DC salt-fog tests on glass and porcelain cap and pin insulators

Cap and pin insulators in HV DC transmission lines are reported to suffer 10 times higher rates of failure than in AC lines at comparable voltage. Although failure rates are higher in coastal than in rural areas, previous investigators discounted surface degradation as the cause of failure of glass and ceramic insulators. Recently, however, a form of tracking and erosion was observed on toughened glass and on porcelain insulators when subjected to `inclined plane tests´ with DC at service stresses (Mason et al., 1987). Electrolytic corrosion at the positive electrode causes a marked polarity effect with failures at 30% lower stress when the upper electrode is positive, compared with the reverse polarity. As this corresponds with service experience, it seemed probable that similar effects would occur during salt fog tests of long duration. The possibility of reducing surface degradation during such tests and in service, by protecting the metal components on outdoor insulators from electrolytic corrosion is investigated