Physico-chemical analysis of transformer oil and stability studies of their blends with synthetic and natural antioxidants.

Abstract

This study is focused on the analysis of transformer oil commonly used in South African transformers and the characterisation of the blends of these oils with either synthetic antioxidants or those extracted from natural sources. The virgin naphthenic based uninhibited transformer oil was shown to have no acidic or saponifiable constituents. Although the oil used in this study is naphthenic based it was found to contain bicyclic compounds identified as tetralin (aromatic) and decalin by GC-MS. The oils were void of typical wear metals and only additive metals/non-metals were present. The blending of transformer oils with synthetic antioxidants was successful as confirmed by ¹H NMR, ¹³C NMR and FT-IR data. The blending showed increased antioxidant activity of the oils as well as thermal stabilities (DPPH assay and TGA respectively). Apparent activation energy for the decomposition reaction was determined by means of the Ozawa, Flynn and Wall (OFW) and Kissinger models. The apparent activation energy for BHT-B, DBP-B and VO were found to be 57.0, 63.7 and 69.3 kJ molˉ¹ , respectively for 10% mass conversion obtained from TGA data (OFW model). The dissipation factor for the oil blends was established to be favourably lower than that of unblended oils; however an undesirable decrease in the dielectric breakdown voltage of the blends was observed (16, 26 and 50 kV for BHT-B, DBP-B and VO respectively). The blending of transformer oil with synthetic antioxidants is a common practice, however greener approaches are always favoured. In this study, a greener approach was employed by blending the transformer oil with turmeric plant extract in an effort to improve the oil’s performance. Turmeric is known for its antimicrobial, antioxidant and anticarcinogenic properties. Solvent extraction of turmeric powder and subsequent blending into transformer oils, revealed the dissolution of 5 components from turmeric into the oil, viz., curcumene, sesquiphellandrene, ar-turmerone, α-turmerone and β-turmerone (curlone). In this work, we analysed novel turmeric-oil blends in terms of oxidation and thermal stability in comparison to that of virgin transformer oil. The DPPH assay confirmed that addition of these blends into the oil increased its ability to scavenge the free radical DPPH. Thermogravimetric analysis showed an increase in thermal stability when compared to that of virgin unblended oil. Evidently blending of synthetic antioxidants/turmeric extract has potential to enhance the performance and increase the life-span of transformer oils.