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Dependence of space-charge trapping threshold on temperature in polymeric DC cables

This paper shows that the charge detrapping rate follows an Arrhenius-type law when the poling electrical field is large enough to cause copious injection of electrons, so that the transport and trapping process is dominated by electronic carriers. At fields close to the charge accumulation threshold, the tested materials, LDPE and XLPE, suggest that the conduction mechanism takes into account both ionic and electronic charge carriers. The threshold field for space charge accumulation significantly decreases as the temperature increases, at least in the range 20 to 60 /spl deg/C, even if it seems that the temperature dependence becomes smoother above 50 to 60 /spl deg/C. The increase of mobility with temperature and, thus, of charge depletion rate, after each polarisation voltage reversal, also follows an Arrhenius-type law, with smaller activation energy values for XLPE. These results must be considered when designing dc polymeric insulation systems, such as HVDC cables, since charge accumulation is a degradation acceleration factor and the temperature distribution in the cable can change significantly with loading conditions.