Research On Ultrasonic Propagation And Monitoring Of Partial Discharge In Cable Joints

With the continuous advancement of urbanization and industrialization in China,the construction of transmission and distribution networks has never stopped.In short-distance transmission networks and distribution networks at all levels,power cables have gradually become the dominant mode of transmission and distribution due to their advantages of high reliability and small footprint.However,most of the cables are placed underground,making routine maintenance and trouble shooting difficult.Partial discharge is an important manifestation of cable joint in the early stage of fault.Therefore,it is of great engineering value and research significance to monitor the partial discharge phenomenon of cable joints.Among all kinds of partial discharge detection methods,ultrasonic detection has the advantages of non-invasive and non-electromagnetic interference,and has become an important way for on-line monitoring of partial discharge in electrical equipment.In this paper,the research on the ultrasonic propagation law of partial discharge in cable joints was used to improve the difficulty of acquisition caused by weak signal and serious attenuation,and further obtain the ultrasonic signal acquisition scheme in the joint.The partial discharge comprehensive monitoring device and on-line monitoring platform were researched and developed to realize the active perception and real-time monitoring of cable joint discharge information.Firstly,the propagation law of ultrasonic waves in cable joints was obtained.The mechanism of acoustic propagation was introduced,and an equal-scale three-dimensional simulation model of cable joint was constructed.The transient sound pressure cloud diagram inside the model and the time-domain and frequency-domain characteristics of sound pressure signals at each observation point are analyzed.It was found that the thin layer structure has little effect on ultrasonic propagation.The higher the frequency of the sound source,the more dispersed the energy.The optimal monitoring range was within 40 mm of the projection of the joint surface from the sound source.Secondly,a comprehensive monitoring device for partial discharge of cable joint was designed and developed.Considering the weakness of partial discharge signal and the deficiency of hardware signal conditioning,a signal processing program using FIR digital filter and wavelet denoising was designed.In order to make the discharge state of the cable joint more intuitive,four kinds of characteristic information of discharge peak-peak value,rise time,duration and kurtosis were extracted,and the discharge intensity was classified by K-Nearest Neighbor classification algorithm.The above work meets the whole process requirements of partial discharge data real-time acquisition,processing,storage and communication from the hardware.Finally,an integrated monitoring platform for cable joints was constructed based on NB-Io T network and One NET.Through this platform,the visualization of real-time monitoring data,historical data query and early warning information issuance can be completed,and the comprehensive remote monitoring of the operation status of cable joints can be realized.Various experimental tests on the whole system show that the monitoring system can operate reliably.