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Ac power lines return path

  1. Aug 17, 2010 #1
    So i know the whole picture starting from power from the generating plants that go all the way to our homes..and the ground serves as a return path to complete the circuit..What i dont get is...how does that current find its way back to the source through ground?..is the source somehow atttracting the same amount of current from earth that it is transfering to the load...
    are all circuits that are grounded share an interconnection network of currents??
     
  2. jcsd
  3. Aug 17, 2010 #2
    The ground does not form the return path it is purely used for safety.

    The neutral provides the return. That means that there are at least two wires provided to make a supply

    Phase (or line).

    Neutral (or return).
     
  4. Aug 17, 2010 #3
    Sorry i didnt mention, Iam talking about high voltage A.C tranmission lines. What your saying is correct regarding distribution power going into a house
     
  5. Aug 17, 2010 #4

    berkeman

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    The only difference then would be that high-voltage transmission lines use 3-phase power transmission. Ground is still not involved.

    http://en.wikipedia.org/wiki/AC_Power_Transmission

    .
     
  6. Aug 17, 2010 #5
    ^ ground is involved, but the load is balanced in all three phases so the ground current would be nearly zero. in HV transmission system all transformers are star connected with neutral grounded. on distribution side delta/star connected transformers eliminate ground currents.
     
  7. Aug 26, 2010 #6
    Though the Earth is not usually used as a return, it can be. Look up SWER (Single Wire Earth Return). This is used in some rural areas to save money on conductors (i don't think in the US.) The grounding rods need to have a large surface area at the power plant and at distribution where is changes back to a two wire system. The earth is a terrible conductor (which is why you can't put two wires into the ground to easily complete a circuit); however it has a huge surface area. The cross sectional area of the transmission wire will be about the same as the surface area of the grounding rod contacting the earth multiplied by the difference between their conductivity. Since the earth is a poor conductor the surface area contacting the earth must be much larger than the cross sectional are of the standard wire conductor. It the entire earth is working in the same way as the wire. Current can flow through anywhere in the earth finding paths of least resistance (possibly following paths of water) to complete the circuit on the other end. Since the earth dries out when its sunny, and gets wet in the winter, the conductivity of the earth can vary greatly, causing electricity in a SWER system to act funny. That's why SWER is rarely used; even though it is cheaper. A regular US power grid has a similar grounding system, but the earth return is expected to only be used when there is a problem with the system, not as the primary return path.

    Then again im certainly no expert, im sure someone can correct me.
     
  8. Aug 26, 2010 #7
    The ground does not and cannot form a return path.

    A ground is a special sort of circuit element that maintains its potential regardless of the current it sources or sinks, it is not a circuit connection element.

    Because it is so large the earth can sink current at one location (the power station) whilst simultaneously sourcing it somewhere else ( the local supply ). If you were to compare the potentials (thought experiment) at each location they might well not be identical, but they would both be constant.
     
  9. Aug 26, 2010 #8
    I think alot of people living in rural areas with their power provided by SWER (earth RETURN) would disagree with you.

    http://www.ruralpower.org/

    Electricity doesn't "see" a different between a wire conductor and the earth conductor other than its resistance.

    From - http://findarticles.com/p/articles/mi_qa3726/is_200809/ai_n29493085/

    "Use of the earth as a circuit conductor goes back more than two and a half centuries, long before "grounding" became an issue in power distribution. The first practical transmission of electrical signals over long distances was in the summer of 1747, when experiments in England confirmed that when current was sent through a wire across the Thames River, ". .. the water of the river would serve as a return path."

    They eventually learned even the river isn't needed, the earth works too. Even the air is not a perfect insulator and conducts some electricity and could therefore theoretically act as a return path (lightning?)
     
  10. Aug 26, 2010 #9
  11. Aug 26, 2010 #10
    You are not understanding what I said.

    I was trying to explain that using the earth invokes a different mechanism from a neutral return.

    In fact it would still work if the (single) cable was strung between two planets with the source on one and the sink on the other. There would be no 'return current path'.
     
  12. Aug 26, 2010 #11
    That is an interesting thought (A SWER between two planets.) From the way i understand electricity it wouldn't work. So if i am wrong, I would need to do more research. Do you know of any articles which would back up this thought experiment?

    I would tend to think it is the exact same mechanism. If you filled two insulated cylinders full of dirt and ran it between a power plant and a house. It would work the same as copper/aluminum, so long as the diameter of the cylinder was large enough. I don't see why the earth would be so special.
     
  13. Aug 26, 2010 #12
    As I write this, an average summer electrical storm is providing plenty of visual proof that the earth is capable of sinking millions of amps.

    That would be because an 'earth' is a slippery concept to get hold of and very difficult to explain.

    However I repeat my comment that its main property is that its potential does not alter, regardless of the current flows into or out of it.

    Given this property, try doing a circuit analysis on your tube of dirt and you will see what I mean.
     
  14. Aug 26, 2010 #13

    russ_watters

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    The reason the SWER does not require an actual return path and only a sink or a virtual return path is that we're talking about alternating current: the electrons don't flow in a continuous loop, they just oscillate back and forth. So you don't need a continuous loop, you only need a sink that can absorb/release elctrons as needed to allow the oscillation through the single wire.

    You can actually do this at home, using the hot wire of your home electrical service (don't actually do this!). I'll do some digging, but I've seen Youtube vids of people using a metal ball as the sink to demonstrate powering a light.
     
  15. Aug 26, 2010 #14
    Fair point Russ.
     
  16. Aug 26, 2010 #15

    stewartcs

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    The ground can certainly, albeit inadvertently sometimes, create a return path. When it does it is typically called "ground fault current" and is usually a bad thing.

    However, the electric utility provider typically grounds the primary and secondary neutral conductor to the earth at multiple locations to create a parallel path so as to reduce the impedance of the return neutral current path. This is mostly to prevent unexpected surges in the system (e.g. lighting strikes, downed lines). However, a secondary effect is that the earth acts as a parallel network and reduces the line losses.

    CS
     
  17. Aug 26, 2010 #16
    I can't agree with that interpretation. It is a simple matter of impedance.

    It is also important to distinguish between a circuit earth and the planet earth, which are two different things. The planet is a good, but not perfect, circuit earth.
     
  18. Aug 26, 2010 #17
    So does that mean:
    1) SWER would not work with DC (If the earth freely gives up or takes extra electrons, couldn't a single wire essentially suck them up from the ground at the power plant and dump the into the ground at the load or visa versa (DC SWER?)
    2) The dirt tubes I described would work similar to regular conductors, not the metal ball u describe, or would it not work at all?
    3) This is something like a tesla coil (wireless electricity) using the earth instead of air?
    4) The generator descirbed in https://www.physicsforums.com/showthread.php?t=333863" thread didn't work only because one leg of the circuit didn't allow electrons to oscillate?
    5) A SWER between two planets would actually work in theory (this seems to be the only one of my questions that would actually differentiate between a "virtual return" and an "actual return".)
    6) If all you need is a "virtual return" and not an actual one. That would lead me to believe that my earlier statement that a SWER is not used because of changes in the earth's resistance due to water is false. In that case, why would it not be used more often?
     
    Last edited by a moderator: Apr 25, 2017
  19. Aug 26, 2010 #18
    Seems like this is the same thing as regular conductors, that is why i said the SWER between two planets seems to be the best thought experiment to explain why there is a different between actual and virtual return.

    This would also lead me to believe if you cut one wire wire of a transmission line (somewhere near the middle), it should still transmit electricity since the electrons can still oscillate on the cut wire. Maybe not because they would have to "fall off" the end of the wire, then get sucked back up again. However, if the line is not insulated as is typical. Could they not oscillate in the air contacting the long cut wire?
     
  20. Aug 26, 2010 #19

    mheslep

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  21. Aug 26, 2010 #20
    Earlier in the thread I invited you to do some calculations??????

    Anyway here are some sketches to help

    In A we have a single supply connected to one end of a load, the other end of which is locally earthed. 1 amp is flowing to earth.

    I have shown this supply as being 100 volts above earth, but what happens if the current increases to, say 1000 amps as at B?

    How many volts above earth is the supply now?

    Further what happens if the voltage at earth is not constant? How do we then define any voltage?

    Now in case C we have a locally earthed power station supplying a load (earthed as before at c) by a supply line ab. This line is very long and has an impedance of 10 ohms.
    The power station and the load are situated on a large granite formation so the impedance between the two is very high - I have shown 1018 ohms.

    Now suppose 1, 10, 100 ,1000 amps flows down the supply what will the effect be on the voltage across bc and more importantly

    If 1, 10 100 or 1000 amps enters the power station connection via an earth 'return', what is the voltage across this vey high impedance? and how is it generated?

    These examples are designed to show the absurdity of thinking of a 'return current' via earth.
    A return current must needs equal the supply current.

    The model, however, works well in terms of earth being at a fixed potential, regardless of current sourced or sunk.
     

    Attached Files:

  22. Aug 26, 2010 #21
    The earth is a lousy conductor. If the earth was used for conduction, losses would be prohibitively high. Also, the neutral on Y-connected systems carries no load current. It is located above the 3 phase hot wires to provide lightning protection.

    But during fault conditions, the neutral & the earth can carry current. If a line touches ground or a grounded object, fault current is conducted through eart and/or neutral.

    During unfaulted conditions, the 3 phase hot wires carry the load current. As long as each transformer has at least 1 delta connected winding, 3 wires can support any 3 phase load, balanced or unbalanced. Or, a 3 phase 3-legged "E-core" type of transformer can support unbalanced loading with only 3 wires, even w/o a delta.

    Any power generation/distribution reference book covers this in detail. Westinghouse & GE manuals come to mind. Uni libraries should have 1 of them.
     
  23. Aug 26, 2010 #22
    @Studiot: Earth can form return path for electrical currents, and you can not change it with false assumptions like 1018 ohms earth resistance.
    supply voltage V=100
    Current = 1000A
    Load resistance = 100/1000 = 0.1 ohm
    that is again impractical. do you know the cross section of conductor required for 1000A current? such high loads are feed on higher voltages to avoid losses in cables.
    what does that mean. voltage at earth is taken as zero for reference.
    earth resistance should be less than ~5 ohms. In normal wet soil it is easily achieved by simple ~3m pipe earthing. for rocky areas more surface area of earthing electrode is required or earthing can be done at some location away from load where lower earth resistance can be achieved. In practical Power station / loads are not situated on large granite formation.
    A simple distribution system with one load connected through earth as return is given below, the earth current is shown in red dotted line. the generator is not shown in the diagram as it will not carry any ground currents:
     

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  24. Aug 26, 2010 #23

    stewartcs

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    That's not an interpretation that's a fact - current can certainly flow through the ground. What exactly do you not agree with? Are you suggesting that the earth does not allow current to flow through it?

    CS
     
  25. Aug 26, 2010 #24
    I think he is saying that an electron does not flow from one ground to another (or does not need to). But this could be true for a regular circuit. So it seems like a matter of semantics to say earth is a return as opposed to sinking in one grounding and propagating in another.

    The only potential difference i can see in the two interpretations is whether or not electricity could flow if the two groundings were electrically insulated/isolated from each-other (provided each grounding area is sufficiently large.
    I would be interested in the possibility of this.
     
  26. Aug 27, 2010 #25
    @aks786

    There is an enormous difference between the 'earth resistance' and the resistance of a particular conduction path or channel through the earth from one point to another, they are quite different things.
    I'm sure that the manuals mentioned by Cabraham make this difference abundantly clear.

    The resistance of the path or channel is found in the normal way by multiplying the resistivity (summing or integrating if necessary) by the path length and dividing by the cross sectional area. It is independent of the current flowing.

    earth resistance, on the other hand, is a property of a particular point in the earth to source or sink a particular level of current as defined in some local code. It does not imply that current passes from any point A to another specified point B many kilometres away. It is a prime use of the property of a circuit earth to remain at constant potential, whilst sourcing or sinking current.

    Incidentally the electric storm I mentioned earlier is a good example of the effect at DC.

    @Good4you

    I think you are beginning to get the idea. Of course our planet is not a perfect earth, it just seems that way for the levels of current that we can apply. So yes the single cable between planets would work at those levels.

    The charge imbalance implied in assuming that a circuit earth remains at constant potential is prety good for a planet. However for lesser earths, for example in electronic circuitry, it has to be taken into account in design calculations.

    I think the concept of earth is sufficiently fundamental but also so widely ill understood as to be worth debating so I started a thread when I first joined here.

    https://www.physicsforums.com/showthread.php?t=382007&highlight=earths
     
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