Transformer insulating oil is a liquid medium widely used in electrical equipment such as transformers, current transformers, switchgear, and rectifiers. Its primary roles include insulation, cooling, arc extinguishing, and protection against corrosion and moisture. Below is a detailed overview of the types and functions of transformer insulating oil:

Main Types of Transformer Oil

1. Mineral Oil Transformer Oil

Mineral oil transformer oil has been used in oil-immersed transformers for nearly a century due to its excellent electrical and thermal properties. It remains an ideal choice for ultra-high and extra-high voltage power transformers.

Composition: Mineral oil transformer oil is mainly processed from naphthenic and paraffinic oils, containing straight-chain alkanes, isomerized alkanes, naphthenes, and aromatics.

• Straight-chain alkanes: High freezing points and easily produce hydrogen under high voltage, making them less suitable.
• Isomerized alkanes: Low freezing points, high flash points, moderate gas evolution, and good oxidative stability, making them ideal for transformer oil.
• Naphthenes: Low freezing and flash points but meet electrical performance requirements.
• Aromatics: Single-ring aromatics enhance gas evolution properties, while polycyclic aromatics are less stable and harmful, requiring removal.

2. Vegetable Oil Transformer Oil

Vegetable oil transformer oil is derived from natural oil crops through pressing, refining, and modification.

Advantages: High flash points, excellent electrical performance, renewable, and biodegradable.

Challenges: Higher cost (3–4 times that of mineral oil), high viscosity, prone to hydrolysis and oxidation, poor low-temperature performance, and lack of standardized indicators and maintenance practices.

3. Silicone Oil Transformer Oil

Silicone oil transformer oil offers excellent electrical insulation, high flash points, and stable viscosity across temperatures.

Applications: Typically used in transformers below 35kV.

Limitations: Sensitive to partial discharge byproducts, reduced insulation stability under repeated electric stress, and higher environmental and cost drawbacks, leading to reduced usage globally. It remains used in specific industries, such as shield machine transformers, and regions like the U.S.

4. Synthetic Ester Transformer Oil

Synthetic ester transformer oil, primarily using polyol ester, stands out for its environmental benefits.

Key Features:

• Highly biodegradable, breaking down into CO₂ and water.
• Excellent fire safety, electrical insulation, oxidation resistance, and anti-aging properties.
• Strong polarity, maintaining high breakdown voltage even in moist conditions.
• Ideal Use: Recommended for environmentally sensitive applications such as offshore wind farms.

Primary Functions of Transformer Oil

1. Insulation

Transformer oil serves as an electrical insulating medium, preventing current flow between parts with different potentials. This ensures the safe and reliable operation of electrical equipment.

2. Cooling

During operation, transformers generate significant heat. Transformer oil effectively dissipates this heat through circulation, cooling the core and windings, thereby improving operating conditions and preventing overheating.

3. Arc Extinguishing

Electrical arcs may form when circuits are interrupted, posing a risk of damage or fire. The high thermal conductivity and rapid cooling of transformer oil help extinguish arcs, protecting equipment and ensuring safety.

4. Protection Against Corrosion, Oxidation, and Moisture

Transformer oil contains additives that prevent internal metal components from corrosion and oxidation, extending the lifespan of equipment. Additionally, by isolating the insulating structure from air, it helps avoid moisture absorption, maintaining system integrity.

Understanding the types and roles of transformer insulating oil is crucial for maintaining the performance and longevity of electrical equipment while ensuring system reliability and safety.