Permissible Winding and Oil Temperature Indicator values

[zkz]

Hello,
We have recently installed a 15MVA , 66/11kv, Dyn11 transformer of which cooling is ONAN. Unfortunately we lost its manual from our station. When it was fully loaded we found that the Winding Temperature and Oil Temperature is compared other generator transformers in the same station. Which are ONAF and of different make. When we checked the name plate details we found the following details:
Maximum permissible Ambient Temperature: 50 degree Centigrade
Maximum Temperature rise above Ambient Temeprature for Oil/Winding: 50/55 degree Centigrade.
When we checked maximum Ambient temperature, we found that it was 34 degree.
Also we found that Winding Temperature is about 18 degree higher than Oil Temeperature.
My doubt is: Which is permissible maximum Winding Temperature Indicator reading? Is it 105 (=50+55)degree centigrade or 89 (=34+55) degree centigrade? Similarly which is maximum permissible Oil Temperature Indicator , 100 or 84 degree centigrade?
We have to set the alarm and trip settings to both Oil Temperature Indicator and Winding Temperature Indicator of the transformer. Please provide some suggestions in this regard too.
Thanks in advance,
zkz

 

[edison123]

Trip - 100 C for oil and 105 C for winding

Alarm - I would set 10 C above your normal observed values to give you enough early warning.

Muthu

http://www.edison.co.in

 

[TurbineGen]

At the power utility I worked, we set our alarms at 90C for both oil and winding temp. We tripped at 105 C winding temp or oil temp. I completely agree with edison's settings.

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If it is broken, fix it. If it isn't broken, I'll soon fix that.

 

[prc]

Your setting has to be decided based on your operating conditions ie are you overloading the transformer etc. IEC 60076-7 (2005)Overloading guide allows oil/hot spot temps of 105/120 C for cyclic overloading and 115/160 C for short time overloading.But it will be risky to overload a wet transformer or too aged ones.When name plate says 50/55C over 50C ambient max means,allowable top oil rise of 50C &average winding rise of 55C.Limiting temp is usually the winding hot spot.Winding Hot spot will be 5-20 C over the average winding rise.

So in your case,assuming no overload ever at max ambient condition, take max ambient (T),top oil(to),average wdg rise(tw),drop across cooler (tc)from temperature rise report.
Then Winding hot spot setting is T+ to + 1.3[tw-(to-1/2 tc)]Make sure that WTI is calibrated for this hot spot gradient corresponding to rated load current.Top oil setting will be T+to.While selecting setting, some may take + 5C margin over those maximum attainable temperatures.

 

[zkz]

Hello,
Thanks for all who gave me advices. As per your suggestion, we do not have to consider actual ambient temperature(which was 34 C) but to consider max.permissible temp. indicated in name plate (which is 55 C). Please correct me if I am wrong.
Let me ask one (perhaps silly) question then. If we are setting everything based on the name plate details, why should the name plate indicate maximum permissible ambient temperature, max. permissible oil temperature rise and max. permissible winding temperature rise separately? Why do not the name plates be displayed like the ones given below?
Maximum permissible oil temperature: 100 C
Maximum permissible winding temperature: 105 C.

Thanks in advance.
regads,
zkz

 

[TurbineGen]

That's because of the design of the transformer. It is designed to operate at full load in a maximum of 50 degrees C ambient temperature. Higher temperatures will require derating of the transformer.

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If it is broken, fix it. If it isn't broken, I'll soon fix that.

 

[prc]

The top oil & average winding rise indicated in name plate are the guaranteed max values based on customer requirements.Customer decides this based on the average weighted annual air temperature for the location.IEc specifies 60 &65 C based on a weighted annual average temp of 20C and maximum ambient air temp of 40C.In case ambient temp where you are putting the trf is 30&50C(weighted average &max) then customer will specify 50 &55 C as the max limit.Actual oil and winding rise will be different and given in test report.

 

[zkz]

Thanks a lot for informations.
regards,
zkz