Reverse fed 480/277-2300 delta secondary

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My understanding of the 'don't connect the primary neutral' is that the neutral if a wye:delta transformer has very low impedance to zero sequence current. This is another way of saying that the transformer reliably derives a neutral from the three phase voltages.

Wye:delta transformers are used as grounding transformers to derive a neutral point to ground ungrounded delta systems.

When you use the wye side as the primary and connect the wye neutral to the system neutral, then this transformer's neutral will be placed in parallel with the source neutral. If the phase to neutral voltages do not match between the two transformers (system source and connected load transformers) then neutral current will flow to try to balance things.

Weather or not this is a problem depends upon the situation. Utilities often intentionally use wye primary transformers as part of 'effective grounding'; the re-derived neutral is of benefit to stabilizing phase-ground voltages.

If this load transformer is of similar rating to the system source, then any circulating currents might be unimportant.

But if you have a bad combination of source to transformer size, and phase imbalance, then the transformer might be overloaded simply trying to fight the supply system balance.

Jon
 
Xptpcrewx said:
Im leaning on this primary neutral isolation thing for Y-Δ transformer being mostly a myth. Here is an interesting IEEE analysis showing its fine either way in case anyone can follow it.

Click to expand...
I can tell you from experience that supplying a wye -delta with a neutral on XO is very bad. This happened at a job I was working on. Blew this nice hole in the 2" EMT feeder serving the transformer. Obviously there are situations where nothing will happen and it will work fine, depends on the amount of imbalance.
 

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electrofelon said:
I can tell you from experience that supplying a wye -delta with a neutral on XO is very bad. This happened at a job I was working on. Blew this nice hole in the 2" EMT feeder serving the transformer. Obviously there are situations where nothing will happen and it will work fine, depends on the amount of imbalance.
Click to expand...

Nice picture. Are you sure it’s because the neutral imbalance or could this be due to ferro-resonance?
 
Xptpcrewx said:
Nice picture. Are you sure it’s because the neutral imbalance or could this be due to ferro-resonance?
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I am pretty sure it was the source neutral connection to XO. Here is a thread on it from 6 years ago if you are interested. Its a bit into the weeds at first but seems to narrow in on it toward the end.

Meltdown. What happened?

A building where I did a service upgrade a few months ago had a fault in a feeder feeding a transformer. Presumably at about the same time, the 2 C phase conductors broke/melted off the utility transformer secondary terminals. Here are the details: 1000 amp 120/208 overhead to underground...
forums.mikeholt.com
 
jminer99er said:
My understanding with ferro-resonance is typically the cancelation of reactances would blow fuses when re-energizing the xfrms?
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It could be triggered by phase loss, blowing of fuses, and opening or closing switches. All systems are susceptible to ferro-resonance, but some transformer connections are more prone to it.
 
electrofelon said:
I am pretty sure it was the source neutral connection to XO. Here is a thread on it from 6 years ago if you are interested. Its a bit into the weeds at first but seems to narrow in on it toward the end.

Meltdown. What happened?

A building where I did a service upgrade a few months ago had a fault in a feeder feeding a transformer. Presumably at about the same time, the 2 C phase conductors broke/melted off the utility transformer secondary terminals. Here are the details: 1000 amp 120/208 overhead to underground...
forums.mikeholt.com
Click to expand...

Based on that thread, it sounds like you had a phase loss at the utility which consequently caused an overload condition. I don’t think it was due to a primary neutral connection, although I could see that contributing to the meltdown. Think of it this way, it was probably in service with a primary neutral on X0 for a long time with no issues.
 
Xptpcrewx said:
Says who?
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I have always said the primary wye connection should be left floating on a reverse fed transformer unless it is part of a utility grid. This comes from the predominant use of dry type transformers with a single three legged core, to supply 480V delta.

You are correct there is no absolute prohibition against connecting a primary neutral. However, there are many considerations therefore these should be left to installations that have been throughout engineered.
 
jim dungar said:
I have always said the primary wye connection should be left floating on a reverse fed transformer unless it is part of a utility grid. This comes from the predominant use of dry type transformers with a single three legged core, to supply 480V delta.

You are correct there is no absolute prohibition against connecting a primary neutral. However, there are many considerations therefore these should be left to installations that have been throughout engineered.
Click to expand...


As a general rule I agree no one should be installing or connecting a wire on something they don’t understand (which includes using non-standard transformer connections and providing advice to remove the primary neutral), but this rationale is to discourage unqualified persons from working outside of their competency rather than providing a technical reason against utilizing a primary neutral.

Some members seem to be afraid of using the neutral based on a misunderstanding of past experience. It sounds like most cases are a result of primary phase loss and undersized wires. For example:

One advantage of a primary neutral on a wye-primary delta-secondary configuration is you still get balanced secondary voltage when you experience a phase loss upstream. This behaves similar to an open-delta bank and increases current draw on the remaining phases by a factor of sqrt(3). Unless you account for this increased current by adequate transformer capacity and larger primary feeder size, there will be overloading and eventual damage (to the wiring and transformer).

Removing the primary neutral obviously eliminates this possibility and could be one reason everyone thinks it should be removed without not really knowing why…

Just trying to challenge the “I’ve always done it that way” mentality since a few threads on this subject have came up with no apparent resolution.
 
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Xptpcrewx said:
Removing the primary neutral obviously eliminates this possibility and could be one reason everyone thinks it should be removed without not really knowing why…

Just trying to challenge the “I’ve always done it that way” mentality since a few threads on this subject have came up with no apparent resolution.
Click to expand...

As part of the 'don't connect the primary neutral' crowd, I'd appreciate a more extensive education on the situations where one would connect the primary neutral.

On the other hand, if we go back to the original post, it sure appears that the neutral _and_ the EGC are landed on the primary neutral terminal, which strongly suggests to me that this was not an intentional connecting of a primary neutral, but rather a mistaken connection by someone who 'always does it that way' on the secondary of a transformer. Additionally there are questions about the proper grounding of the high voltage side that I don't think have been clearly addressed. So perhaps a discussion of the why or why not to connect the neutral of a primary wye belongs in a different thread :)

-Jon
 
I am inclined to agree that probably someone hooked it up the "same" way they would hook up a step down transformer.

It also looks to me like they landed the bonding wires from the conduits on XO. That would mean there is a connection of the EGC to neutral that should not be there.

Whether the neutral should be run to X0 is another question. I think on the whole leaving it floating is best because there is no downside, at least that I can see, and there are potential downsides to connecting the neutral to the X0.
 
petersonra said:
I am inclined to agree that probably someone hooked it up the "same" way they would hook up a step down transformer.

It also looks to me like they landed the bonding wires from the conduits on XO. That would mean there is a connection of the EGC to neutral that should not be there.

Whether the neutral should be run to X0 is another question. I think on the whole leaving it floating is best because there is no downside, at least that I can see, and there are potential downsides to connecting the neutral to the X0.
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I think you're correct.20210729_091810.jpg
 
This type of lug intentionally deflects a portion of the strands outward to its open slots on the sides in order to withstand higher pull-out forces during a fault.
I agree that the discoloration, also evident at X3, should be investigated further.

Patent Public Search | USPTO

pdfpiw.uspto.gov pdfpiw.uspto.gov
 
Putting aside the bonding on the primary (480V) side and answering the question you CAME HERE to ask; there is no requirement for Medium Voltage (1kV and up) to have a Delta system grounded or to have GF monitoring. That NEC requirement (250.21.B) ONLY pertains to systems below 1000V. That's not to say it isn't a good idea, that's just answering your question as to satisfying NEC requirements.

Most of the Delta MV systems I have worked on are not corner grounded but do have protective relaying that would detect a GF and therefore would meet the same intent as 250.21.B. However in your specific application, i.e. stepping up and back down just for long distances, it is in fact quite common to not have that and do not violate anything the NEC. The first time I came across it (this is done a lot with MV rated down-hole pumps) I had the same concern, so I added a $3k protection relay that nobody wanted to pay for and had the answer beaten into me by my boss after he had to eat the relay cost...
 
Jraef said:
Putting aside the bonding on the primary (480V) side and answering the question you CAME HERE to ask; there is no requirement for Medium Voltage (1kV and up) to have a Delta system grounded or to have GF monitoring. That NEC requirement (250.21.B) ONLY pertains to systems below 1000V. That's not to say it isn't a good idea, that's just answering your question as to satisfying NEC requirements.

Most of the Delta MV systems I have worked on are not corner grounded but do have protective relaying that would detect a GF and therefore would meet the same intent as 250.21.B. However in your specific application, i.e. stepping up and back down just for long distances, it is in fact quite common to not have that.
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That's what I said! It really depends on available fault current. If arc flash is a huge concern, I'd go ungrounded.

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