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Power plant generators

  1. Aug 11, 2014 #1
    I'm curious to know whether these huge generators [700 MW, 2235 MW] are same in construction as small generators, except bigger frame, coils and magnets?

    Also they say Generator is Motor and vice versa. So does it mean those big generators can be used as Motor too?

    In reality is it wise to use Motor as Generator and vice versa? Do they differ in design? If so how?
     
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  3. Aug 11, 2014 #2

    OmCheeto

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    Yes.
    Sometimes.
    When the situation requires it, yes.
    Not the ones I dealt with.
    Due to associated electronics, some motors cannot be turned into generators, and vice versa.

    For instance, an automotive alternator uses diodes to convert AC into DC. If one were to apply a DC voltage to an alternator, you would not have a motor. Though if you stripped out the circuitry, and added new circuitry, I'm pretty sure you could make an AC motor out of one.

    Some motors and generators are specifically designed to act as both. When connected together, they form a device called a motor-generator, aka an "MG" to nuke submariner nerds. On the submarine I was on, if the reactor scrammed, then the MG became a DC motor and AC generator, running off the storage battery. When the reactor was restarted, the steam turbine driven generator would supply power to the MG, turning it into AC motor and DC generator, so we could recharge the battery. When the reactor was up and running, I could also fiddle with the knobs to make power go in which ever direction I wanted. It was quite fun. :smile:

    For more information regarding "motors vs generators", you may want to check out a previous thread: Confused with back emf, where we discussed a similar question.
     
  4. Aug 12, 2014 #3

    jim hardy

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    When they get bigger than a few tens of megawatt size you'll find extreme measures taken to cool them .

    The case is pressurized and filled with hydrogen. That's because hydrogen with its light molecules is more effective at carrying away heat than is air, and the mechanical work required to pump it through the machine is similarly less (see "Fan Laws") .

    The conductors are hollow. Our 894 MVA machine blew hydrogen through the conductors as well as around them. Some machines liquid cool the conductors by circulating water through them(very pure water of course). My utility preferred oil for its liquid cooled 400 megawatt units, for obvious reasons..

    The bearings have an oil circulation system.

    But OM is right - the differences are not in principles of operation, but in the mechanics of holding the huge parts together against centrifugal force and keeping them cool.

    old jim
     
  5. Aug 12, 2014 #4
    Thank you OM and JIM. Excellent explanation.

    I wasn't aware of cooling the conductors by using hollow conductors.

    Isn't Hydrogen explosive? If the temperature goes up [I don't know what temperature], it could blow whole generator, right?

    Could I get link of PDF or Book name, which I can refer to learn about Generator design?

    What might be the Wire size [awg] used in these 700+ MW generators? I guess they might be copper wires. Are the Magnets Neodymium [N52 or Better] or some thing else?
     
  6. Aug 12, 2014 #5

    sophiecentaur

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    Hydrogen is very flammable but it is only 'explosive' when in a good mix with oxygen. A leak will produce a flame rather than a bang - not good news, of course but it has so many advantage that the extra care needed is worth it. (For a start, it vents very quickly from anywhere it leaks into - unlike Methane and Butane etc.)

    Did you know that saturation divers breathe a mixture of Hydrogen and Oxygen? As they prepare for a dive with this mixture, they breath the mix in various proportions, including the explosive one. Once there is a lot of H2, the mix is no longer explosive.
     
  7. Aug 12, 2014 #6
    There's only so much conductor material you can make use of due to the skin effect. That's why, as Jim wrote, it makes sense to use hollow conductors. You can look up the skin depth of various conductors on Google, etc.

    In that power range you won't see permanent magnets used to provide the rotor field. One reason is that you have fewer degrees of freedom with a PM generator in terms of voltage regulation. This isn't a concern for PMSG's in the power range of, for instance, wind turbines since they're grid connected through an AC-AC converter, but one does not simply build a 700+ MW inverter. PMSG's are also typically more "delicate" than their field-wound counterparts - you have to take extra care to regulate the temperature and mechanical stress of the PM assembly.
     
    Last edited: Aug 12, 2014
  8. Aug 12, 2014 #7

    sophiecentaur

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    PF is such a good source of mind enhancing information. That stuff about PM generators is really interesting. I wondered whether there are any hybrid arrangements for the field? There could be advantages ???
     
  9. Aug 12, 2014 #8
    Generators in that power range isn't really my field, so I can only speculate. I guess you could have a PM field provide a baseline and then windings for regulating it, but my gut tells me you'll inherit all the problems associated with PMSG's without much to show for it. I think PMSG's generally also have a large initial cost associated with them - a cost that probably scales rapidly with size.
     
  10. Aug 12, 2014 #9

    jim hardy

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    A search on "turbogenerator construction" yielded quite a few hits for me. Mostly sales literature with pretty pictures.

    If you're curious, surf the net for an hour .
    This book has more nitty gritty details than most sales brochures
    http://books.google.com/books?id=Rp...onepage&q=turbogenerator construction&f=false
    use it in conjunction with lighter reading
    like the 'application details' bullets at end of this page
    http://www.electricmachinery.com/products-Turbo-Generators.html
    (here's one of them
    http://www.electricmachinery.com/_files/LR10004.gb.11-09.01_SA155S_2-Pole Product Sheet.pdf)

    and slideshows like the ones here
    http://www.slideshare.net/srutipn/shru-tg-ppt

    and sales literature for eye candy
    http://www.alstom.com/Global/Power/Resources/Documents/Brochures/turbogenerators.pdf [Broken]

    and you'll appreciate a little more what's behind your electric wall socket.

    As Sophie says hydrogen can't burn without oxygen. Our machine operated at 75 psi. The hydrogen is kept well above 95% pure by 'feed and bleed' .

    The hydrogen purity meter was an interesting little gizmo you might enjoy - basically it measures the dp(differential pressure) across a centrifugal blower with hardly any flow.
    Fan laws tell you that dp at near zero flow is in proportion to density of the gas being pumped.
    Since pure H2 is less dense than an air-hydrogen mix, dp goes down with increasing purity.
    But it goes up with pressure.
    The gizmo was a mechanical analog computer that:
    measured dp across the blower in units "inches of water";
    also measured pressure in units of "atmospheres";
    numerically divided dp by atmospheres (by a clever arrangement of springs, levers and cams) thus arrive at what would be the dp across blower were its pressure one atmosphere;
    then displayed that result on a meter.

    Now : since hydrogen has Molecular Weight(MW) of 2 and air has 29, and density is in proportion to MW,
    the fraction of hydrogen in the mixture can be readily calculated from pressure corrected dp.
    Well,that is, provided you know the relation between density and the pressure characteristic of your blower.
    This blower was sized to produce dp (in inches of water) numerically equal to 1/10th the molecular weight of whatever it is pumping- honest it made
    0.2 inches of water for H2(MW=2), 2.9 inches for air(MW=29)
    The calculation from apparent molecular weight/10 to %hydrogen in air is just ninth grade algebra
    So the meter scale was numbered not in inches of water but %hydrogen in air 0 to 100.

    Reason for digression - just thought you might enjoy a real world application of freshman physics and chemistry.
    Another fellow and i figured out how that Art-Deco era mechanical analog computer worked and replaced it with electronics.
    The mechanical skills to keep it working were getting scarce. Hardly anybody could calibrate it.
    Plus it had about thirty pounds of mercury in the dp sensor so leaks were messy.
    Plus we added temperature compensation.

    Maintenance work can be fun.

    old jim
     
    Last edited by a moderator: May 6, 2017
  11. Aug 13, 2014 #10
    Thank you all. I found pretty good info on Voith website. They have lot of material, brochure as well as magazines too. Here is the animation of their large Turbo Generator.

    http://voith.com/en/products-services/hydro-power/generators-557.html

    Doesn't give details about the Magnets and Wire type/size used, but hey you don't get everything for free :)
     
  12. Aug 13, 2014 #11

    jim hardy

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    Huge wires are designated not by a gage number but by their cross sectional area. Usualli expresses as "circular mils", that is multiple of the area of a 0.001 diameter wire.
    http://www.ihiconnectors.com/AWG wire sizes.htm
    Biggest round wire i ever handled was 500MCM, maybe an inch diameter.

    For your question on generator conductors,

    try this one
    https://www.eiseverywhere.com/file_uploads/ed091f32dd86e9c9bc4237451681bf80_GenerationOverview.pdf

    page 9 of 66 has a cross section photo of a medium sized individual wire.. It's a bundle of individual strands each maybe 3/4 inch wide. In our machine the bundle was perhaps a foot tall .


    The rotor is an electromagnet as represented on page 10 of 66, ours was brushless as in lower drawing.
    There are some actual photos starting on page 35 of 66 .

    Here's a picture of an old GE machine with some people in it for perspective.
    Don%20Albright%20infront%20of%20stator%20-%20web.jpg
    courtesy http://www.generatortech.com/index.html

    old jim
     
  13. Aug 13, 2014 #12
    Just to reiterate, permanent magnets aren't used in generators of that size. Among many others, one huge problem with permanent magnet synchronous generators (PMSG) is that the amplitude of the phase voltages they output is fixed in proportion to their rotational speed. For installations as large as those, you typically want them to run at some fixed frequency (e.g. 50/60 Hz for conventional power plants, depending on where you live), but for a PMSG you have no way of controlling the amplitude of its phase voltages independently of its frequency of rotation. That's a bit of a problem, since you want to match both the output frequency and voltage amplitude of the generator to that of the grid.

    Now, the reason that PMSG's are used so heavily in, for instance, wind turbines, is that you have no control over their rotational speed in the first place (the wind determines it), so you need an AC-AC converter in between the generator and the grid. That converter gives you (almost) total freedom in selecting output frequency and voltage amplitude, but building such a converter to handle 700+ MW is a monumental undertaking. Even if we disregard cost, there are also severe disadvantages in putting such a converter in between the generator and the grid, e.g.:

    A huge generator has a lot of inertia, i.e. it takes a lot of effort (work) to change its rotational velocity. If you connect it directly to the grid, the generator itself will resist changes on the grid. It thus has an intrinsic stabilizing effect.

    If you place an AC-AC converter in between the generator and the grid, you effectively decouple this inertia from the grid. You'll get a lot more freedom in terms of what you can do with the voltage/current waveforms on both sides of the converter, but you also better make absolutely sure your control system is up to the task of regulating both sides. This can be another monumental undertaking.
     
  14. Aug 13, 2014 #13
    That means Electromagnets... so we need electricity to generate electricity.

    I know Neo Magnets have flux upto 14 KGs. What is the max Flux value we get from Electromagnets?
     
  15. Aug 13, 2014 #14

    anorlunda

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    It's not just the strength of the field. The magnetism (called excitation) is varied to regulate the generator voltage.


    There have been hydrogen explosions in rare circumstances. However, the risk of that is small compared to the fire/explosion risk of the lubricating oils, not to mention the mechanical risk of so much heavy machinery spinning at 3600 RPM, lots of superheated steam, and the meltdown possible it you short circuit 25,000 amperes.

    Working with highly concentrated energy and big heavy things in motion, it is always dangerous. Safety is a very big deal in design and operation, and despite some accidents, the safety records are very good.
     
  16. Aug 13, 2014 #15

    nsaspook

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    I remember a few years ago one of the Russian hydro generators failed. The results were catastrophic.

    http://eandt.theiet.org/magazine/2011/07/siberia-hydro-disaster.cfm [Broken]
     
    Last edited by a moderator: May 6, 2017
  17. Aug 13, 2014 #16

    dlgoff

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  18. Aug 13, 2014 #17
    Those 14 kGs is the remanence of the magnet alloy, i.e. it's the flux density you'd get if you made a closed loop of it. A motor/generator includes an air gap in the magnetic circuit (it would be pretty useless otherwise), which greatly reduces the flux density you'll get. With an air gap, you'd need a huge volume of magnet alloy to even begin to approach those 14 kGs.

    The cores of electromagnets are typically built using non-oriented electric steel, which also usually tops out around 14 kGs = 1.4 T if heavily saturated:
    http://www.arnoldmagnetics.com/uploadedFiles/Library/PTM/Arnon%205%20NGOES%20Hysteresis%20Curves.pdf [Broken]

    The difference is that you can actually get near this limit in a practical motor/generator if it's field-wound.
     
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  19. Aug 13, 2014 #18

    jim hardy

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    Yes you need current to make the magnetic field. It could originate in a small PMG on same shaft as in that sketch on page [STRIKE]9[/STRIKE] 10 of 66.

    Iron itself has a limit on how much flux it can carry and it's in that range, around 10 to 16 kGauss (1 to 1.6 Teslas).

    220px-Magnetization_curves.svg.png

    In a generator stator where flux is cyclic (sinewave) high flux levels heat the iron. You want to stay away from saturation for it can melt holes in a stator core.. With good quality steel for stator material i'd guess peak flux is close to maybe as much as 15 kilogauss, 1.5 Teslas, but that's a guess. That's a design tradeoff - cost of steel versus cost of apparatus to cool it, and more heat loss = less efficiency. And you have to leave room for utility to adjust voltage up and down a little.

    Wow Don - 45 Teslas? Sound like enough to push neutrinos super-c !:biggrin:
     
    Last edited: Aug 13, 2014
  20. Aug 13, 2014 #19

    dlgoff

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    [hijack]Being it's in Florida, I assumed you had done some experiments there. o:)[/hijack]
     
  21. Aug 22, 2014 #20
    I'm also curious about the cause(s) of Heat in conductors? Is it because of Back EMF or Just Current drawn? or something else?

    Virtually, What can be done to avoid the heating of conductors?
     
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