Methods for Direct and Indirect Condition Assessment of Insulating Paper in Transformers

Consequently, indirect assessment methods based on ageing markers in the insulating fluid are commonly employed. Traditionally, the concentration of 2-furfuraldehyde (2-FAL) has been utilized for this purpose, with DP values estimated using empirical correlations such as those proposed by Chengdong and De Pablo.

This work presents post-mortem analyses of 30 transformers of TenneT TSO GmbH to evaluate the correlation between 2-FAL concentration and DP values. Calculation methods by S.D.

Myers, Heisler & Banzer, Chengdong and De Pablo were compared. Results indicate a strong correlation with average DP values, although deviations occur for minimum DP values.

Additionally, the study examines the mitigating effect of oil regeneration on 2-FAL concentrations in two cases.

In recent years, alternative ageing markers such as methanol and ethanol have gained attention.

These parameters have been monitored in the German TenneT grid since 2019, yielding 846 measurements. Comparative analysis reveals that methanol concentrations generally exceed ethanol concentrations. While no definitive trend emerges when comparing methanol and 2-FAL, ethanol concentrations are consistently lower than 2-FAL.

A limitation of indirect methods is the loss of ageing markers during oil regeneration, which compromises DP estimation. To address this, a novel optical sensor has been developed for direct DP measurement. The system employs a light-emitting diode (LED) as a light source, with optical fibers guiding light to the insulating paper and returning reflected light to a compact spectrometer. Spectral analysis enables determination of paper ageing, exhibiting a strong linear correlation between sensor readings and DP values. This approach facilitates continuous monitoring during transformer operation.

Both the indirect and direct methods ultimately improve operational safety. This in turn allows more economical operation of the assets, as it promotes the process of condition-based assessment.