If you've ever wondered why wind turbines stop moving, it is reasonable to believe that there just isn't enough wind to power the blades.
Although this is a valid point, it isn't the solitary reason.
Wind turbines are complex structures, designed to produce maximum renewable energy only when it is safe to do so.
What is a Wind Turbine
A wind turbine is a tall, tower-like structure that harnesses kinetic energy from the wind and converts it into electrical energy.
The wind is captured in the blades, and via a system of shafts and a generator, where voltage is added, it is converted into electricity. In the case of large, commercial wind turbines, the electrical power created is sent to substations and then on to the National Grid.
Domestic turbines are much smaller and are designed to generate enough power to run individual houses or businesses.
Why Do Wind Turbines Stop?
It is strange seeing stationary wind turbines, particularly when the wind seems to be high.
Some of the contributing factors are as follows:
Not Enough Wind
For rotors and blades to turn there must be significant and sustained wind speed; this is usually around 5mph. This slow rotational speed isn't great enough to supply any usable power.
Once the cut-in speed is reached, 6-10mph, the generator produces electricity.
Too Much Wind
Conversely, high wind speeds resulting from storms can have a very harmful effect on turbines.
If blades were allowed to spin out of control the generator and motor would overload, resulting in system failure.
Rotating blades also suffer from significant damage in high gusts where-as stationary blades fare much better.
To this end, wind turbines have inbuilt automatic shut-off mechanisms, designed to trigger when wind speeds exceed 65mph.
Maintenance or Mechanical Fault
Routine repairs and maintenance work demand that wind turbines be shut down.
The structures have fail-safes designed to stop rotation and therefore electricity production, in the event of such issues.
Demand Has Been Reached
Occasionally wind farmers are asked to power down their turbines if they are producing more electricity than the grid can cope with.
When production outreaches demand, turbine operation is curtailed to prevent major transmission blocks.
There are extremely rare cases where poor blade design prevents them from turning, regardless of the wind speed.
What Happens When Wind Turbines Stop?
In the UK, when electricity production outweighs demand, wind farmers are asked to turn off their turbines.
During this period, they won't earn money from the turbines on their land. As such, the government has a compensation scheme to attempt to reimburse the farmers for any out-of-pocket costs.
Constraint payments, as they are known, have sky-rocketed in recent years. A decade ago the amount paid out by the government was negligible, by 2018 they were paying out well over £100,000,000, and fast-rising.
The American government combats these huge charges by paying the landowner a single annual payment per turbine. These are called Power Purchase Agreements and are legally binding contracts.
The amount usually depends on the rated capacity of the wind turbine, but some savvy farmers also get a small percentage of the value of any electricity produced written into their contract.
How Wind Turbines Work
- Small sensors at the top of turbines measure the speed and direction of the wind. This indicates to the yaw motors which direction to point the nacelle or nose so that it is facing into the oncoming wind.
- The kinetic energy in the wind causes the blades to rotate, which, in turn, causes a shaft to spin at a low speed.
- This shaft is connected to a gearbox that increases the speed.
- The second, high-speed shaft, holds the generator. The fast rotational power converts the kinetic energy into electricity.
- A frequency converter synchronizes the electricity to the power grid where a transformer amps up the voltage before it can be used.
How Wind Turbines Stop
Now that we know why wind turbines grind to a halt, it is useful to know how it is achieved.
The most common reason for a wind turbine to stop is because the maximum wind speed is being exceeded and it is unsafe to continue. Automated braking systems are triggered to prevent undue stress on the motor and damage to the turbine.
Eat wind turbine has a cut-in speed of between 6 and 9mph, that is when the wind speeds achieve these rates, the blades will begin to rotate and produce electricity.
As the wind speed increases, so does the amount of electricity generated.
It will continue in this manner until maximum rated speed or maximum rated power is reached and generation remains constant.
When wind speed exceeds a rating based on the individual design (between 56 - 65 mph), an automatic shutdown is triggered.
Initially, the feather-edges of the blades will splay and point into the oncoming wind, to increase surface area. This begins the gradual slowing process without any sharp jerks that put an unnecessary added load on the motor.
There are three methods employed:
Aerodynamic or Pitch braking is the most practical braking design and usually the first port of call for turbine controllers.
Once the anemometer registers exceptionally strong winds, the pitch control function begins to rotate the blades in the opposite direction to the current wind flow.
The wind catches a different area of the blades, resulting in less lift and momentum.
As the rotors slow, the gap between them increases and more wind slips between them. The turbine and motor resume control of the slowing down process.
Once the blades are completely free of the wind's path, they slow to a natural standstill.
This is by far the safest method and guarantees a longer usable life for the turbine and its components.
Mechanical braking is mostly used as a backup for the aerodynamic braking system and as a parking brake once the blades have stopped. However, if the aerodynamic braking system should fail or the winds are too strong for it to work effectively, mechanical braking is the next option.
The brake disc is a circular piece of metal with holes around its perimeter. Once activated, a stopper peg plugs one hole to bring the system to an emergency stop.
Although a fast way to stop the rotors, this system causes undue grinding of the metal components, which can cause damage to components, and greatly reduce the service life of a wind turbine.
The system is sometimes relied upon during maintenance work and repairs.
Electrical braking works in a similar way to aerodynamic braking, whereby some of the generated electricity is used to turn the rotating blades in the opposite direction of the wind.
It isn't available in all turbines as it is a more expensive system to operate. However, electrical braking slows rotors down until they stop faster, usually before any friction damage is done to internal components.
The reasoning behind the stoppage of a wind turbine is down to much more than lack of wind, although, that is often a mitigating factor.
Surprisingly, too much wind is a much more likely reason for the blades to be at rest and not generating energy.