Ambient temperature and transformer performance

[ters]

Smaller oil field transformers (300 MVA) 34.5/480 V may occasinally be exposed to a minimum ambient temperature of –40 DEG C, while the manufacturer stated that the transformer operating temperature range is -30 C _ 50 C. Could someone comment what are major consequences if an oil transformer is exposed to temperatures below the range they are designed for?

 

[dpc]

As long as it has been energized for a while and warmed up, and stays energized, I wouldn't think there would be much of an issue. The core losses will keep the oil viscosity at an acceptable level.

If it had been sitting de-energized at that temp, I would be concerned that the oil would be too thick for thermosiphon effect to keep oil moving.

 

[ters]

Thank you dpc. There will be instances when the TR will be sitting de-energized maybe for several days during the winter.

How do they deal with this (transformer design for very low winter temperature conditions)- is this just the matter of using different type of oil or there are other considerations related the transformer geometry and similar?

 

[DanDel]

I would contact the manufacturer and ask them.

Mineral oil thickening is one thing, but other coolants, like R-Temp, would definitely have flow problems.

I believe you may find that even when the transformer is running, an ambient temp of -40° C would cause problems if the manufacturer lists -30° C as the limit.

 

[BJC]

Somewhere in the old Westinghouse T&D book it gives a figure for rating and derating transformeres.
I believe for cold ambients it was something like an increas of 1.8% per degree below freezing ( 32 Degrees ) and 0.8% decrease. for temps over 104 degrees.
I think those figures are in the GE distribution transformer booklet. That booklet is somewhere on the web.
I don't have any of the books here, I can take a look this weekend.
I recall the parking lot transformer ( for vehicle heaters ) in NoDak could be loaded about 250% on the colder days.

 

[ters]

Thank you all for useful inputs. About your comment BJC (lower temperature - more output is avaialbe), if it is a dry transformer that would seem to be a logical assumption, but for oil transformers, as dcp and DanDel indicated, cooling seem to depend on the ability of oil to move. If the oil does not move at all, or starts to develop wax and other sediments, etc, there could be a completely different case including development of the overheated areas at ?.

Does anyone know if such condition (when the oil starts to solidify at very low temperatures) has a negative impact on the oil dielectric strength?

 

[tinfoil]

Another issue is that cold oil holds less moisture than warm oil. If its not in the oil, the water goes into the paper, or collects at the bottom of the oil.

I recall reading a paper where the startup procedure for a cold-environment substation included circulating the xmfr oil through a heater to bring it up to a min temp, but I think this was for the DEW line in Northern Canada. I'll see if I can find it again.

 

[ters]

The more comments I read, the more questions I seem to have . I just realized based on tinfoil’s comment that I don’t seem to understand maybe some basics: when a manufacturer gives a temperature rating of say – 20 C _ +50 C, what does it mean? Does it mean that this is an operating range when the transformer is energized (with or without load)? If that is the case, and going back what dpc said (no problem as long as it is energized – oil will be much warmer than outside temperature anyway) , I’m wondering what then the actual “non-energized” temperature range may be. Say if in this example, - 20 C is OK when transformer is online but no load, that probably means that the oil temperature is not – 20C but something much higher, perhaps more than 0 C? So to make things equal, how do you translate this rating to a non-energized state to begin with?

 

[cuky2000]

Regarding the transformer capacity vs. ambient temperature, see the enclosed link

http://cuky2000.250free.com/Tr_Load.jpg

One possible option is to use supplementary heater and/or using cooling fluid with lower frozen temperature.

Beware that sub-zero temperature is not a usual condition and has to be addressed with the transformer manufacturer.


Paraphrasing Yogi Berra, engineering is 90% technical and the other half is administrative.

 

[dpc]

I think you're going to need to talk with a transformer designer/engineer. I have to believe that the lower ambient limit would be based on a de-energized unit, but I don't know that for a certainty.

 

[MJR2]

For colder applications we use Voltesso C50-97 class A transformer oil. It has better low temperature properties than standard transformer oils.

Mike

 

[ters]

Update. One of the manufactures commented that they use mineral oil until - 30 C and below that a special type of oil is needed but the transformer is still essentially the same. However, they indicated that extremely low temperatures (no definition of "extremely low" is given though), require a completely different transformer design.

I’m just trying to digest some related info reading IEEE transformer book (C57), which has about 2000 pages, and still it does not seem to say much if anything about very low ambient temperature conditions...

In terms of rating, ANSI seem to suggests that at low ambient temperatures the transformer could be uprated for 1% for each DEG Celsius below +30 C but above 0 C, however it doesn't seem to elaborate what happens below 0 C ("consult with the manufacturer", they say).

 

[able357]

I am a test engineer and perform electrical tests in the field and regurlary see xmfrs at cold temperatures. Actually just two days ago I tested a transformer using a system called DFR (dielectric frequency response) which tests the power factor. The only time I see an issue with a transformer at low temps is in the resistance and PF and all it takes to correct this is letting the transformer run for a little while! Other then this I dont know why the transformer would be effected if it has cold oil. When you start the transformer it heats up and dissipates through the oil and in the process warms the oil begning the thermo process.