High electrical field degradation in polymeric dielectrics: Electrical tree formation and growth in polyethylene

Date of Completion

January 1999


Engineering, Electronics and Electrical|Engineering, Materials Science




Undoped low-density polyethylene conductivity is given by, s=m˙NcN T˙rtot˙e-W/ kT˙eEqa/2kT where m is carrier mobility, W = 0.87 eV is the trap depth, a = 2.8 nm is the trap site separation. For doped polyethylene and high-density polyethylene, the conductivity is given as, s=2nl NciNT rtoti+Ncd NTr totde- 4+W/kT 1Esinh Eql2kT eEqa/2kT where W = 0.87 eV, 4 = 0.3 eV is the hopping barrier, a = 3.03 nm, l=n-3d , where nd is the doping concentration. ^ The analysis, based on the hypothesis that a minimum extent of material along the direction of the electric field must be damaged through exposure to a space charge limited field within the dielectric in order to initiate an electrical tree, suggests that the tolerable Laplacian field at a defect increases by roughly a factor of two as the defect radius decreases from about 5 m m to about 1.5 m m. ^ A mechanism which can explain the transition from branch to bush-type electrical trees with increasing applied voltage and have presented computations to support plausibility. ^ The details of a guarded needle instrument, measurement technique, and data analysis methods are presented. Space charge limited field measurements of laboratory manufactured polybutadiene and commercial polybutadiene are presented. The frequency effects of the limiting field are analyzed. An approach of refining the conduction model at high fields is discussed. ^ A technique for making very low variance measurements of the discharge inception field from a defect in a polymer is demonstrated. Using these measured PD inception voltages and the known geometry for which they were measured, we have modeled the discharge process based on the energy available from the electrostatic field during tree channel formation. We have demonstrated a self-consistent mechanism for channel formation, in that the energy available from the electrostatic field is sufficient to form the channel size observed. ^