What Is an Oil-Immersed Transformer?

An Oil-Immersed Transformer is an electrical transformer in which the magnetic core and windings are immersed in insulating liquid within a transformer tank.
An oil-immersed design uses transformer oil as both an electrical insulation medium and a heat-transfer medium, unlike a dry-type transformer that depends mainly on air and solid insulation.
Electrical losses in the core and windings produce heat during operation. The insulating oil absorbs this heat and passes it toward the tank walls, radiators, or other cooling equipment. The dielectric insulation strength between energized components is simultaneously increased by the oil.
The combination of insulation and cooling makes the Oil-Immersed Transformer suitable for a wide range of medium- and high-voltage applications, including utility substations, industrial plants, renewable-energy projects, commercial infrastructure and distribution networks.
An oil-immersed transformer can be built as a distribution transformer, power transformer, single-phase transformer, three-phase transformer, hermetically sealed transformer, conservator-type transformer or specialized transformer depending on the design.

How Does an Oil-Immersed Transformer Work?

The working principle of an Oil-Immersed Transformer is based on electromagnetic induction.

The basic energy-transfer process can be simplified as:

AC Input → Primary Winding → Magnetic Flux → Transformer Core → Secondary Winding → AC Output

When an AC voltage is applied to the primary winding, it produces an alternating magnetic flux in the transformer core.

The magnetic flux links the secondary winding and induces an electromotive force according to the transformer turns ratio.

The relationship between voltage and winding turns can be expressed approximately as:

V₁ / V₂ ≈ N₁ / N₂

V₁ = primary voltage

V₂ = secondary voltage

N₁ = primary winding turns

N₂ = secondary winding turns

In general, if the secondary winding has fewer turns than the primary winding, the transformer will be a step down transformer.
If the secondary winding has more turns it is a step up transformer.
At the same time the transformer heats up because of:

• Core losses 

• Loss of copper 

• Stray Losses 

• Losses from dielectric
  

The insulating oil absorbs this heat and carries it through the transformer cooling system. Depending on the design, circulation may be natural or assisted by pumps, and heat may be released through radiators and fans.

Therefore, an oil-immersed transformer has two closely related operating systems:

This combination allows oil-immersed transformers to operate continuously under demanding electrical loads.

Main Components of an Oil-Immersed Transformer

It is important to know the main components when evaluating an Oil-Immersed Transformer for a power distribution or industrial project.
1. Magnetkern
The transformer core provides a low reluctance path for the magnetic flux.
Modern transformer cores are typically made of laminated electrical steel to reduce the eddy-current losses.
2. HV and LV Winding
The high voltage and low voltage windings are responsible for the transfer of electrical energy by electromagnetic induction.
The windings are usually made of copper or aluminum, according to the electrical, thermal, mechanical and cost characteristics required.
3. Transformer Oil
Transformer oil serves two vital functions: 

Electrical insulation

Heat transfer
Oil must have good dielectric strength, thermal properties, chemical stability and moisture control.
4. Transformer Tank
The tank holds the core, windings and insulating liquid.
It must be strong enough and seal well enough to support the operating conditions and the environment.
5. Radiators
Radiators provide a greater surface for the transfer of heat.
Hot oil flowing through the radiator walls transfers that heat to the surrounding air.
6. Reserve tank
Larger oil-immersed transformers may have a conservator tank to accommodate thermal expansion and contraction of the insulating oil.
7. Respite
In conservator-type transformers, a silica-gel breather may be used to reduce moisture entering the transformer as air enters and exits during temperature changes.
8. Buchholz Relay
Some conservator-type designs can detect some internal faults by the Buchholz relay for gas accumulation and oil-flow abnormalities.
9. Release Device for Pressure
A pressure relief device is used to protect the transformer tank from excessive internal pressure.
10. Tap Changer

Depending on the application, the voltage ratio of a transformer may be varied by means of off-circuit or on-load tap-changing equipment.

What Are the Main Types of Oil-Immersed Transformers?

Oil-immersed Transformer has no single classification system.
Transformers can be categorized based on application, cooling system, phase configuration, construction, oil preservation method and electrical function.

1. Oil Immersed Distribution Transformers

Distribution transformers that are filled with oil are usually located closer to the electricity consumers.
Its main function is to convert medium voltage to a lower voltage that can be used for:

• Distribution residential

• Commercial buildings

• Facilities for agriculture

• Small and medium sized industrial plants

• Local distribution networks

Common designs are three-phase and single-phase oil-immersed transformers.

2. Oil Immersed Power Transformer

In general, an oil immersed power transformer is rated for higher power ratings and transmission or substation applications.
These transformers are applicable in:

• Power generation plants

• Substations for transmission

• Interconnection projects with the grid

• Large industrial plants

• Substations for Renewable Energy

Large power transformers may have advanced cooling systems such as ONAF or OFAF, depending on the required capacity and thermal performance.

3. Single Phase Oil Immersed Transformer

A single phase transformer is used in single phase electrical systems.
It is widely used in: 

• Rural Electrification

• Distribution in the home 

• Systems for small businesses 

• Special electrical installations

4. Three phase oil immersed transformer  

Three phase oil immersed transformer is widely used in industrial and utility power systems.
The three-phase design would be an integrated solution and would have a more compact overall design than three separate single-phase transformers.
Typical applications are:

• Factories Distribution substations

• commercial infrastructure 

• Solar farms 

• Wind farms Utility networks

Oil-Immersed Transformer Cooling Methods

The most important factor in transformer design is the cooling.
The heating caused by transformer losses. High temperature can speed up the aging of the insulation.
Some common oil immersed transformer cooling classifications are : ONAN, ONAF, OFAF and OFWF.

ONAN

ONAN is one of the most common cooling arrangements for distribution transformers.

As the oil heats up it rises . Cooler oil moves down . This creates a natural circulation.

The radiators give off heat to the outside air.

ONAF

An ONAF system still depends primarily on natural oil circulation, but the fans help increase the flow of air through the radiators.

This leads to a better heat dissipation than the standard ONAN cooling.

OFAF

OFAF employs oil pumps and forced-air cooling.

It is valid for larger transformers where natural circulation is not capable of providing the required thermal capacity.

What Types of Transformer Oil Are Used?

Transformer oil is a necessary component of oil immersed transformer.

As the oil heats up it rises . Cooler oil moves down . This creates a natural circulation.

The radiators give off heat to the outside air.

Mineral Oil

Mineral oil is widely used due to its well known electrical and thermal performance and relatively competitive cost.

Natural Ester Oils

Natural ester fluids are renewable and are increasingly being considered where biodegradability and fire-safety characteristics are important.

Synthetic Ester Oil

Synthetic ester fluids may provide useful fire safety and insulation properties for applications with special installation requirements.

Silicone Oils

Silicone fluids may be considered for special applications where thermal and fire-performance properties are required.

The choice of the appropriate liquid should be based on the transformer design, relevant standards, operating environment, fire safety requirements, and project specifications.

Advantages of Oil-Immersed Transformers/Why are oil-immersed transformers still widely used?

The answer is not only the price. As the oil heats up it rises. Cooler oil moves down. This creates a natural circulation.

The radiators give off heat to the outside air.

ONAF

An ONAF system still depends primarily on natural oil circulation, but the fans help increase the flow of air through the radiators.

This leads to a better heat dissipation than the standard ONAN cooling.

OFAF

They are applicable to many utility and industrial applications, featuring a combination of cooling capability, insulation performance, scalability and long established field experience.

1. Effective Heat Release

In many transformer applications, oil is a better medium for heat transfer than air.

This makes oil-immersed designs suitable for relatively high electrical loads.

2. High Voltage Capability

Liquid insulation offers high dielectric performance and can be designed for medium, high and extra high voltage applications.

3. Long useful life

An oil immersed transformer, with proper design, installation, monitoring and maintenance can provide many years of service.

The actual service life depends on thermal stress, insulation aging, oil quality, loading, environmental conditions, maintenance practices, etc.

4. Suitable for High-Capacity Uses

Oil cooling can be supplemented with radiators, fans, pumps and other cooling equipment.

This makes it possible to use the technology from distribution level transformers to large power transformers.

5. Design Freedom

An oil immersed transformer can be customized according to:

Power Rating

Mains voltage

Second voltage 

Frequency

Impedance

Tap Range

Cooling method

Oil Type Installation environment

Protection requirements

6. Proven Technology

Oil immersed transformer technology has been used in electrical grids for decades.

This means that there is a lot of experience with its design and operation among manufacturers, utilities, EPC contractors, engineers and maintenance teams.

Where Are Oil-Immersed Transformers Used?

Oil-Immersed Transformer has broad application scope.
Electricity grids
The oil-immersed transformers are used by utility companies for transmission and distribution networks.
Typical applications are:

Switch yards
Substations for distribution

Grid interconnections

Industrial Plants

Voltage conversion
Heavy industrial plants often require large amounts of electrical power.
Oil Immersed transformers may be used to supply:

Motors

Production Lines

Pumps

Furnaces Processing Equipment

Industrial control systems (ICS)
Renewable Energy Projects
Solar and wind projects require transformers to connect the electricity they produce to collection and grid systems.
Oil-immersed transformers are often used in:

Solar farms Wind farms

Renewable energy substations

Energy storage and battery infrastructure
Commercial Facilities
Oil-immersed transformers are also suitable for external electrical infrastructure, for example: 

Centres commerciales

Large commercial buildings 

Logistics park 

Airport 

Infrastructure development work/projects
Railroad and Transportation Systems
Special transformer designs can be employed in railroad electrification and traction power systems requiring voltage conversion and reliable power delivery.
Specifications for Standard Oil Filled Transformers
Buyers asking for quotes from a transformer manufacturer should state more than the kVA required.

A complete technical specification can significantly reduce quotation errors and redesign work.

The final specification should always be confirmed according to the applicable project standard and manufacturer's technical design.

How to Choose the Right Oil-Immersed Transformer?

When choosing an oil-immersed transformer, the electrical system should be the starting point, not the catalog.
Step 1: Understand your capacity needs
Calculate the actual and the expected load.
Think about it:

Load at present 

Future load growth 

Motor startup current 

Renewable energy production 

Load diversity

Required reserve balance
Don't just size a transformer based on the load you measure today.
Step 2: Check Primary and Secondary Voltages
The transformer must be compatible with the electrical systems upstream and downstream.
E.g. 11 kV to 400 V or 33 kV to 11 kV
Check the actual voltage from the electrical design of the project.
Step 3: Set Up the Phase Configuration
Select from: Single phase Three-phase depending on the distribution system.
Step 4: Choose the Cooling Method
ONAN may be adequate for many distribution transformers.
Larger transformers may require forced-cooling arrangements, such as ONA.
Cooling should be chosen based on: 

Size of transformer

Environment temperature 

Load profile 

Installation height

Overload capability required 

Room to dissipate heat
Step 5: Choose the Insulating Liquid
While mineral oil is still widely used, ester-based liquids can be considered if fire safety or environmental considerations are significant.
Step 6: Take the installation environment into account
Please inform the manufacturer if the transformer is to be installed in any of the following environments: coastal areas, tropical regions, desert areas, high-altitude locations, areas with industrial pollution, high-humidity environments, or urban substations.

Environmental conditions affect insulation, corrosion protection, cooling, enclosure design, and material selection.

Step 7: Look at Standards & Certifications
International projects may be subject to standards like: IEC、IEEE、ANSI、UL、National electrical code
From the project specification and destination market, the appropriate standard shall be determined.

Conclusion

Oil-immersed transformers are a mature and versatile technology utilized across power generation, transmission, distribution, industrial and renewable energy applications.
Its basic merit lies in the two-fold function of the insulating liquid, as an electrical insulator and as a medium for heat transfer.
Different types of transformers such as distribution transformers, power transformers, single-phase transformers, three-phase transformers, ONAN transformers, ONAF transformers, conservator-type transformers and hermetically sealed transformers allow for the adaptation of the technology to different electrical and environmental requirements.
But selection of oil immersed transformer should not only depend on capacity.
Consideration should be given to voltage, load profile, cooling method, insulating liquid, installation environment, fire protection, applicable standards, maintenance demand and future expansion.
For project buyers, EPC contractors, distributors and electrical engineers, the best procedure is to provide a complete technical specification to the manufacturer and to evaluate the transformer on the basis of initial cost and long-term operating requirements.

Frequently Asked Questions About Oil-Immersed Transformers

1. What is Oil Immersed Transformer?
An oil-immersed transformer is a transformer having its core and windings immersed in insulating liquid. The liquid is an electrical insulator and carries the heat away from the active parts.
2. Oil-pressurized transformer or oil-filled transformer?
In most commercial transformer applications, the oil-immersed transformer, the oil-filled transformer and the oil transformer are used to describe similar liquid insulated transformer design.
The exact terms may vary from manufacturer to manufacturer and technical document to technical document.
3. What is the major advantage of Oil Immersed Transformer?
Its capability to combine effective heat dissipation, electrical insulation, scalable design and medium- to high-capacity application is useful in a number of power-system applications.
4. What is the life of Oil-Immersed Transformer?
The service life is affected by loading, temperature, aging of insulation, oil condition, design quality, environment, maintenance and operating practices. With proper maintenance, a transformer can last decades.
5. What is ONAN in Transformer?
ONAN is Oil Natural Air Natural.
The insulating oil circulates in the transformer by natural convection and heat is transferred to the surrounding air by the tank and radiators.
6. What is ONAF?
ONAF is Oil Natural Air Forced
Oil circulation is still natural but fans blow air over the cooling surfaces to improve heat transfer .
7. Which is better, oil-immersed transformer or dry-type transformer?
The two technologies are meant for different operating conditions. Oil-immersed transformers are also used frequently for outdoor, utility and higher capacity applications. Dry-type transformers are often used for indoor or fire-sensitive applications. The choice depends on the project conditions and not on one technology being universally preferable.
8. What is the oil used in Oil-Immersed Transformer?
Common insulating liquids are mineral oil, natural ester, synthetic ester and special fluids such as silicone based liquids. The right choice is determined by electrical performance, thermal and fire safety, environment and project specifications.
9. What maintenance is required for an Oil-Immersed Transformer?
Maintenance may include oil testing, leak inspection, temperature monitoring, bushing inspection, electrical testing, cooling-system inspection and condition monitoring such as DGA where applicable.
10. Selecting The Proper Oil-Immersed Transformer
Start from the required capacity, primary and secondary voltage, phase, frequency, cooling method, installation environment, insulating liquid, applicable standards, protection requirements. For International Projects forward these details to the transformer manufacturer prior to requesting a quotation.