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#66471 - 06/05/06 04:01 AM Odd transformer reading
LearJet9 Offline
Member

Registered: 04/24/05
Posts: 196
Loc: Millis, MA USA
While checking a SqD 75KVA 480Pri - 208/120Sec transformer I got some strange readings. (All readings taken several times with more than 1 [TRMS] meter)
A=109.1 B=120.4 C=132.2
AB=208.1 AC=209 BC=208.4
I've not seen this much of a difference on the low side before. Any ideas??

[This message has been edited by LearJet9 (edited 06-05-2006).]

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#66472 - 06/05/06 04:26 AM Re: Odd transformer reading
e57 Offline
Member

Registered: 05/27/03
Posts: 2837
Loc: S.F.,CA USA
Taps in right locations? A could move up, C could move down if taps are available to do so.

Or did this "just" happen on its own? Maybe lost a few windings between A-C.

Is the high side the same way?

[This message has been edited by e57 (edited 06-05-2006).]
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Mark Heller
"Well - I oughta....." -Jackie Gleason

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#66473 - 06/05/06 05:09 AM Re: Odd transformer reading
Zog Offline
Member

Registered: 09/13/05
Posts: 116
Loc: Charlotte, NC
Did you do a TTR test? What were the phase current readings? You probally have either unbalanced loads or open/shorted windings.
_________________________
MV/HV Testing Specialist, "BKRMAN"

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#66474 - 06/05/06 05:28 AM Re: Odd transformer reading
togol Offline
Member

Registered: 09/15/05
Posts: 426
Loc: NW In. USA
gotta good neutral?
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#66475 - 06/05/06 08:10 AM Re: Odd transformer reading
LearJet9 Offline
Member

Registered: 04/24/05
Posts: 196
Loc: Millis, MA USA
Unknown on the neutral at the xmfr but a good neutral in the panel we accessed. No TTR test. High side OK 277/278/277. Don't know when it happened, could have been that way since the original installation 4 years ago. We were there to T.S. a problem with the sign and found these readings downstream from the xmfr. We did not have access to the xfmr itself. I just thought that was odd to get a good 208 but not on each phase. So, the taps effect the individual phase but not the 208??

Added note: We had access to view the xmfr, but not to open or work on it.

[This message has been edited by LearJet9 (edited 06-05-2006).]

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#66476 - 06/05/06 09:35 AM Re: Odd transformer reading
Radar Offline
Member

Registered: 04/30/04
Posts: 349
Loc: Los Angeles, CA
Not so odd - I've seen similar readings (actually larger variations) on a buncha new transformers just installed where the installer did not realize he needed to bond the neutrals and the grounds. He assumed they just came that way, ready to go. Phase to phase readings look OK, phase to ground readings vary according to relative load imballance on the particular transformer. In our case, we had installed 10 of these things, all were unbonded. We got some really wierd readings.

You need to have a look into that transformer, check out the N-G bonding.

Radar
_________________________
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#66477 - 06/05/06 04:39 PM Re: Odd transformer reading
e57 Offline
Member

Registered: 05/27/03
Posts: 2837
Loc: S.F.,CA USA
 Quote:
Unbalanced secondary voltages:
Overload; tap connections not on identical tap positions; neutral ungrounded

Re: Page 26 of Squar D manual
_________________________
Mark Heller
"Well - I oughta....." -Jackie Gleason

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#66478 - 06/05/06 05:08 PM Re: Odd transformer reading
luckyshadow Offline
Member

Registered: 01/04/05
Posts: 305
Loc: Maryland USA
I had a transformer giving me goofy readings - It turned out to be XO was not bonded . Tightened the XO Bond and everything straightened right up.
If I was questioning the voltage readings I would test them at the transformer if possible.

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#66479 - 06/06/06 02:45 AM Re: Odd transformer reading
LearJet9 Offline
Member

Registered: 04/24/05
Posts: 196
Loc: Millis, MA USA
We told the customer they should have the terminations checked at the xmfr. However, their general philosophy of; "If it isn't affecting business, who cares." Will win out I'm sure.

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#66480 - 06/06/06 03:52 AM Re: Odd transformer reading
RODALCO Offline
Member

Registered: 12/08/05
Posts: 863
Loc: Titirangi, Akld, New Zealand
Hi, you need to get access to the transformer itself.

1. Check neutral, earth connections from the TF.
2. Also do a current and voltage check on each phase and check for overload at the TF.
3. Check if tappings are set correctly and not one tap has been plugged or connected differently than an other tap.
4. Check if there are any signs of overheating, discoloured windings, smell of hot varnish.
5. Check temperature of TF, it will certainly tell you if it's overloaded.
6. Do an insulation check on TF if they allowe you to shut it down. All wires disconnected. ( make a note of where the wires go before you take them off )

BTW is this a dry TF or oil filled TF?

If oil, check oil level.

edited for typo's RF.

[This message has been edited by RODALCO (edited 06-06-2006).]
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The product of rotation, excitation and flux produces electricty.

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