Investigation on electrical, thermal and mechanical properties of thermally aged pressboard impregnated with mixed mineral oil and synthetic ester fluid

Abstract

The proposed study compares the electrical, thermal and mechanical performances of thermally aged cellulose pressboard material impregnated in mineral oil, synthetic ester fluid and mixed oil. The electrical insulation characteristics of oilimpregnated pressboard (OIP) are studied by means of surface discharge inception voltage (SDIV) using an ultra-high frequency technique and surface potential measurement analysis. It indicates that OIP aged in mineral oil has higher surface potential due to higher charge trap density compared to that of the mixed oil and synthetic ester fluid. Mechanical and thermal characteristics of the aged OIP are studied by means of tensile strength measurement and thermo-gravimetric analysis, respectively. The change in the chemical composition of OIP is studied by means of laser-induced breakdown spectroscopy, which indicates lower plasma temperature with mineral oil-based OIP indicating higher degradation state. To further understand the microscopic structure of the thermally aged OIP in different oils, scanning electron microscopy and X-ray diffraction studies have been performed. The pressboard thermally aged in synthetic ester fluid and the mixed oil has better electrical, thermal and mechanical properties as compared with the pressboard aged in mineral oil.

A new technique for recovering aged mineral oil to promote lifetime and sustainability

“…in a reaction tank with 1 L of oil (Amizhtan et al, 2020), where it was submerged at 90 °C for further 24 h to achieve impregnation. The weight proportion of the vessel pressboard, copper, and oil was 10:1:1.…”

“…Ageing is likely to cause the interaction of water molecules and the hydroxyl groups in the cellulose chain, with the consequence of the breakdown of polymers hydrogen bonds, and scission of the lengthy cellulose chains, which leads to a lower degree of polymerisation and decrease of their mechanical strength (Amizhtan et al, 2020;Coulibaly et al, 2013). After ageing, impregnated pressboards exhibit a decrease in percent tensile strength and elongation at break, respectively, in the following rank: unaged PB (4.78%:4.8 mm), FMO (4.35%: 4.64 mm), RAMO (4.32%: 4.33 mm), and AMO (3.9%: 3.2 mm).…”

“…The presence of contamination in oil (which can increase with ageing under operation) and metal particles in oil or paper, delamination, and gas bubbles in the oil can cause accelerated extrinsic insulation ageing (Safiddine et al, 2017). Polar molecules, including a group of carboxylic acids, will develop in paper due to electrothermal stress (Lelekakis et al, 2014;Amizhtan et al, 2020), damaging cellulose chain structure. Further degradation reactions are induced by moisture, ultimately fracturing the cellulosic structures.…”

“…Acids and sludges are produced when oil oxidises, which causes oil to degrade. Pressboard absorbs low molecular weight acids, facilitating the protonation that damages the pressboard matrix (Amizhtan et al, 2020;Liao et al, 2008).…”

Degree of polymerisation estimation for insulation paper immersed in silicone oil using frequency domain dielectric spectroscopy

“…Degree of polymerisation (DP) and tensile strength are the main ageing indexes of oil-paper insulation, among which DP is most widely used, and it is the most intuitive and recognized target to reflect the ageing state of insulation paper [3][4][5]. Many and the most widely used zero-order ageing kinetic model was proposed.…”