Research On Fire Risk Assessment And Online Monitoring System Of Power Cable

With the continuous development of urban construction,the use of XLPE power cables is also increasing.However,the power cable channel is usually limited in space and the cables are densely arranged with multiple loops.Fires are easily induced when the cable has arc faults,core overheating or external heat sources.Especially when a single-phase grounding fault occurs in the low-current grounding system,it is allowed to run for 1 to 2 hours with the fault.Therefore,based on the theories and diagnosis methods of relevant factors affecting the internal fire of power cable,such as singlephase grounding arc and conductor temperature,this thesis studies the fire risk assessment and on-line monitoring system of power cable.Firstly,combined with the structure and function of each part of the three-core XLPE cable,the internal loss of the cable and the thermal resistance and heat capacity of the cable body are expounded,which provides a theoretical basis for the analytical calculation of the temperature field of the power cable.Focusing on the analysis of single-phase grounding arc,the distribution law of sheath grounding current is analyzed when single-phase grounding fault occurs in 10 k V cable distribution network.Based on this,a single-phase grounding arc self-extinguishing criterion and a fault location method based on the grounding currents at the beginning and end of the cable section for the low-current grounding system are proposed.Secondly,an analysis is carried out for overheating of the cable core.In order to obtain the temperature of the cable conductor timely and accurately,a calculation model was adopted to estimate the conductor temperature of the power cable according to the structural parameters of the power cable,the temperature of the outer skin and the internal loss.The multiphysics coupling simulation software COMSOL Multiphysics was then used to analyze the effect of load current,external environment,short-term overload operation and sheath ground current on the cable temperature field.On the one hand,the reliability of the conductor temperature calculation model is verified,and on the other hand,the correspondence between the cable current,the external environment,the conductor temperature and the cable skin temperature is analyzed.Thirdly,the ignition mechanism of the fire in the power cable is combined with the evaluation basis for the influencing factors such as single-phase ground arc,conductor temperature,and insulation aging.A fire risk assessment model for power cables is established from three aspects: cable operation parameters,cable faults and cable laying by using the analytic hierarchy method and fuzzy comprehensive evaluation.The results show that the assessment model can scientifically and accurately quantify the fire risk level of power cables.Finally,based on the power cable fire risk assessment,an online monitoring system for power cables is designed.The system uses a variety of sensors to monitor the cable body and joint temperature,zero-sequence voltage,sheath grounding current and ambient temperature and humidity,and uploads the monitoring data to the master station through RS485 and Lo Ra wireless network,so as to realize the diagnosis of cable running state and fire risk.Through the testing and installation of the equipment,the results show that the monitoring system can effectively monitor the running state of the cable and the risk of fire.