Online Monitoring Method For Initial Arc Fault Inside Cable Based On Transient Waveform Matchin

Power cables are widely used in various fields of power transmission and distribution.The online prediction of power cable faults can provide preventive maintenance decisions to ensure the safe and stable operation of the power system,and prevent power accidents before they occur.With the development of local insulation defects,the local defects of power cables often evolve from partial discharge to internal initial arc,and eventually form permanent faults.Among them,the initial arc fault is the most predictive stage of cable fault development for its relatively obvious electrical quantity to easily detection and long duration.In this paper,the real fault waveform of cable initial arc fault is tested by experiment.Based on the unique waveform characteristics of the initial arc fault voltage extracted from experiments,a two-stage efficient transient waveform identification method of initial screening and fine judgment matching is proposed.And the variable scale search of the transient waveform matching extremum is used to achieve accurate positioning of the initial arc fault.Based on the occurrence mechanism of initial arc faults in cables,this paper established a practical high-voltage experimental platform for internal arc faults in cables in the laboratory.Taking 8.7/15 k V(4.5mm insulation thickness)cross-linked polyethylene cables commonly used in distribution network systems in China as experimental objects,the actual measurement of the waveform of internal arc faults in cables is conducted.The characteristic analysis of the obtained experimental waveform shows that the initial arc fault inside the cable has its own unique waveform shape which is significantly different from the conventional cable to ground breakdown arc.The waveform shape is similar to a half arc,which can be characterized as a unique shape that drops near the voltage peak point and generally presents a decline and concave shape.The fault duration is about 1/4 power frequency cycle.The feature extraction of this unique transient waveform provides a powerful basis for identifying and locating initial arc faults in cables.Based on the initial arc fault waveform obtained from experimental measurements,and the idea of transient waveform matching,this paper proposes a two-stage initial arc fault identification method based on initial screening and fine judgment matching.This method describes the shape characteristics of the core waveform of the initial internal arc in the stage of the cable,and takes the perturbed voltage waveform at the head end of the power cable as the matching object.The propoesed method performs a rapid preliminary matching screening between the perturbed voltage waveform and the shape characteristics of the core waveform based on the discrete Fréchet distance,and then uses an artificial hummingbird algorithm to accurately fit the perturbed voltage that has not been accurately screened to achieve efficient judgment.By simulating four types of interference disturbances,namely,load switching,capacitor switching,metallic ground fault,and no-load line switching,the paper validates the proposed two-stage matching method of preliminary screening and fine judgment,which shows that,under the premise of accurate fault reference waveform,the proposed two-stage transient waveform matching method of preliminary screening and fine judgment can quickly eliminate disturbances with significant waveform differences,and accurately determine the types of interference with similar waveform shapes.And the accurate fitting results contributes to further accurately identifying the initial arc fault inside the cable from various common interference phenomena.And this method has a good ability to resist noise interference.After initially identifying the initial arc faults within the cable,the paper proposes a variable scale search method based on transient waveform matching extremum for rapid location of initial arc faults within the cable.With the voltage and current of the metal shield layer at the head end of the cable and the cable core,this method builds differential equations based on the cable distributed parameter model,and solves the time-domain calculation waveform of the initial arc voltage inside the cable along the cable.The waveform characteristics of the initial arc fault are matched with the time-domain calculated waveform to determine best matching point by the searching matching extremum via variable scale hierarchical search method,that is,the location of the initial arc fault point.The PSCAD/EMTDC simulation analysis shows that the fault location method based on transient waveform matching extremum can effectively achieve the initial arc fault location,and the proposed variable scale search method accelerates the location speed.Moreover,this method has strong practicality,high positioning accuracy and robustness,and strong anti-noise ability.