Insulating liquid used in power and distribution transformers Identify the standard insulating and cooling liquid commonly used as transformer oil in electrical power apparatus.

Introduction / Context:
Transformer oil serves dual roles: electrical insulation and heat removal. The correct choice affects dielectric performance, cooling, oxidation stability, and long-term reliability of transformers in power systems.

Given Data / Assumptions:

• We refer to conventional mineral-oil-filled transformers.
• Special applications (e.g., silicone or ester fluids) exist but are not the standard default.
• Performance metrics include dielectric strength, flash point, pour point, and chemical stability.

Concept / Approach:
Historically and presently, refined mineral oil (naphthenic or paraffinic base stocks with additives) is the most common transformer oil. It offers good dielectric strength, cooling properties, and is cost-effective. Alternatives such as silicone oils and natural esters (vegetable-based) are used in niche applications for fire safety or environmental reasons but are not the ubiquitous standard.

Step-by-Step Solution:
Identify the primary functions: insulation + cooling.Match with commonly specified fluid in standards and utilities: refined mineral oil.Conclude that refined mineral oil is the correct answer for typical power equipment.

Verification / Alternative check:
Utility specifications and IEC/IEEE guides on oil testing (breakdown voltage, acidity, interfacial tension) predominantly address mineral oil-filled transformers.

Why Other Options Are Wrong:

• Refined vegetable oil: used for natural ester fluids in specific cases but not the generic default.
• Fixed mixture: not a standard requirement.
• Silicone oil: used for high fire safety but not the general standard.
• Castor oil: not a mainstream transformer oil today.

Common Pitfalls:

• Assuming eco-friendly esters have fully replaced mineral oils; adoption is growing but not universal.
• Confusing fire-point and pour-point advantages of specialty fluids with standard practice.

Final Answer:
Refined mineral oil