PDA

View Full Version : Electrical generation


AlbieSenior
27th Mar 2014, 20:15
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.

west lakes
27th Mar 2014, 21:00
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.

Shaggy Sheep Driver
27th Mar 2014, 21:12
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.

Fox3WheresMyBanana
27th Mar 2014, 21:13
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?

ORAC
27th Mar 2014, 21:16
As I had it explained to me in my youth this all works through the application of FM*

*Fuc*king Magic.....

tony draper
27th Mar 2014, 21:17
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
:rolleyes:

Donkey497
27th Mar 2014, 21:19
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.

ORAC
27th Mar 2014, 21:19
Thereby hangs a tail.......

p.p. the other acronym was was WMM. but that's a bit non-PC...

500N
27th Mar 2014, 21:19
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 !

Flash2001
27th Mar 2014, 21:24
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...

TomJoad
27th Mar 2014, 21:44
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).

Shaggy Sheep Driver
27th Mar 2014, 21:53
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.

funfly
27th Mar 2014, 22:34
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.

http://www.funfly.co.uk/images/truppence.png

Fox3WheresMyBanana
27th Mar 2014, 23:36
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.

TomJoad
27th Mar 2014, 23:49
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.

Windy Militant
28th Mar 2014, 00:00
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!:uhoh:

TomJoad
28th Mar 2014, 00:07
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:E

awblain
28th Mar 2014, 08:41
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.

wings folded
28th Mar 2014, 09:04
Electrical generation


I am of the electrical generation, but my great grandfather wasn't.

MagnusP
28th Mar 2014, 09:31
As the Sensational Alex Harvey Band sang, "There's no lights on the Christmas tree, Mother, they're burning Big Louie tonight".

Bzzzzzzzzt.

Shaggy Sheep Driver
28th Mar 2014, 10:04
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.

Of course, but my 'stalling' scenario was the 60w to sudden many MW, not the power station 'no stall' scenario you describe.

421dog
28th Mar 2014, 10:18
The reason the lights flickered when an electrocution was carried out had nothing to do with increased load stressing the system.
The utility companies of yore were much like today's drug companies and had serious reservations about their product being used to do in convicts. Therefore, the prison would turn on the standby generator to do the deed.

Lon More
28th Mar 2014, 12:30
Sensational Alex Harvey Band-There's no lights on the Christmas tree

was it really 1972?

arcniz
28th Mar 2014, 13:03
The "new" electrical supply model does not use distant spinning things driven by megatons of electrical bulk and rotating physical mass.

The modern alternative to 19th-century turbine and motor-generators, wires, and hierarchies of transformers is a distributed sea of electronic "charge-switches", operating at microsecond speeds and driven from incoming near and far "charge pools", that collectively can and do shuffle bits of energy around in packets like grains of sand, a milliwatt here, megawatt there, cleverly arranging for the right-stuff to arrive at the right destination at exactly the right moment.

Such is the method and mechanism that allows nuclear generators and solar panels and various sorts of butterfly wings to all contribute some bit of share
(from each according to their ability) to local and also to far distant loads in an equitable and constructive way. Just coming on, it is... a hundred-year process, at minimum, to displace the Victorian infrastructure now existing in many places.

cattletruck
28th Mar 2014, 13:24
Any producer system coupled to highly variable consumer system will often incorporate a buffer inbetween. Be it computers, hi-fi amplifiers, hydraulic power, water supply or electricity generation.

Interesting point about the "new" electrical supply model, I guess it scales well and can take compliant input from many different forms of electricity generation. It's distribution seems to be modelled on data communications.

SawMan
28th Mar 2014, 22:42
Looking at the thread title I thought the topic was about today's kids, who unlike us oldsters who could play happily with a stick and a ball, need electricity for their everything- I-pad, I-pod, I-phone, I-gamebox, I-gotta-have-one-because-everyone-else-does. Were it not for rechargable batteries parents would starve from excessive battery purchases for their progeny :ooh:

The grid system is not as good an idea as it seems- the worst regional blackouts in N. America were all caused by single-point failures which could not be switched out in time to prevent the rest of the regional grid from collapsing. It needs to be able to switch out as fast as it switches in and that capability is not possible with current designs in N. America :ugh: As the "I-gotta-have" world increases the electrical loads, something is going to have to give and it won't be them, so plan to see more Nukes because that's about all we've got that can keep up with their pace!

ricardian
28th Mar 2014, 22:47
Here's an interesting site (http://www.dynamicdemand.co.uk/grid.htm) where you can watch the mains frequency varying as the load changes

tony draper
29th Mar 2014, 00:30
I read or heard somewhere that a decade or so after the war when people started buying fridges washing machines fridges televisions and such the industry extrapolated the power needs ten twenty years in the future and built the power stations accordingly that would be needed,however the invention of the transistor and more efficient kit meant we ended up with a surplus.
:uhoh:

west lakes
29th Mar 2014, 00:35
With the predicted reduction in the availability of gas, the industry is now worried that the network etc. will not be able to cope!


I must admit to stalling a 250kVA diesel generator on full load one day. That was quite exciting and very smoky!

Solid Rust Twotter
29th Mar 2014, 06:13
We're experiencing daily blackouts over large parts of SA. No forward planning was done so no new power stations were built, combined with maintenance funds being diverted for more worthwhile projects such as building a new home for the prez and padding the bank accounts of the pilferati. It's going to be even more fun in winter when demand goes up.

Meanwhile, cheap electricity continues to be exported over the border to our neighbours while the SA consumer (who bothers paying for it instead of stealing it) is getting screwed with constant price increases, while coping with those regular blackouts.

Oh, another thing... The Grande Fromage at the electricity supply company is also one of the highest paid people in the country and receives regular bonuses, despite the failures mentioned.

rh200
29th Mar 2014, 09:56
I must admit to stalling a 250kVA diesel generator on full load one day. That was quite exciting and very smoky!

Ahh memorys, watched a Cummins on a pontoon have a fight with Flygt pump one day, the Cummins won. Musn't have had under speed protection because it was touch and go for a while. Didn't think the gensets could be pulled that low, and more smoke, than you could believe, but after about 30 seconds, she went back up to high idle with no load on it.

Scratch one pump:p

tony draper
29th Mar 2014, 10:16
Worked with a chap once who was a engineer on the Harlepool Power Station when it was under construction, he said as part of the project they were obliged to test one of the generators to destruction before installation.
Well I assume he meant before the remaining ones were installed.
:uhoh:

arcniz
29th Mar 2014, 11:34
A relative had responsibility for witnessing acceptance tests on HV power generation & distribution kit of various sorts. Destruction was optional, it seemed, but the voltages and currents were spec'd in advance, and product survival was TBD. Occasionally I got to tag along.

Can still see the scene, standing in a faraday cage built of welded chain and lined outside and inside 100 percent with very fine gleaming copper mesh some four storeys above a test floor holding a transformer the size of a modest house. Metal balls maybe 2m in diameter and spaced a precise distance apart -- maybe 2 or 3 meters --- just forward outside the cage, quite near under our feet, it seemed to me, with a trigger electrode smaller ball in-between.

For the test, bells rang, horns, lights flashing, all as prep. One half expected a Glockenspiel next, but instead there was a really brilliant flash, a line of incandescing plasma maybe 20 cm diameter in a slightly ziggy line between the spheres -very bright white in the middle , and what seemed minutes later a sharp-edged kerwhack noise of
cannon-blast proportions filtered into the mind through the heavy-duty earmuffs.

Quite a rush that was! The transformer survived, smiles spread all around, and shortly after a really great lunch, ears still ringing.

VFD
29th Mar 2014, 19:31
I must admit to stalling a 250kVA diesel generator on full load one day. That was quite exciting and very smoky!Testing Motor/Alternator packages with a resistor bank to 2000KVA is one thing VFD has done.
The early design diesels up to Tier 2 EPA requirement did definitely produce massive amounts of smoke when you hit them with a large loads. Most relied on mechanical type fuel controls.
Since Tier 3 and now Tier 4 interim requirements engines use electronic injection techniques and do a pretty good job of with the smoke and particulate matter although there is still visible evidence.


Worked with a chap once who was a engineer on the Harlepool Power Station when it was under construction, he said as part of the project they were obliged to test one of the generators to destruction before installationWell I have to say I have seen some pretty spectacular failures, although usually on engine side but not always. I have seen some huge engine blocks with large windows put in them from rod failures. However, those issues usually crop up long before rated output if it is an engine problem.


Since electronics have come along way, most voltage regulation will stop exciting at about 57Hz on a 60Hz system and 47Hz on a 50Hz system. So if you drag down the engine the voltage regulator will just turn off the excitation to the alternator and it quits producing electricity to protect the alternator.


The biggest problem I run into is trying to convince the customer that has requested a load bank test is that his 10 year old 1365KVA engine/alternator set is only producing 700KVA is that the problem is the engine not the alternator.


VFD

John Hill
29th Mar 2014, 19:33
We were installing a no break power supply system (battery and inverter) at Pyongyang but the electronics in the controller refused to put the battery charger on line due to low mains frequency. This was finally worked around by setting the battery charger to accept down to 15Hz which apparently is a European railway standard.

According to our instruments the mains frequency there was 41.6Hz and rock steady. It was 50Hz at the hotel. I have an idea why the mains frequency might have been low at the site.

llondel
29th Mar 2014, 21:08
Consider also the reverse scenario, all those pumps drawing full load and suddenly being disconnected. All the energy being put into the turbine has to go somewhere, so it starts spinning faster. As it goes faster, bits start flying off until it disintegrates. (This is how my power lecturer described the process at uni.)

In practice you wouldn't design a system with such a load change. If you were going from 300MW to 330MW as a step, that's not too bad, but 0.00006MW to 30.00006MW in one go is a bit drastic.

Of course, if there's a power cut, they have to be prepared for a surge on the grid as stuff is brought back on-line, but even then it's usually done one breaker at a time.

cockney steve
29th Mar 2014, 21:35
A late friend became a Chartered Engineer and was Charge-engineer at several power stations up and down the UK. He finished his carrer in Largs, just up the road from a nuclear power station which was shut down for political reasons.

He said they used to read the paper to find out what events would affect demand...a generator would be bought up to speed and the phase had to be synchronised, before switching it into the grid.....get it wrong and the bang would shake the whole building.....anyway, come the advert break on the telly, several million 2-Kw kettles would suddenly switch on...but they were ready!
n interesting fact, the generators were filled with Hydrogen....the concentration had to be maintained high enough that it could not explode....why?....the rotating mass had a lot less power -absorbing drag in Hydrogen, than air, presumably it was easier to fill with gas than to attempt a vacuum! IIRC, it also had something to do with spark-quenching.

He also told of a large, reinforced -steel switchgear door which blew a few hundred feet, demolishing a wall along the way........ the chamber was, again, gas-filled and a seal somewhat akin to the airlock familiar to home-brewers, was incorporated.....oversight allowed the water-level to drop, air got in, the ratio got critical ....then the switch operated :eek:

many fascinating yarns about an industry that has changed massively in the last 30 years.....Alan Binns MICE remembered with affection.

Flash2001
29th Mar 2014, 21:36
Once again, consider the energy stored in the rotating mass of the generator as you add load or the energy necessary to accelerate that mass once the load is removed. In practice the system has many sources and many loads all connected by links of varying impedance. Some generators are kept spinning just for PF correction and these add to the ability of the system to absorb transients most of the time. Sometimes the system becomes unstable and when that happens there is hell to pay as the system has to be broken down into manageable sizes of source and load and then reassembled.

After an excellent landing etc...

Little cloud
29th Mar 2014, 21:45
'the pilferati', 'The Grande Fromage'


Thanks SRT, I'll be using those! Got any more?

Flash2001
29th Mar 2014, 21:52
'Grand Fromage' Is that anything like Big Kahuna?

After etc...

Sallyann1234
29th Mar 2014, 23:11
There was I reading the thread title and thinking one of our more mature contributors from the Mechanical Generation was complaining about the younger ones and our new-fangled wired gadgets. :E

arcniz
30th Mar 2014, 10:23
Cockney Steve wrote:He said they used to read the paper to find out what events would affect demand...a generator would be bought up to speed and the phase had to be synchronised, before switching it into the grid.....get it wrong and the bang would shake the whole building.....anyway, come the advert break on the telly, several million 2-Kw kettles would suddenly switch on...but they were ready!

A step along the way to change in power distribution and use is the proliferation of "smart" electric power meters that keep track of time of use for increments of power consumption.

On the power-provider's side, having a logging device with its own timers and rules can allow a great variety of schemes for charging different cost rates for power at different times -- higher prices during peak demand periods is the common example, but the possibilities are wide open, with the granularity of the process likely feasible down to increments of a second or less, possibly even individual alternating voltage cycles.

An advantage to the provider from this is it makes possible and practical the purchase of power from outside sources at what-the-market-will-bear rates when demand surges for short or long intervals. Many end-of-life jet turbines seem destined to be coupled onto generators to provide very pricey on-demand peak power boosters during peaks in climate and diurnal cycles. Since the cost of power used is passed through directly to the consumer, the power company potentially has benefit of vast available reserve capacity without the capital cost, etc., of owning and operating it in-house. Downside to the consumer is that one may discover after-the-fact that one has purchased a small or large quantity of power at 100x or 1000x the basic going rate.

Seems likely that a side-effect of the above quandary will be competitive offers for PREPAID peak power at discounted rates, useable like money in the bank for inevitable demand surges. What follows from that likely is a very lively market in power contracts and futures, and a whole new set of markets and rules for clever folk to decipher and game.

And likely there'll be thrifty folk that buy little boxes or programs to watch the ups and downs of moment by moment power rates and store or use as much as possible at the lowest rates -- for the oven or the boiler or the grow-room.

A A Gruntpuddock
30th Mar 2014, 10:58
"what events would affect demand".

Recall reading many years ago that one of the biggest tests for the grid was the Morecambe & Wise Xmas shows.

Apparently everything was fairly static because so many people were watching it then all hell broke loose at the interval when the kettles were switched on to make a cuppa before the next segment!

alisoncc
30th Mar 2014, 11:26
When I was 'ittle we had a DC electricity supply to our house in Nottingham. No problems with frequency variation and the voltage used to go up and down regular. My father ran wires down the gas lighting pipes and fitted bulb sockets to where the gas mantle had been, with bits of string hanging down to switch them on and off. Don't know why they changed. :confused:

PingDit
30th Mar 2014, 11:29
There's a pretty good explanation of the theory here:


http://www.assct.com.au/media/pdfs/16-74.pdf

MikeBanahan
30th Mar 2014, 16:58
Demand management in a real power distribution system is a serious issue. Fortunately they don't have to deal with going from no-load to full-load, it's just not tenable on a national scale. In the case of the UK power distribution system (the one I know something about) the planners are pretty good at predicting demand and having capacity ready for when the load is needed and they bring it online or spin it down according to long experience and forward planning based on weather forecasts and the like.

To smooth out their inaccuracies in load prediction there are rapid-demand facilities kept ready and waiting like the stored-water system at Dinorwig which can be brought online in seconds to fill a sudden gap while the slower-response generators catch up. If they are caught by surprise they are allowed to drop the grid frequency and voltage for a few seconds as long as it's made up later (within limits) and in cases of serious problems there are well-prepared plans for load-shedding - though how well-rehearsed they are might be another question. A small drop in frequency and voltage gets rid of a lot of demand and it does vary throughout the day.

Andy_P
30th Mar 2014, 23:19
There is a minimum load on power generation of that scale, and in reality the minimum is not far off the maximum in order to retain efficiency. In a real power station, there is alternators of various sizes. You start with small and bring on more as needed. Also in a modern network, power can be switched in from other stations as required. This can be done whilst another alternator is bought online.

awblain
31st Mar 2014, 10:04
As Mssrs Crun and Banahan say,

UK demand peaks on a minute timescale can be accommodated using the Dinorwig storage station in North Wales.

It also provides a dark-start capability for the whole grid should it ever collapse.

Capn Notarious
31st Mar 2014, 10:18
Consider this.
If tea were made in flasks before a programme started: would there be the surges and sudden demands?

MagnusP
31st Mar 2014, 11:17
I did a year or so working in the test department of a large motor, generator and transformer manufacturer. Got in one morning to a scene of destruction when a LARGE DC motor's series field winding had failed, the rotor flew apart, locked in the stator, ripped the mounting bolts off the floor and the whole lot crashed through the wall and ended in the car park. Night shift, fortunately. We also had a 15 ton rotor/flywheel assembly (for ventilation in a gold mine) up to full speed when a bearing failed. These things are meant to keep turning if power fails so that the mine can be excavated; stopping them means shutting off power, swapping 2 phases and powering up again. Fun. Sparks. Smoke. :eek:

Andy_P
31st Mar 2014, 11:23
Consider this.
If tea were made in flasks before a programme started: would there be the surges and sudden demands?

Yes. For example, big sport game (soccer for you pommie folk), half time = increased loads. But your power companies are wise to this, and will bring alternators online in anticipation of your tea making.

I did some work for a base load power station many years ago, they are pretty dam good a predicting load usage.

tony draper
31st Mar 2014, 11:26
Those three words Testing to Destruction always reminds me of this clip.:E
Chinook ground resonance - back - YouTube

Flash2001
31st Mar 2014, 20:38
Another interesting thing is that when starting a large rotating mass, it is the whole rotating mass already connected and spinning in the system whether source or load that is available to pick up the new load.

After an excellent landing etc...

Andy_P
31st Mar 2014, 22:30
Another interesting thing is that when starting a large rotating mass, it is the whole rotating mass already connected and spinning in the system whether source or load that is available to pick up the new load.


Not sure if you are still talking about power generation, but, the steam turbines in a base load plant are started with no load and no excitation and are run up to 3000rpm (provided you are on 50hz). Once up to speed the alternator is synced to the grid then connected to the grid. When I was back on the tools, it was mostly all computer controlled, guessing these days its probably all computer controlled.

Flash2001
31st Mar 2014, 23:44
Andy

Quite so. All I'm saying is that the whole rotating mass of the system is available to spin up additional rotating load. The ability of rotating mass to contribute is, of course, reduced in proportion to transmission impedance, thus this effect is more important to small systems. As an aside I heard that one of Ontario Hydro's specs for a certain standby generator was that it must be able to withstand a bolted short suddenly applied. The maker's tech rep asked that he be informed when this would be tried so that he could heigh himself into the next county before the test as this would effectively lock the rotor to the frame at 3600 RPM.

After an excellent landing etc...

Andy_P
1st Apr 2014, 03:33
Andy

Quite so. All I'm saying is that the whole rotating mass of the system is available to spin up additional rotating load. The ability of rotating mass to contribute is, of course, reduced in proportion to transmission impedance, thus this effect is more important to small systems. As an aside I heard that one of Ontario Hydro's specs for a certain standby generator was that it must be able to withstand a bolted short suddenly applied. The maker's tech rep asked that he be informed when this would be tried so that he could heigh himself into the next county before the test as this would effectively lock the rotor to the frame at 3600 RPM.

Ahh I got yah.

I think its a pretty common myth that putting a short on a generator will stall it (or at least one the size you find in a power station). More likely the short would fuse. I would not want to be in the room when it happened though, there would be molten metal everywhere.

Mind you, if anyone can show me a photo of a generator of significant size that has stopped and removed itself from its mounts I will believe you, until then I will just assume its a myth. What is more likely to happen is the coupling would fail, or various parts of the drive (which could still cause significant damage mind you).

Solid Rust Twotter
1st Apr 2014, 07:50
Some pics around somewhere of a large generator in SA which was started by someone not qualified to do so. It all went TU and the genny fired half its guts out through the roof of the power station.

Andy_P
1st Apr 2014, 10:20
Some pics around somewhere of a large generator in SA which was started by someone not qualified to do so. It all went TU and the genny fired half its guts out through the roof of the power station.

Bit different though. The only pics I have seen of a generator detached itself from its mounts was a water turbine that had a bearing failure. Apparently it was vibrating for a week and fractured the mounting bolts. From my experience outside of power generation any detected vibration in a large system like that means an immediate shutdown. I used to work on gear for detecting that vibration. Maybe the pilots of Boeing and Airbus can pipe up here, but I believe the same monitoring occurs on jet engines.


I have never seen a massive failure of any motor or generator personally, but I have seen pics.

MagnusP
1st Apr 2014, 11:09
Apart from the aforementioned DC motor runaway and the 3-phase motor bearing seizure, we had a substantial fire when running 5MVA and 40MVA alternators in parallel. There was a short in the windings of the 5MVA one, and the 40MVA basically dumped into the smaller machine. Not only did it start the fire, the heat was so intense it effectively brazed the covers shut and the fire brigade had to cut the covers off before they could tackle the fire. Great fun.

Technical note: we ran the alternators in parallel to test vertical motors as we didn't have a vertical DC generator to provide the load for tests. By running the alternators at different frequencies we could produce a "beat", so the motor under test was constantly accelerating and decelerating and its own rotor then became the load.

west lakes
1st Apr 2014, 11:47
Then there were the two 1650kVA diesel sets we used to support supplies during a 33kV outage.
Nice brand new V16 CAT diesel prime movers!
Day one we started them, tested them and shut them down for the next day to run them in synchronism (with each other & the grid) for a week, started No1 and left it warming up, started No2 and BANG!
Overnight fuel had filled up one bank of 4 cylinders (the fuel tank was mounted on the front of one of the trailers they were on) which it objectyed to!

Both taken away for No2's engine to be rebuilt, when they came back it took a full day for all the fuel & oil to burn out of the exhaust system so it looked more like a steam train in operation!

Andy_P
1st Apr 2014, 11:53
Apart from the aforementioned DC motor runaway and the 3-phase motor bearing seizure, we had a substantial fire when running 5MVA and 40MVA alternators in parallel. There was a short in the windings of the 5MVA one, and the 40MVA basically dumped into the smaller machine. Not only did it start the fire, the heat was so intense it effectively brazed the covers shut and the fire brigade had to cut the covers off before they could tackle the fire. Great fun.

Technical note: we ran the alternators in parallel to test vertical motors as we didn't have a vertical DC generator to provide the load for tests. By running the alternators at different frequencies we could produce a "beat", so the motor under test was constantly accelerating and decelerating and its own rotor then became the load.

I was never fortunate enough to see any major machine failure. I did see the aftermath of an arc flash though, but the guy was fully suited at the time so no major injury! I was not far away and it scared the crap out of me.

WRT to diesel and gas generators, best I saw was three in series with buggered load sharing control. They would all start, one would switch on and just stall. Next would come online and stall... and so on and so forth.. Each as presented with huge loads, but nothing catastrophic happened. They were all supposed to spin up, sync and switch online.

Windy Militant
1st Apr 2014, 19:23
Some pics around somewhere of a large generator in SA which was started by someone not qualified to do so. It all went TU and the genny fired half its guts out through the roof of the power station.
It was a test of the overspeed protection that went wrong.
Duvha powerstation turbine blowup: SA | Space van Adriana (http://nolstuijt.wordpress.com/2011/11/27/duvha-powerstation-turbine-blowup-sa/)

NWSRG
1st Apr 2014, 19:49
Fascinating stuff this electricity...made a career out of it!

One analogy I always had in my head...Power Station West in Belfast had an output of 240MW in its heyday. Now that is about the same as a Nimitz class aircraft carrier. So imagine the USS Nimitz forging her way through the Atlantic at 30 knots, pedal to the metal. That's effectively the amount of energy that a 33kV circuit breaker at Power Station West would have to contain in the event of a short circuit...and remember, in a short circuit situation, the Nimitz equivalent is a complete and sudden removal of all water resistance...
And with large steam turbines having so much stored energy in their inertia, you can feed a short circuit fault for a significant period...but a modern circuit breaker should be able to interrupt things in less than 50ms.

cockney steve
2nd Apr 2014, 17:16
@ Basil, #48said The nuclear station at Hunterston B is still operating but the oil fired station at Inverkip, due to the oil price rise in 1973, was never used much and is now being demolished.

Quite right, Sir. It was ,indeed, Kip- with the NSM factory built next door...the tobacco company abandoned it , when the Gov't. of the day reneged on a deal and announced the product would be taxed at the same rate as tobacco.........It was to Hunterston he was offered a transfer,but the redundancy and retirement package meant he would be working for about 50 a week :ooh:
Funnily enough, I just met up with his widow today,to catch up.Shortly before he died, the authorities at "B" pleaded with him to come back on his own terms at his own price.(his son was doing consultancy there at the time!

As with many UK industries, the skills were "let go"..now the bean-counters have suddenly realised you cannot buy experience, it's lessons have to be passed on.

Ban must have been norty words in a post :O no PM, no email....tried my other browser and it announced I was banned until today and have to moderate my own language :\

I'm a proper Pprooner, now!:}

flying lid
2nd Apr 2014, 17:30
Be careful with big generators and magnetic fields !!

The Philadelphia Experiment 1984 - YouTube

Lid

OFSO
3rd Apr 2014, 13:45
the lights in the nick used to dim when they scragged someone in the electric chair?

I understand that people have widely varying internal resistance. Sometimes the lights would dim slightly when the old knife switch was thrown, and other times dim a lot. On one occasion they actually got brighter, or so I heard.

Do we have anyone on PPRuNe in the business of administering the ultimate deterrent in the USA ?

Lon More
3rd Apr 2014, 14:14
Is this anything like the beatnik generation?

Do we have anyone on PPRuNe in the business of administering the ultimate deterrent in the USA ?

I don't know but there are a number of right wingers who are probably willing to do so in the UK

probes
6th Apr 2014, 10:22
how come no-one has mentioned electric eels?

John Hill
9th Apr 2014, 09:39
Current will be significant!

Remember that the typical size petrol station dispenses energy at about the same rate as a typical size power station! (Or so I was told.)

oldpax
9th Apr 2014, 10:39
At 500 megawatt our station used @240 tons of coal per hour with an ash content of 14%,work that one out! I can get the calorific value for you ,its somewhere in my training notes!!To be near a turbine that disconnects from the grid at full load is an awesome experience!!I survived many such occasions!

MagnusP
9th Apr 2014, 10:57
Fine stuff, current. When we were doing the high power tests with motors over 10MVA, the cables for the 440V dc generators were thicker than my arm, and we'd have 2 or 3 in parallel. If we ran the 40MVA machine in winter (different building) you could tell where the cables were buried as the snow would melt.

Only had one flash accident when I worked there, and I wasn't on shift at the time. Someone had removed an AVO from a test rig where it was being used to measure slipring voltages, checked resistance with it, and returned it to the rig still set to measure resistance. Engineer put the probes on the sliprings and BANG was the result. Nasty. The bloke was off for weeks with keratoconjunctivitis.

oldpax
8th Jul 2016, 00:46
Another "large power station"has closed down,Rugeley "B"was a late 60s standard 2x500 MW station(where I started my career!),reason stated "no more coal(cheap imported).I assume they have to give nat grid notice so they can plan the extra load needed from other sources!Not long since this company fitted flue gas cleaning equipment at great cost!!

TURIN
10th Jul 2016, 20:39
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.

I do believe that the Boeing 787 uses a similar system on its heavy motors such as the combined starter generators.

I also apologise for the aviation content.

oldpax
11th Jul 2016, 00:41
Large industrial gas turbines also use the alternators as starter motors.Another interesting point,on the standard 500MW alternators the stator windings are water cooled!

IFMU
11th Jul 2016, 03:50
The 787 Variable Frequency Starter Generator (VFSG) and all the modern generators we make at Sundstrand are all oil cooled. We also use industrial motor drives in our test labs. Facility power comes in at 60 hz, turns to DC, then turned into variable frequency AC to run our motors at variable speeds. The days of the DC motors are done.

KenV
11th Jul 2016, 13:52
Do we have anyone on PPRuNe in the business of administering the ultimate deterrent in the USA ? I don't want this to turn into a political discussion, but for the record, the death penalty in the USA is NOT about "deterrence." I'll leave it at that.