| Growing focus is put by many researchers on the nonlinear insulating dielectrics with enhanced conductivity by electric field,which is self-adaptive to uniform the electric field distribution.A comprehensive study of the dielectric properties of nonlinear dielectrics is not only crucial in improving dielectric theory,but is conducive to promote the development of relevant technology as well,that leads to the question of which kind of measurement and characterization approaches could be applied in making a better understanding of the dielectric properties of nonlinear dielectrics.The main theme of this dissertation is to explore the measuring technique,characterization and analysis method of the dielectric properties of nonlinear insulating dielectrics.And in order to achieve these objectives,the typical nonlinear dielectrics silicon carbide/low-density polyethylene（Si C/LDPE）composites is studied based on dielectric spectrum in time domain and space charge measurement.The current measurement technology in time domain is adopted to study the nonlinear dielectric properties of Si C/LDPE composites with Si C filler concentration of 3 phr in temperature range of 20℃ to 50℃.The nonlinear relationship between the conductivity,polarizability and the applied electric field is acquired in different temperature.The results show that conductivity may decrease with the field increasing when the applied electric field is less than the threshold electric field.After the electric field exceeds the threshold value,the conductivity increases rapidly with the field increasing and the conductive nonlinear coefficient decreases with the temperature increasing.The conductive and polarization of Si C/LDPE composites are thermally assistant activated process.The hopping potential barrier energy of conduction and the thermal activation energy of polarization is related to the applied electric field.Referring to the asymmetrical double-well model and Dissado-Hill linear dielectric response theory,the phenomenological model and its mathematical expression are proposed to describe the nonlinear relationship between steady polarization property,transient current property of nonlinear dielectrics and electric field in theory.The model can describe the experimental trends well.The space charge may be formed in insulating dielectrics when electric field is high.The experiments confirmed that the reversed absorption current in depolarization can’t characterize the dielectric response when there is space charge in dielectrics.The space charge behavior is a typical slow polarization,which will change the local electric field and affect the apparent dielectric response so that the current or potential measurement can’t describe the dielectric properties alone.So it’s necessary to realize the simultaneous measurement of current,potential and space charge in order to study and characterize the dielectric properties of insulating dielectrics.Enlightened by the present simultaneous measurement approach of current and space charge,the simultaneous measurement method of current,voltage decay in open-circuit and space charge based on the ‘physical twin-samples’ is proposed and the corresponding measurement system is designed.The second post-modification of space charge measurement results is proposed to modify the deviation of charge and electric field distribution from the physical reality in the vicinity of electrodes in order to provide more reasonable results.In order to obtain the dielectric behavior of dielectrics by using simultaneous measurement results,the lumped equivalent circuit model is proposed to describes the apparent dielectric behavior.The equivalent space charge current and the relaxation current are decoupled by the lumped equivalent circuit model.The current decoupling results indicate that the equivalent space charge current and the relaxation current don’t fully obey the universal law.The lumped equivalent circuit model overlooks the space information of the real electric field distribution in dielectrics,which results in the deviation from the physical reality when decoupling the space charge behavior and the relaxation behavior.To overcome the above defects in the lumped equivalent circuit model,the spacial distributed equivalent circuit model that makes full use of the space charge information is proposed.The spacial distributed equivalent circuit model conforms the real polarization process much better with considering the effect of field distortion in different positions.The proposal of the distributed equivalent circuit model makes the focus of insulating dielectric research from the overall external appearance to the details of the spatial and temporal evolution inside the dielectrics.The circuit equations of dielectric volume elements are built according to the spacial distributed equivalent circuit model.The numerical method for obtaining the dielectric parameters and the evolution of the polarization in time and space is proposed based on the piecewise linear concept.Based on the above-mentioned simultaneous measurement technology,distributed equivalent circuit model and corresponding data analysis method,the spatial and temporal evolution of conductivity,polarization,space charge as well as energy storage and dissipation during polarizing,short-circuit depolarization and open-circuit depolarization after polarizing with different electric field are obtained.These spatial-temporal distributions interpret the detailed information of the dielectrics internal charge transport process under different circuit states from a full perspective,which is helpful for the in-depth understanding of the dielectric characteristics of the nonlinear insulating dielectrics.Meanwhile,the trapped charge and the gradient of polarization can be decoupled from the space charge measurement results,which provides a brand-new method for the in-depth study of the characteristics and mechanism of space charge in the future.The analysis of the simultaneous measurement results shows that the high-frequency polarizability of the Si C/LDPE composites is basically independent of the position in the dielectrics and the applied electric field.During the polarization and depolarization,the slow polarization of the space charge changes the local micro-environment of the dielectrics,which not only affects the characteristics of polarization and conductivity at different positions,but also affects the energy distribution and transfer in dielectrics.During the polarization process,the polarization in time domain may be non-monotonic.The conductivity at different positions satisfy almost the same function of electric field,but the specific parameters in the function are different.The energy distribution and dissipation in the composites show a non-uniform distribution.Comparing the decoupled trapped charge and space charge measurement results,it is found that there is a big difference between them.Based on the trapped charge obtained by charge decoupling,it is expected to obtain more realistic and accurate trapping characteristics and charge transport characteristics of the insulating dielectrics and further to improve the dielectric theory. |
