Unusual electrical service wiring

I was looking at a webpage that showed typical power transformer
connections for different types of services. In addition to
the usual 120V/240V single phase and 120V/208V three phase
configurations, they showed a few odd ones One was 3 240V
transformers in a delta configuration with the center tap
of one of them grounded. The leg not connected to the
grounded transformer was referred to as the 'wild' leg
and apparently carries a voltage of 208V to ground.
What sort of devices would one have that would cause
one to request such a service from the electric company?
3 phase motors with a requirement for 240V delta seem
like they'd be kind of oddball, esp with 208V equipment
being common. A motor connected in a Y would be even
odder, it would get 138V (if my math is correct) and its
neutral would be 70 volts above ground.
Also in the examples shown sometimes one of the
3 transformers (not the one with the ground) was
omitted.
Even weirder was a "Scott" arrangement, two
transformers fed from 3 phases so to produce
two phases at 90 degrees. What would want to be
fed with _that_?
I've also wondered about a small industrial
building. It has 3 individual heavy wires from
the pole to the building, the pole has 3 "cans".
I found this odd, there should be 4 wires. Looking
closer the secondaries are in a delta configuration
and none are grounded as far as I could see.
The same building has a second feed which appears
to be a standard Y setup, 3 hots and a neutral.
So the first must be some special equipment.
Also what are the advantages of Y and delta
configurations for power wiring? Y provides a
natural neutral for one. For secondary wiring
(the 11,000 volts or whatever that run down your
street) it appears most of it is Y connected but
older is delta. Why? From what I can see the
very high voltage power lines are always delta.
Why? Also, if a power line is described as
carrying 345kV, is that phase to phase or
345kV phase to ground?
Reply to
Michael Moroney
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That is a 240V power bank. It provides a three phase 240V delta service as well as 120V from the center tapped transformer for "lighting". That transformer is usually larger than the other two and is used to serve all single phase load in the facility. Nothing odd about it. It is a quite common service for commercial facilities.
Open delta. Provides three phase service with only two transformers. Works well when the three phase load is well balanced and there is little, or no, single phase load. Again, not unusual at all.
Scott T was developed to supply three phase loads from a two phase distribution system. This still happens from time to time. It is rather rare. I know of a few radio/television broadcasting locations with this type of service since it was cheaper than running all three phases the multiple miles to the top of the mountain.
Delta service is not for "special equipment". Motor drives run on three phases, no need for a neutral in most cases.
You sure have a lot of questions. The comparison of the advantages/disadvantages of delta vs wye could keep this group hopping for months. One advantage of delta is a savings in the amount of wire that must be run.
The "11,000 volts that runs down your street" is not secondary, it is primary and is referred to as medium voltage. It is not 11kV, but usually 12.47, 13.2, 13.8, 25, or 34.5kV. Some 4kV also exists. Most of the US uses a 4 wire multigrounded wye setup. There are many reasons; two of which are that most load is single phase and the other being that it is easier to detect single line to ground faults (these are the most common faults).
345kV refers to the line to line voltage.
Charles Perry P.E.
Reply to
Charles Perry
You certainly *do* have a lot of questions.. :-)
It depends which country you are talking about (silly Americans think they run the whole world! ;-) :-)
Down under, the most common sight around the back blocks certainly is 11kV Delta (no neutral or ground). As you get further out of town you find 22kV, 33kV, 66kV and finally 132kV. The easiest way to check the voltage in use is to count the number of disks on the insulators.
Pole-mounted transformers (D/Y) convert the high-voltage primary to low-voltage (415V) secondary with a neutral and an earth at each pole. The main reason for doing this is to save on cable costs.
Oh, and in the country you might find a 22kV SWER line, but that's another story.
Cameron:-)
Or phase to phase if you prefer ;-)
Reply to
Cameron Dorrough
One was 3 240V
We call this 240 - 4 wire, the wild leg has no reference to ground and can be most any voltage. I usually see 140-160 volts to ground, tho it varies. Actually, there is not a lot of 208 equipment, many, many 230 volt motors are in use at 208...which is often about 215, thanks to the boys at the power company. They usually work fine and the voltage is within the tolerance of the motor. 208 is becoming more common in motors due to hi-efficiency design, but 208 shines in small offices and factories where both lighting and power can be balanced from one source. It is also often desired where owners/managers do not feel their maintenance personnel should deal with higher voltages.
Reply to
User 1.nospam
Well, I did say that I was talking about the US and he did say there was 4 wires, which means he is not in Australia.
SWER is some odd stuff, but it does work.
Charles Perry P.E.
Reply to
Charles Perry
I know you did... but you also said "It is not 11kV, but usually 12.47, 13.2, 13.8, 25, or 34.5kV. " and he said "it appears most of it is Y connected but older is delta."
I was merely pointing out that 11kV delta *does* exist and makes sense and is in use in other countries. I was just not automatically assuming that the OP was from the US and didn't know his supply voltages.
Works well too.. especially over ridiculously long distances.. Strangest-looking transformers you will ever likely see.
Cameron:-)
Reply to
Cameron Dorrough
"Nukie Poo @verizon.net>"
Single Wire Earth Return. It's a system for supplying single phase power over very long distances using only one wire. :-)
Cameron:-)
Reply to
Cameron Dorrough
You forgot one older service voltage - 2400V delta :) Not a factor though - these systems are probably as old as if not older than the 4kV wye systems mentioned by Charles...
Our municipal utility finally made the push last year to upgrade the last remaining 2400V lines to 13.8 kV (apparently culminating a 20+ years of transition)... yet the IOU that *also* operates here in town still has miles and miles of 4kV in use.
Dave
Reply to
Dave Dahle
Whattya smoking?
The largest of the 3 (or 2 in the open delta setup) transformers is the singlephase (or "lighting") side, and it is the center leg of this transformer that is grounded. Going between any 2 phases, you will see 240V, but using the grounded leg as a reference, the 2 phases on the lighting side will measure 120V, while the wild leg WILL show 208V to GROUND.
Whattya smoking?
Dave
Reply to
Dave Dahle
Except that there usually are only TWO transformers used to provide the service. One BIG transformer for the 120/240 "lighting" and one "dogleg" to provide the third phase node.
It is very uncommon for secondaries to be connected in delta. A delta load is usually driven by a Y transformer bank.
With two transformers, it is VERY common for the service for larger restaurants. The distribution utilities seem to be pushing the 120/208 service over the 120/240/240/240 service described. They want the "balance" but the customers end up with inferior power for their heavy loads (208 v 240 volts).
Reply to
John Gilmer
Are you serious? That sounds like an old telegraph setup. That doesn't sound too safe with the "return" current running through the ground like that. I remember reading in an old IEEE transaction about stray currents near the Brooklyn bridge causing severe corrosion problems (there's a power plant nearby on the Long Island side in Ravenswood). About 20yrs ago, I had a customer that complained about getting zinged when his foot was immersed in his swimming pool whilst the other was still on the ground. To make along story short, I wound up temporarily disconnecting his service drops to prove that his problem was caused by stray current and a utility issue (he had a private well so it wasn't coming in on any water main). SWER sounds like an invitation for such problems.
Reply to
Nukie Poo
That is an intersting link.
At one time {still may be} In southern Alberta, Canada, SWER was used for rural farmhouses. At each house, a single-bushing 14.4kV-120/240V transformer had two driven rods?one at the transformer pole and one at the second pole out. For each transformer there was one lower neutral span between the first and second poles for interconnecting the two rods.
Based on a phone call to the local utility, they conducted periodic ground-resistance tests by governmental mandate. An engineer there mentioned that each transformer was limited to something like 15kVA, making the hi-side ground current a bit above 1 ampere at full secondary load.
Of course, ground-overcurrent relaying at the distribution-circuit source would be like an ashtray on a motorcycle.
--s falke
Reply to
s falke
yeah, and I left some out because the note was getting too long.
What is the correct terminology (and voltage ranges) for:
The high voltage stuff where (around here, northeast US) the wires hang from large insulators of many disks, usually on two large poles with a crossarm which support the cables, or latticework towers (older) with 6 conductors (two circuits I guess);
Circuits on single wooden poles, that have the crossarm below the wires but the insulators are not the bell-shaped ones seen on the primary lines running down the streets, and the insulators are rather large, they seem to run from large substations to small substations in rural areas to feed the local primary feeds, usually no neutral but sometimes it (or a lightning wire?) is at the very top of the pole, not half way down. These do not have transformers for individual houses, they just go substation to substation.
I have to admit I've never climbed one of these poles with a voltmeter! I can only guess from the occasional news story where someone gets electrocuted by coming in contact with an xx,xxx volt wire.
Also I often see setups where two poles are connected to each other with beams and 3 transformers sit on those beams. This is almost certainly to convert higher voltage primary to lower voltage (esp. since everything "downstream" of the setup is older looking), but within a few poles of this setup are two more transformers, each with 3 wires, one wire goes to the center phase and the other 2 go to one of the other phases on each side of an insulator spaced inline. Usually with a cable leading down the pole to a box at about eye level. Sometimes this setup exists in the middle of nowhere without the bank of 3 transformers. What is this? Mostly seen in rural NY in Niagara Mohawk territory. Seems redundant in the first case.
Reply to
Michael Moroney
It is very common in the coalfields of West Virginia and Kentucky to have 480 delta, often with a "corner ground", but not always. Then they run the ground thru a resistor and monitor the current, if it exceeds a predetermined level, an alarm sounds and maintenance takes over.
Some mines even take 4160 into the mine where a unit substation reduces it to utilization voltage.
Reply to
User 1.nospam
Substation to subtation = Transmission lines.....usually 120KV and up....to 750KV. Substation to transformer poles = Distribution lines....12,470 volts up to 35KV, depending on your utility.
Don't, you will never get close enough to measure the voltage before you are dead.
You may be looking at a capacitor bank, used for power factor correction on long lines. Common in rural areas. These do not provide power to any user.
Reply to
User 1.nospam
Those are not stepdown transformers, they are voltage regulators. They adjust the voltage on the conductors.
Most lines with 2 poles per structure are either subtransmission or transmission. Most lines with a single pole are distribution but could be subtransmission or transmission.
Go here for a decent explanation:
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Charles Perry P.E.
Reply to
Charles Perry
"Nukie Poo @verizon.net>"
Interesting stuff.. stray currents can be a hassle and IIRC most old telegraph setups were two-wire DC (at least the ones here were)..
But to answer your question - yes, I'm quite serious. For more information, try a Google search on "Single Wire Earth Return". Happy reading!!
Cameron:-)
Reply to
Cameron Dorrough
Quite correct. BTW, it was not uncommon to see mist rising from pools of rainwater around the base of the transformer pole early on a winter's morning.. It was a great place to warm the feet when a long way from the house.
Cameron:-)
Reply to
Cameron Dorrough

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