Empirical based approaches to evaluating the residual life for oil-immersed transformers - A case study

The transformer cellulose insulating material is a key benchmark for evaluating the remnant transformer service lifetime. The decomposition of the cellulose paper may be assessed by using the degree of polymerization (DP) which necessitates that paper samples be extracted from the unit in- service. Whereas, extracting the cellulose paper is particularly complex, alternative indicators have been employed to indirectly divulge the level of DP in the cellulose paper. For case in a point, the measurement of the Furan compounds in the oil samples extracted from the unit in-service. Notwithstanding, the existing DP models have proven to be unreliable in the assessment of the DP value in the cellulose paper on account that for the same furan concentration, these models lead to varying values of the DP. This is attributed to the fact that the statistical survey of these models studies different transformers with varying blends of insulating oil and application. In this work, new formulae for estimating the DP value of cellulose insulation for transforms particularly in the South African electrical grid are formulated. Measured DP values are used as a benchmark for the new formulae and further compared with the exiting DP models. The results yield an error of estimate of less than 3% against the measured data. The study further evaluates the remnant life of the studied transformers and yields an error of estimate of 2% against the measured data.