What type of liquid is most commonly used for insulation in transformers?

Mineral oil is the most commonly used liquid for insulation in transformers due to its excellent electrical insulating properties, high flash point, and good heat dissipation capabilities. It acts as both an insulator and a coolant, efficiently managing the temperature within the transformer during operation. Mineral oil also has a significant advantage in terms of availability and cost-effectiveness, making it a longstanding choice in the industry.

Silicone oil, while used in some applications due to its thermal stability, is more expensive and less common than mineral oil. Vegetable oil is being researched and used increasingly as an alternative due to its biodegradability and lower environmental impact, but it is not yet as prevalent as mineral oil in most conventional transformers. Distilled water is not used for insulation in transformers; it lacks the necessary insulating properties when compared to oils.