输电线路的防雷（英文文献翻译）.docVIP

The Lightning of Transmission Line Overvoltages on power systems are traceable to three basic causes, lightning, switching, and contact with circuits of higher voltage rating. The power system designer seeks to minimize the number of these occurrences ,to limit the magnitude of the voltages produced,and to control their effects on operating equipment. Lightning results from the presence o{ clouds which have become charged by the action of falling rain and vertical air currents, a condition commonly found in cumulus cloudsVoltages may be set up on overhead lines due to direct strokes and due to indirect strokes . In a direct stroke, the lightning current path is directly from the cloud to the subject equipment--an overhead line. From the llne, the current path may be over the insulators and down the pole to ground. The voltages setup on the line may be that necessary to flash over this path to ground. In the direct stroke, the lightning current path is to some nearby object, such as the tree shown In Fig. 10 lb. The voltage appearing on the line is explained as follows As the cloud comes over the line, the positive charges it carries draw negative charges from distant points and hold them bound on the line under the cloud in position as shown. The voltage on the ]ine is zero assuming that the line is not energized, IF the cloud is assumed to discharge on the occurrence of the stroke in zero time, the positive charges suddenly disappear, leaving the negative charges unbound. Their presence on the llne implies a negative voltage with respect to ground. On the occurrence of a stroke, lightning clouds do not discharge in zero time. Instead,the stroke current rises from zero value to maximum value (perhaps 50, 000 amperes) in a few microseconds and is completed in a few hundred microseconds. Direct lightning strokes to lines as shown in Fig. lO-la are of concern on lines of all voltage class ,as the voltage that may be set up is in most instances limited by the fla