A space-charge based method for the estimation of apparent mobility and trap depth as markers for insulation degradation-theoretical basis and experimental validation

Assuming that aging of polymeric insulation consists of irreversible chemical-physical and microstructural modifications, effective ageing markers could be achieved from measurements of properties, which should vary with material microstructure. Among these properties this paper focuses on the estimation of apparent trap-controlled mobility and trap distribution, based on the results of space charge measurements. The depolarization characteristic obtained recording space charge magnitude when, after poling, electric field is removed and electrodes are short-circuited and grounded, is the experimental reference for the application of the expressions for apparent trap-controlled mobility and trap depth calculation derived in this paper. A simple relationship is proposed for apparent mobility, which is affected by significant approximations, but can be applied successfully for material characterization and aging investigation. It is applied to experimental results coming from space charge measurements carried out by the pulsed electroacoustic (PEA) technique on cross-linked polyethylene (XLPE) specimens, unaged and aged, cut from high-voltage cables. The trap depth distribution is calculated for the same material, and the results are cross-correlated with mobility estimates, revealing good agreement. Both apparent trap-controlled mobility and trap depth distribution reveal noticeable modifications with aging. The proposed methodology permits finding information on charge trapping/detrapping processes in polymeric materials, using a simple experimental procedure, as well as to extract quantities that seem very attractive as aging markers.