The Wind Turbine is Renewable Energy biggest fan and I am a fan of wind turbines! Now since we are on the topic of fans, I decided to share with you the answer to a burning question I had some time ago regarding wind turbines. My mind boggling question was, “why don’t the turbines behave like a big fan when the wind stops?”
And rather than simply turning to Google for the answer, I decided to ask the professionals and so I cold called Wigton Windfarm Limited instead. Unfortunately, there weren’t any Engineers available to tend to my question so I was put onto the project manager. It took few emails back and forth, but in the end, the simple answer was: at low wind speeds (or no wind condition) the wind generators are disconnected from the grid so as to prevent them from behaving like Big Fans.
And here is the more detailed explanation: the Wigton wind farm consists of three separate installations namely, Wigton I, II and III. Wigton I, for example, consist of 23 NEG Micon NM52 – 900 kW wind turbines to give a total capacity of 20.7 MW. The electric generators are asynchronous (induction) generators. Therefore the rotor must turn faster than the synchronous speed of the electricity grid to generate power for the grid. Otherwise, if connected, it will consume power from the grid and in turn, operates like a Big Fan.
The following diagram shows how power is generated by these massive turbines and is routed to the public electricity grid:
The NEG Micon NM52 – 900 kW wind turbine contains 2 generators (double speed), as shown, based on an old Dutch manufacturing principle. The basis of which, is to optimize power generated at both low and high wind speeds. The small generator (g) generates power in the range 0 – 200 kW, while the large (G) generator generates power in the range 200 – 900 kW.
When the wind speed is such that the turbine-generator reaches the number of revolutions needed to generate electric power a thyristor-based circuit breaker connects the generator to the electricity grid in a fashion known as soft connection. The thyristors are bypassed by mechanical switches if more than 50 kW of power is generated to avoid excessive stress on the thyristors.
The two generators cannot be connected at the same time. This is because the large generator is a 4-pole generator which must run at just over 1500 rpm (the synchronous rotational speed for a 4-pole machine). The small generator is a 6-pole generator which must run at just over 1000 rpm (the synchronous rotational speed for a 6-pole machine).
At low wind speeds, i.e under 8 m/sec, the wind turbine starts up on the small generator. If the wind now freshens, it automatically connects to the large generator by disconnecting from the small one, “speeding up” and connecting to the large generator.
Conversely, if the wind has been strong for a long time and suddenly drops, it is no longer worthwhile running on the large generator, and therefore the small generator is reconnected by folding out the blade tips and applying the rotor brake. Once revolutions have been reduced to a suitable level for the small generator, the process of connecting the small generator is started.
These wind turbines do not always start up on the small generator. If the wind speed is 8-9 m/sec, as measured by an anemometer, the wind turbine will start up directly on the large generator completely by-passing the small one.
So there you go….! No power is generated to the electricity grid until the generator is forced by the wind to run faster than its synchronous rotational speed, i.e. the rotor will run a little faster than the rotating magnetic field in the stator. And if the generator was already connected and wind stops or the drops below the cut-in speed of the turbine, the generator is disconnected from the grid.
Thanks again to Wigton Windfarm Limited for taking the time to answer my question and to provide the technical details about the operating principle of their NEG Micon wind turbines…. I hope this helps if you have the same “stupid question (for an electrical engineer)” that I had the heart to ask!