Partial Discharges in Cavities in Insulation Impregnated with Supercritical Helium

Apparatus has been designed and constructed to evaluate the high voltage parameters of assemblies of flat discs of insulation incorporating artificial voids or cavities impregnated with supercritical helium for pressures up to 1.5 MPa (15 bar) and for temperatures down to 4.2 K. The materials studied are possible insulations for a superconducting cable and are high density polyethylene and Valeron(R) (cross laminated polyethylene film). These were selected on previously published parameters related to mechanical properties, lappability, and low loss (tan¿< 2 × 10-5). Polyethylene terephthalate also was investigated because its permittivity is higher than that of polyethylene. The variation of inception stress (1 pC and 5 pC) with helium density for various pressures is obtained for circular and square section cavities and the relevance of the results to the design of insulation for a superconducting cable is discussed.