Transformer %IZ Change
Transformer %IZ Change
(OP)
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".
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".





RE: Transformer %IZ Change
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.
RE: Transformer %IZ Change
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.
RE: Transformer %IZ Change
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.