Nowadays, ester oils have become an alternative to conventional mineral transformer oil. These ester oils provide insulation and cooling to the transformer. We may use them, particularly where fire safety is crucial. Also, we may use ester oil where we essentially require environmental sustainability and improved insulation life. In this article, we have covered natural esters and synthetic esters.
What Are Ester Oils?
There are mainly two major categories of ester oils: natural ester oil and synthetic ester oil. Oil producers derive natural ester from renewable vegetable oils. On the other hand, they produce synthetic ester from polyols and carboxylic acids. Both ester insulating liquids possess fire points above 300°C. We can follow the standards IEC 62770 / IS 16659 for natural esters. We follow IEC 61099 / IS 16081 for synthetic esters.
Improved Fire Safety
One of the major advantages of ester oils is their higher fire point. The fire point is approximately 360°C for natural ester. The same for synthetic ester oil is 310°C. Both fire points are much higher than that of mineral transformer oil. Actually, the fire point of mineral oil is about 160°C . Needless to say, the high fire points can significantly reduce fire and explosion risks in substations. Also, the lower fire risk with ester oils allows transformers to place much closer to each other without fire walls. Therefore, using ester oil in transformers may be an alternative where availability of space is a constraint for installations.
Longer Insulation Life and Overloading Capability
The condition of cellulose insulation (paper) greatly influences transformer life. An ester oil can provide 5–8 times more life to solid paper insulation compared to a mineral oil. Ester oils also have substantially higher moisture tolerance. This means their affinity for moisture absorption is significantly higher than that of mineral oil.
So, ester oil effectively draws water from the paper and helps keep transformer paper insulation drier. Further, an ester oil-filled transformer may operate 15–20°C warmer without degrading paper and oil life. This characteristic enhances the overload capability of the transformer up to about 20%. On the other hand, the continuous overloading capacity of a mineral-oil-filled power transformer is only 10%.
Environmental Advantages
Natural ester is readily biodegradable. The biodegradability of ester is up to approximately 99%. Obviously, this makes ester oils non-hazardous to soil and water. Synthetic ester also has a lower environmental footprint than mineral oil.
Cost of Ester Oils
However, ester oils have a higher initial cost. The cost of natural ester oil is approximately 1.5–2 times higher than the cost of mineral insulating oil. The cost is around six times that of mineral oil for synthetic ester.
Conclusion
Overall, ester oil offers a combination of enhanced fire safety, improved moisture tolerance, potentially longer insulation life, higher overload capability and environmental benefits, making it an important option for modern transformers.