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In this letter, we experimentally demonstrate that the lightning impulse breakdown characteristics of natural ester liquids can be significantly enhanced by dispersing molecular additives possessing lower values of both the ionization potential and 1st excitation energies as compared to the base liquid. One such additive contributed to an increase in the breakdown and acceleration voltage of the base liquid by 32% and 90%, respectively. Apart from the expected influence of the low ionization potential of the additives, results also indicate a positive effect of lower value of 1st excitation energy.

Hydrolysis of transformer insulating Kraft paper is experimentally investigated. Paper moisture content was varied and five different acids -representative of what may be found in an aging transformer -were added to the oil. The low molecular weight acids were to a large degree absorbed by the paper, contrary to the high molecular acids that remained in the oil. The three low molecular acids accelerated the aging of the paper, and a clear synergy between these acids and water content was found. The high molecular acids did not influence the paper aging significantly. The results support a theory explaining the hydrolysis as governed by acid catalysis. Examples from measurements on real transformers are shown. It is concluded that low molecular acids constitute an important aging factor for transformer windings and should be included in diagnostic schemes, and when estimating the pay-back from maintenance actions.