ABSTRACT
Having described in detail how to calculate the aerodynamic loads on a wind turbine, the following text concerns the structural issues that need to be addressed to ensure that the construction will not break down during its design lifetime of typically 20 years. Normally a breakdown is caused by an inadequate control system, extreme wind conditions, fatigue cracks or a defective safety system. A very dangerous breakdown may occur if power to the generator is lost. In this case there is no braking torque on the rotor, which, in the absence of a safety system such as mechanical or aerodynamic emergency brakes, is free to accelerate. Because the aerodynamic forces increase with the square of the rotor speed, the blades will bend more and more in the downwind direction and might end up hitting the tower or flying off due to centrifugal forces. It has been estimated (Sørensen, 1983) that tornoff blades from an overspeeding wind turbine could land up to about 300 m from the tower. Fortunately, violent failures are extremely rare and no humans have, to the author’s knowledge, ever been reported to have suffered injuries from debris flying off a wind turbine. Safety standards such as IEC 61400 (2004) exist to ensure that wind turbines operate safely. The standards define load cases, such as extreme gusts, which a wind turbine must be able to survive. Lightning is also known to have caused disintegration of blades.
