Generally, when the thundercloud discharge close to the ground, it makes the ground's electric field distorted. At the top of the overhead ground line, the formation of local electric field strength concentration space influences the development of lightning pilot discharge direction, guiding lightning to the overhead ground line or tower top discharge. Then the lightning current is introduced into the earth through the grounding lead wire and grounding device so that the protected object is protected from lightning strikes. Such lightning strikes the ground in the form of direct lightning is called a counterstrike. (Thundercloud according to its height and amplitude is divided into high altitude high amplitude thundercloud, high altitude low amplitude thundercloud, low altitude high amplitude thundercloud, low altitude low amplitude thundercloud four, which refers to a high height greater than 1000 m, lightning current greater than or equal to 100kA, low refers to the height of less than 600 m, lightning current less than 100kA).
Overhead ground line so that the thundercloud pilot discharge electric field distortion range (i.e., height) is limited. When the lightning pilot has just begun to form, the overhead ground line can not affect its path of development. Only when the lightning pilot channel is developed to a certain height H from the ground (called the directional height), the overhead ground line may affect the development direction of the lightning pilot. So that the lightning pilot channel along the direction of the maximum electric field strength to hit the overhead ground.
From high altitude down, the graded development process of high altitude high amplitude value or high altitude low amplitude value thunderclouds. If intercepted by the high grounding body head-on pilot, it will be with the top of the pole tower or overhead ground line to produce a backlash direct lightning overvoltage. When the overhead ground line can not affect the development path of the thundercloud, the downward grading development process, if not intercepted by a high grounding body head-on pilot, will continue downward. Reaching low altitude, especially into the side of the line tower, the ground shield effectiveness of the lightning pilot in the region. In the electromagnetic field of the wire with voltage and lightning field together, to its positioning development. From high altitude to low altitude, long-distance development of free process makes the pilot amplitude greatly reduced. When the electric field at the top of the low-altitude grounding body generated by the head-on pilot is not enough to make the lightning pilot it's positioning. That is, can not intercept the lightning pilot while the side of the grounding body can attract the lightning pilot. At the same time should be moving to a certain range from the grounding body and the pilot amplitude is sufficient to reach the remaining air gap breakdown discharge. At this point must occur around the strike.
Low-altitude high amplitude lightning pilot due to reach the grounding body of the time is short. So that the grounding body near the induction field strength is higher, the top of the grounding body is easy to produce a head-on flow injection and its intersection. And high amplitude value pilot front-end free zone radius is larger and can be positioned at a higher level. Therefore, the general height of 50 m below the plain, hilly areas of the transmission line tower is not easy to produce around the strike, while in mountainous areas, hillside tower can not only occur around the strike. And there are great chances of induced lightning overvoltage. Low-altitude low-amplitude lightning pilots on the low lightning resistance level of transmission lines will have a backlash hazard, and easy to meet their conditions around the strike. In the grounding body at the same time to meet their winding conditions, the chance of a winding strike is higher.
From the above, we know that the main reason for bypassing: is the low amplitude lightning current, its amplitude just penetrates the ground to hit the wire to produce bypassing. Operation of the transmission line ground protection angle is larger, coupled with the mountainous section of the slope of the ground inclination is larger. Tower height higher towers (such as some double-circuit towers, spanning towers, extra-high voltage towers), are erected along with the steep mountainous areas, so that the line to the ground height increases, reducing the shielding effect of the ground. These three situations can lead to lightning bypassing the overhead ground line and hitting the wire cause. This bypasses the overhead ground lightning strikes, the wire type called around the strike. The lightning overvoltage around the type of strike and counterstrike are direct.
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