Research On The Icing Growth Model And Key Parameters On Overhead Transmission Lines

Transmission line icing is a highly hazardous natural phenomenon for power grid safety.The severe ice load,dynamic aspects(mechanical traveling waves),and insulation deterioration have provoked a number of serious problems,such as flashover,structural failure,and even breakdown of the electricity grid.The icing growth model is an effective method of the icing disaster prevention,which has important theoretical significance and engineering application value for improving the safety of transmission lines.The key point of the ice growth model is the calculation of the collision quality of supercooled droplets and the heat balance on the ice surface,while the latter one is mainly affected by icing surface heat dissipation and internal heat conduction.So in this paper,the key parameters such as the collision coefficient of supercooled droplets,the surface heat dissipation and the heat conduction in icing are studied,using the numerical calculation approach and specialty experimental method.On this basis,a more practical icing growth model would be established and verified.The main researches in this thesis are as follows:The collision characteristic of supercooled droplets is a basic parameter for predicting the growth of transmission line icing.Whereas,the empirical formula of collision coefficient only can be used for the condition of smooth cylinder,but not for the noncircular cross-section object and local collision characteristic.In this paper,firstly,two simulation models of collision coefficient,the Euler-Lagrange method and Euler-Euler method,are studied and employed to the collision characteristic of supercooled droplets.On this basis,the collision coefficient for the elliptical and crescent cross-section shape are analyzed with different cross-section parameters and wind attack angles,which are really exists but cannot be calculated by the empirical formula.The research results would be one more useful academic reference for the icing growth model and the icing disaster prediction method.Convection heat transfer and icing gasification are determined to be the main factors of heat loss in the process of icing growth,which are usually calculated by the coefficient of convective heat transfer.In this paper,the finite element model of iced objects are established and solved by the Calculation Fluid Dynamics method,especially for the nonideal cross-section shape iced objects which could not be calculated by the empirical formula,such as elliptical shape,crescent shape,bare conductor,icicle,and roughened surface.The average and local values of the Nusselt number on the objects are acquired and analyzed under different Reynolds numbers.And then,some of the local averaged Nusselt number formulas are obtained by fitting these numerical simulation data.The heat conduction in icing can be calculated by the Fourier equation,and the thermal conductivity of icing is the basic parameter of this equation.However,this basic parameter is ignored by most of the existing icing models which set it as a constant.In this paper,an experimental study is conducted to investigate the thermal conductivity for overhead transmission line icing.The results show that the icing density can be regarded as the major influential parameter of thermal conductivity.When icing density is lower than 800kg/m~3,the relationship between thermal conductivity and density can be considered as a quadratic function.When icing density is larger than 800kg/m~3,the icing thermal conductivity can be expressed by the parallel structure model.Finally,the equation of icing thermal conductivity is employed to calculate the significant characteristic parameter in icing growth and ice-melting process,such as the critical condition of icing wet-dry growth,critical ice-melting current,and ice-melting time.The results of the experiment and simulation show that the thermal conductivity can improve the prediction accuracy of icing growth and thermal de-icing projects.Based on the research of the key parameters mentioned above,a mass balance equation is presented including the parameters of droplet collision,water runback,evaporation,sublimation;and then,the icing heat balance equation is established on the basis of mass balance equation and thermal conductivity.Multiple unknown variables of the model are acquired by using multiple hypothesis and one dimensional search.Moreover,an experimental apparatus for transmission line icing growth are designed in artificial icing laboratory,in which,one homemade simulation conductor and several Aluminium Conductor Steel Reinforced are employed as icing objects with different structure parameters.The comparison of the simulation results and the experimental data show that this method is feasible in predicting the icing growth.This research has focused on the icing growth model and its key parameters,which may provide some valuable reference on the study of transmission line icing.However,the physical models of conductor torsion and icing thermal conductivity are not included in this thesis,which needs more analysis in future theoretical and experiment research.