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Breakdown in Gaseous insulators
and the PASCHEN'S LAW
These notes are prepared from the material presentedin HIGH VOLTAGE
ENGINEERING text book by the authors M. S. Naidu
and V. Kamara, and also from the class notes of Dr.Pedrow
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Conduction and Breakdown in Gases
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Ionization Process
Initially, an electron is produced by the ultra-violet light falling on tothe cathode. This event is called photo ionization.
Then, this free electron starting at the cathode will acceleratetowards the anode. The electrical field applied between theelectrodes exerts a force on the electron and causes it to move.
While moving, it gains kinetic energy (Ek). The moving electron may collide to gas molecules during this travel
towards the anode.
If the energy (Ek) gained during this travel between collisionsexceeds the ionization potential, Vi, which is the energy required toremove an electron from its atomic shell, then ionization takes
place. This process is called impact ionization and can berepresented as
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Ionization Process
Every collision generates a new electron and apositive ion.
These additional electrons also make ionizing
collisions and process repeats itself. In addition, the positive ions also reach the
cathode and fall on to it giving rise to secondaryelectrons.
When the electrons reach to the anode, a currentflows between the electrodes and electricalbreakdown of the gas occurs.
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PASCHEN'S LAW
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PASCHEN'S LAW
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PASCHEN'S LAW
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PASCHEN'S LAW
This equation shows a relationship between Vand pd, and implies that the breakdown
voltage varies as the productpd varies.
V= f(pd)
This equation is known as Paschen's law and
has been experimentally established for many
gases, and it is a very important law in highvoltage engineering.
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PASCHEN'S LAW
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PASCHEN'S LAW
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PASCHEN'S LAW %Paschen's curve for Argon and Tungsten.
gamma=0.095;
pd_start_1=0.01;
pd_stop_1=1;
pd_inc_1=0.001;
pd_start_2=2;
pd_stop_2=1000;
pd_inc_2=1;
A=13.6;
B=235;
pd_1=pd_start_1:pd_inc_1:pd_stop_1;
pd_2=pd_start_2:pd_inc_2:pd_stop_2;
pd=[pd_1 pd_2];
term1=B*pd;
term2=log(pd*A);
term3=log(log(1+gamma^(-1)));
vb=term1./(term2-term3);
for count=1:1:length(vb);
if vb(count)
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PASCHEN'S LAW
100
101
102
103
104
101
102
103
104
105
Paschens Spark Curve for Argon and Tungsten
pd (Torr*mm)
Vb,
BreakdownVoltage(volts) A=13.6 (Torr*cm)(-1)
B=235 V/(Torr*cm)
gamma=0.095
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PASCHEN'S LAW
The existence of a minimum sparking potential inPaschen's curve may be explained as follows:
For values of pd > (pd)min, electrons crossing the gapmake more frequent collisions with gas molecules thanat (pd)min, but the energy gained between collisions islower. Hence, to maintain the desired ionization morevoltage has to be applied.
Forpd < (pd)min, electron may cross the gap without
even making a collision or making only less number ofcollisions. Hence, more voltage has to be applied forbreakdown to occur.
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PASCHEN'S LAW
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CORONA
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CORONA
Corona, also known as partial discharge, is a type oflocalized emission resulting from gaseous ionization inan insulation system when the voltage stress, i.e.,voltage gradient, exceeds a critical value.
The ionization is usually localized over only a portion ofthe distance between the electrodes of the system.Corona can occur within voids in insulators as well as atthe conductor/insulator interface.
Under certain conditions, the localized electric field
near energized conductors can produce a tiny electricdischarge or corona, that causes the surrounding airmolecules to ionize.
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CORONA in Transmission Lines
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Problems caused by CORONA
Corona results in a real power loss
Communication interference
Audible noise (Hissing or cracking sound)
Ultraviolet radiation Ozone
Nitric and various other acids
Salts, sometimes seen as white powder deposits
Mechanical erosion of surfaces by ion bombardment
Carbon deposits, thereby creating a path for severearcing
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There are 3 ways to prevent corona:
1. Bundling ---This is a common way of increasing the effective
diameter of the conductor.
2. Elimination of sharp points --- Electric charges tend to form onsharp points; therefore when practicable we try to eliminate sharp
points on transmission line components.
3. Corona rings ---- These rings have smooth round surfaces which are
designed to distribute charge across a wider area, thereby reducing the
electric field and the resulting corona discharges.
Methods to prevent CORONA
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Preventing CORONA on Transmission
lines using Bundle conductors
We use bundle conductors to prevent corona
on the EHV transmission lines.
Bundle conductor means using more than oneconductor per phase.
2 conductors per phase in 345 kV lines
3 conductors per phase in 500 kV lines 4 conductors per phase in 765 kV lines
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Bundle Conductors
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Preventing CORONA on Transmission
lines using bundle conductors
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Preventing CORONA on Transmission
lines using bundle conductors
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PRACTICAL CONSIDERATIONS IN USING GASES FOR
INSULATION PURPOSES
The preferred properties of a gaseous dielectric for high
voltage applications are:
(a) high dielectric strength,
(b) thermal stability and chemical inactivity towardsmaterials of construction,
(c) non-flammability and physiological inertness,
(d) low temperature of condensation,(e) good heat transfer, and
(f) ready availability at moderate cost
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Sulphur hexafluoride (SF6)
Sulphur hexafluoride (SF6) possess most of the above requirements.
Dielectric strength is the most important property of a gaseous
dielectric for practical use
From the figures and the table, it can be seen that SF6 has high
dielectric strength and low liquification temperature, and it can beused over a wide range of operating conditions.
SF6 was also found to have excellent arc-quenching properties.
Therefore,
It is widely used as an insulating as well as arc-quenching mediumin high voltage apparatus such as high voltage cables, current and
voltage transformers, circuit breakers and metal enclosed
substations.
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VACUUM INSULATION
According to the Townsend theory, the growth ofcurrent in a gap depends on the drift of thecharged particles. In the absence of any suchparticles, as in the case of perfect vacuum, there
should be no conduction and the vacuum shouldbe a perfect insulating medium.
However, in practice, the presence of metallicelectrodes and insulating surfaces within the
vacuum complicate the issue and, therefore, evenin vacuum, a sufficiently high voltage will cause abreakdown.
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What Is Vacuum?