In this series, we will discuss the different condition monitoring options for the analysis of transformer component performance where natural esters are used as lubricants in transformers.

Lubricants can be categorised in many different ways. One of the most common classifications is by the constituent base oil: mineral, synthetic or vegetable. Mineral oil, which is derived from crude oil, can be produced to provide a range of qualities associated with the oil’s refining process.

Natural esters, derived from 100% renewable vegetable oils, are superior to mineral oil for use in distribution and power-generation transformers of all voltage classes, both new and retrofilled.

Natural esters and mineral oils are miscible and mostly compatible; they are also compatible with halogenated hydrocarbon insulating fluids. Mixing mineral oil and natural esters may or may not significantly impact the typical properties of impact performance. If the property values change, it may or may not be proportional to the ratio of the content of the fluids.

NOTE: If the purpose of using the natural ester oil is to comply with the National Electrical Code, that would require that less-flammable fluids have an ASTM D92 fire point of not less than 300°C and that the installation complies with all restrictions provided for in the product listing of the fluid.

Too much mineral oil contamination of the natural ester might fail to meet the requirements of the Safety Code. The natural ester manufacturer should be contacted to determine the maximum mineral oil content range, that is allowed, to ensure that flammability parameters are met. Typically, a maximum of 7% mineral oil is acceptable.

As a rule, it is not advisable to mix synthetic esters, synthetic hydrocarbons, and high molecular weight hydrocarbons, although they are miscible. Silicone fluid is not miscible with natural ester oils, so cross contamination should be avoided. Typically, natural esters are miscible with non-flammable halogenated hydrocarbons, like PCBs. This might occur when retro filling older transformers that are filled with this insulating fluid. It would be advisable to consult the manufacturers in such a case

To read more about testing guidelines for natural ester oils in transformers, click here: https://www.wearcheck.co.za/shared/TB85.pdf