Partial discharges in liquid-nitrogen impregnated, taped-cable insulation

An investigation into the parameters affecting partial discharges in liquid-nitrogen impregnated taped insulation using cylindrical-cable models is described. The inception voltage increases with pressure and for all but the thinner tapes studied little change occurs after 197.8 × 105N/m2 (14 lbf/in2g.). The inception stress also decreases with increase in tape thickness. Several plastics have been used in this work. Nomex, polyethylene and polycarbonate tapes are seen to be reasonably discharge resistant whereas Tyvek, Tenax and l.l.p.p./paper are not. The effect of stress on discharge magnitude is investigated and initial work on insulation life as a function of discharge activity reported. It is postulated that the discharges occur in nitrogen gas bubbles in the butt gaps of the insulation. Possible theories for this initial formation of these bubbles are discussed but no mechanism at this stage appears pre-eminent.