What is a Hermetically Sealed Transformer? (Oil Type Transformer)
A hermetically sealed transformer is a special type of oil type transformer that is completely sealed from the external atmosphere. It is the most widely used type of oil-filled transformer due to its lower cost, high reliability, and excellent performance in outdoor and harsh operating conditions.
Unlike conventional oil type transformers with an expansion tank, a hermetically sealed transformer is filled with transformer oil under vacuum and fully sealed. This design prevents contact between the insulating oil and outside air, significantly reducing risks of moisture, oxidation, and contamination.
Hermetically Sealed Transformer vs Expansion Tank Type (Dry and Oil Type Transformer)
Oil type transformers are divided into two main categories:
- Hermetically sealed transformer
- Expansion tank (conservator) type transformer
In a hermetically sealed transformer, the transformer tank has flexible corrugated walls (wave walls) that allow for thermal expansion and contraction of the oil without any contact with the atmosphere. This design eliminates the need for an expansion tank, Buchholz relay, and silica gel breather.
Expansion tank type transformers have an open connection to the atmosphere through a conservator tank, making them more susceptible to moisture and contamination over time.
Structure and Components of Hermetically Sealed Transformer
The main components of a hermetically sealed transformer include:
- Copper or aluminum windings
- Grain-oriented silicon steel core
- High-quality transformer oil for insulation and cooling
- Corrugated wave-wall tank
- Off-load or on-load tap changer
- Cable bushings
- Electronic protection relay (for units above 400 kVA)
- Pressure relief valve
- Temperature indicator
What type of oil is used in transformer? High-grade mineral insulating oil is used. This specially refined oil provides excellent dielectric strength, high thermal stability, and effective heat transfer.
Advantages of Hermetically Sealed Transformer
- Completely sealed design prevents moisture and oxygen ingress
- Lower maintenance requirements (no silica gel to replace)
- Longer service life
- Better suited for humid and dusty environments
- High reliability in outdoor installations
- Reduced risk of oil contamination and insulation degradation
Cooling System
Hermetically sealed transformers are typically ONAN (Oil Natural Air Natural) cooled. The corrugated walls increase the cooling surface area. For higher loads, ONAF (Oil Natural Air Forced) cooling with fans can also be used.
| Letter | Meaning |
|---|---|
| O | Oil |
| A | Air |
| W | Water |
| N | Natural |
| F | Forced |
Technical Specifications and Standards
In Turkey, hermetically sealed transformers must comply with TEDAŞ MLZ/99.032E technical specification. They are commonly manufactured up to 2500 kVA for distribution networks and are widely preferred in concrete kiosk substations up to 1600 kVA.
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