Investigation on thermal aging characteristics of vegetable oil-paper insulation with flowing dry air

In order to investigate the thermal aging characteristics of oil-paper insulation in open-type transformers, a thermally accelerated aging experiment with flowing dry air is performed. The vegetable oil, NOMEX paper, pressboard, copper and silicon steel are sealed into the aging vessels and further aged, for 216 hours, by supply of flowing dry air at 170 °C. Several important parameters associated with the aging are observed and evaluated during the aging process. It is shown that the degree of polymerization (DP) of pressboard gradually decreases and fits well into the second-order kinematic model. Little deterioration of the NOMEX paper under 170 ° is confirmed by the tensile strength which stabilizes at a certain level after aging. In addition, the logarithm of the furfural concentration in oil is found to have a good linear relationship with DP of pressboard. Interestingly, when the aging time is about 150 hours and DP is 329, the aging process reaches an inflection point at which the DP approaches the leveling off degree of polymerization (LODP) value, the moisture content and the furfural content of oil begin to fall down while the acid number and the dissipation factor of oil increase with a faster rate. However, the viscosity of the fluid, one of the important parameters of transformer operation, does not change significantly. The obtained results demonstrate that exposure to air leads to serious oxidation of oil and rapid increase of acid and dissipation factor, and this way oxygen should be avoided for the open-type transformer filled with vegetable oil.