Electricity: conductors and insulators – Insulators – With conductor holding means
Reexamination Certificate
2003-01-07
2004-07-06
Dinkins, Anthony (Department: 2831)
Electricity: conductors and insulators
Insulators
With conductor holding means
C174S019000, C174S169000, C174S176000, C174S178000, C174S211000
Reexamination Certificate
active
06759595
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
2. Description of the Related Art
The present invention relates to an outdoor termination for a high voltage cable and a manufacturing method for such an outdoor termination.
Outdoor terminations for high voltage cables typically consist of an insulator body in which the high voltage cable with its insulated cable core is accommodated. The high voltage cable is typically insulated with a polymeric material and the interior of the insulator body must be filled with an insulating filling compound to provide the necessary insulation.
FIG. 1
shows a typical construction of a inventional outdoor termination OT. It comprises an insulator body
2
having an upper cover
10
and a lower cover
11
, preferably made of metal, i.e. an upper metal work
10
and a lower metal work
11
. At the upper metal work
10
a conductor stalk
9
is provided with which the cable core
5
.
1
is connected. The lower metal work
10
is also connected to the insulator body
2
at a bottom portion thereof, for example by means of nuts and bolts
11
.
1
.
Within the interior of the insulator body
2
the cable CA extends wherein the cable core
5
.
1
is surrounded by an insulation
5
, which is typically made from a polymeric material.
At the lower portion of the insulator body
2
the high voltage cable CA is surrounded by an antikinking protection
7
to avoid a breakage of the cable. Also provided at the lower metal work
11
(the base plate) there is an entrance bell
8
having a connector
20
.
1
.
Through the entrance bell
8
the filling compound
3
can be injected into the interior of the insulator body
2
such that the filling compound
3
fills at least a portion of the space between the interior walls
2
.
1
of the insulator body
2
and the cable insulation
5
. Typically the outdoor termination is mounted in a substantially upright position such that a cavity
1
is formed at an upper portion of the insulator body
2
.
Furthermore, an electric field control means
4
in the form of stress cone
4
is provided at a lower portion of the insulator body
2
around the cable insulation
5
in order to appropriately set the electric field conditions inside the insulator body
2
. Typically, the insulator body
2
is made of porcelain or is a composite insulator of reinforced epoxy resin and siliconee sheds.
The critical components in the outdoor termination OT shown in
FIG. 1
are of course the upper and lower metal work
10
,
11
and in particular the filling compound
3
itself, with respect to the liquid/gas-tightness and with respect to possible temperature fluctuations and pressure variations.
The filling compound
3
, e.g. conventionally an insulating fluid, must possess the required dielectric properties and it must be chemically inert with respect to the material of the insulator body
2
, the cable insulation
5
and the material of the stress cone
4
.
The insulating liquids are quite expensive. It must in particular be observed that such outdoor terminations for high voltage cables comprise rather voluminous insulator bodies
2
such that a cost-intensive filling compound
3
will drastically increase the total cost of the outdoor termination OT. Typically, the outdoor termination is between 2 and 5 m long and its interior volume is between 50 and 1000 liters.
It must furthermore be considered that the outdoor termination OT is arranged in open space and thus exposed to all kinds of environmental influences, in particular large changes in temperature and/or large stresses due to snow or wind. Temperature changes cause changes in the volumina of the filling compound
3
accompanied by pressure changes. Even when large temperature changes occur, it must be avoided under all circumstances that a leakage occurs at the bottom part of the insulator body
2
. On the other hand, when there is a large drop in temperature, it must be avoided that air is sucked into the cavity
1
at the top of the insulator body
2
. Generally, it is accepted that the interior of the insulator body
2
must be protected against pressures of several bar.
To ensure that neither a leakage occurs at the bottom portion of the insulator body
2
nor air is sucked into the cavity
1
, the insulator body
2
must be effectively sealed and this requires components with high precision. In turn, several mounting steps are necessary when mounting the outdoor termination OT in the environment and this adds to the cost of the outdoor termination OT.
Even after installing the outdoor termination OT regular maintenance must be performed since loss of insulating liquid can substantially influence the operating characteristics of the outdoor termination.
Thus, on the one hand the substantial amount of expensive insulating liquid
3
increases the cost of the outdoor termination and on the other hand the provision of a liquid inside the insulator body
2
requires complicated mounting steps and requires regular maintenance. This applies to all high voltage cables CA, not only plastic insulated high voltage cables.
A conventional solution to reduce the cost of the outdoor termination OT is to attempt-to reduce the amount of necessary insulating fluid
3
. For example, insulator bodies
2
have been suggested which have a tapered shape towards the upper portion of the insulator body
2
such that the interior volume of the insulator body
2
is decreased. In principle, this is possible because the electric field strength decreases towards the upper portion of the insulator body
2
. Whilst in this manner the interior volume of the insulator body
2
can be decreased to some extent, on the other hand the manufacturing of a tapered insulator body
2
again increases the cost.
The maintenance work is essentially caused by the filling compound
3
being in a liquid state. Thus, conventionally also outdoor terminations OT have been suggested that do not require the use of an insulating fluid inside the insulator body
2
. Three different possibilities have been investigated:
A first example is an outdoor termination where a rubber-elastic insulating body including an integrated stress cone is pressed into a solid insulating casing which is connected with an insulator and central conductor. Whilst the rubber-elastic insulating body provides the necessary electric insulation, this construction requires several components whose dimensions must be matched very accurately. This leads to high costs and requires several manufacturing steps also leading to an increase of the costs. Whilst the use of an insulating liquid is not necessary, on the other hand the assembly is rather rigid and thus does not easily allow some movement due to winds and short-circuits.
A second example uses a rubber-elastic or heat-shrinkable insulating sleeve. This leads to a less rigid assembly which cannot tolerate the above mentioned mechanical impacts.
A third example is suggested in “Elektrizitätswirtschaft, Jg. 99 (2000), Heft 11: Trockene Freiluftendverschlüsse mit Stützeigenschaften” by R. Eitle and J. Kaumanns. Here, a dry rigid type outdoor termination is suggested. The solution presented here comprises a siliconee-based liquid insulating material which cross-links in the outdoor termination only after having been filled in. The insulating material is compressible. This property is achieved by “micro-spheres” with which the insulating material is filled. These “micro-spheres” are hollow and filled with gas. They have a size of about 100 &mgr;m. The “micro-spheres” serve as compensating volumina. Thus, even at extreme temperature changes no damages due to cracks or gaps are caused. Furthermore, the solid insulation material has some electrical properties.
However, even in the third example with the cross-linking siliconee-based material, there are disadvantages. Firstly, the cross-linking polymer is prepared on the basis of a siliconee elastomer and spheres are hollow and are filled with gas. The thermal conductivity of the siliconee elastomer is about 60 mW/mK and is thus only about 20% of the usual
Goehlich Lothar
Görk Claude
Dinkins Anthony
Finnegan Henderson Farabow Garrett & Dunner L.L.P.
Lee Jinhee
Pirelli Kabel und Systeme GmbH & Co. KG
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