Charging behavior of polyethylene and ionomers

Space charge may lower the breakdown strength of solid insulation by making local electric fields bigger than the applied field. One extensive study[1] of this phenomenon led to a proposal to blend crosslinked polyethylene (XLPE) insulation with an ionomer to reduce space charge build-up. The authors used contact electrification (no applied field) as a measure of the tendency for a polymer to charge under an applied field and found that the XLPE acquired negative charge, the ionomer positive charge and a blend of the two acquired much less net charge than either component. The reverse-polarity breakdown strength of the blend was found to be higher than that of XLPE and this was related to its reduced contact charging. We have extended this work to test the above ideas and proposal for improving the performance of XLPE-insulated cables. In our study we measured electric field distributions in films of polyethylene, ionomers and blends charged by applying 0 to 50 V/μm at temperatures between 20 and 80°C for times up to 100 hours. These electric field measurements show more directly, than do contact charging data, the charging behavior that occurs in the bulk of coaxial DC power cable insulation in service under an applied field.