Hermetically sealed transformers: how the corrugated tank replaced the conservator

Look at a PNACHO-O transformer and you will see a tank of steel fins and a flat lid – no expansion vessel on top, no silica-gel breather on the side. The transformer is hermetically sealed: the oil never touches the outside air. This design has become the European standard for distribution transformers, and for good reasons.

Why the old design needed a conservator

Transformer oil expands by roughly 0.07 % per kelvin. Between a cold winter night and full load on a summer afternoon the oil temperature can swing by 80 K – about 6 % of the volume. Classic transformers took up this expansion in a conservator tank above the main tank, open to the atmosphere through a breather. The price was constant breathing: moist air entered on every cool-down, the silica gel had to be changed, the oil absorbed water and oxygen, and the dielectric strength dropped over the years. Oil sampling and replacement became a maintenance routine.

How the hermetic tank works

The corrugated tank is made of thin steel sheet (1.2–1.5 mm) folded into fins that act as cooling radiators and, at the same time, as an elastic membrane. The tank is completely filled with oil under vacuum and welded shut. As the oil expands the fins flex outwards by a fraction of a millimetre; as it cools they return. The internal pressure varies only within a few tens of kPa. There is no gas cushion, no air and no moisture ingress – ever.

	Conservator type	Hermetically sealed
Contact with air	Through breather	None
Oil maintenance	Sampling, drying, replacement	None over the service life
Height	Higher (vessel on top)	Lower – fits compact substations
Corrosion risk inside	Yes	No
Ratings	All	Standard up to ~5 MVA

What protects the tank

• Pressure relief device: opens at about 30–50 kPa overpressure, for example after an internal fault, and prevents tank rupture.
• Integrated protection relay (DGPT2 / DMCR): combines gas detection, pressure, temperature and oil-level monitoring in one device on the lid. Standard on PNACHO-O from 1 000 kVA, optional below.
• Tightness test: every tank is tested at 30 kPa for 12 hours before filling – documented in the routine test report.

Practical benefits for the owner

1. No oil analysis, no drying, no breather cartridges – zero scheduled maintenance.
2. Outdoor installation without a housing (IP00 tank, RAL 7033 C3 coating for corrosion class C3).
3. Lower overall height – simpler compact substations and kiosks.
4. Service life above 30 years with stable dielectric properties.

All PNACHO-O transformers from 25 to 2 500 kVA are hermetically sealed corrugated-tank designs with a 60-month warranty. The data sheet of each unit lists tank dimensions, oil mass and the fitted protection devices.