The Public Service Commission of Wisconsin on Nov. 18 approve a July 6 application from Wisconsin Power and Light (WP&L) for authority to replace main-shaft bearings in the wind turbines at the Bent Tree Wind Farm, located in Freeborn County, Minnesota.
The main-shaft bearings on the project’s Vestas V82 wind turbines are exhibiting premature wear that has led to several bearing failures. The estimated cost of the project is nearly $37.8 million.
The existing Bent Tree facility began commercial operation in February 2011, and includes 122 Vestas V82 wind turbines, each with a nameplate capacity of 1.65 MW. The facility is located in a project area of approximately 32,500 acres.
The most common type of bearings used in wind turbines are spherical roller bearings (SRB). All 122 turbines at Bent Tree are equipped with SRB bearings, many of which are exhibiting premature wear and unexpected failure. Typically, SRB bearings achieve an expected design life of approximately 20 years. WP&L states that the SRB bearings at Bent Tree have been susceptible to premature wear because of uneven axial-to-radial load ratios, which leads initially to micro pitting wear, and later to more catastrophic failure of the bearings. At the extreme, the bearings fail by crack or fracture and the wind turbine becomes inoperable.
To-date, bearings on seven of the wind turbines have failed due to premature wear. Based upon an analysis performed in May 2015, WP&L anticipates that most of the bearings at Bent Tree will fail and require replacement by 2025. WP&L anticipates that the annual peak number of wind turbines with bearing failure would occur in the years 2016 and 2017.