Occasionally we get a request from a customer asking for oxidation inhibitor and metals testing of natural ester dielectric fluids. Natural ester fluids are vegetable oil-based fluids. The most common brands are Cooper’s Envirotemp FR3 fluid and ABB’s Biotemp fluid.
We don’t test for inhibitor in natural ester fluids for several reasons:
- To our knowledge, no natural ester fluid manufacturer has been forthcoming about what inhibitors are used in their fluids, when they were used, at what concentrations, or whether any of the standard methods will quantify them.
- The common standard method (ASTM D 2668) for testing for inhibitor by FTIR (Fourier Transform Infrared) spectrometry does not work well for determining the inhibitor content in these fluids (on the assumption that they even contain at least some DBPC or DBP – the common inhibitors that are used for mineral oil-based dielectric fluids). If this standard method is used on FR3 or Biotemp to try to quantify DBPC/DBP, there is interference that prevents obtaining an accurate result. This interference is also referenced in the literature on this topic.
- Finally, inhibitor testing of natural ester fluids would be of very limited use. Oxidation of the natural ester fluid (that inhibitor would try to prevent) is not the major source of aging products in these fluids. Fluid breakdown, forming fatty acids and fatty alcohols, is the primary mechanism of fluid aging/degradation. (When such compounds are present, they degrade the results of the acid number and IFT tests.) It is possible to have this type of aging in the presence of what we would consider to be an acceptable level of DBPC.
In summary, we don’t test for inhibitor in natural ester fluids because there is no available method for doing so, because the manufacturers haven’t revealed the identity of the inhibitor(s), and because it would be of limited use anyway.
We also do not test natural ester fluids for metals content, for various reasons:
- For metals testing, we follow the commonly used method ASTM D 7151. That method states that white oil or mineral oil is to be used as a base oil for instrument calibration, so the method is not applicable to natural ester fluids.
- Natural ester fluids may cause problems with the ICP (inductively coupled plasma) instrument that is used in ASTM D 7151, making extensive cleaning necessary, and possibly even causing damage.
- To our knowledge, there are no standards for the interpretation of the results of metals analysis on natural ester fluids.
- To our knowledge, no transformer fluid testing laboratory performs metals testing on these fluids, so it does not appear to be a needed or relevant test.
In conclusion, the existing standard tests for natural ester fluids are sufficient to gauge the condition of the fluid and of the transformer. For critical transformers, appropriate testing includes Oil Screen, Dissolved Gas Analysis (DGA), Karl Fischer Moisture, Liquid Power Factor, and Furans Analysis. For non-critical transformers, the Liquid Power Factor and Furans Analysis can be omitted.
