Variations in Gear Fatigue Life for Different Wind Turbine Braking Strategies

A large number of gearbox failures have occurred in the wind industry in a relatively short period, many because service loads were underestimated. High-torque transients that occur during starting and stopping are difficult to predict and may be overlooked in specifying gearbox design. Although these events comprise a small portion of total load cycles, they can be the most damaging. Theseverity of these loads varies dramatically with the specific configuration of the wind turbine. The large number of failures in Danish-designed Micon 65 wind turbines prompted this investigation. The high-speed and low-speed shaft torques were measured on a two-stage helical gearbox of a single Micon 65 turbine. Transient events and normal running loads were combined statistically to obtain atypical annual load spectrum. The pitting and bending fatigue lives of the gear teeth were calculated by using Miner's rule for four different high-speed shaft brake configurations. Each braking scenario was run for both a high- and a low-turbulence normal operating load spectrum. The analysis showed increases in gearlife by up to a factor of 25 when the standard high-speed shaft brake isreplaced with a dynamic brake or ,modified with a damper.