Research On Identification Of Defect States In High-voltage Cable Metal Sheath And Current-carrying Loop

In recent years,high voltage(HV)cable has been widely used in urban transmission and distribution network,and the requirements of reliability and operation life for the HV cable are constantly improved.Identifying the defects of the HV cable and preventing the defects from developing into faults is of great significance to ensure the safe and reliable operation of the HV cable and improve the pertinence and economy of the cable’s maintenance and repair.Identification methods of the HV cable defects have been extensively investigated in recent researches.However,the researches mainly focused on the insulation defects identification based on partial discharge characteristics,and few research focused on the metal sheath defects identification and current-carrying loop thermal defects identification.And the existing researches on the metal sheath defect identification are mainly based on the time-domain characteristics of the measured currents in the metal sheath.Therefore,taking the major section of the cross-bonded(CB)HV cable as the research object,considering temperature characteristics and frequency dependent characteristic of the HV cable,the identification method of the HV cable metal sheath defects and current-carrying loop thermal defects has been studied,and the main contents of this paper are as follows:(1)An equivalent thermal circuit model and a broadband circuit model of the HV cable are established.Considering the influence of cable structure on heat transfer process,a simplified equivalent thermal circuit model of the HV cable is established,the temperature state equations of cable is derived,and the calculation method of thermal circuit model parameters is given.The thermal circuit model of the HV cable is verified by historical operation data of the HV cable.In addition,based on the major section structure of the CB HV cable,the equivalent broadband circuit model of the HV cable for metal sheath defect identification is established.Considering the influence of cable temperature,the calculation method of broadband circuit model parameters is proposed,and the parameters of the HV cable broadband circuit model are solved by vector fitting method.(2)A method for identifying defects in the HV cable sheaths based on the coding of equivalent impedance spectrum characteristics is proposed.The proposed equivalent broadband circuit model of the HV cable is decoupled in three phases.On the basis of the decoupled equivalent broadband circuit model,the functional relationship between the sheath equivalent impedance and the on-line monitored sheath currents is analyzed,and the parameter identification equation of the sheath equivalent impedance is derived.Based on the spectrum analysis of the HV cable metal sheath quivalent impedance,an identification method of defects in the HV cable metal sheath by the sheath equivalent impedance spectrum feature coding is proposed.The simulation analysis of the broadband circuit model of the HV cable considering the temperature characteristics under 12 types of sheath defects is carried out to verify the proposed method.(3)A joint identification scheme for identifying the defect states of the HV cable metal sheath and current carrying circuit is proposed.Combined with the practical engineering,the process methods of the equivalent thermal circuit model and the broadband circuit model of the HV cable under different conditions are proposed.The relationship between the HV cable metal sheath defects and the thermal defects of current carrying circuit is analyzed,the identification processes of metal sheath defects and thermal defects of current carrying circuit are improved,and the joint identification scheme of metal sheath and current carrying circuit defects of the HV cable is furtherly proposed.The joint identification system of metal sheath and current carrying circuit defects is designed to realize the application of the proposed defect joint identification scheme.