1. Basic Principle of Lightning Protection for Line Surge Arrester

During lightning strikes on towers, part of the lightning current flows through the lightning protection wire to adjacent towers, while the other part enters the ground through the towers. The grounding resistance of the towers exhibits transient resistance characteristics, which are generally represented by the impulse grounding resistance.

When lightning strikes a tower, the potential at the top of the tower rapidly increases, with the potential value beingUt=iRd+L.di/dt(1)

Equation: i - lightning current

Rd - Impulse Ground Resistance

L.di/dt — Transient component.

When the potential difference between the tower top potential Ut and the induced potential U1 on the conductor exceeds 50% of the discharge voltage of the insulator string, a flashover will occur from the tower top to the conductor. That is, Ut - U1 > U50. If the influence of the line's frequency voltage amplitude Um is considered, it becomes Ut - U1 + Um > U50. Therefore, the lightning resistance level of the line is related to three important factors: the 50% discharge voltage of the line insulators, the intensity of the lightning current, and the surge grounding resistance of the tower body. Generally, the 50% discharge voltage of the line is fixed, the intensity of the lightning current is related to the geographical location and atmospheric conditions. Without installing lightning arresters, improving the lightning resistance level of the transmission line often involves reducing the grounding resistance of the tower body. This is particularly difficult in mountainous areas, which is why transmission lines are frequently struck by lightning.