Aging of transformer insulation — experimental transformers and laboratory models with different moisture contents: Part I — DP and furans aging profiles

The paper presents the results of a comparative study on accelerated aging of specially designed experimental transformers and their corresponding laboratory models. The experimental transformers are 300 kVA, with insulation in line with medium size and large mineral oil immersed power transformers, having different initial water contents in the standard Kraft paper insulation of ca. 1 and 4 %. These transformers were subjected to the rated current and voltage conditions, operating in the irregular parallel operation, whereby the temperature of windings were measured with fiber optics sensors and kept at 105 °C for 460 days. Each transformer was equipped with a removable tank which allowed access to the outer surface of the HV windings. Laboratory experiments were simulating experimental transformers and were performed in Erlenmeyer flasks, containing oil, paper materials, copper and iron in the same proportions as in the active part of the experimental transformers. The parameters that were monitored during aging were the following: degree of polymerization of paper, hot-spot temperature of transformers, water content in paper, and water and furans content in the oil. Estimations of water content in solid insulation of experimental transformers using frequency domain spectroscopy and the Perrier-Lukic moisture equilibrium chart were performed. The experimental results show that shortening of the insulation life, defined by the value of degree of polymerization, increased 2.5 times with increasing moisture content in the cellulose from ca. 1 to 4 %. For both experimental transformers and laboratory models, the relationships between the paper degree of polymerization and the furans content for dry and wet insulation was developed and is presented in the paper.