Max. temperature oil-powertrafo at full load

[JETO]

What is a maximum practicle value (°C) for an oil-trafo
5 MVA (natural cooling) at full load.
The transfomer has never been at full load for 35 years.
The values given by different manufactors seems to me very
low.

Thanks, JETO - Belgium

 

[DanDel]

First, check the nameplate value for max rise above ambient.

 

[JETO]

DanDel,

The only information on the nameplate is the trafo-impedance
at 75°C ( 4.8 %).

Thanks for responding

 

[DanDel]

Are you sure there is no Temp Rise value on the nameplate? Sometimes it is near the manufacturer's info. It is not necessarily specific to each transformer, and may be part of the 'boilerplate'.

 

[gordonl]

Mineral Oil Transformers are generally rated for 65C rise above ambient, although 55C rise is common to give extended life and available margin for overloading. This gives a typical max temperature of 95C. (30C average ambient) Exceeding this generally shortens expected trasformer life.

 

[DanielElias]

IEC 76 rules European transformers. According this
60°C: Top oil temperature rise (measured by thermometer)
And
40°C: Maximum ambient temperature
So, with these values maximum temperature for oil is 100°C, but as Gordonl said is common to be conservator to extend life of the transformer. Why don't you ask to your Control documentt Office for the test report, probably you'll find extra information from manufacturer.

 

[busbar]

An applicable standard may be IEEE C57.91-1995 Guide for Loading Mineral-Oil-Immersed Transformers described at

http://standards.ieee.org/reading/ieee/std_public/description/dtransformers/C57.91-1995_desc.html

 

[jlazucena]

First the transformer is at least 35 years old.

You do not want to exceed the cooling capacity of the transformer oil, and must also consider that in an old transformer you have other limitations like oil expansion (the oil is not that great any more), pressure in seals (seals are not flexible any more), oil pressure in bushings, tap changer condition, etc.

Because the year, this transformer may be a 55º-Celsius raise from an ambient temperature of 30º Celsius, then the oil thermometer would be at 85º-Celsius at full load (When the ambient temperature is 30º-Celsius).

You can exceed this value of temperature but you will accelerate XFMR aging. (Make sure that the overload and tem raise protection do not operate for that over-loading)

If you want to load the XFMR during winter you have extra capacity (Ambient temp less than 30º C), but in a hot summer week you have less capacity (Ambient temp more than 30º C).

For North America, the industry recommendation (and explanation for the “55º-Celsius raise”) was that during rated load, the temperature of the winding hot spot should not surpass 100°C or rise 70°C above a 30°C ambient temperature, as follows: 30°C ambient temperature, plus 55°C allowed oil temperature raise, plus 15°C that is the expected difference between the oil temperature and the winding hottest spot,

Please note that the thermometer can be at 85º-Celsius only, but the hottest spot can be at 100º-Celsius, because the thermometer may not be reading the hottest spot.

 

[JETO]

Thank you all for your information,

This trafo has to run in overload (max 10 %) during e faw weeks,now I have confirmation about the idees that I had
myself (alarmsetting at 85°C)

Thank you all and greetings from Belgium

JETO