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Joined: May 2004
Posts: 697
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Dave55 Offline OP
Member
I have a question for any line-workers (U.S.A.).

I'm installing 800A Delta 120/208/240 3PH services and was wondering about the size of the transformer fuses. I use gear with an 800A breaker. In another thread is a discussion about fuses and breakers.

My curiosity is this...if there were a short in this service and it flashed, the fuse at the transformer might be more reliable in disconnecting than the main breaker, so why have a main breaker? There will be six or less disconnects in this service (usually 4 or 5).

There are two pole-mounted transformers for each of these 800A services.

Another question I have is...does anyone have a wiring diagram for how they get 3PH from two HV wires. I've seen winding diagrams for Delta secondaries, but not for the primary with the secondary.

Any help or comments would be greatly appreciated.

Dave

Joined: Apr 2002
Posts: 2,527
B
Moderator
Who owns the transfomers? Are you saying there are two 1ø 120/240V-secondary transformers serving an 800A service?

Joined: May 2004
Posts: 697
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Dave55 Offline OP
Member
I feel like an idiot about this, but there are two things hanging on the pole, one larger than the other. They are owned by the POCO.

Joined: Apr 2002
Posts: 2,527
B
Moderator
Distribution-transformer installations in the US are generally designed under ANSI C57 standards, with engineering review adjusted for regional practices and local adaptations. There a thousands of electric utilities on the North American continent, and few are the same when it comes to distribution construction and operating practices.

Joined: Apr 2002
Posts: 2,527
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Moderator
One very general distribution-transfomer configuration reference is www.cooperpower.com/Library/pdf/R201902.pdf It sounds like you may be describing an open-wye-primary, open-delta-secondary bank similar to figure 21.

If the two 1ø transfomers are not of equal kVA ratings, it is because the utility intends for the lighting/general 1ø 3-wire loads directly connected to the larger transfomer via lo-side “bø” and “cø.” Additional 3ø 3-wire loads like motors are fed from both 1ø transformers. In figure 21, the transformer with X2 unconnected is typically the smaller transfomer. {The non-symmetrical bank is anticipated for intentionally unbalanced secondary-phase currents.} In figure 21, lo-side “aø” is connected only to 3ø loads [that are also served from lo-side “bø” and “cø.”] Lo-side “aø” in the diagram is also called the “stinger”/”wild”/”power” leg. [It is more common for US utilities to designate lo-side bø (or cø) as the high leg.]

It is normal for an open-delta-secondary bank to experience some ø-ø voltage unbalance on the loaded transfomer bank, but is typically limited to no more than 5%. In the usual 3-wire delta and 3- or 4-wire wye systems where all transfomeres are sized equally, phase currents are best equally balanced.

www.cooperpower.com/library/pdf/02022.pdf and, www.cooperpower.com/library/pdf/R240306.pdf on basic distribution-transformer fuse data.




[This message has been edited by Bjarney (edited 05-21-2004).]

Joined: Jul 2002
Posts: 8,407
Member
Great post Bjarney!.
Us guys over here had the 2 transformers up on a set of poles + a platform for the Xformers.
Purely 22kV Line stabilisation, but as far as I know they were configured in a Scott Connection, where 2 HV windings feed 3 HV windings of the same voltage.
Can you shed some light on this Bjarney?.
Not long before I left Temuka, to come to Ashburton, they were Undergrounding all the services and the Scott Connection was lost, in that part of the world.
Changed for plain vanilla Delta-Star Xformers.
Damn, take all the fun out of life why don't they!. [Linked Image]

Joined: May 2004
Posts: 697
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Dave55 Offline OP
Member
Thanks, Bjarney. This is exactly what I was looking for!

Dave

Joined: Dec 2000
Posts: 4,287
Member
Thanks, Bjarney!!
Those are some very informative links.
(You sure can find 'em)...S

Joined: Apr 2002
Posts: 2,527
B
Moderator
Variations of Scott(-Tee) connections are covered in the Cooper R201902 bulletin in figures 25+.

More importantly — Although it’s slowly getting better in some respects, relying on the web for information can have its perils. Be very careful about making decisions that affect your own and others’ safety where information is web-based. Do not forget it may be worth exactly what you paid for it—or a lot worse—it may have very serious costs. Providers of “free” information don’t always have your best interests in mind. In the longer run, you may have to critically question their motives for providing “free” stuff. Web information is almost always terse and can be notoriously incomplete or incorrect.

Free advice from anonymous sources can be a very dangerous way of doing business.

Joined: Jul 2001
Posts: 599
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JBD Offline
Member
Three phase T-connections are created by using two transformers with center taps on the primary and secondary windings. The "teaser" transformer is connected to the center tap of the "main" transformer. If the transformers have 10% taps available the output voltage can be made much more symmetrical than a standard open-delta configuration. Most dual voltage transformers can be connected in a T arrangement.

The Scott connection is a modified T connection which uses a standard center tapped "main" transformer and a special "teaser" transformer with a tap at 86.6% instead of at the center. This special tap location provides a "neutral" connection for a 3-phase 4-wire output. Most commercially built three phase transformers smaller than 15KVA are built using arrangement. Another common use is for creating a true 2-phase system from an existing 3-phase grid.


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