Space charge studies on mid-voltage cable by thermally stimulated depolarization currents in the melting temperature range

In the present work, a XLPE mid-voltage cable from General Cable co. has been studied by thermally stimulated depolarization currents. Systematic measurements have been carried out in order to compare the conductive processes in this cable with previous results. Depolarization current as a function of thermal annealing, thermal history, polarizing field and polarizing time and temperature has been obtained. The results show the presence of a broad and complex heteropolar process between 60 and 120 °C as expected. Annealing of the sample at temperatures above 80 °C develops an homopolar contribution associated to chemical components diffused from the cable semiconducting layers into the XLPE bulk. For annealing times of 60 min at 140 °C and 2 days at 90 °C, the homopolar current intensity reaches a maximum, decreasing and recovering the heteropolar sign with further annealing. Experiments performed with different polarizing times and temperatures show as well the presence of an homopolar contribution, overlapped to the heteropolar behavior, that increases continuously with polarizing time. These results indicate that conductive processes within the XLPE are probably responsible of homopolar charge injection.