Applications of Base Oils in Electrical Insulation and Transformers

Functions, Benefits, Types, and Technical Requirements in the Power Industry

Base oils have long been one of the most critical components in medium- and high-voltage electrical systems, especially transformers, circuit breakers, bushings, and various insulation equipment. Their primary function is to provide reliable electrical insulation and efficient heat dissipation, but in practice, their role extends far beyond these two tasks.

This article provides a comprehensive and technical overview of the structure, properties, types, selection criteria, performance expectations, and operational challenges of base oils used in electrical insulation and transformer applications.

1. Importance of Electrical Insulation in Power Equipment

Transformers and other electrical equipment are exposed to:

• Thermal stress

• Electromagnetic fields

• Partial discharges

• Electrical arcs

• Environmental contamination

Proper insulation can extend equipment life by 30–50 years. Poor insulation can lead to:

• Reduced dielectric strength

• Higher risk of electrical arcing

• Fire hazards

• Equipment failure and network downtime

High-quality base oils serve as the first line of defense against these risks.

2. Primary Functions of Base Oils in Transformers

2.1 Electrical Functions

• Increase dielectric strength

• Prevent partial discharge

• Provide effective electrical spacing between windings

• Reduce corona effects in high-voltage systems

• Protect conductors from oxidation

2.2 Thermal Functions

• Transfer heat from windings to the transformer tank

• Prevent hot spots

• Stabilize operating temperature under heavy load

2.3 Chemical and Physical Functions

• Inhibit oxidation and corrosion

• Form a protective layer on cellulose insulation

• Absorb moisture and reduce water content

• Maintain long-term chemical stability

3. Essential Properties of Base Oils for Electrical Insulation

Even small amounts of water, acid, or contaminants can reduce dielectric strength by 50% or more.

4. Types of Base Oils Used for Electrical Insulation

4.1 Mineral Base Oils

The most widely used oils for transformersAdvantages:

• Cost-effective

• Good dielectric performance

• Easy to produceTwo major types:

• Naphthenic Base Oils

• Paraffinic Base Oils

Naphthenic oils are generally preferred due to their lower pour point and better stability.

4.2 Synthetic Oils

Used in high-performance or special-purpose transformersTypes include:

• Synthetic esters

• Polyalphaolefins (PAO)

Advantages:

• Outstanding thermal stability

• High environmental resistance

• Very high fire point

4.3 Natural Esters / Bio-based Oils

An emerging and environmentally friendly option

• Excellent dielectric performance

• High moisture tolerance

• Biodegradable and renewable

• Increasingly used in environmentally sensitive regions

5. Thermal Performance and Heat Dissipation

Approximately 70% of transformer electrical losses convert to heat.Base oils support cooling mechanisms such as:

• ONAN (Oil Natural Air Natural)

• ONAF (Oil Natural Air Forced)

• OFAF (Oil Forced Air Forced)

Proper viscosity plays a key role in ensuring efficient thermal circulation.

6. Degradation Mechanisms and Challenges

Over time, insulating oils degrade due to:

6.1 Oxidation

• Formation of acids

• Sludge buildup

• Viscosity increase

6.2 Moisture

Every 1% rise in moisture = 10–15% drop in dielectric strength

6.3 Excessive Heat

Accelerates chemical decomposition.

6.4 Contaminants

Metal particles, fibers, or carbon residues increase the risk of arcing.

7. Standards for Selecting Insulating Base Oils

• IEC 60296 – Mineral insulating oils

• IEEE C57.106 – Guidelines for oil evaluation

• ASTM D3487 – Specifications for mineral oil

• IEC 61099 – Synthetic insulating oils

Using oils outside these standards can significantly reduce equipment reliability.

8. Key Tests for Evaluating Oil Quality

• Dielectric Strength (BDV test)

• Acidity (Neutralization number)

• Moisture (Karl Fischer test)

• Dissolved Gas Analysis (DGA)

• Interfacial tension (IFT)

• Flash point

• Viscosity

• Color and appearance

9. Benefits of Using High-Quality Base Oils

• Extends transformer service life by up to 20 years

• Reduces thermal degradation

• Minimizes filtration and maintenance needs

• Improves high-voltage stability

• Enhances heat dissipation and efficiency

• Lowers risk of fire and failure

10. Future Trends in Insulating Oils

• Shift from mineral oils to bio-based alternatives

• Increasing environmental regulations

• Demand for high-temperature resistant oils

• Development of low-viscosity, high-stability formulations

• Oils with better moisture tolerance and improved dielectric behavior

Conclusion

Base oils play a vital role in the electrical insulation of transformers and other high-voltage equipment. Choosing the right oil significantly improves operational efficiency, safety, and service life. While mineral oils remain the most commonly used option due to their affordability and performance, synthetic and environmentally friendly bio-based oils are becoming increasingly popular in modern power systems.

This article was researched and written by AmiPetro

The use of this article is permitted by citing the source.