Experimental investigation into the thermal-ageing of Kraft paper and mineral insulating oil

The state of insulating oil in a transformer can be directly evaluated because it is easily accessible for sampling. The same is not true for insulating paper for which the condition must be evaluated using an indirect method. International experience indicates ageing of insulating paper leads to furanic compound formation;. mainly furfuraldehyde, also named furfural, or 2FAL. These compounds are soluble in the insulating oil and may be measured using a standard IEC method. However, interpretation of the results and their relation to transformer ageing is not clear. A dynamic heating experiment was conducted to examine the influence of temperature on the ageing and degradation of insulating paper in transformer oil. The furfural and gases generated were measured and their relation to paper ageing investigated. The results allow concentration of furfural in oil and furfural absorbed by paper, to be related to degree of polymerisation of the paper and to paper ageing.