Lightning arresters are protective devices for limiting surge voltages due to lightning strikes or equipment faults or other events, to prevent damage to equipment and disruption of service. Also called surge arresters.Lightning arresters are installed on many different pieces of equipment such as power poles and towers, power transformers, circuit breakers, bus structures, and steel superstructures in substations.
Lightning, is a form of visible discharge of electricity between rain clouds or between a rain cloud and the earth. The electric discharge is seen in the form of a brilliant arc, sometimes several kilometres long, stretching between the discharge points. How thunderclouds become charged is not fully understood, but most thunderclouds are negatively charged at the base and positively charged at the top. However formed, the negative charge at the base of the cloud induces a positive charge on the earth beneath it, which acts as the second plate of a huge capacitor.
When the electrical potential between two clouds or between a cloud and the earth reaches a sufficiently high value (about 10,000 V per cm or about 25,000 V per in), the air becomes ionized along a narrow path and a lightning flash results.
Many meteorologists believe that this is how a negative charge is carried to the ground and the total negative charge of the surface of the Earth is maintained.
The possibility of discharge is high on tall trees and buildings rather than to ground.
Buildings are protected from lightning by metallic lightning rods extending to the ground from a point above the highest part of the roof. The conductor has a pointed edge on one side and the other side is connected to a long thick copper strip which runs down the building. The lower end of the strip is properly earthed. When lightning strikes it hits the rod and current flows down through the copper strip. These rods form a low-resistance path for the lightning discharge and prevent it from travelling through the structure itself.
The lightning arrestor protects the structure from damage by intercepting flashes of lightning and transmitting their current to the ground. Since lightning strikes tends to strike the highest object in the vicinity, the rod is placed at the apex of a tall structure. It is connected to the ground by low-resistance cables. In the case of a building, the soil is used as the ground, and on a ship, water is used. A lightning rod provides a cone of protection, which has a ground radius approximately, equal to its height above the ground.
Surges due to lightning are mostly injected into the power system through long cross-country transmission lines. Substation apparatus is always well shielded against direct lightning strokes. The protection of transmission lines against direct strokes requires a shield to prevent lightning from striking the electrical conductors. Adequate
drainage facilities and adequate insulation structures must be provided so that the discharge can drain to ground without affecting the conductors. This prevents any arc from line conductor to ground. A ground wire placed above the phase conductors of a transmission line shields the phase conductors from the lightning strokes. A shielding angle of about 300 gives adequate lightning protection.
Terminal equipment at the substation is protected against by surge diverters, also called surge arrester or
lightning arresters. A diverter is connected in parallel or shunt with the equipment to be protected at the substation
between the line and ground. Ideally, it should
• become conducting at voltage above diverter rating
• restrict the voltage across its terminal to the design value;
• become non conducting again when the line-to-neutral voltage becomes lower than the design value. In
other words, it should not permit any power follow-on current;
• not conduct any current at normal or somewhat above normal power frequency voltages.
earthing screen and ground wires can well protect the electrical system against direct
lightning strokes but they fail to provide protection against travelling waves, which
may reach the terminal apparatus. The lightning arresters or surge diverters provide protection against such surges. A lightning arrester or a surge diverter is a protective device, which conducts the high voltage surges on the power system to the ground
One end of the diverter is connected to the terminal of the equipment to be protected and the other end is effectively grounded. The length of the gap is so set that normal voltage is not enough to cause an arc but a dangerously high voltage will break down the air insulation and form an arc. The property of the non-linear resistance is that its resistance increases as the voltage (or current) increases and vice-versa. This is clear from the volt/amp characteristic of the resistor shown in Fig 7 (ii).
The action of the lightning arrester or surge diverter is as under:
(i) Under normal operation, the lightning arrester is off the line i.e. it conducts no current to earth or the gap is non-conducting
(ii) On the occurrence of over voltage, the air insulation across the gap breaks down and an arc is formed providing a low resistance path for the surge to the ground. In this way, the excess charge on the line due to the surge is harmlessly conducted through the arrester to the ground instead of being sent back over the line.
(iii) It is worthwhile to mention the function of non-linear resistor in the operation of arrester. As the gap sparks over due to over voltage, the arc would be a short-circuit on the power system and may cause power-follow current in the arrester. Since the characteristic of the resistor is to offer low resistance to high voltage (or current), it gives the effect of short-circuit. After the surge is over, the resistor offers high resistance to make the gap non-conducting.
The function of the lightning rods consists of emitting an ascending electrical unloading to influence the effect of the descendent tracer. When one propagates towards the cloud, this ascending unloading generates an electric field sufficient to modify the trajectory of the descendent tracer, allowing the unloading of the ray to earth. This process can be realized naturally but the action of lightning rods IONIFLASH allows a fast activation but, providing an effective protection but.
This it is the concept of the fattened advance of.
1.Level of protection classified of very high
2.It does not need external power supply.
3. It does not need special maintenance.
History and Significance
Lightning arrestors became a more crucial facet to our modern lifestyle in the 1980s as the rapid proliferation of consumer technology resulted in the widespread use of electronic devices, both at home and in the commercial sector, thus increasing the need for arrestors. Computer chips in televisions, DVD players, radios, PCs, printers, telephones, garage door openers and the like are all susceptible to even small electrical surges and can be irreversibly damaged if not protected. It is, however, important to note that arrestors are not lightning rods. While both devices seek to channel the current from a lightning strike into the ground, lightning rods are used to protect entire buildings from fires and structural damage as opposed to targeted electrical devices.
How it works
At the heart of all arresters is Metal Oxide Varistors (MOV). The MOV disk is a semiconductor that is sensitive to voltage. At normal voltage, the MOV disk is an insulator and will not conduct current. But at higher (extreme) voltage caused by lightning or any surges, it becomes a conductor.
The usual construction of a typical surge arrester consists of disks of zinc oxide material sized in cross-sectional area to provide desired energy discharge capability, and in axial length proportional to the voltage capability. The disks are then placed in porcelain enclosures to provide physical support and heat removal, and sealed for isolation from contamination in the electrical environment.
When a surge of electricity such as a lightning strike hits an electrical system, it naturally seeks a way to equalize and dissipate itself as quickly as possible, taking the path of least resistance. A resistor provides the most efficient route for the surge by diverting the electricity away from the equipment's insulation and channeling it into the ground via grounding rods. In order to do this, the resistor uses a metal oxide varistor (MOV), which is a component with a diode-like nonlinear current-voltage characteristic that is triggered when voltages reach sensitive levels. Specifically, the varistor's symmetrical, sharp breakdown characteristics allow it to achieve a high level of transient electrical suppression performance. The word "varistor" is a portmanteau that combines "variable" and "resistor."
Lightning can occur with both positive and negative polarity. An average bolt of negative lightning carries an electric current of 30,000 amperes ("amps") — 30 "kiloamps" (kA), and transfers five coulombs of electric charge and 500 megajoules of energy. Large bolts of lightning can carry up to 120 kA and 350 coulombs. An average bolt of positive lightning carries an electric current of about 300 kA — about 10 times that of negative lightning.
WHAT EXACTLY DOES A SURGE ARRESTER DO?
1. Surge Arresters does not absorb the lightning.
2. Surge Arresters does not stop the lightning.
3. Surge Arresters divert the lightning to ground.
4. Surge Arresters clamp (limit) the voltage produced by lightning.
5. Surge Arresters equipment electrically in parallel with it.
About Lightning
Lightning is an atmospheric discharge of electricity accompanied by thunder, which typically occurs during thunderstorms, and sometimes during volcanic eruptions or dust storms. In the atmospheric electrical discharge, a leader of a bolt of lightning can travel at speeds of 220,000 km/h (140,000 mph), and can reach temperatures approaching 30,000 °C (54,000 °F), hot enough to fuse silica sand into glass channels known as fulgurites which are normally hollow and can extend some distance into the ground. There are some 16 million lightning storms in the world every year.
Location of Arresters
The ideal location for lightning arresters,from the standpoint of protection, is directly at the terminals of the equipment to be protected. At this location, with the arrester grounded directly at the tank,frame or other metallic structure which supports the insulated parts, the surge voltage applied to the insulation will be limited to the discharge voltage of the arrester. Practical system circumstance and sound economics often dictate that
arresters be mounted remotely from the
equipment to be protected. Often, one set of arresters can be applied to protect more than one piece of equipment. Low BIL apparatus (certain dry-type transformers and rotating machines) will often require surge protective devices be connected directly at the terminals of the equipment being protected.
In many switchgear installations, the
only exposure to lightning will be
through a transformer located on its up
stream side. When the transformer has
adequate lightning protection on its
primary, experience has shown that the
surge transferred through the transformer is usually not of a magnitude that would be harmful to the switchgear. Hence, it is generally not necessary to provide arresters in the switchgear.
When arresters are located away from
the terminals of the protected equipment, the voltage wave will reflect positively on the equipment terminals and the voltage magnitude at the terminal point will always be higher than the discharge voltage of the arrester. This, as discussed earlier, is due to the fact that the protected equipment usually has a
higher surge impedance than the line or cable serving it.
If the circuit is open at the protected equipment (infinite surge
impedance), the voltage will be double
the arrester discharge voltage.
The actual surge voltage appearing at the protected equipment depends, in part, on the incoming wave magnitude at the instant of arrester discharge. If a positive reflected surge from the protected equipment arrives back at the arrester before arrester discharge, it will add to the incoming wave to produce discharge at a lower incoming wave magnitude.
The reflected wave, in this case, results in improved protection. The closer the
arrester is to the protected equipment, the greater the effect of the reflected
surge on arrester discharge and
the better the protection.
Conclusion:
All electrical equipment in an electrical system needs to be protected from voltage surges. The rating of the arrester, the class of arrester and the location of the arrester all play a part in the surge protection. Modern metal oxide arresters provide markedly superior protective characteristics and energy absorption capability, compared to previous generation arresters. The application and selection of metal oxide arresters requires a thorough review of the power system, including voltage, system stresses, switching surges, grounding method and MCOV.
References :
· www.scribd.com/search/lightning arrester
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