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#127564 08/31/01 09:54 AM
Joined: Jul 2001
Posts: 599
J
JBD Offline
Member
We are getting into complicated areas where the slightest imconsistent word usage can change the whole meaning of the discussion.

There are true 2-phase systems where the phases are 90 deg apart from each other. Motors on these systems can create a large amount of torque, similar to DC. A (2) transformer Scott-Tee connections is used to convert 3-phase to 2-phase and 2-phase to 3-phase.

There is also a standard Tee transformer connection which is used for normal 3-phase to 3-phase voltage conversion.

In your case you are describing using 2 hot lines of a 3-phase system (The engineering equivalent circuit of 2 hot lines is a 1-phase system). You need some type of phase convertor system to go from 1-phase to 2-phase and finally to 3-phase. A "static" system would contain 1-phase capacitors to create a 4-wire 2-phase system and then a Scott-Tee transformer arrangement to provide the 2-phase to 3-phase conversion.

This type of phase conversion has some definite drawbacks (including effciencyand regulation) but it is better than nothing.

Hope this helps.

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#127565 08/31/01 11:28 AM
Joined: Nov 2000
Posts: 2,148
R
Member
Utilities use a connection with two transformers in an open delta fed with two hots and a ground to provide 3 phase power. I don't know what it is called but I have one right outside my construction trailer. The two primary hots feed the ends of the open delta. The ground is connected to the point of the open delta.
Don(resqcapt19)


Don(resqcapt19)
#127566 08/31/01 01:20 PM
Joined: Oct 2000
Posts: 2,723
Likes: 1
Broom Pusher and
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Steve,

Is it anything like the 2 phase schematics I have posted???

I think Don has hit on this one, where one Primary feeder is a grounded conductor and it's also common to the secondary. Kind of strange, but I have seen such Animals in Las Vegas and Oregon.
A 3 phase transformer bank was fed [primaries] with only two conductors that were suspended by Insulators. These two were on the "ends" of the open delta. In the "middle" [the paralleled connections], a bare conductor [which was run like the ground bonding conductors found on top of Steel Towers for long distance power distribution] made connection to the "middle" of the primary delta AND was bonded to ground at the pole, plus to the secondary's grounded conductor.

Though I was seeing things, so after having a long look [plus a few others to verify it existed], that was the setup!

It has benifits! It just looks really odd!

Scott SET


Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!
#127567 08/31/01 05:11 PM
Joined: Jul 2001
Posts: 599
J
JBD Offline
Member
Utility transformer connections are not much different than ours. Some utilities may use a grounded phase system (same features as our Grounded B phase), they may even call it an "earth return" or a "common neutral". I believe low voltage grounded-B systems are much more common than similar medium voltage utility arrangements.

25 years ago I saw a 1-phase utility system that had only 1 wire on the pole, the earth was the only return path on the utility side.

#127568 08/31/01 09:49 PM
Joined: Mar 2001
Posts: 2,056
R
Member
Quote
Originally posted by JBD:

25 years ago I saw a 1-phase utility system that had only 1 wire on the pole, the earth was the only return path on the utility side.
Pardon my ignorance of distribution transformers. I see single phase transformers with 1 wire feeding the primary all the time. Can someone explain?

#127569 08/31/01 10:23 PM
A
Anonymous
Unregistered
Can you provide a photo? Are you saying that there is only one wire on the supply pole?

I would like to see this!

#127570 09/01/01 06:08 PM
Joined: Mar 2001
Posts: 2,056
R
Member
Quote
Originally posted by Dspark:
Can you provide a photo? Are you saying that there is only one wire on the supply pole?

I would like to see this!
I'm not into shutterbugging. But, I swear, I can only see 1 wire going into the top of the can. I've thought that maybe they were some sort of autotransformers with a common, single ground point coming down to earth.
Once, I even asked a utility worker who was working on one and his reply was "they're single phase, they only need 1 wire."

#127571 09/01/01 08:20 PM
Joined: Oct 2000
Posts: 5,392
S
sparky Offline OP
Member
Scott,
I did search thru the technical references you have provided , but no soap, sorry.
[Linked Image]

#127572 09/02/01 11:22 AM
Joined: May 2001
Posts: 717
G
Member
Guys, Also do not forget the neutral sizing on a Scott (2phase) connection. Remember it is only 90 deg. out of rotation. Scott, where do you live ? I have seen some VERY WRONG utilities in PA using a single wire on rural connections years ago, utilizing a ground return path, which is EXTREMELY hazardous. I would hope they have all been changed by now. No, they can have a single energized (ungrounded) line, and a grounded line, but must have 2. You guys keep overlooking "Uglys", they used to have the schematic for old 2 phase Scott circuits, one reason is that a lineman used to have to connect it and 'splain it to pass his Class A test, I don't think anyone does this anymore.

#127573 09/02/01 03:28 PM
Joined: Mar 2001
Posts: 2,056
R
Member
Quote
Originally posted by George Corron:
I have seen some VERY WRONG utilities in PA using a single wire on rural connections years ago, utilizing a ground return path, which is EXTREMELY hazardous. I would hope they have all been changed by now. No, they can have a single energized (ungrounded) line, and a grounded line, but must have 2.

That is what I'm wondering. Can the 2nd line just be the same wire that grounds the secondary. In other words getting the 2nd primary wire by just plugging into the earth?

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