TRANSFORMERS - A solid connection from primary to secondary ?

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mtnelect

mtnelect

HVAC & Electrical Contractor
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Contractor, C10 & C20 - Semi Retired
Solid electrical connection
from the primary neutral to the secondary neutral. Such a
connection is unthinkable and unheard of in the industrial,
commercial and residential electrical engineering fields.
 

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mtnelect said:
Solid electrical connection from the primary neutral to the secondary neutral. Such a connection is unthinkable and unheard of in the industrial, commercial and residential electrical engineering fields.
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Not unthinkable, and allowed by the NEC. Transformers don't have to be installed as SDSs, you can solidly connect the primary neutral to secondary neutral instead.

Cheers, Wayne
 
The issue is not the connection of primary neutral to secondary neutral. As others have noted, the NEC allows this connection in a number of situations.

The issue is the re-grounding of this non-isolated neutral, which the NEC prohibits but the POCO does all the time.

The NEC prohibits regrounding the neutral precisely because this would inject neutral current into the EGC/ground system.

The POCO has good reasons for re-grounding their system, but this certainly injects current into the soil, leading to the sort of problems @mtnelect is concerned about.

I don't know enough to describe how to eliminate the problems of multi-grounding while still meeting the POCO grounding needs.

Jon
 
I don't know how old the article is, but I know I read it more than 20 years ago. I think it was posted on an old forum on eng-tips.com

Either in that forum or in the continuation of the article that is not posted here, it was mentioned that the solution to the re-grounding problem is for the power companies to run five wires instead of four, three phase wires, the neutral wire which would then be insulated and isolated from ground except at its single grounding point where it originates, and a protective earth wire. Just like the way we do it in commercial/industrial wiring. They would then have to treat the neutral just as they do a phase conductor when working on the system - assume it may have dangerous voltage on it.

After reading this two decades ago, I was left with the impression that utilities in some areas actually were being required to install that five-wire system. Such as California. But I guess I was wrong.
 
Mr. Serious said:
I don't know how old the article is, but I know I read it more than 20 years ago. I think it was posted on an old forum on eng-tips.com

Either in that forum or in the continuation of the article that is not posted here, it was mentioned that the solution to the re-grounding problem is for the power companies to run five wires instead of four, three phase wires, the neutral wire which would then be insulated and isolated from ground except at its single grounding point where it originates, and a protective earth wire. Just like the way we do it in commercial/industrial wiring. They would then have to treat the neutral just as they do a phase conductor when working on the system - assume it may have dangerous voltage on it.

After reading this two decades ago, I was left with the impression that utilities in some areas actually were being required to install that five-wire system. Such as California. But I guess I was wrong.
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Or run an ungrounded system. Both will increase costs over an MGN and it's easy to write off the cost aspects until you consider it is us that would pay for it not the utility.
 
Yeah and how do we know what all the problems would be with ungrounded or 5 wire until we build it? Just because we have small models of these systems doesn't mean it will scale up to the size of the North American power grid
 
I'm not sure that simply running a ground isolated neutral and separate 'EGC' in a utility power distribution situation (the way we run feeders and circuits in buildings under the NEC) would actually increase safety.

It would certainly improve the 'stray current' issue. However keep in mind that utility conductors run for miles and miles outside and are exposed to transients such as lightning.

The multi-earth-neutral system is an installation where every transformer is acting as a shunt to permit transients to flow to the ground. In an isolated earth neutral system, the outside wire system becomes a great tool for coupling transients into structures.

Like I said I don't know the engineering trade-offs. I'm sure one could design a transient resisting isolated earth neutral distribution system, but I've no clue the details or cost of such a system. It might be cheaper and sufficient to continue to use the multi-earth-neutral system but add additional bonding to shunt the resulting soil current away from sensitive situations...however that also changes who pays the costs. Maybe if the utilities had to pay for equipotential bonding around swimming pools and dairy facilities and anywhere else someone felt a tingle, they would design their distribution systems differently.

-Jon
 
Joethemechanic said:
Yeah and how do we know what all the problems would be with ungrounded or 5 wire until we build it? Just because we have small models of these systems doesn't mean it will scale up to the size of the North American power grid
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Well I would respond that as far as ungrounded goes, that's how the grid started and there is still lots of delta distribution. Most of upstate NY is 4800 ungrounded Delta. I have never heard of any practical disadvantages of these systems, other than cost.
 
mtnelect said:
Solid electrical connection
from the primary neutral to the secondary neutral. Such a
connection is unthinkable and unheard of in the industrial,
commercial and residential electrical engineering fields.
Click to expand...


mgn.png

As others have said over 1000V its not even a NEC code violation.


Can you post the full paper? It would be interesting to read.
It would be interesting to see what source they cite for this part:
In Chicago in 1932, the transformers were failing. By this
time, the NEC was requiring grounding of the neutral in the
electric service to homes. The earthing connection was made
to the metallic water lines.
The Utilities Research Commission of Chicago and the
Engineering Experimental Station at the University of Purdue
conducted an investigation of surge protection of distribution
circuits as to why transformers were failing. Test involved the
interconnection of the primary lightning arrester ground and
the grounded neutral of the secondary main circuit.
The conclusions reached were, “Measurements of the
voltage between primary phase c lead and secondary neutral
have shown that the interconnection of the secondary
neutral with the lightning arrester ground is, in general,
beneficial to the transformer. In particular, with a low
resistance secondary neutral ground and a high resistance
lightning arrester ground, the interconnection reduced the
above voltage by 30 to 50 per cent.” [16]
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If they(or you) are just suggesting utilities add more ground rods at utility poles (not ditching the MGN system) that is not a big ask.
Its hard to know what they are suggesting without reading the full paper.
 
electrofelon said:
Well I would respond that as far as ungrounded goes, that's how the grid started and there is still lots of delta distribution. Most of upstate NY is 4800 ungrounded Delta. I have never heard of any practical disadvantages of these systems, other than cost.
Click to expand...

I always thought when you use deltas on a large scale you ended up with circulating currents do to the 3rd harmonic. I could be wrong, it was something I was told by a PECO engineer over 30 years ago
 
Joethemechanic said:
I always thought when you use deltas on a large scale you ended up with circulating currents do to the 3rd harmonic. I could be wrong, it was something I was told by a PECO engineer over 30 years ago
Click to expand...
Certainly maybe. I wasn't using the fallacy "I haven't heard of a problem so there isn't a problem", I was just saying there is a lot of history with these systems so we should have good data on them.
 
Back when I was a kid working on 240 and 480 volt motor controls I noticed that you couldn't get shocked by the output of the control transformer even though it was 120 volts unless you touched both conductors. I was told this was because it was ungrounded. So for the longest time I kinda thought that N-G bonds and grounding in general was a bad idea. I even had the idea that kitchens, bathrooms should be fed by an isolation transformer. This was in the mid 70's and I'd never seen or heard of a ground fault device.

Since then I've learned a lot more and even with it's problems I see a lot of good arguments for a MGN system
 
I recently participated in a discussion on another site where the design engineer was going to connect the neutral from the primary feeder to the XO on the secondary of a delta/wye transformer to avoid any requirements to provide a grounding electrode system for the transformer secondary. With the feeder neutral connected to the secondary XO, the transformer is not a separately derived system and the requirements in 250.30 do not apply.
 
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