Author Topic: Where does the power go ?  (Read 30106 times)

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Offline rs20

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Re: Where does the power go ?
« Reply #25 on: August 23, 2017, 12:04:37 am »
In Belgium, they store 1.2 GW in "Coo", a pumped-storage hydroelectric power station:

https://en.wikipedia.org/wiki/Coo-Trois-Ponts_Hydroelectric_Power_Station

Looks like someone needs to brush up:

 

Offline petergebruers

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Re: Where does the power go ?
« Reply #26 on: August 23, 2017, 12:52:17 pm »
Ha! Power versus Energy! You are quite right. I do now the difference, but I did not pay attention. Stupid me, posting too fast  ;)

I do not think the maximum energy transfer is mentioned anywhere, but I seem to remember it can deliver that 1.2 GW for "a few hours" (2 hours?). My memory is rusty, I studied energy production in 1990 and at that time it didn't generate that amount of power, but it was smaller too, so run time might be the same.

They do mention: "It generates about 1 million MWh annually and consumes about 20 percent more in pumping mode."
« Last Edit: August 23, 2017, 12:55:02 pm by petergebruers »
 

Offline jmelson

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Re: Where does the power go ?
« Reply #27 on: August 23, 2017, 09:32:28 pm »
In Belgium, they store electricuty in "Coo", a pumped-storage hydroelectric power station, it can generate 1.2 GW:

Ameren has a pumped energy storage facility in central Missouri (US).  They pump water up a mountain at night, and then it runs downhill during peak energy usage times.  The plant is called Taum Sauk, the upper reservoir is Proffit Mountain.

Jon
 

Offline Codebird

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Re: Where does the power go ?
« Reply #28 on: August 30, 2017, 07:35:52 pm »
No-one here has addressed the more fundamental question: Where does unused energy go? Or, more practically, why do grid operators go to such lengths to carefully balance power flow to ensure that there is no excess energy to go anywhere?

Whenever energy is drawn from the grid, it is drawn from the kinetic energy of the generators: Hundreds of tons of rapidly spinning metal gets a tiny bit slower. The governor mechanisms quickly detect this and a few steam valves open to push the turbines a tiny bit harder. Likewise if consumption drops, the metal spins faster and the governor systems close those valves.

If you could somehow convince everyone in the country to run around turning off their factories, appliances, and lights at once, then the resulting sudden drop in consumption could result in generators spinning so fast that turbine blades become shrapnel flying through the halls before the governor systems can react and get those valves closed. Fortunately this is not likely to happen.
 

Offline IanB

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Re: Where does the power go ?
« Reply #29 on: August 30, 2017, 09:54:48 pm »
If you could somehow convince everyone in the country to run around turning off their factories, appliances, and lights at once, then the resulting sudden drop in consumption could result in generators spinning so fast that turbine blades become shrapnel flying through the halls before the governor systems can react and get those valves closed. Fortunately this is not likely to happen.

I think a generator with suddenly no load is indeed likely to happen, and fortunately the damaging consequences do not actually occur. If generators started spinning so fast that turbine blades started flying around then the engineers who designed them would not be worth their job titles. So no, not gonna happen.

A generator can end up with no load if its output circuit trips open for any reason. The control system around the generator is designed to ensure this will not be a catastrophic event.
 
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Offline Mr. Scram

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Re: Where does the power go ?
« Reply #30 on: August 30, 2017, 09:55:58 pm »
No-one here has addressed the more fundamental question: Where does unused energy go? Or, more practically, why do grid operators go to such lengths to carefully balance power flow to ensure that there is no excess energy to go anywhere?

Whenever energy is drawn from the grid, it is drawn from the kinetic energy of the generators: Hundreds of tons of rapidly spinning metal gets a tiny bit slower. The governor mechanisms quickly detect this and a few steam valves open to push the turbines a tiny bit harder. Likewise if consumption drops, the metal spins faster and the governor systems close those valves.

If you could somehow convince everyone in the country to run around turning off their factories, appliances, and lights at once, then the resulting sudden drop in consumption could result in generators spinning so fast that turbine blades become shrapnel flying through the halls before the governor systems can react and get those valves closed. Fortunately this is not likely to happen.
I have trouble believing that our technology wouldn't intervene before that happens, but I like the picture you painted :)
 

Online Ian.M

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Re: Where does the power go ?
« Reply #31 on: August 30, 2017, 10:11:40 pm »
Isn't that scenario equivalent to the power station's grid interconnect tripping out - an event that the turbines and control systems must be designed to survive?  Due to the damage that a serous turbine over-speed would do, I suspect that it would be a requirement that it would take three faults to cause damage, as loss of load occasionally (rarely) happens and steam valves do occasionally jam.   
« Last Edit: August 30, 2017, 10:16:26 pm by Ian.M »
 

Offline Jr460

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Re: Where does the power go ?
« Reply #32 on: August 30, 2017, 11:31:48 pm »
No-one here has addressed the more fundamental question: Where does unused energy go? Or, more practically, why do grid operators go to such lengths to carefully balance power flow to ensure that there is no excess energy to go anywhere?

Answered early on in the thread.   The energy comes from the fuel.  If you don't need that load level, you don't put the fuel into the boiler to start with and don't generate excess energy.

If you had no grid operators, a system of generators with changing loads will follow the overall system load.  That is as long as your control system keeps each unit spinning at 3600RPM.  (This is why any movie or tin foil hat person that talks about hacking the power plant/system computers is an idiot.  Plants can and do run just fine without gird operators or computers of any kind.)

Grid operators are balancing several things.  One is you can't overload feeder lines.  Another big one is overall system efficiency.  You have a bunch of units, built at different times, and they all have a different curve for heat rate vs. load.  In fact it is not the operator that does this but a routine that runs abut once a second that computes the best load for each unit and then bumps one ups 1MW and another down the same amount.   Overall the total generated power is the same, but now it is more efficient overall.

The other thing they do is sell or buy power.  Some it short term, some longer term contracts. 

Say you have a city with several units generating power for it.  Outside of your city you have tie lines to other utility companies.  Just sum up the power flow on the tie lines with outbound being negative and in being positive.   When it sums to zero, then you know that what you are generating is the load of your city.  If someone is buying power from you, then all you do is bias the equation, rather than zero, the amount you are buying or selling.   Then about once every tens seconds you make up for any over or under that flowed on the tie lines.   

Now in terms of sudden load loss.   Yea that happens, like when a unit trips and the generator breaker opens.  The control system is very very fast, and the hydraulics that move the valves are to be respected.  Not much overspeed is tolerated by the controls.  The emergency  all mechanical trip is set at 110% percent of rated speed if I remember right.

Normally you have the main steam feed split into two "stop valves".  As the name implies they are meant to stop the flow, right now.  Either open or closed.  Pressure is required to keep them open, and large springs and the steam flow wants to push them closed.  Pull the top off of one and take the stuff out and you can sit inside one.  From that point it goes to 4 control valves.   These modulate the steam flow.   Once a unit trips these 6 along with the reheat control and stop valves slam closed

Or if you want to lose your job there is a big red button on the front of the turbine that is an emergency trip.   Just push it.

That button does get tested, also the turbine overspeed trip is tested, and then the backup mechanical overspeed trip is tested.  This happens after an outage, (once a year for 2 or more weeks) after the turbine is buttoned back up and just before it is put back online.  Of course during the test the generator breaker is open, so the trurbine is free to run at any speed.  The "load" is just the frictional losses.
 
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Offline Codebird

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Re: Where does the power go ?
« Reply #33 on: August 31, 2017, 04:36:21 pm »
I did exaggerate a little to make the point. Flying turbine blades is very much the worst case scenario - while it is a distant possibility, there are indeed multiple safety mechanisms in place to prevent that from happening, all of which would have to fail at once.
 

Offline Jr460

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Re: Where does the power go ?
« Reply #34 on: August 31, 2017, 10:09:00 pm »
I did exaggerate a little to make the point. Flying turbine blades is very much the worst case scenario - while it is a distant possibility, there are indeed multiple safety mechanisms in place to prevent that from happening, all of which would have to fail at once.

With all the stuff I talked about, it can and has happened.  However a unit trip or sudden shedding of load is rarely the cause.  It requires a several things to go wrong.

I learned a lot about turbines from Bill.  I was young and just starting out.  Bill was old as dirt and worked for GE.  He died in 2006 at the age of 83.

If I had a question he took the time to explain why things were designed the way they were, and what could go wrong.   I think he had a few stories of commissioning new turbines in Israel and one of them due to a sudden imbalance tossing blades through the turbine casing.
 

Offline Old Don

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Re: Where does the power go ?
« Reply #35 on: September 02, 2017, 10:22:59 am »
The electrical grid in the USA is divided into 4 sub-grids. Power in each grid is controlled by adding or removing power generating units. Some generators are able to adjust their output quicker than others (nuclear are slow to change) and are used shift power as loads vary. To shift power from one unit to another the operators first parallel the outputs to match frequency and phase. Once the two generators are in sync they are connected via breakers and the two are in parallel with each other and share the total load. To drive power to the new oncoming unit they try to raise the speed of that generator and/or lower the speed of the other unit and the causes the new generator to pick up part of the load. This can continue until all the power load is on the new generator or they can remain in parallel and split the loads. This is how power is shared in each grid and total load is shared by many generators at any one time. Should there be a sudden change of load (up or down) the change is shared by all. In the case of a major failure the generators are supposted to split, but there has been several times in the past where one plant takes down the whole grid. Babies occur 9 months later in these cases.  ;) But other than screw ups like these the load continues to shift between the various power generators and there is no excess power generated. Hydro plants can shift generators and turn some into pumps and so at night when power needs drop they pump water back up into storage lakes and also at the same time maintain output on the operating generators. 
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Offline G7PSK

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Re: Where does the power go ?
« Reply #36 on: September 03, 2017, 05:10:11 pm »
I don't think that a turbine and alternator weighing several hundred tons is likely to so suddenly increase speed so rapidly that it will burst before some one or something applies the brakes. In my experience with gen sets (up to half Mw) you switch the load of and the voltage only spikes by a few volts and the frequency (RPM) only increases a nominal amount before the regulators and governors get back in control.
 

Offline retrolefty

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Re: Where does the power go ?
« Reply #37 on: September 03, 2017, 05:25:17 pm »
To try and keep it on a simpler example, if a generator needs to connect to the grid they first synchronize the generator to the grid frequency before closing it's output breaker, then close the breaker. If they want to take on load they slowly increase it's phase angle Vs the grid and to decrease the load on the generator they slowly decrease it's phase output. Automatic (usually PID based) control keeps the generator at it's operator's desired setpoint value.

 

Offline SeanB

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Re: Where does the power go ?
« Reply #38 on: September 03, 2017, 06:26:22 pm »
Get the synchronising wrong ( it was possible with older synchroscopes to have it be exactly 180 degrees out of phase and no error shown on the display, though the addition of 3 incandescent lamps to the circuitry to show the much larger differential voltage was always done to tell the difference between right and wrong on the dial otherwise) and close the contactor and you will have several hundreds of tons of turbine and alternator exit the building is very large chunks. Been done at quite a few power plants, either by accident or with a faulty synchroniser. Generally this also results in fatalities, as the pieces are going to be sent in all directions, along with large chunks of structure as well.

With wind turbines there is the added issue of the brakes not really being able to dissipate the full load power as well, relying on the feathering system to provide power reduction so that the brakes only have to dissipate and hold a minimum power from having zero angle of attack. Been a few turbines that have burned when the feathering mechanism failed due to leaking seals and losing all the hydraulic fluid and thus control of both blade positioning and slewing of the nacelle. They do not put a brake capable of dissipating 1MW at the top of the tower, it would be as large as the alternator itself and a lot heavier as well.
 

Offline G7PSK

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Re: Where does the power go ?
« Reply #39 on: September 03, 2017, 08:54:52 pm »
A lot of modern wind turbine use inverters so the speed regulation is not so critical, I was watching one the other day and noticed that as two blades were in the rising side the rota ion speed dropped and as one went over the top dead center the speed picked up until there were again two blades rising.
 

Offline jmelson

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Re: Where does the power go ?
« Reply #40 on: September 07, 2017, 07:27:49 pm »

If you could somehow convince everyone in the country to run around turning off their factories, appliances, and lights at once, then the resulting sudden drop in consumption could result in generators spinning so fast that turbine blades become shrapnel flying through the halls before the governor systems can react and get those valves closed. Fortunately this is not likely to happen.
I have trouble believing that our technology wouldn't intervene before that happens, but I like the picture you painted :)
This is called a load dump, and is one of the conditions that the power plants are designed to handle.  If a circuit breaker on an HV transmission line trips, the plant can lose its load.
They have a fast-acting emergency steam valve that shuts off the steam input to the turbine in a fraction of a second.  Since the turbine-alternator set had truly MASSIVE inertia, it won't speed up much before the valve can be shut.

Jon
 

Offline jmelson

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Re: Where does the power go ?
« Reply #41 on: September 07, 2017, 07:34:44 pm »
A lot of modern wind turbine use inverters so the speed regulation is not so critical, I was watching one the other day and noticed that as two blades were in the rising side the rota ion speed dropped and as one went over the top dead center the speed picked up until there were again two blades rising.
I think this must be an optical illusion, easy for this to happen if the shaft was not pointing right at you.  The blade set is well-balanced, so there is never a heavy side and a light side.  The rotational inertia of these blades is hard to comprehend, they just don't permit the rotor to speed up and slow down quickly.

Yes, you'd think that with one blade sticking straight out to the right, and two angled up and down to the left, the left side would be heavier.  But, those blades on the left are angled, and therefore their centers of gravity are closer to the hub on the horizontal plane, and EXACTLY balance the weight of the single blade's center of gravity on the right, because it is farther out.  You can check this with any 3-bladed household fan, or work out the relatively simple trig yourself.

Jon
 
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Offline dr.diesel

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Re: Where does the power go ?
« Reply #42 on: September 07, 2017, 08:28:24 pm »
Overspeed is protected by multiple redundant systems, and these systems are individually tested on regular intervals.  This testing is performed at the end of an outage or maintenance cycle as well to ensure no outage activities messed with the operation.  Now before anybody says it, yes these systems can fail, but when you look at historical events, these are extremely rare and usually caused by human error.

Even if a unit is at max power and the load instantly disappears (I have personally witnessed this many many times) there is no danger of overspeed.  The steam valves slam shut lightning fast, and scare you half to death when not expecting it!  (also first hand experience ha ha)

Another interesting bit, many generators are hydrogen gas cooled, so if one does happen to explode it will be spectacular.   :scared:

 
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Offline Jr460

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Re: Where does the power go ?
« Reply #43 on: September 07, 2017, 10:20:31 pm »

Even if a unit is at max power and the load instantly disappears (I have personally witnessed this many many times) there is no danger of overspeed.  The steam valves slam shut lightning fast, and scare you half to death when not expecting it!  (also first hand experience ha ha)

Another interesting bit, many generators are hydrogen gas cooled, so if one does happen to explode it will be spectacular.   :scared:

Yep seen it happen.  This is what I remember as it was many many years ago.

The unit had three 480V 3 phases busses, A, B and C with most systems split across two busses.  So two pumps for a system would be on different buss.  This was the case the stator cooling water pumps. 

One day the 4160V to 480V transformer for buss B smoked.  Normally not a big problem, with loss of half of things, the controls would have run the unit back to no more than 50% load.  However, 3 months before the same thing happened on buss C.  The issue was quickly fixed by opening the breaks to the C buss transformer and closing a buss tie from B to C buss.  In this case the two pumps in questions were on buss B and C, so when the buss B failed, it took down B and C and both pumps....  Along with tons of other things.   No stator cooling and the controls ran the turbine back to 10% load in less than a second.

Off course the boiler controls followed and shut the coal feed down, but the thermal mass of boiler drove the pressure up until it hit the upper limit, with then caused a full unit trip.  Main steam pressure still when up, and the large pop off values on the roff opened shaking the whole 14 story high structure.

Funny thing was the replacement for the smoked transformer for C buss had just shown up the week before and plans were being finalized on putting it in place and going.

As to hydrogen cooling in the generator, even the oldest units I saw from the 1930s had that feature.  Keep it above 95% pure and it will not burn.  If I remember right about 10% to 90% and a spark can make it ignite.
 

Offline Ghydda

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Re: Where does the power go ?
« Reply #44 on: September 27, 2017, 06:34:53 pm »


A lot of modern wind turbine use inverters so the speed regulation is not so critical, I was watching one the other day and noticed that as two blades were in the rising side the rota ion speed dropped and as one went over the top dead center the speed picked up until there were again two blades rising.
I think this must be an optical illusion, easy for this to happen if the shaft was not pointing right at you.  The blade set is well-balanced, so there is never a heavy side and a light side.  The rotational inertia of these blades is hard to comprehend, they just don't permit the rotor to speed up and slow down quickly.

I beg to differ.
The rotor plane is weight balanced, that part is correct. And obvoius. The rotational inertia is very big. But the shaft torque is incomprehensible too.
And define quickly by the way.

Anyhow, wind rushes past a turbine at different rates at the top of the rotor plane and the bottom. Thus when two blades occupy the upper half the the rotor plane, the rotor speeds up, and when two blades occupy the lower half the rotor slows down. A little.

This lead to frequency variation on the generator and the power converter handles the difference. If not, then the stresses on the drivetrain (gearbox) would quickly eat said gearbox.

This speed change is indeed perceivable when standing inside the nacelle looking into the hub of the rotor.
From afar, outside, I find this is much less visible to the point I doubt it happens. There is something about the lowest blade rushing past the tower, that screws with my brain.

The amount of speed pickup and drop (3 times per rotor plane rotation) is most pronounced in big turbines with huge rotor planes on fairly short towers, as the ratio of wind difference between top and bottom gets big and the low angular rotor speed helps to make it perceivable.

On a side note, the amount of rocking back and forth of the nacelle/tower on a big turbine is really disconcerting. The brain insists the motion is somehow wrong and not meant to be. This is again a result of different wind speeds across the rotor plane.

Emergency stops in a big wind turbine under full load is also not for the faint hearted. The tower sways so much you can't see from the top to the bottom. Yikes.

Cheers!

If we learn from our mistakes then I reckon I'm getting a great education!
 

Offline Ice-Tea

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Re: Where does the power go ?
« Reply #45 on: September 28, 2017, 08:26:01 pm »
What happens instead is that the generator with too little load advances its phase angle slightly relative to the grid, while still keeping the same rotational speed. The increase in phase angle causes more power to be pulled from the generator, thus balancing the situation. Therefore the generator remains "locked" to the grid frequency even when there is a load imbalance.

AKA droop.

OT: Check out the "electricity map" app for some insights. Great stuff.

Offline IanMacdonald

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Re: Where does the power go ?
« Reply #46 on: October 02, 2017, 09:15:11 am »
Here, the politicos are talking about aiming for 100% renewable energy by 2050. I just wonder if they've thought through what happens on a cloudy windless day. Or more importantly, through a month of cloudy windless days. 

The greens are putting-out claims about energy storage being available soon to solve all these issues, but I don't see any evidence of that being the case. If the politicos sign a massive wind contract on the strength of pure vapour 'energy storage' predictions, then they could be wasting a huge amount of public money. Did a few calculations on the basis of the Tesla Powerwall, and I reckon that it would cost a trillion UP Pounds to provide a week's backup that way.  :-DMM

Point of fact a week wouldn't be enough anyway unless there were plans to un-mothball fossil fuel plant in such a contingency, because wind outages can last a month or two, not weeks. (and by Sod's Law it's a fair bet that some of the mothballed plant wouldn't start, compounding the problem!)  :-BROKE  :palm:

http://iwrconsultancy.co.uk/renewables-backup

Anyone see a missteak in my reckoning. by all means say so.
« Last Edit: October 02, 2017, 09:21:39 am by IanMacdonald »
 

Offline DenzilPenberthy

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Re: Where does the power go ?
« Reply #47 on: October 03, 2017, 11:13:11 am »
I just wonder if they've thought through what happens on a cloudy windless day.

Yeah I think they've probably thought of that.  :=\

Anyone see a missteak in my reckoning. by all means say so.

Well for a start you are analysing a technical solution to something 33 years in the future using current day technology and pricing.  At the moment today the grid is 22% wind powered and 13% solar powered. There is almost no coal used in the UK grid any more. This would have been unthinkable 33 years ago and proposing that level of renewable infrastucture would have got me laughed out of the room by people such as yourself. What makes you think that the grid in 33 years' time will be powered by £50 car batteries bought from the local Halfords?  :-DD
 

Offline Someone

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Re: Where does the power go ?
« Reply #48 on: October 04, 2017, 12:27:58 am »
Anyone see a missteak in my reckoning. by all means say so.
Money is not the primary constraint as renewables are getting lifecycle energy production costs well below traditional generators. Read this:
https://www.withouthotair.com
The constraints for the UK are real but they aren't dominated by the economics.
 

Offline Old Don

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Re: Where does the power go ?
« Reply #49 on: October 04, 2017, 01:05:51 am »
Where has all the power gone?
Long time passing
Where has all the power gone?
Long time ago
Where has all the power gone?
Computer nerds used it all
When will they ever learn?
When will they ever learn?

 >:D
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