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Electrical generation

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Electrical generation

Old 27th Mar 2014, 21:15
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Electrical generation

Consider this,
A steam driven turbo alternator is supplying a network at 11,000V, 50Hz through various transformers, switchgear and distribution systems.
The only load on the network is a 60w bulb, it is then required that 10 pumps are started which increase the required load to, say 3mW.
My question is this,
What happens at the generating plant to accommodate the increase in current,
The speed must remain the same to keep the frequency.
The excitation volts must stay the same to keep the voltage.
Thanks.
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Old 27th Mar 2014, 22:00
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Given that in real world systems there are permitted variations in frequency & supply voltage, they would initially drop until the governor reacts and picks the alternator speed back up.
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Old 27th Mar 2014, 22:12
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The situation would be greatly eased if the pumps are switched in gradually through an electronic system which runs them from zero to full speed gradually, and perhaps one at a time.

Old fashioned electric trains and trams used banks of resistors to limit DC traction motor current on starting, the resistors being gradually switched out as the train accelerates, being all out of circuit by the time the current through the motors is acceptable without them. Modern electric trains (and presumably trams) instead use electronic drives to optimise the waveform (rectify the AC line current to DC then chop it to the requisite AC waveform) to AC traction motors.
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Old 27th Mar 2014, 22:13
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Indeed, the steam plant is specified to cope with a defined rate of increase of load whilst maintaining the voltage/frequency within specified short term limits.

p.s. I don't think you mean 3mW = 3 milliWatts. 3MW perhaps?
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Old 27th Mar 2014, 22:16
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As I had it explained to me in my youth this all works through the application of FM*

*Fuc*king Magic.....
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Old 27th Mar 2014, 22:17
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Isn't that the reason the lights in the nick used to dim when they scragged someone in the electric chair?
We never had that problem here, we used gravity not lecky
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Old 27th Mar 2014, 22:19
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What happens - the superheat steam flow control valve on the boiler cracks a but further open.


It won't have been fully closed, as the system will have had a fair load on it in any case due to transformer & transmission (I squared R) losses which will likely have been in the several 100 kW range for a full network, if not the MW range.
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Old 27th Mar 2014, 22:19
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Thereby hangs a tail.......

p.p. the other acronym was was WMM. but that's a bit non-PC...
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Old 27th Mar 2014, 22:19
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Tony

" Isn't that the reason the lights in the nick used to dim when they scragged someone in the electric chair?"


That was planned, it sent a message to all the other inmates !
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Old 27th Mar 2014, 22:24
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Just think of the angular momentum of the power plant and its rotors and flywheels etc. vs that of the pumps and clutching them all together through a shaft of medium stiffness, that's the instantaneous effect, after that you have to shovel more coal.

After an excellent landing etc...
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Old 27th Mar 2014, 22:44
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Originally Posted by AlbieSenior View Post
Consider this,
A steam driven turbo alternator is supplying a network at 11,000V, 50Hz through various transformers, switchgear and distribution systems.
The only load on the network is a 60w bulb, it is then required that 10 pumps are started which increase the required load to, say 3mW.
My question is this,
What happens at the generating plant to accommodate the increase in current,
The speed must remain the same to keep the frequency.
The excitation volts must stay the same to keep the voltage.
Thanks.
Albie, good question and with electrical generation the answer is not as intuitive as say an equivalent mechanical system. That said, it would be worthwhile to consider a mechanical analog first. What you are asking is in effect how does the generator "feel" and "react" to the change in load (demand). Consider what happens in a car when driving at a steady speed and the load on the engine suddenly increases as would happen when the level road changes to an upward hill climb. The engine feels the additional load and unless you supply more fuel the rpm of the engine and hence your speed will start to fall. To maintain the engine rpm and your forward speed you need to supply more fuel - in other words the engine needs to do more work. The same thing happens at the power station. As the generator feels the step change in load (starts to climb the hill) the extra load is in turn felt by the turbine as it struggles to maintain it previous effort on the generator. The turbine control system will drive the shaft rpm to increase against any drop off in rotational speed. The turbine is now rotating at its original speed but against a higher back load (it is now producing a higher torque) - it and the generator are now producing more power (Power = Torque x Rotational Speed).

If you want to feel the effect of electrical loading get one of those hand held wind up generators. Hook it up to some incandescent lamps (you will need to match the voltage) and then compare the effort when you swap the incandescent lamps for LED lamps. You will quickly appreciate that to maintain the same rpm you need to provide more torque for the incandescent lamps (higher load).
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Old 27th Mar 2014, 22:53
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Yeah, but if the system is whistling along just driving a 60w bulb and suddenly 10 big hairy-ar5ed pumps are switched in... the alternator will stall and the steam valve will be whacked open by the governor. The circuit breaker will trip as the alternator will 'see' a dead short (no back emf yet from the pump motors). If the trip happens before alternator stall, the alternator will keep going but disconnected from the load.

Then you have to bring the load on as I described earlier to keep the current in limits until the pumps are up to speed.
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Old 27th Mar 2014, 23:34
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AlbieSenior.

Can I remind this 'New Boy' that questions of this depth may or may not get answers that may seem sensible. This is a forum of knowledgable serious people who's main interest in common seems to be tits. After a few replies the rest will inevitably tend towards this subject like filings to a magnet.

This is totally unlike the Flight Deck Forum's 'Rumours and News' area reserved for high minded, highly qualified and serious pilots but seemingly populated mainly by high minded, high qualified simm pilots who seem to post most of the responses to any observations that verge on the serious.

We need to lower the tone of Jet Blast otherwise the ramblings in the other sections will overtake us in the plunge to banality. Our world on here will be challenged and, who knows, we even might get our posts counted.

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Old 28th Mar 2014, 00:36
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As SSD says, a gentle load increase is best. The rate the generator slows down at (angular deceleration) depends on the applied load and the moment of inertia of the generator/turbine combination. One option is to have a relatively high inertia. The maximum rpm drop (and so frequency change) before the load change is accommodated will also depend on how fast "the steam valve can be whacked open by the governor". The final factor is making the machinery strong enough to handle the sudden torque changes.
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Old 28th Mar 2014, 00:49
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Originally Posted by Shaggy Sheep Driver View Post
Yeah, but if the system is whistling along just driving a 60w bulb and suddenly 10 big hairy-ar5ed pumps are switched in... the alternator will stall and the steam valve will be whacked open by the governor. The circuit breaker will trip as the alternator will 'see' a dead short (no back emf yet from the pump motors). If the trip happens before alternator stall, the alternator will keep going but disconnected from the load.

Then you have to bring the load on as I described earlier to keep the current in limits until the pumps are up to speed.
SSD the power station control system are constantly monitoring and reacting to load changes at the global level across the grid. In a modern power station "stalling" of a generator is very unlikely and would in reality result in considerable damage to both generator and grid. Real time monitoring of the base load is continuous with anticipatory loading accommodated by bringing on load fast start-up stations such as the hydro stations and gas fired turbines. The step change from 60W to hypothetical MW as described by the op is of course very unlikely in the extreme across the national grid but the control principles are the same.
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Old 28th Mar 2014, 01:00
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I'll have to ask our resident sparks but I suspect that if you had that type of grid set up you'd need to have a bigger base load than 60w to balance the system. I'm not sure, but I think the generator would probably just run away without a suitable load to work against. Which is why we had off peak electric and why the Black pool illuminations came to be to provide a dump load when the trams weren't working.

I do know that the Nimrod linear accelerator used a flywheel system to generate the pulse required to fire the particles down the system. They had a Hotline to the control room at Didcot power station to alert them that they were about to start the flywheels which were 30 tons apeice. Even so and running during off peak periods they used to regularly brown out Didcot and surrounds.
Apparently they did a worse case failure assessment which reckoned that if one of the flywheels did break free at full speed it wouldn't stop till it got to Newbury, which is around 20 miles away!
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Old 28th Mar 2014, 01:07
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Originally Posted by Windy Militant View Post
I'll have to ask our resident sparks but I suspect that if you had that type of grid set up you'd need to have a bigger base load than 60w to balance the system. I'm not sure, but I think the generator would probably just run away without a suitable load to work against. Which is why we had off peak electric and why the Black pool illuminations came to be to provide a dump load when the trams weren't working.
Absolutely Windy, the generator would destroy itself without suitable no load protection - which is of course there in reality. It would be a bit like cycling in a sprint to the finish line and then the chain on your bike snaps! Mind the cross bar
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Old 28th Mar 2014, 09:41
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It's already been said,

If you're running only 60W from the system, the generator spins with a torque required to power losses in the system (and a 60W bulb), controlled by only having a dribble of steam going into the turbine.

In such an unmatched system, the chances of keeping a reliable 60Hz output aren't good.

If you up the power demand, then you need much more steam to provide torque from the turbine. In the meantime, the shaft will slow - frequency will drop, and the voltage will drop until the turbine is set correctly.
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Old 28th Mar 2014, 10:04
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Electrical generation


I am of the electrical generation, but my great grandfather wasn't.
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Old 28th Mar 2014, 10:31
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As the Sensational Alex Harvey Band sang, "There's no lights on the Christmas tree, Mother, they're burning Big Louie tonight".

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