| Submarine cable is important for power transmission between coastal islands and cities.It plays an important role in realizing the internationalization of power grid and the process of regional power grid interconnection.The submarine cable is vulnerable to damage by external forces,and it cannot be inspected routinely.It will suffer huge losses once there is a sudden failure.Problem of submarine cable can be found with on-line monitoring of running states,and preventive actions can be taken before failure occurred,which is of great significance to protect the safety of power grid.The vibration signal generated by external disturbance on the internal fiber of the submarine cable is measured using the vibration sensor based on ?-OTDR technology in this thesis.The disturbance tests for different position,type and distance on the submarine cable is carried out.The disturbance position error is within 10 m by measuring the waterfall figures of the vibration monitoring system.The vibration signal of different types of disturbances has different frequency and duration,and the waterfall figures are different.The disturbance signals can be detected within distance of 4m.The results show that the position accuracy of the monitoring system is 5m,which reduces the time and energy for positioning the disturbance.The system can distinguish the disturbance type according to the frequency and duration of the signal and the disturbance can be eliminated more effectively.The system can detect the disturbance of the submarine cable itself,also its early warns to the perimeter disturbance.The safety of submarine cable is sensitivity enhanced.?-OTDR technology can send out early warns before submarine cable being damaged,which reduce the chance of damage to the cable.Secondly,the finite element analysis software is used to simulate the stress field of the submarine cable.The strain informations of the armored layer and the optical fiber layer are analyzed when the cable is hit or dargged by an anchor.The strain curve is analyzed and the strain curve of the armored layer can be divided into three stages: elastic stage,yield stage and strengthening stage.The strain curve of the optical fiber layer increases linearly.When the strain of the armor layer reaches the elastic limit,the corresponding strain value of the optical fiber is 0.00269,which is the first warning value.When the strain of the armor layer on yield stage reaches the maximum value,the corresponding fiber strain value is 0.00378,which is the second warning value.When the armor layer strain reaches the yield limit,the corresponding fiber strain value is 0.00455,which is the third warning value.The results show that the damage degree of the submarine cable can be judged by three warning values.The armored strain corresponding stage can be given by measuring the strain of the fiber,which is used to determining whether the cable needs to be repaired.According to the three values,timely measures can be taken to prevent large power outages caused by serious damage to the cable.Then,the ground induced loop current and capacitor current of the grounded metal sheath at both ends are calculated theoretically.Let B phase line core currents are 356.9?0°A and 130?0°A respectively,and the amplitudes of the loop currents are 354.05 A,357.37 A,356.42 A and 128.96 A?130.17 A?129.82 A.The results show that when the cable operates normally,the three-phase currents value change with the core current value,and the capacitance current value is very small.If the insulation of the protective layer is damaged,the two current values will increase greatly.Therefore,it is an effective method to detect the fault of the cable by monitoring grounding current.Finally,the states according to several monitoring techniques are evaluated synthetically.The evaluation criteria are divided into normal,attention,abnormal and serious.According to different grades,action is taken to reduce the probability of serious damage to the submarine cable and improve the safety of the power grid. |
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