Note: Descriptions are shown in the official language in which they were submitted.
:
The present invention relates to a high voltage cable
insulated with synthetic insulation around which a semiconduct-
ing layer is applied, and more specifically to a high voltage
cable in which the semiconducting layer has a predetermined
specific conductance.
In a typical high voltage cable, an inner semiconducting
tape or layer is wrapped or extruded around a metal conductor
and a layer of insulation is then extruded over the inner layer.
An earth screening or grounding element is thereafter applied
concentrically over the insulation, which element usually con-
sists of a semiconducting layer and a metallic earth or ground
return screen, whereby a uni~orm e~uipotential surEace around
the insulation is createcl. ~ stud~ O:e the c~lrrent proportions
in the layer shows that the outer semiconducting layer conducts
a capacitive current across the layer in a radial direc-tion from
the conductor to the surrounding screen. Furthermore, a resis-
tive current appears in the layer and equalizes the voltages
which, due to an eventual non-uniform field distribution, appear
in a peripheral direction. The surrounding cable sheath can be
applied on the metallic screen nearest the outer semiconducting
layer.
In synthetic cables, i.e. cables having some form of
extruded plastic or rubber insulation, the inner semiconductor
is usually applied in the same operation as the insulation. It
has been found preferable to also apply the outer semiconductor
in the same operation, a so-called triple extrusion. The semi-
conducting layer and the insulation material then adhere well
together and provide a mechanically and electrically reliable
product. Triple extrusion has primarily been used on cables ~or
high voltage applications and moreover when specially trained
installation personnel are used.
One problem associated with known high voltage cables
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relates to reliable cable preparation at an assembling works.
The preparation re~uires that parts of the cable sheath or
jacket be removed together with the screening layers and the
insulation in order to reach the conductor. In some known cable
construct ons it is re~uired, in order to facilitate the prepara-
tion of the cable, to manufact-ure the outer semiconductor in the
form of tapes which are directly applied on the insulation or as
layers painted or sprayed outside the insulation Eollowed by
semiconducting tapes wrapped around the painted layers. It is
also known in the art to extrude around the insulation a "t~re"
or sleeve of semiconducting rubber, for example, which tightens
around the insulation. A ~rincipal disadvantage with the ore-
going known methods is that a corona discharge can arise in the
air slots or spaces which are created at the overlap portion o~
the semiconducting tapes. ~lso, clearances can arise between a
loosely applied semiconducting layer and the insulation at points
of mechanical and thermal stress. Furthermore, the semiconduct-
ing paint can he difficult to remove, especially if it has burnt
or bonded firmly onto the layer underneath in the event of an
overheating condition.
At the ends of the cable, where the semiconductor has to
; be removed a predetermined length from the connection point,
high longitudinal field strengths appear at the thus created
screen edge. It is known to decrease the Eield strength at an
abruptly terminated screen by arranging layers having a prede-
termined ~ ~ resistancehout~side the insulation, which lavers
extend Erom the screen edge to the conductor. ~ccordingly, a
part of a leading earth or ground return current of the cable
will flow through the resistive layer which creates a dispersed
potential rise that decreases the field strength and prevents
corona discharge.
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Other field strength equalizing methods are also known.
The methods used require special material or special accessories,
a skilful assemble~ and time consuming work. Especially
troublesome is the removal of substantially all traces of the
semiconducting layer, particularly when it adheres well to the
insulating surface. Manufacturers therefore endeavour to
make semiconducting layers which can be simply separated from
the insulation surface without leaving portions thereon.
However, the easier it is to strip away the semiconductor,
the greater is the risk for damage occurring at points of stress.
The object of the present invention is to provide a
cable having one or more cable cores, each of which includes
an outer semiconducting layer having good electrical and
mechanical stability which substantially eliminates the foregolng
inconveniences and problems when terminating the aable.
Accordingly, the invention provides an electrical
cable, including a conductor and a surrounding insulation of
synthetic material together with an outer layer of semiconducting
material disposed in good adherence on the surface of the
insulation and extending substantially along the whole length
of the cable, the outer semiconducting layer being formed
of synthetic material having a resistance at the surface
thereof that is substantially voltage independent and which
exce~ds a value of 106 ohms per square.
The invention will now be more particularly described
with reference to an embodiment thereof shown, bv way of example,
in the accompanying drawings in which:
Fig. 1 is a plan view of an end portion o~ a cable
according to the present invention; and
Fig. 2 is a side view, partly in section, of a cable
end portion according to the present invention together with a
graph showing a voltage gradient thereat.
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The description of the invention as follows covers
a single core cable but is applicable to a separate core of
a multiple core cable.
In the cable according to Figure 1 an inner conductor
1 comprises a plurality of twisted and packed wires. Around
the conductor 1 an inner semiconducting layer 2 is applied
in the form of semiconducting tapes or extruded semiconducting
material to equalize any ~oltage stresses from the lndividual
wires of
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the conductor 1. A layer o insulation 3 is provided which
consists of, for example, polyethylene material having a thick-
ness that is determined by the rated voltage of the cable and
is applied around the layer 20 Outside the insulation 3 an outer
semiconducting layer 4 is applied, usually by extruding and
subsequent vulcanizat.ion.
In known cables, the resistance, of the outer semi-
conducting layer, measured longitudinally along the outer surface
of the cable core, is low and at most about 106 ohms per square.
Thus, a field pattern in the cable is obtained wh.ich shows small
voltage gradients occurring in a tangential direction at
frequencies higher than the power frequency, for example, during
transient e~ents such as lightning discharges ancl the like.
Calculations show, however, that the resistance of the layer can
be increased to considerably more than the usual values oE 102 _
104 ohms per square, commonly used in practice while maintaining
rellability of service. According to the invention, a value
greater than 106 ohms per square, is chosen, for example, 107
ohms per square. Accordingly, the advantages of the layer 4
as a field equali2ing device is essentially maintained and
certain further advantages will be atained at terminal ends of
the cable as will be apparent from the description in connec~ion
with Figure 2.
The semiconducting layer 4 having a resistance of high
value may consist of polyethylene material doped with admixtures
of carbon, for example carbon black, to obtain the desired
specific resistance value. The layer is then applied suitably
by extrusion or preferably by triple extrusion which gives the
best electrical and mechanical stability. It is also possible
to apply the outer semiconductor layer ~ by means of continuous
lacquering with a lacquer having a high resistance in principally
the same manner as it is applied on layers having low values o
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resis~ance. For example, the layer can be applied by spraying,
dipping or by electrostatic painting. In this way a good
adherence to the underlying insulation layer can be attained.
The extruded outer semiconductor 4 according to the invention
can be an elastomer, thermoplastic or cross-linked plastic,
(vulcanized) which, in the manufacturing process, can be made to
completely adhere to the insulation surface so as to virtually
eliminate the possibility of corona discharge. In certain cases
the semiconducting layer 4 can be covered with an applied layer
5 of conducting plastic, textile, synthetic fibre or the like
having a resistance per square value of conventional magnitude.
In this event, the object of the invention has not been abandoned,
as this outer material can be simply removed when preparincJ a
cable termination.
In Figure 2 the volt~ge dlstribution along a cable
termination is shown in a graph to illustrate the advantage of
the invention. The cable termination is shown in a longitudinal
section and as in Figure 1 there is shown an inner conductor 1,
on which the inner semiconducting layer 2 is applied. The outer
semiconducting layer 4 is disposed on the insulation 3 and in
order to discharge any field currents to earth an earthed
metallic screen 6, fabricated of copper wire, is applied in a
known manner. The screen 6 is surrounded by a cable sheath 7.
Beneath the screen 6 there is provided an extra semiconducting
layer 5. In the graph, the potential of the conductor is de-
signated~ro, for example 1?/ ~ or 24/ ~ kV, the screen 6 having
the potential 0. ~t the termination of the cable, -the earthed
screen 6 is removed together with the semiconducting layer 5 so
that an end portion of length R of the outer semiconducting
layer 4 is uncovered. Furthermore, part of the inner semi-
conducting layer 2 and the insulation 3 has been removed,
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so that the conductor l is uncovered at the very end of the cable.
Th~ outer semiconducting layer 4 is brought into electrical
contact with the conductor at the free end of the cable, for
example, by applying some layers of semiconducting tape 8.
Of interest is a voltage distribution which arises in
the outer semiconducting layer 4 along the portion Q between the
screen 6 and the inner conductor 1. At too high a voltage
gradient, an undesirable corona discharge arises in voltage tests
which are prescribed by standard references. The discharge will
occur at the screen edge of conventionally fabricated cables,
i.e. the edge which is formed when the insulation of the cable
i9 uncovered ~rom a conducting surface covering if no special
measures are taken. With a cable construction according to the
present invention this surface covering, i.e. the outer seml-
conducting layer 4 is not removed, but 1~ mainta~ned and Eulfills
its function to equalize the longitudinal field between the outer
screen 6 and the inner conductor 1. From the graph of Fig. 2 it
appears that by means of a semiconducting layer, the resistance
per square of which has been chosen according to the idea of the
invention, an even voltage distribution along the uncovered cable
path having the length Q is obtained. The wave forms in the
diagram are shown for different values of resistance per square
~ for the outer semiconducting layer 4, and for a predetermined
value ~ = lO ohms per square an approximately linear voltage
distribution can be obtained. With a layer o~ lower resistance,
i.e. about the value range lO2 - 104 ohms per square, too great
a power is developed in the semiconducting layer which could
cause a fire. As a precaution it should be noted that concen-
trated creeping current paths and surface flash-over will arise
as a consequence of the semiconducting layer 4 being completely
homogeneous in its resistance per square.
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