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Question about electric generators

  1. Jan 17, 2013 #1
    Generators in power stations are usually connected to step-up transformers to boost their output voltages for long distance power transmission, but why not just use a generator that has been designed and built to produce really high voltages and just connect the generator directly to transmission lines or connect multiple generators in series directly to the transmission lines to produce the really high voltages?
     
  2. jcsd
  3. Jan 17, 2013 #2
    The insulation requirement - turn to turn in the generator makes it impractical. The spacing between the actual conductors (turns).

    Although I believe ABB tried to make a HV Cable based generator - some years ago, obviously never caught on.
     
  4. Jan 17, 2013 #3
    That's a good question, but you're missing a teeny-tiny detail. Large generators have to be excited by the grid and that's why they operate at the same voltage. They return energy in the form of current, while maintaining the same voltage. You want to think of the step-up/down transformers as an interface for the special case that the energy is transmitted at a large distance (hundreds of kilometers). Additionally, if we followed your logic, then what if the generator is near a consumer area, and your generator created a high voltage, in that case you'd have to use a step-down transformer. Or manufacturers would have to design high-voltage and low-voltage generators depending on where a generator is used. Not practical at all! Transformers keep things real :)
     
  5. Jan 17, 2013 #4
    The step down transformer will still be used. It is only at the power producing end where the voltage has to be raised where the transformer and standard voltage generator will be replaced with a high voltage generator. But as was mentioned previously, this would cause an insulation problem in the turns of the generator because the high voltages would overcome the dielectric threshold voltage of the air gaps and contact resistance of the turns and create shorts which could produce arcing. This problem could be solved by insulating the turns but that would require impractically thick insulation which would make the generator needlessly large and heavy and interfere with the exposure of the turns to the rotating magnetic field of the generator's rotor by blocking the magnetic field and distancing the turns from the rotor which would lower or completely prevent the generator from producing an electrical output.

    But why not just use an electrostatic generator that uses triboelectric and electrostatic inductive effects to produce the electricity to avoid using induction coils in the generator?
     
    Last edited: Jan 17, 2013
  6. Jan 17, 2013 #5
    Hassan - I beg to differ - large generator are not excited by the grid - they are 99.9% ( every one I have seen) of the time synchronous generators with separate exciters - necessary to manage reactive power (KVARS). Anything that is grid excited would need to be connected to the grid to be started -with systems this large that just can not be done.
    Actually the MV level (15-22KV) generation causes other problems because the current levels are very high ( >10KA... >100KA Fault current etc) and difficult to control, so large generators are almost always stepped up immediately to make the currents more manageable. In some cases the Gen is connected directly to the step up transformer with no breaker, and then the primary CB is at the higher voltage.
     
  7. Jan 17, 2013 #6
    Windadct, I was under the impression that most generators are asynchronous. I guess I was mistaken.
    Bararontok, if by electrostatic generator you mean DC generators, I can tell you about the last power generation, transmission, & distribution conference I attended. There was a lot of babble about HVDC power transmission (in Europe). If I understood correctly, over larger & larger distances it's more efficient to do so, especially now that the Old Continent's grids are becoming increasingly interconnected, with projects in renewable energy carried out all over by the European Commission in renewable. The way I see it, HVDC generation, and transmission is becoming more relevant but still a long way to go. Of course this is all debatable, and viewpoints clash regarding this subject, and even at the conference I attended. But in my opinion, the HVDC proponents have a lot of ground to support them. I hope I'm not starting the old Tesla vs Edison debate now! :rofl:
     
  8. Jan 17, 2013 #7
    Yes, the electrostatic generators produce DC power. This is usually done by using similar or different materials that can have their balance of charges altered by placing them close together while being electrically grounded and then disconnecting the circuit or rubbing the materials together to transfer charge to the other material. The electricity is then produced by letting the charged material discharge the electric current until the charges are balanced and then the process is repeated. The constant mechanical movement needed by the materials is of course provided by a source of mechanical power. But is it not possible to also extract energy from the source of power so that the first movement cycle of the generator transfers charge to material 1, the current is then discharged and the charges balance, the charge is then forcibly transferred to material 2 and the current is discharged but because it is being discharged by material 2, there is a reversal in the polarity of the current and this produces AC power as a result. After all, why does the charge transporting only have to occur in one direction?
     
  9. Jan 17, 2013 #8
    Sorry, but electrostatic genrators do not produce DC.
     
  10. Jan 17, 2013 #9
    So the electrostatic generators produce AC after all.
     
  11. Jan 17, 2013 #10
    Static is it's own form of electricity. If you figure out how to convert it to useable energy, don't tell anyone and let me know.
     
  12. Jan 17, 2013 #11
    There is a type of device on this link that explains how the electrostatic generator works:

    http://en.wikipedia.org/wiki/Electrostatic_induction#Explanation

    The neutral material is placed in closed proximity to the electric field of a charged material. This causes the electrons in the neutral material to move to the end that is closer to the charged material. When the neutral material is rotated and the other side is placed in close proximity to the charged material, the electrons that previously accumulated on one side will now flow to the other so constantly rotating the neutral material while it is close to the charged material will cause the electrons to constantly shift from one side to the other, producing an alternating current. By placing electrical contacts on the neutral material, is it not possible to make the current flow through a circuit?
     
  13. Jan 17, 2013 #12
    I understand electrostatic generators - but the stored charge can not be used for anything - at least not easily. As for large generators per the OP - the is nothing close to harnessing static that even comes close. You can maybe light a neon bulb - but not run an air conditioner.
     
  14. Jan 17, 2013 #13
    What about shaping the neutral material into an induction coil so that when the charges oscillate back and forth, electromagnetic radiation is discharged and this radiation can be absorbed by a nearby material such as an induction coil or photoelectric material to convert the energy back into electricity?
     
  15. Jan 17, 2013 #14
    Static is just accumulated charge producing an electric field that want to be discharged (like when you touch the doornob in a cold winter). There is energy in there, but it's discharged instantaneously as soon as it finds a path to do so, and that's not useful in any way shape or from. Unless you're talking about harnessing static discharge like from lightning bolts, and then converting that to DC or AC, then that's alright, and there are people working on that actually. But if you mean using static in lieu of AC and DC then that really makes no sense. We need a steady flowing current in our devices, and not just a quick jolt of energy that would do nothing but make it chaotic.
     
  16. Jan 18, 2013 #15
    In addition to what "Windadct" said about the problems in construction because of the very large number of turns, there is also a problem which that not all MV and HL voltages are the same, so in some cases you want to increase the voltage up to 6.6kv, while in some networks you want to increase up to 11KV, so with transformers you will have this advantage also.
     
  17. Jan 18, 2013 #16
    So this means that static discharge is not as consistent as current produced from other types of phenomenon and because of this it is not advisable to use it to run loads. But what about my suggestion on this post:

    https://www.physicsforums.com/showpost.php?p=4232549&postcount=13

    Where the radiation emitted by the constantly oscillating flow of static electricity will be converted into electricity by a photoelectric material or induction coil.

    Or what about intentionally placing a spark gap in the electrostatic generator to produce an electric arc and converting the heat energy and radiation emitted by the arc into electrical energy by some type of material that can perform such an energy conversion?

    Although the problem with such a system would be the high operating voltage of the device. Since the material that will convert the energy of the electrostatic rotor is close to it, the high voltage would cause a static discharge into the device lowering the net charge of the rotor. When this happens, the rotor's ability to be influenced by the electric field of the charged material would weaken until the charged material can no longer influence the rotor and there would be no more static electricity in the rotor. Connecting the rotor into contacts to make an electrical circuit would also cause the same problem. The static electricity will discharge into the external circuit until charges balance and lower the net charge that is close to the electric field of the charged material. This will also lower or eliminate the charged material's capability to influence the rotor and no more power output will be produced as a result.

    Perhaps the only way to harness static electricity's energy would be to hypothetically get enough electrically charged material if it even exists and connect an external circuit between it and the ground. Or connect an external set of circuitry between a lightning rod and the ground as suggested in this post:

    https://www.physicsforums.com/showpost.php?p=4232624&postcount=14

    And use that energy. But then transformers would still be needed because different transmission lines use different voltage levels since there is no universal standard for high voltage transmission.

    But what about substituting the rotor magnet of generators that use the electromagnetic induction phenomenon with an electrically charged material and spinning that instead, using its rotating electric field to move the electrons in an external circuit? But then again, the high voltages induced would pose insulation problems. And, since to convert the mechanical energy into electricity efficiently requires the rotor to be as close to the coils as possible without making contact, the close proximity of the coils to the spinning charged material could cause the charged material to discharge static electricity into the external circuit, eliminating the electric field of the charged material and rendering the generator unable to produce an output, again posing insulation problems.
     
    Last edited: Jan 18, 2013
  18. Jan 18, 2013 #17
    You need to go back to first principles, and you'll see why everything you've proposed becomes awful complicated.

    Charge exists everywhere, but for practical purposes, it isn't useful whatsoever unless it is moving nicely in a predictable manner, with amplitude and direction. Your typical copper wire has 8.5×1028 free e-/m3--that's a lot. You can harness them all you want using a static generator but what for? You want them moving down the cable that's what powers your light bulb.

    The point of electric power generation is to get electrons moving in a closed loop. It's true on any scale. That's what your battery does, small/large generator, etc. So to say let's replace the battery with a charge accumulator, then create some mechanism to make that charge flow nicely is, in my opinion, as strange as saying let's suck all the fresh water from a stagnant river, pump it onto an uphill pond (at a higher potential), then let it flow again. But I assure you, it will stagnate again. It's a lot smarter (& efficient) to use the water pump to give it that lil nudge to move in its preferred direction.
     
  19. Jan 18, 2013 #18
    This is what I mentioned in this post:

    https://www.physicsforums.com/showpost.php?p=4233161&postcount=16

    It turns out to produce so many complications and inefficiencies that it is just not practical. Things would actually work more efficiently with the current electrical generation methods. Besides an induction alternator has an efficiency of up to 97% and a transformer has an efficiency of >90% so the grid is already very efficient as it is.

    In conclusion, what you said about using the conventional induction electromagnetic generator to give the electrons a steady nudge is better and more controlled than forcing the charge distributions to change and letting the distribution equalize then repeating the process which is inefficient and produces an output that is unstable and difficult to control.
     
    Last edited: Jan 18, 2013
  20. Jan 18, 2013 #19

    jim hardy

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    What's in a name?
    That's why, when first starting out with beginners i stress that "E" for Voltage is short form of "EMF",
    which is the abbreviation for Electro Motive Force", a good name for it
    which should be thought of backwards: Force that Motivates Electrons (to move).

    Then one can introduce the concept of potential, absolute and differential, with finger on the table,
    and from that it follows naturally that work = Force X Distance...
    Some people need that kind of visual aid in order to gain confidence in the equations.

    Not lecturing you guys, just i've seen this method help struggling newbies. Hope it helps you help someone.
     
  21. Jan 18, 2013 #20
    It now seems we have 2 threads in one ....

    Back to the OP to clarify "Generators in Power Stations" - to be large scale > 100MVA, this is where the issue of a step up transformer is most relevant.

    The issue of practical energy harvesting from static - is a completely different topic. -- really should not be in the same discussion.
     
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