2. What are Overvoltages?
According to IEEE standard for Insulation
Coordination, Overvoltage is defined as:
“ Voltage between one phase and
ground or between two phases, having a
crest value exceeding the corresponding
crest of maximum system voltage.”
Overvoltages may be classified by shape
and duration as either temporary or
transient.
3. What are Overvoltages?
Temporary Overvoltage:
An Oscillatory phase-to-ground or phase-tophase overvoltage that is at a given location of
relatively long duration(seconds, even minute)
and that is undamped or only weakly damped.
Temporary overvoltage usually originate from
switching operation or faults (e.g load rejection,
single-phase fault, fault on a high-resistance
ground or ungrounded system) or from
nonlinearities (ferroresonance, harmonics), or
both. They are characterized by the amplitude,
the oscillation frequencies, the total duration or
the decrement.
4. What are Overvoltages?
Transient Overvoltage:
A short-duration highly damped,
oscillatory, or nonoscillatory overvoltage,
having duration of few milliseconds or
less. Transient overvoltage is classified as
one of the following types:
Lightning Overvoltage
Switching Overvoltage
Very fast front, short duration
overvoltage
5. What are Overvoltages?
Lightning Overvoltage:
A type of Transient voltage in which a fast
front voltage is produced by lightning or
fault. Such overvoltage is usually
unidirectional and of very short duration. A
typical waveform is shown in figure.
6.
7. What are Overvoltages?
Switching Overvoltage:
A transient overvoltage in which a slow
front, short-duration, unidirectional or
oscillatory, highly damped voltage is
generated (usually by switching or fault). A
typical waveform is shown in figure
9. What are Overvoltages?
Very fast front, short-duration overvoltage:
A transient overvoltage in which a short
duration, usually unidirectional, voltage is
generated (often by GIS disconnect switch
operation or when switching motor). Highfrequency oscillation or often
superimposed on the unidirectional wave.
A typical waveform is shown in the figure.
11. Causes of Overvoltage
The overvoltages on a power system
may be broadly divided into two main
catagories:
1) Internal Causes
(i) Switching surges (ii) Insulation
Failure
(iii) arcing Ground (iv) Resonance
2) External Causes i.e. lightning
12. Causes of Overvoltage
Internal causes do not produce surges* of large
magnitude. Experience shows that surges due to internal
causes hardly increase the system voltage to twice the
normal value. Generally, surges due to internal causes
are taken care of by providing proper insulation to the
equipment in the power system. However, surges due to
lightning are very severe may increase the system
voltage to several times the normal value. If the
equipment in the power system is not protected against
lightning surges, these surges may cause considerable
damage. In fact, in a power system, the protective
devices provided against overvoltages mainly take care
of lightning surges.
* A sudden rise in voltage for a very short duration on the
power system is known as a Voltage surge.
13. Lightning Facts
A strike can average 100 million volt of
electricity
Current up to 100,000 amperes.
Can generate 54,000 oF.
Lightning strike somewhere on the
earth every second.
14. Mechanism of Lightning Discharge
In an electrical storm, the storm clouds
are charged like giant capacitors in the
sky. The upper portion of the cloud is
positive and the lower portion is negative.
When there is a charge separation in a
cloud, there is also an electric field that is
associated with the separation..
15. Mechanism of Lightning Discharge
The strength or intensity of the electric field
is directly related to the amount of charge
buildup in the cloud. As the electric field
becomes more and more intense -- so
intense, in fact, that the electrons at the
earth's surface are repelled deeper into the
earth by the strong negative charge at the
lower portion of the cloud. This repulsion of
electrons causes the earth's surface to
acquire a strong positive charge.
17. Mechanism of Lightning Discharge
All that is needed now is a conductive
path for the negative cloud bottom to
contact the positive earth surface. The
strong electric field, being somewhat selfsufficient, creates this path.
18. Mechanism of Lightning Discharge
When the electric field becomes very
strong (on the order of tens of thousands
of volts per inch), conditions are ripe for
the air to begin breaking down. The
electric field causes the surrounding air to
become separated into positive ions and
electrons -- the air is ionized.
19. Mechanism of Lightning Discharge
The importance of this separation is that
the electrons are now free to move much
more easily than they could before the
separation. So this ionized air (also known
as plasma) is much more conductive than
the previous non-ionized air.
The ionization of air or gas creates plasma
with conductive properties similar to that of
metals. After the ionization process, the
path between the cloud and the earth
begins to form.
20. Mechanism of Lightning Discharge
Once the ionization process begins and
plasma forms, a path is not created
instantaneously. In fact, there are usually
many separate paths of ionized air
stemming from the cloud.
These paths are typically referred to as
step leaders. The step leaders propagate
toward the earth in stages, which do not
have to result in a straight line to the earth.
21. Mechanism of Lightning Discharge
As the step leaders approach the earth,
objects on the surface begin responding to
the strong electric field. The objects reach
out to the cloud by "growing" positive
streamers. These streamers also have a
purplish color and appear to be more
prominent on sharp edges. Next to occur
is the actual meeting of a step leader
and a streamer.
23. Mechanism of Lightning Discharge
After the step leader and the streamer
meet, the ionized air (plasma) has
completed its journey to the earth, leaving
a conductive path from the cloud to the
earth. With this path complete, current
flows between the earth and the cloud.
This discharge of current is nature's way
of trying to neutralize the charge
separation.
28. More on Lightning Phenomenon
Lightning is a discharge of electrical
energy.
It may occur:
Between cells in the same storm as Intercloud Lightning or within a cloud as Intracloud Lightning (80%)
Cloud to Air (1%)
Cloud to Ground (19%)
29. More on Lightning Phenomenon
During a collision between heavy graupel
particles and lighter ice crystals:
Negative charge is transferred to the graupel
(electrons gained)
Positive charge is transferred to the ice
crystals (electrons removed)
Graupel falls to the bottom of the cloud
bringing the negative charge with it
Ice crystals are transported to the upper
levels of the thunderstorm
30. More on Lightning Phenomenon
++ +++
- - -- - -+
-
---- +
+ + +
+
The strong negative charge
at the base of the thunderstorm
induces a positive charge at
the surface by repulsion of electrons
--
31. More on Lightning Phenomenon
If the electric field, or the difference between the
negative and positive charge regions, is large
enough, the insulator between the charge regions
(the air) “breaks down” and the lightning
discharge can occur between the regions of
positive and negative charge.
The break down voltage for air is about 10,000
Volts/meter
32. More on Lightning Phenomenon
The lightning stroke begins when the electric fields
exceed the break down voltage.
Initially streams of electrons surge from the cloud base
toward the ground in steps of 50 to 100 m.
Start and stop steps as the stepped leader progresses
toward ground.
This occurs over a few microseconds and is relatively
invisible.
34. More on Lightning Phenomenon
Streamer:
When the stepped leader gets near the ground
within 100 m or so...
Positive charge moves from the ground up toward the
stepped leader -- these are called streamers.
The streamers may come from almost any pointed
object on the ground:
Trees
Antennas
Grass
Flagpoles
Telephone Poles
People
Really Tall Towers
Electric fields are stronger around pointed objects.
35. More on Lightning Phenomenon
A streamer rising from a part of a
tree. The return stroke was created
when the stepped leader met with a
streamer from another part of the
tree.
36. More on Lightning Phenomenon
Stroke:
An electrical current of about 20,000 Amps
flows, depositing the electrons on the ground.
The current generated over the short time
interval heats the surroundings to
approximately 30,000 K (The sun’s surface ~
6000 K)
