A Guide to Understanding the Most Common Types of Wind Turbine Generators

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As you drive into work in the morning, you can see that the turbines at your wind farm are spinning and are on-line. They are producing power. But can you tell how much power?  I can’t. Why is that?
   
That’s because the turbine turns pretty much the same speed or revolutions per minute (rpm) when the turbine is producing minimal or maximum power.

How much does your team understand about how the turbine’s generator produces power?  Let’s find out.   

There are many different types of generators used today in wind turbines, but the most common types are asynchronous generators. The two types most commonly used are the squirrel cage induction generator and the wound rotor induction generator—also known as a doubly feed induction generator (DFIG).  

Both types work pretty much the same way, with DIFGs having some additional capabilities.

I will start with explaining the operation of the squirrel cage induction generator and then explain an attribute of the other.

For most turbines today, if the turbine is safe, has no faults or errors, and sufficient wind is present, the turbine will face into the wind and the blades will start to rotate as they absorb energy.  As the rotor turns the gearbox, the generator also rotates.  But as we all know, the turbine is not generating power if it is spinning at less than the connecting speed of the generator. The turbine can pinwheel for hours or days if there is not enough wind to get it up to spinning to the generator connection speed. We know that the turbine—that is, the generator shaft—has to get up to a certain speed before we connect it to the grid by applying power to it. That speed is called synchronous speed and is the speed at which the generator neither consumes power nor makes power (other than reactive power, but that’s a topic for a whole other article).

In reality we could apply power to the generator at any time.  But if we do so before we reach the connecting speed, we will be running the generator as a motor.  If we run the generator as a motor, then we will be consuming energy and that will cost us money. Why is this?

The connecting speed of the generator is determined by the number of poles in the generator.  It is also a function of the frequency of the grid. The frequency in the U.S. is 60hz (or cycles per second). Other parts of the world use 50hz. The generator is constructed is such a way that there is a relationship between the number of poles in the generator and the frequency of the power supplied by the grid.  This relationship is what determines synchronous speed of the generator.  A six-pole generator has a synchronous speed of 1,200rpm @ 60hz, and a four-pole generator has a synchronous speed of 1,800 rpm @ 60hz.  

You may be asking: “What does that all mean?” Synchronous speed means that the shaft of the generator rotates at the same speed of the rotating magnetic field that is formed when the generator has power applied to its stator when it is connected to the grid. If the shaft rotates slower than the magnetic field in the stator, then the generator will be working as a motor and will consume power.  That is why we don’t connect the generator at rpms much lower than the synchronous speed.  We typically connect just under or at synchronous speed—the point at which the turbine is expected to produce power.  When we connect power to the generator just at synchronous speed, the wind tries to push the generator faster than the rotating speed of the magnetic field in the generator.  Instead of the generator spinning faster, the system produces power. If the wind pushes soft against the magnetic field or pushes hard against the magnetic field, the generator maintains pretty much the same rpm but produces more power the harder the wind pushes.

Here is a good practical example of what is happening. The rotating magnetic field in a 4-pole generator rotates at 1,800rpm at 60hz here in the U.S. That magnetic field is basically a wall that prevents the generator shaft from spinning faster. To illustrate this point, choose a wall in your office. This wall will represent the generator’s magnetic field spinning at 1,800 rpm. Go ahead and push against that wall with your hand. Does it move? No. Push harder. You may be able to cause the wall to flex, but it won’t move—no matter how hard you push against it. The same principle applies with the magnetic field in the generator. Once the turbine gains speed and connects to the generator, the wind pushes, but the magnetic field in the generator doesn’t let the generator rotor shaft turn any faster. Instead, power is produced according to how hard the wind pushes. The wind spins the turbine and in turn pushes against the rotating magnetic field of the generator.

If you were to remove the wall and push in the place where it once existed, you would go tumbling forward (presumably into another room. The same thing would happen if you removed the magnetic field from the generator (disconnected the generator). If you removed the magnetic field while the wind is blowing, the blades would still be absorbing energy. That energy has to go somewhere.  In this case, the energy transfer would increase the rotational speed of the turbine, resulting in the turbine entering overspeed.  

A DFIG works the same way as a squirrel cage generator, except that it allows you to move the “wall” you’re pushing against. We can move the generator’s magnetic field by adjusting the power to the rotor through slip ring connections. Instead of the wall being fixed at 1,800 rpm, it can be adjusted electrically. By adjusting the power to the rotor, it can move forward to say 900 rpm and backward to 2,000 rpm. The advantage of being able to move the wall allows us to produce power at lower rpms and to absorb some gust loads by allowing the wall to move back or faster, absorbing the additional load.  The way we move the wall or the magnetic field in the generator is by adjusting the power to the wound rotor with power electronics.

I hope this increases your understanding of how the generators in your turbines produce power, and explains why you can’t tell from looking at the spinning turbines how much power is being produced. I’ll leave you with one last tip about wound rotors. If the turbine experienced a true overspeed, it would be prudent to perform a bore scope inspection of the windings for expansive movement.  At the minimum, the situation necessitates a climb to the turbine to listen to the generator up-close at very low rpms in order to detect rubbing of the rotor windings on the stator. If rubbing is present, the rotor could be damaged, and you could take steps to prevent damage to the stator.

As always work as safe as possible and work to prevent surprises.