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Pole Transformer Mystery

Discussion in 'General Electronics Discussion' started by timl, Aug 5, 2010.

  1. timl

    timl

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    Aug 5, 2010
    I have seen pole transformers that have one high voltage bushing that is tapped into a single phase of the power companies three phase mains. From what I gather the high voltage line goes into the primary winding and then the other end of the primary winding is connected to the split phase neutral. Here is what I don't understand, the primary winding is say 11 kV, if it is connected to neutral and/or ground isn't that creating a short circuit? Why doesn't an immense power flow through the primary winding?
    To put it another way, if one side of the primary connects to a 11kv wire and the other side is connected to neutral, what stops power from flowing freely? This seems the same to me as if I connected the hot wire of an outlet in my house directly to the neutral. No resistance, major current.
     
  2. Mitchekj

    Mitchekj

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    Jan 24, 2010
    Ah but there is resistance. Reactance, in this case. The primary would be a (near) short... to DC. Throw a 60Hz AC wave at it, and you've definitely not got a short.

    Read up on inductors/transformers for further info.
     
  3. timl

    timl

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    Aug 5, 2010
    Thus I see the power companies circuit is completed by the earth itself. I have scoured the Internet and never read this anywhere!
     
  4. Resqueline

    Resqueline

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    Jul 31, 2009
    Neutral, Protective Earth, and the Earth itself are three different things, be careful not to confuse them with each other. I'm not sure what split phase neutral is though.
    You have Delta and Wye connection. Delta has no Neutral wire, whereas Wye would have 11kV (or 220V) between phases and 6.35kV (or 127V) between Phase & Neutral.
    Neutral is connected to Protective Earth at the transformer (if the 5-wire system is applied). Can you make up a wiring diagram of how it's connected at your place?
     
  5. trobbins

    trobbins

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    Jun 15, 2010
    timl,

    re the pole transformer - are you saying there are three bushing/cables going to normal 3ph (LV ?) distribution - are you also saying the 'transformer' also has another separate single HV bushing cable? Does that cable go anywhere - i.e. is it a single wire earth return distribution system?

    Ciao, Tim
     
  6. timl

    timl

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    Aug 5, 2010
    When I say split phase, I really mean (according to wikipedia) 3-wire, single-phase, mid-point neutral. My transformer is in someone elses yard, but here is what I've seen on other poles. There are one or two bushings. The high voltage line always goes into a bushing to form the primary winding; the other end of the primary comes out at the top (either with or without a bushing) and connects to the LV neutral. I will have to more closely inspect a pole (not easy from the ground) to see how the whole system is grounded.
    Things to keep in mind: this is an urban environment. There is only one HV phase (1 wire) from the power company (farthest up on the pole). It is of course three phase distribution, but quite often they only send one phase to split off and feed several pole mounted transformers.
     
    Last edited: Aug 6, 2010
  7. timl

    timl

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    Aug 5, 2010
    Here is an image that is very similar, but not exactly, what I see on the pole mounted transformers in my area. [​IMG]
     
    Last edited: Aug 6, 2010
  8. trobbins

    trobbins

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    Jun 15, 2010
    It looks like 1ph MV coming in on the HT and neutral, and split 120-0-120 1ph coming out for local distribution. Not sure what they do with the neutral in your country, but suggest it is reticulated back to the originating 3ph MV transformer N connection (most likely a star secondary).

    If so, then the primary winding on your pole transformer is MV to 'local' neutral.

    Ciao, Tim
     
  9. timl

    timl

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    Aug 5, 2010
    I'm sorry not sure what 'MV' means? Although I can assure you there is no neutral 'path' back to the substation. There are local neutrals as far ad I can tell that are grounded, but nothing conveys the circuit back to the substation. As far as high voltage distribution lines (11kV or greater) there are never neutrals. Three wires only.
     
  10. trobbins

    trobbins

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    Jun 15, 2010
    timl,

    MV refers to Medium voltage - I think that is now an archaic term, as it is either LV or HV. MV was the next step up from LV (eg. 600V to 11kV).

    Commonly the HV distribution source has a star(wye)-neutral-earth connection at the source transformer secondary. In that case the 'return' path for the HV can be earth - aka earth return distribution. But there could also be a delta transformer secondary and a far-end neutral-earth distributed interconnection of phases and loads - a little ugly, but possible. So many options - doh.
     
  11. timl

    timl

    6
    0
    Aug 5, 2010
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