Assessment of Solid Insulation by Analyzing Oil in Service Transformers and Laboratory Ageing Studies

Transformers are the most critical and costliest equipment in the power grid system. Therefore, for a reliable power supply, it is crucial to ensure the power transformer is operating at its best level. The ageing of oil-filled power transformers causes several flaws in the insulating materials. The remaining life of a transformer is assessed by the ageing status of its liquid and solid insulation. The quality of liquid insulation is maintained by processes like filtration, reclamation, replacement, etc., depending on the level of degradation of the oil, whereas severe deterioration of solid insulation necessitates more expensive activities such as rewinding or replacement of the transformer. Therefore, effective diagnostic methods are essential to analyze the insulation status of the transformer. Presently, there are various techniques in practice for the assessment of insulation status. Among the methods, degree of polymerization (DP) is the most significant one. Nevertheless, obtaining a representative paper sample from an operational transformer is difficult. In this paper, an attempt is made to assess the insulation status of a transformer through the estimating oil samples. Oil samples of transformers in service are brought to the laboratory and investigated for various electrical and physicochemical properties, including furan content, CO
2
/CO ratio, and alcohol content. The relationship between furan content and various oil parameters is analyzed. The study is further extended to the assessment of laboratory aged test samples of oil/paper/copper systems, aged at 130°C for up to 14 days. In field samples, a correlation between furan content, CO
2
/CO ratio, and oil property is obtained where oil-paper equilibrium is undisturbed by treatment of oil. In the case of laboratory aged samples, the concentration of furan is found to increase with an increase in the duration of thermal exposure, varying with oil property. This investigation demonstrates that accurate and systematic condition monitoring of oil can be considered as an alternate to DP measurement.