A hermetically sealed transformer has a fully welded, airtight tank with no conservator and no breather. The oil never touches the surrounding air, so it cannot absorb moisture or oxygen, and the two mechanisms that age insulating oil fastest are removed at the source. The trade-off is that the tank must absorb the thermal expansion of the oil itself.
How a sealed tank handles oil expansion
Mineral oil expands measurably with temperature: roughly 0.07 % of its volume per kelvin. Over the working range of a loaded transformer that amounts to a few percent of the total oil volume, which has to go somewhere. In a conventional design a conservator tank at the top provides the space, with an air gap above the oil and a silica gel breather to dry the air moving in and out.
A hermetic design instead uses corrugated fin walls that flex elastically as the oil expands and contracts. The tank breathes mechanically rather than pneumatically. Some designs use a nitrogen cushion above the oil instead, but the principle is the same: no path exists between the oil and the atmosphere.
Why keeping air out matters
Oxygen in contact with hot insulating oil drives oxidation. The reaction products are acids and sludge: acids attack the cellulose of the paper insulation, and sludge settles on windings and in cooling ducts where it obstructs heat transfer. Both effects compound, because a hotter transformer oxidises faster.
Moisture does the second kind of damage. Water lowers the dielectric strength of the oil and, more seriously, accelerates the hydrolysis of the paper insulation. Since the paper is what actually determines the life of the unit and cannot be replaced, keeping its water content low is the single most effective form of life extension available.
What it changes about maintenance
The routine tasks tied to a breathing system simply disappear:
- No silica gel to inspect, regenerate or replace
- No conservator oil level to check and top up
- No oil drying or reconditioning cycles driven by moisture ingress
- Longer intervals between oil samples, since oxidation and moisture ingress are largely eliminated
What remains, and becomes more important, is the integrity of the seal itself. Gaskets, welds and the flexibility of the fin walls determine whether the design keeps its advantage. A visual leak inspection, a check of the pressure relief device and monitoring of tank pressure and top-oil temperature give a reliable picture of internal condition. Sudden pressure change is a meaningful early signal in a sealed unit precisely because the volume is closed.
When a conservator still makes sense
Hermetic sealing suits distribution transformers and smaller power transformers well, and is standard for outdoor and unattended sites. Above a certain rating the oil volume, and therefore the expansion volume, becomes too large for fin walls to absorb, and a conservator returns. An on-load tap changer also needs its own oil compartment and usually its own breathing arrangement.
The practical rule: for a distribution unit that will sit outdoors for thirty years with few site visits, a sealed tank is almost always the better choice, provided you accept that it cannot be topped up and that seal integrity becomes the thing you monitor. Protrafo supplies hermetically sealed units as standard and can add Protrafo Insights to log tank pressure and oil temperature continuously, which turns that seal integrity from an inspection round into a measured value.
