what are the reasons for abnormal (at no load) heating of power transformers?

Question

a power transformer is showing abnormal temperature rise (oil temp) even at no load. its all electrical test results are satisfactory.

Answer 1

If you are certain that all electrical tests are satisfactory, check for non-electrical problems:

Check the oil temperature meter.
Check for inadequate air flow around the transformer.
Check for loss of oil.

Answer 2

from the link:
No-load losses are caused by the magnetizing current needed to energize the core of the transformer, and do not vary according to the loading on the transformer. They are constant and occur 24 hours a day, 365 days a year, regardless of the load, hence the term no-load losses. They can be categorized into five components: hysteresis losses in the core laminations, eddy current losses in the core laminations, I2R losses due to no-load current, stray eddy current losses in core clamps, bolts and other core components, and dielectric losses. Hysteresis losses and eddy current losses contribute over 99% of the no-load losses, while stray eddy current, dielectric losses, and I2R losses due to no-load current are small and consequently often neglected. Thinner lamination of the core steel reduces eddy current losses.

The biggest contributor to no-load losses is hysteresis losses. Hysteresis losses come from the molecules in the core laminations resisting being magnetized and demagnetized by the alternating magnetic field. This resistance by the molecules causes friction that results in heat. The Greek word, hysteresis, means "to lag" and refers to the fact that the magnetic flux lags behind the magnetic force. Choice of size and type of core material reduces hysteresis losses.

Answer 3

what's the reading of your abnormal temperature?
is it beyond the temperature rise of your transformer?
what tests did you do that satisfied you?

Answer 4

Have you checked for harmonics in the primary supply?
Clogged oil passages?