Why are transformers submerged in oil?

Oil-immersed transformers are widely used in power systems for medium to high voltage applications. One defining feature of these transformers is the use of insulating oil, which plays a critical role in ensuring safety, reliability, and performance. This article explores the reasons why transformers are submerged in oil and the benefits it provides.

What Is Transformer Oil and What Are Its Types?

Transformer oil, also known as insulating oil, is a critical fluid used in oil-filled transformers to provide electrical insulation, cooling, and arc suppression. Without it, transformer windings would rapidly overheat and break down under voltage stress, leading to failure or fire. Choosing the right type of transformer oil—based on operating conditions, environmental safety, dielectric strength, and fire resistance—is essential for ensuring long-term transformer performance and compliance with international standards.

Transformer oil is a specialized insulating and cooling fluid used in transformers to dissipate heat, prevent electrical discharges, and protect internal components from oxidation and moisture. The main types include mineral oil (the most common), synthetic and natural ester oils (biodegradable and fire-resistant), and silicone-based fluids (high-temperature tolerant and fire-retardant). Each type has unique properties suited to specific applications such as indoor, outdoor, environmentally sensitive, or high-load environments.

Selecting the right transformer oil is as important as selecting the right transformer.

Why Is Transformer Oil Used?

Transformer oil fulfills three essential functions:

1. Electrical Insulation
   Prevents arcing and breakdown between energized components like windings and core laminations.

2. Cooling
   Circulates via convection to carry heat away from the windings and dissipate it through radiators.

3. Moisture and Oxidation Protection
   Forms a protective film on metal parts and reduces oxygen ingress, slowing aging and corrosion.

Main Types of Transformer Oil

1. Mineral Oil: The Industry Standard

• Most widely used due to cost-effectiveness and good insulation/cooling properties.

• Two types:

  • Naphthenic: Better low-temperature performance, more common globally.
  • Paraffinic: Higher oxidation stability but wax formation at low temperatures.

Pros:

• Affordable
• Proven performance
• Easily processed/refined

Cons:

• Low fire point (\~155°C)
• Non-biodegradable
• Environmental risk in case of leakage

2. Natural Ester Oils: Biodegradable & Fire-Safe

• Derived from vegetable oils (e.g., soybean, rapeseed, sunflower).
• Increasingly popular for distribution transformers in urban, forested, or water-sensitive zones.

Pros:

• Fire-safe (fire point >300°C)
• Biodegradable and eco-friendly
• High moisture tolerance

Cons:

• Higher viscosity (slower circulation)
• Limited availability in some regions
• Shorter oxidation life without inhibitors

3. Synthetic Ester Oils: High-Performance & Stable

• Man-made esters with excellent oxidation stability and fire resistance.
• Suitable for sealed and hermetically sealed transformers.

Pros:

• Extremely high fire point (\~300°C)
• Long service life in closed systems
• Eco-friendly (readily biodegradable)

Cons:

• Expensive
• Sensitive to water contamination in open systems

4. Silicone Oils: Thermal Resilience and Fire Resistance

• Often used in transformers in fire-prone or confined indoor environments (e.g., hospitals, subways).

Pros:

• Extremely high fire resistance (\~360°C)
• Stable at high temperatures
• Electrically stable

Cons:

• Very expensive
• Moderate biodegradability
• Poor oxidation resistance in air-exposed systems

5. Hydrocarbon-Based Synthetic Oils: Niche Use

• Blends of refined hydrocarbons with good low-temperature flow and oxidation resistance.
• Suited for arctic climates and specialty installations.

Pros:

• Excellent cold-weather performance
• Stable oxidation profile
• Non-toxic alternatives to mineral oil

Cons:

• More costly than mineral oil
• Less widely available

Table: Comparison of Key Properties

Visual Guide: Transformer Oil Use Case Map

Use Environment      → Recommended Oil Type
-------------------------------------------
Urban Substation     → Natural Ester / Synthetic Ester
High Fire Risk       → Silicone Oil / Synthetic Ester
Eco-Sensitive Area   → Natural Ester
Outdoor High Voltage → Mineral Oil
Extreme Cold         → Synthetic Hydrocarbon
High Load Industrial → Synthetic Ester

Oil Testing and Maintenance to Ensure Efficiency

Transformer oil performance degrades over time due to oxidation, heat, and moisture. Regular testing ensures it retains efficiency-enhancing properties.

Best Practice: Test oil annually, filter or replace every 5–10 years depending on condition.

How Does Transformer Oil Provide Electrical Insulation?

Transformer oil is the unseen hero of power distribution systems—essential for preventing dangerous electrical faults, arcing, and equipment failure. In oil-filled transformers, the windings and core are constantly exposed to high electrical voltages and magnetic flux. Without an effective insulating medium, voltage stress can cause flashover between energized components, damaging insulation paper, deforming windings, or even causing fires. That’s where transformer oil plays a vital role: providing a highly resistive, stable dielectric environment that shields internal components from electric breakdown.

Transformer oil provides electrical insulation by filling the spaces between energized windings and core components with a fluid that has high dielectric strength, low conductivity, and excellent arc suppression capabilities. It prevents short circuits by resisting electrical breakdown under high voltage, displacing air (a weaker insulator), and quenching any initial ionization that could lead to arcing. Additionally, its chemical stability ensures that it does not degrade quickly under electric stress, making it a reliable medium for long-term insulation.

No transformer insulation system is complete without a properly selected and maintained dielectric oil.

How Transformer Oil Functions as an Electrical Insulator

1. High Dielectric Strength (BDV)

• Clean transformer oil has a breakdown voltage (BDV) between 30 kV and 70 kV per 2.5 mm gap.
• This means it can withstand high voltages without conducting electricity, acting like a non-conductive barrier between components.

Oil fills all voids and penetrates insulation layers, providing uniform voltage distribution and eliminating air pockets (which could become arc paths).

2. Fills Gaps Between Energized Components

In a transformer, windings and conductors are closely spaced. The oil:

• Displaces air, which is more prone to ionization under voltage stress.
• Immerses windings and insulation paper, allowing voltage gradients to be distributed evenly.
• Maintains insulation even at elevated temperatures, when dry solid insulation may begin to degrade.

Visual Explanation:

Before Oil Fill:
[Winding]      <— air gap —>    [Winding]
   |                            |
   ↑ Flashover risk at HV

After Oil Fill:
[Winding] ====Transformer Oil==== [Winding]
   |                             |
   ↑ Arc suppressed, BDV increased

3. Arc Quenching & Discharge Suppression

Transformer oil suppresses the formation of free electrons and ionized gas molecules, which are precursors to arcing and corona discharge.

• During voltage surges, oil resists breakdown better than air or other media.
• In the case of a partial discharge attempt, oil’s molecular inertia slows down electron movement, suppressing avalanche formation.

4. Acts as a Moisture and Oxidation Barrier

Water and oxygen reduce the insulation properties of solid materials like cellulose and degrade the dielectric system.

Transformer oil:

• Forms a physical barrier against moisture ingress.
• Protects the insulation paper and windings from oxidation, preserving their insulating capacity.

Conclusion: Clean oil = strong insulation. Dirty oil = high failure risk.

5. Works with Solid Insulation as a Composite System

Most transformer windings are wrapped in cellulose-based insulation (kraft paper or pressboard). Transformer oil impregnates these materials to enhance their insulation.

This composite insulation system is essential for withstanding the combined electrical, thermal, and mechanical stresses over decades.

Table: Dielectric Properties of Common Transformer Oils

6. Routine Testing to Maintain Insulating Performance

To ensure oil continues providing high insulation, regular testing is required:

Failing any of these tests may require filtration, reconditioning, or replacement of the oil.

Diagram: Transformer Oil Insulation System in Action

        [ HV Winding ]
             ||
     --- Insulation Paper ---
             ||
      <<< Transformer Oil >>>
             ||
     --- Insulation Paper ---
             ||
        [ LV Winding ]

• Oil surrounds and impregnates all insulation materials.
• Prevents direct contact of conductive parts.
• Maintains dielectric integrity across the system.

Standards Governing Transformer Oil Insulation Properties

These standards define minimum BDV, moisture limits, chemical content, and test protocols for safe transformer operation.

What Role Does Oil Play in Transformer Cooling?

As transformers operate, they generate significant heat due to electrical losses in the windings (copper losses) and magnetic core (core losses). Without proper heat dissipation, this thermal energy can cause insulation breakdown, accelerated aging, reduced efficiency, and even catastrophic failure. In oil-immersed transformers, transformer oil plays a dual role—it not only insulates but also functions as a crucial cooling medium, removing heat from the transformer's internal components and safely transferring it to the external environment through radiators or heat exchangers.

Transformer oil acts as the primary cooling medium by absorbing heat generated from windings and the magnetic core, then transferring that heat through convection to external radiators or coolers. This fluid-based thermal management keeps the transformer's operating temperature within safe limits, prevents thermal degradation of insulation materials, and maintains high efficiency. Depending on the transformer’s design, oil circulates naturally or is forced through pumps to enhance heat dissipation. Effective oil cooling ensures long service life, reliability, and thermal stability of the transformer under load.

Cooling is not optional—it is a non-negotiable requirement for safe transformer operation.

1. How Transformer Oil Absorbs and Transfers Heat

When current flows through the transformer windings, two primary types of losses generate heat:

• Copper (I²R) losses in the windings
• Core losses (hysteresis and eddy currents) in the laminated steel core

The oil absorbs this heat through direct contact with hot surfaces and distributes it via thermal convection. As the hot oil rises, it moves to the top of the transformer tank and flows into the cooling radiators. Cooled oil then returns to the bottom of the tank, creating a natural circulation loop.

Heat Flow Path:

[Winding/Core] → [Transformer Oil] → [Radiators/Fins] → [Ambient Air]

2. Cooling Modes in Oil-Filled Transformers

Transformers use a variety of oil-based cooling methods depending on size, load, and application.

Table: Comparison of Transformer Cooling Modes

3. Thermal Properties That Make Oil Ideal for Cooling

These properties enable oil to safely transport and dissipate thermal energy, unlike air or solid insulators.

4. Design Features That Support Oil Cooling

Diagram: Transformer Oil Cooling Loop (ONAF Mode)

  [Hot winding/core]
        ↓ Heat absorbed
   → Transformer oil
        ↓ Natural rise
   → Radiator (cooled by fans)
        ↓ Heat dissipated
   → Cool oil returns to bottom

Oil completes a closed convection loop, removing heat and returning for reheating.

5. Impact of Oil Cooling on Transformer Efficiency

Transformer efficiency drops when:

• Heat increases winding resistance (R↑ → I²R loss↑)
• Insulation degrades due to high temp
• Load is derated to avoid overheating

Effective cooling:

• Maintains optimal winding temperature (\~65–85°C)
• Preserves dielectric strength of oil and insulation paper
• Prevents accelerated aging and failure

Efficiency Gains:

6. Monitoring and Maintenance of Cooling Systems

Failure of cooling systems is among the top causes of transformer failure.

How Does Oil Help Prevent Moisture and Contamination?

Moisture and contamination are the silent killers of transformer reliability. They don’t cause immediate failure—but over time, they corrode conductors, reduce dielectric strength, weaken insulation, and accelerate transformer aging. In oil-filled transformers, the insulating oil serves as a first line of defense, acting as both a barrier and a cleansing medium. Without effective oil protection, moisture infiltration or particle contamination can lead to internal arcing, dielectric failure, or costly unplanned outages.

Transformer oil prevents moisture and contamination by acting as a sealed, hydrophobic, and oxygen-resistant medium that blocks external impurities from reaching energized components. Its low water solubility and excellent chemical stability reduce moisture ingress and oxidation, while its ability to flush internal surfaces continuously removes particulates, metal fragments, and by-products of insulation degradation. Additionally, oil-immersed systems include conservators, silica gel breathers, and oil purification processes to maintain purity and dryness, which are essential for preserving dielectric integrity and operational safety.

Transformer oil doesn’t just fill the tank—it shields your system from failure.

1. How Moisture Enters Transformers and Why It’s Dangerous

Sources of moisture and contamination include:

• Atmospheric humidity entering through breathing or poor seals
• Aging cellulose insulation, which releases water as it breaks down
• Cooling system leaks or poor maintenance
• Contaminants introduced during oil filling or repairs

Even 20–50 ppm of moisture in transformer oil can cut dielectric strength in half.

2. Transformer Oil’s Protective Properties

3. Oil-Filled Conservator System: Sealed Moisture Protection

Most large transformers use a conservator tank system, which allows oil expansion without exposing the main tank to the atmosphere.

Dry air entry and oil separation = moisture-proof insulation.

Table: Moisture Impact on Transformer Oil Performance

4. How Oil Prevents Contamination-Induced Failures

• Oil carries away and suspends metallic particles, carbon, and fiber debris from insulation degradation.
• These suspended impurities are captured via filters or replaced during oil purification.
• High-quality oils are also designed to resist sludge formation, ensuring continued fluidity and cleanliness.

Regular oil sampling and filtration restore cleanliness and dielectric capacity.

5. Oil Conditioning and Purification Techniques

To maintain oil's moisture resistance and cleanliness:

Best Practice: Perform oil purification every 5–7 years or as tests indicate degradation.

6. Oil Testing to Monitor Contamination and Moisture

Routine testing ensures oil remains a robust moisture and contamination shield.

Diagram: Transformer Oil Protection System

[Ambient Air]
   ↓    (Moisture)
[Silica Gel Breather]
   ↓ (Dry air only)
[Conservator Tank] ←→ [Main Oil Tank]
   ↓ Oil barrier
[Windings & Insulation] (Protected from moisture and O₂)

Does Oil Extend the Transformer’s Service Life?

Transformers are capital-intensive, mission-critical assets expected to function reliably for decades—often 30 to 50 years or more. But heat, moisture, electrical stress, and environmental factors constantly threaten this longevity. The hidden but vital role of transformer oil is to defend against all of these degraders simultaneously. Far more than a coolant or insulator, high-quality transformer oil serves as the lifeblood of the transformer, actively protecting and preserving its internal components from premature aging and catastrophic failure.

Yes, transformer oil significantly extends the service life of a transformer by protecting internal insulation, dissipating heat, preventing moisture ingress, and suppressing electrical discharges. When kept clean and chemically stable through proper testing and maintenance, oil prevents deterioration of cellulose insulation and windings, which are the primary determinants of transformer lifespan. Well-maintained oil systems can extend a transformer's life by 10 to 25 years beyond the expected average, depending on usage and load conditions.

Transformer longevity is directly linked to the health of its oil.

1. Core Mechanism: Oil Protects Insulation and Conductors

The primary limiting factor in transformer life is the degradation of solid insulation, particularly cellulose-based kraft paper and pressboard. Once degraded, these materials cannot be repaired, and the transformer must be decommissioned.

Transformer oil:

• Removes heat that accelerates insulation aging
• Prevents moisture from entering and weakening the insulation
• Suppresses electrical arcing that can burn insulation
• Reduces oxygen exposure to limit oxidation of materials

2. Aging Rate of Insulation: Oil Quality Is Key

The aging rate of cellulose insulation follows an exponential curve, driven by temperature and moisture. This is described by the Arrhenius Law:

Rate of Aging ∝ e^(–B/T)
Where B = constant, T = absolute temperature

Life Expectancy Impact (Rule of 6°C):

Oil’s cooling role alone can double or quadruple the transformer's lifespan by keeping temperatures under control.

3. Impact of Moisture and Contaminants on Lifespan

Moisture accelerates depolymerization of cellulose. The Degree of Polymerization (DP) indicates insulation condition:

Water content above 2% can reduce lifespan by 50% or more. Clean oil keeps paper dry and preserves DP levels longer.

4. Case Study: Life Extension Through Oil Reconditioning

Life extension cost was 5% of transformer replacement—proving oil management is cost-effective.

5. Oil-Related Maintenance That Preserves Lifespan

Proper documentation and trend analysis are also vital for predictive maintenance and lifespan projection.

Chart: Lifespan vs. Oil Condition

Service Life (%)
|
| 100 ────────■────■──■──■──■ (Clean Oil)
|        \
|         \
|          ■──■────■─────■ (Contaminated Oil)
|_________________________________________
           Time (Years)

Transformers with well-maintained oil last 1.5–2x longer than those with ignored oil health.

6. How Oil Types Affect Service Life

Ester oils also preserve paper insulation longer by absorbing water and reducing aging rate.

7. Oil as a Diagnostic Indicator of Aging

Oil carries the chemical "signature" of transformer aging:

• Increased acids = insulation breakdown
• Carbon particles = internal arcing
• Elevated moisture = paper degradation
• Gases (CO, CO₂, H₂) = insulation overheating

Tracking these markers allows early action to prevent irreversible damage and extend lifespan.

Standards Supporting Oil's Life-Preserving Role

Are There Environmental and Safety Concerns?

Transformer oil, while essential for reliable and efficient transformer operation, also carries significant environmental and safety risks if not properly managed. Spills, leaks, combustion, or improper disposal of used oil can lead to soil and water contamination, fire hazards, and regulatory violations. These concerns are especially critical in urban, industrial, and environmentally protected zones. Understanding and mitigating these risks is essential not only for safety compliance but also for preserving environmental integrity and public health.

Yes, transformer oil poses several environmental and safety concerns, including fire risks due to its flammability, ecological damage from leaks or spills, and toxic exposure during improper handling or disposal. Mineral oils are particularly hazardous if released into soil or waterways, while some older transformer oils may still contain toxic substances like PCBs. Environmental regulations require strict containment, spill response protocols, and end-of-life treatment, while fire codes mandate flame-retardant systems and oil selection based on application. Using biodegradable oils and following ISO 14001, EPA, or EU REACH standards helps mitigate these risks.

Transformer oil safety is a regulatory, technical, and environmental imperative.

1. Fire and Explosion Risks

Why It’s a Concern:

Mineral transformer oils have flash points between 140–160°C, making them combustible under fault or overload conditions.

Historical Fact: Many substation fires are traced back to oil ignition after faults.

Mitigation:

• Use of Buchholz relays, pressure relief devices, flame arrestors
• Fire barriers and blast walls in urban installations
• Ester or silicone oils for low-flammability applications

2. Environmental Contamination from Spills or Leaks

Transformer oil, especially mineral oil, is classified as hazardous waste if spilled due to its toxicity and persistence.

Even small leaks (1–5 liters/day) can result in hundreds of liters per year, accumulating below tanks or in cable trenches unnoticed.

Prevention Measures:

• Oil-tight bunds (containment dikes) that can hold 110–120% of transformer oil volume
• Impermeable linings in containment pits
• Oil level monitoring with alarms
• Routine inspections and maintenance
• Biodegradable oils in ecologically sensitive areas

3. Health Hazards During Handling and Disposal

PCBs (Polychlorinated Biphenyls) were banned globally by the Stockholm Convention, but transformers manufactured before 1980 may still contain them.

4. Oil Waste and Disposal Regulations

Used transformer oil may be classified as hazardous waste depending on its contamination level.

Table: Environmental Risk Comparison of Transformer Oil Types

Natural and synthetic esters are preferred for LEED buildings, wind farms, and water protection zones.

5. Design Best Practices for Safety & Environmental Protection

Smart design + smart monitoring = minimum risk

6. Case Example: Substation Retrofit to Eco-Friendly Oil

Conclusion

Transformers are submerged in oil to achieve superior insulation, efficient heat dissipation, and enhanced reliability. The oil serves as both a dielectric medium and a coolant, enabling the transformer to operate safely under high voltage and thermal stress. While there are environmental considerations, proper design, containment, and modern oil alternatives help manage these risks effectively. Oil immersion remains a proven and widely adopted method for maximizing transformer performance and longevity.

FAQ

Q1: Why are transformers submerged in oil?
A1: Transformers are submerged in oil to serve three essential functions:

Electrical Insulation: The oil insulates internal live parts and prevents arcing

Heat Dissipation: It absorbs and transfers heat from windings and core to the tank walls or radiators

Moisture Protection: Oil prevents moisture ingress that could degrade insulation and cause dielectric breakdown
This enhances the safety, performance, and lifespan of the transformer.

Q2: What type of oil is used in transformers?
A2: Common types include:

Mineral Oil: Widely used due to its cost-effectiveness and good thermal properties

Natural Ester (Biodegradable): Eco-friendly, high flash point, used in fire-prone areas

Synthetic Ester: Offers superior thermal aging and environmental safety

Silicone Oil: Used in high-temperature or indoor applications
Each type is selected based on environmental, thermal, and fire safety requirements.

Q3: How does transformer oil help with cooling?
A3: Transformer oil performs convection cooling by:

Absorbing heat from the core and windings

Circulating through the tank or radiators (natural or forced circulation)

Transferring heat to the surrounding environment
Cooling methods include ONAN, ONAF, OFAF, depending on transformer size and cooling needs.

Q4: What happens if transformer oil degrades or leaks?
A4: Oil degradation or leakage can lead to:

Reduced insulation strength, increasing risk of electrical faults

Overheating and accelerated aging of insulation

Fire hazards due to flammable vapors

Environmental contamination
Regular oil testing (DGA, moisture, acidity, BDV) and maintenance are vital for safe operation.

Q5: Why not use air instead of oil in transformers?
A5: Air lacks the dielectric strength and thermal capacity of oil. Oil offers:

Superior insulation and arc suppression

Better thermal conductivity for heat removal

Sealing against moisture and contaminants
Dry-type transformers use air but are limited in voltage and power capacity compared to oil-immersed types.

References

"Why Transformers Use Oil" – https://www.electrical4u.com/transformer-oil-purpose

"IEEE C57.106: Guide for Acceptance and Maintenance of Insulating Oil" – https://ieeexplore.ieee.org/document/8571027

"NREL: Analysis of Cooling Fluids in Transformers" – https://www.nrel.gov/docs/transformer-cooling-analysis.pdf

"Doble: Transformer Oil Testing and Monitoring" – https://www.doble.com/transformer-oil-testing

"ScienceDirect: Insulating Oils and Their Application in Power Transformers" – https://www.sciencedirect.com/transformer-oil-study