Seminar report on protection of lightning arrester

7/29/2019 Seminar report on protection of lightning arrester

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CHAPTER 1

INTRODUCTION

1.1 General

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 voltages and direct

lightning strokes. The induced over voltages can be counteracted by installing a single

arrester on an overhead line support (pole). 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.


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CHAPTER 2

LIGHTNING

2.1 WHAT IS LIGHTNING

Lightning is an electrical discharge between cloud and the earth,

between clouds or between the charge centers of the same cloud. Lightning is a

huge spark and that take place when clouds are charged to at a high potential with

respect to earth object (e.g. overhead lines) or neighboring cloud that the dielectric

strength of the neighboring medium(air) is destroyed.

Fig 2.1 .A lightning flash during athunderstorm.

Lightning is a massive electrostatic dischargebetween electrically charged

regions within clouds or between a cloud and the Earth's surface. The charged regions

within the atmosphere temporarily equalize themselves through a lightning flash

commonly referred to hits an object on the ground.

There are three primary types from a cloud to itself intra-cloud or IC from

one cloud to another cloud (CC) finally between a cloud and the ground (CG).

Although lightning is always accompanied by the sound of thunder distant lightning

may be seen but be too far away for the thunder to be heard .Lightning occurs

approximately 4050 times asecond worldwide resulting in nearly 1.4 billion flashes

per year. Many factors affect the frequency, distribution, strength, and physical

properties of a "typical" lightning flash to a particular region of the world. These

factors include ground elevation, latitude, prevailing wind currents, relative humidity,

proximity to warm and cold bodies of water, etc. To a certain degree, the ratio

between IC, CC and CG lightning may also vary by season in middle latitudes.

Because human beings are terrestrial and most of their possessions are on the Earth,where lightning can damage or destroy them, CG lightning is the most studied and

best understood of the three types, even though IC and CC are more common.
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Lightning's unpredictability limits a complete explanation of how or why it occurs,

even after hundreds of years of scientific investigation. A typical cloud to ground

lightning flash culminates in the formation of an electrically conducting

plasma channel through the air in excess of 5 km . The actual discharge is the final

stage of a very complex process.A typical thunderstorm has three or more strikes to

the Earth perminute at its peak.

Fig 2.2 Four-second video of a lightning strike, Island in the

Sky, Canyonlands National Park, Utah, United States

Lightning primarily occurs when warm air is mixed with colder air masses

resulting in atmospheric disturbances necessary for polarizing the atmosphere. However,

it can also occur during dust storms, forest fires, tornadoes, volcanic eruptions, and even

in the cold of winter, where the lightning is known as thunders now. Hurricanes typically

generate some lightning, mainly in the rain bands as much as 160 km (100 mi) from the

center. The science of lightning is called fulminology. The fear of lightning is

called astraphobia.

2.2 Distribution of lightning

Lightning is not distributed evenly around the planet, as seen in the image

on the right. About 70% of lightning occurs over land inthe tropics where atmospheric convection is the greatest. This occurs from both the

mixture of warmer and colderair masses, as well as differences in moisture

concentrations, and it generally happens at the boundaries between them. The flow of

warm ocean currents past drier land masses, such as the Gulf Stream, partially

explains the elevated frequency of lightning in the Southeast United States. Because

the influence of small or absent land masses in the vast stretches of the world's oceans

limits the differences between these variants in the atmosphere, lightning is notably

less frequent there than over larger landforms. The North and South Poles are limited

in their coverage of thunderstorms and therefore result in areas with the least amount

of lightning. In general, cloud-to-ground (CG) lightning flashes account for only 25%

of all total lightning flashes worldwide. Since the base of a thunderstorm is usuallynegatively charged, this is where most CG lightning originates. This region is typically

at the elevation where freezing occurs within the cloud. Freezing, combined with
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collisions between ice and water, appears to be a critical part of the initial charge

development and separation process. During wind-driven collisions, ice crystals tend

to develop a positive charge, while a heavier, slushy mixture of ice and water

(called graupel) develops a negative charge. Updrafts within a storm cloud separate

the lighter ice crystals from the heavier graupel, causing the top region of the cloud to

accumulate a positive space chargewhile the lower level accumulates a negative space

charge. Because the concentrated charge within the cloud must exceed the insulating

properties of air and this increases proportionally to the distance between the earth and

the ground, the proportion of CG strikes (versus cloud-to-cloud (CC) or in-cloud (IC)

discharges) becomes greater when the cloud is closer to the ground. In the tropics,

where the freezing level is generally higher in the atmosphere, only 10% of lightning

flashes are CG. At the latitude of Norway (around 60 North latitude), where the

freezing elevation is lower, 50% of lightning is CG.

Lightning is usually produced by cumulonimbus clouds whose bottoms are

56 km above the ground and that are themselves up to 15 km (9.3 mi) in height. On

Earth, the place where lightning occurs most often is near the small village of Kifuka

in the mountains of the eastern Democratic Republic of the Congo, where

the elevation is around 975 m (3,200 ft). On average, this region receives 158

lightning strikes per 1 square kilometer (0.39 sq mi) per year. Other lightning hotspots

include Catatumb lightning inVenezuela, Singapore,Teresina in northern Brazil,and

"Lightning Alley" in central flourdia.

2.3 Establishing conditions necessary for lightning

In order for an electrostatic discharge to occur, two things are necessary: 1)

a sufficiently high electric potential between two regions of space must exist; and 2) a

high-resistance medium must obstruct the free, unimpeded equalization of the

opposite charges. It is well understood the thunderstorm is able to separate and

aggregate charges in certain regions of the cloud, however the exact processes by

which this occurs are not fully understood .The atmosphere provides the electrical

insulation, or barrier, that prevents free equalization between charged regions of

opposite polarity. This is overcome by "lightning", a complex process referred to as

the lightning "flash".

2.4 Establishing the electric field in CG lightning

As a thundercloud moves over the surface of the Earth, an equal electric charge,

but of opposite polarity, is induced on the Earth's surface underneath the cloud. The

induced positive surface charge, when measured against a fixed point, will be small as

the thundercloud approaches, increasing as the center of the storm arrives and dropping

asthe thundercloud passes. The referential value of the induced surface charge could beroughly represented as a bell curve. The oppositely charged regions create an electric
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field within the air between them. This electric field varies in relation to the strength of

the surface charge on the base of the thundercloudthe greater the accumulated charge,

the higher the electrical field.

As a thundercloud moves over the surface of the Earth, an equal electric charge,

but of opposite polarity, is induced on the Earth's surface underneath the cloud. The

induced positive surface charge, when measured against a fixed point, will be small as

the thundercloud approaches, increasing as the center of the storm arrives and dropping

as the thundercloud passes. The referential value of the induced surface charge could be

roughly represented as a bell curve. The oppositely charged regions create an electric

field within the air between them. This electric field varies in relation to the strength ofthe surface charge on the base of the thundercloudthe greater the accumulated charge,

the higher the electrical field.

Fig 2.3 View of lightning from an airplane flying above a system.

2.5 Upward streamers

Fig 2.4 A lightning flash terminates & discharge occur on a tree while an un- attached

streamer is visible on the earth surface projection to the left.
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When a stepped leader approaches the ground, the presence of opposite

charges on the ground enhances the strength of the electric field. The electric field is

strongest on grounded objects whose tops are closest to the base of the thundercloud,

such as trees and tall buildings. If the electric field is strong enough, a positively

charged ionic channel, called a positive or upward streamer, can develop from these

points. This was first theorized by Heinz Kasemir. As negatively charged leaders

approach, increasing the localized electric field strength, grounded objects already

experiencing corona dischargeexceed a threshold and form upward streamers.

2.6 TYPES OF LIGHTNING STROKES

There are two main ways in which the lightning may strike thepower system . They are:

2.6.1. DIRECT STROKE

In the direct stroke, the lightning discharge (i.e. current path) is directly from the

cloud to the subject equipment e.g. an overhead line. From the line, the current path may

be over the insulators down the pole to the ground. The over voltages set up due to thestroke may be large enough to flash over this path directly to the ground. The direct

strokes can be of two types:

(i) Stroke A and (ii) stroke B.

(i) In stroke A:

The lightning discharge is from the cloud to the subject equipment i.e. an overhead

line in this case as shown in Fig 3.1 .The cloud will induce a charge of opposite sign on

the tall object eg. an overhead line in this case. When the potential between the clouds

exceeds the breakdown value of air, the lightning discharge occurs between the cloudand the line.

Fig 2.5Lightning from cloud
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(ii) In stroke B:

The lightning discharge occurs on the overhead line as a result of stroIj4 between

the clouds as shown in Fig. 3.2. There are three clouds P, Q and R having positive,negative and positive charges respectively. The charge on the cloud Q is bound by the

cloud R. If the cloud P shifts too near the cloud Q then lightning discharge will occur

between them and charges on both these clouds disappear quickly.

2.6.2 INDIRECT STROKE

Indirect strokes result from the electrostatically induced charges on theconductors due to the presence of charged clouds. This is illustrated in fig 2.6. A

positively cloud is above the line and induces a negative charge on the line by

electrostatic induction. This negative charge will be only on that portion of the line right

under the cloud and the portions of the line away from it will be positively charged as

shown in Fig 2.6. The induced positive charge leaks slowly to earth via the insulators.

When the cloud discharges to earth or to another cloud, the negative charge on the wire

is isolated as it cannot flow quickly to earth over the insulators. The result is that a

negative charges rush along the line is both directions in the form of travelling waves.

Majority of surges in transmission lines are caused by indirect lightning strokes.

Fig 2.6 Charge on conductor due to charged cloud


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CHAPTER 3

LFA

3.1THE LFA PRINCIPLE

When a lightning surge gets to an insulator, the insulator may flashover

depending on the overvoltage value and insulation level of the line. Probability of

power arc flow depends on many parameters: nominal voltage of the line Unom,

length of the flashover path L, moment at which lightning stroke occurred, lightning

current magnitude, line parameters, etc.

It was found that the main factor, which determines the probability of PAF, is themean gradient of operational voltage along the flashover path.

E = Uph/L

L= length of flashover,

The probability of PAF sharply decreases with a decrease in E. An analysis of

available data on spark over discharge transition to PAF concluded that for E=7 to 10

kV/m probability of PAF is practically zero. The flashover length, L is greater for

lines with wooden structures rather than steel or concrete structures, because wooden

Cross-arm increases the flashover path. As a result probability of PAF for wooden

structures is sufficiently lower than for steel or concrete supports. From the short

analysis presented above, it is clear that it is possible to improve the protection against

lightning by increasing the length of lightning flashover path. The suggested LFA

accomplishes this principle. The LFA's length may be several times greater than that

of an insulator . Due to a special inner structure the LFA impulse flashover voltage is

lower than that of the insulator and when subjected to lightning overvoltage the LFA

will flashover before the insulator.


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3.2 DESIGN OF LFA-M

Fig 3.1 Design of LFA

An LFA-M arrester consists of two cables like pieces. Each cable piece

has a semi conductive core of resistance R. The cable pieces are arranged so as to

form three flashover modules 1,2,3 as shown in figure1.Semiconductive core of upper

piece, whose resistance is R ,applies the high potential U to the surface of the lower

piece at its middle. Similiarly, the semi conductive core of the lower piece of the same

rsistance R applies the low potential 0 to the surfaces of the upper piece, also at its

center. 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 in to a single long flashover channel.

Tests have been shown that, as the line conductor is stressed by lightning

over voltage impulse, flash over channel develop at different rates.Modules1 and 3

flashover first, followed by module 2 ,and thus, forming a rather long flashover

channel along the LFA. Due to long flashover path, a flashover does not give rise to a

power arc as the arc extinguishes when the power frequency current crosses zero. This

assures uninterrupted power supply of a LFA protected over head line.


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3.3 FLASHOVER PERFORMANCES

The flashover performance of modular long flashover arresters of two

different flashover lengths and the voltage-time characteristics of LFA loop arresters,

as well as those of the most common Russian insulators ShF 10-G and ShF 20-G with

lengths 17 and 23 cm respectively were studied. The 50% flashover voltages of these

units are 130 and 160 KV when stressed by 1.2/50 lightning impulses of negative

polarity. therefore these units will be referred hereafter as INS 130 and INS

160,respectively.

The voltage-time characteristics of the arresters and insulators can be approximated by

the expression

U=a tb

Where U=flashover voltage in kilovolt.

t=time to crest in microseconds.

a,b are empirical coefficients whose values are given in the

Table 1.Empirical coefficient values

test object impulse polarity A b

insulator ins130 + 190 -0.352

insulator ins 130 - 185 -0.285

insulator ins160 + 243 -0.407

insulator ins160 - 280 -0.28

LFA-M,L=1m +,- 109 -0.784

LFA-M,L =2 m +,- 173 -1.05

LFA-M,L=0.8m + 159 -0.5

LFA-M,L =0.8m - 107 -1.64


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3.4 PROTECTION AGAINST DIRECT LIGHTING STROKES

The physical phenomena associated with a direct lighting stroke on an

unprotected power line causing line tripping. The general pattern is as follows. For an

overhead line in delta configuration shown in fig, 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. A fairly high voltage drop develops at the points where the lines equivalent

resistance equals half of the surge impedance Zs/2; this point is the closest to insulator

unit of the lightning struck phase conductor. 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.

3.5 PROTECTION USING LFA-M

It follows from the previously described sequence of events that direct

lightning stroke causes flashover of all the insulators on the affected pole. 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 reducedand it is given by the following equation.

Pfail = exp

9

40

ho

ho= the pole height in meters. It follows from the previously described sequence of

events that direct lightning stroke causes flashover of all the insulators on the affected

pole. 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.


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Pfail = exp

9

40

ho

Where, =protection angle between the top and bottom phases in degrees

ho= the pole height in meters.

Fig 3.2 Protection using lfa

For example, for ho = 10 m and = 300, the probability of a lightning

stroke on the outer phases can be as slow as 0.001. 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

than applied to the top phase insulator consequently, an outer phase arrester

operates under much easier coordination conditions than a top phase arrester. With

one orboth 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.

L= Ul/Ecr


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Where Ul is the maximum operating line voltage Ecr is the critical gradient of the

power frequency voltage that rules out PAF.

3.6 CRITICAL GRADIENT AND LENGTH OF LFA-M

Fig 3.3 characteristic between critical gradient & fault current

Some results of published experimental studies on the critical gradient are

shown in fig3.3. As seen in fig 3.3. the critical gradient depends greatly on the line

fault current. As the fault current increases from 20 300A, the critical gradient drops

abruptly from 207kV/m. The rate of decrease of the critical gradient slows down forlarger fault currents. Over the 1000-10000A fault current range, the critical gradient

decreases from 5-4kV/m. Phase-to-phase faults on a pole can give rise to fault current

in order of a few kilo ampers. Therefore the critical gradient can be assumed to be

4kV/m. for a 10kV power line operating at maximum voltage (20% higher than

nominal), the total flash over length is equal to L=12/4=3m. With 1-m flash over

length of the top center phase LFA, the length of the LFA protecting an outer phase

must be 2m.


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3.7. PERFORMANCE ANALYSIS OF DIRECT LIGHTNING

STROKE PROTECTION

The direct lightning performance of the modular arresters was carried out

using the equivalent circuits of fig .The arresters are connected between the pole and

all of the phase conductors in parallel with the insulators. The arresters are assumed to

be variable resistors whose resistance changes step wise from infinity to zero, in steps

of, R , R/2, 0.

Fig 3.4 Performance analysis of direct lightning stroke protection

where LA o is the length of one cable piece of phase A arrester, LFA A,

and Rl is the per-unit- length resistance of the cable. As tests have shown, module 3 of

phase a arrester will usually flash over after module 1. At this instant t2, the resistance

RAO of the second cable piece gets connected in parallel with the resistance of first

piece and the total equivalent resistance of the arrester becomes RA =RAO/2. 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


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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. Therefore, starting from instant t3 the arrester resistance RA is zero a

lightning stroke on the phase A include voltages on the outer phases B & C . However,as shown by calculations, until arrester LFAA flashes over, none of the modules of

phase B arrester, LFAB, flash over. In other words , in time interval t3 < t t4 when

LFAA flashes over completely, none of the LFABs modules are affected. The effect of

the power frequency voltage of a 10-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. Phase B and C

arresters are represented by a variable resistance RB/2 while the surge impedance of

phases B and C on both sides of their arresters are represented by resistance ZS/4

where ZS is the line conductor surge impedance. The pole inductance is replaced by

the concentrated inductance is the per-unit-length inductanceof the pole and hpole

is

the pole height.


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CHAPTER 4

PROTECTION

4.1 FLOW CHART OF LIGHTNING OVERVOLTAGE

PROTECTION

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. The calculation is carried out

for a linear increase of the lightning current, that is

I1=Il1

t

Where Il1

t is the lightning current steepness and it is the time

Time is incremented in equal steps t. The equivalent EMF e is calculated as follows

e = I1Z1

where Z1 is the surge impedance of the lightning channel .

The next step is to calculate flashover voltages for the individual dischargecomponents or modules. Initially the equivalent resistance of phase A and B arresters

respectively is infinitely large.

The channel lengths in the LFAAs modules are compared to the modules

lengths. 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 flashoverbased 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 at which the insulator flashed

over. If insulator InsA does not flashover, the calculation restarts at a new time step of

where ti and ti+1 are the instants of iterations i and i+1, respectively. After a module of

the LFAA flashes over across resistance RA, the pole reactance Xpole and Rg get

involved, and the voltage and the rate of voltage rise on InsA, LFAA, InsB, and LFAB

are calculated. The rate of channel propagation on arrester modules is determined, and

the modules are checked for flashovers. In case of a flashover, the respective

resistance RA and/or RB is changed. Finally, the calculation is checked for completion.

If both the LFAA and LFAB arresters flashed over the lightning protection system

performed successfully: If a flashover occurred on at least one of the insulators InsA or


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InsB. 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.

4.2 VOLTAGE-TIME CHARATERISTICS OF ARRESTERS

AND INSULATORS

Fig 4.1 Voltage & time characteristic

Fig. 4.1gives typical voltage versus time curves for phase A and B

insulators and arresters. It is seen from Fig. 9.1 that until module 1 of the LFA A

arrester flashes over (t t1) the rate of rise for phase A voltage is quite high. When

module 1 flashes over at instant t1, the voltage first drops abruptly but insignificantly

and then it starts increasing but at a slower rate in the t1< t t2 interval. When module

3 flashers over at instant t2, the voltage drops abruptly again and then, over the t 2 < t

t3 interval. It keeps increasing at a still slower rate until time t3. At t 3. Phase A voltage

curve crosses the VTC curve of the LFAA arrester and the second (middle) module of

the LFAA flashes over. (i.e., the arrester is now fully flashed over and the voltage

drops on both the insulator and phase A arrester). At instant t3 an opposite polarity

surge takes rise on insulator InsB and arrester LFAB of phase B. After the LFAA fullyflashed over, the lightning current travels through the pole and its footing. Thus, the


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voltage on phase B rises at a much slower rate than on phase A before the LFA A

flashed over. The pattern of voltage rise on the LFAB is similar to that on the LFAA

but features a slower rate of rise. At instants t4, t5, and t6 the first, third, and second

modules of arrester LFAB

flash over, respectively, changing the resistance of the

arrester. Fig.9.1 shows that the VTC of insulators and arresters cross at relatively

small times to crests tcr For a line using INS160 insulators and phase A LEFs with a

flashover lengthLa= 1m. the critical time is tcr.t = t3 =0.3 s. The average span length

Ispan of a 10-kV line is usually about 70m. The travel time of a reflected wave from

the nearest pole to the lightning-struck pole is given by Ttr = (Ispan+Ispan)/vs

=(70+70)/300 = 0.5 s.

Where Vs = 300m/s is the speed of propagation of an electromagnetic

surge along the line. Thus, Ttr is larger than Tcra and a voltage surge reflected from

the nearest pole comes to the lightningstruck pole only after the arrester has operated

or the insulator flashed over. Therefore, the nearest pole is not to be taken into account

in the coordination analysis of the LFAA. For a line using phase B arresters with a

flashover length lb = 2 m the critical time is Tcbr = t6 0.8 s (i.e., a voltage surge

reflected from the nearest pole will be able to reach the lightning struck pole and

lower the voltage applied to insulator InsB and arrester LFAB). 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. Fig. 7 puts in evidence a lightning protection hazard of steep lightning over

voltages. The voltage rate of rise U is proportional to steepness of the lightning

current. This is the reason why the calculation takes into account the critical values ofthe lightning current steepness at which the insulator flashes over for a given set of

parameters.

4.3 EFFICIENCY OF LFA-M

Fig4.2 Shows the critical lightning steepness. It can be clearly seen that as the grounding resistance increases, the critical lightning current steepness Il,cr

decreases.

Fig. 4.2 Critical lightning current steepness


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The number of lightning outages n o caused by direct lightning strokes

(DLS) on conductors of an unprotected line can be estimated by the following

equation n0=NDLS P( Il)Parc(1-Prc).Where Ndls is the number of direct lightning stroke

on a line P (Il) is the probability of lightning current likely to cause flashovers of the

line insulation. Parcis the probability of a power are caused by an impulse flashover an

insulator and Prc is the probability of successful line breakers enclosures.

It is shown that the steepness and not the magnitude of lightning current ILl Il is the

important factor in the performance if a LFA protected line thus(1) can be written in

the following form.

n= NDLS p( Ilcr)Parc(1-Prc).

Where n is the number of lightning outages on an LFA protected line caused by direct

lightning strokes on the phase conductors and P (Ilcr) is probability of a lightning

current with steepness greater or equal to Ilcr.The efficiency of LFA lightning

protection against direct lightning strokes can be expressed as the ratio of the numberof lightning outages n0 for unprotected line to n for lines protected by LFA.

K =

n

n

1

0

0= I crlp

1

,

1

Where k is the outage reduction factor of lightning outages caused by direct lightning

strokes.


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CHAPTER 5

REDUCTION FACTOR

5.1. GROUNDING RESISTANCE

Figure 4.2 shows the outage reduction factor of a line protected by LFA

10-M arresters (IA=1M; LB=LC=2M) versus the grounding resistance for the INS 160

and INS 130 insulators. A line with LFA arresters and INS 160 insulators is shown to

have a good lightning protection performance for direct lightning strokes. For

grounding resistance Rg = 10. LFA 10-M arresters assure a 200-fold decrease oflightning outages, virtually ruling them out. As the grounding resistance increases the

outage reduction factor k decreases faster up to Rg = 50 and then more slowly . for

Rg = 50 . K is approximately equal to 20 and for Rg = 80 , k= 10. 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.In the case of INS 160 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 INS130 insulators the number of lightning outagesis 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. LFA A is

coordinate with an INS130 insulators at much lower values if the lightning current

steepness than with an INS160 unit. It was also shown that, after the LFA a 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 100- 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.


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CHAPTER 6

FUTURE

6.1. FUTURE EXPANSION OF LFA

The LFA-M described here consists of three flashover modules. 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.


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CHAPTER 7

7.1. CONCLUSIONS

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 apole.

The conditions for the efficient protection of a medium voltage (e.g. 10-kv) overhead

line against direct lightning strokes, are as follows:

i. Delta phase configuration of phase conductorii. Mounting of LFA-M arresters on all poles in parallel with each insulators ;

iii. A relatively short flashover path (for example, 1 m for a 10-kv line) for the top

phase LFA-M arrester

iv. A longer flashover path (for example 2 m for a 10-kv line) for the bottom phase

LFA-M arresters


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REFERENCES

1).IEEE TRANSACTIONS ON POWER DELIVERY VOL.18,NO 3,JULY 2003

2) .PRINCIPLES OF POWER SYSTEM BY V.K. MEHTA

ROHIT MEHTA

3). www.ieee.org

4) http://ws9. Iee.usp.br/sipdax/paper six /session 05/5.6 pdf

5) www.seminars.com
http://ws9/http://ws9/http://ws9/


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ACKNOWLEDGEMENT

I express my sincere gratitude to Asst. prof. Mr. Manoj Khuswha Prof. &

Head, Department of Electrical. VCE College of Engineering, Bijnor for his

cooperation and encouragement. I would also like to thank my seminar guide ProfMr.

R.S Pal (Lecturer, Department of EEE), Asst. Prof. Mr. Lokesh. ( Department of

EEE) for their invaluable advice and wholehearted cooperation without which this

seminar would not have seen the light of day.Gracious gratitude to all the faculty of the

department of EEE & friends for their valuable advice and encouragement.


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Documents

Seminar report on protection of lightning arrester