Lightning is an electrical discharge between cloud and earth, between clouds or between the charge centers of same cloud. Lightning is a huge spark that takes place when clouds are charged to a high potential with respect to earth object e. 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.
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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. 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. Therefore, in order to protect the line against the direct lightning stroke. LFAs should be mounted on the pole in parallel with each line insulator. A delta arrangement of conductors maximizes direct lightning strokes on the top center phase, which acts as shielding wire for the bottom outer phases.
The shielding failure of the outer phases is reduced and it is given by the following equation. An LFA mounted on the top phase must flash over before the top phase insulator. It is stressed by fairly steep over voltage impulses associated with direct lightning strokes on a conductor. Therefore, this arrester should be relatively short. After a top phase LFA flashes over, lightning current will flow, through the affected conductor and through the pole to the ground. Thus, the voltage on the cross arm increases at a much slower rate than it does on the lightning struck conductor before the flashover of the top phase LFA.
On the other hand, the potential of the adjacent phases also increases due to electromagnetic coupling between conductors but at much slower rate than that applied to the top phase insulator consequently, an outer phase arrester operates under much easier coordination conditions than a top phase arrester. With one or both outer phase arresters activated, a two or three phase lightning flashover is initiated.
To prevent transition of an impulse flash over to a PAF, the total flashover path L must be long. It can be calculated from the formula. As seen the critical gradient depends greatly on the line fault current.
The rate of decrease of the critical gradient slows down for larger fault currents. Phase-to-phase faults on a pole can give rise to fault current in order of a few kiloampers. The arresters are connected between the pole and all of the phase conductors in parallel with the insulators.
As illustration, let us consider the operation of phase A arrester. Due to different propagation rates of flashover channels for lightning impulses of positive and negative polarity, the first module to flashover is module 1 with a flashover length of l1.
Before flash over, the total resistance of the arrester can be assumed to be infinitely large. As tests have shown, module 3 of phase a arrester will usually flash over after module 1. When the central part of the arrester flashes over at instant t3, the arrester sparks over through a single spark channel of very low resistance.
Since the resistance of the flash over channel is low compared to other resistance affecting lightning over voltages surge impedance of the conductor and of the lightning channel, etc. It was assumed to be equal to zero. The effect of the power frequency voltage of a kV line on discharge process on the arrester surface is negligible. Since phases B and C and their arresters operate under identical conditions, it is practical to combine them in an overvoltage analysis.
First, the line parameters are entered, including the arrangement and radius of the conductors, the pole height, the grounding resistance, etc. Next, the insulators and arresters voltage-time characteristics VTCs are entered in an analytical form. Finally, the overvoltage calculations are performed for a given lightning current steepness in order to determine the lightning protection performance.
It is shown in the figure 5 The next step is to calculate flashover voltages for the individual discharge components or modules. Equation A4 of the Appendix is used to find the rate of propagation of discharge channels in modules 1 and 3 of arrester LFAA and the distance covered by these channels over time t.
If the channel length is greater or equal to the module length, a flashover is assumed to have occurred for that particular module and the equivalent arrester resistance abruptly becomes equal to the resistance of the respective semi conductive cable section. Furthermore, the arresters and insulators are checked for flashover based on their voltage-time characteristics.
Flashover of insulator InsA indicates lightning protection failure. At this point, the calculation is stopped and the output is printed, including the steepness of the lightning current I11 at which the insulator flashed over.
The rate of channel propagation on arrester modules is determined, and the modules are checked for flashovers. Finally, the calculation is checked for completion. The lightning protection failed. Both results put an end to the calculation, and printout is produced. If only a partial flashover of the arrester occurred. The calculation is restarted as a new time step t. It is seen that until module 1 of the LFAA arrester flashes over t t1 the rate of rise for phase A voltage is quite high.
It keeps increasing at a still slower rate until time t3. After the LFAA fully flashed over, the lightning current travels through the pole and its footing.
At instants t4, t5, and t6 the first, third, and second modules of arrester LFA B flash over, respectively, changing the resistance of the arrester.
The average span length Ispan of a kV line is usually about 70m. Thus, ttr is larger than tcr. Therefore, the nearest pole is not to be taken into account in the coordination analysis of the LFAA. The above calculation does not take into account the effect of near-by poise: thus, the calculated lightning performance of LFA-protected overhead lines can be regarded to have a certain margin. The voltage rate of rise Ul is proportional to steepness of the lightning current.
This is the reason why the calculation takes into account the critical values of the lightning current steepness Ill,cr at which the insulator flashes over for a given set of parameters.
It can be clearly seen that as the grounding resistance increases, the critical lightning current steepness Ill,cr decreases. Where n 0 is the number of lightning outages on an LFA protected line caused by direct lightning strokes on the phase conductors and P Il,cr is probability of a lightning current with steepness greater or equal to Il,cr The efficiency of LFA lightning protection against direct lightning strokes can be expressed as the ratio of the number of lightning outages n0 for unprotected line to n 0 for lines protected by LFA arresters.
A line with LFA arresters and INS insulators is shown to have a good lightning protection performance for direct lightning strokes. Thus, the number of outages caused by direct lightning strokes can be lowered with the use of LFA arresters by an order of magnitude or more even for high values of grounding resistance. As shown by calculations, in the case of INS insulators, it is important to coordinate the performance of phase B arrester and insulator because the voltage rate of rise, and thus, the lightning protection efficiency at direct lightning strokes depends heavily on the grounding resistance.
With the INS insulators the number of lightning outages is lowered by a factor of five, the outage reduction factor KDLS being practically independent of the grounding resistance. In the case, it is essential to coordinate the arrestors and the insulators on the lightning struck phase A.
As indicated before, the coordination of arrester LFA A is not depend on the grounding resistance because the pole does not get involved in the path of the lightning current until the insulator or the arrestors have flashed over. It was shown by the calculation that a 1-m- long arrestors. It was also shown that, after the LFAA arrestors has successfully operated, the voltage rate of rise on phase B insulator and its arrestor becomes low and this facilities successful operation of the LFAB arrestor, at least, over the 10 to grounding range.
It should also be remembered that even large lightning currents do not present any hazards to these arrestors because the discharge develops in the air and not inside the device. Therefore, this new lightning protection system is thought to feature simple design, low cost, and high reliability.
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. 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. 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.
To protect a line against induced over voltages; a single arrestor must be mounted on a pole. The conditions for the efficient protection of a medium voltage e.
LIGHTNING PROTECTION USING LFA-M PDF
Vitthal Patnecha 13 January 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 voltage and direct lightning strokes.
LIGHTNING PROTECTION USING LFA-M
Mikakus 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. After the LFA A fully flashed over, the lightning current travels through the pole and its footing. At instants t 4t 5and t 6 the first, third, and second modules of arrester LFA B flash over, respectively, changing the resistance of the arrester. Therefore the total voltage U is applied to each flashover module at the same moment, and all three modules are assured conditions for simultaneous initiation of creeping discharges developing lighttning to a single long flashover channel. The next step is to calculate flashover voltages for the individual discharge components or modules. The rate of channel propagation on arrester modules is determined, and the.