Research On Modeling And Measurement Method Of Vibration Of Three-core Submarine Cable Based On BOTDR

Submarine cable is the main means of cross-sea power transmission.In recent years,people have embedded optical fiber in the submarine cable to form the optic-electric composite submarine cable,which realizes the transmission of grid information while transmitting power.Distributed optical fiber sensing technology has the advantages of high temperature resistance,strong anti-electromagnetic interference ability,long monitoring distance and continuous monitoring,is the effective means to realize the optic-electric composite submarine cable on-line monitoring.BOTDR(Brillouin Optical Time Domain Reflectometer)based on Brillouin scattering is better than other distributed optical fiber sensing technologies in many technical indicators,and is more suitable for online monitoring of submarine cables.At present,submarine cable monitoring systems based on BOTDR technology generally adopt the frequency sweep method to obtain the Brillouin spectrum,and the long measurement time makes it difficult to realize the dynamic strain monitoring.In addition,there are few simulation studies on three-core submarine cable vibration,and most of them ignore the vibration of the submarine cable themselves caused by electrodynamic force.This paper simulates the vibration of the three-core submarine cable during normal operation,overload operation and when the submarine cable is suspended,and the vibration frequency and strain of the fiber under different vibration are analyzed emphatically.Then,the slope-assisted method,which can realize the rapid strain measurement,is introduced into the online monitoring method of submarine cable based on BOTDR technology,and its effectiveness is verified and the optimal working point is given.Finally,it evaluates whether BOTDR technology based on slope-assisted method can realize submarine cable vibration monitoring.The COMSOL Multiphysics was used to establish a 36 k V ZS-YJQF41 three-core submarine cable solid mechanics and magnetic field coupling two-dimensional finite element model,the normal operation and overload operation of the submarine cable were simulated,and the strain/stress data of the optical fiber under the vibration of the submarine cable caused by electrodynamic force were obtained.The finite element analysis software ANSYS was used to model the vibration of the same type of submarine cable when a suspended fault occurs,Pipe288 and Solid element are selected to construct the outer layer and other components of the submarine cable respectively,and the characteristics of Pipe288 element are used to apply marine environmental loads.The displacement,stress and strain nephograms of different components of the submarine cable,as well as the acceleration-time curves of the center node of the fiber are calculated.Combined with the results of the above model and the measurement performance of BOTDR technology,the feasibility of the vibration measurement of the submarine cable is preliminarily analyzed,the results show that the optical fiber strain of the submarine cable in suspension reaches the minimum value that can be measured by BOTDR,while the optical fiber strain under normal and overload operation will not affect the monitoring system of BOTDR.In order to improve the real-time performance of submarine cable strain measurement based on Brillouin scattering,the slope-assisted method that only needs to measure the corresponding gain of one frequency point is introduced into the submarine cable online monitoring.Aiming at the numerically generated Brillouin spectrum of different SNR(Signal to Noise Ratio)and the measured Brillouin spectrum of bare fiber/submarine cable composite fiber,this paper compares the accuracy and real-time performance of the slopeassisted method and the spectrum fitting method based on frequency sweeping,studies the selection of working point in varying degrees of fluctuation of Brillouin frequency shift along the fiber,and the feasibility of BOTDR technology based on slope-assisted method in the field of vibration monitoring of three-core submarine cables is evaluated from the perspective of measurement time.The results show that the measurement time of the BOTDR technology based on the slope-assisted method is greatly reduced when the error is only slightly increased,the best working point is the average Brillouin frequency shift along the fiber minus half the line width,this technique can be used to measure the vibration of submarine cable suspended fault in short length.The research in this paper provides a reference for the vibration monitoring of the three-core submarine cable.