Hi there freethase, we have to be more PC than your utility, so we are obliged to staff our air blast breaker stations in cold weather ;o)
And SF6 breakers don't handle the cold at all well; in our jurisdiction it seems nigh impossible during periods of sustained cold to provide enough heat to prevent gas liquefaction...overnight we once had all of the SF6 breakers in a switchyard trip themselves out of service one at a time on low SF6 density, interrupting a number of radial ckts and the customer load that was on them, and we were powerless to do anything about it until the daylight hours when the sun warmed the breakers, the alarms and trips cleared, and we were able to return the breakers to service.
But I digress.
Pretty well all of our transformers, some as high as 750 MVA FCR, typically have all fans shut down by the time the oil temp drops to ~ +40 C.
Most of our stepdown transformers are equipped with under load tap changers for output voltage regulation. One sometimes very unwelcome protective feature on these banks is that the ULTC's lock out if the oil temp gets too cold - - which is understandable, since you wouldn't want taps moving when they're submerged in insulating oil that's halfway to being treacle.
We have specific instructions to follow during very cold weather; a transformer low oil temperature alarm is one thing, but a lightly loaded bank may annunciate with low conservator oil level due to shrinkage, which means rousing some poor soul out of bed in the wee hours to confirm the bank hasn't developed an oil leak.
Some of us old guys will deliberately skew the tap positions on paralleled transformers to deliberately produce circulating currents just for the warming effect; often enough this is enough to clear the low level alarm.
Anecdote:
At a large hydraulic generating station where I worked we once had an 86 MVA single-phase bank of 135 kV L-N primary develop a fault during a particularly severe and sustained period of cold weather. Maintenance crews came in and swapped it out for our spare bank. When it came time to place the spare forced oil, water cooled transformer in service, our supervisor obstinately insisted that the cooling water had to be in service on the bank since an instruction stipulated that this type of transformer could not be placed in service without cooling water.
Us lowly operators protested that since the oil in the transformer was at about -20 deg C, the first thing that would happen was that the water side of the heat exchangers would freeze solid since the oil would cool the water, not the other way around, and that it would be better to leave the cooling water out of service until the oil temperature was on the plus side of the freezing point of water...but we were overruled, and being subject to discipline if we did not comply, we did as directed.
Result? Both HXRs pronmptly froze solid; we knew this was the case because return cooling water from the HXRs only flowed into the tundishes inside the plant for about five minutes. Since maintenence personnel were still on site, we had them remove the HXRs from service one at a time and melt the ice out of the water boxes with a steam cleaner [there was no need to de-ice the tubes, because without any cooling water in service that transformer's share of the 250 MW of generation being stepped up to system voltage soon enough melted the ice in the tubes] ...but this took time, and the oil temperature in the transformer had risen to between 75 and 80 deg C before we managed to get both HXRs back in service.
We operators figured it was our supervisor who should have been disciplined...
A sage gentleman acquaintance of mine told me of an axiom he had once heard early in life and had applied often during his thirty years as a police officer. The axiom was: "Instructions are written for the guidance of the wise...and for the blind obedience of all others."
CR
![[smile]](/data/assets/smilies/smile.gif "[smile] [smile]")
. DRWeig, the second one was a pretty interesting read--thanks for that.