Wind turbine rotor fragments: impact probability and setback evaluation

Abstract

With increasing installation of wind turbines, the exposure to the hazard of impact from blade fragments increases. Local authorities use setbacks to reduce the risk by limiting the distance from wind turbines to adjacent property lines and dwellings. Unduly conservative setbacks are a deterrent to wind energy development. To determine appropriate setbacks, the authors developed a fragment trajectory model based on fragment rotation and aerodynamics. The model was used to simulate fragment trajectories at various rotor speeds, with randomly generated inputs for wind speed, wind direction, rotor azimuth, and rotor break position. Four sizes of wind turbines were studied, with rated power of 750 kW, 1.5, 3 and 5 MW. A sensitivity analysis showed that a fragment trajectory is highly dependent on the input parameters. However, for multiple trajectories from a given turbine and rotor speed, the sensitivity of the impact probability to most inputs was negligible. The results indicate that the range increased with turbine rating and rotor speed. When the range was normalized by overall turbine height, the probability of impact at a particular normalized range decreases with turbine rating. Planning agencies use the normalized range for setbacks, and the results indicate that using a common setback for all turbine sizes would be reasonable. Existing setback standards of 2–3 overall turbine heights offer better than 1 in 1,000,000 probability of impact per year; however, setbacks approaching 1 turbine height will have an order of magnitude higher probability of impact.