Single Wire Utility Transformers?

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Alfont1120

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Dighton, MA
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Electrical Project Manager/Journeyman Electrician
They are replacing the poles and upgrading transformers in my area and while observing the new xfmr from the ground I noticed it only has one wire on the primary and 3 on the secondary side. I'm struggling to understand how this could work. Are they using a high voltage single phase feed (one wire the hot, the other wire the ground?) and using that to create 120/240V through the xfmr? Does this mean the ground wire is also playing the role of the neutral? I feel like I should understand how this works lol, i've been an electrician way too long to be this ignorant of the utilities!
 
The other side of the primary winding is connected to the transformer's metal case.

It's likely that there is a second primary wire, which is not readily visible because it's stapled directly to the pole.
 
drcampbell said:
The other side of the primary winding is connected to the transformer's metal case.

It's likely that there is a second primary wire, which is not readily visible because it's stapled directly to the pole.
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Wouldn't that be a ground if its connected to the transformers metal case?
 
Alfont1120 said:
Wouldn't that be a ground if its connected to the transformers metal case?
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You may not realize it, but that question strikes a nerve.

Many utilities use a distribution system called "Multiply-Grounded Neutral". There's a Y secondary, often ~15kV phase-to-phase and ~8kV phase-to-neutral, and the neutral is connected to a ground rod at each pole where there's a transformer.

It's controversial because the inevitable voltage drop on the neutral* wire is imposed on the Earth (the planet, the soil; not the GEC or EGC) and creates voltage gradients in the Earth. These can wreak havoc, not the least of which is electric-shock drowning. Most people who have ever chased down one of these stray-Earth-voltage gremlins would like to see transformer primaries fed from two ungrounded phases and isolated from the Earth, but that conversion won't happen overnight.

* (they're widely called "neutral" wires, but they're not actually neutral unless all the ungrounded phases are perfectly balanced; (harmonics and all) they're almost always carrying a significant amount of current)

That said, implementing multiply-grounded neutral was a big improvement over Single-Wire, Earth Return. It could be a cost saver (one copper wire, not two) when servicing a small far-away load, but the energy losses, stray-voltage problem, and farms no longer being small users led to it being mostly abandoned.
 
Single-bushing primary transformers are connected line-to-neutral, so the winding is something like 7200:120/240. Pretty common in many parts of the country. Slightly cheaper than the more common two-bushing transformers.
 
Alfont1120 said:
Are they using a high voltage single phase feed (one wire the hot, the other wire the ground?) and using that to create 120/240V through the xfmr?
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Yes. A transformer (and most other loads) don't care whether one of the supply conductors is grounded, only that the correct voltage is applied.

Does this mean the ground wire is also playing the role of the neutral?
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More the other way around: the primary is a line-to-neutral load, and that neutral is grounded.
 
Alfont1120 said:
So a hot and neutral on the primary with x1, x2 and neutral on the secondary? With both neutrals being the same?
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Yes. A single grounded conductor is often shared between the primary and secondary systems.

If a secondary triplex is strung between poles, its bare conductor can also serve as the primary neutral.

That conductor also serves to ground (bond) metallic parts. There is no separate grounding conductor.
 
drcampbell said:
* (they're widely called "neutral" wires, but they're not actually neutral unless all the ungrounded phases are perfectly balanced; (harmonics and all) they're almost always carrying a significant amount of current)
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I beg to differ; carrying imbalanced-load current defines a neutral conductor's purpose.

A perfectly-balanced load (theoretically) has no need for a neutral conductor.
 
Larry Fine is very brave to accurately read this article. I challenge you to all, also take the time to read this well thought out article.
 
Okay, opinions as I'm reading:

1. Nobody is recommending SWER anymore.

2. It gives way too much credit to earth as a good neutral-current pathway.

2. "Remember Ohm’s Law which states in order to have a voltage we must have a current flowing through a resistance; Voltage = Current times Resistance."

Really??? The current causes the voltage??? You can't have a voltage without current??? OMG!!!

3. I guess that's it.

My conclusion is that excess earth current is a sign of a resistive problem in the MGN pathway.

Would an open-neutral situation be better without electrodes? I think it would be worse.

As for the OP, I don't think it would it be practical to convert all distribution systems to delta.
 
LarryFine said:
Okay, opinions as I'm reading:

1. Nobody is recommending SWER anymore.

2. It gives way too much credit to earth as a good neutral-current pathway.

2. "Remember Ohm’s Law which states in order to have a voltage we must have a current flowing through a resistance; Voltage = Current times Resistance."

Really??? The current causes the voltage??? You can't have a voltage without current??? OMG!!!

3. I guess that's it.

My conclusion is that excess earth current is a sign of a resistive problem in the MGN pathway.

Would an open-neutral situation be better without electrodes? I think it would be worse.

As for the OP, I don't think it would it be practical to convert all distribution systems to delta.
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Thank you for taking your time to read the article.

With all the talk about gradients around swimming pools and electrocutions. I couldn't help to publish it again.
 
You're welcome. Any idea how long ago it was written?

mtnelect said:
With all the talk about gradients around swimming pools and electrocutions. I couldn't help to publish it again.
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Bonding and bonding grids/mats are equally important and valuable for any electrical installations around water and livestock environments. I doubt elimination the MGN would change that.
 
Alfont1120 said:
They are replacing the poles and upgrading transformers in my area and while observing the new xfmr from the ground I noticed it only has one wire on the primary and 3 on the secondary side. I'm struggling to understand how this could work. Are they using a high voltage single phase feed (one wire the hot, the other wire the ground?) and using that to create 120/240V through the xfmr? Does this mean the ground wire is also playing the role of the neutral? I feel like I should understand how this works lol, i've been an electrician way too long to be this ignorant of the utilities!
Click to expand...
Years ago we called this an auto transformer. To me its dangerous to supply say a 75 KVA transformer from a pole that already has three phase 13,200 power but lazy cheap ultility company only feeds it with one wire therefore in probably all cases causes not only the house grounded conductor ( neutral ) but copper water service, steel gas service and cast iron sewer pipe to carry current back to transformer. I live in a large city and when taking a few block walk will come across at least two pole mounted distribution transformers to have 6 to 8' of missing aluminum wire that should attach to ground rod at bottom of pole. Lazy ultility company no longer installs heavy plastic guards over ground rod conductor to prevent it slow down pipers from stealing the aluminum wire. Worst part while upgrading residential services with no power to panel still would get a small current flow on copper water & steel gas line services.
 
Not an auto transformer if the primary winding's "neutral" or low side is simply connected to the "neutral" connection of the secondary winding! ❗

The neutral is run all the way back to the substation. i.e. out in the country the poles have three high voltage wires (typically 34.5KV between phases, 19.9 KV phase to neutral) and a neutral conductor that is on very small insulators. Wye feeds. I hardly ever see a two bushing transformer on poles around here for the last 60 years I have been looking at them.
 
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