Transformer overheating protection

[Derm]

Hi!
We're reviewing specs for new power transformers and
I've forgotten the basic theory behind temperature monitoring on transformers. For large power transformers the tank thermometer is temperature compensated by a ring CT on one of the LV bushings. ?
What is the exact reason for doing this? Is it to keep the thermometer temperature relative to the oil temperature? And is the conservator tank thermometer temperature compensated also?

Any info would be greatly appreciated!

 

[edison123]

Derm,

Since actual winding temperatures cannot be read transformers due to HV, CT's are used to simulate copper loss in transformers. I think the CT secondary amps is used to heat a heater fitted in an oil packet and heater output is designed to reflect the transformer winding temp, which is done at design stage. Pls refer J&P book.

 

[Derm]

Thanks edison123, that jolts the memory now! I just need to find the J&P handbook..!

thanks

 

[mvcjr]

Normally, there are two thermometers in a power transformer. One monitors the top oil temperature while the other monitors the winding temperature. In the first, there is no need for a CT since the thermowell is in direct contact with the top oil. In the second, the thermowell could not be physically in contact with the windings (it is only in contact with the top oil) so a resistor is heated by current derived from the load current to simulate the winding temperature.

BTW, I've never seen a thermometer used to measure conservator tank temperature.

God bless!

 

[mta2003]

Hi Derm!
The explanation given by mvcjr is very correct. bravo. your suggestions and responses are very useful.