high current on grounds at transformer secondary

[dch921]

The place I work we have a pair of 480/208 3 phase transformers delta-wye. These are tied together with a tie switch in case of failure of one of the transformers. Current have loads of 300+ amps on one and 150+ on the other(secondary sides).

We have come to find high current on the main grounds that are bonded to the secondary neutrals.

Transformer 1 has around 40amps on its ground and #2 has 60amps on its ground.

From our main panels on each transformer to the tie switch panel we have around 10 amps on each ground. From the tie switch grounds to each transformer is where we are seeing the high current.

So far have not been able to find any ground fault issues and not even sure how long this have been a problem. Only started looking into it after seeing neutrals that would spark off to ground. Any ideas or help would be great

 

[ggunn]

The place I work we have a pair of 480/208 3 phase transformers delta-wye. These are tied together with a tie switch in case of failure of one of the transformers. Current have loads of 300+ amps on one and 150+ on the other(secondary sides).

We have come to find high current on the main grounds that are bonded to the secondary neutrals.

Transformer 1 has around 40amps on its ground and #2 has 60amps on its ground.

From our main panels on each transformer to the tie switch panel we have around 10 amps on each ground. From the tie switch grounds to each transformer is where we are seeing the high current.

So far have not been able to find any ground fault issues and not even sure how long this have been a problem. Only started looking into it after seeing neutrals that would spark off to ground. Any ideas or help would be great

Maybe a stupid question but I have to ask - the switch isn't closed, is it?

 

[don_resqcapt19]

First, with a grounded system, it is normal to see a spark if you touch the neutral to ground. You are creating a parallel path for the neutral current via the ground and the system bonding jumper. The amount of current that you have on the grounding conductor appears to be excessive.
One thing that will cause current on the grounding conductor is neutral to ground connections on the load side of the system bonding jumper.

 

[GoldDigger]

So far have not been able to find any ground fault issues and not even sure how long this have been a problem. Only started looking into it after seeing neutrals that would spark off to ground. Any ideas or help would be great

1. Is there a high current in the neutral normally? That is, is the load on the wye unbalanced? If so, you need to take precautions to confine that neutral current to the grounded conductor(s) and you must make sure that they are adequately sized and tightly connected.
2. If there is a high neutral current, then you will have current in the ground system if there is a neutral to ground bond anywhere other than at the transformer. This will not show up as a ground fault if you are just looking for phase to ground connections, but it will result in the normal neutral current dividing between the grounded conductor and the EGC or other ground connection.
3. This can result in high ground currents, but not dangerously high voltages. If you are seeing high voltages to ground on any neutral wires, it is an indication that you have an open or high resistance neutral, forcing the center point of the wye at the load(s) to move away from ground potential.

An open neutral, by itself, will not necessarily show up as a ground fault via a current detector in a wye system since the current in the three phase wires can still be balanced even though no current flows in the neutral. But once you allow current to flow through the ground system instead, it should trip a Residual Current Device (RCD) or Ground Fault Detector (GFD).

 

[dch921]

Yes loads are unbalanced.

Xmfr 1readings
A 235a
B 196a
C 357a

Xmfr 2
A 120
B 125
C 59

The grounds from the neutrals of both transfers are bonded together in the panel with the tie switch. That ground is then bonded to each main Low Voltage distribution panel. The neutrals from both transformers are also bonded together in the tie switch panel.

Since finding this issue we have be trying to figure out the best course of action to correct this. One thing we are unsure about is what affect the neutrals being bonded together from 2 different transfer causes.

 

[GoldDigger]

Yes loads are unbalanced.

Xmfr 1readings
A 235a
B 196a
C 357a

Xmfr 2
A 120
B 125
C 59

The grounds from the neutrals of both transfers are bonded together in the panel with the tie switch. That ground is then bonded to each main Low Voltage distribution panel. The neutrals from both transformers are also bonded together in the tie switch panel.

Since finding this issue we have be trying to figure out the best course of action to correct this. One thing we are unsure about is what affect the neutrals being bonded together from 2 different transfer causes.

With the tie switch open, there should not be any current between the two transformer secondary windings themselves, but you are providing several alternate paths through the grounding/bonding system for neutral current going back to whichever transformer is sourcing the current in question. Multiple neutral-ground bonds should be avoided on the customer side of the service disconnect or separately derived system. This is an argument for switched neutrals when using a transfer switch rather than a tie switch, but you are not in that situation.

 

[cadpoint]

Why don't you have the transformers tested? IMO you'd have too have a power study done both actual and mathematical to understand where your picking up this current.

 

[GoldDigger]

Why don't you have the transformers tested? IMO you'd have too have a power study done both actual and mathematical to understand where your picking up this current.

If there is no ground current when there are no loads connected to the transformer(s), then you can pretty well rule them out as a cause.
If there is ground current which is independent of the loads connected to the transformers, then either it is originating in the transformers or is being introduced into the ground system from another supply/source.

Unfortunately you cannot dye-color the electrons leaving each transformer to see what color comes back on the ground. Plumbers do have some advantages.

Since we know that you have a neutral conductor current because of your unbalanced wye loading, what you need to do is figure out where it is leaving the neutral conductor and getting onto a ground conductor.
If there is a high impedance neutral somewhere, then the ground system may provide an alternate path and so carry current. But that would require multiple ground/neutral bonds in some form.
One more thing to look at is whether the GEC current is still there when you de-energize the primary of the transformer.

 

[mgookin]

...

Unfortunately you cannot dye-color the electrons leaving each transformer to see what color comes back on the ground....

Not yet, but we're getting close!

 

[cadpoint]

The Op has no point of reference, as I read their second post.

I can almost guess that there is not a complete set of documents on the job. IE no road map and no records.

I'll also assume that the problem surfaced due to the plant now requiring some documenting that has never happened before.

But alais, far be it for me to assume anything here.

 

[GoldDigger]

I'll also assume that the problem surfaced due to the plant now requiring some documenting that has never happened before.

But alais, far be it for me to assume anything here.

Well, the OP said that it started when they noticed neutrals sparKing to ground. Which is not necessarily a bad thing by itself.

 

[dch921]

The Op has no point of reference, as I read their second post.

I can almost guess that there is not a complete set of documents on the job. IE no road map and no records.

I'll also assume that the problem surfaced due to the plant now requiring some documenting that has never happened before.

But alais, far be it for me to assume anything here.

This all started when an electrician that was wiring up some solenoids said the neutral would spark off to ground. These come off of a 120vac output card on a PLC in a PLC panel, then to a breaker then a ct then out to the field. This is a new panel and the person thought I might have powered up the panel and came to check. The entire panel was off. We then assumed must be some inductance since the shields on the tray cable hadn't all been landed yet. I checked voltage between the neutral and ground and only saw 3 vac. Thinking it was odd to see a spark like that with only 3 vac I jumped the ground to the neutral in the panel and had put an amp clmap on it and got 26Amps.

This is when I realized something didn't seem right. From there we went back to our source at the transformer to find that we had high current on our grounds going to each transformer.

This low voltage system has been up and running for 5+ years like this, and since we have found an issue with it we are working to resolve it correctly. This isn't because of documentation or anything like that.

Our current plans are to run a new ground from the neutral on each transformer to out main grounding electrode. Then to remove any parallel paths to ground. We have begun to inspect some of the low voltage panels but with 40+ panels it will take a few days.

Powering down one of the transformers is possible with some planning the other transformer is supplied with 480v UPS power and handles all of our controls which makes it difficult to unload.

Everyone that has helped thanks. Has given me some good ideas to follow up on.