Comparative Analysis of Transformer Solid Insulation Drying Methods

The quality of the drying process of the transformer winding insulation can be directly correlated to the reliability and longevity of the transformer as a whole. Therefore, efficient removal of moisture from paper insulation is one of the most important stages in the manufacturing process of transformers. Throughout transformers service life, several factors (oxygen, water, temperature, etc) will have a negative effect in the insulation and will reduce its performance characteristics in a process referred to as aging. Residual moisture in the solid insulation of the transformer in combination with heat and oxygen will accelerate cellulose aging, which ultimately may cause early transformer failure. An ideal insulation drying process in factory will reduce the residual moisture to levels below 0.5% content in the bulk solid insulation while retaining a high degree of polymerization between 1000 and 800. In this article, authors look into the reasons for the degradation of cellulose insulation; consider the differences and advantages of using electrical and low-frequency heating for the drying process of solid insulation inside the transformer, as well as the effect of vacuum on the evaporation of moisture. Based on the experimental work, authors classify the most effective methods in reducing moisture content in solid insulation depending on the type and voltage of the transformer.