Differnt type of lightning stroke and its different harmful effect on transmission line
Answers
There are two Types of Lightning Strikes may strike the power system (e.g. overhead lines, towers, sub-stations etc.), namely;
Direct stroke
Indirect stroke
1. Direct stroke: In the direct stroke, the lightning discharge (i.e. current path) is directly from the cloud to the subject equipment e.g. an overhead line. From the line, the current path may be over the insulators down the pole to the ground. The overvoltages set up due to the stroke. may be large enough to flashover this path directly to the ground. The direct strokes can be of two types viz.
2. Indirect stroke: Indirect strokes result from the electrostatically induced charges on the conductors due to the presence of charged clouds. This is illustrated in Fig. 24.6. A positively charged cloud is above the line and induces a negative charge on the line by electrostatic induction. This negative charge, however, will be only on that portion of the line right under the cloud and the portions of the line away from it will be positively charged as shown in Fig. 24.6. The induced positive charge leaks slowly to earth via the insulators. When the cloud discharges to earth or to another cloud, the negative charge on the wire is isolated as it cannot flow quickly to earth over the insulators. The result is that negative charge rushes along the line is both directions in the form of travelling waves. It may be worthwhile to mention here that majority of the surges in a transmission line are caused by indirect Types of Lightning Strikes strokes.
Overhead Ground Wires:
The most effective method of providing protection to transmission lines against direct lightning strokes is by the use of overhead ground wires as shown in Fig. 24.7. For simplicity, one ground wire and one line conductor are shown. The ground wires are placed above the line conductors at such positions that practically all lightning strokes are intercepted by them e. ground wires). The ground wires are grounded at each tower or pole through as low resistance as possible. Due to their.proper location, the ground wires will take up all the lightning strokes instead of allowing them to line conductors.
Types of Lightning Strikes
When the direct lightning stroke occurs on the transmission line, it will be taken up by the ground wires. The heavy lightning current (10 kA to 50 kA) from the ground wire flows to the ground, thus protecting the line from the harmful effects of Types of Lightning Strikes. It may be mentioned here that the degree of protection provided by the ground wires depends upon the footing resistance of the tower. Suppose, for example, tower-footing resistance is R1 ohms and that the lightning current from tower to ground is I1 amperes. Then the tower rises to a potential Vt given by ;
Types of Lightning Strikes
Since Vt (= I1R1) is the approximate voltage between tower and line conductor, this is also the voltage that will appear across the string of insulators. If the value of Vt is less than that required to cause insulator flashover, no trouble results. On the other hand, if Vt is excessive, the insulator flashover may occur. Since the value of Vt depends upon tower-footing resistance R1, the value of this resistance must be kept as low as possible to avoid insulator flashover.
Advantages
It provides considerable protection against direct lightning strokes on transmission lines.
A grounding wire provides damping effect on any disturbance travelling along the line as it acts as a short-circuited secondary.
It provides a certain amount of electrostatic shielding against external fields. Thus it reduces the voltages induced in the line conductors due to the discharge of a neighbouring cloud.
Disadvantages
It requires additional cost.
There is a possibility of its breaking and falling across the line conductors, thereby causing a short-circuit fault. This objection has been greatly eliminated by using galvanised stranded steel conductors as ground wires. This provides sufficient strength to the ground wires.