Study of low molecular weight chemicals as paper ageing indicators in transformer liquid and paper insulation system

For over a century, liquid and solid insulations have been widely used in high voltage power transformers. Degradation of paper insulation and the loss of its insulation integrity are one of the major causes for transformer failure. This has stimulated research into the ageing of paper insulation and led to the development of ageing indicators. An ageing indicator is a detectable parameter whose value is directly linked to the state of transformer paper insulation. This thesis is devoted to an experimental investigation of the behaviour of low molecular weight chemicals in the thermally accelerated ageing process of Kraft paper. A particular focus is directed towards acidity as a paper ageing indicator. Although acidity is a well-known indicator for assessing the ageing condition of the transformer liquid, only more recent studies found that acidity is mainly made up of short and long chain carboxylic acids. The short chain acids, otherwise known as low molecular weight acids (LMA), play a significant role in the paper ageing. In this thesis, a perfectly sealed oil-paper ageing system was designed and developed. The proposed test and measurement strategy uses oil samples of 10 ml and paper samples of 0.5 g. The perfectly sealed samples completely eliminate mass exchange between the samples and their environment, allowing the realization of strict ageing conditions. Additionally, specific array formations are used to enable a uniform thermal ageing environment for a large number of samples to be aged concurrently. A high extraction efficiency of LMA from transformer liquid, containing a mixture of LMA and HMA (high molecular weight acids), has been achieved by the development and use of a water based acid extraction system that uses Erlenmeyer flasks equipped with internal mixers. An LMA detection efficiency of >80% for synthetic ester fluid and >90% for mineral was achieved for LMA concentrations as low as 0.017 mg KOH/g of liquid. The change of LMA concentration during the ageing process of Kraft paper at an elevated temperature of 130 °C in two types of transformer liquids with different initial conditions were systematically measured in parallel to the measurement of moisture, 2-FAL and methanol concentrations. This enabled the study of the ageing process from different angles based on experimental results. Results from samples with different ageing durations show that LMA are produced from the ageing of Kraft paper. The LMA concentration in mineral oil and synthetic ester is different because of the polar nature of the latter, but the liquid LMA concentration is strongly coupled to the change in degree of polymerization (DP) of the paper, providing pivotal evidence for the development of LMA concentration in liquid as a transformer paper ageing indicator. The effect of the presence of excessive LMA in transformer liquids was also studied. It was shown that an initial LMA-rich ageing environment (with formic acid of 0.4 mg KOH/g added to the liquid) greatly accelerates the ageing of Kraft paper, by a factor of 5 for mineral oil and 2 for synthetic ester. The lower ageing acceleration factor of LMA in synthetic ester manifests its ability to mitigate the catalytic effect of LMA in paper ageing.