Partial discharges in lapped polymer taped insulation impregnated with supercritical helium

Apparatus has been designed and constructed to evaluate the high-voltage parameters of lapped cable models impregnated with supercritical helium for pressures up to 1.5 MPa and for temperatures to 4.2 K. The tape materials studied are as follows: high-density polythene, Valeron (cross-laminated polythene film), polypropylene, Nylon 11 and polysulphone with limited tests using polycarbonate. These were selected on previously published parameters related to mechanical properties, lappability and low loss. The variation of the maximum value of discharge with applied alternating voltage is obtained, and also the variation of inception stress (1 pC and 5 pC) with helium density and pressure. Although most attention has been given to tapes wrapped on a stainless-steel electrode, the influence of various conductor screens has also been investigated. In an AC superconducting cable the attainment of the highest possible inception stress is important, and the order of merit of the materials investigated is: Valeron, polypropylene, high-density polythene, polysulphpne and, finally, Nylon 11.