Report Lightning Arrester 27435g

LIGHTNING ARRESTER

VISVESVARAIAH TECHNOLOGICAL UNIVERSITY BELGAUM, KARNATAKA

A Seminar Report On “Lightning Arrester” A seminar report submitted in partial fulfillment of the requirements for SEMINAR WORK Of th B.E. 8 SEMESTER ELECTRICAL & ELECTRONICS ENGINEERING During the academic year 2010-2011 Seminar by HARISH G 3VC07EE012

DEPARTMENT OF ELECTRICAL & ELECTRONICS ENGINEERING

RAO BAHADUR Y MAHABALESHWARAPPA ENGINEERING COLLEGE (d To Visvesvaraya Technological University, Belgaum) (NBA Accredited) Bellary - 583104, Karnataka

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VEERASHAIVA VIDYAVARDHAKA SANGHA’S

RAO BAHADUR Y MAHABALESHWARAPPA ENGINEERING COLLEGE Page | 2

LIGHTNING ARRESTER (D TO VISVESWARAIAH TECHNOLOGICAL UNIVERSITY, BELGAUM & APPROVED BY AICTE & ACCREDITED BY NBA, NEW DELHI) BELLARY - 583104, KARNATAKA

DEPARTMENT OF ELECTRICAL & ELECTRONICS ENGINEERING

CERTIFICATE This is to certify that HARISH G bearing USN 3VC07EE012 of 8 t h semester has successfully completed the seminar entitled “Lightning Arrester” , in the partial fulfillment of the requirements for the aw ard of degree of Bachelor of Engineering in Electrical & Electronics Engineering, for the year 2010-2011.

Mr.Hanumanth Rao.A M.Tech. Asst.Professor Seminar Coordinator

Mr. M.B. Mallikarjuna Ms Prof. & H.O.D

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ACKNOWLEDGEMENT The famous English poet in Stratford-upon-Avon said “Pupil Thy Work Is Incomplete, Till Thee Thank the Lord and Thy Master”, which means a student’s

work, is incomplete until he thanks the Almighty and his Teacher. I sincerely believe in this. So before introducing my seminar I would like to thank the people without whom the success of this seminar would have been only a dream.

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I express my sincere thanks to our beloved H.O.D Mr. M.B.MALLIKARJUNA, Electrical & Electronics Department for his inspiration and constant for the seminar. It also gives immense pleasure to express my sincere thanks to Asst.Professor Mr. Hanumanth Rao, Department of Electrical & Electronics Engineering, for his valuable guidance and encouragement during the course of this seminar. His keen interest, patient hearing and constructive criticism has instilled in me the spirit of confidence to successfully complete this seminar. I’m also thankful to the entire staff of the Department of Electrical & Electronics for help in some way or the other. I’m also thankful to all my classmates and family encouraged me to work out this seminar.

- HARISH G

Abstract 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 Page | 5

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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.

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Introduction 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

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Fig 7(i) shows the basic form of a surge diverter. It consists of a spark gap in series with a nonlinear resistor. 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.

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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 Page | 9

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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 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."

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. Page | 10

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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.

FIG.Lightning

strikes the Eiffel Tower, in 1902.

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.

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FIG. Cloud to Ground Lightning

Types o f Lightning Arrestors Station Class Page | 12

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Fig.Station Type Surge Arresters Station class arrestors are typically used in electrical power stations or substations and other high voltage structures and areas. These arrestors protect against both lightning and overvoltages, when the electrical device has more current in the system than it is designed to handle. These arrestors are designed to protect equipment above the 20 mVA range. Intermediate Class Like station class arrestors, intermediate class arrestors protect against surges from lightning and over-voltages, but are designed to be used in medium voltage equipment areas, such as electrical utility stations, substations, transformers or other substation equipment. These arrestors are designed for use on equipment in the range of 1 to 20 mVA. Distribution Class Distribution class arrestors are most commonly found on transformers, both dry-type and liquid-filled. These arrestors are found on equipment rated at 1000 kVA or less. These arrestors are sometimes found on exposed lines that have direct connections to rotating machines. Secondary Class Secondary class lightning arrestors are designed to protect most homes and businesses from lightning strikes, and are required by most electrical codes, according to ,Inc., an electrical power protection company. These arrestors cause high voltage overages to ground, though they do not short all the over voltage from a surge. Secondary class arrestors offer the least amount of protection to electrical systems, and typically do not protect solid state technology, or anything that has a microprocessor.

Applications Lightning arresters can be used for a wide variety of applications. For example, lightning arresters are used on tall buildings to prevent lightning from causing physical damage to Page | 13

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infrastructure. Lightning arresters are also used on power lines to protect the fragile cables from lightning, electromagnetic forces, downed power line poles, and other natural phenomenon. Additionally, lightning arresters are used on power outlets to protect electronic devices from surges of electricity. 3

Arrester Selection The objective of arrester application is to select the lowest rated surge arrester which will provide adequate overall protection of the equipment insulation and have a satisfactory service life when connected to the power system. The arrester with the minimum rating is preferred because it provides the greatest margin of protection for the insulation. A higher rated arrester increases the ability of the arrester to survive on the power system, but reduces the protective margin it provides for a specific insulation level. Both arrester survival and equipment protection must be considered in arrester selection. The proper selection and application of lightning arresters in a system involve decisions in three areas: 1. Selecting the arrester voltage rating. This decision is based on whether or not the system is grounded and the method of system grounding. 2. Selecting the class of arrester. In general there are three classes of arresters. In order of protection, capability and cost, the classes are: Station class Intermediate class Distribution class Page | 14

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The station class arrester has the best protection capability and is the most expensive. 3. Determine where the arrester should be physically located.

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 s 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 Page | 15

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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.

Three types of surge diverter are described below in principle, construction and application. (i) Rod gap arrester This is constituted of a plain air-gap between two square rods (1cm2) bent at right angles and connected between the line and earth as shown for the case of a transformer bushing in fig 5. The gap may also be in the form of horn or arcing rings.

When the surge voltage reaches the design value of the gap, an arc appears in the gap providing an ionised path to ground, essentially a short-circuit. The gap suffers from the defect that after the surge has discharged, power frequency current continues to flow through the ionised path and the arc has to be extinguished by opening of circuit breakers resulting in outage. Rod gap is therefore generally used as back-up protection. For a given gap, the time to break down varies inversely with the applied voltage. It is normally recommended that a rod gap should be so set that it breaks down to a voltage not less than 30 percent below the voltage withstand level of the equipment to be protected. (ii) Expulsion type arrester (protective tube): Page | 16

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It is improvement over a rod gap and is commonly used on system operating at voltages up to 33KV.

The series gap is set to arc over at a specified voltage lower than the withstand voltage of the equipment to be protected as shown in fig 6. The follow-on current is confined to the space inside the relatively small fibre tube. Part of the tube material vaporizes, and the high pressure gases so formed are expelled through the vent at the lower end of the tube, causing the power follow-in arc to be extinguished. The device, therefore, has the desired self-clearing property. Because of vaporization of the tube material and weathering effect, the protector tube requires frequent replacement and lack of proper maintenance may lead to occasional outage. It has not, therefore, found favour in application and is practically out of use now. (iii) Value type (nonlinear) arrester

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The value type lightning arrester consists of two assemblies namely series spark gaps and nonlinear resistor discs (made of material such as thyrite or metrosil) in series.The value type lightning arrester consists of nonlinear resistors in series with spark-gaps, as shown in fig 7. The spark-gap assembly acts as a fast-switch, which gets ionized (conducting) at specified voltage. Both the assemblies are accommodated in tight porcelain container. The spark gap is a multiple assembly consisting of a number of identical spark gaps in series. Each gap consists of two electrodes with fixed gap spacing. The voltage distribution across the gaps is linearised by means of additional resistance elements (called grading resistors) across the gaps. The spacing of the series gaps is such that it will withstand the normal circuit voltage. However, an over voltage will cause the gap to breakdown, causing the surge current to ground via the nonlinear resistors. The nonlinear resistor discs are made of an inorganic compound such as thyrite or metrosil. These discs are connected in series. The nonlinear resistors have the property of offering a high resistance to current flow when normal system voltage is applied, but a low resistance to the flow of high surge currents. In other words, the resistance of these nonlinear dements decreases with the increase in current through them and vice-versa (5). Under normal conditions, the normal system voltage is insufficient to cause the breakdown of air gap assembly. On the occurrence of an over voltage, the breakdown of the series spark gap takes place and the surge current is conducted to earth via the nonlinear resistances. Since the magnitude of surge current is very large, the nonlinear elements will offer a very low resistance to the age of surge. The result is that the surge will rapidly go to earth instead of being sent back over the line. When the surge is over, the nonlinear resistors assume high resistance to stop the flow of current. The advantages of value type arrester are: i. They provide very effective protection (especially for transformers and cables) against surges. ii. They operate very rapidly taking less than a second iii. The impulse ratio is practically unity. The limitations of value type arrester are:

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i. They may fail to check the surge of very steep wave front reaching the terminal apparatus. This calls for additional steps to check steep fronted waves. ii. Their performance is adversely affected by the entry of moisture into the enclosure. This necessitates effective sealing of the enclosure at all times. According to their application, the value type arresters are classified as station type and line type. The station type arresters are generally used for the protection of important equipment in power stations operating on voltages up to 220kV or higher. The line type arresters are also used for stations handling voltages up to 66kV. The development of current limiting gap arresters was in considerable measure responsible for a number of improvements in arrester performance including: 1. Reduction in liitning discharge voltages made possible by reducing valve element resistance; 2. Reductions in arrester size because of the reduction in energy absorbed fora given surge level and because of energy sharing between gaps and valve elements; 3. A reduction in protective levels on HV and EHV systems in particular because of the overvoltage reseal capability of current-limiting gap arresters. Current limitinggap arresters have been widely applied with an excellent service record; however, they do possess certain inherent limitations most of which are related to the fact that the arrester voltage is not at the level shown by Fig. 1 during the whole arrester operation. Instead, the arrester voltage falls to the valve element voltage immediately after sparkover, and does not rise to the level in less than a few hundred microseconds after sparkover. (')Untilg ap voltage is developed,t he arrester current is given by:

When the circuit in which the arrester is operating has low surge impedance (multiple lines, capacitor banks, or cable circuits), the initial arrester current can become very high. It is possible in some cases of very low surge impedance for the initial current to be so high that it does not move away from the point of initial arc strike into the normal arcclearing region of the gap units. As a result the arrester may fail to clear, or the erosion of the electrodes may be sufficiently severe to cause a change in arrester sparkover.' A s i m i problem may arise during operation of current limiting fuses because of the abiiity of such fuses to switch high fault currents very rapidly into arresters of low rating compared to fuse rating. Page | 19

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The new arrester offers some very significant advantages: a) improved reliability due to simplicity of the design. b) Superior protective characteristics. C) Minimum energy absorption commensurate with the protective levels Provided. d) Superior performance compared to present arresters on low impedance circuits (e.g. cable and capacitor applications). e) Gentle operation -protected equipment is not subjected to sparkover transients; f) Superior energy absorption capability compared to presently available arresters.

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 Page | 20

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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.idoub.com/search/lightning arrester www.wikipedia.com www.Arresterworks.com How Does a Lightning Rod Protect a House eHow_com.htm file://SURGE%20ARRESTERS%20APPLICATION%20IN %20TRANSMISSION%20and%20DISTRIBUTION%20LINES %20%20TRANSMISSION%20LINES%20DESIGN%20HUB.htm

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