SOLIDS AND COMPOSITES

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  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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