Temperature dependence of methanol and the tensile strength of insulation paper: kinetics of the changes of mechanical properties during ageing

Abstract

This paper reports the temperature-dependence of methanol generation and the tensile index under ageing conditions for two paper/oil systems: one consisting of a standard wood Kraft paper and the other of a thermally-upgraded Kraft paper (TUK). A linear correlation between methanol and the tensile index for these paper/oil systems was observed in a previous study at 170°C. In the current study, the correlation was extended to a large range of temperatures (150-190°C). The experimental data were fitted with a modified Calvini's kinetic model, while the Arrhenius equation was used to model the kinetic of changes of the paper's mechanical properties during ageing. Good agreement with the temperature (lnk vs. 1/T) was observed for the rate constants of the chainend group production, methanol formation, and the decrease of mechanical properties. The Arrhenius equation applied to this data grouping showed activation energy of 130 ± 29 kJ/mol for the decrease in mechanical strength of the standard Kraft paper, while the value was 118 ± 55 kJ/mol for the TUK paper. These values are of the same order of magnitude as those obtained in this study and in the literature for the depolymerization of cellulose and the generation of methanol that was reported. This confirmed the close relationship between cellulose depolymerization, methanol generation and the reduction of the paper's mechanical strength during ageing, dominated by the acid hydrolysis mechanism. The results, which can be seen as a benchmark and pave the way to further research, provide an opportunity for applying the proposed correlations to estimate the insulation condition in operating transformers.

Methanol Marker for the Detection of Insulating Paper Degradation in Transformer Insulating Oil

“…As shown in Figure 3, a relationship exists between methanol formation and the breaking of 1,4-β-glycosidic bonds in cellulose [1]. Therefore, studies have been performed to confirm the kinetic correspondence between these two processes [27][28][29][30].…”

“…Nevertheless, it is practically impossible to measure both parameters for power transformers in service. Studies of thermally accelerated ageing of paper impregnated with mineral oil revealed a quasilinear, temperature-independent correlation between the mechanical strength of paper (using the zero and wide span modes) and the cellulose's degree of polymerization [30,41,45,46,52,57,61,62,71], as seen in Figure 5. In the first part of the master curve, up to around DPv = 700, for different temperatures and types of paper, the mechanical strength of paper does not decrease with the decline in degree of polymerization [41].…”

“…Different techniques are used to determine the changes in paper's mechanical performance during ageing, such as tensile strength, flexural strength, folding endurance strength, edge tearing resistance and bursting strength [55][56][57][58][59]. Tensile testing is the technique used in several studies to track the loss of mechanical strength during paper ageing [30,41,45,46,55,57,[59][60][61][62][63][64][65][66][67]. It has been shown that the condition of paper, when chain scission is involved, can be monitored using tensile strength as a reference [41].…”

“…However, the same path is followed by the paper independent of temperature. In addition, the accumulation of points at the end of the curve marks the end of the paper's service life at around a DP v of 150 or 10-20% of the initial mechanical strength [30]. Based on experimental results and this linear correlation, some equations were developed to correlate the two parameters [46,72].…”

Dual-Temperature Evaluation of a High-Temperature Insulation System for Liquid-Immersed Transformer

“…The water content in FR3 at the end of the aging test was even lower than at the start, which means that the water was consumed during the process. Similar changing of water content is shown in [21,22], which suggests an accelerated aging rate in mineral oil compared to FR3, because hydrolysis, for which water is the reactant, is the main cause cellulose degradation of both Kraft and T910 [23]. Aramid has excellent chemical stability and is resistant to hydrolysis and oxidation, whose by-products, gas and water, are lower than the cellulose [24].…”