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Thread: Neighborhood Transformer ?

  1. #1
    Join Date
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    Default Neighborhood Transformer ?

    Ok...this has been bugging me for a long time. I understand the local neighborhood transformer has a primary and secondary winding. Secondary having 2 terminals that are out of phase with each other, each providing a 120 volt hot leg, and a center tapped neutral that is grounded. 240 volts between the hot leads, 120 from each hot leg to the neutral. Ok so far.
    Here is where I don't get it....the primary coming in. A guy at work says there are 2 primary terminals...one for the high voltage coming in, and the other one leaving the transformer and going downstream, with these 2 terminals being out of phase with each other. It appears to me, from looking around the web at various sites, that there is ONE primary lead coming into the transformer at "high" voltage (7200?) and none leaving, that forms the primary coil. I even saw one diagram that showed the other end of the primary wire terminating on the neutral tap of the secondary side!
    How does it work??
    Last edited by rain252; July 2nd, 2005 at 10:53 PM.

  2. #2

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    There are one bushing transformers and two bushing transformers. The two bushing type (most common) have two primary terminals on top of the pole can for for both primary conductors to connect to. The one bushing type has one terminal for the primary hot to connect to. The second primary terminal in on the back side of the can (180 degrees opposite the secondary terminals). One bushing transformers are used mainly for settup up three phase transformer banks, since the terminals are located in a little better position for this type of setup.
    "Nothing is foolproof to a sufficiently talented fool"

  3. #3
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    It appears to me, from looking around the web at various sites, that there is ONE primary lead coming into the transformer at "high" voltage (7200?) and none leaving.
    I think that maybe you are not seeing that the primary is usually fed with a cable, not individual conductors. The secondary lines on the pole 240V/120V are of course individual conductors.

    The three phase supply entering the neighborhood is typically split three ways. So you may have a 4160Y/2400V or 12400Y/7200V 3-phase supply that will be used to power single phase transformers at 2400V or 7200V.

    If you look closely at the fuse holder on the pole in your neighborhood you will notice that the feeder has two conductors, not one. The second conductor is a concentric braid. The center conductor is fused while the neutral (braid) is not. You may then see a piece of this cable connecting to the transformer (appearing like a single wire). Since the voltage is high (current low) the gauge of wire needed is small.



    Homer
    Last edited by Homer; July 3rd, 2005 at 01:27 PM.
    Depending on your skills, doing your own electrical work may risk the health and safety of the community. Always find out how to do things safely before beginning.

  4. #4
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    I guess you learn something every day. I thought the primary was one wire and the "neutral" was a common wire used for both the primary and secondary. Is a common wire ever used in power distribution for one side of the primary and secondary?
    Mark
    Kent, WA

  5. #5
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    Quote Originally Posted by suemarkp
    I thought the primary was one wire and the "neutral" was a common wire used for both the primary and secondary. Is a common wire ever used in power distribution for one side of the primary and secondary?
    I guess that in a sense they are a common wire since they're both grounded. I suspect that they are both grounded but not directly connected. I will have to take a closer look the next time I have the opportunity.

    The 4-wire WYE 4160Y/2400V feeder has its neutral grounded (4 times per mile). This neutral along with one of the phase conductors supplies the primary. The neutral on the secondary is also grounded.

    I believe that there are some 3-phase distribution configurations that do actually use this common wire. Here's a link to an interesting document.

    Homer
    Depending on your skills, doing your own electrical work may risk the health and safety of the community. Always find out how to do things safely before beginning.

  6. #6
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    Quote Originally Posted by suemarkp
    I guess you learn something every day. I thought the primary was one wire and the "neutral" was a common wire used for both the primary and secondary. Is a common wire ever used in power distribution for one side of the primary and secondary?
    Around here the primary and secondary neutral are one in the same.

    I just looked at several transformers. Both ones with single primary terminals and those with 2.

    Those with one have a single lead from the primary lug to the cutout.

    One the secondary they have 3 terminals (all appear to be insulated). But below the CT is a ground lug to the case. A bare wire goes form the ground lug to the CT lug and then down the pole to the ground electrode under the pole.

    3 leads from the secondary to the secondary lines.

    At the end of that circuit the primary/secondary netural continues on to the next transformer.

    Also at the tap for the cutout there is what appears to be an arc gap arrestor connected to between the primary and ground wire.

    Those transformers with 2 primary side connections are wired the same, except the 2nd connection goes to the ground wire, which is also connected to the neutral at the secondary CT. There is no simple single ground/neutral bonding point like there is in a panel.

    As I was walking the poles I came to the one place where one circuit ended and another started. The pole happened to be on a corner and had the 3 secondary from one circuit land on one side of the pole and feed one house. The other other circuit land 90 degrees from the first and feed another house. Then there was a single jumper from neutral on one circuit to that of the other.

    Note - this is for all above ground distribution I am sure that under ground is handled differently.

  7. #7

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    Many single bushing transformers have the H2 terminal internally connected to X2 and X3. This means that there is no external appearance of the H2 terminal. Regardless, H2, X2 and X3 are always connected to each other and to earth. You might also note that many single bushing transformers do not have a non-load break cutout or fuse on the pole. They are internal to the transformer. If the fuse blows on a single bushing transformer (as from a lightning strike), the transformer is sent back to the transformer shop and a new one installed. Double bushing transformers are sometimes used to completely seperate the primary and secondary grounds to eliminate stray current on a dairy farm or where the customer is located very near a rural sub station.
    "Nothing is foolproof to a sufficiently talented fool"

  8. #8
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    Here is the drawing of a transformer I saw....... does this look how it works?
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