The oil conservator (see 1 fig below) is an auxiliary tank that allows the dielectric liquid to flow into and out of this reservoir as the fluid in the main tank expands and contracts. Some design may have air bags sometime called bladders or diaphragms.
Also you have much lower N2 in solution for conservator than for N2-blanketed transformer . I haven't ever heard of N2 being a problem but in theory it can come out of solution during rapid cooling of oil.
Also you can expect to see different behavior in fault gases measured by DGA since the headspace on top of transformer is not there in a conservator. I'm not sure if this is advantage or disadvantage... just different.
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Thanks for the comments! I have another question: If the conservator tank breathes moisture-laden air from the outside, isn't or shouldn't this be a concern?
My own gut feeling is I would prefer having a sealed system with an N2 bottle to change, than having the oil exposed to outside air.
I know the conservator tank allows the use of the Buchholz relay to detect dissolved gases in the oil, but with the advent of modern electronic dissolved gas systems, wouldn't a sealed N2 system with such a gas detection system be preferable to an air-breathing conservator tank?
Conservator designs have breathers which use either silica gel or a Peltier cooler dryer to maintain dry air within the conservator. A direct vent to atmosphere is asking for trouble and is not a feature of any normal conservator design.
The bladder that Cuky spoke of keeps the oil and air separated. Sometimes the silica gel is used in addition so that only dry air meets the bladder.
One design uses another oil chamber to keep moist air away from the transformer oil. Air>oil>N2>transformer oil. DGA from this transformer shows high O2, so this method is not as effective.
We had a bladder fail at an industrial substation (no dehydrating breather). I believe the quality of the atmosphere hastened it's demise.
I like the N2 blanket with a bottle, gages, and alarms. No bladder to fail, and if a bushing gasket should leak, it will leak N2 rather than oil, and the condition is alarmed.
15 years ago I had to install wooden platforms under every every transformer N2 setup. The platforms were designed so N2 bottle changeouts only required 1 man. If there wasn't a platform for him to roll the bottle it was a two man job. After that I changed the transformer spec to conservator tanks only. Hasn't been a problem whatsoever.
