Oil-immersed transformers play a vital role in electrical power systems around the world. These devices are essential for stepping up or stepping down voltage levels in power transmission and distribution networks. The term "oil-immersed" refers to the cooling and insulation method used in the transformer—specifically, the use of insulating oil to manage heat and protect internal components.

This article covers the structure, working principles, advantages, and key considerations in the use of oil-immersed Transformer.

What Is an Oil-Immersed Transformer?

An oil-immersed transformer is a type of power transformer where the core and windings are immersed in a special insulating oil—typically mineral oil or synthetic ester. This oil serves two primary purposes:

1. Insulation: Prevents electrical breakdown between components.
   
   

2. Cooling: Transfers heat generated in the windings and core to the outer radiator or cooling fins.
   
   

These transformers are widely used in high-voltage and medium-voltage applications, especially where reliability and long service life are required.

Construction of an Oil-Immersed Transformer

A typical oil-immersed transformer consists of the following components:

1. Core

• Made of laminated silicon steel sheets.
  
  

• Provides a low-reluctance path for magnetic flux.
  
  

2. Windings

• Primary and secondary windings are made from copper or aluminum.
  
  

• Insulated with cellulose-based paper.
  
  

3. Insulating Oil

• Dielectric fluid used for both insulation and cooling.
  
  

• Commonly used oils include mineral oil, synthetic ester, or silicone oil.
  
  

4. Tank

• Hermetically sealed steel container filled with insulating oil.
  
  

• Protects internal parts from environmental conditions.
  
  

5. Conservator (in some designs)

• A cylindrical tank mounted above the main tank.
  
  

• Accommodates the expansion and contraction of oil due to temperature changes.
  
  

6. Breather

• Contains silica gel to absorb moisture from the air entering the conservator.
  
  

7. Radiator or Cooling Fins

• Helps dissipate heat to the surrounding air.
  
  

8. Buchholz Relay (for protection)

• Gas-actuated relay placed between the transformer tank and conservator.
  
  

• Detects gas accumulation from internal faults.
  
  

Working Principle

The working of an oil-immersed transformer is based on the principle of electromagnetic induction. When alternating current flows through the primary winding, it creates a magnetic field in the core. This magnetic field induces a voltage in the secondary winding based on the turns ratio between the windings.

As current flows and losses occur (mainly I²R and core losses), heat is generated. The oil absorbs this heat and circulates within the tank. As it heats up, it rises and transfers heat to the radiator or cooling fins, from where it is dissipated into the environment.

Types of Oil-Immersed Transformers

1. Distribution Transformers

• Low to medium voltage.
  
  

• Commonly used in residential and commercial electrical distribution.
  
  

2. Power Transformers

• High voltage, typically above 33 kV.
  
  

• Used in transmission substations and power plants.
  
  

3. Hermetically Sealed Transformers

• No conservator tank.
  
  

• Fully sealed to avoid contact with air and moisture.
  
  

4. Free Breathing Transformers

• Include a conservator and breather.
  
  

• Open to atmospheric pressure, with silica gel to dry incoming air.
  
  

Advantages of Oil-Immersed Transformers

• High Reliability: Excellent insulation and cooling result in long life.
  
  

• High Load Capacity: Better heat dissipation supports higher load operation.
  
  

• Durability: Can withstand harsh environmental conditions.
  
  

• Proven Technology: Used reliably in power systems for decades.
  
  

Disadvantages

• Fire Risk: Mineral oil is flammable, posing a fire hazard.
  
  

• Maintenance Requirements: Needs regular oil testing and monitoring.
  
  

• Environmental Impact: Oil leaks can contaminate soil and water.
  
  

• Bulkier: Larger and heavier than dry-type transformers.
  
  

Maintenance Practices

Maintaining oil-immersed transformers is crucial for ensuring long-term reliability and safety. Common maintenance tasks include:

• Oil Testing: To check for dielectric strength, moisture content, and dissolved gases.
  
  

• Visual Inspection: Look for leaks, rust, or damage to cooling fins.
  
  

• Buchholz Relay Testing: Ensures fault detection functionality.
  
  

• Cleaning and Tightening: Keeps terminals clean and connections tight.
  
  

• Thermal Imaging: Identifies hotspots and irregular heating.
  
  

Safety and Environmental Considerations

• Fire Protection: Use of fire-resistant oils (e.g., natural esters) in sensitive areas.
  
  

• Spill Containment: Installation of bunds or catch basins around transformer base.
  
  

• Oil Recycling: Old transformer oil must be handled according to environmental regulations.
  
  

• Arc Flash Protection: Proper grounding and isolation to prevent arcing accidents.
  
  

Oil vs. Dry-Type Transformers

Applications

• Power Generation Plants
  
  

• Substations (urban and rural)
  
  

• Industrial Facilities
  
  

• High-rise Buildings (with oil containment)
  
  

• Renewable Energy Systems (e.g., wind and solar farms)
  
  

Conclusion

Oil-immersed transformers  remain a cornerstone of modern electrical infrastructure due to their robustness, efficiency, and ability to handle high power loads. While they require careful handling and regular maintenance, their long lifespan and performance make them indispensable in both transmission and distribution networks.

As technology evolves, the use of safer insulating fluids and smarter monitoring systems is helping extend the functionality and reliability of oil-immersed transformers into the future.