Research On Cable Online Monitoring Method Based On Time-Frequency Domain Analysis

Cables are buried deep underground and are inevitably affected by high temperature,mechanical stress,radiation,chemical corrosion,etc.in different operating environments.The economic losses caused by failures may be more serious than traditional overhead lines.This requires us There are higher requirements for the accuracy of cable fault location.The time-frequency analysis method combines the joint distribution information of the signal time domain and frequency domain,and has been widely used in the fields of earthquake prediction,medical diagnosis,and fault analysis.Time-frequency domain reflectometry(TFDR)based on Wigner-Ville Distribution(WVD)is the latest application of time-frequency analysis in the field of cable fault diagnosis.TFDR obtains incident and reflected signals.Time-frequency cross-correlation(TFCC)function,using the peak value of the time-frequency cross-correlation function to judge the cable fault.In this paper,based on the principle of time-frequency reflection,the operating status of XLPE cables is studied.The main contents of the work are as follows:1.The structural aging phenomena and causes of cross-linked polyethylene cables are studied.Through the study of the dielectric properties of insulating materials,the dielectric constant of XLPE materials is analyzed.The degree of change trend has deepened.2.Establish an equivalent mathematical model by analyzing the cable distribution parameters and transmission line theory to lay a theoretical foundation for the following cable simulation modeling,and derive the TFDR theory based on the WVD and TFCC methods in detail.The research shows that the pseudo WVD can be compared to WVD.Effectively reduce the generation of cross terms and provide positioning accuracy.3.Set up a simulation model in PSCAD,conduct simulation experiments on cable faults and local aging defects,and import the collected data into MATLAB for time-frequency analysis.The experimental results show that TFDR has better positioning accuracy when cable faults such as line open or short circuit occur.The measurement error is mostly distributed between 0.1%-0.3%;for local cable aging,the peak value of TFCC can characterize the aging state of the cable.With the deepening of the aging degree(increasing dielectric constant),the normalized TFCC local peak value changes from 0.002 to 0.0033,the peak value of TFCC in the control experiment(increase of conductor volume resistivity)cannot characterize the change in the degree of cable aging.Its normalized local peak value is at the level of 3 ‰ as the aging deepens,which proves that the dielectric constant is correlation of cable aging.The research in this paper shows that the cable monitoring method based on the time-frequency reflection principle has accuracy and sensitivity,and is of great value for the location of power cable faults and aging.