Testing Guidelines for Natural Ester Oils in Transformers

Insulating fluids can be categorised in many different ways. One of the most common classifications would be by identification of the constituent base oil: mineral oil, synthetic, or natural oils. Mineral oil is derived from crude oil, this type of oil is known for specific characteristics - chemical and electrical characteristics - that are highly reliant on the specific process of refining that is used during the manufacturing of the oil.

Natural esters are derived from a 100% renewable source, vegetable oils are superior to mineral oil in some instances and for some applications. They can be used for power transformers and are especially well suited for distribution transformers. These types of transformers can be retro-filled using natural or synthetic ester fluid if previously filled with mineral oil. The manufacturer should be contacted to confirm if the retro-fill is advisable for that specific design, as ester fluids might flow slower through the transformer, causing lower heat dissipation.

In this article transformer consultant Corné Dames discusses the different condition monitoring options for the analysis of transformer component performance, where natural esters are used as insulating fluids in transformers, to determine the degree of contamination and degradation.

Before getting into the various types of tests conducted on transformer fluids that are based on esters, it is imperative that we understand some background information on mixtures of natural ester oils and other dielectric fluids.

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 significantly impact the typical properties and impact performance. If the fluids are mixed, the concentration of the different oils will determine the chemical and electrical properties of the fluid mixture now in the transformer. This might not adhere to the desired properties of the ester fluids, but this might enhance the properties of the mineral oil. This ensures a lower flammability and lower acid formation, but the environmental impact upon spillage would still be devastating. The other problem would be testing the mixed oil, as there are different acceptable levels for ester and mineral fluids to determine the quality and safety of the insulating fluid for the intended purpose.

It is worth noting that, a natural ester oil must comply with the National Electrical Code, which stipulates that less-flammable fluids have a fire point of not less than 300°C and that the installation complies with all the restrictions provided in the product listing of the fluid.

Too much mineral oil contamination causes the mixture of oils not complying with the required 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 still 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 Polychlorinated biphenyls (PCBs). This might occur when retro-filling older transformers with this insulating fluid. It would be advisable to consult the manufacturers in such a case.

To classify natural ester oils that are in service, the following laboratory screenings are recommended:

• Visual condition of the oil (American Society for Testing and Materials (ASTM) D1524)

• Colour of the oil (ASTM D1500)

• Dielectric breakdown voltage (ASTM 1816)

• Water content (ASTM D1533)

• AC loss characteristics (dissipation factor) (ASTM D924)

• Fire point (ASTM D92)

• Viscosity (ASTM D445)

The functional tests outlined below although not required, they can be carried out:

• Interfacial tension (ASTM D971)

• Relative density (ASTM D1298)

• Pour point (ASTM D97)

• Volume resistivity (ASTM D1169)

• Neutralisation number (ASTM D664 and ASTM D974)

Types of oil tests and the significance of each test

After classifying the ester oils, the following tests are applied to natural ester insulating fluid and compares them to standards set for mineral oils. Due to inherent differences, standards may require updates. This article delves into key tests:

a. Sampling Practices (ASTM D923): Emphasises the critical role of accurate sampling to ensure valid diagnostic evaluations, preventing misleading results and unwarranted expenses.

b. Acid Number (ASTM 664 and ASTM 974): Describes the unique paths of acid formation in natural esters, addressing the impact of hydrolysis, pyrolysis, and oxidation on acid levels—differentiates between harmless long-chain acids and potentially harmful short-chain acids.

c. Dielectric Breakdown Voltage (ASTM 1816): Highlights the importance of measuring an insulating fluid’s ability to withstand electrical stress and the potential impact of contamination on dielectric values, especially when using natural ester oil.

d. AC Loss Characteristics (ASTM D924): Discusses dissipation factor and relative permittivity, emphasising that natural ester oils inherently exhibit higher dissipation factors and relative permittivity than mineral oils.

e. Interfacial Tension (ASTM D971): Explains the measurement of interfacial tension against water, noting its sensitivity to surfactants and the need for further ASTM limits for new natural ester oils.

f. Colour (ASTM D1500): Compares colour considerations between the natural ester and mineral oils, highlighting the importance of additional tests for oil deterioration or contamination in natural ester oils.

g. Kinematic Viscosity (ASTM D445): Addresses the impact of viscosity on cooling and performance, noting the higher viscosity of natural esters and the potential for increased viscosity over time due to polymerisation.

h. Flashpoint and Fire Point (ASTM D92): Emphasises natural ester oils’ higher flash and fire points than mineral oils and their significance in fire risk assessment.

i. Relative Density (ASTM D1298): Explains that relative density is not a significant indicator of fluid quality but may be relevant for specific applications.

j. Pour Point (ASTM D97, D5949, D5950): Discuss the pour point’s significance in fluid circulation and its role in fluid identification and equipment selection.

k. Volume Resistivity (ASTM D1169): Explores the electrical insulating capability of fluids, noting the lower resistivity of new natural esters compared to mineral oils.

l. Gas Analysis (ASTM D3284, D3612): Details the importance of dissolved gas analysis (DGA) in identifying faults, with a focus on the unique gas profiles of natural ester oils compared to mineral oils.

Conclusion

There are differences in gas solubility coefficients between the various natural esters and mineral oils, and their respective values should be used for data interpretation.

Thus, mixing the two types of oil might lead to a lot of confusion and misinterpretation of possible faults, or indicate dangerous scenarios, as pointed out when performing oil diagnostic testing. We should wait on International Electrotechnical Commission (IEC) or Institute of Electrical and Electronics Engineers (IEEE) or ASTM to supply us with guidelines to identify dangerous levels of gases, moisture etc effectively when performing diagnostics for mixed insulating fluids.

This article is a summary of the full Wear-Check’s Technical Bulletin 85, which may be downloaded via this link: https://www.wearcheck.co.za/shared/TB85.pdf