16-04-2012, 04:44 PM
EE4270 High Voltage Breakdown and Testing
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High Voltage Breakdown Phenomena [6 hrs]
Breakdown Characteristic in gases:
Electron Avalanche Mechanism, Townsend Breakdown Process,
Streamer Mechanism, Time lags of Spark breakdown.
Corona Discharges, Mechanism of corona formation, Power Loss
due to Corona
Breakdown in Liquids:
Breakdown of Commercial liquids; Breakdown due to gaseous
inclusions, liquid globules, solid particles;
Breakdown of Solid Insulating Materials:
Electro-mechanical breakdown, Breakdown due to internal
discharges, Surface Breakdown, Thermal Breakdown, Electrochemical
Breakdown, Chemical Deterioration, Breakdown of
Composite Insulation.
2. High Voltage Cables [6 hrs]
Power loss in the cable: Dielectric loss, Conductor loss, Sheath loss,
Intersheath Loss, Cross-bonding of Cables
Impregnated paper insulation: Properties required, Principle
underlying the design, Paper insulated power cables,
Single core and three core cables: Insulation Resistance,
Capacitance, Copper Space Factor
Dielectric stress in a single core cable: Cable Grading for Uniform
Stress Distribution, Capacitance Grading, Intersheath Grading
Pressurised high voltage cables: Oil-pressure cables, Gas-pressure
cables, External Pressure Cables, Internal Pressure Cables
Thermal design of cables: Current rating, Thermal Resistance -
single-core cables, three-core cables, protective coverings, ground
around cable, Cables exposed to air
High voltage bushings: Simple cylindrical bushing, Condenser
bushing
3. High Voltage Generators for Testing [6 hrs]
Generation of High Alternating Voltages:
Cascade arrangement of transformers
Resonant Transformers
High frequency high voltages
Generation of High Direct Voltages:
Rectifier circuits
Voltage Multiplier Circuits
Electrostatic generators: Van de Graeff generator, Sames
Generator
4. High Voltage Measurements and Testing [6 hrs]
Electrostatic voltmeter, sphere gaps, potential dividers,
matching, peak reading meters, Klydonograph
Type tests, Sample Tests, Routine Tests
Oscilloscopes for the measurement of fast transients
Measurements of capacitance and loss tangent: High Voltage
Schering Bridge, Dielectric loss measurement,
Detection of internal discharges
Measurement of dielectric constant and dissipation factor of a
liquid dielectric
General tests carried out on High voltage equipment.
Testing of solid dielectric materials.
1
Breakdown of
Gaseous Insulation
3 J R Lucas Breakdown of Gaseous Insulation
Electrical Insulating Materials
referred to as Dielectrics
electrostatic fields can remain almost indefinitely
offer very high resistance to the passage of direct currents
cannot withstand an infinitely high voltage.
○ when applied voltage across dielectric exceeds a critical value the
insulation will be damaged
may be gaseous, liquid or solid in form.
1 J R Lucas Breakdown of Gaseous Insulation
Can a person swim one length without hitting some one ?
If the pool is almost empty
If the pool is crowded
Will he be able get sufficient momentum in these cases to hurt someone?
1 J R Lucas Breakdown of Gaseous Insulation
1.1 Ionisation of Gases
Gaseous dielectrics are not free of electrically charged particles, including
free electrons.
Free electrons
may be caused by irradiation or field emission
can lead to a breakdown process to be initiated.
on the application of an electric field are accelerated from cathode to
anode by the electric stress applying a force on them.
Force = mass´ acceleration, Force = charge´ electric field
acquire a kinetic energy (½ mu2) as they move through the field.
moving towards the anode, collide with gas molecules present between
the electrodes.
part of the kinetic energy of the electrons is lost in these collisions, and
part is transmitted to the neutral molecule.
3 J R Lucas Breakdown of Gaseous Insulation
Energy is usually expressed as a voltage, Ei = e Vi
in electron-volt (eV) as the energies involved are extremely small,
where e is the charge on an electron = 1.6 x 10-19 C.
1 e V = 1.6 x 10-19 J
If molecule gains sufficient energy (more than
the ionisation energy Ei), it may ionise by
collision.
Mean number of ionising collisions by one
electron per unit drift across the gap is not a
constant but subject to statistical fluctuations.
The newly liberated electron and the impinging
electron are then accelerated in the field and an
electron avalanche is set up.
3 J R Lucas Breakdown of Gaseous Insulation
Further increase in voltage results in additional ionising processes.
Ionisation increases rapidly with voltage once these secondary processes
take place, until ultimately breakdown occurs.
It is worth noting that in uniform fields, the ionisation present at voltages
below breakdown is normally too small to affect engineering applications.
In non-uniform fields, however,
considerable ionisation may be present in
the region of high stress, at voltages well
below breakdown, constituting the well
known corona discharge.
1 J R Lucas Breakdown of Gaseous Insulation
1.1.1 Ionisation processes in gas discharges
Electrical breakdown of a gas is caused by various processes of ionisation.
gas processes involving the collision of electrons, ions and photons
with gas molecules, and
electrode processes which take place at or near the electrode surface
[Electrons can be emitted from the cathode at stresses around 100 –
1000 kV/cm due to field emission].
Ionisation is the process by which an electron is removed from an atom,
leaving the atom with a nett positive charge (positive ion).
Since an electron in the outermost orbit is subject to the least attractive
force from the nucleus, it is the easiest removed by any of the collision
processes.
1 J R Lucas Breakdown of Gaseous Insulation
The energy required to remove an outer
electron completely from its normal state in
the atom to a distance well beyond the
nucleus is called the first ionisation
potential.
The reciprocal process of an electron falling
from a great distance to the lowest
unoccupied orbit is also possible. In this
case, a photon will be emitted having the
same energy as previously absorbed.
3 J R Lucas Breakdown of Gaseous Insulation
1.1.2 Relevant gas ionisation processes
(i) Ionisation by simple collision
When the kinetic energy of an electron (½ mu²), in collision with a neutral
gas molecule exceeds the ionisation energy (Ei = e Vi) of the molecule,
then ionisation can occur.
(i.e. the necessary but not necessarily sufficient condition is ½ mu² > Ei)
M + e- (½ mu²) M+ + 2 e-
In general, a positive ion and 2 slow moving electrons will result.
The probability of this process is zero for electron energies equal to the
ionisation energy Ei, but increases almost linearly at first, and then
gradually with electron energy up to a maximum.
When the gas molecules are bombarded with electrons, other electrons
bound to atoms may be freed by the collision with the high energy
electron.