Research On Evaluation Method Of Local Defects In Power Cables Based On Head End Impedance Spectrum

Cables play an extremely important role in the transmission of electrical energy and communication signals in modern large electrical systems.Cables in operation are prone to permanent failures.Once cable failures occur,large electrical systems will be shut down or even out of control,causing serious economic losses and social impact.The occurrence of permanent cable faults often starts from local latent defects.The local latent defects of the cable are mainly divided into the following causes: local aging of the cable due to local high temperature,high humidity,partial discharge or even nuclear radiation,and cable due to mechanical external forces The local damage occurred.If the local defects of the cable cannot be eliminated in time and effectively,it will pose a serious threat to the safety and stability of the large electrical system.Therefore,it is of great significance to diagnose the local latent defects of the cable.At present,the diagnostic methods for local latent defects in cables are insufficient.Based on the related research,this paper proposes a cable local defect diagnosis method based on the cable head end impedance spectrum.The vector network analyzer is used to measure the curve of the cable head end input impedance as a function of frequency.This is called the cable head end impedance spectrum.When it is high enough,the impedance spectrum is mainly related to the characteristics of the cable itself,and the relevant information of the cable running state is extracted through the analysis of the impedance spectrum data,and then the local defect diagnosis of the cable is realized,and the method is verified by simulation and experiment.The main contents of the research are:1.Understand and explain the causes and types of the formation of local defects in the cable;study the transmission line theory;analyze the traveling wave reflection and transmission phenomena of the cable defects;deduce the cable end input impedance calculation model based on the transmission line theory,analysis and summary Based on the characteristics of the cable's head-end impedance spectrum,it is found that the characteristics of the cable's head-end impedance spectrum are mainly determined by the propagation coefficient that characterizes the cable's characteristics,thereby linking the impedance spectrum with the cable's characteristics.In order to strengthen the understanding of the characteristics of impedance spectroscopy,the relevant measurement principles of the impedance spectroscopy instrument vector network analyzer were studied.2.The mechanism of the local cable defects on the cable distribution parameters is studied.An impedance spectrum calculation model with local defects was established,and the relationship between the local defects and the head-end impedance spectrum was clarified.Studies have shown that after the cable has local defects,changes in the local distribution parameters of the cable will cause changes in the local propagation coefficient at the defect.The change in the propagation coefficient directly affects the impedance spectrum.It can be seen that local defects will directly affect the head-end impedance spectrum.When the transmission signal propagates in the defective cable,the head-end impedance becomes a function of the relationship between the characteristic parameter at the local defect and the defect position.3.A local defect location method for cables in frequency domain reflection is proposed,and how to extract the position information of the defect from the impedance spectrum data is studied.Since the first-end impedance is a function of the characteristic parameter and position relationship of the local defect,the integral transformation process is used to obtain The diagnosis function of the change of the cable propagation coefficient along the length of the cable realizes the simultaneous positioning of multiple defects,and the simulation test proves the effectiveness of the method.4.The method of obtaining the initial characteristic parameters of the actual cable is studied,which provides a basis for evaluating the severity of the local defects of the cable,and the method of evaluating the defect state is studied.The molecular structure of the cable is changed due to the defect of the cable,which is manifested as a complex dielectric constant macroscopically.If the change occurs,the defect state can be characterized by the complex permittivity.Since the reverse solution of the complex permittivity from the impedance spectrum data involves multiple unknown variables,the process is very complicated.Now,using the method of hypothesis,first assume the complexities of each defect location The dielectric constant and the length of the aging section are brought into the impedance spectrum calculation model containing the defective section,and the calculated and measured values of the model are used as the difference,and the difference function is optimized and searched through the ant colony algorithm to find the global optimal solution.At this time,the complex dielectric constant is the closest to the true value,which can be used to effectively assess the severity of cable defects and provide a basis for the formulation of cable maintenance strategies.Simulation experiments verify the effectiveness of the method.5.An actual cable local defect diagnosis experiment was carried out.The test results show that the cable local defect diagnosis method proposed in this paper has certain effectiveness and operability.