Abstract
The insulation of transformer oil comprise of two practices: paper and dielectric fluid (such as mineral transformer oil). Ageing of these oils occur mainly by thermal, electrical and oxidative stresses. This paper describes the determination of kinetic parameters of a naphthenic based transformer oil and its blends based on the Ozawa, Flynn and Wall (OFW) and Kissinger models using data obtained from thermogravimetric analysis (TGA) and differential scanning calorimetric (DSC), plots. Virgin oil and its blends were analysed from an initial temperature range of 20-25 ºC to 300 ºC at temperature ramps of 1, 2, 4, 6, 8 and 10 ºC min-1. The OFW method in conjunction with TGA data was used to calculate the apparent decomposition activation energies at each of the selected weight loss (% conversion or α) values. The activation energies at 10% weight loss were found to be 57.0, 63.7 and 69.3 kJ mol-1 for BHT-B (virgin oil-2,6-di-tert-butyl-4-methylphenol blends), DBP-B (virgin oil-2,6-di-tert-butylphenol blends) and VO (virgin transformer oil), respectively. However using this method the activation energy (Ea) of the oils showed no significant difference between virgin oil and its blends. DSC curves revealed that the decomposition reaction was exothermic in nature. From DSC data the activation energy was determined using two separate models: OFW and Kissinger models. Decomposition activation energy obtained from DSC data showed no significant difference when applied to these two models. Dissipation factor tests showed superior results for the blends when compared to those of virgin transformer oil. However, the blends showed substantial reductions in their dielectric breakdown voltage. The implication of the reduced breakdown voltage is that the blends show poorer dielectric strength in comparison to virgin transformer oil.