Hermetically Sealed Transformer

The hermetically sealed oil transformers are normally manufactured with a sealed tank equipped with fins that allow for expansion due to temperature variations. The tightness of the tank is up to 0.5 Bar. 

This type of transformer is the most widely used in the world. In the hermetically sealed transformer, the oil does not come into contact with the air and its electrical properties are therefore not compromised, ensuring a long life span of the transformer. 

For powers exceeding 3150kVA, or in case of a transformer with radiators, the transformer can still be hermetically sealed by means of a nitrogen cushion.

What is a hermetically sealed transformer?

A hermetically sealed transformer is an oil‑immersed transformer built with a closed, gas‑tight tank and external fins to accommodate thermal expansion; the tank is sealed to a specified tightness (typically up to 0.5 bar) so the insulating oil never contacts air, which helps prevent oxidation and prolongs the transformer’s life.

How does a hermetically sealed transformer work?

The transformer operates like a conventional oil‑filled unit, but the oil and windings are enclosed in a sealed vessel so the dielectric fluid remains isolated from atmospheric oxygen and moisture; as the oil heats and expands the fins (or an internal gas cushion) absorb the volume change, and for larger ratings a nitrogen cushion is introduced to maintain pressure balance while keeping the tank hermetic.

What are the benefits of a hermetically sealed transformer?

Key benefits are longer oil life and reduced maintenance because the oil is not exposed to air or moisture, improved reliability in corrosive or humid environments, and lower risk of contamination and oxidation; hermetic designs are widely used worldwide for these reasons and can be adapted for higher powers or radiator‑equipped units using a nitrogen cushion to preserve the seal.

Where are hermetically sealed transformers used?

They are used across utilities, industrial sites and commercial installations where low maintenance, long service life and resistance to environmental contamination are priorities. Especially in coastal, high‑humidity or polluted locations, larger installations above typical kVA thresholds or those with radiators can still be hermetically sealed by adding a nitrogen cushion to manage expansion and pressure.