Single Wire Utility Transformers?

[David Castor]

Years ago we called this an auto transformer.

It's a two-winding transformer, same as the two-bushing cans, just a different primary voltage. Quite common.

 

[garbo]

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.

Having a hard time following how can any transformer with only 1 primary winding can have the neutral travel all the way back from the substation on its own system. Meaning it always appeared that if every copper water service that I ran one of my amprobes on had current flow they ultility company is using that to provide the other side of primary that is center tapped to grounded conductor ( neutral ). Know that electricity takes the easiest path but it appears in this situation to take numerous paths when a one bushing transformer is used to supply power to say 10 houses like mine. At least 30 paths when you consider 10 of each: copper water service, steel gas service pipe, & cast iron leaded jointed soil pipe. Everybody knows that a 8' ground rod in the middle of cold winter and to make matters worse there is nothing but concrete & blacktop within 12' of ultility pole the ground resistance would be very high so current flow on copper water service would increase.

 

[David Castor]

Having a hard time following how can any transformer with only 1 primary winding can have the neutral travel all the way back from the substation

The primary hot leg goes all the way back to the substation. Why would the neutral be different?

 

[kwired]

Having a hard time following how can any transformer with only 1 primary winding can have the neutral travel all the way back from the substation on its own system. Meaning it always appeared that if every copper water service that I ran one of my amprobes on had current flow they ultility company is using that to provide the other side of primary that is center tapped to grounded conductor ( neutral ). Know that electricity takes the easiest path but it appears in this situation to take numerous paths when a one bushing transformer is used to supply power to say 10 houses like mine. At least 30 paths when you consider 10 of each: copper water service, steel gas service pipe, & cast iron leaded jointed soil pipe. Everybody knows that a 8' ground rod in the middle of cold winter and to make matters worse there is nothing but concrete & blacktop within 12' of ultility pole the ground resistance would be very high so current flow on copper water service would increase.

The primary and secondary grounded conductors both tie to the same conductor. Is sort of redundant to run two grounded conductors between two poles (one primary and one secondary) if you are going to tie them together on both ends. If you did that then all you did was create a parallel set for what is effectively still one conductor.

When we do this with NEC installations we usually can't do the same, but that is partly because we aren't running MGN type systems and we generally are not allowed to do that sort of thing other than with service conductors.

 

[mtnelect]

This is asinine to me. So, the utilities say "So What" we have willing victims that don't know anything about electricity.

 

[kwired]

I have posted this before, and everyone thinks this is BS. Except for me.

I only read the Abstract and just glanced at the rest of the document so far. From what I have seen I think writer is barking up the wrong tree to some extent. Current in the earth isn't the problem, it is voltage drop on the grounded conductor that is the problem that causes the "stray voltage".

tie an ungrounded conductor onto a ground rod, the only place you have risk of shock from this is in the immediate vicinity of the rod where voltage gradients are close to one another which allows you to contact different areas of potential within your reaching distance. The further away from the rod you are the farther those potential zones are from one another and you simply can not physically put yourself across any potential differences.

 

[tortuga]

I dont think thats the entire article, what I got only goes to page 5. But I think the whole thing can be summed up by figure 3.

The author uses 14kv as the primary voltage in the example, so if its 50 kVA transformers fully loaded (as the xformer can handle a 3X peak overload) the neutral current would be about 4 amps per transformer.
Now I suppose if we had a long primary feeder with say 50 distribution transformers, the first poles close to the substation could have ~200A of primary current and ~200A of secondary current on the same wire.
It does not show all the possible parallel paths such as a water system, phone and cable TV.
I suppose the biggest problem is if you had a hi resistance connection near the beginning of the system in a urban environment with lots of parallel metallic paths.
The thing is utilities use ground fault sensing relay technology so its seems like any dangers from this would be detected.

I say the more dangerous thing would be if you had situation like kwired described a hot conductor attached to a grounded object that had a hi resistance path or no path back to the equipment grounding conductor.

 

[garbo]

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.

To me its dangerous because every copper water service and steel gas service that I have measured have current flow on them. Believe its back feeding the cheap lazy utility company of only supplying one high voltage line to primary of their transformers. Sounds like something last tried by Tesla 90 years ago. I have connected over a hundred single phase control & power transformers and had to provide two wires to the primary in order to produce a voltage on the secondary. Yes everybody knows that Ultility companies do not have to follow the NEC and get away with too many short cuts.

 

[winnie]

My understanding is that the neutral wire _does_ go all the way back to the substation, except in the very rare case of 'single wire earth return'.

However it is 'multi-grounded' meaning it is connected to ground electrodes all over the place.

So the soil becomes a parallel path, and if the utility has too much voltage drop on that neutral _wire_ then significant current will flow through the soil.

I don't believe the utility is _simply_ being lazy/cheap for using that MGN as a primary circuit conductor; I think there are some benefits with respect to voltage stability and transients during faults ('effective grounding'). With that said, I _don't_ know the details, and I think I'd prefer delta distribution for single phase loads. Perhaps the MGN should only be used for three phase loads which tend to be better balanced.

-Jon

 

[kwired]

My understanding is that the neutral wire _does_ go all the way back to the substation, except in the very rare case of 'single wire earth return'.

However it is 'multi-grounded' meaning it is connected to ground electrodes all over the place.

So the soil becomes a parallel path, and if the utility has too much voltage drop on that neutral _wire_ then significant current will flow through the soil.

I don't believe the utility is _simply_ being lazy/cheap for using that MGN as a primary circuit conductor; I think there are some benefits with respect to voltage stability and transients during faults ('effective grounding'). With that said, I _don't_ know the details, and I think I'd prefer delta distribution for single phase loads. Perhaps the MGN should only be used for three phase loads which tend to be better balanced.

-Jon

Earth becomes a parallel path, but is not like we are trying to send all the earth current through one electrode far away and one at the substantion either. We have neutral taps to earth on nearly every structure in the distribution system. Many are single ground rods at poles or other small structures with probably 25 plus ohms each. By the time you factor all them scattered around the system you have a very low resistance to earth, until you start putting current on the neutral, then you get a rise in neutral to earth volts because of basic voltage drop that will vary depending on conditions at any particular point in the system.

 

[tortuga]

The earth is a weak parallel path. In a urban or even suburban environment any other metallic path that heads towards the substation, such as a old water, gas main or CATV coax could be a very good parallel path if they pass or connect to the substation GEC or a closer service / pole GEC. And its out of the POCO's control at that point. I would hope the POCO would install / configure ground fault sensing relays however I don't know how they would work in that scenario.

 

[electrofelon]

. I have connected over a hundred single phase control & power transformers and had to provide two wires to the primary in order to produce a voltage on the secondary. Yes everybody knows that Ultility companies do not have to follow the NEC and get away with too many short cuts.

Utilities supply their transformers with two conductors too. A delta or two bushing system isnt really different than an MGN or single bushing system except the latter has transformers that are fed L-N while the former is fed L-L. In a MGN, the neutral is connected to a grounding electrode at minimum every 1300 feet, BUT the actual neutral conductor is still continuous back to the source (substation).

 

[ggunn]

Know that electricity takes the easiest path but it appears in this situation to take numerous paths...

No, electricity takes all paths available to it. More of it travels the "easiest" (lowest resistance) path than others.

 

[Frank DuVal]

This is asinine to me. So, the utilities say "So What" we have willing victims that don't know anything about electricity.

?????

Two wires go from substation to pole pig. What does it matter if we call one wire a phase or a neutral if the voltage feeding the pole pig primary matches? NO WHERE is the return current from the pole pig going through the earth unless the "return" conductor is snapped free, same as if a phase is snapped free!

 

[electrofelon]

?????

Two wires go from substation to pole pig. What does it matter if we call one wire a phase or a neutral if the voltage feeding the pole pig primary matches? NO WHERE is the return current from the pole pig going through the earth unless the "return" conductor is snapped free, same as if a phase is snapped free!

An MGN does result in the possibility of stray current traveling through the earth, however negative effects are quite rare. This is one of those things that, as people often do, they overstate the risks and understate the benefits. That is my issue with that Zipse paper: it's not that the stray current issue doesn't exist and is made up, but he drastically overstates the risks and problems and doesn't talk about the benefits.

 

[Frank DuVal]

I think there are some benefits with respect to voltage stability and transients during faults ('effective grounding').

Think LIGHTNING! California has no lightning compared to the number of strikes we have here in Virginia!

Cloud-to-Ground Lightning Strikes by U.S. State & County in 2019

This interactive map explores cloud-to-ground lightning data for U.S. state & counties in 2019 as detected by the Earth Networks Total Lightning Network.

get.earthnetworks.com

2019 cloud to ground: VA 220,000 CA 55,000.

 

[zbang]

NO WHERE is the return current from the pole pig going through the earth unless the "return" conductor is snapped free, same as if a phase is snapped free!

Almost- Unless the intended return conductor has zero resistance, Kirchhoff's law tells that some, albeit probably quite small, current will flow through the grounding conductor to the dirt; granted that it could be 5-6 orders of magnitude smaller, but it'll still be there.

BTW, a couple of days ago while driving through rural PA, SWER systems came to mind for some reason.

 

[Frank DuVal]

And if you think of square miles of surface, 220,000 / 42,000 = 5.23 strikes per square mile 55000/163000 = .3374 strikes per square mile. 16 times as many strikes in Virginia per square mile. We need to ground a lot! Or should I say we need to earth our system a lot for lightning dissipation/protection.

And systems in the middle of the country have more strikes than us. Kansas 1,480,000 strikes, 82,000 square miles or 18 strikes per square mile...!

 

[ramsy]

any other metallic path that heads towards the substation, such as a old water, gas main or CATV coax could be a very good parallel path

Similar electrocution today, eight people involved at a public fountain.

 

[electrofelon]

Similar electrocution today, eight people involved at a public fountain.

I see there is another thread on that incident, but I don't think it's appropriate to post that here when we have no idea whatsoever if that was caused by a utility stray current or an mgn issue. You seem to be implying that.