Influence of EVA-carbon black based SC-shield in free charge in LDPE

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Abstract
Several semiconducting (SC) shields used in medium voltage cables were analyzed. Besides, four experimental SC shields were prepared in the laboratory and were studied as well. PEA measurements were performed in LDPE films, in contact with the SC shields, with applied fields up to 140 MV/m. These measurements show the presence of a threshold electric field, above 100 MV/m, from which charge packets formation is observed. Packed size and its evolution, however, is quite different depending on the SC shield. In four cases the packed charge size show a decrease as it propagates, not observed in the other cases. Propagation speed is as well notably dependent on the SC shield with transit speeds from 1.4×10 -7 m/s to 4×10 -7 m/s. With regard of packed size, the biggest charge packet observed is almost five times bigger than the smallest one. Obtained TSDC spectra are in good agreement with all these results. Some correlation is obtained between the `packet generation ability' and the nature and composition of the SC shields. Break-down tests in three of the original cables show good correlation with free charge measurements. The presence of free charge shows up as the main cause of poor breakdown results. Infrared spectroscopy show an increase of diffusion of some components from the SC shield into the LDPE in the case of the poorest test results.