What is a sealed transformer?

What is a Hermetically Sealed Transformer? Technical Working Principle and Advantages

In power distribution networks, maintaining the purity of insulating oil and its cooling capacity within a transformer is the most critical factor for ensuring a long operational life. Traditional distribution transformers use an external conservator tank that allows the insulating oil to breathe and expand as temperature rises. However, this process exposes the oil to oxygen and moisture in the atmosphere, leading to oxidation and aging.

To eliminate this weakness, modern engineering has introduced the hermetically sealed transformer, also known as a hermetic transformer or sealed-type transformer. In this guide, we will explore the technical design, working principles, and distinctive industrial advantages of hermetically sealed transformers.

What Does "Hermetically Sealed" Mean?

A hermetically sealed transformer is a completely enclosed distribution transformer, where the internal core, windings, and insulating fluid are 100% isolated from the external atmosphere.

Unlike conventional transformers, a hermetically sealed transformer does not have a conservator tank, a breather, or silica gel filters to absorb moisture. The internal tank is completely filled with high-quality electrical insulating oil and then sealed either by welding or using special airtight gaskets, under strictly controlled factory conditions.

Technical Working Principle: How Does It Handle Expansion?

When a transformer operates under high load or in high ambient temperatures, the internal oil naturally heats up and expands. Conversely, when the load decreases, the oil cools and contracts. Without a traditional conservator tank, a hermetically sealed transformer manages this thermal volumetric expansion using one of two primary engineering methods:

1. Elastic Expansion of Corrugated Tank Walls, or Corrugated Fin Hermetically Sealed Type

The transformer tank walls are made from corrugated steel fins that also act as cooling fins and possess elastic properties. As the oil expands due to heat, the corrugated walls flex outwards elastically to accommodate the increased volume. When the oil cools, the flexible walls return to their original position. The structural design of these fins is precisely calculated to withstand continuous pressure cycles without material stress or damage.

2. Gas Cushion Insulation, or Gas-Cushion Hermetically Sealed Transformer

In larger hermetically sealed designs, the tank is not entirely filled with oil. Instead, a specific volume of dry, inert gas, often nitrogen, is injected into the space above the oil level. When the oil expands, it compresses the nitrogen gas cushion. The gas absorbs the pressure changes, keeping the mechanical stress on the steel tank within safe limits.

Key Technical Advantages of Hermetically Sealed Transformers

1. Zero Maintenance Costs

Because the system is completely sealed, moisture or oxygen cannot enter the tank. This eliminates the need for regular inspection, drying, or replacement of silica gel breathers. It also prevents the need for periodic oil purification or degasification, significantly reducing maintenance work and costs throughout the transformer's lifespan.

2. Significantly Longer Operational Life

In conventional transformers, oil's contact with oxygen leads to oxidation of the insulating oil, which produces sludge and acidity that degrades the paper insulation around the copper windings. By completely preventing contact with the external atmosphere, the oil in a hermetically sealed transformer retains its dielectric strength and chemical properties for decades, greatly extending the transformer's life.

3. High Resistance to Environmental Hazards

Hermetically sealed transformers are the ideal choice for harsh, high-humidity, or polluted environments. Since the internal components are isolated from the outside world, they are fully protected from:

High humidity and salt spray in coastal areas, preventing internal rust and oil contamination by water.

Heavy dust, sandstorms, and industrial chemical fumes.

Significant fluctuations in ambient temperatures.

4. Compact Size and Low Height

Since hermetically sealed transformers do not have a large top-mounted conservator tank, their overall height is much lower, and their physical footprint is more compact. This makes them exceptionally easy to install within compact distribution substations, underground vaults, wind turbine towers, and industrial rooms with limited space.

Technical Comparison Between Hermetically Sealed Transformer and Conventional Conservator-Type Transformer

Feature: Contact with External Atmosphere
Hermetically Sealed Transformer: No contact whatsoever; system is completely sealed
Conventional Conservator-Type Transformer: Continuous breathing via breather

Feature: Oil Oxidation and Aging
Hermetically Sealed Transformer: Completely prevented
Conventional Conservator-Type Transformer: Occurs over time due to oil-oxygen contact

Feature: Moisture Absorption
Hermetically Sealed Transformer: Zero risk
Conventional Conservator-Type Transformer: Requires regular monitoring of silica gel

Feature: Maintenance Requirements
Hermetically Sealed Transformer: Maintenance-free under normal operating conditions
Conventional Conservator-Type Transformer: Requires periodic oil tests and filtration processes

Feature: Physical Dimensions
Hermetically Sealed Transformer: Compact and lower height
Conventional Conservator-Type Transformer: Taller due to presence of top conservator tank

Feature: Primary Application
Hermetically Sealed Transformer: Compact distribution substations and harsh environments
Conventional Conservator-Type Transformer: Large substations and heavy-duty high-voltage grids

Common Industrial Applications

Due to their high reliability and low maintenance requirements, hermetically sealed transformers are widely used in the following sectors:

Renewable Energy Plants: Within wind turbine towers and solar power plants, where remote locations make regular maintenance difficult and costly.

Compact Distribution Substations: Highly suitable for urban distribution networks due to their limited space requirements.

Heavy Industrial Facilities: Cement factories, chemical plants, and steel mills, where dust and corrosive fumes can damage conventional air-exposed transformers.

Oil & Gas and Mining: Extremely robust designs suitable for explosive or underground environments, where oil leakage or sparking must be strictly prevented.

Conclusion

Hermetically sealed transformers represent a significant advancement in low and medium voltage power distribution. By creating a permanently sealed environment for the core and insulating oil, the primary causes of transformer degradation—moisture, oxidation, and contamination—are eliminated. For industrial operators seeking to minimize downtime and reduce maintenance costs, investing in highly efficient hermetically sealed transformers is an engineering strategy for long-term grid stability.

If you need assistance choosing between a corrugated fin hermetically sealed transformer or a gas-cushion hermetically sealed transformer, or if you require custom voltage specifications for your next project, contact our applications engineering team today for a technical consultation and direct price quote.

https://enerjitemglobal.com/