Residential Street Wiring Transformer Quest. ?

[Robert11]

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

 

[Charles Perry]

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

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.

 

[jriegle]

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

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

 

[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.

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.

 

[Charles Perry]

So they quit doing delta-wye? Is that kust for cheaper transformers?

--

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.

 

[Don Kelly]

| [email protected] writes:
|
|>On Mon, 19 Jul 2004 01:06:07 +0000 (UTC) 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.
|
| 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?

-----
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.