Transformer Rated Ambient Temperature De-rating

[ThePunisher]

Hi all,

Table 3 IEEE C57.91-2011 indicates

1.5% decrease of load for each 1 Deg C increase in ambient temperature beyond 30 deg C for ONAN and

1% decrease of load for each 1 Deg C increase in ambient temperature beyond 30 deg C for ONAN/ONAF

My interpretation would be say, for a 30 Dg. C rated transformer applied to a 40 Deg C ambient, the total de-rating would be 100% - (10 x 1.5%) = 85%

Therefore, the transformer site rating is = Nameplate kVA at 30 Deg C * 0.85. Is this correct?

My application is here in Canada and I am looking into the CSA C88. However, I seem not to see any standard rated ambient temperature specified there. Are CSA manufactured liquid-immersed power transformers also 30 Deg C ambient rated?

Our Meteorilogical Data specifies 36 Deg C maximum ambient on summer, is it ok to consider 40 Deg C for transformer and cable operating ambient for design purpose?

Thanks

 

[dpc]

The ANSI ambient temperature rating for transformers is based on a 24 hr average temperature - not the peak, IIRC. There's a huge thermal mass in an oil-filled transformer and it take a while for the ambient temperature changes to have much impact.

Also, if this is a large new transformer, you can specify any ambient you want.

 

[prc]

As per ANSI C57.12.00-2006, the average winding temperature rise of 65C and hot spot winding temperature rise of 80C is specified based on a maximum ambient air temp of 40C and average daily ambient temperature of 30C( 24 hour period)

For ambient conditions other than above, C57.12.91-2011 Loading guide, recommends as below for ambient temperatures from -30C to 50C.

Clause 6.3 &Table 3, for every degree reduction in average ambient temperature the rating can be increased by 1% of kVA rating and for every one degree rise in average ambient temperature, the load shall be decreased by 1.5 % of kVA rating. This is approximate variation in rating and for detailed calculation reference may be made to other clauses in C57.12.91

So you have to find out the average daily ambient temperature for your site.