Dielectric material for influencing electric fields, and stress control devices made therefrom

A dielectric composition for influencing electric fields comprising a dielectric base material having electrically conductive and electrically insulating platelet-shaped particles and stress control devices made therefrom.

BACKGROUND OF THE INVENTION 
The invention relates to a dielectric material for influencing electric 
fields, a method for preparing such material, and stress control devices, 
and particularly stress control elements, manufactured utilizing that 
material. 
In electrical installations for medium and high voltage, i.e., from about 
10 kV, high potential gradients may occur in areas where the radial field 
has discontinuities due to changes in the field borders at the locations 
which are under voltage and which are separated by a dielectric. In such 
areas, overburdening of the dielectric can easily occur due to field 
amplification and densification which the dielectric cannot withstand. 
Terminations, angular plugs, connecting plugs or other connecting elements 
of shielded high voltage cables are considered as typical examples of such 
locations; there, the electric field which exists between the exposed end 
of the conductor and the end of the shield concentrates in the areas close 
to the shield, such that there is a danger in those areas that the 
breakdown voltage of the cable insulation and/or the adjacent air layers 
may be exceeded. 
That danger is increased by the fact that in practice, the transmission of 
electrical energy deals almost exclusively with alternating voltages in 
which the dielectric losses in dielectric materials can cause rather 
substantial temperature increases. Such increases in turn normally result 
in an increase of the dielectric losses so that eventually an accumulation 
of the factors which stress the dielectric may exceed the dielectric load 
capability. It is known to counteract this phenomena and resultant dangers 
by arranging, in the endangered areas, materials or shaped bodies of 
various geometric design made therefrom, which due to particular 
properties, e.g. a resistivity which decreases with the potential gradient 
(U.S. Pat. No. 2,666,876) or a high permittivity (dielectric constant) of 
e.g. more than 20 (U.S. Pat. No. 4,053,702), are capable of influencing 
the electric field to make it more uniform. Such materials typically 
consist of an electrically insulating base material, particularly an 
elastomeric plastic, with particles embedded therein which give the 
material the desired properties. 
In the case of voltage-dependent resistivity, for example, particles of a 
semi-conductive material, e.g. silicium carbide, or metal particles may be 
embedded in the material. In the case of materials exhibiting a high 
dielectric constant, it is typical to insert particles of a material 
having a high dielectric constant, for example barium titanate or titanium 
oxide. 
Furthermore, it is known to influence the field distribution in endangered 
areas in a capacitive manner by arranging metallic electrodes 
correspondingly. For example, alternate layers of conductive and 
insulating foil may be wrapped about a cable termination (U.S. Pat. No. 
2,276,923), or a sleeve-shaped stress control element which includes 
annular electrodes (U.S. Pat. No. 3,673,305) may be placed around the 
cable termination. Still further, deflectors are known for influencing the 
field distribution capacitively, which comprise a conductive outer portion 
of elastic material, and a highly insulating elastic interior portion 
which is gap-free, connected thereto (U.S. Pat. Nos. 3,243,756 and 
3,344,391; German Pat. No. 1,465,493). 
The above-described known possibilities for providing resistive, refractive 
or capacitive field control have some disadvantages, even when applied in 
combination. The embedding of particles of a material having a high 
permittivity (dielectric constant) normally increases the dielectric 
losses considerably. The heating caused thereby in turn increase the 
tendency of thermal decomposition or premature aging of the insulating 
materials and thus can lead to thermal break-throughs. In the capacitive 
field control, the edges of the electrodes form strong discontinuities so 
that the edges must be arranged in areas having little stress, and thus 
the field control device as a whole must be dimensioned correspondingly 
large. It has also been found in the application of conical deflectors on 
cable terminations that between the cone and the parts of the cable 
termination, small cavities are likely to occur which are highly 
undesirable because of their unfavorable influence on the electrical field 
intensity along the deflector casing. Furthermore, there is the danger 
that proper positioning of such cones cannot always be ensured. 
Finally, another solution of the above-described problems has become known, 
which offers considerable advantages over the above-described known 
countermeasures. According to that solution, a material is used for 
influencing electrical fields which comprises a dielectric base material 
and a conductive material which is finely distributed therein and consists 
of platelet-like particles of an electrically conductive substance, 
particularly metal, the platelets being oriented substantially parallel 
with each other so that when measuring the dielectric constant, different 
values are obtained when the measuring electrode arrangement used is 
applied parallel to or perpendicular to the platelet planes (U.S. Pat. No. 
3,349,164). As a base material, preferably an easily workable, 
particularly pourable or die-castable material is used, which also may be 
resilient and/or temperature resistant and/or weather-proof, if desired. 
Examples of such materials include polyvinyl chloride in hard or softened 
forms, butadiene-acrylonitrile elastomers, and the like. Conductive 
particles such as carbon black, and filler substances may be added; 
however, the particle size thereof is considerably smaller than that of 
the electrically conductive platelets. The material is used either in the 
form of a wrapping band containing the material, or as a pastable or pasty 
suspension. 
In the case of cable terminations, protective coatings are typically 
prepared at the application site by wrapping around the band, or applying 
multiple coatings of the suspension. The desired parallel orientation of 
the conductive platelets is effected in the manufacture of the band by 
calendering or by the brushing process in the case of painting with the 
suspension. The material has been found to be very useful in practice. It 
has low losses and is insignificantly influenced by possible air 
inclusions. The mode of function is basically capacitive, i.e., 
homogenization of the field is obtained through the orientation of the 
conductive particles together with the relatively high permittivity of the 
base material. 
In practice, however, the wrapping-around of a band, or the brushing-on of 
a paste is frequently difficult, and in any case is troublesome. 
Furthermore, coatings made in this manner are strongly dependent on the 
skill of the respective worker with regard to their geometry and their 
electrical properties. 
It has also been proposed to manufacture prefabricated elastic shaped 
bodies from materials in which particles of a platelet-shaped conductive 
material are embedded in a dielectric base material (U.S. Pat. No. 
3,515,798), as it was known in the case of materials having embedded 
particles of insulating material of high permittivity (U.S. Pat. Nos. 
3,823,334 and 4,053,702). In practice, however, such has not been followed 
because the orientation of the platelets of conductive material, which was 
desired and was considered indispensable, could not be obtained in a 
simple manner in the manufacture of such shaped bodies. More particularly, 
however, it has been found that the range of application of the 
above-described material with embedded parallel-oriented particles of 
conductive material was limited toward high operating voltages. Still 
further, the break-through voltage decreases strongly with increasing 
content of conductive platelets. On the other hand, however, it appeared 
desirable to take advantage of the favorable properties of the described 
known material with embedded particles of conductive material also in 
electrical installations with high operating voltages. 
SUMMARY OF THE INVENTION 
It is the object of the invention to provide a composition which is 
suitable for influencing, and, more particularly, equalizing electric 
fields at low losses in medium and high voltage installations, which is 
easily prepared, makes possible, even with the application of small 
amounts, a highly effective electrical relief of endangered areas, is 
useful at high operating voltages, and moreover may be easily worked to 
form resilient shaped bodies. 
According to the invention, that object is achieved with a dielectric 
material for influencing electric fields comprising a dielectric base 
material wich preferably is easily workable, particularly pourable or 
die-castable, and, if desired, resilient and temperature-resistant and/or 
weather-resistant and/or of good heat conductivity, and a finely 
distributed conductive material of platelet-shaped particles of a 
substance having good electrical conductivity, particularly metal like 
aluminum, in an amount below a limit concentration at which the material 
still has an electrical volume resistivity as is characteristic for 
insulating materials. Such material according to the invention also 
contains an insulating material which is different from the base material 
and has a higher electrical breakdown voltage than the base material, in 
finely distributed form as platelet-shaped particles, particularly of 
mica, which according to number and size are comparable with the 
platelet-shaped particles of the conductive material. 
It has been found that the composition according to the invention 
illustrates all the advantages of the known material having embedded 
platelets of conductive material, and moreover offers, under comparable 
conditions of application, an appreciably increased safety against glow 
phenomena, flash-overs, and breakdown, even at high operating voltages 
and/or with relatively considerable contents of conductive material 
therein. Surprising and particularly advantageous is the fact that the 
material shows the highly advantageous properties particularly in cases 
where its permittivity is set to relatively low values, particularly up to 
8, by a corresponding selection of the base material and the embedded 
insulating material. Then, the dielectric losses are particularly low. 
DETAILED DESCRIPTION OF THE INVENTION 
When applied at terminations or other connecting elements of shielded high 
voltage cables, a breakthrough safety is observed which is increased by 
about 50%, as compared to known insulating materials for field control 
devices with comparable dimensions. Even when applying high weight 
concentrations of conductive material, e.g. up to about 30 percent by 
weight aluminum platelets, which give the material a correspondingly high 
effectiveness for the field control, the electric volume resistivity 
remains at values which are typical for good insulating materials, in any 
case at values above 10.sup.10 Ohm.multidot.cm. 
The amounts and particle sizes of the insulating material naturally are in 
similar ranges. Often, the concentration of embedded platelet-shaped 
insulating material may be as high as 60 percent by weight, an upper limit 
being due to possible undesirable changes in the mechanical properties of 
the material. 
The reason for the surprisingly advantageous properties of the material 
probably is due to the fact that the platelets of insulating material 
intersperse between the platelets of conductive material and effect a 
particularly high mutual electrical insulating of the platelets of 
conductive material. The concentration of electrically conductive bridges 
between a plurality of successive platelets of conductive material is 
thereby strongly reduced. This results in a more homogeneous structure of 
the material, and a larger total length of the electrical field lines 
which extend along curved paths between the platelets of conductive 
material. It is particularly surprising that the advantages of the 
composition are obtained without the platelets having a preferred 
orientation, i.e., when the platelet-shaped particles are contained with 
an essentially random-distributed orientation of their platelet plates. 
Thus, the composition or shaped bodies consisting thereof can be 
manufactured without the application of orienting working steps like 
calendering or the like, e.g. by simple casting or die-casting. This 
simplifies the manufacture and enlarges the range of possible 
applications. 
It has been found that it is favorable for the dielectric properties of the 
material, particularly the flash-over safety obtained therewith, if the 
embedded insulating material used has a permittivity which is at least 
equal to that of the base material. Generally, good properties are 
obtained if the base material and the insulating material each have a 
permittivity below 10. 
The size of the conductive platelets is of importance for the obtained 
homogeneity of the composition in relation to the dimensions of the 
structural parts which are used at and in the composition in the 
particular case of application. With the dimensions and flash-over 
distances for alternating voltages from about 15 kV, the platelets of the 
conductive material may have a transverse dimension, measured transverse 
of their thickness, of about 5 to 75 .mu.m; an advantageous intermediate 
range is 10 to 25 .mu.m. The thickness of the platelet-shaped particles 
should be no more than about 1/10 of the transverse dimension to retain 
the character of a platelet. 
Of course, similar considerations hold also for the platelets of insulating 
material. The transverse dimensions thereof preferably should be in the 
same order of magnitude, particularly about 15 to 75 .mu.m, preferably 
between about 20 and 40 .mu.m. Normally it is difficult to purchase 
insulating material in platelet-shaped particles of very small transverse 
dimensions. A particularly suitable insulating material is mica which 
according to its nature has platelet structure. 
The composition of the invention may be prepared in a very simple manner 
by: 
(a) preparing a pasty or fluid or die-castable material by mixing the 
conductive material with a base compound which is capable of being 
hardened or cured to form the dielectric base material, 
(b) bringing the platelet-shaped insulating material additionally into the 
base compound, 
(c) forming a parison from the prepared material compound, and 
(d) hardening or curing the parison. 
The expression "hardening" in this context shall include any process which 
provides solidification from a pasty, flowable or die-castable condition, 
particularly the curing or solidification of plastics which are capable of 
polyaddition or poly condensation, the drying of solvent-containing 
compounds, the solidification of molten compounds, vulcanization, the 
thermal setting of plastics, and the like. 
A particularly simple mode of preparation for the composition is to form a 
premix of the conductive material and the insulating material, following 
which the premix is mixed with the base compound. 
In order to ensure and improve the mutual electrical insulation of the 
particles of conductive material, it may be recommendable to prepare a 
premix from the conductive material and a liquid electrically insulating 
auxiliary substance which is compatible with the base material, and to 
then further process the premix and the insulating material with the base 
compound to form the inventive composition. As an electrically insulating 
auxiliary compound, a disperging agent for the conductive material may be 
advantageously used. Often, it is also possible to use as an auxiliary 
substance a plastisizing and/or curing and/or catalyzing agent which is 
compatible with the base material. That agent moreover may act as a 
disperging agent for the conductive material. This means in practice that 
in many cases the constituents which are per se required to obtain the 
desirable properties of the base material may also function as the 
electrically insulating auxiliary substance. 
The composition of the invention can be processed and worked, depending 
upon the nature of the constituents selected, to shaped bodies having a 
definite configuration, or also as an amorphous or thixotropic mass which 
particularly may be capable of being brushed-on, poured or die-cast. 
Dielectric stress control devices can be manufactured which are designed 
with respect to their dielectric properties and their geometric 
configurations in accordance with desirable modifications of an electric 
field present at the respective site of application. These stress control 
devices consist at least partly of the composition of the invention. The 
manufacture of the stress control device is not difficult because 
orienting working procedures are not required. Particularly useful is a 
dielectric stress control element which consists of a shaped body, 
preferably a sleeve, which can be resiliently shifted onto an end of a 
cable insulation and/or shield. In such a case, it is possible to design 
the sleeve so that it includes an annular constituent of electrically 
conductive resilient material. This often is desirable to establish a 
well-defined potential at the end of the stress control element. 
Particularly, the annular constituent may be arranged at an end of the 
sleeve so that upon shifting the element on the end of a cable shield, 
electrical contact with the shield is established. 
In many cases, stress control elements having other geometric 
configurations may be useful to prevent unacceptably high local field 
concentrations, for example in break elbows, transition or throughgoing 
connections, feed throughs and branchings of high tension cables. 
It is particularly advantageous and therefore preferred, that the 
composition have elastomeric properties. Accordingly, dielectric stress 
control devices may be manufactured which are suited for different 
dimensions or sizes of electrical structural components. For example in 
the case of sleeves, same may have sufficient resilience to be applicable 
with cable insulations and/or dimensions of various thicknesses.

In FIGS. 1 to 3, three digit reference numerals are used wherein the first 
digit designates the figure, and the second and third digits designate the 
respective part. 
FIG. 1 illustrates an axial section of a sleeve-shaped stress control 
element destined to be shifted upon an end of a shielded high voltage 
cable. It consists of a unitary sleeve body 101 of resilient material of 
the kind described. At one end, the sleeve body has a cylindrical recess 
103 having a diameter d.sub.1, which extends toward the other end into a 
passage 105 having a constant smaller diameter d.sub.2. Approximately from 
the mid of its length L, the outer diameter D of the sleeve body 101 
continuously decreases toward the interior diameter d.sub.2 of the passage 
105. 
FIG. 2 illustrates an axial section of a sleeve-shaped stress control 
element comprising a sleeve body 201 which is equal to the sleeve body of 
the element according to FIG. 1. In the recess 203, a sleeve-shaped insert 
body 207 of electrically conductive or semi-conductive resilient material 
is inserted in a gap-free manner, particularly by vulcanization, the 
insert body having at the end which is remote from the passage 205 an 
outwardly cranked sleeve-shaped skirt 209 extending beyond the end of the 
sleeve body 201. 
In case the sleeve body 201 consists of silicon rubber, conductive silicon 
rubber of the type "Conductive Rubber Silastic Q 41602" supplied by the 
Dow Corning company may be used as the base material for the manufacture 
of the sleeve-shaped insert body 207. It may be added at a weight ratio of 
50:50 to an organosiloxane such as "Silastic 133 BU" of the Dow Corning 
company, and crosslinked with a suitable catalyst, e.g. "Catalyst B" of 
the Dow Corning company, at 140.degree. C. (10 minutes). The product so 
obtained has a specific volume resistivity of 80 to 
100.OMEGA..multidot.cm. 
FIG. 3 ilustrates an axial section of a sleeve-shaped stress control 
element of the kind shown in FIG. 2, operatively shifted onto an end of a 
shielded high voltage cable 311. The cable conventionally consists of a 
conductor 313, a dielectric insulation 315 surrounding the conductor, a 
cable sheath which may also consist of insulating material and is 
interiorly and exteriorly provided with a metal shielding 319, and a 
radiation protection layer 321 of semi-conductive material. At the 
illustrated end of the cable, the sheath 317 is cut away. Onto the 
thus-exposed end of the insulation 315, the stress control element 301 has 
been shifted into place under elastic expansion to obtain a tight fit so 
that the cable insulation 315 extends through the passage 305 without any 
gap, if necessary utilizing a pasty filler mass like silicon grease to 
avoid air inclusions, and the electrically conductive or semi-conductive 
insert body 307 is electrically connected to the metal shield 319. The 
electrical contact is improved in that the skirt 309 of the insert body 
307 tightly fits around the outer layer of the metal shield 319. Outside 
of the other end of the stress control element 301 the cable insulation 
315 is removed after a set-off length A so that the conductor 313 is 
exposed. 
For the dielectric material, the following examples are given: 
EXAMPLE 1 
To 58 parts by weight of a branched polyalcohol having ether and ester 
groups and a hydroxyl content of about 5% (a suitable commercial product 
is "Desmophen" supplied by Bayer AG), are added 10 parts by weight of 
aluminum flake consisting of platelet-shaped particles having a mean 
transverse dimension of about 20 .mu.m, and 10 parts by weight of mica 
powder consisting of platelet-shaped particles having a means transverse 
dimension of about 35 .mu.m under stirring, and are homogeneously mixed 
with 22 parts by weight of diphenyl methane-4,-4'diisocyanate (MDI). 
The composition may be easily formed to shaped bodies and forms a resilient 
material after curing. 
EXAMPLE 2 
10 Parts by weight aluminum flake and 20 parts by weight mica of the nature 
as described in Example 1 are added, with stirring, to a mixture of 17.4 
parts by weight of a branched polyether of the kind stated in Example 1 
and 30.6 parts by weight of a trifunctional polyalcohol (molecular weight 
3000). After a homogeneous mixture has resulted, 22 parts by weight MDI 
(see Example 1) are added. This results in an easily moldable material 
compound which forms a resilient material after curing. 
EXAMPLE 3 
10 Parts by weight aluminum flake and 20 parts by weight mica of the nature 
described in Example 1 are homogeneously mixed with 64.64 parts by weight 
of a reactive organosiloxane, suitable commercial products being "Sylgard 
184" of Dow Corning and "HTV 100/25" of the Wacker company, and 
thereafter, 6.36 parts by weight of a metal catalyst are added. An easily 
moldable compound is obtained which hardens to form a resilient material. 
EXAMPLE 4 
10 Parts by weight aluminum flake having a mean particle transverse 
dimension of 25 .mu.m, 5 parts by weight mica having a mean particle 
transverse dimension of 35 .mu.m are homogeneously intermixed on a 
masticizing cylinder with 84.2 parts by weight of a reactive 
organosiloxane of the type stated in Example 3. Thereafter, 0.8 part by 
weight dicumyl peroxide is added as a vulcanizing agent. An easily 
moldable compound is obtained which after 15 minutes of vulcanization at 
165.degree. C. forms a rubber-elastic material. 
EXAMPLE 5 
10 Parts by weight aluminum flake (platelet structure) and 5 parts by 
weight mica powder are added to 75.55 parts by weight of a OH 
group-containing polybutadiene (OH number 1.3, "Poly BD R45HT" supplied by 
Arco Chemical Comp.). After 0.01 parts by weight of dibutyl tin laurate 
have been added, the mixture obtained is reacted with 9.44 parts by weight 
of pure MDI (see Example 1) (NCO content=34%). 
EXAMPLE 6 
26.6 Parts by weight of an internally castor oil-plastified DGEBA 
(diglycide ether of bisphenol A) (a suitable commercial product being 
"Beckopox Spezialharz EP151" supplied by Hoechst AG, having an epoxy value 
of 0.22) are added to a mixture of 10 parts by weight of aluminum flake 
and 10 parts by weight of mica powder. For flexibilization, 26.66 parts by 
weight of a diglycide ether (a suitable commercial product being "LER 736" 
supplied by Dow Corning, having an epoxy value of 0.53), and 11.74 parts 
by weight of a liquid hydrocarbon resin (a suitable commercial product 
being "Epodil L" supplied by Anchor Chemical Comp., Ltd.) are added. 
The mixture obtained is cured with 14.94 parts by weight of a modified 
polyamine having an amine equivalent of 70 (a suitable commercial product 
being "Beckopox Spezialharter VEH 629" supplied by Hoechst AG). 
EXAMPLE 7 
61 Parts by weight of a nonylphenol-blocked isocyanate (prepolymer) (a 
suitable commercial product being "Desmocap 11" supplied by the Bayer 
company) are intermixed with 15.3 parts by weight of diundecyl phthalate 
(which is sold under the name "DUP" by the Monsanto company). 
Subsequently, a mixture of 12.5 parts by weight aluminum flake and 7.5 
parts by weight mica powder are added. The mixture obtained is crosslinked 
by means of a bifunctional aliphatic polyamine (amine equivalent 60, a 
suitable commercial product being "Laromin C260" of the BASF company). 
The hardened materials obtained in accordance with Examples 1 to 7 have 
properties as listed in the following table I. 
TABLE I 
______________________________________ 
Permittivity (Dielectric Con- 
Volume Resistivity 
stant) at the following Temp- 
at Room Temperature 
eratures in .degree.C. 
Example 
Ohm . cm 20 50 80 
______________________________________ 
1 5 . 10.sup.12 7.8 8.1 9.5 
2 4 . 10.sup.13 6.5 7.1 8.2 
3 4 . 10.sup.15 6.6 6.2 5.5 
4 7 . 10.sup.14 7.8 7.6 6.9 
5 4 . 10.sup.13 4.6 5.3 6.3 
6 2 . 10.sup.12 6.5 7.2 8.1 
7 1 . 10.sup.11 7.1 7.9 -- 
______________________________________ 
The materials listed had electrical break-through voltages between 10 and 
15 kV/mm. 
From the compositions of Examples 1 to 4, shaped bodies according to FIG. 1 
were molded and hardened to form resilient sleeve bodies. Additionally, 
further sleeve bodies were, for comparison purposes, prepared from 
materials which contained 10 percent by weight ("comparison E") and 5 
percent by weight ("comparison 6"), respectively, aluminum flake of the 
kind stated in Example 1 but without platelet-shaped insulating material, 
and in other respects were composed as the material according to Example 
1. The geometric dimensions in all cases were D=40 mm, d.sub.1 =30 mm, 
d.sub.2 =25 mm, L=90 mm. The sleeve bodies were provided with 
semi-conductive resilient insert bodies 207 according to FIG. 2, and the 
so-formed stress control elements were shifted onto an end of a shielded 
high voltage cable, as illustrated in FIG. 3. The shield 319 was grounded, 
and voltage pulses were applied to the conductor 313 in accordance with 
the prescriptions VDE 0472 511, the free end of the conductor being 
provided with a spherical electrode. 
Tests were made with different set-off lengths A (FIG. 3) of the cable 
insulation. In every series of tests, eventually flash-overs eventually 
occurred between the exposed end of the conductor 313 and the end of the 
shield 319 when the pulse voltage was increased. The length of the 
flash-over distance is approximately equal to the structural length L of 
the stress control element, plus the set-off length A of the cable 
insulation. In every case, the "50% flash-over pulse voltage" was 
determined, i.e., the pulse voltage at which flash-overs occurred in 50% 
of the voltage pulses applied. In no case were break-throughs through the 
cable insulation 315 observed. 
FIG. 4 illustrates the mean values of the test results in a graphical 
representation. It can be seen that the stress control elements having 
short structural lengths and small dimensions provide a high degree of 
safety against overvoltages as may for instance occur in energy 
transmitting systems by the influence of flashes. Corresponding tests with 
materials which were prepared as in Examples 1 to 4, however, with only 5 
percent by weight aluminimum flake, showed results which were only 
slightly less advantageous.