The designation of cooling types of the oil distribution transformers depend on the fire point of the oil being used in the transformer. Mineral oils are symbolized with "O" since their fire point is below 300°C. Non-mineral oils such as silicone oil, Synthetic Ester (which is also known in the market by the famous brand "Midel") and Natural Ester (vegetable oils) are designated with "K" since their fire point is above 300°C. Non-mineral oils are more expensive but they have different advantages based on the application area of the transformers asked. Non-mineral oils can be devided into two types: synthetic ones and natural ones. Both have higher flashing points and thus are more safe to use in explosive areas defined as Zone 1 and Zone 2. KNAN transformers do have a bigger footprint compared to a mineral oil filled transformer. The heat dissipation factor for mineral oil compared to a non-mineral oil is higher; thus, the tank design is kept bigger. Higher-temperature-withstanding-active parts can be designed with non-mineral oils. Mineral oils are suitable for A-class design. (IEC-60076-14-2009)

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