By Frank May, Service engineer
The last decade has led to strong growth in the wind industry. The overall capacity of wind turbines installed worldwide at the beginning of 2018 was 539 GW, according to the Global Wind Energy Council. This means that all of the turbines installed covered more than five percent of electricity demand worldwide. There is a clear demand for a clean, renewable energy source and reduced harmful emissions.
Such successful wind development is possible, in part, to the continuous improvements and enhancements to turbine equipment and component design. Engineers have had to keep up with demands for increased wind-turbine sizes and power outputs, which have meant advances in operation and reliability of key turbine components, such as the gearbox.
Typical gearbox problems
Although there are several uses for gearboxes in wind turbines — including the yaw-drive gearboxes that position the nacelle relative to the wind direction and pitch drives that adjust the blade angle of attack — the main gearbox has garnered critical attention from engineers. It is designed to speed the slow, high-torque rotation into a much faster rotation for the generator. And few of these machines make it past the 10-years mark.
Today’s geared turbine typically has a three-stage gearbox with a low-speed planetary stage and two parallel stages. Engineers use planetary gears to design gearboxes that are durable enough to withstand the harsh loading conditions and small enough to fit into a nacelle.
The harsh conditions typical of wind-farm operation can take a toll on a turbine and its components. Factors that may adversely affect the main gearbox include variations in wind speed and direction, temperature, vibration, and the ingress of moisture. These factors make the gearbox a vulnerable component of a turbine’s drivetrain.
Problems that occur in gearboxes include surface damage such scuffing and micropitting. Cracks at the surface of gears may also lead to pitting or tooth-root breakage, and possible bearing failure. Micropitting is a starting point for a chain of destructive events that, if left unchecked, may result in tooth breakage and the need to replace the gears. Along with the stress of a constantly changing load, micropitting is also caused by insufficient lubricant film thickness, incorrect lubricant viscosity, and foaming.
The cost of gearbox maintenance and replacement — including the cost of turbine downtime for O&M and repairs — are a significant part of wind farm’s operating losses. The condition of gearbox oil in the wind turbine is one critical factor in saving on these costs.
Gearbox oil health
The main purpose of gearbox oil is to reduce friction. However, it also keeps the gearbox clean and protects the metal surfaces from corrosion. A higher quality oil or lubricant typically means a more reliable gearbox.
A lubricant is selected based on such factors as:
- Viscosity: the oil’s consistency or resistance to flow
- Viscosity index: used to characterize the viscosity-temperature behavior of lubricating oils, for example
- Pour point: the lowest temperature at which the oil will flow under given conditions
- Additives: protective properties such as anti-scuffing and anti-foaming
Synthetic oils have a reputation for improved performance over conventional mineral-based oils. For example, synthetics have a lower pour point than mineral oils, which let the gearbox operate more efficiently in colder ambient temperatures. Synthetic oil also has higher a viscosity index — its viscosity changes less with temperature compared with that of mineral oil.
Gearbox oil deterioration is associated with oxidation due to heat, contamination, and additive exhaustion. Water is the most troublesome of the contaminants. While particulate matter and sludge may be filtered out, it is difficult to remove water from the gearbox without a complete oil change.
For most wind-turbine gearbox oils, the critical concentration of water is less than 500 ppm. When water content exceeds this limit, the lubricant degrades with additive sedimentation. Water also causes film breakdown, oil oxidation, and corrosion. The recommended purity of wind-turbine gearbox oil is at least 16/14/11, according to ISO 4406.
When to change the oil
The oil filter in the gearbox should be changed after the first 500 hours of operation. After one year of operation, an oil sample should be taken to determine its viscosity, water content, acid number, contamination with particulate matter, and additive exhaustion. After two years, some recommend a complete oil change, even if its quality approximates the standards.
Periodic analysis of the oil is ideal because even a high-quality product is no guarantee of gearbox health. However, manually changing the oil is a challenging and time-consuming task. Workers lift and lower oil in special canisters using lifting devices (either elevators or ladders). This requires a team of three or four technicians and about eight hours of work time.
Time and labor costs can be reduced by using special oil-change equipment that facilitates quick servicing by using an integrated oil containment and transportation tanks; an oil heater to reduce viscosity and speed oil pumping, gearbox cleaning with special flushing oil, and mitigation of oil spills and contact between the oil and the environment.