Artificial and natural ageing tests of 275-kV class full scale insulator strings

Semi-conducting glaze porcelain and polymer insulators have higher flashover voltages under contaminated and wet conditions compared with conventional porcelain or glass insulators. However, these insulators cannot be exempted from ageing deterioration mainly caused by partial discharges along their surface under contaminated and wet conditions. Various kinds of artificial accelerated ageing tests are conducted to evaluate long term performance of these insulators all over the world. We have conducted full scale artificial ageing tests of 275-kV semi-conducting glaze and polymer insulators under combined stress conditions for about four years, together with outdoor exposure tests under the same test voltage. Erosion occurred on the trunk surface along the parting line initially at the energized end where electric stress is highest and extended up to the middle part of the specimens irrespective of the specimen effective length. Surface erosion occurred at the tips of the first and the second ribs from the pin of semi-conducting glaze disc insulators not only in accelerated ageing test but also in outdoor exposure test in a relatively clean area.