Deterioration mechanism of paper base phenolic insulators for power distribution equipment

We verified by analyses of new and used paper base phenolic insulators and an acceleration test that the main reason for surface resistivity reduction in paper base phenolic insulators was the permeation of deliquescent ion compounds into the insulators. The main elements were Na and Cl, which were detected by element mapping analysis both on the cross section and surface of insulators, though Al and Si were detected only on the insulators' surface. The mechanism was presumed from the results of these analyses as follows. (1) Deliquescent ion compounds such as sodium chloride, calcium sulfu¿rate, etc. adhere to the insulators' surface. (2) They absorb water in the air. (3) Aqueous solutions of sodium chloride, calcium sulfu¿rate, etc. permeate into the insulators. (4) Though the deliquescent ion compounds adhering to insulators can be removed by cleaning, it is difficult to remove the permeated compounds. Therefore, the insulators deteriorate progressively over time. To verify this mechanism, a deliquescent ion compound (ex. sodium bromide) was put on the used insulators and a composite temperature/humidity cyclic test was executed. The above mechanism could be verified because Na and Br elements were detected by element mapping analyses on the cross section of the accelerated test samples and the permeated Na and Br elements could not be removed by cleaning.