AN INVESTIGATION INTO ELECTRICAL AGING OF INSULATING POLYMERS BY DETECTION OF CHARGE INJECTION UNDER ALTERNATIVE DIVERGENT FIELD

Our aim is to understand the electrical ageing of insulating polymeric materials under a.c. voltage and in discharge-free situation, i.e. in the absence of any pre-existing gaseous cavities in the bulk of the d ielectric. In this report, we use a needle electrode moulded into the insulant to simulate defects producing local field enhancement. An exp erimental technique for measuring the charge flow injected by the need le into the dielectric under a.c. voltage is described. The transition between the discharge-free electroluminescent state to micro-partial discharge state (early electrical tree propagation phase) is investiga ted with a sensitivity reaching 0.01 pC. Optical diagnosis is carried out simultaneously. Special emphasis is given to synthetic insulation of power cables. Low-density polyethylene is used as a dielectric with moulded metallic needle electrode. Semi-conducting electrodes prepare d from the shield of a cable are also used in this investigation. Stat istics on times to tree inception show that the needle tip cannot be c onsidered as a homogeneous injecting surface. The conclusion is double : (i) the onset field for massive charge transfer derived from the me asured curvature radius of the needle is not the absolute value and (i i) the discussion of the results obtained by a needle test must includ e a Weibull statistical analysis with determination of the location pa rameter.