Optimization And Application Of Analytical Calculation Model Of Underground Cable System

Power cable is the main equipment for transporting electrical energy.The accurate calculation of its temperature field is very important for accurately evaluating its thermal state and improving the utilization rate of cable assets.The cable external soil thermal resistance of underground cables is affected by several factors,such as arrangement mode of buried cable,buried depth,the thermal conductivity of soil,the thermal characteristics of backfill medium around cables,and so on.Therefore,the calculation of external soil thermal resistance is always been a difficult point in the accurate calculation of carrying capacity,and also a key point to determine the accuracy of analytical calculation.Because multiple groups of power cables with high load are located in a same place,the accurate calculation model of external thermal resistance is of great significance for improving the prediction accuracy of cable core temperature and carrying capacity.In order to solve the problem of inaccurate calculation of external thermal resistance of buried cable,the finite element method is used to obtain the sample data,and then the external thermal resistance calculation model is optimized by regression analysis.The finite element modeling is carried out with buried metal bar instead of cable.The outside diameter,loss and buried depth of buried cable are taken as independent variables.The external thermal resistance of soil as dependent variable.Finite element software ANSYS is used to calculate sample data.The relationship between each variable and the external thermal resistance of soil is analyzed from the sample data.Then the soil external thermal resistance model is established.The cable core loss is considered at first.Then loss is neglected.The temperature of single loop and multi-loop XLPE power cable systems are calculated.It is proved that the influence of loss value on soil thermal resistance can be ignored within the range of effective cable load,and the validity of the optimized calculation model of soil external thermal resistance is verified.For the cable in duct,there is a closed air layer between the cable and duct.Metal bars are used to replace cables.Outer diameter and heat of metal bars are taken as independent variables.Equivalent thermal resistance of air layer as dependent variables,and finite element software ANSYS is used to calculate sample data.From the data obtained by FEM,the functional relationship between the thermal resistance of the air layer and each variable is defined,and a custom function is established to calculate the equivalent thermal resistance of the air layer.Taking the finite element calculation results as the reference standard,the optimized calculation model is used to calculate the temperature of cables in the single and double ducts.The calculation results are basically consistent with the finite element results,reduced the difference between the analytical calculation method and the finite element method.The comparison proved the effectiveness of the air layer equivalent thermal resistance calculation model.Through the optimization of the external thermal resistance of the cable group under the conditions of direct embedding and drainage,the accuracy of the analytical calculation is improved,which provides a reference for the management of underground power cable load.