The cooling ways of the oil immersed 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 Esterand 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.
| Aspect | ONAN Transformer | KNAN Transformer |
|---|---|---|
| Insulating liquid | Mineral oil | Ester fluid (natural or synthetic) |
| Cooling method | Natural oil + natural air | Natural ester + natural air |
| Fire safety | Lower fire point (~160 °C) | Very high fire point (>300 °C) |
| Environmental impact | Poor biodegradability | Biodegradable, eco-friendly |
| Indoor suitability | Limited (fire risk) | Excellent |
| Moisture tolerance | Lower | Higher (esters absorb moisture better) |
| Cost | Lower | Higher (fluid cost) |
Most common for outdoor substations
Lower initial cost
Mature and widely standardized technology
Requires fire protection measures indoors
Preferred for indoor, urban, tunnel, marine, and renewable energy applications
Much safer in fire-sensitive locations
Longer insulation life due to better moisture management
Often used where environmental regulations are strict
