Partial discharges in polymeric insulation systems at cryogenic temperatures

The generation and transmission of electric power using equipment operating at cryogenic temperatures has been considered to be economically and technically feasible within ten to fifteen years. Cryogenic generators have the ad vantage of smaller size yielding greater generated power per unit volume. Superconducting or cryoresistive underground cables can transmit more power and with less losses than conventional oil/paper cables. However, the operating electric stresses of the electrical insulation have not been determined and studies of ageing phenomena are limited. A project has begun at the National Research Council of Canada to study the mechanisms of deterioration of polymeric tapes immersed in a cryogenic fluid and subjected to high electric stresses. The first part of the study is to determine the discharge inception and extinction stresses of various polymers immersed in pressurized liquid nitrogen and liquid helium. The initial results using liquid nitrogen are reported in this paper. Subsequent tests will determine the effects of partial discharges on the life of the insulation.