Study on Shielding Failure Flashover Rate for EHV Transmission Line

Lightning is the main reason that endangers the safety and reliability of transmission line, but the existing models of estimating shielding failure flashover rate can not be in agreement with field experience at present. This paper deals with shielding failure flashover rate of the EHV transmission lines with many parameters by the improved electric-geometry model. The parameters consist of phase wires, shield wires, and the ground angle. In the improved model, "strike distance" is calculated with the expression recommended by IEEE, "the striking distance factor" is introduced to describe the difference of electric intensity between lightning stroke on the transmission tine and the ground, and the "uncovered distance" is adopted to calculate shielding failure flashover rate for ultra high voltage transmission line. Taking 500kV transmission line as example, the improved electric-geometry model gets the result in agreement with field experience. Compared with the order of the method, it is more accurately. At the same time, the paper analyzes the influence of the shielding angle, the height of wire, the grounding obliquity on shielding failure flashover rate. The results of studies show that the shielding failure flashover rate is a function of the shielding angle, ground angle and the height of tower. It will increase with the increase of the grounding obliquity, protection angle and the height of tower. The simulation calculation shows that when the shielding angle is -6.24 degrees, the shielding failure flashover rate reduces from 0.2135 to 0 in this paper. It will help designers in making the correct economic judgment.