OT Mega windfarm power question

[RickH]

I was driving down I65 this weekend and went through about a 10 mile stretch of wind farm in Indiana. And I was wondering how they got all that power back on the grid. Each one of those windmills has a generator on top of it I assume. So each generator is generating an alternating current, and no way in hell are all of those literally thousands of generators going to be in phase... So I was wondering to get the generated current back on the main grid efficiently dont they have to have some kind of master controller to align the phase of all those generators back to the timing of the main grid and lock it in? If the phases were all random for those 1,000 windmills wouldn't there be cancellation of the current flow when you try to pump it back to the grid individually? Because by probability half the generators would be out of phase with the other half and simply cancel each others current flow. Or do they just time each generators phase angle individually off a small control current from the grid, and let it feed the grid right there?

Any electricians can shed some light on this?

[leonard hofstadter]

there's this from the company that runs that farm:

[dpb]

Why not? All the other thousands of generators all over the country are.

Yes, they are in phase when started; the small amount (relatively) of generation of each generator is effectively locked in by the grid.

In order to "parallel" the 3 phases, each A-B-C phase of the power source (whether it's 100 watts or 1,000 megawatts, it makes no difference) has to be in "synchronization" with the A-B-C phases of the system.

In large units, this used to be done manually by using what's called a 'synch' scope with a rotating arm (representing the phase angle of the 3 phases as one, in relationship to the phase angles of the system. When they are at "unity", that is the arrow on the meter facing straight up, the switchyard breaker is closed. Now, such can be done automagically w/ servomechanisms and online measurement.

When the generator goes on grid, the system "grabs", quite literaly, the generator and the three phase angles move in harmony/synchronization. The 'speed' of the turbine/generator, whether 1800 RPMs or 3600 RPMs is then forced driven by the system and not the primary energy source (wind/steam/whatever). No matter how much more steam (or water as in hydro) you add, the speed will always stay the same. Which is how power is increased because the additional input wind force, while not adding speed, adds torque, which, by increasing the DC field, increases megawatts.

(Not EE/electrician but NE w/ 30+ yrs working w/ power utilities)

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[RickH]

So it appears that each individual generator would have to synch with the grid on its own at that spot and feed the grid on its own at that spot. Because if you tried to synch all the generators at once and feed the grid from a collection of generators, a more distant generator will be out of phase with a point a mile away that is being used as the reference time simply because of the distance. Yes all the blades were spinning at slightly different rates based on the wind there I guess but that does not mean the generators are at different RPM's if they have a transmission of sime kind or adjusters for the prop blades, to keep the generator speed constant and some controller that calculates available torque at that moment to control how much the generator can be saturated.

I must have been passing through Meadow Lake I, II, III and IV farms looks like a total of 500 MW. It was really a head turner for many miles there.

[gfretwell]

These are DC generators with a grid tie inverter like the solar arrays. They clock right off the line they are feeding

[dpb]

On 9/6/2011 5:59 PM, [e-mail] wrote: ... ...

I don't think so...from the other poster's link to the utility's site under the "How Does A Wind Turbine Work?" heading it says (in part)--

"..The fast spinning shaft turns inside the generator, producing AC (alternating current) electricity. Electricity must be produced at just the right frequency and voltage to be compatible with the utility grid."

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[dpb]

On 9/6/2011 4:09 PM, RickH wrote: ...

Each does synch itself, but they're all tied to one (or at least a relative few) common point(s); each one is not tied directly to the grid but to a switchyard that is, in turn, tied to the transmission grid itself (at least all the large farms around here are and I really can't imagine there are any that aren't that way altho it is possible, I suppose, but would be prohibitively expensive w/ all the switchgear that way I think).

It's really no different than any other generator; the reactance of the grid is so large compared to the inertia of the individual generator that once it is on grid it is forced to stay in synch. This happens essentially "for free" by the nature of the field once the initial synch is achieved.

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[dpb]

NB that the synchronization is important only at the point of connection--what time lag there is from one portion of the grid far remote from another is immaterial; the local reactance that the generator field "sees" from its own connection is all that is of significance.

Also with respect to your earlier observation that rotor speed may be slightly different owing to local variations in windspeed; as noted that translates into more/less torque (and hence MWe) but the generator shaft itself remains at a constant rpm. There is, of course, gearing that translates the slow rotor speed to the generator shaft.

The consistency of the generator speed comes from the internal feedback of the field and is forced by the much larger reactive response of the overall grid that the individual generator is simply too small relatively to influence significantly on its own. Hence there isn't need for the complicated logic you're thinking must be there; only while the individual generator is disconnected from the grid is there any actual control of the generator rpm in order to achieve initial phase-matching; once it's then connected to the grid it simply follows along. As described in my first response, this is the basic operation the same as what happens w/ any other AC generator tied to the grid whatever its size or power source.

Many conventional power stations have multiple turbine-generators; Kingston Fossil in TN has nine separate boilers as a relatively small example. There's no fundamental difference between that and the wind farm; it's simply more of the same w/ more smaller individual units.

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