A novel optical diagnostic tool for condition assessment of insulation oils

Synthetic and natural oils are used in many items of high voltage plant, both as an electrical insulator and as a coolant. In addition, analysis of the oil may provide a convenient means of monitoring plant health-as in the case of dissolved gas analysis (DGA). Although it is well appreciated that failure of plant occurs when the insulating properties of the dielectric are reduced through ageing, a fundamental understanding of the ageing process is still lacking in many areas. To explore ageing processes in one technological oil system, dodecylbenzene (DDB) cable oil was thermally aged under air and nitrogen in the laboratory under controlled conditions. Oils were also aged identically in the presence of different additives that would typically be found in high voltage plant. The ageing behaviour was then characterised by various analytical techniques including ultra violet/visible (UV/Vis) spectroscopy and dielectric spectroscopy. It was found that when oil was aged in the presence of copper, the dielectric loss remained comparable to that of oils aged in the presence of aluminium or paper or with no additives until a characteristic absorption at 680 nm (termed "red absorbers") was detected by the UV/Vis spectrometer. The appearance of red absorbers appears to be preceded by an increase in the acidity of the oil. This paper discusses the ageing behaviour of DDB and relates this to results obtained from a commercial mineral oil. Evidently, the first appearance of red absorbers is linked to a catastrophic increase in dielectric loss of the aged oil and hence could form the basis of a novel optical diagnostic indicator to signal the end of the useful life of the oil