Oil cooled transformer

Reliable voltage transformation, efficient heat dissipation and long-term stable operation are the requirements for power systems that use oil-cooled transformers. The insulating oil is used for dissipation of heat from the core and windings and for electrical insulation. This makes them particularly well suited for outdoor substation, industrial, utility distribution, renewable energy and medium to high capacity transformer applications.

But the right transformer is much more than a kVA or MVA rating. In addition, buyers should consider voltage class, cooling method, vector group, impedance, losses, insulation requirement, applicable standards, installation conditions, testing requirements and total cost of ownership.

This guide will explain what oil cooled transformers are, how they work, how to choose the right model, what specifications to look for as a buyer, and how to assess reliable transformer manufacturers.

What Is an Oil Cooled Transformer?

Oil cooled transformer(Oil Immersed Transformer)in which insulating oil is used as a cooling medium and electrical insulating medium.
In operation heat is produced in the windings by the current passing through them and in the iron core by the core losses. If this heat is not removed efficiently, high temperatures can increase the aging of the insulation and reduce the transformer service life.
Oil-filled transformers have their iron core and windings submerged in insulating oil. The oil takes the heat produced inside the transformer and carries it to the tank and cooling system of the transformer. Depending on the transformer design, the heat is then dissipated by radiators, cooling fans, heat exchangers or other cooling equipment.

The main components typically include:

Transformer oil quality and condition are essential for reliable long-term operation as the oil performs both cooling and insulating functions .

How Do Oil Cooled Transformers Work?

The basic principle on which an oil cooled transformer operates is electromagnetic induction. An alternating voltage applied to the primary winding creates an alternating magnetic flux in the transformer core which, in turn, induces a voltage in the secondary winding. At the same time electrical and magnetic losses heat up.

This is what the cooling process generally looks like:

Core & Windings → Transformer Oil → Tank/Radiators → Ambient Air

The oil takes heat from the core and windings. The heated oil is circulated by natural or forced circulation to cooler areas as the oil temperature increases. Then, the heat is dissipated to the surrounding environment through the surfaces of the transformer tank or radiators. This process enables the transformer to operate in a proper temperature range.

Core and Windings

Magnetic cores are usually made from high quality electrical steel to minimize iron losses. Windings are made to the required voltage, current, insulation class and transformer capacity.

The design of the core and windings directly influence:

• No-load losses

• Load losses

• Temperature rise

• Efficiency

• Short-circuit strength

• Service life

Transformer Oil

There are two essential functions of transformer oil.

1. Electrical insulation

Oil also serves to improve the dielectric strength between energized components and between the windings and the grounded transformer tank.

2. Thermal transfer

The oil transports heat from the core and windings to the cooling surfaces.

This combination makes oil cooled technology especially valuable for transformers operating at higher capacities.

Main Types of Oil Cooled Transformers

Depending on the transformer capacity, operating conditions, installation requirements and project specifications, different cooling systems are used.

ONAN Transformer

ONAN means: Oil Natural Air Natural

The transformer oil is circulated naturally by temperature difference inside the tank and heat is dissipated naturally into the surrounding air.

The ONAN (Oil-Immersed Natural-Air-Cooled) method is extensively used for distribution transformers and many medium capacity transformers, due to its simple construction, low maintenance requirements and reliable operation.

Advantages of ONAN Cooling：

• Simple design

• No cooling pump required

• No cooling fan required under normal conditions

• Lower maintenance requirements

• High reliability

• Suitable for many outdoor distribution applications

ONAF Transformer

ONAF means: Oil Natural Air Forced

The oil circulation is natural but fans are fitted to force air through the radiator.

The ONAF method provides better heat dissipation than the ONAN cooling method, which means that the same physical size transformer can carry higher loads or be of larger capacity.

This cooling method is normally used for large distribution transformers and power transformers.

OFAF Transformer

OFAF means: Oil Forced Air Forced

The transformer oil is circulated by an oil pump. Fans are used to force air over the cooling radiators or heat exchangers.

This arrangement gives more cooling capacity and is used for large power transformers where the natural circulation of oil is not enough to fulfilll the cooling requirements.

OFWF Transformer

OFWF means: Oil Forced Water Forced

This type of cooling uses oil to flow through a heat exchanger . Water is used to dissipate the heat .

The OFWF system is suitable for large capacity transformers and some industrial or power applications, i.e. where normal air cooling is not satisfactory or where it is desirable to have water cooling due to installation conditions.

Oil Cooled Transformer Cooling Method Comparison

For most conventional distribution transformer applications ONAN cooling provides a practical balance of reliability, simplicity and cost. Larger projects may require more complex cooling systems.

When Should You Choose an Oil Cooled Transformer?

Oil cooled technology is often a good choice when:

1. Requires large capacity

2. To be installed in outdoor

3. Efficient heat dissipation is essential

4.The project comprises a utility substation.

5. The transformer will be working with heavy load all the time

6.A long service life is a priority

7. The installation environment permits oil-filled equipment.

Why Choose Oil Cooled Transformer?

Dry type transformers are preferred when indoor installation, fire safety, space limitations or specific environmental requirements are more important.

Where Are Oil Cooled Transformers Used?

1.Utility Distribution Networks

Electric utilities use oil-cooled distribution transformers to reduce medium-voltage electricity to levels used by residential, commercial and industrial consumers.

Typical applications include:

• Distribution substations

• Utility networks

• Rural electrification

• Industrial distribution systems

2.Industrial Plants

Manufacturing plants often need transformers to convert medium-voltage utility power into usable low-voltage electricity.

Applications include:

• Manufacturing plants

• Steel mills

• Mining facilities

• Chemical plants

• Machinery factories

• Processing facilities

3.Solar Power Projects

Solar farms have photovoltaic systems that generate electricity. Transformers are needed to collect and change the electricity so that it can be connected to a medium- or high-voltage grid.

Oil cooled transformers are widely considered for:

• Solar power plants

• PV collection systems

• Step-up transformer applications

• Renewable energy substations

4.Wind Power Projects

Wind farms also need transformers to step up the voltage they produce so that it can be transmitted efficiently.

Oil cooled transformer solutions can be used in:

• Wind farms

• Wind turbine transformer systems

• Collection substations

• Grid connection projects

5.Infrastructure and Commercial Projects

Depending on local requirements, oil cooled transformers can also be used in large infrastructure projects where outdoor substations and reliable medium voltage distribution are needed.

How to Select the Right Oil Cooled Transformer？

A systematic review of electrical and environmental needs requires the selection of an appropriate transformer.
Step 1: Determine How Much You Need
The first step is to find the required transformer capacity.
The typical ratings are: 100 kVA, 160 kVA, 250 kVA, 315 kVA, 400 kVA, 500 kVA, 630 kVA, 800 kVA, 1000 kVA, 1250 kVA, 1600 kVA, 2000 kVA, 2500 kVA
High MVA ratings for large power applications
The transformer should not be selected on the basis of today’s load only. Other considerations include future load growth, starting currents, seasonal variations and required operating margins.
Step 2: Find the High-Voltage Level
The primary voltage must be compatible with the upstream power system.
The medium voltage levels are usually: 6.6 kV 10 kV 11 kV 13.8 kV 20 kV 22 kV 33 kV 35 kV
The actual voltage should always be taken from the local grid and project specifications.
Step 3: Identify the low-voltage level
The secondary voltage is dependant on the downstream electrical system.
For instance, local regulations and equipment requirements may differ between industrial and commercial projects, and different LV configurations may be required.
Step 4: Choose the Cooling Method
ONAN is good for many standard distribution applications.
Larger transformers may need: ONAF, OFAF, OFWF
The choice of a transformer is based on its capacity, temperature rise, ambient conditions and project specifications.
Step 5: Find the Vector Group
The vector group describes the relation between the primary and secondary winding connections and phase displacement.
Typical configurations are: YNd11, Yyn0, Dyn11
The vector group must be compatible with the existing electrical grid.
Step 6: Think About the Install Environment

Important environmental factors include:

• Ambient temperature

• Altitude

• Humidity

• Dust

• Salt spray

• Coastal conditions

• Outdoor or indoor installation

• Solar radiation

• Ventilation

• Seismic conditions

For projects situated in hot climates or at high altitudes, transformer design may have to be modified accordingly.
Step 7: Identify the Relevant Standards
The transformer shall be manufactured and tested as per project specified standards.
Depending on the target market, buyers may require IEC, IEEE/ANSI or applicable local standards.

Oil Cooled Transformer Specifications Buyers Should Check

Before placing an order, B2B buyers should review the complete technical specification rather than focusing only on transformer capacity.

A professional transformer supplier should be able to provide a detailed technical datasheet before order confirmation.

What Affects Oil Cooled Transformer Price?

There is no one standard price for an Oil Cooled Transformer, as the final cost depends on technical specifications and project requirements.
The main cost factors are:
1. Capacity of Transformer
The construction and materials of a 100 kVA transformer and a 5 MVA transformer are vastly different.
2. Voltage Level
Transformers of higher voltages generally need more sophisticated insulation systems and components.
3. Core Material
The losses and cost of the transformer are directly affected by the quality and design of the core steel.
4. Wire, winding
The electrical, mechanical and the cost characteristics of copper and aluminum windings are different.
5. Cooling System 

ONAN systems are generally simpler than forced air cooling systems.
6. Tap chager
The costs of an off-circuit tap changer and an on-load tap changer can be quite different.
7. Requirements regarding efficiency
Transformers with low losses may need more optimized designs and higher-grade materials.
8. Testing and Standards
Special testing requirements, certification and standards unique to a project can impact manufacturing costs.

9. Accessories
Additional components like:

• Buchholz relay

• Pressure relief valve

• Oil level indicator

• Temperature indicator

• Ventilatores

• Monitoring equipment

10. Logistics and Place of Destination
Other factors that can affect the final project cost are packaging, inland transportation, port handling, shipping distance and destination requirements.
That’s why comparing suppliers only on the initial purchase price can be misleading.

What Standards Apply to Oil Cooled Transformers?

International transformer projects commonly reference recognized technical standards.

Depending on the destination and project requirements, buyers may need to consider:

• IEC standards

• IEEE standards

• ANSI requirements

• National or regional grid standards

• Utility-specific specifications

Standards can cover areas such as:

• Transformer design

• Insulation

• Temperature rise

• Electrical performance

• Routine testing

• Type testing

• Safety

• Energy efficiency

Transformer Oil Selection: Mineral Oil vs. Ester Oil

Transformer oil is an important part of oil cooled transformer performance.

Mineral Transformer Oil

Mineral oil remains widely used because of its established performance, availability, and cost-effectiveness.

It is suitable for many conventional distribution and power transformer applications.

Ester-Based Transformer Fluids

Natural and synthetic ester fluids can provide advantages in applications where fire safety and environmental considerations are especially important.

Potential benefits can include:

• Higher fire point

• Improved environmental profile

• Biodegradability for suitable natural ester fluids

• Application flexibility

However, ester based insulating fluids may be more expensive initially. Choice should depend on transformer design and project requirements.
It is important to consult with the transformer manufacturer to specify the proper insulating fluid.

How to Choose a Reliable Oil Cooled Transformer Manufacturer

Choosing the manufacturer can be just as important as choosing the transformer specification.

B2B buyers should evaluate suppliers according to several factors.

1. Manufacturing Capability

Check whether the manufacturer has dedicated production facilities and suitable equipment for transformer manufacturing.

2. Quality Control

Ask how the manufacturer controls:

• Incoming materials

• Core production

• Winding

• Assembly

• Oil processing

• Final testing

3. Testing Capability

A professional manufacturer should have appropriate testing equipment and documented test procedures.

4. International Standards

Confirm whether the supplier has experience manufacturing transformers according to the standards required in your market.

5. Export Experience

International projects require more than manufacturing capability.

The supplier should understand:

• Export packaging

• Shipping documentation

• Port logistics

• Destination requirements

• International communication

• Project coordination

6. Customization Capability

Different countries and projects have different requirements.

A capable manufacturer should be able to customize transformer specifications according to:

• Voltage

• Capacity

• Frequency

• Vector group

• Cooling method

• Tap range

• Accessories

• Local standards

Why Choose Shandong Future Electric for Oil Cooled Transformers?

Shandong Future Electric is dedicated to developing and manufacturing transformer and power distribution equipment for the global market.
The company has a smart factory of 30,000+m2 and experience in serving customers in 70+ export countries.
Its oil immersed transformer line includes commonly specified models such as: S11 、S13 、S20 、S22

According to the project requirements of efficiency, capacity, voltage ratings and applicable standards, suitable series of products can be selected.
The value of dealing with the manufacturer direct for overseas B2B clients is more than just the transformer products. It includes services such as technical communication, specification confirmation, production management, factory testing, export packaging and project delivery coordination.
For customized projects, the buyer can provide capacity, voltage, frequency, vector group, cooling method, standards, quantity, destination and other details and then the technical team will develop suitable solutions.

Oil Cooled Transformer FAQ

What is an Oil Cooled Transformer ?
An oil cooled transformer uses insulating oil to insulate and to transfer heat away from the core and windings.
What is difference between oil cooled and oil immersed transformer?
In the transformer industry, the terms are commonly used interchangeably. “Oil immersed” emphasizes the fact that the active parts are immersed in insulating oil, while “oil cooled” emphasizes the cooling function of the oil.
What is ONAN cooling?
ONAN stands for Oil Natural Air Natural. The transformer oil circulates by natural means and the heat dissipates naturally to the surrounding air.
What is cooling ONAF?
ONAF is Oil Natural Air Forced. Air is blown over the radiators to improve heat dissipation, while natural oil circulation is maintained.
Are oil-cooled transformers suitable for outdoor applications?
Yeah. Oil cooled transformers are widely used in outdoor substations and utility distribution systems, provided the transformer and installation meet the applicable environmental, safety and local requirements.
How to Choose the Right Transformer Capacity?
The present load, future load growth, starting currents, operating conditions, required margin and the characteristics of the electrical network will be taken into consideration in determining the capacity.
What is the cost of an oil cooled transformer?
Price varies with capacity, voltage, core and winding materials, efficiency requirements, cooling system, tap changer, accessories, testing requirements, standards and point of delivery.
What is the life of an oil cooled transformer？
The service life is dependent on transformer design, load, temperature, insulation aging, maintenance, oil condition, environmental conditions and operating practices. A well designed and maintained transformer can provide many years of service.
What do I need to give to be able to get a quotation?
Must include, at a minimum: Capacity, voltage (HV/LV), frequency, phase, cooling method, vector group, tap requirements, applicable standards, quantity, and locati0n of installation/delivery.

Conclusion: Selecting the Appropriate Oil Cooled Transformer

Oil-cooled transformers are primarily used in public grid distribution, industrial facilities, renewable energy projects, substations and other applications that require reliable and efficient power transformation.

There is more to choosing the right transformer than capacity and price. Buyers should review technical specifications in detail such as voltage, cooling method, vector group, losses, insulation level, oil type, applicable standards, testing requirements, installation environment and total cost of ownership.

Shandong Future Electric provides oil-immersed transformer solutions including S11, S13, S20 and S22 series supported by a smart manufacturing facility with a footprint of over 30,000 square meters and a global presence in over 70 countries.

Whether it’s designing a new power distribution project, upgrading an existing substation or sourcing transformers for an international project, we can recommend the optimum solution to suit your needs.

Ask now for oil cooled transformers price quote. Our engineering team will help you in selecting the suitable capacity, voltage rating, cooling method and configuration for your project.