TSDC investigations of thermally and electro-thermally aged cable models

Classical TSD and fractional polarisation analyses were carried out on unaged and aged XLPE cable models from -120°C to 120°C and three relaxation processes were found. A low temperature process, identified as β relaxation and ascribed to the polar groups attached to the polymer chains in the amorphous region, gets stronger with duration of the aging and can be mainly ascribed to an increase in carbonyl group concentration. The second polarisation process, that exhibits itself by a strong TSD peak at a temperature range 50-80°C and is only present in the spectra of aged samples, is believed to be caused by a charge relaxation. The high temperature peak, present above 100°C, can be related to melting of crystalline phase in the polymer. This peak should also be related to charge polarisation process. Fractional depolarisation measurements allowed for a precise description of elementary processes (activation energies and preexponential factors)