Study on Shielding Failure Flashover Rate for EHV Transmission Line

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Lightning is main reason that endangers safety and reliability of transmission line, but 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 EHV transmission lines with many parameters by improved electric-geometry model. The parameters consist of phase wires, shield wires, and ground angle. In improved model, strike is calculated with expression recommended by IEEE, the striking distance factor is introduced to describe difference of electric intensity between lightning stroke on transmission line and ground, and uncovered is adopted to calculate shielding failure flashover rate for ultra high voltage transmission line. Taking 500 kV transmission line as example, improved electric-geometry model gets result in agreement with field experience. Compared with order of method, it is more accurately. At same time, paper analyzes influence of shielding angle, height of wire, grounding obliquity on shielding failure flashover rate. The results of studies show that shielding failure flashover rate is a function of shielding angle, ground angle and height of tower. It will increase with increase of grounding obliquity, protection angle and height of tower. The simulation calculation shows that when shielding angle is -6.24 degrees, shielding failure flashover rate reduces from 0.2135 to 0 in this paper. It will help designers in making correct economic judgment.