Note: Descriptions are shown in the official language in which they were submitted.
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WO 99/17315 PCT/SE98/01748
A method and as arrangement for regulating a trane-
forsner/reactor, and a trana~former/reaetor
The present invention relates to a method and an ar-
rangement for regulating an induced voltage in a transformer,
alternatively regulating the reactive power of a reactor.
The present invention relates furthermore to a
transformer/reactor as defined in the preamble of Claim 36.
The present invention relates both to transformers
and reactors having a core, as.described below, as well as
air-cored transformers and reactors.
For all transmission and distribution of electric
energy, transformers are used and their task is to allow
exchange of electric energy between two or more electric
systems. Transformers are available in all power ranges from
a few VA up to the 1000 MVA region. The designation power
transformers normally relates to transformers with a rated
output from a few hundred kVA up to more than 1000 MVA and
with a rated voltage ranging from 3 - 9 kV and up to very
high transmission voltages.
A conventional power transformer includes a trans-
former core, referred to below as the core, made of lami-
nated, preferably oriented sheet metal, usually of silicon
steel. The core consists of a number of core legs connected
by a yoke. A number of windings are provided around the core
legs in the form of primary, secondary and regulating wind -
ing. In power transformers these windings are practically
always arranged in concentric configuration and distributed
along the core legs.
Conventional power transformers at the lower end of
the aforementioned power ranges, are at times manufactured
having air-cooling for the removal of inevitable losses in
the form of heat. Most conventional power transformers are
however oil-cooled and then as a rule by means of so-called
forced oil-cooling. This applies especially to high-power
transformers. Oil-cooled transformers present a number of
known disadvantages. They are among other things large,
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cumbersome and heavy contributing especially to great trans-
port problems. Extensive requirements need also to be met
with regard to security and peripheral equipment, of which
the requirement for an outer tank is especially noteworthy,
in which the transformer is to be contained in the event of
oil-cooling.
It has however largely been shown possible to re-
place oil-cooled power transformers with dry transformers,
i.e. oil-free transformers, of a new type. This new type of
transformer is provided with a winding designed with high
voltage insulated electric conductors, having solid insula-
tion, of a design similar to cables used for transmitting
electric power (for example so-called XLPE cablesy. Conse-
quently, dry transformers of this new type may be used at
considerably higher powers than what was possible with dry
transformers according to prior art.
With regard to reactors, they include a core which
is usually provided with only one winding. Moreover, that
which is stated in the aforementioned about transformers is
generally also applicable to reactors. It is especially
noteworthy that large reactors are also oil-cooled.
For different reasons it is often necessary to be
able to regulate or adjust the voltage of a power trans-
former. This may for example apply to maintaining the secon-
dart' voltage constant with a varying primary voltage; varia-
tion of secondary voltage; providing a reduced voltage in
order to start a rotating machine; providing a neutral point
for earthing or for dealing with out of balance current in
different circuits etc. For this reason transformers are
provided with an adjustable winding, referred to below as a
regulating winding, which may adjust the transformer ratio.
Regarding transformers for low voltages it is previ
ously known from FR 805 544 and GB 1 391 050 to vary the
effective length of the winding by means of a regulating
winding drum onto which the winding is wound or unwound.
However, this application is strictly limited to lower volt-
ages due to the completely different type of winding used in
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3
high-power transformers, where the winding is rigid, as well
as insulation problems associated with such a winding.
Known technique for conventional power transformers
in the higher power ranges, i. e. oil-cooled transformers, is
set forth in, for example, "The J&P Transformer Book" (A. C.
Franklin et al, 11th Edition 1983), describing how regulation
may take place in different ways. The two most common ways
are firstly the use of so-called off-load tap changers in
which tapping may take place between different voltage out-
lets inside the transformer tank, which can only take place
when the transformer is off-circuit, and secondly the use of
so-called diverter switches in which tapping takes place
between different voltage outlets extended to the outside of
the transformer tank and which can therefore take place on-
load.
During this tapping, parts of said regulating wind-
ing are thus connected to the concerned winding side so that
the desired voltage regulation is obtained. This must take
place stepwise where a typical value of the steppings between
taps is 10 winding turns. This arrangement presents the
disadvantage that despite a relatively large amount of taps,
which are necessary to give a reasonable amount of regulation
possibilities, the regulation possibilities are still limited
and far from a stepless regulation.
Correspondingly, reactors may be provided with a
regulating winding by means of which the reactive power of
the reactor may be regulated and which presents the corre-
sponding problem.
The object of the present invention is to provide a
method and an arrangement to solve the aforementioned prob-
lems and which allows for improved regulation possibilities
for transformers, alternatively reactors, especially of the
dry type, in the high power range. Another object is to
obtain such an improved transformer/reactor.
These objects are achieved by means of a method, as
defined in Claim 1, as well as by an arrangement_as defined
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WO 99/17315 4 PCT/SE98/01748
in Claim 10. The object is further achieved by means of a
transformer/reactor as defined in Claim 36.
The present invention relates thus to a method for
regulating induced voltage in a transformer, alternatively
for regulating reactive power in a reactor, wherein a winding
is achieved with an insulating electric conductor including
at least one current carrying conductor. a first layer having
semiconducting properties arranged to surround the conductor,
a solid insulation layer arranged to surround said first
layer, and a second layer having semiconducting properties
arranged to surround the insulating layer, wherein a regulat-
ing winding is arranged around a magnetic flux carrier, and
-.wherein the length of said regulating winding around the
magnetic flux carrier is varied.
The magnetic flux carrier may be a transformer core
or a reactor core, as described above. However, both the
method and the arrangement according to the present invention
are applicable also to air-cored transformers and reactors,
as also mentioned above.
According to the invention a corresponding arrange-
ment is defined, wherein said transformer/reactor includes at
least one magnetic flux carrier and a winding achieved with
an insulating electric conductor including at least one
current carrying conductor, a first layer having semiconduct-
ing properties arranged to surround the conductor, a solid
insulation layer arranged to surround said first layer, and a
second layer having semiconducting properties arranged to
surround the insulating layer, wherein said arrangement
further includes a regulating winding and a regulating means,
by means of which the length of said regulating winding
around the magnetic flux carrier is varied.
The defined method and arrangement have the advan-
tage that the length of the regulating winding may be regu-
lated continuously in a very simple manner, also in high
voltage transformers and reactors. However, an important
precondition to make this possible, is that the winding of
the transformer/reactor is designed with the aforementioned
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type of high-voltage insulated electrical conductor. Through
the use of such a conductor, or cable, the advantage is
achieved that the insulation problem is solved, which would
occur when a conventional winding is wound onto or unwound
from a regulating drum. Thereby, it is made possible to use,
for instance, drum regulation of the winding also for high
voltages, i.e. in distribution and power transformers.
Accordingly, the windings, in the arrangement ac-
cording to the invention, are preferably of a type corre-
lU sponding to cables having solid, extruded insulation, of a
type now used for power distribution, such as XLPE-cables or
cables with EPR-insulation. Such a cable comprises an inner
conductor composed of one or more strand parts, a first,
inner semiconducting layer surrounding the conductor, a
1~ solid insulating layer surrounding this and a second, outer
semiconducting layer surrounding the insulating layer. Such
cables are flexible, which is an important property in this
context since the technology for the arrangement according
to the invention is based primarily on winding systems in
20 which the winding is formed from cable which is bent during
assembly. The flexibility of an XLPE-cable normally corre-
sponds to a radius of curvature of approximately 20 cm for a
cable with a diameter of 30 mm, and a radius of curvature of
approximately 65 cm for a cable with a diameter of 80 mm. In
25 the present application the term "flexible" is used to indi-
cate that the winding is~"flexible down to a radius of curva-
ture in the order of four times the cable diameter, prefera-
bly eight to twelve times the cable diameter.
The winding should be constructed to retain its
30 properties even when it is bent and when it is subjected to
thermal or mechanical stress during operation. It is vital
that the layers retain their adhesion to each other in this
context. The material properties of the layers are decisive
here, particularly their elasticity and relative coeffi-
35 cients of thermal expansion. In an XLPE-cable, for instance,
the insulating layer consists of cross-linked, low-density
polyethylene, and the semiconducting layers consist of poly-
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WO 99/17315 PCT/SE98/01748
6
ethylene with soot and metal particles mixed in. Changes in
volume as a result of temperature fluctuations are com-
pletely absorbed as changes in radius in the cable and,
thanks to the comparatively slight difference between the
coefficients of thermal expansion in the layers in relation
to the elasticity of these materials, the radial expansion
can take place without the adhesion between the layers being
lOSt.
The material combinations stated above should be
considered only as examples. Other combinations fulfilling
the conditions specified and also the condition of being
semiconducting, i.e. having resistivity within the range of
10-1-106 ohm-cm, e.g. 1-500 ohm-cm, or 10-200 ohm-cm, natu-
rally also fall within the scope of the invention.
The insulating layer may consist, for example, of a
solid thermoplastic material such as low-density polyethyl-
ene (LDPE), high-density polyethylene (HDPE), polypropylene
(PP), polybutylene (PB), polymethyl pentene ("TPX"), cross-
linked materials such as cross-linked polyethylene (XLPE),
or rubber such as ethylene propylene rubber (EPR) or silicon
rubber.
The inner and outer (first and second) semiconduct-
ing layers may be of the same basic material but with parti-
cles of conducting material such as soot or metal powder
mixed in.
The mechanical properties of these materials, par-
ticularly their coefficients of thermal expansion, are af-
fected relatively little by whether soot or metal powder is
mixed in or not - at least in the proportions required to
achieve the conductivity necessary according to the inven-
tion. The insulating layer and the semiconducting layers
thus have substantially the same coefficients of thermal
expansion.
Ethylene-vinyl-acetate copolymers/nitrile rubber
(EVA/NBR), butyl graft polyethylene, ethylene-butyl-acrylat a
copolymers (EBA) and ethylene-ethyl-acrylate copolymers
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WO 99/17315 ~ PCT/SE98/01748
(EEA1 may also constitute suitable polymers for the semicon-
ducting layers.
Even when different types of material are used as
base in the various layers, it is desirable for their coef-
ficients of thermal expansion to be substantially the same.
This is the case with the combination of the materials
listed above.
The materials listed above have relatively good
elasticity, with an E-modulus of E<500 MPa, preferably
<200 MPa. The elasticity is sufficient for any minor differ-
ences between the coefficients of thermal expansion for the
materials in the layers to be absorbed in the radial direc-
tion of the elasticity so that no cracks appear, or any
other damage, and so that the layers are not released from
each other. The material in the layers is elastic, and the
adhesion between the layers is at least of the same magni-
tude as in the weakest of the materials.
The conductivity of the two semiconducting layers is
sufficient to substantially equalize the potential along
each layer. The conductivity of the outer semiconducting
layer is sufficiently high to enclose the electrical field
within the cable, but sufficiently low not to give rise to
significant losses due to currents induced in the longitudi-
nal direction of the layer.
Thus, each of the two semiconducting layers essen-
tially constitutes one ~quipotential surface, and these
layers will substantially enclose the electrical field be-
tween them.
There is, of course, nothing to prevent one or more
additional semiconducting layers being arranged in the insu
lating layer.
An example of an insulated conductor or cable suit-
able to be used in the present invention is described in more
detail in WO 97/45919 and WO 97/45847. Additional descrip-
tions of the insulated conductor or cable concerned can be
found in WO 97/45918, WO 97/95930 and WO 97/95931.
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WO 99/17315 $ PCT/SE98/01748
According to the present invention, the method may
further be characterized in that said regulating winding is
arranged on a regulating means, said regulating means being
rotatable around said magnetic flux carrier. As another
feature, a variable part of the regulating winding is trans-
ferred to or from at least one storage means.
A further preferred feature is that the method is
characterized in that the transformer/reactor includes a main
winding which may be connected to the regulating winding. The
method is furthermore characterized in that, starting from a
zero position in which there are no turns on the regulating
winding drum, the induced voltage/reactive power of a trans-
former/reactor respectively, is increased in that the winding
is wound onto the regulating winding means in the same direc-
tion as the direction of the main winding, and that the
induced voltage/reactive power of a transformer/reactor
respectively, is decreased in that the winding is wound onto
the regulating winding means in the direction opposite to the
direction of the main winding, whereby the maximum variation
of the number of winding turns is +-N, where N is the number
of winding turns which are available on the regulating wind-
ing means. The advantage achieved hereby is that the winding
may either be varied stepless or by an optional number of
turns, unlike prior art in which only predetermined combina-
tions of the number of turns was possible.
According to a particular feature the regulating
means includes a rotatable regulating winding drum.
Winding up the regulating winding in one direction
corresponds naturally to an unwinding of the regulating
winding in the opposite direction. Should the whole regulat-
ing winding be rolled-on in one direction, which is assumed
to be the same as the winding direction of the main winding,
obtaining therefore a maximum induced voltage/reactive power,
a reduction of the voltage/power naturally takes place by
unwinding the regulating winding firstly before starting to
wind it up in the opposite direction.
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WO 99/17315 9 PCT/SE98/01748
The arrangement, according to the present invention,
may further be characterized in that the regulating winding
is arranged on said regulating means and that the regulating
means is rotatable around said magnetic flux carrier. As a
further feature, it includes means for the transfer of a
variable part of the regulating winding to or from at least
one storage means. To continue, it includes the preferred
features that the regulating means includes a rotatable
regulating winding drum, and that the storage means includes
a rotatable storage drum. The winding up and the unwinding
preferably takes place by arranging the regulating winding
onto a rotatable means, such as said drum, but other solu-
tions are also possible. Also other solutions regarding the
storage means are conceivable, such as several drums, a reel
or coil, etc. or nothing at all.
According to another preferred feature the regula t-
ing winding may be arranged onto a magnetic flux carrier leg
appertaining to one phase of a polyphase system and the main
winding may be arranged onto a magnetic flux carrier leg
belonging to another phase of the polyphase system. This has
the advantage of enabling a phase shift.
According to yet another preferred feature the stor-
age means may include a second winding arranged around a
magnetic flux carrier belonging to another phase of the
polyphase system than the regulating winding. With this
arrangement both voltage control, by means of the regulating
winding, and phase shift, by means of the second winding, can
be achieved.
The arrangement is further characterized in that the
transformer/reactor includes a main winding and that the
regulating winding is provided with means for electrical
connection to the main winding. The arrangement is also
characterized in a particularly preferred way in that, sta rt-
ing from a zero position, in which there are no turns on the
regulating winding drum, the induced voltage/reactive powe r
of a transformer/reactor respectively, is increased in tha t
the said means for transferring the winding are adapted to
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winding up the winding onto the regulating drum in the same
direction as the direction of the main winding, and that the
induced voltage/reactive power of a transformer/reactor
respectively, is reduced in that said transferring means are
adapted to winding up the winding onto the regulating winding
drum in the direction opposite to the direction of the main
winding, whereby the maximum variation of the number of
winding turns is +-N, where N is the number of winding turns
which is available on the regulating winding drum.
The arrangement displays furthermore the preferred
feature that the transferring means includes a drive means
for the rotation of a regulating drum and a drive means f or
the rotation of a storage drum. These drive means are pref-
erably in the form of at least one motor and a device for
belt driving the respective drum. It is thus possible for a
common motor to drive the regulating winding drum as well as
the storage drum. Each drum having its own motor is another
possibility. The transformer may also be of a polyphase type.
In a transformer of a three-phase type, for example, thereby
having three regulating windings, which may each be indepen d-
ent of the other, it is conceivable that each one of the
regulating windings is driven by its own motor so that in
total there are three, alternatively six motors, or that all
phases are regulated in the same way amounting then to one or
two motors depending on whether the respective storage drum
is also driven by this motor. Alternatives other than belt
driving are naturally feasible.
According to another feature, the regulating winding
drum and the storage drum are respectively rotatable in two
directions.
Said means for electrical connection to the main
winding is, according to one feature, characterized in that
it may include a diverter switch. The winding may be varied
by one winding turn at a time with the aid of this diverte r
switch. This has the advantage of producing a significantly
higher resolution and possibility of more precise regulation
than in the previously known technique.
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WO 99/17315 11 PCT/SE98/01748
The core may alternatively be interrupted by an in-
sulating means, whereby earthing, alternatively connection to
the main winding, takes place by means of a, from the insu-
lating means, outgoing conductor. The insulating means is
preferably designed as a rotatable disk of an insulating
material, or a corresponding device. A stepless variation of
the length of the regulating winding, which is advantageous,
is possible with the aid of the rotatable disk.
As the regulating winding drum is preferably ar-
ranged around the core it is preferable to construct it out
of at least two drum parts which are joined together in the
radial direction in order to form the drum.
According to a particularly preferred feature the
insulating electric conductor of said winding has a second
1~ layer which is connected to a predetermined potential, pref
erably earth potential. As mentioned, this has the advantage
that the electrical field generated by the current carrying
conductor is enclosed within the solid insulation layer.
Since this has the result that no electrical field exists
outside the winding, the further advantage is obtained that
it generally will be possible to apply technique that is
previously only known from the low-voltage range and the
electronics field.
In accordance with the inventive arrangement, the
high voltage electric conductor may be designed to advantage
in several ways. It has 'preferably among other things a
diameter lying in the interval of 20 - 250 mm and a conductor
area lying in the interval of 80 - 3000 mm2. The first laye r
is furthermore essentially at the same potential as the
current carrying conductor. The second layer is preferably
arranged such that it forms a substantially equipotential
surface surrounding the current carrying conduc-
tor/conductors. According to other designs at least two
adjacent layers have essentially the same thermal coeffi-
cients of expansion, the current carrying conductor may
include a plurality of strands whereby only a few strands a re
non-insulated from one another, and finally each one of the
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WO 99/I7315 ~ 2 PCT/SE98/01748
three layers may be securely connected to the adjacent layer
along essentially the whole connecting surface.
Another characteristic defined is that at least one
of, and possibly both, the regulating winding drum or the
storage drum is provided with means for connection of the
said second layer of the winding, having semiconducting
properties, to a predetermined potential, preferably earth
potential. These means may be designed in several ways.
The regulating winding drum is also preferably pro-
vided with a means by which to earth the conductors in the
winding. This means is preferably in the form of a sliding
contact, for example in two halves.
The present invention will now be described in de-
tail, by way of example, with particular reference to the
accompanying drawings showing different embodiments and parts
of the invention in which:
Figure 1 is a diagram showing the principle of an
arrangement according to the invention;
Figure 2 is a diagram showing the principle of an
embodiment of the arrangement according to the invention,
where the number of turns of the regulating winding are
varied by one turn at a time by means of a diverter switch;
Figure 3 is a diagram showing the principle of an-
other embodiment of the invention where the winding may be
varied with stepless control:
Figure 9 is a diagram showing the principle of a
variant of earthing of the winding:
Figure 5 is~a diagram showing the principle of an-
other variant of earthing of the winding;
Figure 6 is a perspective view of a contact suitable
for earthing;
Figure 7 shows an cross sectional view of the con-
tact in Figure 6:
Figure 8 illustrates a detail of the contact in Fig-
ure 6; and
Figure 9 shows a cross sectional view of an insu-
lated conductor suitable to be used in the present invention.
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Figure 1 shows a transformer core 1 consisting of a
yoke and two legs, in which a main winding 2 is applied
around the one leg and a regulating winding 3 is arranged
around the other leg. The main winding may either be formed
s of a primary winding or a secondary winding. The regulating
winding is thus used to vary the ratio of the transformer.
The regulating winding 3 is arranged in the form of winding
turns 5 wound onto a rotatable drum 6. As can be seen, the
drum 6 is divided into two drum halves 7, 8. Other ways of
dividing the drum are also conceivable so as to facilitate
the installation around the legs of the core. The drum is
provided with at least one flange for belt driving by means
of a motor (not shown). The regulating winding functions thus
as a variable coil. The number of winding turns on the regu-
lating winding drum 6 is made to vary with the aid of a
rotatable winding storage drum Z2 for the winding 5. The .
storage drum 12 is likewise preferably belt driven by a
motor.
In the following Figures, the same or corresponding
parts as referred to in Figure 1, are designated by the same
numerals. The symbols A and 8 in Figures 2, 9 and 5 generally
show points of connection for the windings, for example to
the main winding or earth.
The embodiment shown in Figure 2 refers to a regu-
lating winding where the length of the winding is varied
stepwise by one winding~'turn at a time. This takes place by
means of a diverter switch 15 which is known as such
(alternatively termed load coupler).
In Figure 3 another embodiment is shown where the
winding on the regulating winding drum may be varied by
stepless control. A core 18 is shown here which is divided
into two parts 18a, 18b by means of a disk 20 of insulating
material. The disk is rotatable and is connected, at its
centre, to an outgoing conductor 21, 21a, which passes into
and through the core part 18b, and is also connected to a
radial conductor 22a in the disk and which conductor is
connected to the regulating winding 22. The conductors 21,
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WO 99/17315 14 PCT/SE98/01748
21a are thus connected to the winding 22 via the conductor
22a. The winding 22 on the regulating drum 23 is connected to
a main winding, an outgoing conductor or to earth potential,
via the conductors 22a, 21 and 21a passing through said
insulating means 20 such a shown in the Figure. By rotating
the disk a stepless regulated magnetic flux through the last
turn is produced by the conductors 22a, 21 and 21a. This flux
may be varied from zero to the flux through a full turn in
the winding 22. The conductor 21 may either be insulated or
in contact with the core 18b. Contacting of the second semi-
conducting layer in the winding may be achieved by means of a
sliding contact.
In Figures 6, 7 and 8 is illustrated a sliding con-
tact device 60 which is particularly suitable for earthing
IS ofthe second semiconducting layer. The contact is arranged at
one end of the rotatable drum on which the regulating winding
22 is provided. The sliding contact includes an outer tube 62
and an inner tube 63, situated inside the outer tube. Both
tubes are bent to form an substantially annual element lo-
Gated around the core 1. Between the two tubes one or several
upset helicoidal, or canted coil, springs 63 are mounted.
Both tubes as well as the helicoidal springs) are made of an
electrically conducting material. The inner tube is in elec-
trical contact with the outer tube by means of the spring.
The outer tube 62 is provided with a axial slot 67 extending
around its outer circumference and along the entire axial
length of the tube. An outgoing conductor 68 is connected to
the inner tube, for connection with earth. This conductor
extends freely out through the slot. When the regulating dr um
rotates, the outer tube, connected to the drum, rotates als o
and both tubes are in electrical contact with each other by
means of the helicoidal spring, functioning as a sliding
contact. The outer tube is thereby connected to earth. As an
alternative to the above, the inner tube may be the moving
part while the outer tube is the stationary part provided
with the outgoing conductor.
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WO 99/17315 15 PCT/SE98/01748
The outer tube 62 must be divided in order to
achieve an electrical interruption in the circumference
around the core. This may be solved by providing one or
several interruptions 70 close to each other. When the slid-
ing contact, i.e. the spring, passes these interruptions, an
unwanted current may be produced in the spring, which may
damage the spring.In order~to prevent this, the device is
provided with another type of contact 72 which commutates
this current, for example a spring-loaded carbon contact.
Figure 9 illustrates a principle for earthing the
second semiconducting layer of the winding. The regulating
drum is provided with at least one feebly or moderately
conducting ring 28 at its one end. This ring is highly resis-
tant, at least at 10062 and generally at 1000SZ, in order to
l~ prevent short circuit. The resistance of the ring can be
evenly distributed along the ring or concentrated in areas
having high resistance which are connected to well conducting
material. Along the drum, i.e. in the axial direction and on
the outside of the drum, yet under the winding itself, there
are a plurality of elongated means 29, made of a conducting
material, and arranged at regular intervals. These are con-
nected to the ring 28 and thereby connected to each other.
Earthing of the winding takes place through contact at the
outer second layer of the winding having semiconducting
properties. The storage drum 12 may also be provided with a
corresponding arrangement 30.
Figure 5 shows a variant of the earthing of the
outer second layer of the winding having semiconducting
properties. The regulating drum is also provided here with a
ring 38 which is connected to earth potential and located at
the one end of this drum and extending around the circumfer-
ence of the drum. Additional rings 90, made of a conducting
material, are arranged at regular spaced apart intervals,
around the semiconducting layer on the insulation of the
3~ conductor 36, so that the rings of one winding turn are in
contact with the corresponding rings of the adjacent winding
turns. In this way these rings form at least one continuous
CA 02305416 2000-03-30
WO 99/17315 16 PCT/SE98/01748
electric connection 42 across the winding, and said connec-
tion is earthed through contact at the one end of the drum
with the first ring 38 there located. Alternatively, the
storage drum 12 may be earthed in a corresponding way or both
drums may be earthed.
Finally, in Figure 9 is represented a cable which is
particularly suitable to be used as a winding in the trans-
former/reactor according to the invention. The cable 50
includes at least one current carrying conductor 51 sur-
rounded by a first semiconducting layer 52. Outside said
first layer is provided a layer of solid insulation 53.
Surrounding the insulation layer is then provided a second
.. semiconducting layer 54. The current carrying conductor may
include a number of strands 56, of which at least some are
insulated from each other. The three layers of the cable,
i.e. the two semiconducting layers and the insulation layer,
are arranged to adhere to each other even when the cable is
bent. The cable is consequently flexible and this property is
maintained during the entire life of the cable. The illus-
trated cable also differs from conventional high voltage
cables in that it does not have to include any outer layer
for mechanic protection of the cable, nor does it have to
include any metal shield which normally is provided on such a
cable.
The above-mentioned embodiments and variations
thereof are to be considered only by way of example of a non
limited nature and the invention may thus be varied within
the scope of the appended claims.
