Investigation of moisture transport behaviour in ester, mineral, and vegetable insulating oils

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Abstract
This work reports the results of experimental study of moisture migration from cellulose paper, supplied by ABB India Ltd, into six insulating oils under closely regulated test environment. mineral oil (MO), synthetic ester (MIDEL 7131), natural ester (MIDEL eN1215), cottonseed oil, peanut oil, and sesame oil were typically used for the experimentation. First, cellulose paper specimens were dried in oven to constant mass. Next, they were humidified homogeneously to 3.5%, and finally immersed in 500 g of each oil type. The moisture evolution was monitored over 168 h. The moisture in oil was recorded using a Vaisala handheld sensor and moisture in cellulose paper was obtained through destructive gravimetric analysis. The analysis of transient moisture was made using a first-order exponential model. From the analysis, the characteristic time constant ( τ ) and an estimated effective diffusion coefficient ( D ) were derived based on a one-dimensional Fick’s law formulation. The test results show considerable differences in moisture transport among the oils. The fastest moisture equilibration ( τ = 13–16 h) was found in ester oils whereas MO exhibited moderate progression ( τ = 22.7 h). The vegetable oils, on the other hand, showed the slowest transition ( τ = 30–40 h). Estimated diffusion coefficients ranged from 12.2 × 10 −13 m 2 s −1 for MIDEL 7131 to ∼3.9 × 10 −13 m 2 s −1 for sesame oil. These results provide quantitative assessments of moisture transport behaviour in oil–paper insulation systems. They contribute to improved modelling of moisture dynamics in transformer insulation and also support informed selection of insulating oils for transformer applications.

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