This can be explained not only from the theory of electrical geometry, but also from the operating conditions. The UHV overhead transmission lines of the former Soviet Union had trips due to lightning strikes several times during their operation. The basic reason was that lightning shielded the conductors at the tensile corner tower. The lightning trip rate of Japanese UHV overhead transmission lines is also very high during the step-down operation. According to the analysis, the insulator flashover is caused by the line being struck by the side lightning.
Theoretical analysis and operating conditions show that the lightning trip of the UHV transmission line is mainly caused by the failure of the lightning protection line shielding and the lightning shielding wire. Therefore, the use of a good lightning protection line shielding design is the main measure to improve the lightning resistance of UHV transmission lines. At the same time, the influence of the working voltage on the wires of UHV transmission lines on the shielding of lightning protection cables should also be considered. For the mountainous area due to the influence of the terrain, the protection of the lightning protection line may have a negative protection angle.
The insulation configuration of the line mainly refers to two aspects. The first is the configuration of the insulator, and the second is the distance between the live metal parts of the insulator suspension and the tower body after considering the wind deviation. At present, the number of insulators is often selected according to the creeping distance method in China. For UHV lines, it is generally more than 50 sheds, and the length of the insulator string is more than 10 meters. For heavily polluted areas, the length of the insulator string exceeds 15 meters or even longer, which seriously affects the economics of the line. Due to the improvement of domestic polymer insulator technology and the accumulation of experience in use, the proportion of polymer insulators used on AC UHV lines will increase significantly.
The relationship between the amplitude of the air gap withstand operating voltage and the gap length is a saturation curve. When the gap length is 6 meters and the withstand voltage is about 1600 kV, it starts to enter the obvious saturation zone. The specific pressure level is related to the gap shape and the air pressure level. For the test demonstration project, the gap of the middle-phase V string is 6.5 meters.
The above is the main content of how UHV lines protect against lightning over voltage.
For more information of Jecsany products of overhead transmission line, please visit the website: www.jecsany.com