Corona Characteristics of a Large-Sized AC Grading Ring and Prediction of Corona Onset Field Intensity

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.