Performance of dielectric properties due to the presence of moisture in vegetable insulation oils and kraft paper immersed in the oils

Abstract

Vegetable insulation oils in transformers have become a reality. However, they are inherently hydrophilic and can absorb much moisture. Existence of moisture in transformer insulation will generally lower the breakdown voltage (BDV) of oil. Moisture will also accelerate cellulose deterioration by increasing its Direct Current (DC) conductivity. Both oil and cellulose are components of the transformer insulation system. There are few studies that have been, done by other researchers on palm-based oils but they neither address the effect of increasing moisture in the oil types nor do they give understanding of the deterioration of cellulose when immersed in the oil. It is therefore important to understand how these classes of oils retain its dielectric strength and its relationship with kraft paper after absorbing so much moisture. In this study, Fourier Transform Infrared (FTIR) spectroscopy, Karl Fischer titration analysis, IEC dielectric test Standards and established Polarization and Depolarization Current (PDC) insulation assessment technique were used to study moisture increase in four v.egetable-based oils and kraft paper immersed in them. The oils studied were three palm-based oils, Red Palm Oil (RPO), Palm Fatty Acid Ester (PFAE) and Refined Bleached Deodorized Palm Oil (RBDPO) and one soybean-based Envirotemp@ FR3rM. Moisture was varied by up to 0.3% (by vqlume) and the cellulose samples are of 0.2mm, 0.5mm and lmm thickness. Results from these studies show that the hydrolysis of these oil samples causes their moisture level to increase by 100% to 700Yo. [n addition, the moisture increase causes changes in the fatty acid structure of the palm-based insulation oil only and not on the soybean-based oil. Comparing the BDV of these oil types to IEC 60156 standard of 30kV, this study found that FR3, PFAE and RBDPO are capable of remaining above standard at moisture level up to 0.055%, 0.025% and 0.015% respectively while the BDV in RPO was below standard. An equation to estimate the BDVs of vegetable oils from their moisture content was also developed. The estimated BDVs of the oil samples mostly varied between I to 20%o from the experimental BDVs. The study also found that as moisture in the oil increases, kraft paper samples absorb oil and lose moisture to oil. The oil absorption was also a function of paper thickness and moisture in the oil. The conductivity of kraft paper samples immersed in oil types were found to have lowest values at oil moisture level of 0.01% for both RPO and PFAE and 0.02% for both RBDPO and ER3. Thus, for better aging performance, the research concludes that vegetable insulation oils should not be as dry as mineral oils as cellulose has lower conductivity in wetter vegetable oil.