Differences Between Hermetically Sealed and Conservator Type Transformers

Introduction

In the realm of power distribution and energy infrastructure, transformer design plays a crucial role in operational efficiency, longevity, and maintenance. Among the most commonly used types of oil-immersed transformers are hermetically sealed transformers (also known as fully sealed transformers) and conservator type transformers (also called non-fully sealed transformers).

This article offers a comprehensive technical comparison between these two designs, including construction, performance characteristics, applications, and advantages. 

What is a Hermetically Sealed Transformers?

Definition & Structure

A hermetically sealed transformer is designed with a completely enclosed tank system that prevents any air or moisture from entering the transformer. The insulating oil is protected by a welded steel tank and an internal expansion system—such as a rubber bladder, corrugated steel, or flexible bellows—which absorbs oil expansion caused by temperature fluctuations.

Cooling & Insulation

• Insulating Medium: Mineral oil or synthetic ester oil

• Cooling Method: ONAN (Oil Natural Air Natural), optionally ONAF (Oil Natural Air Forced)

• Expansion Control: Sealed bellows or membrane avoids exposure to ambient air

Performance Characteristics

• Voltage Rating: Up to 36 kV (commonly used in medium-voltage applications)

• Power Capacity: Typically ranges from 50 kVA to several MVA

• BIL Rating: Up to 95 kV for 36 kV class

• Temperature Rise: 55°C (average), 65°C (maximum hot-spot)

Advantages

• No Moisture Ingress: Hermetic design ensures extended insulation life

• Minimal Maintenance: No breather, no conservator tank, reduced servicing

• Suitable for Harsh Environments: Ideal for coastal, underground, or humid conditions

• Fire-Safe Option: Compatible with fire-retardant ester oils

Limitations

• Higher Manufacturing Cost: Sealed designs involve more complex fabrication

• Restricted Oil Expansion Volume: May limit performance at extreme temperature rises

• Challenging Repairs: Opening the tank breaks the seal, requiring nitrogen purging and vacuum treatment

What is a Conservator Type Transformers?

Definition & Structure

conservator type transformer, use an air-breathing conservator tank to manage oil expansion. The main tank is connected to this oil reservoir, which exchanges air with the environment through a silica gel breather that filters moisture.

Cooling & Insulation

• Insulating Medium: Mineral oil

• Cooling Method: ONAN or ONAF with external radiators

• Breather System: Air enters through a desiccant chamber, controlling humidity

Performance Characteristics

• Voltage Rating: Up to 220 kV and beyond

• Power Capacity: From 100 kVA to over 500 MVA

• BIL Rating: Up to 550 kV for EHV applications

• Temperature Rise: 55°C or 65°C depending on design

Advantages

• Cost-Efficient: Simpler construction reduces initial investment

• Greater Oil Expansion Handling: Conservator offers more flexibility for thermal variation

• Easier Inspection and Servicing: Accessible conservator and breather components

Limitations

• Exposure to Air and Moisture: Over time, breather effectiveness reduces, risking oil contamination

• Regular Maintenance Required: Silica gel replacement, oil checks, conservator cleaning

• Not Ideal for Polluted Environments: Susceptible to airborne contaminants

Technical Comparison Table

Application Scenarios

Hermetically Sealed Transformers Are Ideal For:

• Underground substations and tunnels

• Marine, coastal, or tropical regions with high humidity

• Urban or indoor installations where fire-safety and low maintenance are crucial

• Renewable energy applications where sealed and ester-oil-filled designs are preferred

Conservator Type Transformers Are Best Suited For:

• Outdoor grid-level substations

• High-voltage transmission and distribution

• Industrial power plants with accessible maintenance teams

• Long-life assets in controlled environments

Conclusion

Choosing between a Hermetically Sealed Transformer（fully sealed transformer）and a Conservator Type Transformer（non-fully sealed transformer）requires careful evaluation of your operating environment, maintenance capability, and voltage/power requirements. While sealed designs offer superior protection and minimal upkeep, conservator transformers remain the most cost-effective and scalable choice for large substations and high-voltage installations.