Transformer Design 4

[safy2004]

I want to give the designer our requirements of MV power transformer, I need some clarifications about the ambient temperature and the temperature rise.
According to IEC, the recommended limits of the temperature are:
Maximum ambient temperature = 40 C
Maximum top oil temperature rise = 60 C
Maximum winding average temperature rise = 65 C
Which means:
Absolute top oil temperature = 100 C
Absolute winding average temperature = 105 C
In the area where we want to use this transformer, the ambient temperature reach 55 C for about six hours a day in summer months.
In our case is it better to use transformer with:
Maximum ambient temperature = 50 C
Maximum top oil temperature rise = 50 C
Maximum winding average temperature rise = 55 C
Or use transformer with:
Maximum ambient temperature = 55 C
Maximum top oil temperature rise = 45 C
Maximum winding average temperature rise = 50 C
Thanks for every one will reply.

 

[RRaghunath]

Safy2004,
The transformer design, if to be based on max ambient temperature, will push up the cost unnecessarily.
The relevant ambient temperature for design of electrical equipment in general is the weighted average temperature.
I don't recollect the IEC no. readily but there is an IEC which explains how the design ambient temperature for electrical equipment is to be computed.
The 40degC specified in IEC generally holds good in most parts of the world except in desert conditions probably.

 

[Marmite]

This is an extract from 60076-2 Temperature rise of power transformers:-
Normal ambient temperature limits (– 25 °C and + 40 °C) for power transformers are given in 1.2 of
IEC 60076-1. With regard to cooling of air-cooled transformers the temperature conditions at the intended
installation site should neither exceed:
+ 30 °C monthly average, of the hottest month; nor
+ 20 °C yearly average.
If the temperature conditions at site exceed one of these limits, the specified temperature-rise limits for the
transformer shall all be reduced by the same amount as the excess. The figures shall be rounded to nearest
whole numbers of degrees.
NOTE The average temperatures are to be derived from meteorological data as follows (IEC 76-1, definition 3.12).
Monthly average temperature:
half the sum of the average of the daily maxima and the average of the daily minima during a particular month, over many years;
Yearly average temperature:
one-twelfth of the sum of the monthly average temperatures.

Regards
Marmite

 

[RRaghunath]

Thanks, Marmite. The post is informative.

 

[msteen]

The ambient temperature rating is a 24 hour average. In desert applications we have succesfully installed transformers at 55 degree ambient temperatures and I specify a 45 degree rise on a 50 degree ambient. If your loads are linear and there are no expectations for large harmonic content you could apply a standard 55 degree rise on a 40 degree ambient.