Faster identification and repair of problems
Ever since the wind power industry was apparently caught unawares by the major series failure of gearboxes and bearings which befell it in the late 1990s, customers have asked if the same problems could hit today's generation of larger wind turbines in the 1.5 MW to 2.3 MW range. So far, there has been no repeat of the problems seen on machines in the sub-megawatt size class. But other problems related to bearings and gearboxes have turned up on the bigger machines.
Denmark's two wind turbine manufacturers, Vestas and the wind power division of Siemens Power Generation, are not about to be caught unawares again, however. Having learned from experience, defects are quickly being identified and retrofits carried out where necessary.
Siemens has already retrofitted the bearings in gearboxes on its 2.3 MW machines in operation, while at the same time exchanging the bearings in wind turbines under manufacture and in its stocks of gearboxes. Vestas has carried out a retrofit program on various turbine models inherited from its takeover of NEG Micon.
Neither company is prepared to reveal the full extent of the retrofit work or what it has cost, although timely repairs mean the sum involved is far less than when NEG Micon was brought to its knees in the late 1990s by gearbox failures on a large number of operating turbines around the world.
At Vestas, poor gearbox and bearing quality is showing up in larger models, the 1.65 MW, 1.75 MW and NM 2 MW, confirms the company's Tom Pedersen, director for northern Europe. The problems are limited to particular gearbox types, he says. Pedersen declines to be specific about how many turbines are affected and which gearbox makes he is referring to, explaining that Vestas has no statistics over gearbox usage and that the company is currently negotiating with the suppliers.
Other sources report that gear units from Jahnel-Kestermann, in particular, have been replaced on the NM 2 MW model and that 2 MW machines not yet retrofitted will most likely have to be. In total, 24 NM 2 MW machines are operating in Denmark, most of them installed in 2002. Jahnel-Kestermann was a family owned German company before its recent takeover by French Arques Industries (Windpower Monthly, July 2005). Pedersen confirms that gearboxes have been replaced on a number of the 2 MW turbines, but not on all of them.
He says that gearboxes on Vestas 1.65 kW turbines have had an "excessive mortality," but that a "statistical excessive mortality" has not been seen on the 1.75 kW turbines. He declines to name the total cost of retrofitting the gearboxes and bearings on Vestas' larger turbine models. "It is a sum that is visible," he says.
In Pedersen's opinion, Vestas is not putting pressure on its gearbox suppliers to get them to pay for the retrofits, although the company is trying to make suppliers live up to their contracts. In future, this will be achieved by working together more closely on product development.
All in all, Pedersen believes that Vestas has the problem of gearbox and bearing failure under control and the same problems experienced in turbines under 1 MW are not appearing in the new and larger machines.
Fears that the economic optimisation of megawatt scale turbines could lead to wind turbines that are not strong enough to last more than ten to 15 years because not all the loads are understood have earlier been expressed by technical consultant to the Danish association of wind turbine owners, Strange Skriver. Today, Pedersen admits that wind turbine construction is "closer to the borders" and around "the far edge of the technology" simply because wind turbines are the largest rotating machines found today. But even though a wind turbine is no stronger than its weakest link, Pedersen offers words of comfort: "We have skilled workers, the right design basis and today also good facilities for testing of our prototypes. I believe we have taken account of all the loads," he says.
At Siemens, which last year bought Bonus, a privately owned Danish wind turbine manufacturer, two types of bearing problem have turned up on its larger machines. But they are not the same as those experienced on smaller turbines, says Henrik Stiesdal, director of special projects in the Siemens wind division.
"One problem was that we had a bearing on the high speed shaft that skated," he says. "Skating is a well known phenomenon which can happen when the loads on a bearing are so light that it refuses to roll. As the loads mount and the bearings have to roll, cold welds can come into the contact zone. In the course of time this can lead to damage on the roller paths. You would expect this phenomenon to occur on all wind turbines because they all experience operation at very low loads, but that hasn't been the case to any significant degree. On the 2.3 MW turbine we breached some or other threshold, and then things went wrong. The solution was to install a bearing of the same dimensions, but with smaller rollers. This specialised bearing was developed by the bearing supplier and all the original bearings have been exchanged with the redesigned bearing."
The second problem was with a bearing on the intermediate shaft, which suffered micro-pitting of the end surface. Diverse tests revealed that the bearing's load carrying qualities did not meet those specified by its manufacturer because the geometry of the rollers was incorrect.
"The solution in this case was to install a bearing that was in large part identical, but which has a better geometry of the rollers. That considerably reduced the contact pressure so that the load carrying ability was in order. Also this new bearing was developed by the supplier and once again all the original bearings were exchanged for the new design," says Stiesdal.
Both the retrofits could be carried out on wind turbines in situ because the large gearboxes in use today are better designed to allow replacement of separate parts than has been the case before.
The work on the Siemens turbines was undertaken last year. Inbuilt condition monitoring equipment on each turbine was used to identify when damage was starting to occur, allowing for speedy repairs and preventing the total bearing failures and associated damage to the gearwheel.
"In fact it was the first real test of the value of condition monitoring. The cost of the system has long ago paid for itself in preventative action. By far the majority of the replaced bearings, however, showed no signs of damage or only minimal damage, but their replacement was necessary to prevent further damage. There has been no further recurrence of damage," says Stiesdal.
The two specific problems, he adds, failed to reveal themselves during test operation of the turbine, on the prototypes or on the zero-series. "They first appeared when a large number of the turbines were already in operation. Taking the precaution of ensuring that individual parts could be replaced was sensible and of course it will be further developed on the next model, the 3.6 MW."
Carrying the cost
Stiesdal declines to say how many wind turbines have been affected or what the repairs have cost, but the cost of identifying the reason for the problems, development of solutions and their implementation has been considerable. "We have had a very satisfactory relationship with our gearbox supplier, Winergy, but clearly we have also had to carry a deal of the cost. None of the costs have been passed on to our customers and the retrofits have been carried out without much inconvenience for them," says Stiesdal.