Interpretation of carbon monoxide formation in liquid impregnated equipment

Carbon monoxide concentrations higher than reference values in international standards have been observed in fault free equipment in service. Since carbon oxides are being commonly considered as degradation products of solid insulation, it is important to find the source of these gases. In a number of cases it seems that it could be due to a higher oil oxidation rate due to moderate, however, continuous bulk oil temperature. Laboratory tests bring evidence that insulating fluids tend to produce higher amounts of carbon monoxide and sometimes also further hydrocarbon compounds (the so called stray gassing), especially under the catalytic influence of copper. Also degradation of passivator in oxidized oils may lead along further gases also to a higher amount of carbon monoxide. Field examples, as well as laboratory verification of carbon monoxide evolution at different temperatures and insulating liquids is presented.