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Old 12-05-2018, 07:39 AM   #1
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Default Transformer Issue

I went to a job today to check a ground current issue on a new UPS, they were measuring 15.6 amps on a ground conductor. The UPS is 480/480 3-wire with a factory supplied 480 delta to 208/120 wye 150 kva shielded isolation transformer. So I am told I could not verify the transformer type.

There is a ground jumper from the frame of the transformer the bottom of a ground bar, from the ground bar there is a bond jumper to a Ground bar in the UPS and then from the ground bar in the UPS there is a bond jumper back to the top of the ground bar in the transformer cabinet. Current seems to be circulating on the from the transformer frame to the UPS ground bar and back to the transformer.

No load on the secondary, Primary current 15/11/8. With 70% load on the transformer and the ground current went to 22 amps. Utilized a resistive load.

Results were the same with the UPS on and with the UPS in bypass and the UPS off.

Removed the top bond jumper in the transformer to the UPS ground bar and current was still on the bottom jumper in the transformer.

The ground bars are mounted to the metallic frame of the transformer and UPS, so the effect is the same with or without the top ground jumper.

No measurable current on the grounding electrode conductor, Neutral-Ground Bond or any EGCs
Any thoughts?
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Old 12-05-2018, 08:16 AM   #2
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doesn't seem to make sense

are there multiple neutral-ground bonds you didn't find yet ? is it possible that there are other grounding electrodes connected to the ups that you didn't find and have current on them ? does the current stop when the transformer is off (ie, is the current coming from somewhere else, like the frame of the building and from other source ?)

that's it, I'm outta ideas
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Old 12-05-2018, 08:27 AM   #3
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Not really a guess but a question, is the neutral bonded in the isolation transformer or in the UPS?
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Old 12-05-2018, 11:40 AM   #4
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magnetic field inducing a voltage onto the conductor, and then it is circulating back home?
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Old 12-05-2018, 12:01 PM   #5
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You said the UPS is 480 3-wire delta, is it corner grounded? Or is the ground just a bond?

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Old 12-05-2018, 12:29 PM   #6
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The staggered primary currents and the fact that the ground current rises with the load makes me wonder if this is autotransformer connected or a transformerless emi filter like Eaton uses and not delta-wye.
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Old 12-05-2018, 02:37 PM   #7
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[quote]
Quote:
Originally Posted by wildleg View Post
doesn't seem to make sense

are there multiple neutral-ground bonds you didn't find y
Complete investigation and NONE

Quote:
is it possible that there are other grounding electrodes connected to the ups that you didn't find and have current on them?

Quote:
that's it, I'm outta ideasThis is a transformer issue t
he UPS was removed from the scenario.

Quote:
does the current stop when the transformer is off (ie, is the current coming from somewhere else, like the frame of the building and from other source ?)
Yes


There is minimal ground current but unrelated and has been isolated from this issue.
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Old 12-05-2018, 02:40 PM   #8
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Originally Posted by MDShunk View Post
The staggered primary currents and the fact that the ground current rises with the load makes me wonder if this is autotransformer connected or a transformerless emi filter like Eaton uses and not delta-wye.
I have seen from the same supplier the use of an AutoTransformer and based on the physical size and kva rating I would bet this is the case.

It is not a filter.
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Old 12-05-2018, 02:40 PM   #9
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Originally Posted by B-Nabs View Post
You said the UPS is 480 3-wire delta, is it corner grounded? Or is the ground just a bond?

Sent from my SM-G920W8 using Tapatalk
No corner ground,

3-wire input fed from a 4-wire source.
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Old 12-05-2018, 02:51 PM   #10
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Originally Posted by MDShunk View Post
The staggered primary currents and the fact that the ground current rises with the load makes me wonder if this is autotransformer connected or a transformerless emi filter like Eaton uses and not delta-wye.
I have to think about the connection of 3 windings to achieve the 480 to 120 would they use the Ground Conductor for X0 (or neutral in this case? COULD THEY?

Page 23 shows a 3 phase wye auto-transformer.

Not sure why the phase imbalance on the primary.

http://www.ece.iit.edu/~flueck/chica...ansformers.pdf
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Old 12-05-2018, 07:10 PM   #11
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what are voltage readings? a winding may be shorted to the core/shield
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Old 12-05-2018, 08:07 PM   #12
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what are voltage readings? a winding may be shorted to the core/shield

Primary A-B 480, B-C 480, C-A 480
Secondary A-B 208, B-C 208, C-A 208, A, B, C to N 120
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Old 12-05-2018, 11:34 PM   #13
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There's a lot of warnings on ups about grounding and back feeding.

"The UPS must be properly earthed/grounded and due to a high leakage
current, the earthing/grounding conductor must be connected first."

In bypass is the ups unit disconnected and isolated or is it still connected.

"Risk of Voltage Backfeed. Before working on this circuit: Isolate the UPS and
check for hazardous voltage between all terminals including the protective earth"


just seems odd that you said

No measurable current on the grounding electrode conductor, Neutral-Ground Bond or any EGCs

that almost sounds like "high leakage" that wants to return to the source a bit like a vfd will. It would also make sense to use a isolation transformer to keep this local to the ups rather than having it use the ground system to return.

Its just a guess but it might be worth a phone call to ask the manufacturer what you would expect to see as ground current.
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Old 12-06-2018, 02:11 PM   #14
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Can you ground the UPS somewhere else? Even though it's an isolation xfmr. it's still a separately derived system, no?
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Old 12-06-2018, 02:38 PM   #15
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Sounds familiar. Sort of like the problem I am still having. Maybe it is a Wye-wye in the UPS not a delta -wye. That seems to be what everyone thinks my problem is.


So if you take your 22 amps on ground and my 22 amp on ground maybe they will cancel out since I'm up north and you are down south.


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Old 12-06-2018, 03:43 PM   #16
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What does the OEM have to say about this?
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Old 12-06-2018, 04:06 PM   #17
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Originally Posted by brian john View Post
Primary A-B 480, B-C 480, C-A 480
Secondary A-B 208, B-C 208, C-A 208, A, B, C to N 120
i would also measure the primary to ground
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Old 12-06-2018, 05:42 PM   #18
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Originally Posted by brian john View Post
No corner ground,

3-wire input fed from a 4-wire source.
Ya try to tell me that the supply is actually a WYE ??
As far we are aware of Wye - Wye set up and the netural or ground current is flowing across on it. And you mention 15/11/8 that tell me that possiblty a connection is crossed from factory.

Did that unit have any capaitor in there ??
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Old 12-06-2018, 06:18 PM   #19
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Quote:
Originally Posted by gpop View Post
There's a lot of warnings on ups about grounding and back feeding.

"The UPS must be properly earthed/grounded and due to a high leakage
current, the earthing/grounding conductor must be connected first."




Its just a guess but it might be worth a phone call to ask the manufacturer what you would expect to see as ground current.
The UPS was taken out scenario COMPLETLY

They aren't not sure
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Old 12-06-2018, 06:19 PM   #20
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What does the OEM have to say about this?
Nothing yet been called and emailed several times.
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