Wear of Key Components

The gearbox is generally considered the most important and vulnerable lubricated component. The US National Renewable Energy Laboratory (NREL) completed a study in 2011 that showed that gearbox damage was the leading cause of downtime and the most expensive repair for the wind turbines in use (Sheng et. al., 2011). The gears are susceptible to surface wear such as micro-pitting and scuffing. Lubricants with antiwear and extreme pressure additives can protect against scuffing and other surface wear. Furthermore, the use of the fluid with the proper viscosity provides an adequate film thickness to prevent contact between the gear teeth. Micropitting can also be exacerbated as an oil oxidizes in service and contains higher levels of acidic compounds.

Along with gearbox damage, damage to the main shaft and bearings or the generator account for the majority of all downtime. Excellent antiwear properties and proper viscosity across the operating temperature range are typical requirements to maintain proper bearing function, for both oils and greases used in wind turbine bearing applications. Furthermore, there is some implication that certain additives may encourage hydrogen absorption in the bearings, contributing to the much-researched but still poorly understood phenomenon of white etching cracks (Aikin, 2020). These cracks are a major concern because they can lead to very premature bearing failures.

Materials Compatibility

In addition to these specific wear problems, wind turbine fluids must exhibit good materials compatibility. For instance, they must not damage seals, which can cause fluid leakage. Compatibility with paints and coatings used in the wind turbine is also important. As with all lubricants, wind turbine fluids need to resist foaming, minimize corrosion, and exhibit good oxidation stability, which extends the drain interval. Finally, the lubricant needs to have good pumpability across the operating temperature range, which can include cold ambient temperatures in higher latitudes. Many of the same tests that are used to validate automotive oils or industrial gear oils can be used to evaluate whether a lubricant is well suited to a wind turbine application. Some possible tests Savant Labs offer and what they tell about a fluid are listed in Table 1.

In-Service Fluid Monitoring

In addition to determining whether fluids are generally appropriate for wind turbine applications, the wind generation industry makes extensive use of in-service fluid monitoring. Many installations use in-line sensors to monitor overall cleanliness or conductivity, which can indicate some level of information about wear and oil degradation. Even more important, the standard practice in the industry is to sample the gearbox oil about every six months during scheduled maintenance and send it to a testing lab, such as Savant Labs, for analysis. The tests conducted can vary, but tend to include fluid cleanliness, viscosity, acid number, water content, elemental analysis, analysis of oxidation, and ferrography. Together, these tests can identify the beginning of a wear problem and can even be used to identify root causes before the gearbox sees catastrophic failure due to wear damage. Tests that indicate the level of insoluble particles due to varnishing or foam can also indicate problems from incompatible fluids being used to top off or fill after a drain. Savant Labs offer many of these tests.

The effectiveness of fluid testing was demonstrated in a research project conducted by NREL. The wind turbine gearbox under test installed for operation at a wind farm saw some temperature excursions above 90 °C, as well as two fluid loss events. Researchers were able to determine based on the laboratory analysis of the fluid that the gear teeth were wearing abnormally after just 300 hours in service. This was later verified by visual inspection after the field-testing component of the project was complete. The researchers were able to retain the gearbox for the remaining stage of testing because they removed it prior to catastrophic damage (Sheng et. al., 2011).

Another benefit of periodic fluid testing is the ability to assess whether a fluid drain interval can be extended in a particular case. A five-megawatt wind turbine can require 700 gallons of lubricant, and costly synthetic fluids are preferred in the industry. Typically, oil change intervals are scheduled for from 9 to 16 months. In a project conducted by OELCHECK and Fraenhofer Institute for Wind Energy Systems (IWES), a gearbox in a wind turbine installed in the field using a polyalphaolefin (PAO) was monitored for almost four calendar years. Only slight degradation was seen in the fluid as a small reduction of additive content and increase in oxidation products. The fluid could continue to be used without replacement, despite the long interval (Coronado and Wenske, 2018).

Savant Provides Fluid Testing Programs

The cost of implementing a fluid testing plan that coincides with regularly scheduled maintenance on a wind turbine installation is minimal compared with the costs of waiting until a failure becomes catastrophic to find it or changing wind turbine fluids more often than necessary. With Savant Labs as the laboratory partner in a fluid testing program, a wind turbine operator would have years of testing expertise, rapid test turnaround, continuous communication, and a quality guarantee statement. Savant Labs are an enthusiastic and capable participants in supporting the growth of wind energy generation.

Because Quality Matters

Savant is positioned to offer some of the most precise data found in the industry to date. Regular instrument calibration and maintenance to meticulous reference values and control chart monitoring is part of the quality regimen. In addition, all test results undergo a stringent quality review process, data is delivered on time and reported via email in a logical format designed for easy interpretation.

Request a quote or contact us to discuss your next lubricant testing project.

References

• US Energy Information Administration (EIA), “International Energy Outlook 2021: Data tables (2020 – 2050).” Released October 6, 2021. Accessed: May 9, 2022.
• US Energy Information Administration (EIA), “Wind Explained: Electricity generation from wind.” Last updated: March 20, 2022. Accessed: May 9, 2022.
• Coronado, Diego and Wenske, Jan, “Monitoring the Oil of Wind-Turbine Gearboxes: Main Degradation Indicators and Detection Methods,” Machines, vol. 6, no. 25, 2018. Accessed: May 9, 2022.
• Sheng, S., Link, H., LaCava, W., van Dam, J., McNiff, B., Veers, P., Keller, J., Butterfield, S., and Oyague, F., “Wind Turbine Drivetrain Condition Monitoring During GRC Phase 1 and Phase 2 Testing,” October 2011, NREL. Accessed: May 9, 2022.
• Aikin, Andrea R., “Bearing and gearbox failures: Challenges to wind Turbines Tribology and Lubrication,” Tribology and Lubrication Technology, Aug 2020, pp. 38 – 39. Accessed: May 9, 2022.