Rotation of Turbine

1. Use it in grid synchronous mode - then RPM will be maintained and the power outlet would be depending on the input power availability (and even it might be minus- ie generator would run as motor and drive the turbine)

2. usage of governors to regulate the input power and hence RPM.

3. usage of DC link.

No the RPM of the Turbine may be much lesser too (or much more) but then the no of poles in the generator is changed (if RPM is less) or an intermediate gear box is used between turbine and generator.

Dear Sir,

Thank you very much for your reply.It was really helpfull

Regards,

Rakesh

In some designs they can also have the generators ran in a sub synchronous or super synchronous mode as well.

That is the rotor is fed three phase power at a variable frequency to allow the rotor to spin at one speed but the magnetic fields of the generator as a whole are still being spun at the correct speed for them to say synchronous to the grid frequency.

Wind is a very inconsistent but with a combination of different disciplines the negative effect can be neutralized. Electronics / CPU's can achieve wonders.

Firstly one can add pitch control to keep the speed constant for AC.

with low or no wind one can "feather" (zero pitch) and use a little bit of power

to keep it turbine running.

There are some other options . Long ago on water pumping windmills we placed the shaft of the wind-wheel slightly off center of the pivot. With high winds the moment around the pivot turned the face of the wheel out of the direction of the wind. That worked for us and it may still work.

Self control may still apply (As a boy I was friends with the diesel power station manager - the scrap yard was my reason) I was always amazed that a Blackstone set could run in parrallel with a Lister set (1800RPM) . As soon as smaller set was attempting to get in front of the other it would take more of the load and are forced to slow down to keep in step.

Your making me go misty eyed with nostalgia. As an apprentice I went to Mirrlees Blackstone's factory in Stockport (now Greater Manchester). It was a fantastic place, I was dumb struck by the size of the engines.

On site we had two Blackstone gen sets for emergency back up. I only ever saw them run once in anger. They'd be run up once a month for a couple of hours but never synchronised to the utility, we'd shed from the utility supply to load the gen sets. Somewhere I've got a photo of a very junior me sat in one of the cylinders.

1 and 4 have already mentioned it, - modern turbines are often AC DC AC, and the DC AC part is an Inverter which responds to the grid frequency and voltage and even re-inforces the grid frequency and voltage, (handy on weak grids). Cheers, Geoff.

When an AC windmill is supplying the mains directly (its going out of fashion I recently read for DC transmissions), then the synchronising "holds" the windmill to the 50 or 60 Hz together.....

Theoretically, if the blades fell off and there was no "reverse current sensing", the windmill would continue to turn by working as a motor with the power from the mains.....the effect is that strong.

Accidentally synchronising "out of phase" can rip a giant generator off its mountings, the forces are IMMENSE.....maybe that's what in Star wars is meant with "May the force be with you!"

When the wind blows harder, the windmill takes more load, but its frequency/speed is held by this synchronising force with the network....it is not "powerful" enough to raise the frequency on its own.......

In the RN, we often ran two alternators in parallel, one in manual taking say 50% of the load, the other in automatic to handle the changing load, could run for days like that!!!!

This wiki article is written for a closed system, such as a ship, but otherwise describes the process well:-

Alternator synchronization

When the line breakers of a BTG are closed in at synchronization, the generator becomes a part of a magnetic circuit. It must follow where the rest of the system "is going". It can add to the magnetic power by adding its strength by way of the steam pressure against the turbine blades. If it has no additional pressure from the steam to turbine blading, it acts as a motor and still has to remain in synchronization with the circuit. If it is running at 50 Hz., it will run at 3000 RPM if it is a two pole machine. If it is a four pole machine, it will remain in synchronization at 1500 RPM.

The same holds true for hydro turbines. Wind turbines, on the other hand are generally connected to a grid in a different fashion. They are generally creating a DC output based on the wind pressure that is available which is processed by an inverter to come up with the proper AC voltage and then synchronized to the grid through the line breakers to provide power to the grid system (circuit). By using an inverter system, the wind turbine can run at speeds other than synchronous speed and still be able to act as an "additive" component to the grid system (circuit).

This has been alluded to in the other posts, but I felt that I should try to put it in more layman terms to point out the differences between BTG, Hydro and Wind. Please do not take this answer as condescending as it is not intended that way. I hope this helps.

This line suggests that you don't fully understand the principle:-

By using an inverter system, the wind turbine can run at speeds other than synchronous speed and still be able to act as an "additive" component to the grid system (circuit).

If you had read my post, it explained that although an AC wind turbine CANNOT turn faster than the frequency, IT CAN TAKE MORE LOAD, eg. current. (Assuming that it is not at its maximum allowed power output already.)

If I have misunderstood your meaning here, please explain again, but that is the only way I could understand "additive" from your post. It was the only line that I felt was wrong.....

Although DC systems are planned and maybe installed, the reasons are more to do with losses with AC particularly when the cables run underwater....

On land, its easy to see the AC ones when several windmills are working together, you notice that the blades are completely synchronised with each other.....DC ones would not remained so well synced, if at all.....the slightest wind strength change would change their speed....

This is one of those happy times when 2 posts in disagreement are actually both correct. In the earlier days of big wind turbines the majority of machines, - particularly coming from Europe, had the grid synchronisation principle Andy refers to, and those machines indeed were fixed in their rotation speed once they locked on to the grid, overspeed being dealt with by increasing the current drawn or when that was maxed, actively pitching the blades to spill some of the wind. That principle went on to become the majority, - Vestas for instance still uses that principle, although they allow a small amount of slippage - just of the blades around the hub, to absorb sudden gusts. However, even then there were also machines using the AC DC AC mode, - which allows variable speed, a good example being the 2 bladed Lagerwey models, - still mostly operating 20 years down the track, another top market company doing that is the German Enercon company, which now makes the 7.5 Megawatt E 126, - I think the largest commercial wind turbine in the world. (although next week another even bigger might take the title). They have long used the Ring Generator direct drive multi pole variable speed Generator system, with advantages in no gearbox, extremely long lived bearings and the ability to harvest more energy in stronger winds, and grid support, ie soft start and their output is just above the grid voltage so they can support the grid against dips (inc. using the momentum in their blades) and reduce their support in a surge. The initial disadvantage this sort of system had in the losses through the AC DC AC conversion has been significantly reduced by modern electronics. Theoretically, I suppose, it can be argued that the most efficient and cost effective turbine will be variable speed passive pitch downwind turbines, - 2 bladed probably, and permanent magnet, and monsters with these characteristics are already on the drawing boards, so if you come across a field of Passive Pitch turbines, they can also be operating at the same speed as when the windspeed is above the rated speed they self regulate to the exact maximum speed however much more the wind increases till say 50m/sec, after which they begin to slow down. However, despite these advantages, the fixed speed grid controlled stall regulated gearbox machines will be with us for quite a while as those companies developed into the worlds biggest and don't feel inclined to change their practises any time soon. What is happening in China may change the ball game though. Cheers, Geoff.

Suggest you follow this thread for edification of your thinking on required RPMs for synchronous speeds of generators/alternators.

http://en.wikipedia.org/wiki/Alternator#Synchronous_speeds_2

As you can see from the link the rotational speed will be a function of the number of poles just as is the case with an induction motor, 2 poles =3000/3600 rpm, 4 poles 1500/1800 rpm etc.

Several of the other posters have clarified how most of the windmills work. One other possibility is an induction generator where the output into a larger system/grid controls the speed. As the speed increases beyond the system's hertz the generator puts out more power which creates a braking-speed governing effect on the turbine. As the wind dies down the system would try to drive the windmill as a fan so a reverse current prevention has to be incorporated to prevent that. In the event of high wind conditions an over current protection would have to be incorporated which would probably involve feathering the blade angles to reduce the speed.