The components of a wind turbine are designed to last 20 years. This means that they will have to endure more than 120,000 operating hours, often under stormy weather conditions. Comparing with an ordinary automobile engine, it usually only operates only some 5,000 hours during its lifetime. Large wind turbines are equipped with a number of safety devices to ensure safe operation during their lifetime.
One of the classical, and most simple safety devices in a wind turbine is the vibration sensor in the image, which was first installed in the Gedser wind turbine. It simply consists of a ball resting on a ring. The ball is connected to a switch through a chain. If the turbine starts shaking, the ball will fall off the ring and switch the turbine off.
There are many other sensors in the nacelle, e.g. electronic thermometers which check the oil temperature in the gearbox and the temperature of the generator.
It is essential that wind turbines stop automatically in case of malfunction of a critical component. E.g. if the generator overheats or is disconnected from the electrical grid it will stop braking the rotation of the rotor and the rotor will start accelerating rapidly within a matter of seconds.
In such a case it is essential to have an overspeed protection system. Danish wind turbines are requited by law to have two independent fail safe brake mechanisms to stop the turbine.
Aerodynamic braking system: Tip brakes
The primary braking system for most modern wind turbines is the aerodynamic braking system, which essentially consists in turning the rotor blades about 90 degrees along their longitudinal axis (in the case of a pitch controlled turbine or an active stall controlled turbine), or in turning the rotor blade tips 90 degrees (in the case of a stall controlled turbine).
These systems are usually spring operated, in order to work even in case of electrical power failure, and they are automatically activated if the hydraulic system in the turbine loses pressure. The hydraulic system in the turbine is used turn the blades or blade tips back in place once the dangerous situation is over.
Experience has proved that aerodynamic braking systems are extremely safe. They will stop the turbine in a matter of a couple of rotations, at the most. In addition, they offer a very gentle way of braking the turbine without any major stress, tear and wear on the tower and the machinery. The normal way of stopping a modern turbine (for any reason) is therefore to use the aerodynamic braking system.
Mechanical braking system
The mechanical brake is used as a backup system for the aerodynamic braking system, and as a parking brake, once the turbine is stopped in the case of a stall controlled turbine.
Pitch controlled turbines rarely need to activate the mechanical brake (except for maintenance work), as the rotor cannot move very much once the rotor blades are pitched 90 degrees.
- Safety (A): general notes on safety systems
- Safety (B): examples of aerodynamic brakes
- Safety (C): examples of mechanical pages
- Trends in safety systems: general notes
- Causes of safety system activation (A): general explanation
- Causes of safety system activation (B): example of failured turbine
- Wind turbine control and safety