UNIT - II
CORONA, ELECTRICAL
BREAKDOWN IN GASES, SOLIDS
AND LIQUIDS.
ELECTRICAL BREAKDOWN IN
GASES, SOLIDS AND LIQUIDS.
Electrical materials constitute an extreme group
of industrial materials that go specifically for
manufacture of electrical machines, electrical
apparatus, instruments, and other elements of
electrical equipment and installations.
By the behaviour in an electric and magnetic
field, or both, we categorize these materials
into either 1. Dielectric 2. Conducting 3.
Semiconducting 4. Magnetic materials
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GASEOUS BREAKDOWN IN
UNIFORM FIELDS
Gas is the most common dielectric. Application
Examples? Characteristics of good gas dielectric?
Conduction of electricity through a gas =
Breakdown of gaseous dielectric
First, the normally neutral gas must create
charges or accept them from external sources, or
both, and Second, an electric field should exist to
produce the directional motion of the charges.
functions of E (electric field) and gas pressure (p)
and are given as follows
3
Low voltage applied, small current flow
between the electrodes and the
insulation is intact.
- Larger voltages, current flowing
through the insulation increases sharply
and an electrical breakdown occur.
The voltage applied to the insulation at
the moment of breakdown is called the
breakdown voltage. 4
Time lag of breakdown
Breakdown is considered as function of
ionization processes under uniform field
conditions.
Practically, the breakdown due to rapidly
changing voltages or impulse voltages is
important
Time lag = difference between the application
of a voltage sufficient to cause breakdown
and the occurrence of breakdown itself. 5
Time lags of breakdown
Time lag of the breakdown in gas
insulation is divided into two parts:
- Statistical time- it is the time elapse between
application of voltage to initiation of electron
in the gas medium.
- Formative time- it is the time elapse
between initiation of electron to formation of
avalanche between electrode causing
breakdown. 6
GASEOUS BREAKDOWN IN
UNIFORM FIELDS
1) Townsend Theory
Ionization mechanism through which
gaseous insulation (dielectric medium)
breakdown occurs
According to Townsend’s theory the increase
in electrons during breakdown is
exponential
Look at Townsend’s growth equation and
the criteria for breakdown 7
2) Streamer’s Theory:
Townsend only considers ionization process as
cause of breakdown and neglects atmosphere and
shape of the medium between the electrodes.
Townsend’s criteria says that time lag is
approximately 1µ seconds but which is small as
estimated is 0.1µ seconds.
Townsend’s criteria says that discharge has regular
shape but practically discharge may be irregular
Streamer theory says that positive space charges
due to ionization process enhances electric field
and tends to cathode very rapidly which is called
as streamer. This forms an second avalanche
between the plates 8
Streamer theory visualized
1- Formation
positive space
charge tending
towards cathode.
2- Field of positive
space charge ionizes
the gas.
3- Formation of
avalanche between
plates.
9
CORONA DISCHARGE
A corona discharge is an electrical discharge
brought on by the ionization of a fluid such as
air surrounding a conductor that is electrically
charged.
A corona will occur when the strength
(potential gradient) of the electric field around
a conductor is high enough to form a
conductive region, but not high enough to
cause electrical breakdown or arcing to nearby
objects.
10
CORONA DISCHARGE
Visual Corona manifests as a bluish/violet colour
with a hissing noise.
It is responsible for considerable loss of power
from high voltage transmission lines,
It leads to the deterioration of insulation of air
around a given conductor and results into a
spark.
The visual glow, hissing sound and spark are
what constitute Corona.
11
12
Factors affecting Corona
Atmosphere: Physical state of atmosphere.
Presence/Quantity of particles that can be
ionized
Conductor: Size (diameter), shape (stranded
or smooth), surface (dirty or clean)
Spacing between conductors
Line voltage
What are the negative and positive aspects
of Corona? 13
BREAKDOWN IN LIQUID
DIELECTRICS
• Examples of application and liquids used?
• Liquids provide protection and cooling functions.
Insulation properties typically higher than that of
gases ,
• Some of the electrical properties which are
necessary to estimate its dielectric strength
include: i. Capacitance per unit volume. ii.
Resistivity of liquid iii. Power factor of the liquid
iv. High electric field stress v. Permittivity of
Liquid
14
Breakdown in liquid dielectrics
• A very large number of external factors affect the
breakdown strength of liquid dielectrics.
• For example, electrode configuration, their
,
material, size and surface finish, the type of
voltage, its period of application and magnitude,
the temperature, pressure, purification of the
liquid and its ageing condition.
This is known as Paschen's law. The scientist,
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Breakdown in liquid dielectrics
• Pachen’s law: breakdown voltage is factor of
pressure and distance. However,
• Dissolved water, gas or the presence of any
other form of contamination and sludge also
,
affect the breakdown strength considerably.
• It is, therefore, not possible to describe the
breakdown mechanism by a single theoretical
analysis which may take into account all known
observed factors affecting the breakdown.
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Process Of Breakdown In
Liquid Dielectric
Pure liquids are the one which has no
contaminants (ideally) while Commercial
liquids are those with impurities like sand
particles, gas pockets, air bubbles, dust etc.
Purification advised before applying liquid as
insulation
17
Conduction & breakdown in
commercial liquids
Commercial liquids breakdown in four
main way:
Suspended particle mechanism
Cavitation and bubble mechanism
Stressed oil volume mechanism
Thermal mechanism of breakdown
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BREAKDOWN IN SOLID
DIELECTRICS
Solid dielectric materials are used to insulate
one current carrying part from another
when they operate at different voltages.
A good dielectric should have low dielectric
loss, high mechanical strength, should be
free from gaseous inclusion, and moisture,
and be resistant to thermal and chemical
deterioration.
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Breakdown mechanism for
solids
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BREAKDOWN IN SOLID
DIELECTRICS
Intrinsic
-ionic breakdown, - streamer
Electromechanical breakdown,
Thermal breakdown,
Failure due to treeing and tracking,
Breakdown due to internal discharges.
Chemical or Electrochemical
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BREAKDOWN IN COMPOSITE
DIELECTRICS
Mechanism of breakdown in
composite dielectric
1. Short-term breakdown
2. Long-term breakdown
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