Oil-Immersed Transformer Structure and Working Principle: A Complete Technical Guide

Oil-immersed transformers play an essential role in today’s power transmission and distribution networks. From utility substations and industrial plants to commercial buildings and rural grid systems, these transformers ensure stable voltage conversion, high reliability, and long service life.

This article provides a clear, professional explanation of the structure and working principle of oil-immersed transformers, helping engineers, procurement teams, and end-users better understand their performance and applications.

What Is an Oil-Immersed Transformer?

An oil-immersed transformer (also called oil transformer) is a type of power or distribution transformer in which the core and windings are fully submerged in insulating transformer oil. The oil performs two primary functions:

1. Insulation: Prevents electrical breakdown between components
2. Cooling: Transfers heat from the windings and core to the tank surface

These transformers are used across both power transmission and distribution networks, commonly classified as:

• Oil Immersed Power Transformer – for high-voltage, high-capacity applications
• Oil Immersed Distribution Transformer – for 10kV/20kV/35kV to 230/400V distribution

Their ability to handle large loads, overload conditions, and harsh outdoor environments makes them the backbone of global power infrastructure.

Structural Components of Oil-Immersed Transformers

Although designs vary among manufacturers, a standard oil-immersed transformer typically includes the following components:

1. Core (Magnetic Circuit)

The transformer core is the magnetic pathway that guides alternating magnetic flux. It is usually made of:

• Cold-rolled grain-oriented silicon steel laminations
• Thickness commonly less than 0.3 mm
• Low hysteresis and eddy-current losses

Two common structural types exist:

• Core-type transformer – windings surround the core limbs
• Shell-type transformer – the core surrounds the windings

Most power and distribution transformers today use the core-type structure due to better cooling and mechanical strength.

Large-capacity transformers include oil ducts inside the core so that circulating oil can efficiently remove heat from the magnetic circuit.

2. Windings (Primary & Secondary Coils)

The windings are responsible for voltage conversion through electromagnetic induction. They are typically made of copper or aluminum conductors, insulated with high-temperature materials.

Common winding structures include:

• Concentric windings
• Interleaved or disc windings
• Layered windings for HV and LV coils

Key considerations for windings:

• Must withstand short-circuit mechanical forces
• Must dissipate heat effectively into transformer oil
• Require robust insulation to prevent inter-turn and phase-to-ground faults

Typical winding failures include:

• Inter-turn short circuits (due to insulation aging or overload)
• Phase-to-ground faults (due to moisture, oil degradation, or overvoltage)
• Deformation caused by short-circuit current forces

Oil circulation helps maintain insulation integrity and dissipates heat generated by the I²R losses.

3. Transformer Oil & Oil Tank

The oil tank houses the core and windings and is filled with high-grade mineral transformer oil.
The tank performs these functions:

• Holds the transformer oil
• Provides a sealed environment to prevent oxidation
• Supports the radiator fins for heat dissipation

Small- and medium-sized oil immersed transformers typically use:

• Sealed oil tanks (hermetically sealed)
• Corrugated tank walls to allow thermal expansion
• Radiators or cooling fins for enhanced heat dissipation

Larger transformers may include oil conservators (oil expansion tank) to maintain pressure balance and prevent oil-air contact.

4. Tap Changer

The tap changer is used to adjust the transformer’s output voltage to match grid fluctuations.

Types include:

• Off-load tap changer (OLTC) – adjusted when transformer is de-energized
• On-load tap changer (LTC/OLTC) – adjusts voltage automatically during operation

Tap changers ensure stable secondary voltage even with fluctuating primary grid conditions.

5. Bushings

Bushings provide high-voltage insulation for conductors passing through the transformer tank wall.

Types:

• Porcelain bushings
• Composite bushings
• Oil-filled bushings for high-voltage applications

They must withstand high voltage, environmental stresses, and mechanical loads.

6. Protection & Monitoring Devices

Oil-immersed transformers include various protection mechanisms:

• Buchholz relay – detects gas accumulation from internal faults
• Pressure relief valve – releases excess internal pressure
• Temperature sensors/thermometers – monitor oil and winding temperature
• Oil level indicator – ensures proper oil volume
• Breather (silica gel) – keeps moisture out of the conservator system

These devices ensure safe, long-term operation.

Working Principle of Oil-Immersed Transformers

Oil-immersed transformers operate based on the principle of electromagnetic induction.

Let’s break down the process:

1. Voltage Is Applied to the Primary Winding

When AC voltage is applied to the primary coil, an alternating current flows, generating a magnetic field in the iron core.

2. Magnetic Flux Links to the Secondary Winding

The alternating magnetic flux passes through the core and links both primary and secondary windings.

The induced voltage obeys the transformer EMF equation: E=4.44fNΦmax​

Where:

• E = induced voltage
• f = frequency
• N = number of turns
• Φmax = maximum magnetic flux

Different turn ratios create different output voltages.

3. Power Transfer Through Magnetic Coupling

• When the secondary side is open-circuit, only magnetizing current flows in the primary winding.
• When the secondary is loaded, current flows in the load, generating opposing flux.
• The primary winding automatically adjusts its current to maintain magnetic flux equilibrium.

This dynamic balance enables energy transfer from primary to secondary without direct electrical contact.

4. Oil Cooling & Insulation

Transformer oil continually circulates due to natural convection or forced oil pumps, performing several vital functions:

• Transfers heat from windings and core to the tank wall
• Enhances insulation strength
• Prevents moisture ingress
• Reduces oxidation and thermal degradation

This oil-based cooling allows oil-immersed transformers to handle much higher capacities than dry-type transformers.

Advantages of Oil-Immersed Transformers

1. Excellent Cooling Capability

Oil provides superior heat dissipation, allowing higher load capacity and longer life.

2. High Electrical Strength

Transformer oil significantly enhances insulation strength between components.

3. Long Service Life

Properly maintained oil-immersed transformers often exceed 25–35 years of operation.

4. Suitable for Outdoor Installation

They perform reliably under harsh weather conditions.

5. Strong Overload Handling

Oil cooling enables transformers to withstand temporary overloads without severe insulation damage.

Applications of Oil-Immersed Transformers

Oil-immersed transformers are widely used in:

• Utility substations
• Industrial manufacturing plants
• Mining operations
• Rural and urban distribution networks
• Renewable energy projects
• Commercial and residential power systems

Conclusion

Oil-immersed transformers remain the preferred choice for global power distribution thanks to their robust structure, high thermal performance, and long-term reliability. Understanding their components and working principles helps users make informed decisions in procurement, operation, and maintenance.