Moisture in Cellulose Insulation of Power Transformers - Statistics

The article presents results of broad investigations aiming at determining moisture content in a large population of power transformers (161 units). The project was carried out during the period 2003 – 2011 in Poland, Germany, and Sweden. Among the transformers tested were grid transformers, generator step-up (GSU) transformers and distribution transformers. According to the classification rules established by IEEE, 30% among the investigated transformers could be classified as “wet” (2-4% moisture content). The observed change of moisture content with age depend on transformer design and their operating conditions. It is concluded that based on the presented statistical results and on the knowledge of transformer type and age it is possible to roughly estimate its moisture content, which thereafter may be useful for taking decisions concerning maintenance schemes