Insulation analysis of instrument transformers using dielectric response

Summary

A high water content in the oil-paper insulation of instrument transformers can lead to failures of the insulation and as a consequence as far as to a complete destruction of an asset. Therefore, it is important to be able to assess the water content of instrument transformers. This proves quite challenging as, in contrast to power transformers, measurement techniques like oil sampling are difficult to conduct due to the small oil volume and the often not simple access to it. This study focuses on the application of dielectric response analysis to assess the condition of instrument transformers as it is a non-invasive and non-destructive measurement technique. The paper describes principles of the analysis of instrument transformer of different types. It also shows how to deal with inevitable influence factors like temperature changes and shows a way how correct temperature compensation can be performed easily. The theoretical considerations are accompanied by a case study where 83 measurements on different types of instrument transformers are analyzed. The results show that if a temperature compensation using frequency shifting was applied, the tan(d) at 50 Hz is a good indicator for a high water content of instrument transformers but less suitable for assessing lower water contents.

Additional informations

Authors

Farhan Mahmood, Datsios Zacharias