Modeling Analysis And Feature Extraction Of The Optical Fiber Temperature/Strain In Submarine Cable With Electrical And Mechanical Faults

The application of submarine power cable has been over hundred years of history.In recent years,the optic-electric composite submarine cable which combines the functions of power transmission,information communication and sensing has emerged,its importance has been further improved.The structure of optic-electric composite submarine cable is complex and its working environment is harsh which result in more fault type and higher rate compared with the traditional cable.With the development of distributed optical fiber sensing technology,getting the operation status information of submarine cable by monitoring the temperature/strain information of composite optical fiber in submarine cable has become a research hotspot.So far,the on-line monitoring system based on distributed optical fiber sensing has been able to effectively monitor the temperature/strain information of the composite optical fiber,but due to the lack of diagnosis characteristics at different operating conditions and the mechanical or electrical fault cases,it is difficult to establish systematic and targeted fault diagnosis criterion of submarine cable.Based on numerical methods and finite element modeling,this paper calculates the thermodynamic and structural mechanics data at normal state and different electrical and mechanical fault cases of optic-electric composite submarine cable,extracts the characteristic information of optical fiber under typical fault types and fault development degrees.The relationship between the temperature/strain information of optical fiber and cable states are built and a more comprehensive quantitative data reflecting the operation and fault status of submarine cables are obtained as the basis for establishing the diagnostic criterion of submarine cable based on the optical fiber sensing.By establishing the thermodynamic finite element model of submarine cable,the effects of ampacity,ambient temperature,buried depth,soil thermal conductivity and heat transfer coefficient on the optical fiber temperature in normal insulation state are analyzed.On the basis of study of the effects of ambient temperature and ampacity on the optical fiber's temperature rise in global insulation degradation,a method for global insulation degradation evaluation of the submarine cable based on temperature rise of optical fiber is proposed.The equivalent circuit model and finite element model of submarine cable with different local insulation states are combined to obtain the relative temperature rise of optical fiber.The effects of ambient temperature,ampacity of cable,volume resistivity of insulation degradation area and the size of the degraded area on the relative temperature rise of optical fiber are investigated.The quantitative relationship between the level of insulation degradation and the temperature rise of the optical fiber is systematically established.According to the above research,the paper discusses the application value of BOTDR(Brillouin Optical Time Domain Reflectometer)-based measurement of optical fiber temperature in on-line monitoring of the submarine cable insulation state.Based on the structural mechanics finite element modeling of submarine cable,this paper established three typical fault conditions of submarine cable with tension,torsion and anchorage.The strain distribution of optical fiber along the submarine cable and corresponding relationship between the strain of optical fiber and strain/stress of important structural components in submarine cable during fault development are analyzed.The functional relationships between optical fiber strain and core of cable,armor strain/stress under the tension and torsion fault are established,and the damage evaluation index of submarine cable under the anchorage is obtained.Based on the above analysis,the optical fiber strain distribution characteristic mode of submarine cable with different types of mechanical faults and the mechanical fault identification method of submarine cable based on the optical fiber strain size and spatial distribution are proposed.