OK, lets see if I've got this...
I turn off the steam tap so the steam turbine directly coupled to the electrical generator slowly stops being the diving power (Mechanical power is almost zero) and slowly becomes a load on the generator which is now taking power from the grid to keep spinning in sync. At this point you can disconnect?

Not sure about this bit...
I disagree, as long as the rotor's spinning it's either generating power or being electrically driven, you've disconnected nothing. If it's spinning it's still connected to the turbine so that's spinning as well even though there's no steam power being input. ie. the force driving the turbine is now coming from the spinning generator rotor rather than the steam. Hmmm...

I think the generated voltage (220V in the UK) would reduce during spin down plus the AC frequency (50Hz in the UK) would reduce at the same time so moving out of sync with the grid? Maybe not, see below...

Let's see if I need to think this through from a different angle... Are you saying the voltage and generating frequency remain constant with the current being supplied to the grid reducing to zero as the generator spins down?

This would allow the powered generator to supply, say 8Mw when running and zero when spun down. Watts = Volts X Amps, though I'm still a bit dubious about the frequency issue. What do you reckon?

Play Bonny!

Hi there,

yes, at the crossover point where the direction of the power flow changes.

Another way to gradually reduce the power is to reduce the generator's excitation. That would cause the power to decrease, while you still maintain a constant spinning speed so that the fading power stays in sync with the grid.


Or it's exactly at the transition between these two conditions.

That's what would happen to a turbine/generator system when it's not connected to the grid and then slowly spins down: Output power (current and voltage) and frequency will drop.

Yes. That's why you have to maintain sync as long as the generator is still connected, because as long as it delivers power, however little, it must be in sync, or you'd be shorting several Megawatts of power. Imagine the damage to the machine and the cabling ...

[X] Doc CPU

We are talking about an alternator here, not a DC generator. As long as a synchronous alternator is connected to the grid it will not spin down. It will keep spinning in sync with the grid. As you reduce the mechanical input power, the alternator will supply less and less power to the grid, eventually passsing the point of zero power and beginning to draw whatever power from the grid is needed to keep it spinning in sync. Yes, it transitions to acting like a motor. At the point of zero power transfer, very little current is flowing and the alternator can be disconnected from the grid with almost no mechanical or electrical transient.

Really, in operation, every motor constantly acts as a generator/alternator, and every generator or alternator constantly acts as a motor. A running motor generates a counter-EMF that bucks the incoming line voltage. That is what limits the current through the motor. The internal impedance of the motor is very low, and the line voltage would push a huge current (the "locked rotor" current) through the motor were it not for that counter-EMF which, in normal operation, is only slightly less than the line voltage. If you start mechanically driving the motor and try to push it faster than it was previously running, either the generated EMF rises above the line voltage (for a DC generator) or the phase angle shifts slightly (for a synchronous alternator), and the motor starts acting as a generator or alternator. As you start drawing more electrical power from that unit, that current flow makes it act like a motor that is fighting against whatever is driving it.

No, no and NO!
This is the most often and most erroneous assumption made.
Reducing the generators excitation reduces the reactive power delivered and starts absorbing reactive power. It has nothing to do with real power to the grid. As a matter of fact, while the generator is coupled to the grid, but not being driven, nor driving, by adjusting the excitation an arbitrary amount of reactive power can be delivered or absorbed. To the maximum of the generator's specs. If does not influence the mechanical forces on the axis in the least.

jlinkels

Thanks rknichols,
A very good explanation, all is now clear, my light-bulb moment. I didn't fully understand that the generator would remain synchronised and unable to spin down while connected to the grid hence my confusion. Disconnecting it from the grid at the point when it ceases to supply current to the grid, where it changes from being a generator to a motor seems so logical I don't know why I didn't understand sooner.

Thanks also to jlinkels: and Doc CPU for highlighting this relationship as well though I still seemed to miss the point.

Shame you can't award Rep points in this particular forum.

That's the problem with getting old, there are three things which happen;
1.) your memory starts going.
... I can't remember the other two.

Play Bonny!

I think you can. There's no "Click here if this was helpful" link, but if you click the scales on the bottom left (next to the icon that shows what OS they posted from), I think it lets you add rep that way.

Yes, I believe you can "Add to Reputation". Funny, I never really "look" and didn't know that was a picture of scales

I didn't either, until I needed to describe it today

All of which discussion makes me wonder ... how, then, do you keep this giant wind-turbine from trying to act as a gigantic fan? Or from, as you say, waffling between the two modes? (Thus, at one moment, the blades exerting torque upon the hub, and the next moment, the hub exerting torque upon the blades.)

The electrical power grid, as we know it today, is a commodity distribution system which requires there to be a consuming load at the other end of that wire at all times. It can't really handle the load being reduced, e.g. by millions of households putting generators on their roofs or abruptly changing how many lights they leave turned-on all the time. It is a system designed and put in place in the 1930's. (And it's now nearly 80 years old; much older in places.)

By feathering the blades when you don't need the power from the wind generator and isolating it from the grid as indicated in previous posts (Thanks again for the enlightenment guys!)
The Power companies constantly monitor the grid and add or remove generating power as required dependent on demand (They've control rooms like the Bridge of the Enterprise, usually bigger.) You're probably talking minutes to bring a wind turbine up to speed and put on stream. The Pumped Storage Hydro stations previously mentioned take about four minutes from opening the sluices to full generating power.

Here's a thought... All these wind turbines are effectively removing energy from the atmosphere to convert into electrical energy (The laws of conservation of energy). Is this therefore having any effect on climate change? (Or maybe like butterflies in chaos theory, affecting the weather but using windmills?)

Yup! I like to confuse very small children by telling them that the windmills are what makes the world turn round...

Oh! And thanks for the "Rep" switch, I didn't realise it was there either.

Play Bonny!

It is almost certainly trivial. The wind turbine blades are relatively small compared to the huge amount of atmosphere there is. And they are very inefficient at capturing air pressure anyway.

Seen from the law of conservation of energy, you are right.
Wind is caused by differences in air pressure on different locations. Differences in air pressure are eventually caused by adding the sun's energy to the atmosphere. Energy is contained in the moving air particles. A wind turbine converts the kinetic energy of the moving air particles into axis rotation, and then into electrical energy.

The electrical energy is eventually converted to heat (everything converts to heat eventually) and contributes to heating up the air at that location. (This has nothing to do with global warming, don't be confused)

While the amount of energy harvested by a wind turbine is totally negligible as compared to the total energy present in the atmosphere, your assumption as such is correct. The wind speed and contained energy directly behind a turbine is less than in front of the turbine. The difference is the energy extracted by the turbine.

And wind turbines are not that inefficient. The theoretical maximum efficiency is 59% (Betz law), efficiencies of 40% are achieved.

I don't think there are ideal solutions; otherwise the perpetual motion machine would truly exist, friction would not cause loss of energy, and power conversion efficiency would be 100%. There's clearly room for improvement in lots of things. I myself read about gas from corn and thought it was great. Not sure. You read firstly about the amount of "bang" for lack of a better term you get out of corn based gas versus fossil fuel based gas. Then further you read about the energy costs of refinement to produce each. And finally there are the by-products of it all. So then you look at hydrogen fuel cells because what could be less harmful then ejecting water as your final by product? And then discover issues surrounding production, distribution, storage, ...

If you're going to generate power using a windmill, you're going to lose some of the energy that the wind gives to you, and you're not going to capture all the sum total of wind energy present in the surrounding area.

To me, the grid, be that localized or national would need to support storage for times of excess production and to support times when usage exceeds production and then they'll have to work at determining the sweet spot where they don't discard too much unused/unable to be stored energy and they'll also not run out. Seems further that something distributed, flexible, and allowing for transfer. I'm also sure that any sweet spot is instantaneous anyways due to the vagaries of personal choice.

For instance, my first house in New England a random town no different than any of our neighbors. We happened to have a town municipal electric company and it was said to me that we bought our power from Canada. I questioned, "Gee ... why are we special from neighboring towns?", who happened to just use Mass Electric. And where do those towns get their power from? I believe without knowing the details are that there are some levels of protection, distribution controls, and so forth, so that if a metropolitan area has a catastrophe in one direction that they can get re-routed services for communications, power, and other distributed services. I have seen the concepts of network protectiveness to ensure that if one node or junction goes down that the network as a whole still can work, just with some affected users. Those concepts came from disaster plans for things like the power grid, the phone system, etc.

So as a result, small and large municipalities, as well as individual sites (homeowners or neighborhoods) could have storage facilities, there will always be limits, and further be safety issues when storing power, obviously. No guarantees that if your neighborhood suffers some problem that they'll be able to be self sufficient; however it may also be more difficult to knock down a very large geographical area without large natural causes being the affect; earthquake, hurricane, etc. But it also might be more able to be re-established in piecemeal form for interim return of capabilities prior to re-establishment of the larger grid support which may take longer.

It is just like with an ordinary generator coupled to a diesel engine. If, for example, the engine shuts down because of an engine failure, like a defective oil pump, the generator would drive the engine. As you can imagine, if the engine has been shut down for a reason this should be avoided.

Therefor there is a protection circuit which detects reverse power, that is power flowing into the generator. When that is detected, the generator is disconnected. The oldest and most primitive device is a disk kWh meter. Once it turns backwards, it pushes a lever to close a contact which causes disconnection.

The reverse power protection in wind turbines is much more sophisticated, but basically the same. In addition, the control system disconnects the generator below cut-in wind speed. This is usually 2-3 m/s.

jlinkels

@ rtmistler
This is one of the major problems. You can store drinking water in a reservoir, you can store gas (not the petroleum stuff, the cooking stuff) in gas holders, or what we in the UK called gasometers though there was no meter involved. Electricity, the AC kind, has a bit of a problem.

You can only store it as a different form of energy, the pumped storage power stations mentioned previously use water held in a header reservoir, potential energy, which will be converted to kinetic energy then back to electricity when dropped through the turbines.
I read somewhere of a scheme where surplus electricity was going to be used to compress air to liquid form which could be stored. The energy stored in the gas/liquid state change would be released through some form of enclosed air turbine to be fed back to the grid when required. I've never heard of such a device being deployed though.

Has anybody heard of any other storage projects?

You can obviously convert the AC to DC and hold it in batteries which is exactly what's done with the UPS' in Data Centres. But they're usually designed for about ten minutes autonomy to allow the servers, etc, to shut down in a controlled fashion. I've seen them go from individual UPS' to ones supplying a single rack to the major room UPS', but I've never heard of any installations which support or are used as storage for the National Grid.

I have been in a Data Centre with individual UPS' when the power failed though! Scary! Audible warnings shrieking, red lights flashing, Aarrgghh! The UPS' held the load till the secondary supply switched over but sourcing clean underware was definitely high on the agenda that day.

Play Bonny!