Analysis Of Dielectric Polarization Conductance And The Study Of Space Charge Injected By Semiconducting

Space charges accumulated in the insulation layer of HV transmission cables will cause substantial failures of transmission lines.Semi-conductive shielding layers in high voltage cable apprehensively utilized to connect insulation layer and conducting metal core for close contact play essential roles of homogenizing electric field and reducing charge injections from metal core to insulation layer.It is of great significance in both scientific theory and engineering applications to impede charge injections into polymer dielectrics by ameliorating semiconductive shielding layer.The surface defects carbon-black infiltration from the traditional semi-conductive shielding layer give rise to electric field distortion and cause space charge accumulations near the interface between the semiconducting and insulation layers.In the present study,a novel scheme of employing an ionic semiconducting electrode(i-electrode)to suppress charge injection is proposed,elaborating on the effect of i-electrode on charge injection characteristics and its mechanism of dielectric polarization and electrical conductance,in comparison with the traditional electron semi-conductive electrode(e-electrode).Two basic physical processes of dielectric polarization and electrical conductance are firstly exploited to describe the electrical properties of engineering dielectrics,in which a new generalized interpretation on the original Faraday definition of dielectrics has been made.According to the binding energy,activation energy and trapping depth of charges in dielectrics,the structure morphology and charge carriers in correlation with dielectric polarization and electrical conductance are analyzed as the fundamental physical elements.Charge injection are tested by individually using e-electrode and i-electrode as the high-voltage terminal.The e-electrode is a semi-conductive composite material composed of low density polyethylene(LDPE)/ethylene-vinylacetate copolymer(EVA)and carbon-black(CB),while i-electrode is an electrolyte aqueous solution prepared from alkali metal inorganic salt.Polyethylene terephthalate(PET)with excellent comprehensive insulation performances is adopted as the prototype material of polymer dielectrics.Short-time electrical breakdown experiments are implemented after charge injections of using i-electrode and e-electrode respectively,in comparison with the pristine PET dielectric breakdown strength.Characteristic breakdown field is reduced by 6.9% and 14.3% after specially injecting charges with i-electrode and e-electrode respectively.It is thus suggested i-electrode that plays the role of inhibiting the injections of negative charges.The complex dielectric functions(dielectric permittivity ε and dielectric loss factor tanδ)in temperature and frequency spectra are tested for PET samples after being injected charges individually with electronic and ionic semiconducting electrodes,the results of which comply with the ideal Debye response theory.The ε and tanδ of the PET samples pretreated by injecting charges with i-electrode are basically identical to that of the pristine PET without charge injections using any semiconducting electrode.By contrast,the charge injections with e-electrode lead to appreciable reductions in both ε and tanδ of PET dielectrics,which is attributed to the evident restrictions on the orientation polarization of dipoles by the considerable space charges from e-electrode injection,as a comparative manifestation of i-electrode competency on suppressing charge injections.Thermal stimulation currents(TSC)tested by injecting charges with ionic and electronic semiconducting electrodes under different electric field strengths indicate characteristic peaks at 95℃ and 105℃ respectively.In comparison with e-electrode,the remarkably lower intensity of TSC peaks implies that the electron injections from i-electrode to be captured by traps accumulating into space charges have been significantly alleviated.The integrals on TSC spectra,which represent accumulations of the thermally released charges from all traps in PET dielectrics,indicate that the total charge quantity injected by i-electrode is only 1/4 of that by e-electrode,confirming the effective restraint on charge injections by i-electrode.Logarithmic curves of current density vs.electric field intensity(J～E)measured individually by i-electrode and e-electrode demonstrate that the variation gradients under low and high electric field conform to Ohm’s law and space-charge-limited conductance(SCLC),respectively.In particular,SCLC gradient for applying e-electrode is significantly greater than the for ielectrode,implying that the less charge carriers injected from i-electrode increases more slowly with increase of electric field intensity,compared with the e-electrode injecting charge carriers more apt to be captured by charge traps and become space charges in PET dielectrics.