Characterization of thermally aged XLPE cable peelings through space charge measurements

Space charge results under DC-voltages have been used to compare samples taken from peelings of cables AC-aged under different specified thermal and electrical conditions with unaged and service-aged specimens. The aim is to identify quantities that have been altered during ageing. All samples were thermally treated to remove volatiles before measurement and ensure reproducible measurements appropriate to just the state of the polymer. Both the pulsed-electro-acoustic (PEA) and thermal-stimulated-current (TSC) techniques were used. This allowed us to obtain information both on the ability of the polymer to transport and accumulate charge, and the trap depth distribution and relaxation barriers in the various XLPE samples. The combination of both techniques gives a more detailed picture of the state of the XLPE specimens than each technique on its own. PEA measurements show that materials that have undergone electro-thermal ageing accumulated the most charge, and that the charge resides in both deep and shallower traps. Differences between the field aged material and the electro-thermally aged ´laboratory´ sample are identified in both the PEA and TSC measurements, and are taken to indicate that a larger fraction of charge lies in the less shallow traps in the service-aged material.