Breakdown in Gases

8/10/2019 Breakdown in Gases

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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

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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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lines using bundle conductors


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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?

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Breakdown in Gases