Transformer ventilation 5

[electricnewbie123]

We have a transformer supplied by a manufacturer which is planned to be installed within a room. The transformer is manufactured to IEC60076 standards.

The question is regarding the ventilation design for the room. IEC60076 states that the cooling air temperature has to be within 40 degrees. The manufacturer has provided a formula for sizing the ventilation fans, etc. We have an engineer who states that as long as we circulate air in and out of the room at the rate of flow of air specified by the manufacturer and the air inflow is within 40 degrees, we satisfy the needs of the transformer and the standard.

However, my problem is that the ventilation sizing is all designed for a change in temperature (between inlet and outlet from the room) of 10 degrees. Therefore, if the temperature on a day is 35 degres, the ventilation system will make sure that the outlet temperature is 45 degrees and it will allow a 10 degree rise in temperature within the room. This means that the transformer will be sitting in an ambient temperature above 40 degrees. I do not think IEC 60076 standard allows the transformer to sit in an ambient temperature above 40 degrees. The reason why I say so is that I believe the winding hot spot temperatures and ratings are all calculated based on the transformer placed in an ambient temperature of 40 degrees and not higher. Therefore, I am saying that the room requires cooling (air conditioners), not just fans.

Can others pls comment on this issue?

 

[jghrist]

It probably would be better to de-rate the transformer for the higher temperature and serve less load or to install a larger transformer.

 

[electricnewbie123]

Thank you jghrist for your idea... unfortunately we have already procured the transformer and it is required to operate at full load occasionally. So, you do agree that operating the transformer at an ambient around the transformer exceeding 40 degrees is not strictly in accordance with the requirements of IEC60076 standards, right

 

[Marmite]

Something has to give. If you have bought a transformer rated for 40C and you put it in a room where the temperature exceeds 40C then in theory you won't get full nameplate rating.
Whether this is a problem in the real World depends on a lot of factors. Is your period of full load coincident with the high ambient temperature?
The ventilation system has to be designed to dissipate the full load losses of the transformer, otherwise it will heat up and eventually fail. Your ventilation exhaust air temperature has to be 40C or less. The ventilation has to be designed to maintain the exhaust at less than 40C regardless of intake temperature, rather than designed to achieve a specific temperature differential. As the intake temperature rises you need to shift more air to get rid of the heat, ie a fan (or several fans).
You need to go back to the ventilation designer and ask them to redesign the system, as it has not been designed with the correct goal in mind.

Regards
Marmite

 

[LDKGR]

Rule of thumb :
Design your ventilation system for a ΔT=5K plus 20% air flow safety factor.

 

[7anoter4]

In my opinion, you are right.
First of all on IEC 60354 Standard Loading guide for oil-immersed power transformers. 2.7 Ambient temperature.
2.7.1 For outdoor air-cooled transformer the actual air temperature is taken as ambient.
For indoor distribution transformer the ambient-temperature correction is given in 2.7.6.
2.7.6 ch. Correction of ambient temperature for transformer enclosure
" A transformer operating in an enclosure experiences an extra temperature rise which is about
half the temperature rise of the air in that enclosure." Attached it is a table for transformers of 250 to 1000 kVA
and up to 3 transformer in the same enclosure. :
Table 3 Correction for increase in ambient temperature due to enclosure.
Second: on IEC 60076-2 ch.4.3.1 Oil-immersed, air-cooled transformer:
“Normal ambient temperature limits(-25 oC and +40 oC) for power transformer are given in 2.1 of IEC 60076-1.
With regard to cooling of air-cooled transformers the temperature conditions at the intended
installation site should neither exceed:
30oC monthly average, of the hottest month; nor
20oC yearly average.

 

[electricnewbie123]

Thank you all for the valuable advice... much appreciated...

 

[prc]

As per IEC60076-2, the maximum permissible winding hot spot temperature rise is 78 C, over a max ambient of 40C, monthly max average of 30C over the hottest month, max 20C of yearly average. If any of one of the above ambient temperatures exceed for the site you are referring to, the winding hot spot temperature rise shall be reduced to that extent to get normal life.

So in the case referred,which I believe is an oil immersed transformer, normal life is expected when hot spot temperature is 78C ie at an ambient of 20C and full load. So it is understood that at 40c ambient there is a loss of life for insulation of the order of double the rate of ageing for every 6C rise over 78C.This will be compensated by gain in life when working at ambient below 20C.So please check whether at ambient of 35C what is the load and duration and if it is less than full load there is nothing to worry about. Also remember when rise in cooling air is 10C, the average ambient air rise inside chamber is 5C.Also see actual temperature rise of the unit as per test report against oil &winding rise as per 60076-2.

Normally the allowed rise in cooling air in such chambers is 10-20 C ( for design details- ABB (earlier BBC) Switchgear Manual-Chapter 4,Power Transformer Hand book- Bernard Hochart(1987) Chapter 10)
IEC 60534 has been superseded by 60076-7 (2005) for oil filled units and 60076-12(2008) for dry type transformers. In 60076-7 clause 8.3.2 covers ambient correction for enclosed chambers. It says- " A transformer operating in an enclosure experiences an extra temperature rise which is about half the temperature rise of the air in that enclosure" as I mentioned earlier. Table 6 of standard gives typical temperature rises in various types of ventilated chambers.

 

[7anoter4]

Thank you, prc !

 

[electricnewbie123]

Excellent info prc - thank you for that!

The transformer I am referring to is a dry type transformer..

 

[prc]

If the transformer is dry type, the standard to be referred is IEC60076-11 . Ambient conditions are same as above, but average winding temperature rise limits are 100 C (Class F) and 125 C (Class H)