Investigation of Distribution Transformers Vibrations in Terms of Core and Winding Condition Assessment

This work refers to the criterion values used to assess the state of the active part of the transformer based on the analysis of the effective value of the total vibration acceleration and the frequency spectrum. It was proved in the work that the criteria values should be differentiated for transformers of different rated power. Transformers with lower rated power are characterized by lower RMS values of vibration acceleration than units with higher rated power, which cannot remain without impact on the criteria values. Trend analysis of the total aRMS values may reveal increasing defects before the currently applicable criteria values are exceeded. In addition, the influence of the position of the sensor on the frequency spectrum of the obtained signals was analyzed. It was proved that the sensors should be mounted in the middle of the transformer tank, between its lower part and the cover. The dependence of RMS value of vibration acceleration on no-load losses was also determined for transformers of different types but the same rated power. This relationship will not have a large share in the total vibrations of the transformer, due to slight changes in the value of aRMS occurring for all analyzed units

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