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).]
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).]
Did you do a TTR test? What were the phase current readings? You probally have either unbalanced loads or open/shorted windings.
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).]
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
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.
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.
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).]
RODALCO - It is a dry xmfr. We have advised the customer the unit should be checked as you indicated, we are still awaiting approval that I suspect will never come. Their motto is; "if it still works, leave it alone."