Transformer Data

[Hileg]

What does it mean for a transformer to have a rating of 1500kva @ 55 deg c and another rating @ 1680Kva at 65 deg c.
I would have thought the cooler temp. would have the higher rating.
Thanks

 

[Sanditech]

The simple reason the transformer has a higher kVA rating is that is allowed to run at a higher temperature, by taking more current the transformer will produce more heat losses I2t etc.
Provided the unit has a dual rating design it should be able to run at the higher load. Cast resin units are popular with dual ratings and usally include cooling fans to achieve there maximum rated duty.
I suspect that if such units were run all the time at the top end ther life span would be reduced.
The upper rating is more of an intermittent duty cycle.

 

[busbar]

One reference is IEEE Std C57.91-1995 Guide for Loading Mineral-Oil-Immersed Transformers

 

[jbartos]

Comment on Sanditech (Electrical) Feb 6, 2004 marked ///\\The simple reason the transformer has a higher kVA rating is that is allowed to run at a higher temperature, by taking more current the transformer will produce more heat losses I2t etc.
///Please, would you clarify the heat losses I2t, which seem to be having unit Ampere^2 x time rather than Watts x time.\\

 

[alehman]

Power loss to heat in the windings is I^2*R = watts.

Many liquid insulated transformers are rated 55/65C rise with two kVA ratings. Additional ratings are commonly available when cooling fans are applied on the radiators or oil pumps are used to improve circulation.

Solid dielectric transformers typically have a single rating unless provided with fan cooling. Temperature rise ratings are commonly 80, 115 or 150C.

 

[jbartos]

Suggestion: The air cooled dry-type transformers also have temperature ratings.
Reference:
IEEE Std 141-1993 Red Book
Section: 10.4 Transformers

 

[jbartos]

Comment on Hileg (Electrical) Feb 6, 2004 marked ///\\What does it mean for a transformer to have a rating of 1500kva @ 55 deg c and another rating @ 1680Kva at 65 deg c.
///Liquid-immersed transformers may be specified with a 55³C/65³C rise to permit 100% loading with 55°C rise and 112% loading at 65°C rise. In addition, 115°C rise, high-fire-point, liquid-immersed transformers are available from some manufacturers.\\