Electrical stress control electrode in combination with a junction end of a shielded insulated electrical conductor

The electrical stress control electrode to be arranged around an unshielded junction end portion of a conductor insulator comprises a sheathing member having several successive adjacent zones each having a non-linear electrical resistance different from the adjacent zones, said resistance typically increasing towards the high voltage parts of the junction. The zones are made of a plastic material support compound or binder including particles of silicon carbide or zinc oxide, the different resistances being obtained by varying the concentration of particles of a same grain size from a zone to another, or by progressively steppingly varying the grain size of the particles for a given concentration from a zone to another.

TECHNICAL FIELD OF THE INVENTION 
The invention relates to connection equipment for electrical power 
transmitting circuits or networks and more particularly to electrical 
stress reduction or control electrodes for termination zones exhibiting 
discontinuities of the constituting elements, more particularly for high 
voltage cables. 
BACKGROUND ARTS 
It is well known that in electrical power transport equipment, such as high 
voltage or extra high voltage cables, there exists, at the level of the 
cable connection zones, such as in cable joints, cable branching or 
connection to cells or transformers, a discontinuity of the electrical 
field adjacent the end of the cable shielding. When the electrical 
discontinuity or potential gradient exceeds a predetermined value along 
the surface of the unshielded electrical conductor insulator surrounded by 
a gas, such as the air, gas discharges may occur which generate ozone and 
other gases which can affect greatly the electrical insulating equipment. 
To prevent such drawbacks, use in generally made at the level of the cable 
shielding interruption of cone-shaped termination equipment or deflective 
cones including or not condensers, which generally show outer dimensions 
far greater than the proper cable terminator, more particularly on extra 
high voltages. Said deflective cones have to be arranged within procelain 
insulators filled with an insulating material, thereby leading to problems 
in handling and connecting said integers in situ. 
It is also known to make use for low voltage cables of materials exhibiting 
non-linear resistance characteristics, high permittivity materials and 
carbon black-based materials having a calibrated resistance. French 
application No. 2,423,036 to the assignee discloses a self-blending mastic 
compound containing silicon carbide particles of a calibrated constant 
granulation or grain-size disposed at the level of the shielding end so as 
to extend onto the outer surface of the unshielded conductor insulator 
along a required distance to prevent electric discharges and to obtain the 
desired electrical characteristics at the termination end. Said different 
products show, however, limited performances as a result of heating in 
over-voltage conditions, and there are no hints to utilize same for 
manufacturing efficient electrical stress reduction or control electrodes 
on high voltages, nor particularly on extra high voltages. 
On the other hand, in manufacturing cable terminators, it has been proposed 
to substitute pre-manufactured junction equipment assemblies of synthetic 
rubbers for the insulators of glass, porcelain or epoxy resins, thereby 
facilitating handling and mounting in situ. However, said junction 
equipment assemblies show limited electric performances, as also the 
junctions and the branching equipment where moulded cast iron boxes have 
been replaced by ribboned or cold-casted resin assemblies. 
SUMMARY OF THE INVENTION 
It is an object of the invention to provide an electrical stress reduction 
or control electrode exhibiting improved electrical properties on high and 
extra high voltages and which allows realization of improved ready-to-use 
prefabricated equipment. 
Another object of the invention is to provide such an electrode which may 
achieve a linear repartition of electrical voltages along the outer 
visible or unshielded portions of the insulator of an electrical 
conductor. 
According to the invention, there is provided an electrical stress 
reduction or control electrode to be arranged at the level of a shielding 
end of an electrical conductor around the periphery of the visible or 
unshielded portion of the conductor insulator, which comprises at least a 
sheathing member including at least two adjacent successive zones in the 
axial direction of the conductor, each zone exhibiting a controlled 
non-linear electrical resistance different from the electrical resistance 
of the adjacent zone. 
According to a feature of the invention, the sheathing member is made of a 
support compound (i.e., binder) of plastic material including particles 
exhibiting non-linear resistance characteritics, two adjacent zones of the 
sheathing member each having a different density of particles in the 
plastic material. 
According to an embodiment of the invention, the successive zones 
incorporate particles of different grain sizes. In an alternative 
embodiment, the different zones incorporate different concentrations of 
particles having substantially the same grain size. 
These and further objects of the invention will be apparent from the 
following specification, taken in conjunction with appended drawings 
forming a part thereof.

DETAILED DESCRIPTION OF THE BEST MODE OF THE INVENTION 
In the following specification, analogue or identical integers of the 
terminators or junctions are identified by the same reference numerals, 
eventually with an indicia. 
The invention will be first disclosed more particularly with respect to a 
prefabricated termination assembly for a high voltage insulated cable 
(such as a 63 kV cable) illustrated in FIG. 2. As seen in FIG. 2, the end 
of the conducting core of the cable is electrically connected to a cable 
lug 2 for connection to a high potential. Said conducting core extends 
within a surrounding insulating sheath 3 which is in turn normally 
surrounded, all over the length of the cable, by a conducting shield 4. 
The above-mentioned problems of potential gradients occur at the level of 
the end of said shield 4 along the visible surface of the insulator 3 of 
the conducting core. The prefabricated cable ending comprises typically a 
series of insulating conical skirts 5, for instance of moulded 
cross-linked EPDM, which are slipped over the insulator 3 above an outer 
insulating sleeve 6 advantageously made of cross-linked EPDM. The 
insulating sleeve 6 extends over the end of the shield 4 while being 
axially closed there by an adaptor 7 of an electrically conducting 
elastomer compound insuring electrical contact with the conducting shield 
4 of the cable. 
According to the invention, the stress control or reduction electrode, 
generally designated by reference numeral 10, is arranged between the 
outer sleeve 6 and the cable insulator 3 and comprises a sheathing member 
having four successively axially joined zones A,B,C,D applied onto the 
cable insulator 3, the successive zones A to D being adjacent one to each 
other in the axial direction of the electrode 10. The end electrode zone A 
is in bounded contact with the adaptor 7. The different zones A to D each 
exhibit, for a determined voltage, a non-linear electrical resistance 
different from the resistances of the adjacent zones so as, from A to D, 
the electrical resistance progressively increases toward the high 
potential end of the terminator. Typically, the stress control electrode 
10 is obtained by embedding within an insulating support compound or 
binder of plastic material finely divided particles of a material 
exhibiting an electrical resistance which is non-linear and varies with 
the applied voltage V. Assuming that I is the current intensity through a 
control electrode zone, there is the following relation: I=kV.sup.Y, k and 
Y being constants and Y being greater than 1. 
According to a feature of the invention, the resistance variation of the 
different zones of the electrode 10 is achieved by progressively 
diminishing the density of the particles embedded within the insulating 
plastic material of the electrode toward the portion thereof remote from 
the conducting members, whereby said variation in the density of the 
particles in the successive zones confers to the electrode a non-linear 
electrical resistance decreasing toward the parts to be protected, i.e. a 
resistance which increases toward the parts at a high potential. The 
variation in density may be obtained by progressively setting the 
concentration of particles having a substantially constant grain size of 
the active material exhibiting a non-linear variable resistance or, 
alternatively, by determining,, for the different zones of the electrode, 
a constant concentration of the particles, said latter having, however, 
different grain-sizes from one zone to another, by taking into account the 
fact that the effect conferred by the particles of active material is the 
more important as the grains are greater for a same overall volume of the 
finished product. In the present specification, there are disclosed stress 
control electrodes which comprise a series of end-to-end successive 
individual elements or zones, but it should be understood that the 
resistance variations may be obtained by varying continuously in the axial 
direction the concentration of particles of a substantially constant grain 
size in a common sleeve-shaped element formed of a support compound or 
binder of plastic material, said concentration of particles in the plastic 
support compound or binder varying for instance from 30 to 70%. 
In the embodiment illustrated in FIG. 2, the binder or support compound 
constitutive of the different elements or zones of the stress control 
electrodes may be a plastic material chosen from the group comprising 
polyethylene, PVC, thermoplastic or cross-linkable elastomers such as a 
PDM, EPR, polyurethane, polyester, elastomers including cold-or-hot 
castable or hardenable components chosen from the group comprising 
silicones, polyurethane, polybutadiene, the resins including cold-or-hot 
hardenable components such as polyester, polyurethane, epoxy resins or 
phenolic resins. Use can also advantageously be made of the elastomer and 
liquid-containing mastic compound disclosed in the above-mentioned French 
application No. 2,423,036, the content of which is incorporated herein by 
reference, of insulating liquids such as naphtenic, aromatic or paraffinic 
oils, silicone oils, as also of ceramics, porcelain, refractory earth, 
enamel, glass, concrete or plaster, or heat-shrinkable plastics which are 
put in place by means of a portable heat generating means. 
The support compounds constitutive of the electrode are determined for 
allowing a controlled incorporation therein of particular active material 
exhibiting variable electric resistance properties, chosen from the group 
consisting silicon carbide, metallic oxides such as natural or 
precipitated silica, titanium oxide, zinc oxide, magnesium oxide, alumina 
or asbestos. Typically, the part members or zones of the stress control 
electrode are manufactured by incorporating within a support compound from 
about 30 to 85% of a mixture of said particular active products. In a 
generic way, the active product is chosen for each zone of the electrode 
with respect to the position of said zone in the electrode and depending 
upon the mesh dimensions of the sieve or the screen defining an average 
grain size for the particles. Use can be made of particles from a screen 
of 8 wires corresponding to an average grain size of 2380.mu. up to a 
screen of 1200 wires corresponding to an average grain size of 3.mu.. In 
the embodiments wherein the progressive electric resistances of the 
different zones of the electrode are achieved by modulating the grain size 
of the active product particles, typically silicon carbide or zinc oxide 
particles, the dimension of the particles will be diminished from zone A 
to zone D so as to distribute in a linear fashion the equipotential lines 
(see FIG. 10), the determination of the grain size of the particles in 
starting end zone A being determined with respect to the serviceable 
voltage of the cable. 
As an illustrative mode for manufacturing the electrode of FIG. 2, the 
cable terminator 3 being made of a 20 kV, 1.times.150 mm.sup.2 
polyethylene, zone A is made of the above-mentioned mastic compound 
including 70% by weight of silicon carbide particles screen 90, zone B of 
such a mastic including 70% of silicon carbide particles screen 180, zone 
C of the same mastic compound including 70% of silicon carbide particles 
screen 280, and zone D of the same mastic compound including 70% of 
silicon carbide particles screen 500. It is to be understood that the 
determination of the grain sizes in the different zones is not limited to 
the above example. The dimensions (length in the axial direction, 
thickness in the radial direction) of the active zones of the electrodes 
are not critical but depend closely on the available space around the 
cable insulator in the terminator or junction. As a rule, the thickness of 
the electrode increases with the serviceable voltage and is chosen greater 
than 5/10 mm for a voltage of 20 kV and greater than 5/8 mm for a voltage 
of 200 kV. Yet improved electrical characteristics are obtained by 
continuously varying the grain size of the particles from the coarser 
screen towards the thinner screen along the axial length of the continuous 
sleeve-shaped element of the electrode. In another embodiment, for a 
similar cable terminator, the variation of resistance is obtained by 
modifying from a zone to another zone the concentration of particles 
having an average grain size. Zones A to D are thus obtained by dispersing 
in the mastic compound 70%, 60%, 50% and 40% respectively of silicon 
carbide particles having a substantially constant average grain size of 
37.mu.. In said particular embodiment, each zone A to D has an axial 
length of 7 cm and a thickness of 3 mm, thereby resulting in a stress 
control electrode having a length of 28 cm for an overall length of the 
cable terminator of 45 cm. The following results have been determined: 
partial discharge at the nominal voltage &lt;1 pico coulomb, 
1/50 shock wave: 250 kV flash-over, 
dielectric strength: 1 hour at 125 kV, 
flash-over voltage: 10 mn at 135 kV, 
i.e. characteristics which are 65% better than the characteristics of the 
prior-art techniques. 
There are plotted in FIG. 1 curves showing the good linearity of the 
distribution of the equipotential lines achieved with a stress control 
electrode according to the invention (curve II) as compared with a stress 
control electrode the support compound of which homogeneously includes 
particles of constant grain size (curve I). The voltage measures along the 
stress control electrodes are in ordinates when the axial distance from 
the shield end 4 are in abscissae. 
The cable terminator illustrated in FIG. 2, including a stress control 
electrode 10 in tightly contacting relationship with the cable insulator 
3, is more particularly convenient for insulated cables on a serviceable 
voltage not higher than 63 kV, thereby leading to a maximum potential 
gradient G.sub.max &lt;4000 V/mm at the electrode/insulator interface. 
There is illustrated in FIG. 3 another embodiment corresponding to a 
terminator for an insulating cable on a nominal voltage of 90 kV and 
including, between the stress control electrode 10 and the cable insulator 
3, an intermediate underlying insulating layer 8 having a thickness close 
to that of the electrode and a relative permittively .epsilon. not lower 
than the permittivity of the cable insulator 3. For higher voltages, 
several underlying layers 8 may be provided, the permittivity 
.epsilon..sub.i of each of which is determined so as to increase 
progressively up to the value .epsilon..sub.1 corresponding to the more 
conducting zone (A) of the stress control electrode 10. Said underlying 
layers 8 radially separate the stress control electrode from the 
conducting core of the cable, so as to maintain, in combination with a 
deflecting cone at the level of the electrode 10, a maximum potential 
gradient which is always lower than 4000 V/mm, as above mentioned. The 
different zones A-D of the stress control electrode 10 in the embodiment 
of FIG. 3 are obtained by dispersing in EPDM silicon carbide particles 
having the following dimensions: from A to D, 60% of particles screen 90, 
60% of particles screen 180, 60% of particles screen 280, and 60% of 
particles screen 500, respectively. The measured performances are as 
follow: 
partial discharge &lt;1 pico coulomb at 80 kV, 
1/50 shock wave: 525 kV flashover, 
dielectric strength: 1 hour at 200 kV. 
Substantially homothetical dimensions may be adopted for stress control 
electrodes for cable terminators on voltages of 110 kV, 150 kV, 225 kV and 
beyond. The intermediate underlying insulating layer 8 may be made of any 
convenient dielectric material such as a solid dielectric material, a gas 
under pressure (for instance nitrogen), sulphur hexafluoride, or confined 
insulating oil or grease. 
There is illustrated in FIG. 4 an embodiment of a similar cable terminator 
comprising three identical radially spaced concentric stress control 
electrodes 10.sub.1, 10.sub.2 and 10.sub.3 each comprising four successive 
zones A to D. The different electrodes are separated from each other and 
from the cable insulator 3 by insulating underlying layers 80 each having 
a relative permittivity .epsilon. greater than that of the cable insulator 
3. 
In the embodiment illustrated in FIG. 5, the electrode 10 is received or 
embedded within a sleeve member 61 which comprises, in addition to its 
usual outer peripheral portion, an inner axially extending tubular 
protrusion 8' which acts as the above-mentioned underlying layer and has a 
relative permittivity .epsilon. greater than that of the cable insulator 
3. In the embodiment of FIG. 6, which is similar to that of FIG. 5, the 
zones A' and D' of the control stress electrode 10 have a thickness in the 
radial direction which decreases from the end of the cable shield 4, the 
adjacent tapered thicker end zone A' further comprising an outwardly 
axially extending skirt portion a.sub.1 which completely surrounds the 
conducting adaptor 7. 
In the embodiment of FIG. 7, similar in some points to that of FIG. 3, the 
distal end zone D of the electrode 10 is prolongated axially outwardly 
beyond the underlying layer 8 to be pressed directly onto the cable 
insulator 3, thereby separating the distal end of the outer insulating 
sleeve 6 from said cable insulator. The dimensions and number of the zones 
of the electrode 10 may vary depending upon the number of the insulating 
skirts 5 of the prefabricated cable terminator. 
There are illustrated in FIGS. 8 and 9 two embodiments of a prefabricated 
integral cable junction utilizing another stress control electrode 
according to the invention. In said figures, the outer sleeve 6' is here 
prolongated axially so as to have its opposite end brought into tight 
bearing contact with the end portions of the shields 4 of the ends of the 
joining cables, said outer sleeve 6' being also made of a conducting 
plastic material. The zones A to D of the electrode 10 are here serially 
arranged or distributed with respect to the junction zone between the 
cable ends where the stripped conducting cores 1 of the cables are 
visible. The distal end zone D is, however, not prolongated up to the end 
of the adjacent shield 4 of the corresponding cable end, the end portion 
of the outer sleeve 6' being interposed therebetween. As in the preceding 
embodiments, the less resistive electrode zone A, which is here a middle 
zone of the electrode, is in bounded contact with the adaptor 7 made of a 
conductive compound which surrounds the joined conducting cores 1 of the 
cables. The electrode 10' is prolongated symmetrically (not shown) toward 
the other end of the shield 4 of the other cable end. FIG. 9 illustrates 
such a prefabricated integral cable junction but including a stress 
control electrode 10' having zones in opposed activity relationship. In 
said embodiment of FIG. 9, the less resistive middle zone A of the 
electrode still contacts the adaptor 7, but the previous zone B is omitted 
so as the electrode 10' presents, toward each cable shield 4, a first 
series of zones C,D and then, a second series of zones C.sub.1 (identical 
to zone C and in symmetrical relationship thereto with respect to zone D) 
and A.sub.1, analogue and symmetric to zone A but in contact with the end 
of sleeve 6'. 
There is illustrated in FIG. 10 a prefabricated integral cable junction for 
adapting different cable diameters, which comprises, as in the embodiment 
of FIG. 9, a stress control electrode 10' the active zones of which are 
arranged in opposed relationship. In said embodiment, there is provided 
between the electrode 10' and the cable insulator 3 an underlying layer 8 
of the above-described character, the electrode 10' being in turn 
surrounded by an insulating sleeve 9 the permittivity of which is also 
greater than that of the cable insulator 3. As in FIG. 9, the electrode 
10' is symmetrical with respect to its middle zone D having a low density 
of silicon carbide particles. Each end zone A and A.sub.1 is in contact 
with an adaptor 7 arranged at the levels of the end of the shield 4 and of 
the conducting core 1 of the cable, respectively, said core being 
electrically connected to a conducting member 2' held within an insulating 
housing 16. The equipotential lines plotted in FIG. 10 illustrate the 
efficiency of the stress control electrode according to the invention. 
There is illustrated in FIG. 11 a prefabricated integral branching wherein 
the conducting core of a first cable having a peripheral conducting shield 
4 (top of the figure) is electrically connected to a connecting block 11 
arranged within an insulating housing 12 and from which extend a main line 
and a derivation line, respectively (bottom of the figure), each having a 
conducting core 1 connected to said connecting block 11. Each of said 
last-mentioned output lines is equipped with a stress control electrode 
10' the active zones of which are in a symmetrical or opposed relationship 
as disclosed in relation with the embodiment of FIG. 10. It should be 
noted that, in said branching, the adaptors 7 are common to both output 
lines and that, in addition to the intermediate surrounding insulating 
sleeves 9 around each electrode 10', an insulating material 13 completely 
fills the housing 6' of the branching. 
There is illustrated in FIG. 12 a plug-in cable end which comprises a bent 
connection portion 15 and an adapting portion including a stress control 
electrode 10' having its active zones in an opposed relationship as in the 
embodiments of FIGS. 10 and 11. 
Although the electrode of the invention is more particularly convenient for 
a compact prefabricated electrical equipment utilizing elastomeric 
materials, it can also be for use in improving the performances of usual 
porcelain insulators. There is illustrated in FIG. 13 a cable terminator 
analogue to that of FIG. 3 but including here a hollow block of porcelain 
skirts 5'. The stress control electrode 10, similar to that of the 
embodiments of FIGS. 2 to 6, is applied onto the inner wall of the block 
5' at a radial distance from the conductor insulator 3. The inner space 
between the cable insulator 3 and the block 5' is axially closed adjacent 
the cable shield end 4 by a frusto-conical adaptor 7 and filled with an 
insulating fluid 14 such as a gas under pressure, for instance air or 
nitrogen, an insulating grease or an insulating oil. 
Although the present specification relates to preferred embodiments, it 
should be understood that the invention is not limited to said embodiments 
and includes all modifications and developments within the scope of the 
appended claims.