Influence of moisture on the negative hetero-charge generation in polymeric dielectrics

Under DC voltage stress, a negative hetero-charge is generated near the positive electrode in cables insulated with cross-linked polyethylene (XLPE). The generation of negative hetero-charges in XLPE-insulated cable results from cross-linking byproducts, antioxidants, and moisture. Among these inclusions, moisture is known to originate from dehydration of a-methyl styrene, which is a byproduct produced during chemical cross-linking of XLPE cable. To investigate the influence of moisture on negative hetero-charge generation, the authors first studied the moisture absorption of XLPE, EPR, and LDPE. The results show a big difference in moisture absorbing characteristics among XLPE, EPR, and LDPE. The authors next studied the effect of moisture absorption on negative hetero-charge generation in XLPE, EPR, and LDPE. Experimental results demonstrated that the charge densities of negative hetero-charges generated in LDPE and XLPE are proportional to moisture concentration in these materials. The hetero-charge generation mechanism in XLPE is discussed.