2 bushing vs single bushing

Discussion in 'Electronics & Electrics' started by Kafoopsy, Mar 11, 2010.

  1. Kafoopsy

    Kafoopsy Member

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    Can anyone explain what is the difference between a 2 bushing, single phase transformer and a single bushing, single phase transformer? Around Perth, you see 2 bushing single phase transformers, whereas in the country its usually single bushing, single phase transformers. I've never worked out why this is as they both appearto have the same output and they are also pretty much the same phsical size.
     
  2. rainwulf

    rainwulf Member

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    What do you mean by "bushing"

    Post a picture perhaps?
     
  3. Mjollnir

    Mjollnir Member

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    bushings used to insulate the transformer winding connections from the metallic enclosure.

    Units that must be grounded on one side of the primary are usually provided with only primary connection bushing.

    Where as transformer with fully insulated coils are provided with two separate primary bushing connections to be connected to phase to phase on 3 phase system.
     
  4. OP
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    Kafoopsy

    Kafoopsy Member

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    This is a 2 bushing transformer
    [​IMG]
    This is a single bushing transformer. Both produce single phase output.
    [​IMG]

    I don't understand that, could you please explain further?
     
  5. LostBenji

    LostBenji Member

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    Mjollnir was off on a different tangent. Don't stress.


    The difference there is that you twin-bush tranny (twin feed) is tied to both the HV feed lines and setup in two ways. The first being that you have an active and neutral wire overhead which is used when soil is bad or in cases where earth return is to be avoided. The second is that trany is actual two phase input (most common) with single phase, 240VAC out. These are very common, more in rural areas for load balancing.

    The other trannies pictured bellow are a pair of single phase trannies that are fed off single phse only being a SWER (single wire earth return) in this case. SWER is used also in rural only positions where there are long line runs but not huge power demands. SWER systems also suffer alot more brownout/voltage fluctuations than two or three phase line return systems. These two are running same phase output but split most likely for load balancing/isolation purposes.
     
  6. Symon

    Symon Castigat ridendo mores

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    LostBenji is on the money.

    Two bushing transformers have a 2-phase input (which is really just single phase - get your head around that) and a single phase output.

    Single bushing transformers have a single phase input, and a single phase output.

    Both are used extensively in SWER systems. What is also common is the output is actually 480V centre-tapped, so you can get two 240V supplies from the one transformer.
     
  7. OP
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    Kafoopsy

    Kafoopsy Member

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    So why is there two different types if they both produce a single phase output? In country WA, single bushing transformers are pretty much all they use. The two bushing transformers are only used in the metro area. I might add that in the country the powerlines always have an earth wire (the wire between the HV and LV on the single bushing photo) - even on three phase lines, whereas metro lines do not have this.
     
  8. Symon

    Symon Castigat ridendo mores

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    Because in rural areas it is more economical to just distribute a single wire and use the ground as the return path. But in metro areas it is easy (and affordable) to use two phases on the primary, this gives MUCH better voltage stability than the SWER system.
     
  9. LostBenji

    LostBenji Member

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    It's also dangerous to rely on earth return in built-up areas. Electricity takes the easiest path and this tends to be water mains and other underground sub-systems. Some poor bastard will find themselves in the wrong spot.
     
  10. OP
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    Kafoopsy

    Kafoopsy Member

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    But in WA, all the country powerlines have an earth wire, whereas in the city they don't.

    But wouldn't it be the same as birds sitting on the powerline? If you touch something that's part of an earth return, where would the power short to?
     
  11. mtma

    mtma Member

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    Birds generally don't come into contact with ground and the power lines simultaneously, as a result no frying current gets to flow when they land on power lines, earth return or not (i.e open circuit)

    It's generally possums and bats which have problems with getting zapped on power lines because they try to move line to line.
     
  12. rainwulf

    rainwulf Member

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    Ah, so not a bushing, but a Feed insulator.


    yea, the single feed transformers use earth as a return, using it in both a capacitive and resistive role, but require the use of a fairly large and properly set up earth wire arrangment.
    Over long distances, its a lot cheaper to run one high voltage wire instead of two.
    here is an article that explains it properly.
    http://en.wikipedia.org/wiki/Single_wire_earth_return
     
  13. Symon

    Symon Castigat ridendo mores

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    That's because they are using the earth as the return path, with a 2-bushing type you don't do that. And it doesn't matter that it's in WA, it's the same all over the bloody country.

    No. Look up a thing called STEP POTENTIAL. It's not uncommon to see people getting shocks when walking around underneath a SWER transformer pole.
     
  14. OP
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    Kafoopsy

    Kafoopsy Member

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    Where is the return path in a 2 bushing system? Or for that matter, where is the return path in normal metro 3 phase systems?

    The reason I mention WA specifically is that in SA, single phase powerlines don't have an earth wire, just the HV conductor. Country WA lines have an earth wire on both single phase and 3 phase lines.

    I read the Wikipedia article on SWER and now I understand how it works. That's what they use in SA with the single HV wire poles. I'm still trying to get my head around the difference between WA metro and WA country systems

    Wow, that's something I didn't know.
     
  15. LostBenji

    LostBenji Member

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    The return is on the other phase. You need to study 3 phase power first.
    http://en.wikipedia.org/wiki/Three-phase_electric_power
    After reading that, you should see where the return path is even with only 2 phases (they are still 120 degrees out from each other).


    The "Earth" wire maybe just that, an earth for lightning protection rather than being a nuetral.
    Earth and Neutral are different little beasties, keep this in mind.

    Alrighty, that now said, the wire over the top is most likely there as an easy target for lightning but may also be there to compensate for the fact that te soil stands a good chance of being dry and less than idea conductor so they spread the earth contact over a bigger area to allow for sufficient current flow to minimise volt drop caused by earth potential rise.
     
  16. aXis

    aXis Member

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    An absolutely tiny amount of current will flow through the bird, but so small as to be insignificant.

    You can approximate it to parallel resistors. The wire between the bird's feet will have a very small resistance, and the path through the bird's body will have a very high resistance. Most of the current will go through the least resistance path.

    You can work out the current this way - the voltage over that length of wire is I x R. The resistance is 0.3ohm/km so about 0.000015ohm over the 5cm between a bird's leg. Lets assume 1000 amps in the wire, so that equals a potential difference of 0.015V between the bird's legs. Assume the bird's resistance is 10kohm, and you have 0.0000015A that will flow through the bird - 1.5 micro amps.


    You can do the same math with SWER and find out the the potential difference between someone's stride is large enough to allow dangerous currents. Appatanly cows copy it badly too due their large span.
     
  17. Symon

    Symon Castigat ridendo mores

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    Just to back up what LostBenji said, the earth wire is more than likely there because the ground isn't as conductive in WA as it is in SA, so you need it to get some kind of voltage stability.

    Now that you've got your head around step potentials and SWER systems, can you see why it's a bad idea to use SWER in metropolitan areas?
     
  18. rainwulf

    rainwulf Member

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    Yea, copper pipes love being earth returns!
    It makes showering a scary idea thats for sure.

    SWER is only for country areas, as the price of running one HV line is a lot cheaper then running two.
     

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