At 7 a.m., a wind turbine blade was partially dislocated. At 6.50 a.m., the supervision centre of the wind farm, located in Germany, received a warning of blade feathering at 90°. It transmitted the information to the maintenance centre a few km from the farm. The personnel was on the spot at around 8 a.m. At around 9 a.m., the operator notified the fire department and established a 150-metre safety perimeter around the tower. The operator locked the 2 entrances to the path near the machine. It informed the farmers of the nearby farmland that they could no longer come on their land. It stopped the 5 wind-farm turbines. The fire fighters gathered the debris. A guard service was arranged. Inspection by drone was performed to visualise the risk of falling pieces, locate debris on the ground and decide how to proceed to dismount the blade.

The blade was in vertical position, torn from the base. Strips of material hung along the tower. The nacelle and the other 2 blades of the wind turbine were damaged. Debris was found on the ground within a 100m radius, and the operator set up an exclusion area. Several components were exposed to the elements. 10 days after the incident, a spell of strong wind caused components to fall to the ground again, and the operator extended the exclusion area to 200m. Two months after the incident, following an assessment of the wind turbine’s stability, the operator accessed the turbine to remove unstable components. At the start of April, all the debris was placed in containers on the site. At the start of July, the two remaining blades and the hub of the wind turbine were dismounted.

The operator mentioned that, during the inspection of the blades by drone in August and November 2020, superficial coating defects were detected; according to it, such defects are customary for 11-year-old blades.

At the time of the incident, the wind turbine was subjected to wind speeds (between 10 and 15 m/s) which require regulation of the power produced by the blade orientation system (pitch control). For the 3 blades simultaneously, this system was ineffective, and the wind turbine then went into overspeed. The aerodynamic braking system was triggered but the pitch control system didn’t respond. The wind turbine continued to rotate at high speed until the blade broke, at around 6 a.m., causing the turbine to stop. The pitch control motor did not receive the order from the programmable controller, because the converter located upstream was “frozen” by an overcurrent protection device. This mode is due to a programming error in the converter control software. The hub was sent for expert appraisal in Germany.

The other wind turbines of the farm restarted 1 month after the incident with updated software and the establishment of a surveillance protocol.

The software update was performed on 240 wind turbines in France. The wind turbine maintenance manager informed the operators of farms equipped with turbines that could be affected by the defect.