Impact of the oil humidity on the AC and LI flashover strength of epoxy resin spacers in transformer oil

This paper reports on experimental investigations dealing with the impact of different water contents on the electrical strength of oil gaps without and with a solid/liquid interface. The measurements at AC stress reveal that in a uniform field an increase of the water content is combined with a significant reduction of the electrical strength which is more pronounced for the open oil gap than for the configuration with a parallel solid/liquid interface. In a non-uniform field this behavior can also be perceived but the breakdown and flashover strength is less dependent on the oil humidity. Moreover it will be shown that at negative LI stress a variation of the water content has a smaller impact on the electrical strength than at AC stress.