Research On Distribution Characteristics And Suppression Of Induced Current Of Field-to-line Coupling

With the rapid development of science and technology,all kinds of electrical and electronic equipment widely used in all fields of the national economy.Although it brings convenience to our life,these devices are electromagnetic interference sources.When it works properly,these devices tend to produce some useful or useless electromagnetic energy that may affect the operation of other equipment,resulting in increasingly complex electromagnetic environment.For example,high-altitude nuclear-burst electromagnetic pulse(HEMP)is a serious electromagnetic interference source,its spectrum range can from very low frequency to hundreds of megahertz,electric field strength can reach 40kV/m or higher,and range of action can run up to several thousand meters.At the same time,cables act as a medium for the effective transmission of energy and information in power,electronic and communication systems.However,these cables are easily coupled with the external electromagnetic field and the dominant interference coupling paths.Therefore,the field-to-line coupling problem has become an important study content in electromagnetic compatibility subject.This thesis focuses on the research on distribution characteristics and suppression of induced current of field-to-line coupling.The main work of this thesis can be summarized as:(1)Applying the Maxwell's equations,the Telegrapher's equations are derived and the equivalent circuits are given for the four transmission line models including Taylor,Agrawal,Rachidi,and Vance model.Then,the frequency Baum-Liu-Tesche(BLT)equations are derived and the analytic solutions of load response excited by plane wave are obtained for the four models.Finally,the similarities and differences of the four models are compared from five aspects including Telegrapher's equations,differential variables,boundary conditions,exciting sources and load response.(2)Based on Agrawal transmission line model,the load induced current response characteristics are researched for the case of overhead cable excited by plane wave or an incident HEMP,the impact of the incident angle(including polarization angle,incident angle and azimuth angle),cable height,terminal impedance,ground conductivity and relative dielectric constant on the load induced current are given.Then,the analytic expression of induced current are derived for the case of both ends of the cable are overhead or one end of the cable is grounded and the other end is overhead,the resonance mechanism is analyzed,and the relationship between resonance frequency and cable length are obtained.Finally,the experimental study is carried out for the case of the condition that the cable height is much smaller than the electromagnetic wavelength is not satisfied,when the longitudinal impedance is amended,the theoretical results of induced current are in good agreement with the measured results.(3)Based on antenna theory,the calculation formulas of the transmission line-mode and antenna-mode induced current are derived for the case of the shielded cable above a lossy ground,the impact of the cable height,ground conductivity and relative dielectric constant on the induced current distribution characteristics are given.The proposed results can provide a data base for the application of bulk current injection technique and direct current injection technique.(4)To suppress both the steepness and the amplitudes of the cable sheath current induced by an incident HEMP,this paper adopts a suppressing method that uses ferrite magnetic rings installed on the shielded cable.Taking into account the material characteristics of the magnetic rings,a nonuniform transmission-line model is presented in the frequency domain for evaluating the suppressing effect of the magnetic rings,using the inverse fast Fourier transform,the terminal response in the time domain can be obtained.First,the impact of the ring cluster length and its installed position on the load cable sheath current are acquired.Then,the relationship between the suppressing effect and the material characteristics is discussed.Finally,the suppression mechanism is analyzed from the point of view of wave propagation and energy coupling,respectively.The validity of the proposed model is verified by simulation,providing guidance for magnetic-ring-type selection and saving the expenditure of the experimental study.