Impacts of wind field characteristics and non-steady deterministic wind events on time-varying main-bearing loads

Abstract

Abstract. This work considers the characteristics and drivers of the loads experienced by wind turbine main bearings. Simplified load response models of two different hub and main-bearing configurations are presented, representative of both inverting direct-drive and four-point-mounted geared drivetrains. The influences of deterministic wind field characteristics, such as wind speed, shear, yaw offset, and veer, on the bearing load patterns are then investigated for similarity scaled 5, 7.5, and 10 MW reference wind turbine models. Main-bearing load response in cases of deterministic gusts and extreme changes in wind direction are also considered for the 5 MW model. Perhaps surprisingly, veer is identified as an important driver of main-bearing load fluctuations. Upscaling results indicate that similar behaviour holds as turbines become larger, but with mean loads and load fluctuation levels increasing at least cubically with the turbine rotor radius. Strong links between turbine control and main-bearing load response are also observed.

Wind turbine main bearing rating lives as determined by IEC 61400‐1 and ISO 281: A critical review and exploratory case study

“…The shear results may be interpreted in the same light, since a higher value of shear exponent leads to a rotor moment that counteracts the rotor weight moment and so reduces the mean radial load on the main bearing. 15,17 Hence, a larger shear exponent leads to a reduction in the main bearing mean load and an increase in the rating life, whereas the opposite occurs when the shear exponent is reduced. This is exactly the behaviour observed in rating life results when the shear exponent is varied.…”

“…¶ ¶ Note that the characteristics and structure of main bearing radial load variability have been studied in detail in previous work. 9,17 ongoing debate surrounding this concept. Especially for the 240 bearing, where loads fall below the fatigue load limit a majority of the time (and hence fall into the "low load" region), there is a direct risk of life overprediction.…”

“…Similarly, the kinematic wind fields used in aeroelastic simulations have the correct second-order statistics but do not contain true turbulent-eddy structure. 17 While experience has shown IEC design standards to provide a strong basis for wind turbine design, it is important to be aware of these limitations.…”

“…This particular problem has further nuances, however, since wind turbine main bearings are typically large-diameter, flexible, greaselubricated, low-speed, high-load bearings that experience large-scale continuous variations in load throughout their operational lives. 9,17,23 As such, it is not clear a priori to what extent the rating life calculations are accurate in this case. This work therefore seeks to consider the insight provided by IEC 61400-1 24 and ISO 281 25 design standards when applied to the problem of rating life assessment for wind turbine main bearings.…”

“…more recent study, 6 utilizing field data from 15.3 GW of wind energy capacity and consisting predominantly of 1 to 3 MW three-point-mounted spherical roller main bearings, predicts that by year 20, replacement will be required for 22%-25% of a main bearing population. Concerns regarding main bearing reliability have led to increased research efforts in recent years focused on this component, [7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22] which seek to address premature failures in current and future systems.…”