Electrica Life Exponent Model Considering Electric Field And Space Charge Effect

High-voltage cable transmission project is an important foundation for the development of global energy internet.With the rapid development of UHV transmission engineering,more stringent requirements are put forward for the reliability of cable.Among them,life exponent is an important parameter of cable factory test and insulation design.However,the traditional method of cable design regards the life exponent as a constant,which is contrary to the experimental fact that the electric field and other factors have a great influence on the life exponent.In addition,the traditional cable design methods only have a lot of experience in the application of AC cable,DC cable space charge is not considered,and the impact of space charge on the life exponent also needs to be carried out in-depth research.Therefore,the establishment of the correlation model between life exponent and electric field and space charge provides a new tool for cable factory test and insulation design,and have a positive guiding significance for the development of cable transmission engineering in the future.Secondly,the study of life exponent model is helpful to further improve the electrical aging theory of solid media and deepen the understanding of the mechanism of space charge accelerating the aging of solid insulation under DC working conditions.In general,the study of life exponent model has important theoretical research and engineering practical application value.To improve the experimental efficiency of life exponent model research,this paper firstly studies the method for obtaining life exponent.A step-stress test parameter setting and data analysis method was proposed,and the effectiveness of this method was verified by experiments.In order to establish a model describing the relationship between life exponent and electric field,the physical meaning of life exponent was analyzed based on the increment of free energy,and the relationship model of electric field and life exponent based on the increment of free energy was established.In order to consider space charge into the life exponent model for DC operation,experiments were designed to study the mechanism of space charge accelerating insulation aging,and then a life exponent model considering the effect of space charge on free energy was established.Finally,the method of applying the proposed model to cable engineering was studied,the test voltage and insulation thickness of 500 k V DC cable were designed,and the differences between the new design method and the traditional method were compared and analyzed.The main innovative achievements of this paper are as follows:(1)A new step stress method based on the cumulative damage curve was proposed.Based on the cumulative damage curve,the aging mechanism of stepping stress method was judged.Then the parameters of step stress test were set according to the aging mechanism of constant and step stress method.The life exponent obtained by this method is equivalent to the constant stress method,and the test efficiency is greatly improved at the same time.The error between the life exponent obtained by this method and the constant stress method is less than 2%,and the required time is reduced to 27%of the constant stress method.(2)The relationship between electric field and life exponent(FEI model)based on free energy increment was established.In this model,the electric field is introduced into the life exponent model based on the concept of free energy increment,so as to realize the analytical calculation of life exponent under any electric field.This model can make up for the deficiency of traditional models which do not consider the effect of electric field on life exponent,and can realize the quantitative calculation of the relationship between life exponent and electric field.(3)A life exponential model(SCE model)considering the effect of space charge on free energy was established.In view of the effect of space charge on the breakdown field under DC,considering the vibration fatigue mechanism of space charge,the space charge is introduced into the FEI model on the basis of FEI model to form the FEI model considering space charge,namely the SCE model,so as to realize the analytical calculation of the DC life exponent.The space charge factor is fully considered in this model,which solves the problem that the common FEI model does not take space charge into account,and improves the accuracy of the model for the quantitative calculation of life exponent under DC.(4)The design method of cable factory test voltage and insulation thickness based on life exponent model was proposed.The aging factor is calculated by life exponent,and then the test voltage and insulation thickness are designed according to the aging factor.This method gives full play to the advantages of FEI model and SCE model,and takes the factors of electric field and space charge into consideration in the cable correlation design.Compared with the traditional method,this method has higher test voltage and thicker insulation thickness.