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 , acidity, aging times, copper particles content, etc. have been measured. It will be shown that while tan 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.
A significant number of minimum-oil breakers has been installed in HV substations and distribution cabinets of regional network operator Stedin. E.g. 35% of all bays in HV stations. Eaton, the successor to the original manufacturers Coq and Hazemeyer intended to end the support of these breakers in 2014. The paper describes the ensuing risk evaluation, options and two-track approach chosen by Stedin to manage the risk: 1. Replacement of switchgear within a period of 20 years, 2. Conservation of the 10 -25kV switchgear that have not been replaced, during 20 years, in cooperation with the successor to the original supplier. For conservation, a long term cooperation has been agreed upon with Eaton. For the other track, replacement, a replacement sequence is elaborated.
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