Define Lightning arrester & Its types
Lightning arrester:
A lightning arrester is a device used on electrical power systems and telecommunications systems to protect them and the conductors of the system from the damaging effects of lightning.
Types of arresters are given below,
1) Rod Gap Arrester
2) Sphere Gap Arrester
3) Horn Gap Arrester
4) Multiple Gap Arrester
5) Electrolyte Lightning Arrester
6) Expulsion Type Lightning Arrester
7) Thyrite Lightning Arrester
8) Oxide Film Arrester
9) Metal Oxide Lightning Arrester
1) Rod Gap Arrester:.jpg "Rod gap arrester")
It is one of the simplest forms of arrest. In such a type of arrester, there is an air gap between the ends of the two rods. The one end of the arrester is connected to the line and the second end of the rod is connected to the ground. The gap setting of the arrester should be such that it should break before the damage. When the high voltage occurs on the line, the gap sparks, and the fault current passes to the earth. Hence the equipment is protected from damage.
The difficulty with the rod arrester is that once the spark has taken place it may continue for some time even at low voltages. A current limiting reactor in series with the rod is used to avoid it. The resistance limits the current to such an extent that sufficient to maintain the arc. Another difficulty with the rod gap is that the rod gap is liable to damage due to the arc's high be temperature, which may cause the rod to melt.
2) Sphere Gap Arrester:
In such types of devices, an air gap is provided between the different spheres connected to the line, and the other sphere is connected to the ground. The spacing between the two spheres is very small. A chocking coil is inserted between the phase winding of the transformer and spheres are connected to the line.
The air gap between the arrester is set in such a way that the discharge must not take place at normal operation. The air will travel up the sphere as the heated air near the arc tends to rise upward and lengthen till is interrupted automatically.
3) Horn Gap Arrester:
It consists of two horns shaded of metal separated by a small air gap and connected in a shunt between each conductor and earth. The distance between the two electrodes is such that the normal voltage between the line and earth is insufficient to jump the gap, but the abnormal high voltage will break the gap and so find a path to earth.
4) Multiple Gap Arrester:
The multiple gap arrester consists of a series of small metal slender insulated from one another and separated by an air gap. The first and the last of the series are connected to the ground. The number of gaps required depends on the line voltage.
5) Electrolyte Lightning Arrester:
Such type of arrester has a high a large discharge capacity. It operates on the fact that the thin film of aluminum plates immersed in the electrolyte. The plat acts as high resistance to a low value but low resistance to a voltage above a critical value.
The voltage of more than 400V causes a puncture and a free flow of current to earth. When the voltage remains at its normal value of 440V, the arrester again offers high resistance in the path, and leakage stops.
6) Expulsion Type Lightning Arrester
Expulsion type arrester is ab improvement over the rod gap in that it seals the flow of power frequency follows the current. This arrester consists of a tube made up of fiber which is very effective, isolating spark gap and an interrupting spark inside the fiber tube.
During operation, the arc due to the impulse spark inside the fibrous tube cause some fibrous materials of the tube to volatile in the form of the gas, which is expelled through a vent from the bottom of the tube. Thus, extinguishing the arc is just like in circuit breakers.
7) Thyrite Lightning Arrester:
Such type of arrester is most commonly used for the protection against dangerous high voltage. It consists the thyrite which is an inorganic compound of ceramic material. The resistance of such material decreases rapidly from high value to low value and from a low value to high value.
It consists of a disc whose both side is sprayed so as to give the electric contact between the consecutive disc. The disc is assembled inside the glazed protection container. It is used in conjunction with the container.
When the lightning takes place, the voltage is raised, breakdowns of the gaps occur, the resistance falls t a very low value, and the wave is discharged to earth. After the surge has passed the value of the thyrite again comes back to its original position.
8) Oxide Film Arrester:
It consists of pellets of lead peroxide with a thin, porous coating of litharge arranged in a column and enclosed in a tube of diameter. Out of two leads, the upper is connected to the line, while the lower is connected to the earth. The tube contains a series of the spark gap.
When an overvoltage occurs an arc passes through the series spark gap and additional voltage is applied to the pellet column and a discharge takes place. After the discharge, the resistance of the pellet gun increases till only a very small current flow through it. This small current is finally interrupted by a series of spark gaps.
9) Metal Oxide Lightning Arrester:
Such types of arresters are also known as gapless arresters or Zinc oxide arresters. The base material used for manufacturing metal oxide resisters is zinc oxide. It is a semiconducting N-type material. The material is doped by adding some fine powder of insulating oxides. The powder into disc-shaped. The disc is then enclosed in a porcelain housing filled with nitrogen gas or SF6.
This arrester consists of a potential barrier at the boundaries of each ZnO. This potential barrier controls the flow of current. At normal operating conditions, the potential barrier does not allow the current to flow. When an overvoltage occurs, the barrier collapse, and a sharp transition from insulating to conducting takes place. The current start flowing and the surge is diverted to the ground.
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