EE 6002 – POWER SYSTEM TRANSIENTS

UNIT III - LIGHTNING TRANSIENTS

1. A. Interaction between lightning and the power system:-
 The study on reaction between the lightning and the power system equipment were carried out
vast number of time in all developed nations.
 It is concerned with evaluation, measurement, efficiency and statics.
 The objective of this study to overcome the interruption due to lightning, so to study the
interaction, a good place to start model of the lightning stroke.
 This can be expressed in the terms of circuit elements and can therefore be used in conjunction
with the circuit elements of the power system.
Development of lightning model
 Transmission tower receives the current by lightning stroke from a cloud and the current
disappearing into the ground.
 A useful concept here is to think of the cloud and earth as forming a vast capacitor which is
being discharged by the stroke.
 When a lightning striking a transmission power line, a current is injected into power system.
 The magnitude of current depends upon its wave shape and impedance through which it flows.

Lightning stroke from cloud to earth discharges a vast capacitor

Finding a surge impedance considering as a lumped circuit

The inductance ‘L’ is given by

The capacitance ‘C’ is given by C=

Ground wire:-
 Whenlightning strikes a power line and the tower, the impedance of the tower will be of
concern.
 The voltage drop down the tower will appear across the line insulation.
  If this excessive, flashover of insulation will occur and a fault will be placed on the system.
 In order to avoid this, one or more ground wires are often stung on the tower above the line
conductors.
Development of power system model
Er . griscorn construct quite a sophisticated model to represent the electricitycurrent flow due to
lightning in the power system.
There are two possible equivalent circuits for the elements involved in the pre-strike.
(ii) Equivalent circuit for lightning stroke to tower based upon travelling wave.

The pre-strike is initiated when the switch is closed, which causes charge in the leader based to
circulate in the circuit.
From this model tower top voltage potential can be determined.

(ii) simplified equivalent circuit-lumped parameter equivalent.

In this circuit, the equivalent to tower, ground and channel are replaced by lumped circuit
constants.
1. B. Transmission line design- important factors:-
The design of transmission line against lightning for a desired performance is practically
independent of operating voltage.
The basic principle underlying the design of a line based on the lightning theory is,
 Ground wires with sufficient mechanical strength must be located.
 Adequate clearance from the line conductor to the tower or to ground must be maintained. So
that fault effectiveness of the insulating structure can be obtained.
 Adequate clearances must be maintained, especially at the midspan, to prevent flashover to
the conductors up to protective voltage level used for the line design.
 Last, but equally as important, tower-footing resistance as low as economically justified must
be secured.
 The ground wire is shown placed high above the line conductors and is well out on the
tower.
 With this type construction, the ground wires are not placed directly over the conductors.
 When the protective angle does not exceed 300 good shielding of the line conductors is
obtained.
 The study and survey show that as this angle increases, the probability of flash to the
conductor increases.
 Low footing resistance can be met depends on the local soil conditions.
Meter ohms Foot ohms Centimeter ohms
General average 100 328 10000
Sea water 0.01-1.0 0.0328-3.28 1-100
Swampy 10-100 32.8-328 1000-10000
ground
2. A. Mathematical Model for lightning:
The lightning stroke consider as a current source.

Where, Zs – impedance of the lightning channel

Z – impedance of stroke strikes object
ZGW – impedance of ground wire
When a streamer discharge occurs to ground by first stroke, followed by main stroke.
If the lightning strikes an object of impedance Z, the voltage built across.
The magnitude of surge voltage is V =IZ
I s∗Z Z S
V=
Z + ZS
If a lightning stroke current as low as 10000A strikes line of 400Ω surge impedance, it may
cause over voltage of 4000kV. This will cause immediate flash over of the line conductor and
insulator.

2. B. Rate of Charge of Thunder Clouds:

Thunder clouds consist of uniform mixture of positive and negative charges. Due to air currents
and hail stones the charges separate.
If the generated charge Qg is being separated Qs at a velocity V, the rate of growth of the
electric field is
dE
+ λ=ρv ----------1
dt
Where, λ – effective air conductivity
v – velocity of separation of charges
ρ – charge density in the cloud

ρv
From equation 1, E= [ 1−e−λt ] ---------2
λ
Generated chargeQ g
Charge density ρ=
Cloud area A∗Height of cloud h
Qg
ρ= -------------------3
Ah
Separted charge Qs
Electric field intensity E=
Cloud area A∗Permittivty of air ε 0
Q
E= s ----------4
A ε0
Substitute the equations 3, 4 in equation 2
 Q s hλ
Q g= -----------5
v ( 1−e− λt )
Qg x h is the electric moment of inertia.
Mλ
Q g= --------------6
v ( 1−e− λt )

3. Mechanism of lightning stroke:-

 An electric discharge between cloud and earth, between clouds or within the same cloud is
known as “lightning”.
 The discharge of the charged cloud to the ground is called lightning phenomenon.
 The large spark accompanied by light produced by an abrupt, discontinuous discharge of
electricity through air, form the clouds generally under turbulent conditions of atmosphere is
called lightning.
 The clouds get charged during thunder-storms, the high potential gradient cause’s breakdown
of insulation of air production a lightning stroke.
 A lightning discharge through air occurs when a clouds is raised to such a high potential with
respect to the ground.
 The total potential difference between the two main charge centers may vary from 100 to
1000mv.
 As the lower part of the cloud is negatively charged, the ground gets positively charged by
induction.
Mechanism:
 When a charged cloud passes over the earth, it induces equal and opposite change on the earth
below.
 The charge acquired by the cloud increase, the potential between cloud and earth increases and
gradient in the air increases.
 The potential gradient is sufficient to breakdown the surrounding air, the lightning stroke starts.
 At this instant, a streamer called a “pilot streamer” starts from the cloud towards the ground
which is not visible.

 The current is 100 ampere and 0.15m/micro sec.

 This process continues until one of the leader strikes ground. When one of the stepped leader
strike the ground an extremely bright return streamer propagates upwards from ground to
cloud.
  After neutralization of the negative charge on the cloud any further from the nearest charge
centre. This streamer of discharge is called “dart streamer”.

 Cold lightning stroke:- the discharge current in the return streamer high it continues for few
micro seconds less energy. It contains less energy.
 Hot lightning stroke:- (Dart leader) – having less discharging current, contains more energy.

Wave shape of lightning stroke

Characteristics of lightning wave shape

  Crest or peak value and it have been observed that the max value of this current is 4, 00,000
amps.
 The wave front line which varies from 1 to 10 micro sec.
 The time at which the stroke current reduces to 50 percent of that crest value and its
estimated limit is 10 to 100µ sec.
Location:-
In the United States, for example Florida experience the largest no. of recorded strikes in a given
period.
Incidence of lightning to power lines.

4. Charge formation in the clouds

 During thunderstorms, positive and negative charges in the clouds separated by heavy air current
with ice crystal in upper part and rain in lower part.
 Charge separation depends on the height of the clouds, which range from 200 m to 10 km with
their charge centers at a distance of 300 m to 2 km.
 Charge inside the cloud 1-100 c, Max potential of a cloud 10 mv to 100 mv, Field gradients 100
v/cm -10 kv/cm
 Energy in a lightning stroke 250 kwhr, Max gradients at ground level 300v/cm,Fair weather
gradient is 1v/cm
Wilson’s theory of charge separation
 Wilson’s theory is based on the assumption.
 A large number of ions are present in the atmosphere
 Many of these ions attach themselves to small particles and water particles
 A normal electric field exists in the atmosphere under fair-weather condition this is generally
directed down ward towards the earth
 A relatively large raindrop (0.1cm radius) falling in this field becomes polarized
 On upper sides = acquires negative charge
 On lower sides = acquires positive charge
 According to Wilson theory
 Large negatively charged drops settle on base of clouds
 Smaller positively charged drops settle on upper positive of the clouds

Simson’stheory:
There are three regions A, B and C in the cloud to be considered for charge formation
 Below region A, air current travel above 800cm/s and no raindrops fall through
 Region B is negatively charged by air current
 Region C only ice crystal exist due to low temp
 The explanation presented on Samson’s theory is not satisfactory Recently, Reynold and mason
proposed modification

Reynolds and mason theory:

 Thunder clouds are developed at heights 1to2 km above the ground level and may extend upto
12 to 14 km above the ground
 Temperature is 0oc at about 4 km from the ground and many reach -50oc at 12 km height
 Water droplets don’t freeze at 10oc, they freeze below -40oonly
  Thunder cloud consisting super cooled water droplets moving upwards and large hail stones
moving downwards
 Splinter moved up by air currents and carry positive charge to upper region
 Hail stones that travel downwards carry an equivalent negative charge to the lower regions in
the clouds
 Once the charge separation is complete, the conditions are now solution for a lighting stroke

5. A. Protection of transmission lines against lighting strokes;

 Effective protection against direct strokes requires a shield to prevent lighting from striking
the electrical conductors, together with adequate drainage facilities over insulated structures
so that the discharge can drain to the ground without affecting the conductors
 The most generally accepted and effective method protecting lines against direct strokes is by
the use of overhead ground wires
 This method of protection is known as shielding method which does not allow an area path to
from between the line conductors and ground
Ground wires:
 Ground wires are conductors running parallel to the main conductors of the transmission line,
supported on the same towers
 Ground wires are made of galvanized steel wires or Aluminum conductor steel-reinforced
(ACSR) conductors
 Overhead ground wires protect the lines by intercepting the direct strokes
 So keeping them off the phase line conductors, and by providing multiple paths for conducting
the strokes current to ground
 The multiple paths for the stroke current result in a reduced voltage drop
 In addition to the above function, ground wires also reduces the voltages induced on line
conductors

Requirement of ground wire for effective protection:-

 There should be an adequate clearance between the line conductors and the ground or the
tower structure.
 There should be an adequate clearance between the line conductors and round wire. It’s come
under transmission line designing.
 It is very important that the tower footings resistance should be as low as possible so as to allow
lightning discharge current to flow to earth easily.
 Ground wire having sufficient mechanical strength must be situated in such a manner so as to
shield the line conductors from direct strokes.
 To avoid short circuit:The round wire s should be placed at such a height above the line
conductors that they should be well out on towers and should not be exactly over the conductor
in order to avoid any possibility of a short circuit occurring in the event of conductor swinging
under lie loading.
 Protective ratio: it is defined as the ratio of the induced voltage on a conductor with ground
wire protection to the induced voltage which would exists on the conductor without ground
wire protection.
 Protection angle or angle of protection: it is defined as the angle formed between the normal
passing through the ground wire.

Protective angle ‘α’ of an overhead ground wire

 Height of the ground wire:- for protecting against lightning, the ground wire should be located at a
height at least 10%
Equation for calculating the height

X= horizontal spacing between the conductor and ground wire

H=height of cloud
Y= height of ground wire
B=height of the conductor
Coupling factor:-
The ratio of the induced voltage on the conductor to the ground wire voltage is known as
coupling factor.
The coupling factor with the effect of corona considered is calculated by the equation
Coupling factor (c) =
Coupling factor is calculated by the equation

C=
C=coupling factor
A= distance from conductor to ground wires
B= distance from conductor to image of ground wire
H= height of ground wire above ground
R=radius of ground wire
Tower footing resistance: reduction
 The effectiveness of ground wire in control the lightning over-voltage on a line to reasonable
value depends to a great extent on lower footing resistance.
 It should be in the range of 10-20 ohm
 The tower footing resistance may be reduced by driving rods near the tower and connecting
them to the tower base or by burying conductors in the ground and connecting them to the tower
base.
 Driven rods are usually of galvanized iron, copper weld or copper bearing steel.
 Counterpoise wires are usually of copper, aluminum, galvanized steel.
 Depends upon the soil conditions either driven rods or counterpoise wires are used; both the
methods are satisfactory if the proper low resistance is obtained.
 Deep rods may be driven in soil free from rocks. Deep-driven rods can be successfully used in
the sandy soil.

5. B.Protection of stations and sub-stations from the direct strokes

  Whenever the direct strokes of lightning occurs, it is likely to strike the line at the station or
near the station, and there is possibility of exceeding high range of surge voltage rise and
large magnitude of surge current discharge.
 If the stroke is severe enough, the margin of protection provided by the protective device may be
inadequate.
 The station installation may, therefore require shielding of the station and the incoming lines
enough to limit the severity of surges particularly in the higher voltage range of 66kv and above.
 By means providing overhead ground wires, masts or rods in the proper place shield the station
from the lightning strokes.
Protection of station by overhead ground wires:-
 If supports are available for overhead ground wires, these may run over the station in such a way
that the station and all apparatus will lie in the protected zone.
 Sufficient ground wire should be stung so that the entire station is covered, including any
apparatus outside the main structure and any structure on top of the structure as depicted.
 The depicts the arrangement of overhead ground wires for small stations, it is sufficient to run
one or two ground wire across the station form adjacent line towers.

Protection of a station by vertical masts or RODS

If it is not feasible to run ground wires over the station, then it may be possible to erect
masts or rods at the corners or over vertical columns, so that the buses and apparatus in the station
will fall within the cone of protection of masts of rods

 If the line ab is represent a rod or mast with a height , then it is said to be protective cone around
the mast the radius on the base line is equal to ky and the protective ratio is ky/y=k