07-10-2014, 10:01 AM
LIGHTNING
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INTRODUCTION
A new simple, effective and inexpensive method for lightning protection of medium voltage overhead distribution line is using long flashover arresters (LFA). A new long flashover arrester model has been developed. It is designated as LFA-M. It offers great number of technical and economical advantages. The important feature of this modular long flashover arrester (LFA-M) is that it can be applied for lightning protection of overhead distribution line against both induced over voltages and direct lightning strokes. The induced over voltages can be counteracted by installing a single arrester on an overhead line support (pole). For the protection of lines against direct lightning strokes, the arresters are connected between the poles and all of the phase conductors in parallel with the insulators.
2.1 WHAT IS LIGHTNING ?
Lightning is an electrical discharge between cloud and the earth, between clouds or between the charge centers of the same cloud. Lightning is a huge spark and that take place when clouds are charged to at a high potential with respect to earth object (e.g. overhead lines) or neighboring cloud that the dielectric strength of the neighboring medium(air) is destroyed.
DIRECT STROKE
In direct stroke, the lightning discharge is directly from the cloud to the an overhead line. From the line, current path may be over the insulators down to the pole to the ground. The over voltage set up due to the stroke may be large enough to flashover this path directly to the ground. The direct stroke can be of two types
INDIRECT STROKE
Indirect stroke result from eletrostatically induced charges on the conductors due to the presence of charged cloud. If 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 on the line right under the cloud and the portion of the line away from it will be positively charged. The induced positive charge leaks slowly to earth. When the cloud discharges to earth or to another cloud, negative charge on the wire is isolated as it can not flow quickly to earth over the insulator. The result is that negative charge rushes along the line is both directions in the form of traveling wave. Majority of the surges in a transmission lines are caused by indirect lightning stroke.
FLASHOVER PERFORMANCES
The flashover performance of modular long-flashover arresters(LFA-M) arresters of two different flashover lengths and the voltage-time characteristics of LFA loop arresters, as well as those of the most common Russian insulators ShF 10-G and ShF 20-G with lengths 17 and 23 cm,respectively,were studied. The 50% flashover voltages of these units are 130 and 160 KV when stressed by 1.2/50 lightning impulses of negative polarity. therefore, these units will be referred hereafter as INS 130 and INS 160,respectively
6.1 GENERAL PATTERN OF DIRECT LIGHTNING STROKES
The physical phenomena associated with a direct lighting stroke on an unprotected power line causing line tripping. The general pattern is as follows. For an overhead line in delta configuration shown in fig2, the top center face is the most vulnerable. For a lightning stoke on a phase conductor, the lighting current propagates both ways from the stroke point overcoming the surge impedance Zs of the line. A fairly high voltage drop develops at the points where the lines equivalent resistance equals half of the surge impedance Zs/2; this point is the closest to insulator unit of the lightning struck phase conductor. The voltage causes the insulator to flashover. A heavy impulse current flows through the flashover channel, the pole, and the pole footing resistance resulting into a large sharp voltage rise at the cross-arm.. Due to electromagnetic coupling between phases, the potential of the healthy outer phases also increases and it can be assessed from the conductor coupling factor. This voltage, however, is not as high as that for the lightning struck-conductor. Thus, the insulators of the healthy phases are stressed and flashed over by a voltage equal to the potential difference between the cross arm and the phase conductor. Phase to phase lightning flashover is also highly probable to occur resulting to a power arc accompanied by heavy short circuit currents, which causes immediate line tripping.
FUTURE EXPANSION
The LFA-M described here consists of three flashover modules. We can increases the flashover modules. If the number of flashover modules increases by increasing the cable pieces this LFA-M can be used for lightning protection of very high voltage lines. When the modules increases the total arrester stressing is distributed these modules also. Then it can withstand very high over voltages.
CONCLUSIONS
1. A long flashover arrestor (LFA) comprising three flashover modules using the creeping discharge effect was presented in this report. Its resistors assure application of the total arrestor-stressing voltage simultaneously to all the modules.
2. The voltage-time characteristics of this modular arrestor assure reliable protection of medium voltage overhead lines against both induced over voltages and direct lightning strokes.
3. To protect a line against induced over voltages; a single arrestor must be mounted on a pole.
4. The conditions for the efficient protection of a medium voltage (e.g. 10-kv) overhead line against direct lightning strokes, are as follows:
• Delta phase configuration of phase conductors
• Mounting of LFA-M arresters on all poles in parallel with each insulators ;
• A relatively short flashover path (for example, 1 m for a 10-kv line) for the top phase LFA-M arrester
• A longer flashover path (for example 2 m for a 10-kv line) for the bottom phase LFA-M arresters