Aging Rate Of Paper Research Articles

Studying the loss of life of natural-ester-filled transformer insulation: impact of moisture on the aging rate of paper

Moisture is one of the most harmful agents for transformer solid insulation. The catalyzing effect of moisture on the degradation process of paper and pressboard has been studied by several authors, who developed models which they used to estimate how the aging process would be accelerated in the presence of water [1]–[5]. In [2] and [3] Lelekakis et al. reported the results of an experimental study in which samples with different moisture contents were aged in the presence of mineral oil (MO). In [3] an equation to determine the reduction of service lifetime of insulation subjected to increased temperature and with increased moisture content is proposed.

Aging rate of grade 3 presspaper insulation used in power transformers

Paper materials are used as the insulation of power transformers. Over time these materials slowly degrade, until they reach a point when they no longer function effectively as transformer insulation, and the transformer has then reached its end of life. The aging rate of paper is affected by temperature, water, oxygen and acids. Investigations have been performed previously on Kraft and Kraft thermally upgraded types of paper. However, a different type of paper used in transformer insulation, Grade 3 presspaper (which contains cotton), has not been extensively tested and compared to Kraft paper. In these experiments we studied the aging rate of Grade 3 presspaper and compared it to our previous studies of Kraft paper. Traditionally, the aging rate of paper has been studied in sealed vessels. The problem with this approach is that the chemical environment within the vessel will change during aging, and so the aging rate will be affected. In our experiment setup we controlled the water and oxygen content to more accurately determine the aging rate. Similarly to Kraft paper, the aging rate of Grade 3 presspaper with the same water content increased with oxygen content in the oil. Life curves were developed based on the water content of the paper and the oxygen content of the oil.

Ageing rate of paper insulation used in power transformers Part 2: Oil/paper system with medium and high oxygen concentration

Utility asset managers and engineers are concerned about the life and reliability of their power transformers which depends on the continued life of the paper insulation. The ageing rate of the paper is affected by water, oxygen and acids. Traditionally, the ageing rate of paper has been studied in sealed vessels however this approach does not allow the possibility to assess the affect of oxygen on paper with different water content. The ageing rate of paper has been studied for dry paper in air (excess oxygen). In these experiments we studied the ageing rate of Kraft and thermally upgraded Kraft paper in medium and high oxygen with varying water content. Furthermore, the oxygen content of the oil in sealed vessels is low which represents only sealed transformers. The ageing rate of the paper has not been determined for free breathing transformers with medium or high oxygen content and for different wetness of paper. In these ageing experiments the water and oxygen content was controlled using a special test rig to compare the ageing rate to previous work and to determine the ageing effect of paper by combining temperature, water content of paper and oxygen content of the oil. We found that the ageing rate of paper with the same water content increased with oxygen content in the oil. Hence, new life curves were developed based on the water content of the paper and the oxygen content of the oil.

Influence of vegetable oil on the thermal aging of transformer paper and its mechanism

With the development of new insulation materials, vegetable oil-the best substitute for mineral oil-has gradually been widely used in the liquid insulation of transformers. To investigate the influence of vegetable oil on the thermal aging rate of oil paper, transformer paper impregnated with mineral oil and vegetable oil underwent thermally accelerated aging at three different temperatures. Degree Polymerization (DP) of paper was measured to indicate the aging degree of transformer paper. The aging rate of paper in mineral oil and vegetable oil was compared quantitatively, and results showed that vegetable oil retarded paper's degradation rate and extended its useful lifetime. The reasons contributing to such phenomenon were analyzed using X-ray Photoelectron Spectroscopy (XPS) and molecular simulation software. Transformer paper in vegetable oil had larger activation energy. Due to the larger interaction force between and natural ester molecules, molecule was easily bonded with natural ester, weakening the hydrolysis process of cellulose. Cellulose was chemically modified by natural ester during thermal aging process, and the reactive -OH (hydroxyl) groups on the cellulose became esterified with fatty acid esters. The molecule was firmly bounded to the ester groups on glucose produced by esterification. The long-chain fatty acid esterified to cellulose was parallel with cellulose chains and acted as a water barrier to further weaken the hydrolysis process.