The document discusses overvoltages in electrical power systems, specifically focusing on lightning and switching surges. It provides details on the mechanism of lightning, including how charges are formed in clouds and the processes of pilot streamers and stepped leaders. Characteristics of lightning strokes like current, rate of rise, and surge voltages are examined. The origins and characteristics of switching surges during operations like line energization are also covered. The document concludes with methods to control overvoltages through line design practices, surge diverters, and protective devices.

1. EE 8701 HIGH VOLTAGE
ENGINEERING

2. UNIT - I
OVER VOLTAGES IN ELECTRICAL
POWER SYSTEMS

3. 3
LIGHTNING
 Causes of over voltage
 Lightning phenomenon
 Charge formation of Lightning
 Rate of Charging of thunder cloud
 Mechanism of lightning strokes
 Characteristics of Lightning strokes

4. 4
LIGHTING
 Factors contributing to good line
design
 Protection afforded by ground wires.
 Tower footing resistance
 Interaction between lightning and
power system
 Mathematical model of Lightning

5. Why study about lightning ?
5

6. 6
Causes of Lightning
 Lightning phenomenon
- peak discharge in which charge
accumulated in the cloud into
neighbouring cloud or to the ground
 Electrode separation – cloud to cloud
or cloud to ground is about 10 km or
more

7. 7
CHARGE FORMATION OF CLOUD
 Positive and negative charges
become separated by heavy air
current with ice crystals in the upper
part and rain in the lower region.
 Charge separation depends on
height of cloud (200 – 10,000m).
 Charge centers at a distance about
300 – 2km

8. 8
CHARGE FORMATION OF CLOUD
 Charge inside the cloud – 1 to 100 C
 Cloud potential – 107 to 108 V
 Gradient within a cloud – 100 V/cm
 Gradient at initial discharge point –
10kV/cm
 Energy at discharge – 250 kWhr

9. 9
CHARGE FORMATION OF CLOUD

10. LIGHTNING
PHENOMENON
 Lightning phenomenon is a peak discharge in which charge
accumulated in the clouds discharges into a neighbouring cloud
or to the ground.
 The electrode separation, i.e. cloud-to-cloud or cloud-to-ground
is very large, perhaps 10 km or more.
 The mechanism of charge formation in the clouds and their
discharges is quite a complicated and uncertain process.
 Nevertheless, a lot of information has been collected since the
last fifty years and several theories have been put forth for
explaining the phenomenon.

11.  During thunderstorms, positive and negative charges become
separated by the heavy air currents with ice crystals in the
upper part and rain in the lower parts of the cloud.
 This charge separation depends on the height of the clouds,
which range from 200 to 10,000 m, with their charge centre's
probably at a distance of about 300 to 2000 m.
 The volume of the clouds that participate in lightning flashover
are uncertain, but the charge inside the cloud may be as high as
1 to 100 C.
 Clouds may have a potential as high as 107 to 108 V with field
gradients ranging from 100 V/cm within the cloud to as high
as 10 kV/cm at the initial discharge point.

12.  The energies associated with the cloud discharges
can be as high as 250 kWh.
 It is believed that the upper regions of the cloud are
usually positively charged, whereas the lower region
and the base are predominantly negative except the
local region, near the base and the head, which is
positive.
 The maximum gradient reached at the ground level
due to a charged cloud may be as high as 300 V/cm,
while the fair weather gradients are about 1 V/cm.

14.  According to the Simpson's theory (Fig. 8.2) there are
three essential regions in the cloud to be considered for
charge formation.
 Below region A, air currents travel above 800 cm/s, and
no raindrops fall through.
 In region A, air velocity is high enough to break the falling
raindrops causing a positive charge spray in the cloud and
negative charge in the air.
 The spray is blown upwards, but as the velocity of air
decreases, the positively charged water drops recombine
with the larger drops and fall again.
 Thus region A, eventually becomes predominantly
positively charged, while region B above it, becomes
negatively charged by air currents.
 In the upper regions in the cloud, the temperature is low
(below freezing point) and only ice crystals exist .
 The impact of air on these crystals makes them negatively
charged, thus the distribution of the charge within the
cloud becomes as shown in Fig.

16.  Reynolds and Mason proposed modification,
according to which the thunder clouds are
developed at heights 1 to 2 km above the
ground level and may extend up to 12 to 14
km above the ground.
 The air currents controlled by the
temperature gradient move upwards carrying
moisture and water droplets. The
temperature is O0C at about 4 km from the
ground and may reach - 5O0C at about 12
km height. But water droplets do not freeze
as soon as the temperature is O0C.

17.  They freeze below - 4O0C only as solid particles on
which
crystalline ice patterns develop and grow.
The larger the number of solid sites or nuclei
present, the higher is the temperature (> -4O0C) at
which the ice crystals grow.
 Thus in clouds, the effective freezing temperature
range is around - 330C to - 4O0C.
 The water droplets in the thunder cloud are blown up
by air currents and get super cooled over a range of
heights and temperatures.
 When such freezing occurs, the crystals grow into
large masses and due to their weight and gravitational
force start
 moving downwards.
 Thus, a thunder cloud consists of super cooled water
droplets moving upwards and large hail stones moving
downwards.

18. 18
MECHANISM OF LIGHTNING FLASH
 Pilot streamer and Stepped leader
 Ground streamer and return stroke
 Subsequent strokes

19. 19
PILOT STREAMER AND STEPPED
LEADER

20. 20
GROUND STREAMER AND RETURN
STROKE

21. 21
CHARACTERISTICS OF LIGHTNING
STROKES
 Current-time characteristics
 Time to peak or Rate of rise
 Probability distribution of current and
time
 Wave shapes of lightning voltage and
current

22. 22
LIGHTNING CURRENT
 Short front time - 10µs
 Tail time – several ms.

23. 23
RATE OF RISE
 50% lightning stroke current –
greater than 7.5kA/µs.
 10% lightning strokes current –
exceeds 25 kA/µs.
 Stroke current above half value –
more than 30µs.

24. 24
SURGE VOLTAGE
 Maximum surge voltage in
transmission line – 5MV
 Most of the surge voltage is less than
1000 kV on line.
 Front time – 2 to 10 µs
 Tail time – 20 to 100 µs
 Rate of rise of voltage – 1MV/ µs

25. 25
LIGHTNING STROKES
 Direct stroke
directly discharges on to
transmission line or line wires
 Induced stroke
cloud generates negative charge at
its base, the earth object develop
induced positive charge

26. 26
OVER VOLTAGE DUE TO SWITCHING
SURGES
INTRODUCTION
 In switching, the over voltage thus
generated last for longer durations and
therefore are severe and more dangerous
to the system
 The switching over voltages depends on
the normal voltage of the system and
hence increase with increased system
voltage

27. 27
ORIGIN OF SWITCHING
SURGES
 Making and breaking of electric
circuits with switchgear may results
in abnormal over voltages in power
systems having large inductances
and capacitances.
 over voltages may go as high as 6
times the normal power frequency
voltage.

28. 28
ORIGIN OF SWITCHING
SURGES
 In circuit breaking operation switching
surges with a high rate of rise of voltage
may cause repeated re-striking of the arc
between the contacts of a circuit breaker,
thereby causing destruction of the circuit
breaker contacts.
 Switching surges may include high natural
frequencies of the system, a damped
normal frequency voltage component, or
re-striking and recovery voltage of the
system with successive reflected waves
from terminations.

29. 29
CHARACTERISTICS OF
SWITCHING SURGES
 De-energizing of transmission lines, cables,
shunt capacitor, banks, etc.
 Disconnection of unloaded transformers,
reactors, etc.
 Energization or reclosing of lines and
reactive loads.
 Sudden switching off of loads.
 Short circuit and fault clearances.
 Resonance phenomenon like ferro-
resonance, arcing grounds, etc.

30. 30
CONTROL OF OVERVOLTAGES
DUE TO SWITCHING
 Energization of transmission lines in one or
more steps by inserting resistances and
withdrawing them afterwards.
 Phase controlled closing of circuit
breakers.
 Drainage of trapped charges before
reclosing
 Use of shunt reactors.
 Limiting switching surges by suitable surge
diverters.

31. 31
PROTECTION AGAINST
OVERVOLTAGS
 Minimizing the lightning over
voltages are done by suitable line
designs,
 Providing guard and ground wires,
 Using surge diverters.

32. 32
PROTECTION AGAINST
OVERVOLTAGS
 Shielding the overhead lines by using
ground wires above the phase wires,
 Using ground rods and counter-poise
wires,
 Including protective devices like
explosion gaps, protector tubes on
the lines, and surge diverters at the
line terminations and sudstations