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#176615 - 04/07/08 09:39 PM
Re: Primaries of pole mounted transformers
[Re: gfretwell]
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Albert
Member
Registered: 08/16/06
Posts: 41
Loc: Falls Church, VA
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I'm not an electrician, but I think most residential distribution systems today are wye, with a multi-grounded neutral common to the primary and secondary. The primaries are connected line-to-neutral.
Most single-phase aerial distribution transformers have a single primary bushing, with the other end of the primary winding internally bonded to the transformer case and the secondary neutral. But I occasionally see a two-bushing transformer, where both ends of the primary are brought out to bushings. This type of transformer can be used on either delta systems (connected line-to-line) or wye systems (where one of the primary terminals is externally connected to the neutral).
I think residential delta distribution was much more common in the past.
Edited by Albert (04/07/08 09:40 PM)
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#176618 - 04/07/08 10:11 PM
Re: Primaries of pole mounted transformers
[Re: gfretwell]
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Albert
Member
Registered: 08/16/06
Posts: 41
Loc: Falls Church, VA
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Are you sure the transformers are really paralleled? Most often (around here at least), you'll see small strain insulators inserted in the two hot wires of the secondary bus to break it into independent sections, each fed by a single transformer. Or there may be two secondary insulator racks on a pole, but the transformer is only connected to one of them. A secondary section may physically span several poles, or it may only serve the houses connected to one "maypole", but in either case it's electrically separate (except for the neutral).
Secondaries *can* be paralleled or "banked", but I think it's pretty rare in aerial distribution. I believe Detroit and someplace in southern California are some of the few places that do it.
In contrast, urban areas with high load densities often do have such "network" systems, usually underground. These feature breaker-like devices called "network protectors" to isolate failed transformers from the interconnected system.
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#176620 - 04/07/08 10:28 PM
Re: Primaries of pole mounted transformers
[Re: Albert]
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noderaser
Member
Registered: 03/01/07
Posts: 169
Loc: Portland, Oregon, United State...
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If you see three transformers of same size on the same pole, it's most likely for a three-phase service. Are there now, or was there previously, and commercial/industrial buildings on your street?
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#176621 - 04/07/08 10:39 PM
Re: Primaries of pole mounted transformers
[Re: noderaser]
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Albert
Member
Registered: 08/16/06
Posts: 41
Loc: Falls Church, VA
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I missed the point that the transformers are on the same pole. That being the case, I'm sure you're correct about the three-phase service.
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#176627 - 04/08/08 12:08 AM
Re: Primaries of pole mounted transformers
[Re: gfretwell]
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Albert
Member
Registered: 08/16/06
Posts: 41
Loc: Falls Church, VA
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Actually, looking again at your picture, I think I see insulators dividing the secondary into two sections. Look directly below the transformer at the two bottom (hot) wires - note the two bulges in each wire, sort of a "dog bone" shape. Those look to me like strain insulators, so the transformer would only feed the wires to the left of the pole.
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#176628 - 04/08/08 12:40 AM
Re: Primaries of pole mounted transformers
[Re: Albert]
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wa2ise
Member
Registered: 11/29/02
Posts: 571
Loc: Oradell NJ USA
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It varies from one POCO to another. Some like to use one high voltage line and neutral/ground, others use a pair of high voltage lines, 2 of the 3 phases.
Also there is variation of how the secondary 120/240V lines are strung along poles. When they use 3 separate wires. Some places will have the neutral as the top of the 3 wires, other places will have the neutral as the middle wire. An arguement for the top wire being the neutral would be that it's the wire that would get touched by falling high voltage distribution wires. Thus shorting directly to ground and less high voltage getting into customers' homes.
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