CHARACTERISATION OF A NAPHTHENIC BASED UNINHIBITED VIRGIN TRANSFORMER OIL AND THE USE OF SYNTHETIC ANTIOXIDANTS. V. Dukhi 1 , A. Bissessur 1* , C. J. Ngila 2 and N. Ijumba 3 1 School of Chemistry, 3 HVDC, University of KwaZulu-Natal, Private Bag X54001, Durban, 4000, South Africa 2 University of Johannesburg, P.O. Box 17011, Doornfontein, 2028, South Africa *bissessura@ukzn.ac.za Abstract: The bulk of insulation material used in a transformer is comprised of mineral oil. Exposure of these mineral oils to oxidative, thermal and electrical stresses [1] induce ageing. Degeneration of the oil imparts poor quality thus causing it to lose its ability to function optimally. This work focuses on the characterisation of a naphthenic based virgin uninhibited transformer oil (VO) commonly used in South African transformers. While antioxidants (AO’s), such as 2,6-di-tert-butyl-4-methylphenol (BHT) (fig. 1) and 2,6-di-tert- butylphenol (DBP) (fig. 2) have been blended into oils to increase its stability, more work needs to be done to highlight the effects on the oil and efficiency of these AO’s. In addition this paper also characterises antioxidant-virgin oil blends as well as the effect of antioxidants on the oil. The common diagnostic chemical characteristics of acid and saponification values showed no detection as expected for virgin transformer oil. The proton and carbon nuclear magnetic resonances ( 1 H NMR and 13 C NMR respectively) as well as fourier transform infra-red (FTIR) data showed no presence of acids, alcohols or esters, which are common by-products of transformer oil ageing. The 1 H-NMR spectra, however indicated the presence of aromatic groups, which later was identified as tetralin and decalin by gas chromatography-mass spectrometry (GC-MS). The blending of antioxidants into virgin transformer oil proved to be successful based on evidence sourced from 1 H NMR, 13 C NMR and IR data. The 2,2-diphenyl-1-picrylhydrazyl (DPPH) assay, served to confirm that blending of BHT and DBP into transformer oils increased the free radical scavenging ability of the oil. Thermal gravimetric analysis (TGA) data revealed that blending of AO’s into transformer oil improved thermal stabilities of the oil. An attempt to identify degradation products of the AO’s by subjecting the oil blends to UV irradiation over a period of 24 hours was successfully carried out using GC-MS. Evidently blending of antioxidants has potential to enhance the performance and increase the life- span of transformer oil. 1 INTRODUCTION South Africa is currently facing a major energy crisis with respect to a shortage in power supply. As a result of urban growth and development, increased pressure has been placed on power service providers for more efficient production and delivery. In addition, the increasing demand for electricity coupled with a limited number of power stations has placed huge emphasis on power conservation and generation. Transformers form an integral part of power-stations and play key role in the transference of power. For efficient transference from power-stations to end-users, the performance of a transformer has to be considered. Transformer oil functions both as an insulator and coolant [2] . However the oil suffers a drawback in which it is susceptible to ageing, whereby the main mechanism entails oxidation. During ageing, oxidation reactions lead to the formation of carboxylic acids as well as oil sludge resulting in poor functioning of the transformer oil and the transformer unit as a whole [3] . There are two classes of transformer oils: a) uninhibited oils (containing no antioxidants) and b) inhibited oils (containing antioxidants at specified concentrations) [4] . This study has focused on naphthenic-based virgin transformer oil with synthetic antioxidants added. Note that these oils are a key component of transformer insulation and for maintaining optimum functioning of the transformer unit. In the methodology, naphthenic based virgin transformer oil was chemically characterised in order to relate its chemical properties to its functioning. The oils are characterised using both conventional and international standard routine oil analysis techniques as well as other analytical techniques such as gas chromatography with mass spectrometry. The procedure used involved the blending of antioxidants (free radical scavengers) with virgin uninhibited oil to enhance oil stability. Currently 2,6-di-tertiarybutyl-4-methylphenol (BHT) (fig. 1) and 2,6-di-tert-butylphenol (DBP) (fig. 2) are used as antioxidants (AO’s) however owing to their carcinogenic and toxic behaviour the International Electro-technical Commission (IEC) governs that these AO’s be used at concentrations not exceeding 0.3 % (w/w) [4] . XVII International Symposium on High Voltage Engineering, Hannover, Germany, August 22-26, 2011