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Just what the title says.

On a wind farm spread across several score hectares, I assume, the wind-speed wouldn't be uniform. The faster spinning turbines would probably induce more electricity than the slower ones ...

Assuming the output from the farm is ganged at some point, how is the output from the faster spinning turbines prevented from interfering with the slower ones? It wouldn't be just a matter of putting a diode, would it?

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2 Answers 2

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You are forgetting that each turbine is connected to the grid, which is much larger than any one turbine. Each generator on the grid runs synchronously. Unless the turbines have variable transmissions, each turbine spins at the same rate. The one with more wind will have more torque, and hence more power, at the fixed speed. As long as there is positive torque on the generator shaft, the generator will be pushing power onto the grid. Some turbines will be pushing more power than others, but since the grid is essentially a infinite sink at this level, they aren't pushing each other.

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  • \$\begingroup\$ Do wind farms use generators with a fixed phase relationship relative to rotational position, or do they use something more like a squirrel cage motor? From my understanding, a squirrel cage motor will have a zero-torque speed which is related to line frequency. If it is mechanically prevented from turning that fast, it will generate torque in the direction of rotation and consume electrical power. If it is driven faster, it will generate torque in the direction opposite rotation and will produce power. Would seem ideal, except I'm not sure about efficiency. \$\endgroup\$
    – supercat
    Sep 15, 2011 at 22:52
  • \$\begingroup\$ @Supercat: As I said, these type of generators are synchronous. Take a look at a wind farm and note that all the turbines are running at the same speed. Speed is fixed by the generator characteristics and gearing. Wind speed then controlls torque, not speed. \$\endgroup\$ Sep 15, 2011 at 23:19
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    \$\begingroup\$ Turbines don't all spin at the same rate, and "running synchronously" doesn't explain how the turbines don't drive each other. Even if they're the same frequency and phase, the voltage of each will be different, no? \$\endgroup\$
    – endolith
    Sep 16, 2011 at 23:15
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    \$\begingroup\$ @endolith: As Dren.dk said, some newer turbines use switching power supplies essentially like infinitely variable transmissions. However, most existing installations are synchronous generators. And yes, the turbines in those do all spin at the same rate. Running synchronously means the generators are synchronized to the power grid. In that configuration, the power frequency dictates the speed. The amount of wind dictates the torque, which is how the power changes. \$\endgroup\$ Sep 17, 2011 at 0:26
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    \$\begingroup\$ @olin for MW class turbines, a doubly-fed generator is most common generator. The stator is connected to the grid and the rotor circuit could be a resistor bank, a power converter to an R or grid (GE/Enercon patent). Permanent magnet generators (direct-drive or gearbox) with full power converters are just entering the market. The blade speed is kept approximately constant by the turbine controller and is determined based on the diameter of the rotor and design of the blades. Same size turbine, same approximate speed. Only very old technology turbines use the grid to fix the rotor speed. \$\endgroup\$ Jun 17, 2012 at 5:17
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Old, turbines were hooked directly to the net, so the generator would be synced to the line frequency, that was what would synchronize the turbines and keep them from over speeding.

Modern windmills contain two switch mode converters:

  • The first converter takes the energy from the generator and charges a capacitor bank to the highest possible voltage.

  • The second converter takes the energy from the capacitor bank and dumps it into the net, at the right phase and frequency and at the highest possible current.

If the network is disconnected for some reason, then the second converter is unable to discharge the capacitor and the voltage will rise (for a very short time, milliseconds) and the first converter will be unable to keep the load on the generator and the windmill will start to over speed.

Once an over speed situation starts (or anything else unplanned) the controller will hit the air brakes (typically hydraulic rams with a huge reservoir which are used to turn the wings out of the wind), this braking is so powerful that the entire tower bends violently, both back-to-front because the load goes off the wings and the tower snaps back upright and from the torque introduced from mechanical and electrical braking.

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    \$\begingroup\$ This is more correct but the turbine blades are feathered first then the clutch between the blades and the gearbox its disengaged and allowed to free spin in the wind to percent damage. \$\endgroup\$
    – Faken
    Sep 16, 2011 at 12:35
  • \$\begingroup\$ Seems like it would be better to brake them gradually, no? \$\endgroup\$
    – endolith
    Sep 16, 2011 at 22:51
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    \$\begingroup\$ @endolith: Gradual braking would reduce mechanical stress, but the blades are going to keep receive energy from the wind as long as they're moving. The longer it takes the blades to stop, the more energy they're going to receive, and thus the more energy the unit is going to have to dissipate somehow. \$\endgroup\$
    – supercat
    Dec 16, 2013 at 17:42
  • \$\begingroup\$ @supercat: well, better for the strain on the tower \$\endgroup\$
    – endolith
    Dec 16, 2013 at 19:01
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    \$\begingroup\$ @endolith: If the tower would survive the strain of hard braking without damage, but the extra energy harvested by slow braking would make it burst into flames, hard braking would be better. If hard braking would damage the tower but the energy from slow braking would be survivable, slow braking would be better. The fact that towers bend violently but snap back may be ugly, but suggests that the hard braking is probably not particularly damaging. \$\endgroup\$
    – supercat
    Dec 16, 2013 at 19:07

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