Transformer Issue

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No load current is 12, 8, 7, which seems unusual as well, normally we see where the two outside coils are high compared to the center winding.

I have pulled my load banks for another job I' will have to see if the customer will pay for a second test.
 
As you already meggared and found insulation resistance of transformer okay, the circulating ground current is probably inductive and abnormal. Why not ask the manufacturer for a replacement?
 
The UPS manufacturer is looking into this issue. I am involved as a subcontractor because we offer electrical testing and often are hired to look into unusual electrical issues.
 
brian john said:
No load current is 12, 8, 7, which seems unusual as well, normally we see where the two outside coils are high compared to the center winding.

I have pulled my load banks for another job I' will have to see if the customer will pay for a second test.
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A Megger test of either a motor or a transformer will detect a line to ground (i.e. line to frame) short inside the windings, bit cannot detect a turn to turn short buried within the winding.
A turn to turn short in one winding will act like an additional load winding on the magnetic circuit in question and will add a resistive component to the current measured for that winding. Depending on whether it is a single or multiple turn short and just what the resistance of the short is, the resulting current could cause asymmetric current and extra heating and maybe noise or could cause enough extra current to trip OCPD.
You can look at it as an autotransformer with a shorted secondary connection.
 
GoldDigger said:
A Megger test of either a motor or a transformer will detect a line to ground (i.e. line to frame) short inside the windings, bit cannot detect a turn to turn short buried within the winding.
A turn to turn short in one winding will act like an additional load winding on the magnetic circuit in question and will add a resistive component to the current measured for that winding. Depending on whether it is a single or multiple turn short and just what the resistance of the short is, the resulting current could cause asymmetric current and extra heating and maybe noise or could cause enough extra current to trip OCPD.
You can look at it as an autotransformer with a shorted secondary connection.
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But his voltage was good.
 
brian john said:
It appears the shield ground is internal and not accessible.
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Is the core strap accessible? if so for the sake of process of elimination, temporarily unground it with no load and confirm this is the problem area. it sounds to me like the source feeding this xfmr is corner grounded secondary, is this correct? whether low impedance or not. i am questioning the insulation resistance test results, it just sounds exactly like a primary winding is shorted and your source is impedance grounded. textbook. i hate asking you because i know you are light-years ahead of me in the testing realm but what voltage did you use primary to core/ground(if core is grounded)?
 
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Wire-Smith said:
Is the core strap accessible?
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No

if so for the sake of process of elimination, temporarily unground it with no load and confirm this is the problem area. it sounds to me like the source feeding this xfmr is corner grounded secondary, is this correct?
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Supply is from a solidly grounded wye



whether low impedance or not. i am questioning the insulation resistance test results, it just sounds exactly like a primary winding is shorted and your source is impedance grounded. textbook. i hate asking you because i know you are light-years ahead of me in the testing realm but what voltage did you use primary to core/ground(if core is grounded)?
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We insulation resistance tested at 1000 VDC

The manufacturer is pulling an identical transformer out of stock and performing some test at the factory. If the electrical contractor can get approval from his customer I would like to perform another load test monitoring primary and secondary voltage and current (phase, neutral and ground current)
 
brian john said:
Supply is from a solidly grounded wye



We insulation resistance tested at 1000 VDC

The manufacturer is pulling an identical transformer out of stock and performing some test at the factory. If the electrical contractor can get approval from his customer I would like to perform another load test monitoring primary and secondary voltage and current (phase, neutral and ground current)
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Gotcha,

Now i am thinking the laminations are unintentionally grounded at two points and allowing circulating current in the surrounding connected metal material. I'm not familiar with shields but it could be something like that with the shield as well. by my understanding of the description given with no accessible ground strap i have no idea how to check this without some dissection.
 
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brian john said:
The manufacturer is pulling an identical transformer out of stock and performing some test at the factory. If the electrical contractor can get approval from his customer I would like to perform another load test monitoring primary and secondary voltage and current (phase, neutral and ground current)
Click to expand...
I am certainly interested to hear about the findings when finished.
 
As a subcontractor, I am not on the job to do any further testing but the electrician on site is filling me in. The manufacturer set up an identical transformer in the same setup, they had current on the frame ground conductor, they rerouted this conductor and the ground current dropped.

The electrical contractor rerouted the conductor and the current dropped from 15.9 amps to 11.7 amps. At the factory, they were able to lower the amperage to 10.0 amps. The manufacturer feels this level of current is acceptable but not say why it is acceptable.

I asked the electrician to extend the ground conductor which would allow him to move the conductor in different directions and see if how the current changes.

The end user wants a letter from the manufacturer stating this is an acceptable level of current.


I would like to get back on site to load test the UPS and transformer and watch this current at different load levels as well as move the conductor as load levels are stepped up and see how that affects the current compared to the primary current..
 
It may also be clarified from the manufacturer whether the transformer shield is grounded at more than one point in which case objectionable induced current would exist.
 
Thanks for the update. Please share additional info if you get any.

Got any pics that might show how the ground wire is being moved around and how it might be interacting with the fields? Must be cutting field lines somewhere to get the induced current but why would the design have it in the way to start with?
 
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