what is lightening? lightening wave shape? theories about lightening lightening protection lightening arrester MOV metal oxide varistors

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Lightening arrester
Concept of lightening:
“Lightening is a visible discharge of static charges between clouds and
earth, between clouds and between the charge centers of same cloud. “
There are different theories that explain how charge is formed in clouds. According to one theory
different kind of ice interact with each other that results in the formation of positive charge on the upper
part of cloud and negative charge on the bottom of cloud. According to second theory warm air moves
upward and due to friction between air and water droplets produces charges on water droplets when
droplets accumulate they
foam cloud carrying
either positive or
negative charge
depending upon the
charge on droplets.
Charge carrying clouds
induces equal opposite
charge on earth or on
tallest grounding object
due to electrostatic
induction. As the
accumulation of charges
increases,voltage stress
on air increases. If
voltage stress crosses the
break down limit of air,
discharge occurs by ionizing the air. It produces luminance and also sounds. This phenomenon is called
lightening.
How the phenomena of lightening occur?
Step 1:As air warms up, it moes up and due to friction clouds get charged as shown in figure.

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There may be more than one charged centers on a cloud as shown in figure.
Clouds may get positive charged but 90 to 95% lightening strokes are due to negative charged strokes and
only 5 to 10% are due to positively charged strokes.
Step 2: due to electrostatic induction clouds create opposite charge on earth.
Note: We will talk about only negative charges because mostly lightening strokes are due to negative
charges.
Step3: as the charged on the cloud increases,the potential difference between cloud and earth increases. If
voltage exceeds the break down voltage of air, air will ionized and provide path to negative charges to
move towards the earth as shown.

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Branches eliminates in mid air
Earth
 Break down strength of air is 30KV/cm in atmospheric air and ~10KV/cm in the presence of
moisture.
 First discharge called leader
discharge also known as
stepped leader because it
moves down ward in discrete
step of 50m to 100m. Stepped
leader moves down ward with
speed 5𝑥105 m/second and a
current of some 100A.
 It may feelthat lightening stokes occur in one step but lightening occurs in steps and branches.
These branches eliminates in the mid air while the main channel moves downward to the earth in
zigzag manner. If we use a high sped camera and takes photographs of lightening it will look like
following figure.
 If you note that stepped leader is not continuous but moves downward in jumps or in steps. In other
words the ionization of intervene air is not continuous and instantaneous but discontinuous.
 also note that dart leader is continuous.

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 It is
not necessary
that lightening always fall on earth if potential gradient E does not provides enough trust to charges
then the main channel also disappear in the air as shown in figure.
 It has been seen that after first
lightening stroke many other strokes occur that follow the same path
as by the first stroke and they are brighter and none branched.
 The leader of Second and subsequent strokes are called “dart
leader” because it moves 10 times faster than stepped leader. Dart
leader are usually brighter and none branched.
Dart leader Returned stroke

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 Step 4: As stepped leader approaches towards earth,distance between negative charge centre and
ground decreases due to which electric field intensity (E=V/d) increases. If any high earthed object
(building, tower or any wire) is present and electric field intensity is high enough then upward
positive discharge will occur that move upward by following the same path as stepped leader. It is
called returned stroke as shown. Return stroke and stepped leader collides producing huge
amount of lightening and sound. It neutralize the charge centre on the clouds such collision occur
many times between dart stroke and return stroke producing large number of lightening flashes.
This complete process of successive strokes is called “lightening flash”
 Return stroke moves with much high speed (50𝑥105 m/sec) and a current of some 250KA.
 Temperature of the lightening stroke channel is about 15000ºC to 20000ºC.

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 During lightening air expands due to high temperature that pushes the nearby air causing storms.
 Lightening discharge may have current in the range of 10KA to 90KA.
 Lightening strokes from cloud to earth are very rare only 10% of lightening strokes. Majority of
discharges take place between clouds.
 To find the energy of lightening stroke let assume
that potential difference between cloud and earth is
95KV and total charge is 50columbs then as W=VxQ
so energy =95000x50=4.75MJ=1.32KWh.
 Shielding wire damaged due to lightening flash is shown in figure.

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Lightening wave shape:
 A large number of experiments determine the Lightening wave shape. Lightening wave is
characterized by two parts as shown in figure. One is wave front (from start point to it peak
value) and other is wave tail (from its peak value to half of it peak value in decay
side) Time to reach its peak value is denoted by 𝑡1and the time is to reach the half of its peak
value in wave tail as 𝑡2 as shown in figure. 𝑡1 is ranges from 1 to 10µsec and 𝑡2 ranges from 50 to
1000µsec
 International standard lightening impulse voltage is define as 1.2/50µsec means 1.2µsec is the
time to reach the peak value and 50µsec is the time to reach half of its peak value in the wave tail
i.e.;
𝑡1 = 1.2µsec ±30% tolerance
𝑡2 = 50µsec±20% tolerance
 Standard peak value of lightening wave is 95KV i.e Em = 95KVas shown in figure.
 Lightening produces step rising voltage and the rate of rising of voltage is from 20KA/µsec to
100KA/µsec
 When such lightening falls on transmission line, produces two travelling waves that moves in
both direction.
Lightening strokes on overhead line:
Lightening can fall on over head transmission line in two possible ways
1) Direct stroke
2) Indirect stroke
Direct stroke:
 In direct stroke lightening directly fall on the over head transmission line producing two
travelling waves moving towards the end poles of transmission line as shown. If there is
lightening arrester at the end of transmission line, lightening wave will move to ground
through tower without any damage provided that tower footing resistance is low.
 Critical flashover voltage(CFO) of self restoring insulator ( that can recover its insulation
property after fault means no damage after flashover) is the voltage where insulator has
50% probability of flash over for a standard lightening wave i.e. 1.2/50µsec.CFO and

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BIL are used interchangeably but there is a slight difference between them. BIL is the
voltage at which there is a 10% probability of flash over for standard lightening wave.
Travelling wave Travelling wave of lightening
Lightening may fall in other manner as shown. There are three clouds P,Q and R carrying charges
positive, negative and positive respectively. If discharge occurs between cloud P and Q then charge on R
will suddenly free and discharge rapidly to earth instead of transmission line. In this case lightening will
fall on earth ignoring tall object.

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Indirect Stroke:
In this case positively or negatively charged cloud induces opposite charge on over head line as shown in
figure.
 Such case occur when lightening is near to over head line
 Positive charge cloud induces negative charge on line and positive charge at the end of
transmission line. This trapped accumulated charge at the end of transmission line will leak to
earth slowly through insulator.
 If charge on cloud discharges to another cloud or to earth, negative charge at the mid of
transmission line is now free. This negative charge will flow towards end of transmission line in
the foam of travelling wave.
 Most of the lightening faults are due to this reason.
Lightening Protection on Overhead Line:

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 To protect the overhead lines we use a wire called “ground wire” or “shielding wire”.
 It cross sectional area is half of the phase conductor.
 It is placed at the top of phase conductor. The reason of it is same that lightening will fall on
tallest ground object which is shielding wire in this case.
 Shielding conductors are placed at the top of phase conductors in such a way that all lightening
will fall on shielding conductor instead of phase conductors.
 Shielding conductors are grounded at each pole end.
 Note in case of overhead line, protection against lightening can also be provided by decreasing
the footing resistance of the tower. If the footing resistance is high it will produce high voltage at
the end of tower causing the line insulator to flashover or puncture causing the outage of line. It
will produce arcing between shielding conductor and phase conductor. It can be removed by
reenergizing the line. This phenomenon is called “backflashover”.it usually occurs when footing
resistance is high.
 No of shielding conductors depends upon voltage level and level of protection.
Lightening fall on the pole:
 Suppose that footing resistance of the tower is 20Ω and if lightening fall on the tower with
current 22KA .some of the current will flow through shielding wire and remaining current
will flow through tower. Suppose 1KA current flow through each shielding wire and
remaining 20KA current will flow through tower then the voltage between tower and phase
conductor will be V=20kAX20Ω=400Kv.
 This voltage will appear across insulator of line.
 If this voltage is greater than the flashover voltage of insulator, insulator flashover will occur.
 If the footing resistance is 40Ω then voltage of tower will be V=20kAx40Ω=800kV.so
footing resistance greatly effect the voltage rise of tower and determines the level of
protection against lightening.
 Tower resistance should be 20Ω or less than it.
Groundingresistance

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Lightening fall on the mid of transmission line:
10000A Z=400Ω
 Suppose that lightening stroke current is about 10000A fall on a transmission line with surge
impedance of Z=400Ω. Half current will flow in both direction so the voltage rise will be
V=5000x400=2MV.
 Note here the voltage rise depends upon the surge impedance of the transmission line.
Conclusion: So to avoid overvoltage due to lightening keep
1) Surge impedance low.
2) Footing resistance of tower low.
5000A5000A

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Lightening Arrester
“Lightening arrester is an electrical protective device that is used to protect the
electrical equipment from lightening by diverting the lightening surges to ground”
Transformer without lightening arrestor when lightening falls transformer burns out
When lighteing fall transformer is saved
Transformer with lightening arrestor

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 Modern types of surge arrester are made of metal oxide varistor (MOV).
 MOV are highly non linear resistors.
 Under normal voltage the resistance of MOV is very high and acts as an open circuit and draws
current in miliamperes
 Wattage consumed by MOV is very small usually 0.05W/Kv
 One of the most important feature of MOV is that it resistance decreasesto few ohms
instantaneously for high voltages and after voltage surge instantaneously return to its high value.
 Metal oxide varistors are zinc oxides.
 In MOV disk there are millions of conductive grains of zinc oxide that are separated by small
distance as shown.
MOV disk
Doping material
Zinc grains
MOV disks
Lightening arrestor

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Current path during lightening
These gaps between zinc conductive grains behave like switches that activates when lightening
falls.
 Spark gap was one of the oldest devices used to protect equipment against lightening. The
principle of gap type lightening arrester was same as that of MOV. Under normal condition gap
act as open circuit. When high voltage surge occur, air between gap ionizes causes gap to short
circuit. But the basic problem with this lightening arrestor was that after the surge has removed
but gap is still short circuited due to ionized air because once the air is ionized very small voltage
is required to keep the ionized path alive. So we have to operate the circuit breaker. Later
modification was made in gap arrester by inserting non linear resistance in series with spark gap.
Resistance of non linear resistance increases to a high value when voltage decreases so current
becomes very small that makes the gap open again.
 Arrester must be placed as closed as possible to the equipment to protect

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Lightening arrestor.
Fuses of transformer
Lightening arrestor
Transformer
Lightening arrestor
Transmission line

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 Every two terminal device must be protected by lightening arrestor like regulators, reclosure,
isolators or switches.
 Arrestor across recloser is very important. Recloser opens when lightening fault occur so surge
reaches open point will double so lightening arrester must be there for the safety of recloser.
 Usually capacitor banks are not protected with surge arrestor because capacitor also act as surge
arrestor but direct lightening sometime fail capacitor to operate.
 Surge arrestor does not stop or absorb the lightening surges. It just divert the lightening surges to
ground.