Residential Street Wiring Transformer Quest. ?

Hello:
This is a sort of follow up to what I posted a while back.
Thanks again for the replies.
Am still wondering, a bit, about residential street wiring, and in
particular the transformer
they use to bring the 12.8 KV line-to-line down to 220 V for a house.
What I am confused about is the following:
They seem to take a single lead off the three wires comprising the three
phase (12.8 KV line to line I am told), and
bring it to the (usually the top) of a pole transformer that serves perhaps
a dozen houses on the street.
Then for the next dozen or so houses, I imagine they tap off a different
phase wire, etc.
**Wouldn't they have to bring in two phases to the transformer on the
primary side ? **
Where's the ground, or return for the primary ?
The secondary seems to comprise three leads, which I guess are 220 V, with
probably a center tap for the ground.
This goes to the house.
It's the primary I don't understand, in that they only seem to use a single
wire off the three phase high voltage 3-wire pole system.
What do they use for a ground (or return) ?
Or, is the primary wired as an auto transformer, with one leg of the primary
connected to one leg of the secondary.
Kind of doubt it, but if not, how is it done ?
Thanks,
Bob
Reply to
Robert11
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Where are you located? If it is the US, mostly likely you have a 3 phase wye primary system. That single bushing on top of the transformer connects to one of the primary phases. The other connection is to the neutral wire through a connection on the transformer can. The use of a line to neutral connection is done because single bushing transformers are generally cheaper than two bushing.
Charles Perry P.E.
Reply to
Charles Perry
The neutral line can be the return for both the high and low voltage sides. It save the cost of extra conductors. The secondary is a center tapped transformer with center as neutral. John
Reply to
jriegle
One end of the primary is bonded to the center tap of the secondary as well as to the transformers grounding electrode conductor. The grounded current carrying conductor of the secondary is also serving as the neutral of the primary. Thus the Multi Grounded Neutral is common to both the primary and secondary systems of the transformers. -- Tom H
Reply to
HorneTD
| One end of the primary is bonded to the center tap of the secondary as | well as to the transformers grounding electrode conductor. The grounded | current carrying conductor of the secondary is also serving as the | neutral of the primary. Thus the Multi Grounded Neutral is common to | both the primary and secondary systems of the transformers.
That would mean you can't use these transformers in a three phase bank unless it would be done to create 2 separate 208Y/120 systems out of phase with respect to each other. But that would still be a wye primary and I don't know if utilities like to do that (wye-wye) for three phase.
Reply to
phil-news-nospam
That's true but so what. The transformers that are used in a three phase array need only an untapped secondary winding that will produce the desired output voltage when the primary is also wired between phase and ground. Wye to wye transformer arrays are very common in all of the areas I have worked so far. I have seen three center tapped transformers used in wye delta arrays but on two of them the center tap was left unused of course.
By the way the two separate center tapped transformers run from different phase would likely provide 240/120 rather than 208/120. That is exactly what is happening in adjacent neighborhood transformers supplying single family homes. -- Tom H
Reply to
HorneTD
In the US, most three phase deliveries are wye, and most of those are from wye/wye transformers. That connection is very common and can be created with three single bushing transformers.
Charles Perry P.E.
Reply to
Charles Perry
| |> |> |> | One end of the primary is bonded to the center tap of the secondary as |> | well as to the transformers grounding electrode conductor. The grounded |> | current carrying conductor of the secondary is also serving as the |> | neutral of the primary. Thus the Multi Grounded Neutral is common to |> | both the primary and secondary systems of the transformers. |> |> That would mean you can't use these transformers in a three phase bank |> unless it would be done to create 2 separate 208Y/120 systems out of |> phase with respect to each other. But that would still be a wye primary |> and I don't know if utilities like to do that (wye-wye) for three phase. |> | That's true but so what. The transformers that are used in a three | phase array need only an untapped secondary winding that will produce | the desired output voltage when the primary is also wired between phase | and ground. Wye to wye transformer arrays are very common in all of the | areas I have worked so far. I have seen three center tapped | transformers used in wye delta arrays but on two of them the center tap | was left unused of course.
But you do avoid wye-wye where harmonic problems exist?
| By the way the two separate center tapped transformers run from | different phase would likely provide 240/120 rather than 208/120. That | is exactly what is happening in adjacent neighborhood transformers | supplying single family homes.
If you have 3 primary phases there, then sure, you can keep things in balance by having different single phase transformers on different phases. But most residential areas have only one phase of primary, so the balance is being done street by street, presumably.
Reply to
phil-news-nospam
|
|> |> |> | One end of the primary is bonded to the center tap of the secondary as |> | well as to the transformers grounding electrode conductor. The grounded |> | current carrying conductor of the secondary is also serving as the |> | neutral of the primary. Thus the Multi Grounded Neutral is common to |> | both the primary and secondary systems of the transformers. |> |> That would mean you can't use these transformers in a three phase bank |> unless it would be done to create 2 separate 208Y/120 systems out of |> phase with respect to each other. But that would still be a wye primary |> and I don't know if utilities like to do that (wye-wye) for three phase. |> | In the US, most three phase deliveries are wye, and most of those are from | wye/wye transformers. That connection is very common and can be created | with three single bushing transformers.
So they quit doing delta-wye? Is that kust for cheaper transformers?
Reply to
phil-news-nospam
Delta-wye is only used in special circumstances. Using wye-wye is cheaper since you can use standard single bushing transformers (which they have hundreds of in stock). Most padmounted service transformers are wye-wye also. The really big transformers are still delta-wye (the ones in stations) but most service level transformers are not.
Now the disclaimers. I did not say ALL at any time. There are exceptions. There are differenct practices from one utility to another, but in general if it is a wye primary line, they will use a wye connected primary service transformer. This is for the US only, where multigrounded wye distribution circuits are the most commonly used types.
Charles Perry P.E.
Reply to
Charles Perry
It seems older distribution systems were mostly delta-wye. I tend to look more closely when I notice them upgrading service along a road. A common upgrade for the delta-wye services is to change them to wye-wye and the voltage boosted by sqrt(3). The distribution transformers don't need to be replaced, just rewired so their primaries are wye connected instead of delta connected. All the transformers (except ones added/replaced later) are two bushing with one grounded.
(one street near me remains a mystery. They upgraded the portion from the substation to a certain point, and left everything beyond that point alone. But the transformer bank they added at that point appears to be delta-delta connected, despite that everything else (both before and after) appears to be wye connected)
Reply to
Michael Moroney
| It seems older distribution systems were mostly delta-wye. I tend to look | more closely when I notice them upgrading service along a road. A common | upgrade for the delta-wye services is to change them to wye-wye and the | voltage boosted by sqrt(3). The distribution transformers don't need to | be replaced, just rewired so their primaries are wye connected instead of | delta connected. All the transformers (except ones added/replaced later) | are two bushing with one grounded.
So a 12470Y/7200 distribution circuit becomes a 21600Y/12470 by making all the 12470 primary voltage transformers go from phase-phase connections to phase-ground? I'd be curious to see how such a thing gets coordinated.
Reply to
phil-news-nospam
If it's anything like when they upgraded power on my street, the power will be out for a while while they run up the road with bucket trucks making the switch from delta to wye. (when they increased the voltage on my street they first reconfigured the old wiring out of the way, installed new wiring (the Hendricks? system with the spacers), shut off power, removed & installed transformers, restored power and removed old primary wiring in that order). Power was off for much of the switchover day and there were lots of bucket trucks. About two miles of lines were upgraded (there is a new transformer bank beyond which things were left alone), and it appears that at some point in the distant past there may have been a delta-wye upgrade, since many of the older transformers (black cans) on the old section have two bushings, one grounded. I don't know the voltages involved, but some of the insulators on the old section are kind of small.
Reply to
Michael Moroney
| snipped-for-privacy@ipal.net writes: | |>| |>| It seems older distribution systems were mostly delta-wye. I tend to look |>| more closely when I notice them upgrading service along a road. A common |>| upgrade for the delta-wye services is to change them to wye-wye and the |>| voltage boosted by sqrt(3). The distribution transformers don't need to |>| be replaced, just rewired so their primaries are wye connected instead of |>| delta connected. All the transformers (except ones added/replaced later) |>| are two bushing with one grounded. | |>So a 12470Y/7200 distribution circuit becomes a 21600Y/12470 by making all |>the 12470 primary voltage transformers go from phase-phase connections to |>phase-ground? I'd be curious to see how such a thing gets coordinated. | | If it's anything like when they upgraded power on my street, the power | will be out for a while while they run up the road with bucket trucks | making the switch from delta to wye. (when they increased the voltage on | my street they first reconfigured the old wiring out of the way, installed | new wiring (the Hendricks? system with the spacers), shut off power, | removed & installed transformers, restored power and removed old primary | wiring in that order). Power was off for much of the switchover day and | there were lots of bucket trucks. About two miles of lines were upgraded | (there is a new transformer bank beyond which things were left alone), and | it appears that at some point in the distant past there may have been a | delta-wye upgrade, since many of the older transformers (black cans) on | the old section have two bushings, one grounded. I don't know the | voltages involved, but some of the insulators on the old section are kind | of small.
So everything was pole transformers along that line? No pad or underground transformers?
Reply to
phil-news-nospam
underground
----- The chances are that this was an older area in need of upgrading and this is a cheap way to go about it. It is unlikely that there would be many pad or underground installations involved, if any. I have known of whole towns of population of 2-3000 which have been uprgarde this way overnight.
Reply to
Don Kelly
This is mostly an older residential area with one small newer subdivision with pad transformers (maybe 8 of them). I actually wondered how they dealt with them and now regret not snooping around when they were working. Everything else is pole mounted, except now for the new school (which is the reason for the upgrade in the first place)
In addition there are a few side streets, each fed with one phase, where they simply put up a large pole pig on the first pole and left everything beyond alone.
Reply to
Michael Moroney

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