Download HIGH VOLTAGE ENGINEERING

HIGH VOLTAGE
ENGINEERING
(HVE)
Dr. Alexey V. Mytnikov
Associate Professor
of Electrical Power
Systems Department
Institute of Power Engineering
COURSE STRUCTURE
• High Voltage Engineering course is
formed form five main parts:
• 1. Electro-physical processes in gases.
• 2. Electro-physical processes in
condensed dielectric materials.
• 3. Generation and measurement of high
voltages.
• 4. Over-voltages and protection.
• Advanced high voltage technologies.
CHAPTER 1.
ELECTROPHYSICAL
PROCESSES IN GASES
Basic fundamentals
• All gases are good dielectric materials. Basic
electro–physical processes of charges appearing
are considered in gases.
• Before proceeding to discuss breakdown in gases
a brief review of the fundamental principles of
kinetic theory of gases, which are pertinent to the
study of gaseous ionization and breakdown, will
be presented.
• The review will include the classical gas laws,
followed by the ionization and decay processes
which lead to conduction of current through a gas
and ultimately to a complete breakdown or spark
formation.
The fundamental principals for the kinetic
theory of gas is derived with the following
assumed conditions:
• Gas consists of molecules of the same mass
which are assumed spheres.
• 2. Molecules are in continuous random motion.
• 3. Collisions are elastic – simple mechanical.
• 4. Mean distance between molecules is much
greater than their diameter.
• 5. Forces between molecules and the walls of
the container are negligible
Distribution of velocities:
Collision–energy transfer
• The collisions between gas particles are of
two types:
• a) elastic or simple mechanical
collisions in which the energy exchange
is always kinetic, and
• b) inelastic in which some of the kinetic
energy of the colliding particles is
transferred into potential energy of the
struck particle or vice versa
Ionization processes
• Breakdown channel in gases is
formed as result of different ionization
processes in volume of anode–
cathode gap
• (volume ionization)
and metal electrode surface
• (surface ionization).
•
•
•
•
•
•
• Main volume ionization
processes are:
impact ionization,
step ionization,
photoionization,
ionization by interaction of
metastabes with atoms,
thermal ionization,
field ionization
Schematic representation of electron
multiplication:
Recombination processes
• Whenever there are positively and
negatively charged particles present,
recombination takes place. The potential
energy and the relative kinetic energy of
the recombining electron – ion is released
as quantum of radiation. At high
pressures, ion – ion recombination takes
place. The rate of recombination in either
case is directly proportional to the
concentration of both positive ions and
negative ions.
Emission processes
• Electrodes, in particular the cathode, play
a very important role in gas discharges by
supplying electrons for the initiation, for
sustaining and for the completion of a
discharge. Under normal conditions
electrons are prevented from leaving the
solid electrode by the electrostatic forces
between the electrons and the ions in the
lattice.
Changing of the potential barrier by
external electric field.
Forms of electrical discharge
• As the voltage between electrodes in
a gas with small or negligible electron
attachment increases, the electrode
current at the anode increases in
accordance with equation:
d
e
I  I0
d
1   e 1


• Taking into account this equation and all
above discussed ionization and emission
processes, we can write main condition
of self–sustaining discharge:
 e  1  1
d
• Growth of electron is arisen very rapidly,
due to exponential law. Such mechanism
of discharge processing is called electron
avalanche.
• So, this phenomenon is named
“avalanche discharge” or Townsend
mechanism of discharge.
• Townsend is English physicist, who
described this discharge form for the first
time.
• In the Townsend avalanche mechanism
the gap current grows as a result of
ionization by electron impact in the gas
and electron emission at the cathode by
positive ion impact. According to this
theory, formative time lag of the avalanche
should be at best equal to the electron
transit time.
STREAMER FORM
• When common quantity of electron in
avalanche reaches 108 (), physical
mechanism of discharge is critically
changed. Self electrical field of avalanche
is too large that begins to screen external
field. Discharge can’t exist so as no
possibility to take energy for electrons and
continue charge generation process.
• In this conditions discharge changes the
form. Avalanche transforms to another
form which is called
• “streamer” or “kanal” mechanism
of electrical discharge.
• In this mechanism for discharge formation
the secondary mechanism results from
photoionization of gas molecules and is
independent of the electrodes.
SCHEME OF STREAMER
FROMATION
LEADER FORM
• Besides avalanche and streamer form of
discharge, one more one exists. This is
leader form. Main conditions of this
discharge form existence are non–uniform
electric field and long enough gap length.
Criteria of streamer-leader transition is
high temperature in discharge channel
area (3 000 – 5 000 K).
• So, every kind of electrical discharge
can exist in one of three form
described above. Discharge channel
formation means that gap looses
electrical strength totally. This
phenomenon has other name –
breakdown of gaseous gap
Paschen’s law
• Equation
VB  F  p  d 
• is known as Paschen’s law, and was
established experimentally in 1889 by
famous German physicist Friedrich
Paschen. Equation (1.77) does not imply that
the discharge voltage increases linearly with
the product pd, although it is found in practice
to be nearly linear over certain regions.
Discharge in non–uniform electric
fields.
Main discharge processes.
• In uniform fields at the normal atmosphere
conditions breakdown voltage is
determined by Paschen’s law. Electrical
strength of atmosphere air is constant
value in this case and equal approximately
30 kV/cm.
• In non–uniform fields like such geometrical
configurations as point – plate, sharp end
– plate, cylindrical surface with sharp edge
– plate and others, breakdown voltage is
much lower that in uniform field. Below
reasons of that are explained.
• Figure below illustrates the case of a
strongly divergent field in a positive point –
plane gap.
• In uniform field and quasi–uniform field
gaps the onset of measurable ionization
usually leads to complete breakdown of
the gap. In non-uniform fields various
manifestations of luminous and audible
discharges are observed long before the
complete breakdown occurs. These
discharges may be transient or steady
state and are known as “corona”.
POLARITY EFFECT
• At the DC Voltage in the strictly nonuniform electric field takes place
unique phenomena which is called
“POLARITY EFFECT”
• So, all electrophysical processes begin with
applying of electrical field, which should be
intensive enough. In uniform electric fields
breakdown voltage is determined by
Paschen’s law and depends on kind of gas,
gas pressure and gap length.
• In non–uniform fields all electrophysical
processes begin at the active electrode area.
Active electrode has small radius of curve
and rod or sharp form.
• Corona discharge is first step at the non–
uniform fields. Depending on polarity in
anode–cathode gap in case of d.c. regime,
breakdown voltages can be quiet different. If
applied voltage is increased, corona passes
to spark discharge. In case when external
field is growth more, spark transforms to arc
discharge.
• Arc discharge is high current (1 – 100 kA),
low voltage (1 – 10 eV). Arcs have unique
physical mechanism of existence which is
based on cathode spots and explosive
emission.