Legal Disclaimer

Wind Energy Conversion Systems



Since a complex set of site and equipment conditions must be addressed (often in remote or inaccessible locations), a major concern becomes the durability of the components of the wind generation equipment. In a typical wind energy conversion system these components include:

• Wind turbine, including rotor and generator

• Control systems

• Gearbox

• Cable collector system

• Brakes

• Sensors

• Hydraulics

• Yawing system


We will examine some in greater detail:


Electronic Wind Turbine Controller

The wind turbine controller consists of a number of computers which continuously monitor the condition of the wind turbine and collect statistics on its operation.  As the name implies, it also controls a large number of switches, hydraulic pumps, valves and motors within the wind turbine.  As wind turbine size increases to megawatt machines, it becomes even more important that they have a high availability rate, i.e. that they function reliably all the time.





Control Strategies

Many of the business secrets of the wind turbine manufacturers are to be found in the way the controller interacts with the wind turbine components.  Improved controller strategies are responsible for an important part of the increase in wind turbine productivity in recent years.  For example, the rotor blades of today’s variable speed wind turbines are adjusted and controlled via three independent electro-mechanical propulsion units for the pitch systems.  The electronic controller on a pitch-controlled wind turbine checks the power output of the turbine several times per second.  When the power output becomes too high, it sends an order to the blade pitch mechanism, which immediately turns the rotor blades slightly out of the wind.  Conversely, the blades are turned back into the wind whenever the wind drops again.



Courtesy: Vestas Wind Systems A/S


Gearbox

Why use a gearbox?  The power from the rotation of the wind turbine rotor is transferred to the generator through the power train, which consists of the main shaft, the gearbox and the high- speed shaft.  So why use a gear box?  Could not we just drive the generator directly, with the power from the main shaft?  The answer is, if an ordinary generator is used, connected directly to a 60 Hz  AC  (alternating current) three-phase grid with two, four or six poles, an extremely high-speed turbine spinning between 1,000 and 3,000 revolutions per minute (rpm) is required.  With a 43 metre rotor diameter, that would result in a tip speed of the rotor blades of far more than twice the speed of sound, which is an impossible situation.

Another possibility is to build a slow moving AC generator with 200 poles to attain a reasonable rotational speed of 30 rpm.  However, this is not an economical solution.

The gearbox in a wind turbine does not “change gears”.  It normally has a single gear ratio between the rotation of the rotor and the generator.  For a typical machine the gear ratio is typically 1 to 50.



Wind turbines are one of the most demanding applications for gearboxes due to variable loads that are extremely difficult to predict.  Massive torque is transmitted through the three-stage planetary gearboxes typically used in multi-megawatt turbines.  They weigh between 50 and 70 tons (45,359 to 53,503 kgs).


Generator

A wind turbine can be designed for constant speed or variable speed operation.  Variable speed wind turbines can produce 10% to 15% more energy output as compared to their constant speed counterparts.  However, they require power electronic converters to provide a fixed frequency and fixed voltage power to their loads.  On balance, the variable speed wind turbine dominates the market.


Upwind or Downwind Turbines?

Upwind machines have the rotor facing the wind.  The basic advantage of upwind designs is that it avoids the wind shade behind the tower.  By far the vast majority of wind turbines are of this design.  The basic drawback of upwind designs is that the rotor needs to be made rather inflexible, and placed some distance from the tower.  In addition an upwind machine needs a yaw mechanism to keep the rotor facing the wind.


Rotors – How Many Blades?

Most modern wind turbines are three-blade designs.  A rotor with an even number of blades will give stability problems for a machine with a stiff structure.  The reason is that at the very moment when the uppermost blade bends backwards, because it gets the maximum power from the wind, the lowermost blade passes into the wind shade in front of the tower.

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