Transformer No Load Loss at Different Temperature

Simply because no load temperature is barely higher than the ambient temperature.

Unless, of course, that you are always working at the highest temperature using some sort of controlled refrigeration.

It might be a negligible amount of variation...if you are operating within the recommended operating parameters....

..."Power transformer losses are a combination of the power dissipated by the core's magnetizing inductance (iron loss) and the winding's impedance (copper loss). Iron losses are a function of the applied voltage and are often referred to as "no-load losses" - they are induced even when there is no load current. Copper losses are a function of the winding current and are often referred to as "load losses". These losses are calculated for any operating condition if a few parameters of the power transformer are known. The transformer manufacturer commonly provides this information on the transformer test sheet:

 rated total kVA of the power transformer (VATXtest).

 rated voltage of the power transformer (VTXtest).

 No-load test watts (LWFeTXtest) - the active power consumed by the transformer's core at the rated voltage with no load current (open circuit test).

 Full-load test watts (LWCuTXtest) - the active power consumed by the transformer windings at full load current for rated kVA (short circuit test).

 %Excitation current - ratio of No-load test current (at rated voltage) to full load current.

 %Impedance - ratio of Full-load test voltage (at rated current) to rated voltage.

The No-Load and Full-Load VAR losses (LVFeTXtest and LVCuTXtest) may not be provided, but are calculated from the above data."...

http://static.schneider-electric.us/docs/Energy%20Solutions/Offers%20by%20Market%20Segment/Utility/70072-0153-08%20Technical%20Note%20Transformer.pdf

But why No load is not corrected in the same way, irrespective of different ?

Loaded transformer losses are due to copper resistance which is temperature dependent. No load losses are due to iron core losses which are not temperature dependent.

"Causes of transformer loss Power transformer losses are a combination of the power dissipated by the core's magnetizing inductance (iron loss) and the winding's impedance (copper loss). Iron losses are a function of the applied voltage and are often referred to as "no-load losses" - they are induced even when there is no load current. Copper losses are a function of the winding current and are often referred to as "load losses". "

http://static.schneider-electric.us/docs/Energy%20Solutions/Offers%20by%20Market%20Segment/Utility/70072-0153-08%20Technical%20Note%20Transformer.pdf (Sorry, link no longer available.)

The iron inductance losses are still temperature dependent both due to temperature dependence of magnetic hysteresis and eddy current production and dissipation. I suspect that the difference is low enough to be considered not significant for rating purposes.