The power generating system – the drive train – consists, roughly speaking, of a low-speed shaft that transmits the rotation and torque from the rotor to the gearbox, increasing the rotational speed and decreasing the torque proportional with the ratio of the gearbox. The increased speed and decreased torque are transmitted to the generator. In other words, the power train transmits energy and increases shaft speeds.
The yaw system – rotates the nacelle on the tower in order to ensure that the turbine at all times points towards the wind within +/-6 degrees deviation ensuring a maximum of power output. The main components of the yaw system are:
• The yaw bearing, which can be of the roller or sliding type and serves as a rotatable connection between the tower and the nacelle of the wind turbine. The yaw bearing should be able to handle very high loads, which apart from the weight of the nacelle and rotor, also include the dynamic loads caused by the rotor during the extraction of the kinetic energy of the wind.
• The yaw drives, which each consist of a powerful electric motor and a large gearbox which increases the torque. The yawing of the large modern turbines is relatively slow, in the range of 0.5 degrees per minute.
• The yaw brake stabilizes the yaw bearing against rotation when the position against the wind direction must be fixed. The most common system is a hydraulically actuated disk brake but the cost of this system makes turbine manufacturers experiment. An alternative is the use of electric yaw brakes, which eliminates the complexity of the hydraulic leakages.
The pitch system – rotates each blade of the turbine to the optimum pitch angle against the wind in order to optimize energy generation.
At lower wind speeds, a perpendicular pitch increases the energy harnessed by the blades, and at high wind speeds, a parallel pitch minimizes blade surface area and slows the rotor. Typically one power unit is used to control each blade. Pitching the blades simultaneously is either done by gear motors or hydraulic cylinders positioned at each blade bearing.
The pitch system is either hydraulically or electrically actuated. In case of power loss, there is a back-up system in the hub either by means of a hydraulic accumulator or an electrical energy storage by means of batteries or super capacitors.
In case of power loss, the accumulators will force the blades to pitch back and bring the turbine to stand still. One major challenge is to get the blades to pitch fast enough should a gust of wind cause sudden acceleration, which is not uncommon on offshore sites.
The pitch angle of all three blades is controlled by an over all pitch controller in the wind turbine. The set points for the pitch actuators are transferred to the rotating hub by a slip ring arrangement.
The cooling system – typically consists of large fans that drive air to convectively cool the generator and gearbox. It exhausts waste heat from the nacelle assembly, and ducting directs cool air to the generator and gearbox.
The cooling system for the gearbox and the generator are separated and serve different purposes: in the gearbox, the oil flow to gears and bearing must be cooled down to a maximum operating temperature in the region of 60° C.
The electrical cooling capacity for the fans where the oil flow passes through represents about 3. 5% of the rated power of the wind turbine.
Generators are cooled separately by fans, chilling the temperature of internal parts through glycol/water passing the fans, and cooling the stator. In principle, this is comparable to the cooling method for car engines, where the cooling fluid chills down the solid parts that surround the internals.