Transient Dielectric Properties And Characterization Of Nonlinear Insulating Dielectrics Under Square Wave Voltage

Nonlinear insulation dielectrics has the advantages of improving the distribution of electric field in the insulation structure.It is widely used in the traditional insulation fields such as the end of high voltage motor and cable terminal,as well as the emerging fields such as pulse power insulation system.The transient problem of nonlinear insulation dielectrics cannot be avoided.Therefore,in this paper,the transient dielectric characteristics of nonlinear insulating dielectrics and related testing technology are studied.At the same time,considering the double-layered insulation structure involved in the engineering application of nonlinear insulating dielectrics,the interface polarization of double-layered nonlinear insulation is studied theoretically and experimentally.Taking 10wt%SiC/PE and 20wt%SiC/PE composite materials as test objects,the polarization and depolarization currents of the two materials under the square wave with the electric field of 5-35kV/mm are measured.A time domain least square fitting method based on the universal law of polarization(Curie formula)is proposed to eliminate the interference in the current time domain spectrum,and the decomposition of polarization absorption current and conduction current is realized.The conductivity under the corresponding field strength is obtained by conducting current.The relaxation polarization strength is obtained by integrating the polarization absorption current with time,and the relaxation polarizability under the corresponding field is obtained by extrapolation according to the trend of its change rule.According to the results under 5-35kV/mm square wave electric field,the conductivity and relaxation polarizability of the two composite materials increase with the increase of electric field,which can be approximately characterized by hyperbolic tangent function,exponential function,power function and polynomial.However,the characterization effect of polynomial is the worst,which shows that high-order conductivity and polarizability are not the effective way to characterize the dielectric properties of nonlinear insulation.To obtain the characterization of the dynamic response of relaxation polarization of nonlinear insulating dielectrics,the polarization absorption current and depolarization current under different amplitude square wave electric field are analyzed.The results show that the relationship between the change rate of relaxation polarization intensity with time during the process of polarization and depolarization and time obeys the universal law.The A and n during the process of polarization are related to the excitation electric field.While the A during the process of depolarization is related to the initial relaxation polarization intensity,and n in the process of depolarization is not related to the initial polarization intensity.In addition,the relationship between dPr(t)/dt and the difference of relaxation polarization strength instantaneous value and the target value ?Pr(t)and the electric field strength E is analyzed.The results show that the relationship between dPr(t)/dt and ?Pr(t)meets hyperbolic tangent function,and the parameters in the function are also related to the excitation electric field or relaxation polarization strength.Therefore,the three-dimensional surfaces about dPr(t)/dt,E and ?Pr(t)can be used to characterize the relaxation polarization dynamic response of nonlinear insulating dielectrics.To obtain the information about fast polarization(non relaxation polarization),the joint measurement technology of current time-domain spectrum and potential time-domain spectrum under the excitation of square wave electric field was proposed.The joint measurement contains the surface potential decay and the surface potential recovery.The non relaxation polarizability is measured by using the potential change rate at the initial time of the potential decay or recovery and the current value before the potential time-domain spectrum test.The non relaxation polarizability test results obtained by the two methods realize the mutual verification,and then the characterization and test of the non relaxation polarizability of the nonlinear insulating dielectrics is realized.The results show that the non relaxation polarizability of the nonlinear insulating dielectrics is independent of the electric field strength,but increases with the concentration of SiC.Through the analysis of potential decay and potential recovery,it can be seen that the relaxation polarization still exists in the process of potential decay,and the relaxation polarization intensity has a peak.The curve of nonlinear relation between the relaxation polarizability and the electric field strength is measured by using the peak information.The results show that the curve of the relation basically coincides with the curve obtained by the time domain spectrum of the polarization absorption current.The mutual verification of the two methods is realized.In this paper,the dynamic characteristics of polarization and depolarization under constant electric field is theoretically deduced by simplifying the physical model of interface polarization of double-layered nonlinear insulation with a+bE type nonlinear conductivity.The analytical solutions of the electric field,the interface charge density and the total current density of each layer dielectrics are obtained.The theoretical study shows that the change law of the electric field and the interface charge density of each layer dielectrics is basically the same as that of linear double-layered dielectrics,which shows monotonic change and tends to be stable.However,the dynamic characteristics of polarization and depolarization currents are obviously different from that of double-layered linear dielectrics,which may be non monotonic decreasing with polarity reversal and peak phenomenon,and no longer obey the universal relaxation polarization law.The polarization and depolarization currents under the electric field strength range of 5-35kV/mm were measured with two kinds of double-layered nonlinear insulating dielectrics,which were composed of polyethylene and 10wt%SiC/PE,and polyethylene and 20wt%SiC/PE.The experimental results verify the theoretical results of the polarization and depolarization current dynamic characteristics of the interface of the nonlinear double-layered dielectrics,and the polarity effect of the dielectric response of the nonlinear double-layered dielectrics were found.