Cable Fault Location Method Based On Frequency Domain Reflectometry

As the "blood vessel" of power system,power cable plays an extremely important role in power system.It is of great engineering value to find a fault detection method that can detect potential faults of cables as early as possible and accurately locate them without damage.Some foreign scholars use the reflection coefficient spectrum of the cable to locate the cable fault(called frequency domain reflectometry)and have achieved some results,which provides a new idea for power cable fault detection.However,the research on frequency domain reflectometry in power cable field is still in its infancy.The intrinsic relationship between the local defects of cable and the reflection coefficient spectrum of cable needs to be further studied.Be aimed at the shortcomings of domestic and foreign research,this paper studies the cable local defect diagnosis based on frequency domain reflection method.The main work includes:1)Based on the transmission line theory,the reflection coefficient spectrum of the cable considering the frequency-dependent effect of the distribution parameters is derived in detail,the correlation between the reflection coefficient spectrum of the cable and the distribution parameters of the cable is analyzed.According to the variation of the distribution parameters after the local defect occurs in the cable,the calculation model of the cable reflection coefficient spectrum with local defects is established,and the relationship between the cable defects and the cable reflection coefficient spectrum is clarified.2)The feature quantity that can represent the local defect position of the cable is extracted from the cable reflection coefficient spectrum to locate the local defect of the cable.In MATLAB,the calculation model of cable reflection coefficient spectrum is used to validate the effectiveness of frequency domain reflection method for localization of local defects in power cables.3)The positioning accuracy of the frequency domain reflection method is affected by the frequency band of the injected excitation signal,so the time-frequency domain reflection method is explored and studied.The Wigner-Ville time-frequency joint distribution with good mathematical properties is used to perform cross-correlation analysis between the reflected electromagnetic wave signal and the incident electromagnetic wave signal to realize the local defect of the cable.The local cable defect simulation model of power cable is established in MATLAB,and the positioning performance of the time-frequency domain reflection method for local defects of the cable is verified.4)An experimental platform for local defects of power cables was built in the laboratory.The diagnostic experiments of local extrusion,local damage,local overheating and multiple local defect points of power cable based on frequency domain reflection method are carried out,and the validity and operability of frequency domain reflection method for localization of local defect are further verified.In summary,this thesis conducts theoretical derivation,simulation and verification experiments on the frequency domain reflection method.The results show that the frequency domain reflection method can locate various local defects of power cable quickly,nondestructively and accurately,but the positioning accuracy of FDR is affected by the frequency band.Therefore,this paper also explores the time-frequency domain reflection method.Although time-frequency domain reflectometry will reduce the localization accuracy of local defects,it can reduce the frequency bandwidth of the injected excitation signal.Therefore time-frequency domain reflectometry can be used as a supplement to frequency-domain reflectometry.