Flexible gas-insulated electrical cable having non-metallic flexible inserts between central conductor and support insulators

A soft silicone rubber insert is compressed between the interior surface of an insulator support in a gas-insulated cable and the outer surface of a soft aluminum central conductor which is supported within the interior surface of an opening in the insulator. The flexible rubber insert prevents scratching of the surface of the aluminum conductor and the production of aluminum particles during relative movement between the insulator and the aluminum tube.

BACKGROUND OF THE INVENTION 
This invention relates to flexible high voltage gas-insulated cable, and 
more specifically relates to a novel soft insert pad disposed between a 
support insulator and the conductor supported thereby within the interior 
of the cable to prevent the generation of conductive particles within the 
housing of the cable. 
Flexible gas-insulated cable structures for transmission and distribution 
of electric power at high voltage are well known and commonly consist of a 
central conductor supported within an outer housing which is filled with a 
dielectric gas such as sulfur hexafluoride under positive pressure. Both 
the outer housing and central conductor are corrugated or are otherwise 
made flexible to enable the reeling of the cable so that it can be shipped 
by truck or railroad car, and to enable the installation of the cable in 
an irregular terrain or ditch such that the cable will conform to the 
contours of the location in which it is installed. 
The central conductor of the cable is frequently aluminum and the support 
insulators which support the central conductor within the outer sheath are 
usually a molded thermoplastic material such as an acrylic Plexiglass 
DR61K. 
A preferred aluminum alloy, which is used for cable of this type, is a 
relatively soft high conductivity aluminum such as a 1100 aluminum alloy. 
The material of the support insulators is harder than the relatively soft 
aluminum so that during the manufacturing process and during the process 
of bending or reeling the assembled cable when there are axial stresses 
and possible movement between the central aluminum conductor and the 
plastic housing insulator, it is possible for the plastic insulator to 
gouge or abrade the outer surface of the soft aluminum, producing 
scractches on the aluminum surface and small chips of aluminum. 
During operation, the aluminum particles, which are in an exceptionally 
high electric field since the small diameter cable might have a rated 330 
kV or higher between the central conductor and outer housing, can cause 
breakdown of the dielectric gas and failure of the cable. Similarly, 
scratches in the aluminum surface can lead to corona discharge and 
ultimate failure of the cable. 
Cables of the general type disclosed above are shown in numerous issued 
U.S. patents including U.S. Pat. No. 4,100,367 in the name of Netzel, U.S. 
Pat. No. 4,095,041 in the names of Netzel and Ponder, U.S. Pat. No. 
4,101,730 in the name of Netzel, U.S. Pat. No. 4,122,298 in the name of 
Brandt. 
BRIEF DESCRIPTION OF THE INVENTION 
While the production of metallic particles in high voltage cable and other 
high voltage apparatus is a known problem, the source of the particles is 
not always known. It has now been recognized that particles are produced 
when using a desirably soft aluminum central conductor with a harder 
support insulating body which can produce the gouging referred to above 
and thus the production of aluminum chips and irregularities in the 
surface of the central conductor. 
In accordance with the invention and following the recognition of this 
problem, a novel intermediate pad is formed between the interior of the 
support surface of the insulator and the exterior of the aluminum 
conductor to prevent rubbing between the two and the gouging of the 
aluminim surface. This pad can take the shape of a molded pad or can be 
produced in any other desired manner and consist of a resilient material 
such as silicone rubber. The pad may be put in place before or during the 
assembly of the cable, wherein a mult-section insulator disk is clamped 
around the central conductor to firmly press the pad between the central 
conductor and the interior surface of the insulator. 
A wide variety of materials has been successful for the pad. The principal 
characteristic of the pad material is that it meets the temperature 
requirements of the cable and will retain its resiliency over a long 
period of time. For example, the material selected should withstand 
temperatures of 150.degree. C. over an extended period of time. Silicone 
rubber satisfies these requirements. 
The novel pad of the invention will also improve the mechanical strength 
and electrical properties of the insulator. Thus, the silicone rubber will 
fill the space between the insulator and conductor to improve its 
dielectric properties by reducing the electric field stress at the 
insulator conductor interface and by eliminating the abrasion and 
subsequent particle generation caused by rubbing contact between the 
insulator and conductor. Moreover, the pad improves the mechanical 
strength of the insulator by spreading the loading force over a wider 
area. Not only does the resiliency of the pad reduce mechanical load 
concentration at the insulator interior, but it also improves mechancial 
shock withstand capability of the insulator. 
The novel insulator pad of the invention can take the form of a molded pad 
which is molded into the interior of the insulator during the manufacture 
of the insulator halves. Alternatively, the pad can take the form of a 
sheet of rubber laid between the insulator and conductor during their 
assembly. Sheets of both silicone rubber and Teflon have been found to 
perform as well as a molded pad, even though the space between the 
conductor and insulator is not completely filled by the sheet material. 
When using sheet material, it can be bonded either to the insulator or 
conductor, or simply held in place between the insulator and conductor 
without bonding. The sheet seam can be either butt-joined or lap-joined 
and both have worked successfully. 
The novel resilient pad can also take the form of a self-fusing silicone 
rubber tape which is wrapped around the conductor and fused into a solid 
mass of silicone rubber surrounding the conductor. The insulator is then 
assembled onto the conductor over the section covered by the self-fusing 
tape. 
It is noted that insulation sheaths have been used in the past between a 
central conductor and its support insulator for rigid cable. The 
insulation sheath covers the entire surface of the rigid central conductor 
and was intended to immobilize conductive particles and control the 
electric field which causes movement of conductive particles. The problem 
of gouging a soft conductor by a hard insulator, however, does not exist 
because the assembly was not flexible. 
The invention can also be carried out by the use of a silcone rubber 
coating applied to the insulator core which is allowed to cure prior to 
insulator assembly.

DETAILED DESCRIPTION OF THE DRAWINGS 
Referring first to FIGS. 1 and 2, there is illustrated therein a flexible 
gas-insulating cable which incorporates the novel resilient pad of the 
present invention (FIG. 2) as will be later described. The gas-insulated 
cable may be of the general type described in any of the patents listed 
above. 
The insulator support is of the type shown in copending application Serial 
No. 82,130, filed Oct. 5, 1979, in the name of Philip C. Netzel, now U.S. 
Pat. No. 4,263,476. 
In FIGS. 1 and 2, the gas-insulated cable consists of an outer corrugated 
aluminum sheath 10 which is filled with sulfer hexafluoride gas under a 
positive pressure, for example, 45 p.s.i.g. A plurality of support 
insulators are spaced along the length of the cable in order to centrally 
support a central conductor 11 within the aluminum sheath 10. Conductor 11 
is corrugated as is aluminum sheath 10 to improve its flexibility. 
Conductor 11 is preferably made of a 1100 aluminum alloy which is a 
relatively soft and flexible aluminum alloy. 
FIGS. 1 and 2 show the central conductor 11 as a single corrugated tube. 
However, the central conductor 11 can take any desired form and can 
consist of a plurality of concentric tubes arranged in the manner 
disclosed in copending application Ser. No. 202,452, filed Oct. 31, 1980, 
in the name of Philip C. Netzel and Edward M. Spencer, entitled MULTIPLE 
WALL STRUCTURE FOR FLEXIBLE CABLE USING TUBULAR AND SPIRAL CORRUGATIONS, 
which is assigned to the assignee of the present invention. 
Regardless of the number of tubes used or structure used for the central 
conductor 11, it is only significant that the outer tube 11, whose surface 
will be gripped by support insulators spaced along the axis of the cable, 
is of a relatively soft conductive material in comparison to the hardness 
of the support insulator. 
Before describing the novel resilient pad of the invention, it is necessary 
first to understand the detailed structure of the support insulator. The 
support insulator structure is best shown in FIGS. 3, 4 and 5 and is the 
structure disclosed in U.S. Pat. No. 4,263,476. 
The insulator in FIGS. 3, 4 and 5 is made in two halves which are latched 
together over the central conductor they support, as will be later 
described, and may be made of any suitable plastic material, for example, 
in an injection-molding process. The resulting product has a hardness 
greater than that of 1100 alloy aluminum used for conductor 11. 
The insulator can be used with flexible gas-insulated cable for a high 
voltage transmission system having a relatively low frequency, for 
example, 60 Hertz, at high voltage, for example, 345,000 volts. The 
insulator outer diameter typically may be 336 millimeters and may support 
a central bus or conductor 11 having an outer diameter which typically may 
be about 120 millimeters in diameter. The insulator permits the bending of 
the cable to a radius having a ratio of reel diameter to cable diameter of 
about 9 to 1. Thus the cable can be wound to a small diameter and requires 
a small shipping reel for shipping the cable. 
The insulator structure generally consists of the two halves 20 and 21. 
Note that half 21 may have an opening therein (not shown) to permit 
passage of gas through the interior of the housing of the completed cable. 
Halves 20 and 21 are otherwise identical and the same identifying numerals 
used hereinafter will identify similar parts. 
Each of the halves is molded from a suitable plastic as a unitary part. 
They each consist of an outer rim 22, an inner rim 23 and a central 
connecting web 24. The inner rim 23 is also shaped for maximum dielectric 
efficiency and contains a central projection 26 which is shaped to fit the 
cooperating corrugation of the central conductor 11 to be supported by the 
insulator. 
The central web 24 is preferably as thin as possible to present as small as 
possible a cross-section to the high electric field which will exist 
between a conductor on the interior of the rim 23 and the grounded 
conductive housing surrounding the exterior of the rim 22. 
In order to join together the two halves 20 and 21, each of the halves is 
provided with integral cooperating latch members shown as the projecting 
latch 30 and a respective latch keeper region 31. During assembly, the 
latch 30 of one half will snap into the latch keeper 31 of the other half, 
as shown in FIG. 3. Note that the latch member 30 is made relatively 
flexible by cutting a notch 32 into the web 24 adjacent the latch 30. The 
keeper 31, however, is rigid. 
Each of the halves 20 and 21 is further provided with keying projections 
and notches which cooperate with one another to hold the halves fixed 
relative to one another after they are assembled. Thus, the half 20 of 
FIG. 5 is provided with key projections 35 and 36 and key depressions 37 
and 38. The key projections and key depressions 35 through 38 of adjacent 
insulator halves are automatically aligned with one another during the 
assembly of the two halves. 
Four reinforcing ribs 40 to 43 are provided for each insulator half. Each 
of the ribs 40 to 43 is identically shaped and tapers outwardly from the 
central web beginning at a point about one-third of the radial distance 
along the web from the central rim 23 and the ribs then taper or flare 
outwardly to join the outer rim 22. The outward flare is a relatively 
gentle flare and, for example, is on a radius of about 113 millimeters. 
Two reinforcing ribs 40 and 43 are preferably located immediately adjacent 
the keeper 31 and just behind the latch 30, respectively. It has been 
found that this placement of the ribs prevents breakage at these points 
which is the most frequent point of failure in insulators of the type to 
which the invention applies. 
The ribs taper over a distance of about two-thirds of the radial dimension 
of the web 24 and begin to taper outwardly only one-third of the radial 
dimension of the web away from the central conductor. The high dielectric 
stress adjacent to the central conductor is then on regions of the web 24 
which have minimum thickness. Thus, the provision of the reinforcing ribs 
40 through 43 does not cause undue dielectric stress within the insulator. 
The placement of the ribs in the latch and keeper areas 30 and 31, 
respectively, as pointed out previously, eliminates breakage at these 
points when insulators are tested for failure under radial loading. 
In accordance with the invention and as shown in FIG. 2, a self-supporting 
pre-molded pad of silicone rubber 50 is fixed to the interior surface of 
the insulator or to the exterior of the conductor 11, where the insulator 
is to clamp onto the outer surface of conductor 11. This pad may have a 
thickness, for example, of 1/8 inch and is compressed between the interior 
surface of the central opening formed between the insulator halves and the 
exterior surface of the aluminum tube 11 during their assembly. 
As a result of this pad, whenever there is relative axial movement between 
any of the insulators and the tube 11, as may occur due to relatively 
axial stresses during reeling and during assembly or during any flexure of 
the cable after installation, there will be no gouging of the relatively 
soft surface of aluminum tube 11 by the relatively harder interior of the 
insulator support. 
The molded pad 50 has the shape of a short cylinder and can be formed of 
two separate halves which are individually fixed to the two halves 20 and 
21 of the insulator. Other materials than silicone rubber can be used and, 
for example, Teflon has been found to serve an adequate function as a pad 
to prevent abrasion of the outer aluminum surface of conductor 11 by the 
harder insulation support. 
The relatively soft pad 50 distributes loading forces between the conductor 
11 and support insulator halves 20 and 21 over a wider area and thus 
improves the mechanical connection of the two components with respect to 
mechanical load concentration and mechanical shock withstand capability. 
The flexible pad 50 can take forms other than a pre-molded form. By way of 
example, pad 50 can take the form of a sheet of silicone rubber wrapped on 
the conductor or wrapped within the insulator. When wrapped on the 
conductor, the ends of the sheet can be either butt-joined or lap-joined 
and both methods have worked satisfactorily. 
Experiments have been carried out using a self-fusing silicone rubber tape 
which is wrapped around the conductor 11 before installation of the 
insulator halves 20 and 21. This tape is wrapped and fuses into a solid 
mass of silicone rubber which is then compressed during the assembly of 
the insulator halves over the silicone rubber covered sections of 
conductor 11. 
Other methods may be employed to connect a resilient pad between the 
opposing surfaces of conductor 11 and insulator halves 20 and 21 which 
will be well apparent to those skilled in the art. 
Although the present invention has been described in connection with a 
preferred embodiment thereof, many variations and modifications will now 
become apparent to those skilled in the art. It is preferred, therefore, 
that the present invention be limited not by the specific disclosure 
herein, but only by the appended claims.