"El aceite dieléctrico del transformador de potencia deberá ser equipado con un sistema que aislé el aceite dieléctrico para su preservación, esto puede ser fabricado en los diferentes tipos de tanque conservador según norma C57-12.00 en el inciso 6.6.2: Tanque sellado, tanque conservador con sello de gas-aceite, tanque conservador abierto y tanque conservador con diafragma."
http://biblioteca.usac.edu.gt/tesis/08/08_0642_EA.pdf

"A preservation system is essential for a liquid-immersed transformer to allow- expansion and contraction of the liquid due to the changes in the temperature with-out exposing the insulating liquid to external contamination. The expansion space is known as the oil preservation system.
There are four different designs:
1. Sealed Tank
2. Gas-oil seal
3. Conservator
4. Conservator diaphragm
https://electrical-engineering-portal.com/download-center/bo...

"Oil Preservation Sealing Systems...
Sealing Systems Types
Free Breathing
Sealing systems have progressed from early designs of “free breathing” tanks, in which an air space on top of the oil is vented to atmosphere through a breather pipe. The pipe typically is screened to keep out insects and rodents and turned down to prevent rain from entering. Breathing is caused by expansion and contraction of the oil as temperature changes. These earlier designs do not use an air dryer, and condensation from moisture forms on inside walls and tank top. Moisture, oxygen, and nitrogen would also dissolve directly into oil from the air. This is not the best design. As mentioned before, a combination of oxygen and moisture accelerates deterioration of cellulose insulation. Moisture also decreases dielectric strength, which destroys insulating quality of the oil and causes sludge to form. If you have one or more of these earlier design transformers, it is recommended that a desiccant type air dryer be added to the breather pipe."
https://electromagneticworld.blogspot.com/2017/10/oil-preser...

"Liquid preservation systems limit the exposure of the transformer insulation structure to the atmosphere while allowing thermal expansion and contraction of the liquid due to load and ambient temperature variations. These liquid preservation systems are:

a. Sealed Tank – A sealed tank system uses a gas space above the insulating liquid level of the main tank to absorb volume fluctuations. It can be regulated or static.

b. Conservator Tank – A conservator tank is an auxiliary tank partially filled with insulating liquid and connected to the completely filled main tank. It uses the gas space above the liquid for volume fluctuations. Conservator tanks can be free breathing or isolated from the atmosphere using a rubber air cell or diaphragm."
https://www.tdworld.com/test-and-measurement/article/2112935...