Transformer %IZ Change

[jwilson3]

We have a 5000KVA 12.47kv-480v delta/wye transformer in a hospital that was recently tested. The Percent Impedance test indicated an average of 9.5% with a calculated X/R = 1.6. The nameplate %IZ is 6.07%. The typical X/R for a power transformer of this size is closer to 9.

Another transformer, a 3000KVA unit, also tested high, but not to the same extent.

What could cause this change? The units have not been subjected to a significant "thru-fault".

 

[nagelec]

Make sure that the tested %Z value and name plate rating is on same MVA base first.
The voltage drop across the transformer will increase and the through fault current will decrease if the %Z increases. If you have large motors in the system, better check the voltage drop during stsrting and running conditions.
I hope that I clarified your doubt.

 

[peebee]

You might have better luck with this question in the Electric Power Engineering forum.

Was there any factory or field test prior to putting the transformer into service to validate the nameplate impedance? How reliable do you believe the recent test data to be?

Something strange is going on.

 

[jwilson3]

To peebee: Yes, you are right. I put it here by mistake.
I'm sure there were factory tests, but I wasn't involved at that time. It seems to be a simple enough test, I don't know how they could have messed it up, but I do doubt that the %IZ has changed that much.