Laboratory assessment and service performance of inhibited insulating oils

The oxidation stability of inhibited insulating oils is usually assessed in the laboratory under one set of highly accelerated test conditions. The service life of these oils under normal operating conditions is now being shown to be very much less than that predicted by these tests, and in some cases it is no better than that of the base oil. As inhibited oils have always been considered capable of giving a satisfactory performance under much more severe conditions, it is necessary to know why their service performance under normal conditions is poor. This will enable them to be assessed correctly in future, and may suggest how they can be improved if more severe operating conditions arise. Commercially available, laboratory-blended and experimental oils were examined by the assessment procedure described by Wilson.1 Results are compared with service performance and with performance in experimental inductors. Tests were also carried out at higher temperatures and with nitrogen sealing to show the comparable effectiveness of inhibited oils and nitrogen sealing. It is shown that the susceptibility of oils to the effects of copper and oxygen is radically changed by the addition of oxidation inhibitors. The temperature susceptibility is, in general, less affected. Inhibitors are shown to be most effective under high-oxygen conditions and when the copper/oil ratio is high (as found in highly accelerated laboratory tests). The combination of an oxidation inhibitor and a metal passivator, or deactivator, is more successful than an oxidation inhibitor alone, but, even so, these combinations of additives have only limited economic advantage, as they will enable the oil to withstand only marginally more severe conditions. Lack of correlation between oil acidity and resistivity is shown to exist after aging in the case of inhibited oils, passivated oils and oils operating under a blanket of nitrogen.