Transformer Impedance and Operating Temperature
Transformer Impedance and Operating Temperature
(OP)
Hello,
Can anyone tell me why a mobile substation transformer can operate at higher temperatures than pad mounted (up to 95°)?
Can anyone tell me why a mobile substation transformer can have impedance as high as 18%?
Thanks for any help!
Can anyone tell me why a mobile substation transformer can operate at higher temperatures than pad mounted (up to 95°)?
Can anyone tell me why a mobile substation transformer can have impedance as high as 18%?
Thanks for any help!






RE: Transformer Impedance and Operating Temperature
Higher temperatures generally mean either shorter insulation life for a given insulation type. Most pad-type transformers are expected to have a 40 year life so the operating temperatures reflect this. Some dry type transformers can run at Class H limits which is blistering hot, far above where an oil-filled design would operate, but they have their own problems.
The impedance is a factor of the winding and core design. 18% is fairly high and would normally seen be on a medium-to-large transformer equipped with a tapchanger to regulate the secondary voltage. Impedance is usually chosen to limit the fault current to within values the switchgear and other equipment can handle.
RE: Transformer Impedance and Operating Temperature
Thanks for the information, I am trying to understand why it would be higher for this application. Maybe because of the shorter time period it is in service?
RE: Transformer Impedance and Operating Temperature
If a transformer with 12% impedance has the KVA rating increased 50% by allowing higher operating temperatures, the impedance doesn't change (much) but the per unit impedance will increase to 18%.
Bill
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"Why not the best?"
Jimmy Carter
RE: Transformer Impedance and Operating Temperature
RE: Transformer Impedance and Operating Temperature
RE: Transformer Impedance and Operating Temperature
Since the unit will be working for short periods only, higher current density is used (with consequent higher winding temperature rise) to reduce the copper weight. Of course it will shorten paper life, but any way it is running only for short periods.Energy efficiency is not important for such short period loadings. Many times nomex of higher thermal class is used as conductor insulation for such trfs.