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Many investigations have confirmed benefit of applying arresters to Transmission Lines. In case of insufficient or non-existing shielding systems, arresters are generally placed on the upper most phase. The frequency of application viz., in every structure or once in two or three structures varies depending on the levels of protection expected. In case where lines have good shielding but poor grounding, the arresters are used on the lowest phase which has a maximum potential for back flash over in the event of lighting striking shield wire.
Overhead transmission lines are the most vulnerable for lightning strokes. More than 50% of electrical faults of overhead lines are known to be caused by lightning. When a lightning strikes the power line, the lightning current may be distributed in different ways through different proportions of the line, depending upon the point of incidence. For example, a lightning strike penetrating the shielding system and terminating on a phase conductor would generate travelling waves of the same magnitude and polarity propagating in opposite directions. When the travelling wave reaches the tower, the line insulator can undergo flashover, resulting in an interruption. Note that several poles are generally subjected to the insulation flashover due to the direct lightning strokes on the power line. The lightning flashover in the transmission lines can be reduced by increasing the insulation, using multiple overhead shielding wires, or by reducing the tower footing resistance. However, these methods do not eliminate the lightning flashover problems completely. Besides, the cost involved in these methodologies is significantly higher. Installation of ZnO arresters across the line insulators (at frequent intervals) would minimize/eliminate the possibilities of insulation flashover.
Transmission line arresters are designed with or without series gap. From the reliability view points, the series gap configuration is preferred as the gap allows the transmission line to perform its function even after the arrester failure.
The line arresters with series gap, therefore, do not require any disconnecting device. Most of the countries such as Japan, USA, Brazil, Mexico and France have already installed the transmission arresters and found to improve the transmission line performance significantly. Besides, usage of the transmission line arresters allows to reduce the insulation level (BIL) of the substation equipments; e.g., power transformers, as the lightning surges are suppressed in the line itself by the line arresters. Hence, the no. of surges experienced by the substation arresters would be reduced and hence the reliability of the entire system would be enhanced.
Transmission Line Arrester is installed parallel to an insulator string. When lightning strikes the steel tower or power conductor, voltage between arcing horns is raised, and this causes a sparkover at a series gap earlier than at the arcing horn. Then, using the effect of a series gap and voltage-current non-linear characteristics of ZnO elements, a follow current from the power system is interrupted within one cycle and insulation is recovered. Thus flashover does not occur between arcing horns and the occurrence of a circuit-breaker tripping is averted. For a double circuit power transmission, either line may be equipped with this arrester. This makes it possible to prevent the occurrence of a double-circuit fault (a service interruption).
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The advantages of transmission line arresters:
- Line compaction: Allows for maximum reduction in width and height of transmission lines by elimination of Shield wires.
- Line uprating: Increased phase conductor clearances required for voltage can be obtained without major structure modifications by elimination of Shield wires.
- Provides improved reliability.
- Low installed cost.
- Improved outage performance: Improves outage performance of existing shielded and unshielded lines which is accomplished by greatly reducing the frequency of line insulation flashovers due to shielding failure and back flash.
- Arrester assemblies can improve the performance of lines with high grounding resistance which is often the main cause of Back flash on line protected with Shield wires.
- The arrester assembly is especially beneficial for very tall structures with long spans like river crossings and other terrains which make Shield Wires impractical or impossible.
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