Transformer Temperature rise rating 2

I’m used to seeing a maximum of a 65C rise on large liquid immersed power transformers, air cooled or water cooled.

55 deg C is top oil temperature and 65 deg C is indirect winding temperature rise via CT and a resistor.

Muthu

1)A temperature rating of 55/68 C was never used. From 1950 to 1980, transformers in the US were rated 55/65 C based on the average winding temperature rise of 55 C for transformers with ordinary paper for conductor insulation and 65 C for thermally upgraded paper. Later, only 65 C was used.
2) Today, the IEC standard 60076-2 dictates the allowable temperature rises for transformers. These are 60 C for oil, 65 C for average winding with non-directed oil flow, and 70 C for directed forced cooling. The upper limit for the rise of winding hot spot temperatures is 78C.
IEEE says the top oil and average winding rise limits are 65C, and the hot spot rise limit is 80C.

Cooling should lower the oil temperature, not raise it. WE specify ratings for both 55 and 65 and for various cooling methods. Clearly the insulation must be suitable for 65, but we stay much lower for longevity. The extra capacity is there if needed for emergency.

Having cooling fully in service allows operation to the higher figure.

I suspect the 68 is just a mis-read or mis-stamp of 65; I've never seen 68 either.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

prc

Aren't 55/65 deg C OTI and WTI alarm limits?


Muthu

The OTI and WTI alarm limits are decided based on the maximum ambient temperature of the particular location plus actual top oil rise and winding hot spot temperature rise during the temperature rise tests.

Maximum ambient air temperatures per IEC/IEE/IS are 40/40 and 50C. The average winding temperature rise was 55/65 C. While the US had these double values for winding temperature rises, the British Standard followed 55/65 C for top oil and average winding rises, and India followed 45/55C due to 10 C higher average weighted ambient air temperature.

The latest guarantee temperature rise figures are as per my earlier post.