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Vegetable oil compatibility study for electrical insulation

Environmental issues have encouraged the use of vegetable oil as alternative insulating fluid for transformers. An important parameter to be evaluated is the vegetable insulating oil compatibility with the materials used in the transformer construction, to ensure that there is no damage to oil and transformer parts under normal operating conditions. Brazil was a pioneer in testing and standardizing compatibility of vegetable insulating oils, through ABNT NBR 16431. In compatibility tests it is considered that variation in vegetable insulating oil dielectric properties is essential to determine oil contamination with the material of transformer parts. ASTM D3455 standardizes mineral insulating oil compatibility tests considering determination of these oil parameters: neutralization index, color, power factor, dielectric strength. All these parameters are included in ABNT standard, in addition to oil viscosity. Actually, Brazilian standard is being reviewed and the inclusion of dissolved gas analysis (DGA) is under discussion. The aim of this study was to assess vegetable oil compatibility, according to ASTM and ABNT NBR standards, using materials from transformer construction, such as silicon steel, paint, rubber, glue, and paper provided by distinct local suppliers. DGA was performed by oil accelerated aging inside a syringe after preparing material proof samples of specific sizes and insert in a glass syringe fulfilled with 40 mL of vegetable oil. A blank test (without materials) is carried out simultaneously, which is a syringe containing only 40 mL of vegetable oil. Both syringes are aged at 100 °C for 164 h in duplicate, followed by the measure of DGA. Main results of this study showed that viscosity, only included in ABNT standard, was a key parameter in investigation of vegetable oil compatibility. This result confirms the importance of maintaining viscosity as an evaluation method in compatibility tests. Besides that, power factor, included in ASTM and ABNT, was significantly altered by the contamination with some of paints and rubbers, showing that regardless the oil nature (mineral or vegetable), this measure should be performed in compatibility tests. Different DGA results were obtained for specific paint tested, which agree with physical-chemical analysis. It is suggested addition of DGA as a monitoring parameter of oil quality in the standards to confirm incompatibility diagnosis. Therefore, differences between vegetable and mineral oil should be considered in compatibility tests, once distinct properties variation in oil contamination should be observed, which is crucial in choice of transformers construction material using different insulating oil. Finally, Brazilian and ASTM standards should include same evaluation parameters in compatibility criteria determination.

Copyright of the above article for X WORKSPOT e EletroEvolução, technical publication of CIGRE - Brasil.

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