Lightning has a certain degree of potential threat to the safe operation of overhead transmission lines. In order to make targeted lightning protection arrangements and reduce the impact of lightning on overhead transmission lines, it is necessary to conduct lightning risk assessments on overhead transmission lines. This paper proposes a lightning failure risk assessment method for overhead transmission lines based on a modified Dempster–Shafer theory. First, analyze the historical lightning failure data of the line, determine the lightning failure impact factors, and use confidence to express the relationship between the lightning failure and the impact factor; then, use entropy weight theory and gray relational theory to calculate the value of mass function, and modify it on this basis; finally, use Dempster–Shafer theory to determine the trust degree and fit this with the calculated lightning trip rate to produce the risk assessment. This paper analyzes the lightning failure data of overhead transmission lines in some areas of Hebei Province. The results show that, compared with the evaluation method of the Dempster–Shafer theory, the accuracy of the evaluation is improved to a certain extent after correcting the mass function value. It can be seen that this method can integrate and comprehensively consider different data and can provide a reference for preventing damage to transmission lines by lightning strike weather.

In order to ensure that corona discharge does not occur in the grading ring under normal operation, this paper studies corona characteristics of large-sized AC grading rings and a prediction method for corona onset field intensity (COI). First of all, a three-dimensional (3-D) finite element simulation model of the electrostatic field is established for the grading ring area where corona discharge is relatively serious in a typical EHV AC substation. The distribution law for electric field intensity of saddle-type and elliptical (circular) grading rings is calculated and analyzed, from which the result shows that pipe diameter is the main factor affecting the maximum electric field intensity (MEI) of grading rings with different structures; with continuous increase in pipe diameter, the MEI of grading rings tends to be saturated gradually. In addition, the COI tests of grading rings are carried out and test results are compared with calculation results obtained based on the Peek formula, which shows that the Peek formula has a large error in predicting the COI of a grading ring. Finally, based on test results of the COI, a formula for predicting the COI of an AC grading ring in a plain area with an altitude of 100 m is proposed by considering the effect of both the pipe diameter and the ring diameter.