Thermal-moisture Coupling And Thermal Properties Of Overlaying Soil On Direct-buried Cable Ampacity

The ampacity of power cable should be accurately evaluated with the propulsion of electricity and direct buried power cable to correct the conservative value of tranditional IEC 60287 method.The traditional calculation method is the IEC 60287standard,which regards the process of thermal losses as heat conduction.The thermal resistances of the heat conduction are the inner thermal resistance in the cable and the outer thermal resistance in the soil.However,the description of soil thermal resistance is inaccurate and neglects the bias caused by moisture transfer and phase change,which often overestimates the ampacity;the lack of research data and methods of overlaying soil thermal physical properties around the cable also shows the necessity of this research.Thus,this thesis takes the 1 kV medium or low voltage direct buried power cable as an example,which combines the methods of experiment in heat conductivity,the theoretic analysis of formula derivation and the finite element modeling,to systematically researches the influence factors about cable ampacity of soil thermal property of soil and thermal-moisturecoupling property.The detailed aspects can be described as follows.（1）This paper collected the soil samples with each soil horizons and soil types（red soil,skeletal soil and paddy soil）in Zhejiang Province,then characterized the dry densityand water content,and drew the particle.The paperstudied the thermal conductivity of various soils with different mass moisture content（0-25%）,dry density(0.7-1.6 g·cm^-1)and variation of aggregate size（0.075-2 mm）by KD2 Pro instrument which is based on thermal probe method.It was found that with the increase of dry density and mass moisture content,the thermal conductivity of soil increased,and the growth rate of thermal conductivity began to slow down in the dry density range of 0.9–1.2 g·cm^-3.In addition,the dry density range of each soil is doubled at the mass moisture content of 25%compares to 0%,and the effect of aggregate size on thermal conductivity is different under each soil type.On this basis,the paper studied the influence degree of various factors on the thermal conductivity by the one-way ANOVA method.It found that the dry density is the most significant effect,while the influence of aggregate size is negligible.In addition,the theoretical values of Johansen et al.were used to verify the accuracy of the experimental values.The ternary model of thermal conductivity was obtained by regression analysis,and its R²is over 98%.The effects of parameters such as moisture content and dry density revealed by this experimental study will be considered in the establishment of the thermal-moisture coupling model.（2）Based on the traditional soil heat and moisture coupling equation and the MAXWELL electromagnetic field equations,the cable loss derived from the electromagnetic field equationswas substituted into the heat source term in the temperature field,and the water and gas diffusion forms were analyzed to derive the three-fields coupling model for the calculation of the ampacity of the buried cable.The specific expressions of electromagnetic field,heat transfer and moisture transfer parameterswere introduced into the model.The boundary heat flux equation of the phase transition process such as solar radiation,natural convection and water evaporation of hot air were established at the upper boundary of soil,which made the entire three-field coupling model closed and solvable.（3）This paper applied the three-field coupling model withthe finite element method and orthogonal analysis method,to numerically simulatedthe ampacity of direct-buried cable YJV22-0.6/1 3×6,in which the soil layer was considered as the single layer,double layer and three layers.The results show that for buried cables,the maximum ampacityis 86.85 A,whichoccurs in the two-layer soil,with the dry density is 1.4 g·cm^-3 and the average particle size is around 2 mm,while the minimum ampacity is only 56.17 A.In the single-layer soils,all the variables have little effect on the ampacity,while in the two-layer and three-layer soils,the dry density had a huge effect,which count for 74.33%and 86.86%.In addition,the air temperature has a negative correlation with the ampacity of the directly buried cable,therefore,the effects of soil type,soil thermal resistance and heat and moisture coupling on the ampacity cannot be ignored.In this paper,a C#calculation program is developed to calculate the ampacity of IEC 60287.The calculated values of the coupled model were compared with the IEC standard and the experimental values of thermal-moisture coupling in the literature.The relative error between the IEC and the model is found at-9%-10%,and the daily variation of temperature and water content is basically consistent with the experimental law,which indicates that the model has a certain precision.