In the world of oil-immersed transformers, there is a seemingly ordinary yet crucial substance—transformer oil. It fills most of the internal space of a transformer, submerging the iron core and windings, and is an indispensable "blood" for the stable operation of transformers. So why oil specifically? Why not water, air, or other liquids?

This article will delve into the two key dual roles of transformer oil in oil-immersed transformers: insulation and cooling, and explore why it has become an irreplaceable choice in power transmission.

I. Insulation: Building an Invisible "Electrical Barrier"

The core function of a transformer is voltage conversion, which means an extremely high electric field intensity exists inside—huge voltage differences are borne between the high-voltage and low-voltage windings, and between the windings and the earthed iron core and oil tank. Without effective insulation measures, breakdown discharge will occur in an instant, leading to equipment damage and system failures.

1. Insulation Properties of Transformer Oil

Transformer oil is an excellent insulating medium mainly due to the following properties:

• High dielectric strength: Pure transformer oil has a much higher dielectric strength (breakdown voltage) than air. Under standard conditions, the breakdown voltage of transformer oil can reach 40-60kV/2.5mm, while that of air is only 3-4kV/mm. This means oil can withstand higher voltages without breakdown.
• Filling micro gaps: The windings of a transformer have complex geometric structures with various micro gaps between turns and among windings. Transformer oil has good fluidity and can penetrate all tiny gaps, eliminate air bubbles, and form a continuous insulating medium. In the field of electrical insulation, the presence of gas is often a weak point of insulation, and oil can effectively eliminate these hidden dangers.
• Oil-paper composite insulation: Oil-immersed transformers adopt the classic "oil-paper insulation structure"—combining insulating paper (cable paper, insulating cardboard) with transformer oil. The fibrous structure of paper can adsorb oil molecules to form an oil-paper composite insulation layer, whose breakdown strength is much higher than that of pure oil or paper alone. This composite structure not only exerts the mechanical supporting effect of paper but also utilizes the filling and self-healing properties of oil.

2. Self-healing Ability

This is a very unique advantage of transformer oil. When partial discharge or slight breakdown occurs in the oil, the generated gas may form air bubbles temporarily. But once the discharge stops, the surrounding oil will flow back quickly, refill the area, and restore the insulation performance. In contrast, once solid insulation is broken down, the damage is often permanent.

3. What If There Is No Oil?

What would happen if we tried to use air as the insulating medium?

• To achieve the same insulation level, the size of the transformer would have to be greatly increased (possibly several times larger), because the insulation strength of air is much lower than that of oil.
• Moisture, dust and pollutants in the air will adhere to the insulation surface, reduce the surface insulation strength, and increase the risk of surface discharge.
• Oxygen in the air will accelerate the oxidative aging of insulating materials and shorten the service life of the transformer.

II. Cooling: A Silent "Heat Mover"

Transformers generate a large amount of heat during operation—hysteresis loss and eddy current loss (iron loss) in the iron core, as well as resistance loss (copper loss) in the windings, are all converted into thermal energy. If the heat is not dissipated in time, the temperature will rise continuously, causing accelerated aging of insulating materials and eventually thermal breakdown.

1. Heat Generation and Hazards

The temperature distribution inside a transformer is not uniform. The windings and iron core are the main heat sources with obvious hot-spot temperatures. According to the standards of the International Electrotechnical Commission (IEC), the thermal aging rate of insulating paper doubles for every 6-8K increase in the hot-spot temperature of transformer windings. Controlling the temperature is controlling the service life of the transformer.

2. Cooling Mechanism of Transformer Oil

The cooling effect of transformer oil is mainly achieved through the following methods:

① High specific heat capacity and thermal conductivityThe specific heat capacity of transformer oil is about 40-50% of that of water, but much higher than that of air. It can absorb a large amount of heat with a limited rise in its own temperature, making it an ideal medium for heat absorption and transfer.

② Natural convection circulationThis is the most typical cooling method for oil-immersed transformers. The process is as follows:

• The oil near the iron core and windings is heated and expanded, its density decreases, and it flows upward.
• The hot oil reaches the top of the oil tank or the inlet of the radiator.
• Heat is dissipated to the external environment through the tank wall and radiators (tubular, finned or corrugated fin type).
• The cooled oil increases in density, sinks and returns to the bottom of the transformer, and re-enters the heating-upward circulation.

This natural convection mechanism without external power enables the transformer to perform continuous and automatic heat exchange.

③ Forced oil circulationFor large high-voltage transformers (such as power plant step-up transformers), natural convection alone is not sufficient to meet the heat dissipation requirements. In this case, oil pumps are installed to force the oil flow through external coolers (air-cooled or water-cooled) for more efficient heat dissipation.

3. Synergy of the Heat Dissipation System

The heat dissipation of a transformer is a systematic synergistic process:

• Oil → Oil tank/Radiator: The oil transfers heat to the metal tank wall.
• Tank wall → External air: Heat is finally dissipated to the environment through radiation and convection.
• Auxiliary enhancement: For transformers with large capacity, cooling fans (air cooling) are usually installed on the radiators to force air flow and significantly improve heat dissipation efficiency.

Cooling Method	Principle	Applicable Scenario
ONAN (Oil-immersed Natural Air Cooling)	Natural convection + natural air cooling	Small and medium-sized distribution transformers
OFAF (Oil-immersed Forced Air Cooling)	Natural convection + forced air cooling	Medium-sized power transformers
OFAF (Forced Oil Circulation Forced Air Cooling)	Forced oil circulation by oil pump + forced air cooling	Large high-voltage transformers
ODWF (Forced Oil Circulation Water Cooling)	Forced oil circulation by oil pump + water cooling	Extra-large capacity transformers, special occasions

 

III. Synergy and Balance of the Dual Roles

The two major functions of transformer oil, insulation and cooling, do not exist independently but interact and restrict each other.

1. The Impact of Temperature on Insulation

This is a typical synergistic relationship: when the oil temperature rises, its viscosity decreases, fluidity improves, and the cooling effect is enhanced—however, high temperature will accelerate the oxidative deterioration of the oil, producing acidic substances and sludge. These deterioration products will reduce the insulation performance of the oil and even block the oil passages, further exacerbating the temperature rise. This is the so-called "thermal-insulation vicious circle".

Therefore, maintaining an appropriate oil temperature (usually the top oil temperature does not exceed 95℃) is the key to ensuring the insulation life.

2. The Impact of Oil Quality on the Dual Functions

Transformer oil will age gradually during use, mainly manifested in:

• Increased dielectric loss factor: Decreased insulation performance
• Reduced breakdown voltage: More prone to discharge
• Increased acid value: Corrosion of solid insulation and metal components
• Sludge precipitation: Deposited on the surface of windings and iron core, hindering heat dissipation and further aggravating temperature rise

For this reason, regular testing of transformer oil (chromatographic analysis, withstand voltage test, micro-water test) and necessary treatment (filtration, regeneration or replacement) are the core work of transformer operation and maintenance.

IV. Other Important Functions of Transformer Oil

In addition to the two major roles of insulation and cooling, transformer oil also plays some supporting but equally important roles:

1. Arc Extinguishing

When an electric arc occurs inside a transformer with an on-load tap changer (OLTC) or protection devices, the oil can extinguish the electric arc quickly. The high temperature of the arc decomposes the oil, generating a large amount of gas (mainly hydrogen), which quickly forms high-pressure air bubbles to blow out the electric arc and restore the insulation strength.

2. Protecting the Iron Core and Windings

The oil submerges the metal components, isolating oxygen and moisture in the air, and slowing down the rusting of the iron core and the hydrolytic aging of insulating materials.

3. Vibration and Noise Reduction

The magnetostrictive effect of the iron core will generate vibration and noise. The iron core and windings submerged in oil have their vibrations transmitted through the oil medium and partially attenuated, making the operating noise of oil-immersed transformers significantly lower than that of dry-type transformers of the same capacity.

V. The Future of Transformer Oil: Environmental Protection and High Performance

With the improvement of environmental awareness and the development of power systems, transformer oil is also evolving continuously:

• High flash point oil: Such as synthetic esters and natural esters (vegetable oil), with a flash point higher than 300℃, which greatly improves the fire safety of transformers and is suitable for places with high fire protection requirements such as underground substations and high-rise buildings.
• Natural esters (vegetable oil): Fully biodegradable, non-toxic, and low in carbon emissions, it is becoming the preferred insulating liquid for environmentally friendly transformers.
• Nano-modified oil: By adding nano-particles to traditional oil, its thermal conductivity and insulation performance are further improved, and it is still in the research stage at present.

VI. Conclusion

Transformer oil plays an irreplaceable dual role in oil-immersed transformers:

• As an insulating medium, it builds an electrical barrier against high-voltage breakdown and has a unique self-healing ability;
• As a cooling medium, it constructs a natural convection heat dissipation system to continuously transport internal heat to the external environment.

The perfect synergy of insulation and cooling enables oil-immersed transformers to achieve high-voltage and large-capacity electric energy conversion in a compact size, making them the mainstay of modern power systems. For this reason, transformer oil is known as the "blood" of transformers, and its quality and state directly affect the performance and service life of transformers.

For transformer users, understanding the role of transformer oil and attaching importance to oil testing and maintenance lay the solidest foundation for the long-term stable operation of transformers.