Abstract
Cellulose paper, whose main component is cellulose polymer, has been widely used in oil-immersed power transformer that gradually deteriorates during transformer operation. Thermal aging is the main degradation form for cellulose paper immersed in insulation oil (oil–paper insulation) in a transformer. One of the most challenging issues in oil–paper insulation is inhibiting the aging of cellulose paper and extending its life. In this work, a comparative study was conducted on the thermal-aging characteristics of cellulose paper immersed in a novel three-element mixed insulation oil and mineral oil at 130 °C for 150 days. The key parameters of cellulose paper were analysed, including the degree of polymerization (DP), thermal-aging rate, surface colour, and AC breakdown voltage. The furfural content and acidity of the oil, as well as the AC breakdown voltage of the insulation oil were also analysed. The results show that the cellulose paper immersed in novel three-element mixed insulation oil had much higher DP values than that immersed in mineral oil after the same thermal-aging time. The mixed insulation oil could significantly inhibit the thermal aging of cellulose paper and prolong its life. The thermal-aging rate of the cellulose insulation polymer immersed in mixed insulation oil is significantly lower than that immersed in mineral oil, whether in the process of oil–paper insulation continuous aging or in the process of aging after oil replacement with unused insulation oil. The furfural generated by cellulose degradation in the novel three-element mixed insulation oil was also less than that in the mineral oil. The mixed insulation oil had a higher acidity value during the thermal-aging process, which was mainly due to the natural esters in the components of the mixed insulation oil. However, the AC breakdown voltage of the mixed insulation oil was always higher than that of the mineral oil. This study offers a new perspective in inhibiting the thermal aging of cellulose polymer in insulation oil.
Highlights
Polymers are widely used in electrical equipment, in which cellulose paper is the most important part of the main insulation in oil-immersed transformers
According to IEC 60450-aml, the specific steps of the degree of polymerization (DP) value test can be described as follows: first, the cellulose-paper sample is degreased with normal hexane; the cellulose paper is cut into pieces, and a certain mass of cellulose paper is weighted in accordance with the standard; the cellulose-paper fragments are placed into the cupriethy lenediamine solution to dissolve, and a vibration gauge is used to promote dissolution; the viscosity of the mixed solution is measured by viscometer, and the DP values are calculated using the measurement results [13]
The cellulose consists of linear, polymeric chains of cyclic, β-D-glucopyranose units that are composed of C, H, and O elements [14]
Summary
Polymers are widely used in electrical equipment, in which cellulose paper is the most important part of the main insulation in oil-immersed transformers. In order to delay the aging rate of cellulose paper in these old mineral-oil transformers, mixed insulation oil containing natural esters has been further studied, and a new three-element mixed insulation oil was successfully developed at the end of 2017 [6]. The three-element mixed insulation oil contains 24% vegetable oil, and its main parameters can satisfy the IEC 60296:2012 for mineral oil This finding indicates that the mixed insulation oil could be directly used to replace mineral oil in field transformers. In addition to the normal thermal aging of cellulose paper impregnated by mixed and mineral oil, oil replacement was conducted using unused mixed oil and mineral oil on mineral oil-immersed samples in the middle of the aging period Parameters such as the degree of polymerization (DP) values of cellulose paper and the furfural content in oil were measured to study the thermal-aging characteristics of cellulose paper immersed in new three-element mixed oil and mineral oil
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