High voltages can cause overvoltage events that exceed the design limits of electrical systems. There are two main types of overvoltage: lightning overvoltage from natural sources, and switching overvoltage caused by changing loads on a system. Lightning overvoltage occurs when a lightning strike induces high voltage in a system. Switching overvoltage happens when large inductive or resistive loads are connected or disconnected, causing voltage spikes. Both types of overvoltage can damage equipment and should be controlled through various techniques like resistors, phase control, and reactors. Uncontrolled overvoltages present a danger, so protection methods are important for system reliability and safety.

1. High Voltage System Over
Voltage
By
Oladokun Sulaiman

2. Slide navigation
• Overview and Source of overvolatge
• Nature of danger
• Lightening overvolatge
• Switching overvolatge

3. Overview and source of overvolatge
• When the voltage in a circuit or part of it is raised above its upper design
limit, this is known as over voltage.
• The conditions may be hazardous. Depending on its duration, the over
voltage event can be permanent or transient, the latter case also being
known as a voltage spike.
• Electronic and electrical devices are designed to operate at a certain
maximum supply voltage, and Considerable damage can be caused by
voltage that is higher than that for which the devices are rated.
• For example an electric light bulb has a wire in it that at the given rated
voltage will carry a current just large enough for the wire to get very hot
(giving off light and heat), but not hot enough for it to melt.
• The amount of current in a circuit depends on the voltage supplied: if the
voltage is too high, then the wire may melt and the light bulb has "burned
out".
• Similarly other electrical devices may stop working, or even maybe burst
into flames if an over voltage is supplied to the circuit of which these
devices are part.

4. Overview and source of overvolatge
• Natural -A typical natural source of transient over voltage events is
lightning.
• Man-made- sources are spikes usually caused by electromagnetic induction
when switching on or off inductive loads (such as electric motors or
electromagnets), or by switching heavy resistive AC loads when zero-
crossing circuitry is not used - anywhere where a large change of current
takes place.
• An important potential source of dangerous over voltages is electronic
warfare. There is intensive military research in this field, whose goal is to
produce various transient electromagnetic devices designed to generate
electromagnetic pulses that will disable an enemy's electronic equipment.
• A recent military development is that of the exploding capacitor designed to
radiate a high voltage electromagnetic pulse. Another intense source of an
electromagnetic pulse is a nuclear explosion.

5. Overview and source of overvolatge
• One of the purposes of electromagnetic compatibility
compliance is to eliminate such sources.
• Conduction path- The transient pulses can get into
the equipment either by power or data lines, or over
the air from a strong electromagnetic field change -
an electromagnetic pulse (EMP).
• Filters are used to prevent spikes entering or leaving
the equipment through wires, and the
electromagnetically coupled ones are attenuated by
shielding.

6. Overview and source of overvolatge
Power systems are always subjected to over voltages that have their
origin
in :
• Atmospheric discharges in which case they are called external or
lightning over voltages, or - The latter type are called internal
over voltages - This classes may be further subdivided into
(i) Temporary over voltages, if they are oscillatory of power
frequency or harmonics
Temporary over voltages occur almost without exception under no
load or very light load conditions.
whereas that of internal or switching overvoltages increases with
increasing the operating voltage of the system.
(ii) Switching over voltages, if they are heavily damped and of short
duration they are generated internally by connecting or
disconnecting the system, or due to the systems fault initiation or
extinction.

7. Overview and source of overvolatge
• The magnitude of the external or lightning over voltages
remains essentially independent of the system’s design, Hence,
with increasing the system’s operating voltage a point is
reached when the switching over voltages become the
dominant factor in designing the system’s insulation
• Because of their common origin the temporary and switching
over voltages occur together and their combined effect has to
be taken into account in the design of h.v. systems insulation.
• Up to approximately 300 kV, the system’s insulation has to be
designed to withstand primarily lightning surges.
• Above that voltage, both lightning and switching surges have
to be considered.
• For ultra. systems, 765 kV and above switching over voltages
in combination with insulator contamination become the
predominating factor in the insulation design.

8. Nature of danger
• The degree of hazard depends on circumstances.
• To minimize the chances of being struck by lightning
during thunderstorm, one should be sufficiently far
away from tall objects likely to be struck, remain
inside buildings or be well insulated.
• A direct hit on a human or animal is rare;
• they are more at risk from indirect striking, usually:
• (a) when the subject is close to a parallel hit or other
tall object,
• (b) due to an intense electric field from a stroke can
induce sufficient current to cause death, and

9. Nature of danger
• (c) when lightning terminating on earth sets up high
potential gradients over the ground surface in an
outwards direction from the point or object struck.
• The potential difference between the person’s feet
will be largest if his feet are separated along a radial
line from the source of voltage and
• will be negligible if he moves at a right angle to such
a radial line.
• In the latter case the person would be safe due to
element of chance.

10. Nature of danger
qualitatively the current distribution in the ground and the
voltage distribution along the ground extending outwards from
the edge of a building struck by lightning.

11. Traveling Wave on Transmission Line
 Any disturbance on a transmission line or system such as a
sudden opening or closing of line, a short circuit or a fault results
in the development of overvoltage or overcurrent at that point.
 This disturbance propagates as a traveling wave to the ends
of the line or to a termination, such as, a sub-station.
 Usually these traveling waves are high frequency
disturbances and travel as waves. They may be reflected,
transmitted, attenuated or distorted during propagation until the
energy is absorbed.
 Long transmission lines are to be considered as electrical
networks with distributed electrical elements.

12. Attenuation and Distortion of Traveling Waves
• As a traveling wave moves along a line, it suffers both attenuation
and distortion. The decrease in the magnitude of the wave as it
propagates along the line is called attenuation
• The elongation or change of wave shape that occurs is called
distortion. Sometimes, the steepness of the wave is reduced by
distortion. Also, the current and voltage wave shape become
dissimilar even though they maybe the same initially.
• Attenuation is caused due to the energy loss in the line and
distortion is caused due to the inductance and capacitance of the
line.

13. Reflection and Transmission of Waves at Transition Points
• Whenever there is an abrupt change in parameters of a
transmission line, such as an open circuit or a
termination, the traveling wave undergoes a transition,
part of the wave is reflected or sent back and only a
portion is transmitted forward.
• At the transition point, the voltage or current wave may
attain a value which can vary from zero to two times its
initial value.
• The incoming wave is called the incident wave and the
other wave are called the reflected and transmitted waves
at the transition point.

14. Lightening
• Physical manifestations of lightning have been noted in
ancient times, but the understanding of lightning is
relatively recent.
• The real incentive to study lightning came when electric
transmission lines had to be protected against lightning.
The methods include measurements of:
(i) lightning currents,
(ii) magnetic and electromagnetic radiated fields,
(iii) voltages,
(iv) use of high-speed photography and radar.

15. lightening
• Fundamentally, lightning is a manifestation of a very large
electric discharge and spark.
• In an active thunder cloud the larger particles usually possess
negative charge and the smaller carriers are positive.
• Thus the base of a thunder cloud generally carries a negative
charge and the upper part is positive, with the whole being
electrically neutral.
• There may be several charge centers within a single cloud.
Typically the negative charge centre may be located anywhere
between 500m and 10 000m above ground.
• Lightning discharge to earth is usually initiated at the fringe of
a negative charge centre together with the current to ground.

16. lightening
• The stroke is initiated in the region of the negative charge
centre where the local field intensity approaches ionization
field intensity (ｾD30 kV/cm in atmospheric air, or ｾ10 kV/cm
in the presence of water droplets).
• To the eye a lightning discharge appears as a single luminous
discharge - although at times branches of variable intensity
may be observed which terminate in mid-air - while the
luminous main channel continues in a zig-zag path to earth.
• High-speed photographic technique studies reveal that most
lightning strokes are followed by repeat or multiple strokes
which travel along the path established by the first stroke.
• The latter ones are not usually branched and their path is
brightly illuminated.

17. Lightening process
Diagrammatic representation of lightning mechanism and
ground current

18. Lightening process
• The current in the return stroke is in the order of a few kA to
250 kA and the temperatures within the channel are 15 000ｰC
to20 000ｰC and are responsible for the destructive effects of
lightning giving high luminosity and causing explosive air
expansion.
• The return stroke causes the destructive effects generally
associated with lightning
• The return stroke is followed by several strokes at 10- to 300-
m/sec intervals. The leader of the second and subsequent
strokes is known as the ‘dart leader’ because of its dart-like
appearance.
• The dart leader follows the path of the first stepped leader with
a velocity about 10 times faster than the stepped leader. The
path is usually not branched and is brightly illuminated.

19. Representation of various stages of lightning stroke
between cloud and ground
• A diagrammatic representation of the various stages of the lightning stroke development from
cloud to ground in Figs 8.2(a) to (f) gives a clearer appreciation of the process involved.
• In a cloud several charge centres of high concentration may exist. In the present case only two
negative charge centres are shown.
• In (a) the stepped leader has been initiated and the pilot streamer and stepped leader
propagate to ground, lowering the negative charges in the cloud. At this instance the striking
point still has not been decided;
• in (b) the pilot streamer is about to make contact with the upwards positive streamer from
earth;
• in (c) the stroke is completed, a heavy return stroke returns to cloud and the negative charge
of cloud begins to discharge;
• in (d) the first centre is completely discharged and streamers begin developing in the second
charge centre;
• in (e) the second charge centre is discharging to ground via the first charge centre and dart
leader, distributing negative charge along the channel. Positive streamers are rising up from
ground to meet the dart leader;
• (f) contact is made with streamers from earth, heavy return stroke proceeds upwards and
begins to is charge negatively charged space beneath the cloud and the second charge centre in
the cloud.

20. Representation of various stages of lightning
stroke between cloud and ground

21. Representation of various stages of lightning
stroke between cloud and ground

22. Representation of various stages of lightning
stroke between cloud and ground

23. Distribution of times to crest of lightning stroke
currents
The data on lightning strokes and voltages has formed
the basis for establishing the standard impulse or
lightning surge for testing equipment in laboratories.

24. Energy in lightning
•Based on many
investigations the
AIEE Committee8
has produced the
frequency
distribution of
current
magnitudes, this
is often used for
performance
calculations.
Cumulative distributions of lightning stroke current
magnitudes: After AIEE Committee.

25. Energy in lightning
• To estimate the amount of energy in a typical lightning discharge let us
assume a value of potential difference of 107 V for a breakdown between a
cloud and ground and a total charge of 20 coulombs.
• Then the energy released about 55 kWh in one or more strokes that make
the discharge.
• The energy of the discharge dissipated in the air channel is expended in
several processes.
• Small amounts of this energy are used in ionization of molecules,
excitations, radiation, etc. Most of the energy is consumed in the sudden
expansion of the air channel.
• Some fraction of the total causes heating the struck earthed objects. In
general, lightning processes return to the global system the energy that was
used originally to create the charged cloud.

26. Over voltage Due to Switching Surges, System Faults and
Other Abnormal Condition
• Unlike the lightning voltages, the switching and other
type of overvoltages depend on the normal voltage of the
system and hence increase with increased system
voltage.
• In insulation coordination, where the protective level of
any particular kind of surge diverter is proportional to
the maximum voltage, the insulation level and the cost of
the equipment depends on the magnitudes of these
overvoltages.
• In the EHV range, it is the switching surge and other
types of overvoltages that determine the insulation level
of the lines and other equipment and consequently, they
also determine their and costs.

27. Origin of Switching Surge
• The making and breaking of electric circuits with switch
gear may results in abnormal overvoltage in power
systems having large inductance and capacitances.
• The overvoltages may go as high as 6 times the normal
power frequency voltage.
• In circuit breaking operation, switching surges with a
high rate of rise of voltage may cause repeated restricting
of the arc between the contacts of a circuit breaker,
thereby causing destruction of the circuit breaker
contacts.

28. Control of Over voltages Due to Switching
• Insertion of Resistors
• Phase Controlled Switching
• Drainage of Trapped Charge
• Shunt Reactor