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An 800 kV electric power substation can have more than one hundred of porcelain multicone type insulators supporting busbar structures. Two or more sections in series, having fitting connections between them, compose these insulators. The ageing process can cause degradation of the cement used in this type of insulator and this fact can affect its dielectric performance under heavy rain. This paper presents results of an investigation based on power frequency high voltage tests performed on 800 kV porcelain multicone type insulators, removed from service after having operated for more than 20 years, as well a non used one that had been stored on site for long time. The insulators were tested in different arrangements: each section at a time and the two sections assembled in series, as actually used insulator columns. The tests were carried out under artificial rain ranging from 1 mm/min to 5 mm/min. The results have confirmed a reduction of up to 30% in the insulator flashover strength under 5 mm/min rain conditions and gave important information to the Utility about radial cracks that were observed in many insulators and about the discharge mechanisms along the insulators under rain.

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