Settings the ONAF operation of Power Transformer

[Marco Antonio]

We have a power transformer with two stages of ONAF, it has two groups of fans.
According to the results of the CO2/CO, the values are greater than 10, and It is suspicion of cellulosic degradation due a over thermal. I want to know, how to set the start of the fans. I want to decrease the start of the fans, however I do not which will be the way the set the new values.

 

[waross]

Find the thermostats that control the fans and lower the settings.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[prc]

Normally fan control is done from the cooler contacts in the oil temperature indicator. You can locally adjust this setting for a lower temperature so that fans will come in to action at a lower temperature. But I don't think this will help you to lower CO2/CO ratio. This ratio is significant only if CO2 & Co values are of the order of 5000/500 ppm.

 

[crshears]

Hello Marco, your post leaves many questions unanswered...

How old is the transformer? Has it been in continuous service since its manufacture? If it has been in storage, what records do you you have of [a] proper mothballing having been performed, proper storage conditions having been maintained during hibernation, and [c] proper re-commissioning steps having been done?

What is the temperature rise of the transformer?

Have you confirmed an over-temperature situation actually occurred? Is/are there mechanical oil or winding temperature gauge with drag hands to confirm this?

Absent such evidence, you may wish to consider whether the xfmr has manufacturing defects that may be covered under warranty...

CR

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

 

[Marco Antonio]

Hello there,

I have found the thermostats that control the fans. But, which will be the setting values? Is there any rule/standard?

According to the IEEE C57.104-2008, greater values of CO2/CO implies cellulosic degradation due a over thermal. Our values are around 19 and 11.

The year of manufacturing is 2001, therefore 17 years old. It begun the operation in June 19,2002. I think, it has a continuously operation, however the period of maintenance is almost 20 days per year. The power transformer (generation) has all the test routines applied, and it seems that everything is good. The temperature rise (in the plate) is 65 C. The transformer has three temperature gauge: T_HV, T_LV and T_OIL. The average values of these in 2016 were: T_HV=60.6 C; T_LV=52.0 C and T_OIL=48.2 C.

The principal concern is the CO2/CO values, greater values implies over-thermal.

Thank you for your help.

Best Regards,
Marco Antonio

 

[bacon4life]

I have seen fan settings based on simulated hot spot winding temperature to come on at 80 C for group 1 and 90 C for group 2. If you find other recommendations, please post them.

IEEE C57.91 gives describes loss of life calculations and a method to calculate the approximate temperature reduction each bank of fans may provide.

Paper degradation due to temperatures is nonlinear, so an average annual temperature doesn't tell you much. Running at 110C degrades the paper approximately 8.5 times faster than running at 90 C.

 

[stevenal]

Are you looking at the ratio or the individual numbers? The forward slash indicates ratio, but it is a low ratio that might be of concern. Are you seeing the values change? If paper degradation is of concern, a furan test might be helpful. Also suggest infrared scans to see if radiators are clogged.

 

[prc]

Marco, Please don't consider high CO2/Co ratio means thermal degradation. There can be other reasons too. Please post complete DGA and whether any history of overloading or high temperature indicator readings during its service. Also arrange a furan content test on oil sample. If furan value is more than one ppm, it may call for serious investigations. There are no standards for the settings of cooler actuation temperatures. Manufacturers recommend the same based on specified hot spot temperature rises and ambient temperature at the installed site.