Medium Voltage transformer temperature

[Francisco Mendonca]

Hello, i am new here and i need help.
I am working on a 6900/400Volt, 225 KVA transformer, with 10 years of use, that feeds a pump house with 8 frequency drivers for 8 motors. This installation don´t have any filters for harmonics, caused by the frequency drivers. Our transformer is oversized for our building, a total of 8X 20KW for each motor, but only works a maximum of 4 pumps each time ( use to be only 2 pumps)
Our transformer is a class of temperature 65 °C. The actual temperature of it is between 70 and 80 °C and environment temperature use to be around 26 °C. We open it a few days ago to check for bad connections but none found. The oil had a bad smell so we replace it. after close it and connect it to the power lines, without any load, the temperature rise for 70 °C in 24 hours. voltage was 235 Volt in al the 3 phases to neutral and 407 Volts in all the 3 phases ( L1/L2, L1/L3 And L2/L3) Perfect except the heat. After connect 2 pumps, total 40 KW, the temperature is around 80 °C. Is this a normal temperature? If not what can we do to fix the issu?
Thank you very much
Mendonca

 

[prc]

Are there radiators with valves? Please ensure all valves to coolers are open. 65 C is the rise. So, compare the temp rise instead of absolute temperatures

 

[Francisco Mendonca]

"prc" Thank you. No radiators on this transformer. It is a pad mounted transformer and the cooling is made by convection

 

[edison123]

For VFD loads, k-factor rated transformer is needed.

https://canadatransformers.com/k-factor/

Having said that, a 70 deg C rise on no-load is steep. What does OEM FAT say about no-load losses and temperature rise? Or did you mean 70 deg C temperature, not the rise, on no-load? In either case, it's too high and doesn't have much allowance for load losses.

Muthu

http://www.edison.co.in

 

[Francisco Mendonca]

edison123 70°C with no load to the transformer. When i connect a load, 40 KW, the temperature rise to 80°C. I think is a litle high, but the cooling oil is good and by another hand the Voltage is balanced between phases and neutral and between phases. If a problem in the coils, for sure the voltage is not balanced. This is not a K factor transformer, at least i don´t see at the transformer plate

 

[itsmoked]

Sounds like you should send in an oil sample for analysis. It can tell you a whole bunch of things about what's happening in the transformer.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[edison123]

@OP

First of all, make sure your temperature indicator is calibrated and accurate lest we are chasing tails.

The no-load loss of such small transformers is around 20%, which is now consuming nearly 45 deg C temp rise, leaving just 20 deg C rise for the 80% load losses. And we have not even covered the additional losses due to VFD's.

It is not a question of winding (yet), it is more of poor quality core with very high losses and/or insufficient oil quantity, both of which indicate a poorly designed and made trafo.

Again, what does the OEM FAT say?

Muthu

http://www.edison.co.in

 

[Francisco Mendonca]

edison123

What do you mean with "Again, what does the OEM FAT say?"
Thank you

 

[unclebob]

Such a small oil-filled transformer. Adding fans could help, but at what cost? Dissolved gas analysis would be useful here. Is it outside? If no, may be replacing it, when it will fail, with a dry-type one, with adequate k-rate?

 

[Francisco Mendonca]

unclebob
Thank you. Yes it´s in the outside, is a Pad mounted Distribution transformer, 6900 Volt to 400 Volt 225 KVA. We add new oil and the rise of temperature still there.

 

[edison123]

OEM - Original Equipment Manufacturer

FAT - Factory Acceptance Test

Muthu

http://www.edison.co.in