In the world of oil-immersed transformers, transformer oil fills most of the internal space – submerging the iron core and windings. It is an indispensable "blood" for the stable operation of transformers. But why oil specifically? Why not water, air, or other liquids?

This article explains the two key dual roles of transformer oil in oil‑immersed transformers – insulation and cooling – and explores why it has become an irreplaceable choice in power transmission.

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1. 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 high‑voltage and low‑voltage windings, and between windings and the earthed iron core and tank. Without effective insulation, breakdown discharge would occur in an instant – leading to equipment damage and system failures.

1.1 Insulation Properties of Transformer Oil

Property	Benefit
High dielectric strength	Pure transformer oil has breakdown voltage of 40‑60kV/2.5mm vs. air at 3‑4kV/mm – withstands higher voltages without breakdown
Fills micro gaps	Good fluidity penetrates all tiny gaps – eliminates air bubbles, forming continuous insulation
Oil‑paper composite insulation	Combining insulating paper with oil creates breakdown strength much higher than oil or paper alone

1.2 Self‑healing Ability

This is a unique advantage of transformer oil. When partial discharge occurs, the generated gas may form bubbles temporarily. However, once discharge stops, surrounding oil flows back quickly – refilling and restoring insulation performance. In contrast, solid insulation damage is often permanent.

1.3 What If There Is No Oil?

• Size increase – to achieve the same insulation level, the transformer would need to be several times larger
• Surface contamination – moisture, dust, and pollutants would reduce surface insulation strength
• Oxidation – oxygen in air would accelerate insulation aging and shorten service life

Therefore, oil is essential for compact, reliable transformer design.

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2. Cooling: A Silent "Heat Mover"

Transformers generate large amounts of heat during operation – hysteresis and eddy current losses (iron loss) in the core, and resistance losses (copper loss) in windings. If not dissipated in time, temperature rises continuously – causing accelerated insulation aging and eventual thermal breakdown.

2.1 Heat Generation and Hazards

The temperature distribution inside a transformer is not uniform. Windings and core are the main heat sources with obvious hot‑spot temperatures. According to IEC standards, the thermal aging rate of insulating paper doubles for every 6‑8°C increase in hot‑spot temperature. Consequently, controlling temperature means controlling transformer service life.

2.2 Cooling Mechanism of Transformer Oil

Method	How It Works	Application
High specific heat capacity	Absorbs large heat with limited temperature rise	All oil‑filled transformers
Natural convection circulation	Hot oil rises, cools at radiator, sinks, recirculates	Most distribution transformers (ONAN)
Forced oil circulation	Oil pumps force flow through external coolers	Large high‑voltage transformers (OFAF/ODWF)

2.3 Cooling Methods Comparison

Cooling Method	Principle	Applicable Scenario
ONAN	Natural convection + natural air cooling	Small and medium‑sized distribution transformers
ONAF	Natural convection + forced air cooling	Medium‑sized power transformers
OFAF	Forced oil circulation + forced air cooling	Large high‑voltage transformers
ODWF	Forced oil circulation + water cooling	Extra‑large capacity transformers, special occasions

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3. Synergy and Balance of the Dual Roles

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

3.1 Impact of Temperature on Insulation

As oil temperature rises, viscosity decreases and cooling improves. However, high temperature accelerates oxidative deterioration – producing acidic substances and sludge. These by‑products reduce insulation performance and can block oil passages, further exacerbating temperature rise. This is the "thermal‑insulation vicious circle."

Therefore, maintaining proper oil temperature (top oil temperature ≤ 95°C) is key to ensuring insulation life.

3.2 Impact of Oil Quality on Dual Functions

Transformer oil ages gradually during use – showing:

Indicator	Consequence
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	Hinders heat dissipation – aggravates temperature rise

Regular oil testing (chromatographic analysis, withstand voltage test, micro‑water test) and necessary treatment (filtration, regeneration, or replacement) are core maintenance tasks.

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4. Other Important Functions of Transformer Oil

Beyond insulation and cooling, transformer oil also:

• Extinguishes arcs – high temperature decomposes oil into gas that blows out arcs in tap changers
• Protects iron core and windings – isolates oxygen and moisture, slowing rust and aging
• Reduces vibration and noise – oil buffers mechanical vibration – noise significantly lower than dry‑type transformers

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

Transformer oil continues to evolve:

Type	Feature	Application
High flash point oil (synthetic/natural esters)	Flash point >300°C	Underground substations, high‑rise buildings
Natural esters (vegetable oil)	Fully biodegradable, non‑toxic, low carbon	Environmentally friendly transformers
Nano‑modified oil	Enhanced thermal conductivity and insulation	Research stage

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6. Conclusion

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

• As an insulating medium – builds an electrical barrier against high‑voltage breakdown with unique self‑healing ability
• As a cooling medium – constructs a natural convection system to continuously transport internal heat to the environment

The perfect synergy of insulation and cooling enables oil‑immersed transformers to achieve high‑voltage, large‑capacity 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 – its quality and state directly affect performance and service life.