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1. Solid
Dielectrics
A good solid dielectric should have
some of the properties mentioned
earlier for gases and liquids and
it should also possess good mechanical
and bonding strengths. Many
organic and inorganic' materials are used for high
voltage
insulation purposes. Widely used inorganic materials are ceramics
and
glass. The most widely used organic materials are thermosetting
epoxy resins
such as polyvinyl chloride (PVC), polyethylene (PE) or
cross linked polyethylene (XLPE). Kraft paper, natural rubber,
silicon rubber and polypropylene rubber
are some of the other
materials widely used as insulate in electrical equipment.
If the solid insulating material is
truly homogeneous and is free from imperfections,
its breakdown
stress will be as high as 10 MV/cm. This is the `intrinsic breakdown
strength', and can be obtained only under carefully controlled
laboratory conditions. However, in practice, the breakdown fields
obtained are very much lower
than this value. The breakdown occurs due to many mechanisms.
In general, the breakdown occurs over the surface than in the solid
itself, and the surface insulation failure
is the most frequent
cause of trouble in practice.
2.
Composites
In many engineering applications,
more than one types of insulation are used
together, mainly in
parallel, giving rise to composite insulation systems.
Examples
of such systems are
solid/gas insulation (transmission line insulators),
solid/vacuum insulation and solid/liquid composite insulation
systems (trans-former winding
insulation, oil impregnated paper and oil impregnated metallised
plastic film etc).
In the application of composites,
it is important to make sure that both the
components of the composite
should be chemically stable and will not react with
each
other under the application of combined thermal, mechanical and
electrical stresses over the expected life of the equipment. They
should also have nearly equal dielectric constants. Further, the
liquid insulate should not absorb any impurities from the solid,
which may adversely affect its resistivity, dielectric strength,
loss factor and other properties of the liquid dielectric.
It is the intensity of the electric
field that determines the onset of breakdown and the rate of
increase of current before breakdown. Therefore, it is very
essential that the electric stress should be properly estimated and
its distribution known in a
high voltage apparatus. Special care
should be exercised in
eliminating the
stress in the regions where it is expected to be
maximum such as in the presence of sharp points.
In the design of high voltage
apparatus, the electric field intensities
have to be
controlled, otherwise higher stresses will trigger or accelerate the
aging of the insulation leading to its failure. Over the years, many
methods for controlling and optimizing electric fields to get the
most economical designs have been developed. Electric field control
methods form an important component of the overall design of
equipment.
·
Electric
Field
A brief
review of the concepts of electric fields is presented, as it is
essential for high voltage
engineers to have knowledge of the field intensities in various
media under electric stresses. It also helps in choosing proper
electrode configurations and economical dimensioning of the
insulation, such that highly
stressed regions are not formed and reliable operation of the
equipment results in its anticipated life.
The field
intensity
E at
any location in an electrostatic field is the ratio of the
force
on an infinitely small charge at that location to the charge
itself as the charge decreases
to zero. The force F on any charge
q at
that point in the field is
given
by
F =
q*E
4
The electric flux density D
associated with the field intensity E is
D
= ε*E
5
Where
E is
the permittivity of the medium in which the electric field exists.
The work done on a charge when
moved in an electric field is defined as the potential. The
potential
φ
is equal to
Where l is the path
through which the charge is moved.
Several relationships between the various quantities in the electric
field can be summarized as follows:
Where
F is
the force exerted on a charge
q
in the electric field
E ,
and S
is the
closed surface containing charge q.
·
Uniform
and Non-Uniform Electric Fields
In general, the electric fields between any two electrodes can be both
uniform and
non-uniform. In a uniform field gap, the average field
E
is the same throughout the
field rigion, whereas in a non-uniform field
gap,
E
is different at different points of the field region.
Uniform or approximately uniform field distributions exist between two
infinite
parallel plates or two spheres of equal diameters when the gap
distance is less than diameter of the sphere. Spherical electrodes are
frequently used for high voltage
measurements and for triggering in
impulse voltage generation circuits. Sometimes, parallel plates of
finite size are used to simulate uniform electric fields, when gap
separation is much smaller than plate size.
In the absence of space charges, the average field
E
in a non-uniform field gap is maximum at the surface of the conductor
which has the smallest radius of curvature.
It has the minimum field
E
at the conductor having the large radius of curvature.
In this case, the
field is not only non-uniform but also asymmetrical. Most of the
practical high voltage components used in electric power systems
normally have
non-uniform and asymmetrical field distribution.
·
Estimation of Electric Field in Some Geometric Boundaries
It has been shown that the maximum electric field Em
in a given electric
field configuration is of importance. The mean electric field
over a distanced between
two conductors with a potential difference
of V12
is
In field configurations of non-uniform fields, the maximum electric field
Em
is always higher than the average value.
For some common field configurations, the maximum value of
Em
and the field enhancement factor
f given by Em/Eav,
are presented Below.
f = Em / Eav
1-Parallel plates
2-
Concentric cylinders
3- Parallel cylinders of equal
diameter
·
SURGE VOLTAGES, THEIR DISTRIBUTION AND CONTROL
The design of power apparatus particularly at high
voltages is governed by their
transient behavior. The transient high voltages or surge voltages
originate in power systems due to
lightning and Switching operations. The effect of the surge
voltages
is severe in all power
apparatuses. The response of a power apparatus to the
impulse or
surge voltage depends on the capacitances between the coils of
windings and between the different
phase windings of the multi-phase machines.
The transient voltage
distribution in, the windings as a whole are generally very non-uniform
and are complicated by traveling wave voltage oscillations set up within
the windings. In the actual design of an apparatus, it is, of course,
necessary to consider the maximum
voltage differences occurring, in each region,
at any instant of time
after the
application of an impulse, and to take into account their
durations
especially when they are less than one microsecond.
An experimental assessment of the dielectric strength of insulation
against the power frequency voltages
and surge voltages, on samples of basic materials,
on less complex assemblies, or on complete equipment must involve high
voltage testing. Since the design of an electrical apparatus is
based on the dielectric strength, the design cannot be completely relied
upon, unless experimentally tested. High voltage testing is done by
generating the voltages and measuring them in a laboratory.
When high voltage testing is done on component parts, elaborate
insulation assemblies, and complete full-scale prototype apparatus
(called development testing), it is possible to build up a considerable
stock of design information; although
expensive, such data can be very useful. However, such data can never
really be complete to cover all future designs and necessitates use of
large factors of safety.
A
different approach to the problem is the exact calculation of dielectric
strength of
any insulation arrangement. In an ideal design each part of
the dielectric would be uniformly
stressed at the maximum value which it will safely withstand.
Such an ideal condition is impossible to achieve in practice, for
dielectrics of different electrical strengths, due to the practical
limitations of construction. Nevertheless it provides information on
stress concentration factors the
ratios of maximum local voltage gradients to the mean value in the
adjacent regions of relatively
uniform stress. A survey of typical power apparatus
designs suggests that factors ranging
from 2 to 5 can occur in practice; when this factor is high,
considerable quantities of insulation must be used. Generally,
Improvements can be effected in the
following ways:
1.
by shaping the
conductors to reduce stress concentrations,
2.
by insertion of
higher dielectric strength insulation at high stress points, and by
selection of materials of appropriate permittivity to obtain more
uniform voltage gradients.
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