1、7 雷击过电压 雷电是由于那些因降雨而带电的云层 ， 以及通常存在于积云中的垂直气流而引起的 。 在架空线上可能会由于直接和间接的雷击而建立起的过电压 。 在直接雷击中 ， 雷电电流的路径是直接从云朵到设备 (架空线 )。 通过架空线 ， 雷电产生的电流可以越过绝缘子 ， 然后 顺着线杆入地 。 架空线上产生的电压或许会在此路径上发生闪络然后接地 。 在直接雷击中 ， 雷电产生的电流流经一些附近的物体 (例如树 )。 架空线上出现电压可做以下解释：当云朵飘到架空线上空时 ， 它所带的正电荷吸引到远处的负电荷 ， 并将这些负电荷附在云层下的架空线 。 假设架空线没有 通电 ， 那么线上的电压就为
2、零 。 假设云层在发生闪电的一刻放电 ， 正电荷突然消失 ， 留下负电荷没有被释放 。 那么架空线上的负电荷就会对地产生负电荷 。 在发生闪络时 ， 闪电层立刻完成放电的 。 相反 ， 产生的电流在几微秒内从零值到最大值 。 并在几百微秒内全部释放 。 在许多情况下 ， 可能产生的电压通过闪络接地而得以限制 。 增加绝缘子串的长度只能在闪络发生前允许建立较高的电压 。 大多数青年概况下可采用架设架空接地线的方法来防止直接雷击 。 接地线放置在电力导线上 ， 使得每一次雷击都要通过接地线而不是电力系统 ， 雷击产生的电流就会流到地 。 之后 ， 电流 的大部分都经过塔脚地面电阻 1R 入地 ，
3、而少部分沿着传输线通过塔脚入地 。 塔架的电压升高到一个值 ， 该值是由电流 1I 和经过的电阻确立的 。 111 RIU 大致上 ， 这个电压出现在塔架和（没有被击穿）电力导线之间 。 如果该电压低于所能一起闪络的值 ， 则不会引起任何麻烦 。 用两条精心放置的接地线和减少塔脚接地电阻的方法 ， 可改善这种保护方法 。 从支撑于 80 90 英尺高的塔架上的传输线的雷击报告中 ， 可以表明上述线路保 护的简单原理 。 但是从 100 英尺以上的铁塔的不太理想的记录表明 ， 或许还必须考虑到其他因素 ， 而对地绝缘水平较高的电压线路就不太重要 ， 这些线路可承受几百千伏的电压而不会发生闪络 。
4、 Over voltage Due to Lightning Over voltage on power systems are traceable to three basic cause： 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
5、to control their effects on operating equipment. Lightning results from the presence of clouds which have charged by the action of failing rain and vertical air currents， a condition commonly found in cumulus clouds.Voltage may be set up on overhead lines due to direct strokes and due to indirect st
6、rokes.In a direct strke， the lightning current path is directly from the cloud to the subject equipment-anoverhead line.From the line， the current path may be over the insulators and does the pole to ground.The voltage set up on the line may be that necessary to flash over this path to ground. On th
7、e occurrence of a stroke， lightning clouds do not discharge in zero time.Instead， the stroke current rises from zero value to maximum value in a few microseconds and is completed in a few hundred microseconds. Direct lightning strokes to lines are of concern on lines of all voltage class， as the vol
8、tage that may be set up is in most instances limited by the flashover of the path to ground， increasing the length of insulator strings merely permits a higher voltage befoe flashover occurs.The most generally accepted method of protection against direct strokes is by use of the overhead ground wire
9、.For simplification only one ground wire and one power conductor are shown. The ground wire is placed above power conductor at such a positon that practically all lightning-stroke paths will be to it instead of to the power conductor.Stroke current then follows to the ground， most of it passing thro
10、ugh the tower footing ground resistance while a smaller part goes down the line and to ground through the adjacent tower footings.The tower rises in voltage due to the current 1I though the resistance 1R to a value whice is 111 RIU Approximately this voltage appears between the tower and the power c
11、onductor (which was not struck).If this voltage is less than that required to cause insulator flashover， no trouble results.Protection by this method is improved by using two carefully placed ground wires and by making tower footing ground resistance of low value. The lightning record of lines suppo
12、rted on towers 80 to 90 feet tall substantiates the simple theory of line protection just presented.The poorer record of lines on towers over 100ft in height indicates that other factors， perhaps the inductance of the path down the tower， should be considered. Indirect strokes produce relatively low volages on lines.They are of real concern on low-voltage lines supported on small insulators.They are of little importance on high-voltage lines whose insulators can withstand hundreds of kilovolts without flas
