Three years is not a long time for a wind turbine, and new wind parks should not have to deal with major repairs such as gearbox-related problems. Yet the cranes are often called back for major service due to gearbox failure.
In Southern California's San Gorgonio Pass, wind turbines that make electricity have been in operation for quite some time. Gearboxes in wind turbines can last 20 years if properly designed. It was done before with two well known models installed here in the 1980s, and these are still running today: the Vestas V15 and the Bonus 65. Some suggest that these turbines lasted because the gearboxes were over-built, but in reality they were built correctly. How do we know? Because they have performed properly for more than the 20 years of their planned lifetime.
Turbine manufactures and design engineers know that gearbox reliability is a problem today. Despite being massive metal components, gearbox housings and shafts, flex and do not stay straight. These deflections are not good for the gearbox bearing and gear alignments. Additional flexing comes from the lighter-built bedplates on which the gearboxes are mounted, and it is possible that this trend contributes to the problem of early gearbox failures we are seeing in the industry. These are engineering problems.
However, despite engineering issues, once that turbine is in your wind park, it is a maintenance task to replace or repair it.
Good intentions, wrong strategy
Some technicians focus most of their maintenance efforts on using the highest-grade lubricants and keeping the oil clean and dry. While these are best practices, they are not an end-in-themselves in terms of preventative maintenance. I say this, knowing that there remain in operation today hundreds – if not thousands – of older wind turbines with less than perfectly clean oil, and without filtration systems. These machines are 20-plus years old and operate in extreme wind areas, and yet they continue to run fairly trouble-free. I have heard a rumour that some have never had an oil change, as the oil samples keep coming back good.
Keeping the oil clean is definitely best maintenance practice, but it will not solve the engineering problems at the root of our industry's current gearbox problems. We are going to have to build the gearboxes bigger and stronger. A gearbox will be destroyed by any workload – or constant load fluctuation – for which it was not properly designed to manage. However, it's worth restating here that once a turbine is selected and installed, it is the job of the O&M team to keep that turbine running – regardless of gearbox design problems – and clean oil alone is not going to accomplish this.
Preventing gearbox failures
While gearbox problems may not be maintenance-induced, maintenance is the group that has to deal with them. In my experience, picking and choosing from among the following suggestions may help improve your chances of a longer run time.
When you first receive the turbine, service the gearbox. Remove the oil and filter it to three microns before you place the turbine into service. This will help remove any residual particles left in the gearbox during assembly. Of course the factory should have done this, and they may have done. But this is now your machine to take care of.
Some gearbox manufactures recommend a complete oil change after 500 hours of operation at half load to rid the system of wear particles that are worked loose during this initial run time. However, for cost reasons and time constraints, this maintenance task is rarely completed. A good oil filtration may satisfy this need to clear the run-in oil of contaminants.
It may also be necessary to check the bearing oil retention areas for metal accumulation at this time.
If you plan to open the gearbox, be sure to clean the outside area of the gearbox near the fill area before you open it. Do not expose the gearbox to outside contamination. This cannot be overstated, as being careless with this task can cause more damage should you introduce contaminants to the inner parts of the gearbox.
- Inspect and remove metal particles
Inspect the inside of the gearbox prior to filling. Look for metal particles accumulating in the bearing oil feed areas. Gear tooth wear debris is splashed up into these oil retention areas and can enter the bearings through the oil feed ports. Obviously metal here cannot be filtered out. But what you can do is place magnets in these areas to catch this metal before it travels down the bearing oil feed ports. Be sure the magnets cannot come free. Glue them down if you must. This will help prevent accumulated debris damage to the bearings. In most cases, metal found in this area will be from gear wear. But if it gets into the bearings, you will also have bearing wear.
Don't overload the gearbox. Do all you can to keep loads within the realm of the turbine design. This means setting the controller parameters appropriately. For example, do not engage the emergency stop if it is not an emergency.
Low-speed stops in which the high-speed brake disc doesn't slip and grabs immediately should be avoided, as this also places high loads through the gearbox. Soft stops are preferred, and hard stops should be avoided if possible.
Improve oil cooling systems in warm areas. If your cooling system is not keeping the temperatures down, then you need to improve its performance. If the gearbox overheats, you chance damaging your oil. If you damage your oil you are then likely to damage your gearbox.
- Change the oil if it oxidises. Oxidised oil means burnt oil. And if it is oxidised, then you need to find out why;
- Keep the gearbox cool by using external oil coolers and force-air fans.
- Repair gear oil leaks
Care must be taken when changing out gearbox oil filters. Protective material should be placed down before any portion of a gearbox filtration system is opened. Spilled oil goes a long way down a tower, and any spilled oil has a chance to make it to the tower.
Wipe up spills as soon as possible. It's unfortunate when a simple maintenance procedure such as changing an oil filter becomes a multi-thousand dollar tower cleaning job.
Note the changes in gearbox noise. Each turbine will have its own machine noise and you should be familiar with them. The same turbine makes different noises in different load or wind conditions. Spend time with your machinery to get familiar with the sounds the machine makes during normal operation. As the turbines age, you will find that being familiar with these sounds will make it easier to recognise new problems.
- Take oil samples regularly
When you take oil samples, send them to the laboratory for analysis – immediately. If you are the technician that services the machine, you should request to see the results of the oil samples. Making this request accomplishes two things. One, it allows you to see the analysis and become familiar with the report. Two, it helps to ensure that the oil sample was submitted to a lab for the analysis. If you don't see the report, how do you know the sample was actually submitted? It may sound incredible, but it happens.
- Perform follow-up procedures – immediately
If the oil sample comes back and shows unsatisfactory results, then perform the necessary actions immediately. Either filter out the contaminants or change the oil. Do what is needed to make the oil right. Every minute you run your machine with faulty oil increases the amount and rate of your gearbox wear and deterioration. Besides, this is why the oil was sampled in the first place.
If the oil is good and you have high amounts of metal in it, then you most likely have wear problems. In that case, prepare for a major repair. Of course, at this point you will need to inform the management of the suspected problem, based on your sampling service.
"Three years is not a long time for a wind turbine, and new wind parks should not have to deal with major repairs such as gearbox-related problems. Yet the cranes are often called back for major service due to gearbox failure".
Jack Wallace Jr.
Additional preventative maintenance suggestions
- Add more filtration to the system;
- Add water removal systems;
- Add metal monitoring systems;
- Use high quality dessicant breathers or seal the system off with bladder system devices;
- Keep the shaft aligned;
- During maintenance, inspect the gearbox oil channels for metal particles.
With so many new machines installed in the past few years, an interesting benchmark will be to see how many make it past the three-year point in their 20-50 year service life – without major service.