in California, US, Henrik Stiesdal, chief technical officer, said: "After more than two decades, 96% of these installations were still running well. Reliability even allowed a reduction in service intervals from two to one time a year."
  
Similar positive experiences have been reported for other early makes and models, with one commentator even suggesting that the internal workings of some 20-year-old gearboxes looked like new.
  
Starting at a modest 10kW power rating, turbines grew incrementally by around 10-25kW each time, later stepping up by 50-100kW for successive models. That gradual process of learning-by-doing proved highly successful and continued until 1993, when former Tacke Windtechik of Germany (now GE Energy) introduced a 600kW model, the TW600. Several competitors, mainly Danish, followed suit, increasing their smaller 450 and 500kW machines to a similar 600kW power rating.
  
That specific development phase is said to have coincided with a turbine export drive, a process that demanded relatively light and compact nacelles capable of fitting inside a standard container. Some wind industry insiders claim that this need for smaller nacelles resulted in higher loads passing through more compact gearboxes that were developed with reduced safety margins.
   
Hot spots

Apart from the mechanical issues, there can be several serious and possibly unforeseen negative consequences. One such effect is that a higher input torque, such as 500-600kW power rating, produces substantially more internal heat, yet there is less lubrication oil available to control the operating temperature. That, in turn, can introduce so-called gearbox hot spots with high bearing temperatures and accelerated degradation in oil quality.
  
The result was that several major and costly gearbox failures have occurred in the 600-750kW class of turbines, but this produced one positive outcome - many affected turbine suppliers realised that they had to strengthen their drive-system design capability and knowledge by employing specialists, often from the automotive industry.
  
Another result of the wind industry scaling up its products by a factor of 2.5 was the introduction of four 1.5MW turbine models between 1995 and 1996 from two German companies (the former Tacke and Enercon) and two Danish suppliers (former Nordtank and Vestas). The geared Tacke and the direct-drive Enercon models both operate with pitch-controlled variable speed, which is now the standard in wind technology. These 1.5MW turbines are the basis of the current 2MW and 3MW mainstream models and even offshore giants up to 6.15MW.
  
One of the key challenges that wind turbine designers still face is keeping the nacelle mass to a minimum while maintaining reliability and manageable costs. Here, perhaps the biggest design uncertainty is how to accurately determine the loads that are exerted by the wind on the rotor. Even today, a serious mismatch between the wind turbine supplier's calculated loads and the actual loads that are imposed in reality is seen to be one of the prime causes for premature, and often catastrophic, gearbox failures.

No overloading

A number of strategies are being adopted in the wind turbine drive systems to protect the gearbox against structural overloading, including those imposed through rotor bending or axial loads. One option is to reduce the dynamic loads themselves by applying cyclic pitch control. This is an advanced technology that continuously monitors the loads on the root of the blade and re-adjusts the blade angle during each revolution of the rotor. Experts claim that this cyclic pitch control allows a substantial reduction in the loads that contribute to material fatigue and the subsequent damages and failures.
  
Today, there is a growing awareness that the gearbox is part of the structural design of a wind turbine, not an isolated drive-system component. As such, it should be treated as integral to the overall dynamic system extending from the rotor to the foundation.
  
For example, when a gust of wind hits the rotor blades, the resulting load is directly passed onto the drive system, with the gearbox deflecting the force by being temporarily pushed sideways into one of its torque supports. And, as the gearbox output shaft is typically located off-centre and set against the main shaft in a non-integrated drive system, this sudden movement can introduce a temporary dynamic misalignment between the gearbox output shaft and the generator input shaft. That, in turn, can have a negative effect on bearing loads and lifetime expectancy.
  
In a non-integrated drive system the challenge is to provide the main chassis with some flexibility, which keeps the nacelle mass within acceptable limits, yet ensure that movement is not too great to cause dynamic misalignment.
  
Static misalignment can also occur in the main components, introduced during assembly, as they settle in following the initial operating period, or during reassembly after replacement of a component.
  
These potential disadvantages associated with the highly popular non-integrated drive systems have prompted several suppliers and technology developers to look into different geared-drive concepts.
  
One of the concepts that is gaining popularity is the semi-integrated system, a large-diameter single rotor bearing with a gearbox attached using a flange. This eliminates the traditional main shaft and produces a shorter, more compact nacelle. Some proponents say this gives a more straightforward design process, and others a much-reduced risk of gearbox misalignment. However, a downside for the single-rotor bearing is the limited number of suppliers, a higher component cost and the uncertain life expectancy of the large diameter friction-type oil-seal.

Pure rotor torque

Another way to avoid misalignment in a fast-speed geared drive system is to fully separate the bending caused by the rotor from the pure torque that passes into the gearbox, as seen in French firm Alstom's wind turbines. While no doubt an effective and elegant system, it is also relatively expensive to manufacture.
  
The Clipper Liberty turbine is a good example of a drive solution where the main shaft assembly and four generators are directly connected to a self-supporting gearbox, which serves as a main load-bearing structure.
  
Medium-speed wind systems are also gaining popularity, some 15 years after German consultancy Aerodyn Energiesysteme developed its patented 5MW Multibrid offshore wind turbine.

This hybrid between conventional fast-speed geared and direct-drive systems aims to combine the main advantages of both concepts, and some analysts predict this system will become the system of choice. Spanish generator developer Indar Electric has developed two medium-speed systems in the 2.5-5MW range, a single-stage gearbox and a generator of around 150rpm (the Areva Multibrid M5000), as well as a faster version with a double-stage gearbox capable of 450rpm. 

Whatever the drive-system preferences, all wind turbine designers without exception are focusing on protecting their gearboxes against unwanted loads, which should result in more reliable long-lasting wind turbines.

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