Transformer Issue

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brian john

brian john

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Leesburg, VA
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?
 
Is the UPS connected to supply or checked during isolation? What are voltage readings, phase-phase and phase-ground? What currents do you get with the transformer de-energized?
 
I noticed you said no EGC currents so that may eliminate common mode signals.

Check the voltages to see if you might have a partial fault in the transformer. You might meg it also.
 
This was brought up on another forum.

I think the transformer may be an autotransformer


Not sure the practical reason for an autotransformer for a voltage change of 480 to 208/120? Seems the copper and steel required would be same as a Delta Wye or close.

Have to think about the XO would that be ground on the primary
 
brian john said:
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.
Click to expand...
What is the 480/208 transformer for?
 
How is the static bypass fed? Is it 480V and fed to its SS on the primary side of the XFMR or 208V onto the secondary?

Since you said it was a factory transformer, that why the bypass voltage is important, because as you know there is a N-G bond up at that source that is providing the needed grounding.
 
ron said:
How is the static bypass fed? Is it 480V and fed to its SS on the primary side of the XFMR or 208V onto the secondary?

Since you said it was a factory transformer, that why the bypass voltage is important, because as you know there is a N-G bond up at that source that is providing the needed grounding.
Click to expand...


The Bypass is on the primary and is 3-wire as well. Isolated the transformer from the UPS, with no connection to the UPS Bypass the issue with the transformer secondary
still existed
 
We had a similar situation with the UPS powered down. Almost 40 amps in our case.
We found main electrical system neutral connected to ground in 7 different locations.

Once they were removed this current disappeared.

Good Luck:D
 
Grounds were checked throughout the distribution system and minimal ground current 2-3 amps. The levels exterior to the transformer remain the same with the transformer on or off.
 
Update to this ground current issue than a question


The transformer was meggered and TTR'ed (Turns Ratio Tested) no problems there.




The transformer is a 480 to 208/120 transformer Delta Wye.


The distribution system is solidly grounded.


The excitation current is A-12, B-8, C-7 Amps, Generally, I believe we see A and C phase higher than B phase.


At no load, the current on the frame ground is 15.9 amps. This current is only measured on the frame ground conductor installed by the factory


No one including the manufacturer can give me an answer.


There is a sticker I noticed today that says due to high leakage current this transformer must be grounded.


Now my question


The manufacturer says to ground the UPS one way if the distribution system is high impedance grounded and ground another way if low impedance grounded. THE UPS IS NOT THE SOURCE OF THE GROUND CURRENT, this has been ruled out by isolating the transformer from the UPS. But I still have a question.


I believe that a low impedance ground is the same as solidly grounded the engineer says no it is similar to a high impedance grounded system just a different value impedance, the manufacturer says that is a design issue and not their concern.


Thoughts?
 
brian john said:
The manufacturer says to ground the UPS one way if the distribution system is high impedance grounded and ground another way if low impedance grounded. THE UPS IS NOT THE SOURCE OF THE GROUND CURRENT, this has been ruled out by isolating the transformer from the UPS. But I still have a question.


I believe that a low impedance ground is the same as solidly grounded the engineer says no it is similar to a high impedance grounded system just a different value impedance, the manufacturer says that is a design issue and not their concern.


Thoughts?
Click to expand...
For generator protection, a low impedance ground is similar to a high impedance ground. Low impedance uses a series resistor in the ground path. A high impedance ground uses a grounding transformer with a resistor on the secondary side that reflects impedance to the primary/ground path side (this is what I am used to). Both will clamp ground faults to levels much lower than a solid ground so are similar in that respect. Think of solid as hundreds to thousands of amps, low as tens to hundreds, and high as tens of amps or less.

It sure sounds like a shield drain issue since the other data does not indicate a fault.
Not sure what is up with the transformer. ​
Do you have a transformer manufacturer contact? I usually don't use shielded units but could ask our rep and he might could hunt down an applications engineer at the plant.
 
mivey said:
It sure sounds like a shield drain issue since the other data does not indicate a fault.
Not sure what is up with the transformer. ​
Do you have a transformer manufacturer contact? I usually don't use shielded units but could ask our rep and he might could hunt down an applications engineer at the plant.
Click to expand...


Tamura Elcomponics Technologies Pvy. Ltd
Pandithanahalli, Karnataka 572104, India

The weird thing about the nameplate, It states Primary 480 Secondary 208, Transformer is a Delta Wye 480 to 208/120. Generally, I would expect the name plate to state 208/120. on a Delta Wye.


 
As mivey pointed out, the issue may be with transformer shield. Is it possible to disconnect the shield from ground temporarily?
 
brian john said:
There is a sticker I noticed today that says due to high leakage current this transformer must be grounded.

Thoughts?
Click to expand...

The leakage current may be due to electronics circuitry in UPS such as filter capacitor and flows to ground via EGC.But the ground current you are observing is not flowing in EGC but circulating and so abnormal.
 
The transformer was isolated from the UPS no connections. The same level current exists when the transformer is totally isolated, in Bypass, on-UPS and on battery.
 
We may check whether it is an induced current. Measure no load transformer current and the ground current. Calculate the ratio between the two. Load the UPS. Measure transformer current and ground current and calculate the ratio for these values also. If the ratios are close, the ground current is induced one.
 
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