Influence of environmental and operational conditions on breakdown voltage of oil in switchgear

In non-hermetically sealed medium voltage oil switchgear (MV OIS), insulating oil will absorb water from the ambient air. Humidity influenced by the acidity of the oil affects the breakdown voltage. Moreover, switching actions causes copper particles from the switching contacts to be released in the oil. To assess and consequently reduce the risk of failures due to breakdown of oil, parameters such as tan δ, acidity, aging times, copper particles content, etc. have been measured. It will be shown that while tan δ and acidity increase during accelerated aging tests, the breakdown voltage of oil is not significantly affected under constant humidity. However, during the condensing process, the acidity does affect the varying humidity, which in turn has a significant effect on the breakdown voltage. Decreases in the breakdown strength have been found between 17 and 29 percent.