Lecture 1 - Introduction,Internal and External Overvoltages.pptx

EEE 2509_HIGHVOLTAGETECHNOLOGY
EEE 2509 - HIGH VOLTAGE TECHNOLOGY
1
 CHAPTER ONE: Introduction, Internal & External
Overvoltages By Mutegi M.

EEE 2509-Course Contents
2
Breakdown mechanism in gases, solids, liquids
Dielectrics: properties, effects of temperature, frequency, pressure, humidity and
voltage. Ionization process and decay. Flashover. Characteristics of liquid and plastic
dielectrics.
Corona: voltages, characteristics, gradient discharges and corona power loss.
Generation of high voltages: transformer over-voltages, behavior and distribution,
oscillations and surges.
Alternator under-voltage surges. Overhead lines: surges, wave propagation,
terminations and surge energy. Lightning and surge protection: shielding, resistance,
surge diverters, horn-gaps, arresters and surge modifiers.
Measurement of high voltages: sphere gaps, cathode ray oscilloscope, rectifier
condenser-current peak voltmeter, potential dividers and tesla coil. High voltage testing
equipment: transformers, direct current testing equipment and impulse generator. Non-
destructive insulation test techniques.

Chapter One: Introduction
3
 HV technology deals with the study and application of the various
phenomena occurring in various mediums as a result of high
potential/voltages.
 High voltage engineering ensures construction of reliable high voltage
insulation and equipment with minimum dimensions at low cost for
optimal energy transmission and distribution.
Causes of Over voltage in Power System
What is an Over voltage/Voltage surge/Voltage transient?
Overvoltage/voltage surge/transient is a sudden increase/rise in
voltage for a very short time (μs) in a power system.

Introduction
4
Causes:
i. Internal Overvoltages: Switching surges, insulation failure, arcing
ground and resonance
ii. External Overvoltages: Lightning
 Internal causes do not produce surges of large magnitude. Mostly 2
times the normal voltage of the system.
 Proper insulation is able to handle the internal over voltages.
 Surges due to lightning (external overvoltages) are very severe and
may increase the system voltage to several times the normal value.
 If the equipment in the power system is not protected, considerable
damage occurs.
 Lightning surges are usually diverted to the ground since insulators
are not designed to have these high voltages pass through them.

Internal Overvoltages
5
 Caused by overvoltages in the power system mainly due to oscillations
set up by the sudden changes in the circuit conditions.These sudden
changes may include a normal switching operation e.g. opening of a
circuit breaker, or a fault condition such as grounding of a line
conductor.
i. Switching Surges:
 There are three scenarios that are considered under switching surges.
a) Case of an open line:

6
 During switching operations of an unloaded line, travelling waves are
set up which produce overvoltages on the line.
 On reaching the terminal point A, it is reflected back to the supply
end without a change in sign.
 This makes the voltage double in magnitude.
 Supposing the RMS value of the voltage is , then the line should be
able to withstand a maximum voltage of .
 Overvoltage is of temporary nature. It is attenuated by the line losses
and settles down to the normal supply voltage.
b) Case of a Loaded Line
 Suppose a loaded line is suddenly interrupted.

7
 A voltage of shall be set up across the switch. is the instantaneous
value of the current at that time t when the switching operation takes
place. is the natural impedance of the line .
 Example: Given a line with Zn = 1000 Ω and carrying a current of
100 A (r.m.s.). Suppose the switching operation occurs when the
current is maximum, calculate.
I. The voltage across the breaker at that instance. (282.2kV)
II. Maximum voltage that the line is subjected to. (Vm + 282.2kV)
c) Current Chopping.
 Current chopping refers to the forceful interruption of current
before the natural current zero is achieved. A voltage of is induced.
Mainly occurs in air-blast circuit breakers.

8
 Example: Given the values of L and C as 4mH and 0·001 F
μ
respectively, calculate the voltage that would be induced for a current
chop of magnitude 50 A. (100kV).
 Solved by shunting the breaker using a resistance (resistance
switching) making part of the arc current flow through it.
 Strength of the arc resistance increases.

9
ii. Insulation Failure
 Most common case for insulation failure is the grounding of a
conductor.
 If a line at potential E is earthed at X, travelling voltages of –E travel
along XQ and XP causing currents and .
 Since both of these currents pass through X to the ground, the
current to ground is .

10
iii.Arcing Ground
 Incase of an ungrounded system, when a line to ground fault is
experienced, the arcing ground phenomenon occurs.
 The arcing ground produces severe oscillations of three to four times
the normal voltage.
 The transients produced due to arcing ground are cumulative and may
cause serious damage to equipment in the power system by causing
breakdown of insulation.
 Arcing ground can be prevented by earthing the neutral.
iv. Resonance
 Occurs when inductive reactance of the circuit becomes equal to
capacitive reactance.

11
 Impedance of the circuit is equal to resistance of the circuit and the
power factor is unity.
 Resonance causes high voltages in the electrical system.
 In the usual transmission lines, the capacitance is very small so that
resonance rarely occurs at the fundamental supply frequency.
 However, if generator e.m.f. wave is distorted, the trouble of
resonance may occur due to 5th or higher harmonics and in case of
underground cables too.

External Overvoltages
12
LIGHTNING
 Is an electric discharge between cloud and earth, between clouds or
between the charge centers of the same cloud.
 Occurs when clouds are charged to such a high potential (+ve or
−ve) with respect to earth or a neighboring cloud that the dielectric
strength of neighboring medium (air) is destroyed.
How do clouds acquire charge?
 During the uprush of warm moist air from earth, the friction
between the air and the tiny particles of water causes the building up
of charges.When drops of water are formed, the larger drops become
positively charged and the smaller drops become negatively charged.
When the drops of water accumulate, they form clouds, and hence
cloud may possess either a positive or a negative charge

13
Mechanism of Lightning Discharge
• When a charged cloud passes over
the earth, it induces equal and
opposite charge on the earth below.
• As the charge acquired by the
cloud increases, the potential
between cloud and earth increases.
• When the potential gradient is
sufficient (5 kV/cm to 10 kV/cm)
to break down the surrounding air,
the lightning stroke starts.

14
• Air near the cloud starts breaking
down, forming a streamer called
leader streamer or pilot streamer.
• The leader streamer will continue
its journey towards earth as long as
the cloud, from which it originates
feeds enough charge to it to
maintain gradient at the tip of
leader streamer above the strength
of air.
• If this gradient is not maintained
the leader streamer will not reach
the earth.

15
• The leader streamer continues its journey
towards earth until it makes contact with
earth or some object on the earth.
• The path of leader streamer is a path of
ionization and, therefore, of complete
breakdown of insulation.
• As the leader streamer reaches near the earth,
a return streamer shoots up to meet it.
• This action can be compared with the closing
of a switch between the positive and negative
terminals.
• This phenomenon causes a sudden
spark which we call lightning and a
resulting neutralization of much of the
negative charge on the cloud

16
Note:
 A lightning discharge which usually appears to the eye as a single flash
is in reality made up of a number of separate strokes that
travel down the same path. The interval between them varies
from 0·0005 to 0·5 second. Each separate stroke starts as a
downward leader from the cloud.
 It has been found that 87% of all lightning strokes result from
negatively charged clouds and only 13% originate from positively
charged clouds.
 It has been estimated that throughout the world, there occur about
100 lightning strokes per second.
 Lightning discharge may have currents in the range of 10 kA to 90
kA.

17
Types of Lightning Strokes:
i. Direct
ii. Indirect
i Direct Stroke
 Lightning discharge (i.e.current path) is directly from the cloud to the
subject equipment e.g. an overhead line over the insulators down the
pole to the ground.Are very rare.

18
ii Indirect Stroke
 Result from the electrostatically induced charges on the conductors
due to the presence of charged clouds-more common.
 Example: In the diagram below, the negative charge will only be on
that portion of the line right under the cloud and the portions of the
line away from it shall be positively charged.

19
 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 negative charge rushes along the line
in both directions in the form of travelling waves.
Harmful Effects of Lightning: Some explanation…
 Produces a steep-fronted voltage wave on the line.The voltage of this
wave may rise from zero to peak value (perhaps 2000 kV) in about 1
s and decay to half the peak value in about 5 s.
μ μ
 Travelling waves shall be initiated along the line in both directions
with the velocity dependent upon the L and C parameters of the line.
 Example: 1/50 s surge reaches its maximum value in 1 s and
μ μ
decays to half of its peak value is 50 μs.

20
 Waveform of aTypical Lightning Surge:

21
Harmful Effects of Lightning
i. The travelling waves produced due to lightning surges will shatter
the insulators and may damage poles.
ii. May damage windings of a transformer or generator incase there is
a strike on these equipment.
iii. If the arc is initiated in any part of the power system by the
lightning stroke, it sets up disturbing oscillations in the line.These
may damage other equipment connected to the line.
Protection Against Lightning
 There are three most commonly used devices for protection against
lightning surges:
i. Earthing screen
ii. Overhead ground wires
iii. Lightning arresters or surge diverters

22
i.The Earthing Screen
 Found in power stations and sub stations.
 Consists of a network of copper conductors-called shield or
screen- mounted all over the electrical equipment in the sub-station
or power station.
 Connected to earth on at least two points through a low impedance.
 Provides a low resistance path by which lightning surges are
conducted to ground.
 Limitation: does not provide protection against the travelling waves
which may reach the equipment in the station.

23
ii. Overhead GroundWires
 Most effective method of providing protection to transmission lines
against direct lightning strokes. Located conveniently above the
transmission lines.
 The ground wires are grounded at each tower or pole through as low
resistance as possible.
*The lesser the
tower-footing
resistance, the
smaller the potential
to which the tower
rises.
Example: For I1 =
50 kA and R1 = 50
, then
Ω Vt = 2500
kV. If R1 = 10 ,
Ω
then Vt =500 kV

24
Advantages:
i. It provides considerable protection against direct lightning strokes
on transmission lines.
ii. Electrostatic shielding against external fields-reduces the voltages
induced in the line conductors due to the discharge of a
neighboring cloud.
iii. A grounding wire provides damping effect on any disturbance
travelling along the line as it acts as a short-circuited secondary.
Disadvantages:
iv. Additional cost.
v. In the event the ground wire breaks and falls across the line
conductors, there is the risk of a short-circuit fault.
vi. No protection against travelling waves.

25
iii. Lightning arresters or surge diverters
 Is a protective device that conducts high voltage surges on the
power system to the ground.
 The lightning arresters or surge diverters provide protection
against travelling waves.
General form of a surge diverter.

26
Characteristics of an Ideal Lightning arrester or surge
diverter.
i. Under normal operation, the lightning arrester does not
conduct.
ii. On the occurrence of overvoltage, the air insulation across the
gap breaks down and an arc is formed, providing a low
resistance path for the surge to the ground. 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. Should not get damaged during its operation.

27
Types of Lightning Arresters
i. Rod gap arrester
ii. Horn gap arrester
iii. Multi-gap arrester
iv. Expulsion type lightning arrester
v. Valve type lightning arrester

28
End

Lecture 1 - Introduction,Internal and External Overvoltages.pptx

More Related Content

Similar to Lecture 1 - Introduction,Internal and External Overvoltages.pptx (20)

Recently uploaded (20)

Lecture 1 - Introduction,Internal and External Overvoltages.pptx

Editor's Notes