Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02373784 2001-11-09
WO 00/77913 PCT/SE00/01069
A METHOD FOR A ROTATING ELECTRIC MACHINE AND A MACHINE FOR
CARRYING OUT THE METHOD
The present invention relates to a method for a rotating electric machine
~ for high voltage and such a rotating electric machine.
The present invention relates especially to a rotating electric machine
having the type of winding as defined in the preamble of claims 1 and 26
respec-
tively.
In this connection, rotating electric machines comprise synchronous ma-
chines, which are mainly used as generators for connection to distribution and
transmission networks, referred to as power networks. Synchronous machines are
also used as motors, in addition to phase compensation and voltage regulation
and then as mechanically idling machines. This technical field also comprises
nor-
mal asynchronous machines, dual-feed machines, alternating current machines,
asynchronous converter cascades, outer pole machines and synchronous flux
machines. These machines are intended for use at high voltages, i. e. voltages
that mainly exceed 10 kV. A typical operating range for such a rotating
machine
may be 36 - 800 kV, and preferably 72,5 - 800 kV.
In conventional types of electric rotating machines the stator body is often
designed in the form of a welded steel sheet construction. The stator core,
also re-
ferred to as a steel core, is normally in larger machines formed of so-called
electric
sheet which is preferably 0,35-0,50 mm thick and divided into stacks. The
stator
core is provided with radial slots for the winding so as to form radial layers
at dif
ferent radial distances from the air gaps between the stator and the rotor.
The term
layer refers to layers of winding at different radial distances from the
central axis of
the stator. A winding turn is formed by that part of the winding, which
extends once
back and forth through the stator between different layers.
Rotating electric machines have conventionally been designed for voltages
in the range of 6 - 30 kV, and 30 kV has normally been considered to be an
upper
limit. This generally means that a generator must be connected to the power
net-
work via a transformer, which steps up the voltage to the level of the power
net-
work, i.e. in the range of approximately 130 - 400 kV.
Different attempts have been made during the course of the years to de-
velop especially synchronous machines, and more especially generators, for
high
CA 02373784 2001-11-09
WO 00/77913 PCT/SE00/01069
2
voltages. Such examples may be found among others in; "Electrical World", Octo-
ber 15, 1932, pages 524 - 525, the article; "Water-and-oil-cooled Turbo-
generator
TVM-300", in J. Elektrotechnika, No.1, 1970, pages 6 - 8, and in the patent
publi-
cations US 4,429,244 and SU 955 369. However, none of these attempts have
been successful or led to any commercially available product.
It has however been shown feasible to use high voltage insulated conduc-
tots as stator winding in a rotating electric machine, which have solid
insulation
and are of a design similar to cables used for transmission of electric power
(e. g.
so-called XLPE-cables). The voltage of the machine can be increased hereby to
such levels that it can be connected directly to the power network without an
in-
termediate transformer. Thus, among other things, the very important advantage
of eliminating the conventional transformer is hereby achieved. A rotating
electric
machine with such a winding is described for instance in the PCT application
WO
97/45919. Additional descriptions of the insulated conductor or cable can be
found
in the PCT applications WO 97/45918, WO 97/45930 and WO 97/45931.
The above-mentioned type of winding, principally corresponding to cables
having solid, extruded insulation of a type presently used for power
distribution,
such as said XLPE-cables or cables having EPR-insulation, comprises an inner
conductor composed of one or more strands, an inner semiconductive layer sur-
?0 rounding the conductor, a solid insulation layer surrounding the inner
semiconduc-
tive layer and an outer semiconductive layer surrounding the insulation layer.
Such
cables are flexible, which is an essential property in this context since the
technol-
ogy for the device, according to the invention, is based primarily on a
winding sys-
tem in which the winding is formed from conductors, which are bent during as-
sembly. The flexibility of an XLPE-cable normally corresponds to a radius of
cur-
vature of approximately 20 cm for a cable 30 mm in diameter, and a radius of
cur-
vature of approximately 65 cm for a cable 80 mm in diameter. In the present
appli-
cation the term flexible is used to indicate that the winding is flexible down
to a ra-
dius of curvature in the order of 4 times the cable diameter, preferably 8 to
12
times the cable diameter.
The winding should be designed so as to retain its properties even when it
is bent and subjected to thermal stress during service. In this connection, it
is vital
that the layers retain their adhesion to each other. The material properties
of the
layers are decisive here, particularly their resiliency and relative
coefficients of
CA 02373784 2001-11-09
WO 00/77913 PCT/SE00/01069
3
thermal expansion. In an XLPE-cable, for example, the insulation layer
consists of
cross- linked, low-density polyethylene and the semiconductive layers consist
of
polyethylene compounded with soot and metal particles. Changes in volume, as a
result of temperature fluctuations, are fully accommodated in the form of
changes
in the radius of the cable and thanks to the comparatively slight difference
be-
tween the coefficients of thermal expansion of the layers in relation to the
resilien-
cy of these materials the radial expansion of the cable can take place without
ad-
hesive failures between the layers.
The material combinations stated above are considered by way of exam-
ple only. Other combinations fulfilling the above-mentioned conditions and the
condition of being semiconductive, i. e. having a volume resistivity within
the range
of 10-' - 10 6 ohm-cm, such as 1 - 500 ohm-cm, or 10 - 200 ohm-cm for example,
naturally fall within the scope of the invention.
The insulating layer can consist, for example, of a solid thermoplastic ma-
terial such as low-density polyethylene (LDPE), high-density polyethylene
(HDPE),
polypropylene (PP), polybutylene (PB), polymethyl pentene (PMP), cross-linked
materials such as cross-linked polyethylene (XLPE), or rubber such as ethylene
propylene rubber (EPR) or silicone rubber.
The inner and outer semiconductive layers may be made of the same ba-
sic material but compounded with particles of conducting material such as soot
or
metal powder.
Ethylene-vinyl-acetate copolymers/nitrile rubber, butyl graft polyethylene,
ethylene-butyl-acrylate-copolymers and ethylene-ethyl-acrylate copolymers may
also constitute suitable polymers for the semiconductive layers.
Even when different types of material are used as a base in the respective
layers, it is desirable that their coefficients of thermal expansion are of
the same
magnitude. This is the case in the above listed combination of materials.
The materials listed above have a relatively good resiliency, with an E-
modulus of E < 500 MPa, preferably < 200 MPa. The resiliency is sufficient for
any
possible minor differences between the coefficients of thermal expansion of
the
materials in the layers to be accommodated in the radial direction so that no
cracks or other damage appear and so that the layers do not loose adhesion to
each other. The materials in the layers are elastic, and the adhesion between
the
layers is at least of the same magnitude as the weakest of the materials.
CA 02373784 2001-11-09
WO 00/77913 PCT/SE00/01069
4
The conductivity of the two semiconductive layers is sufficient to substan-
tially equalise the potential along each respective layer. The conductivity of
the
outer semiconductive layer is sufficiently high to enclose the electrical
field within
the cable, but sufficiently small so as to not give rise to significant losses
due to
currents induced in the longitudinal direction of the layer.
Thus, each of the two semiconductive layers essentially constitutes one
equipotential surface, and the winding constituting these layers substantially
con-
fines the electrical field within itself.
However, nothing prevents one or more additional semiconductive layers
from being arranged in the insulating layer.
One problem arising with the use of an XLPE-insulated conductor and the
like when used as winding is their expansion, because of their relatively high
coef-
ficient of thermal expansion, which occurs as a result of heating when
operating
the machine. The normal operating temperature for a machine of the present
type
is in the order of 70°C, which is considerably lower than that of
conventional ma-
chines that have an operating temperature of approx.100-120°C. The
difference in
temperature between the machine in operation or out of operation, which
tempera-
ture difference is normally in the order of 50°C but may even be
considerably high-
er if the machine is placed outdoors in a cold climate, causes an XLPE-
insulated
conductor that is securely fastened within the stator slots when the machine
is in
operation, to shrink when the operation of the machine is interrupted and the
XLPE-insulated conductor can very possibly loose adhesion to the walls of the
slot
so that it is more or less loosely positioned when the machine is out of
operation.
This loose conductor creates a problem when the machine starts operating
again.
?5 The XLPE-insulated conductor and the stator slots are alternatively
dimensioned
in such a way in relation to each other that the conductor is fastened within
the slot
even when the machine is out of operation. When the machine consequently
starts
operating and the temperature starts rising, the XLPE-insulated conductor in
the
slots expands thermally and risks being damaged in the slots. Special devices
for
securing and maintaining the XLPE-insulated conductor may be used as another
alternative, but which have the disadvantage of being both costly and
difficult to
install.
CA 02373784 2001-11-09
WO 00/77913 PCT/SE00/01069
The present invention intends to solve the above-mentioned problems,
which are caused by differences in temperature and the thermal expansion of
the
winding.
The problem is solved, according to the present invention as defined in the
5 characterizing part of claim1, by a method where the stator is cooled while
during
operation to a temperature T1, and where it is heated while out of operation
to a
temperature T2. A corresponding rotating electric machine solves the problem
as
defined in the characterizing part of claim 26. The rotating machine thus
comprises
a device for cooling the stator, while during operation, to a temperature T1,
and for
heating the stator, while out of operation, to a temperature T2. The advantage
achieved hereby is that said temperature differences are reduced, thereby
reduc-
ing the thermal expansion of the winding. Because the temperature differences
are
reduced to a high degree in this way or, according to an especially preferred
em-
bodiment, are equalised completely, see below, the problems of different
thermal
expansion between the solid insulation and the layers surrounding the
insulation of
the insulated electric conductor in use is also eliminated. Reducing the
tempera-
ture variation in this way, or alternatively equalising the temperature, makes
a freer
choice of material in the insulated conductor possible. Thus, other conducting
materials, having larger temperature coefficients, may be used and materials
hav-
ing different temperature coefficients may be combined in the solid insulation
and
surrounding layers.
According to an especially advantageous feature, this device comprises at
least one cooling and heating system for the stator and one supervision system
comprising means which measure the temperature of the stator both during op-
eration and out of operation respectively, and means which control the cooling
and
heating system such that said temperatures T1, which the stator cools down to
when it is in operation, and T2, which the stator heats up to when it is out
of op-
eration respectively, are obtained and maintained.
According to an advantageous feature, the temperature T2 is preferably
essentially equal to the temperature T1, which means that the temperature of
the
stator is held essentially constant regardless of whether it is in operation
or not.
According to another advantageous feature, the temperature T 2, which
the stator is heated up to when it is out of operation, is somewhat lower than
the
temperature T1, which the stator is cooled down to when it is in operation,
where-
CA 02373784 2001-11-09
WO 00/77913 PCT/SE00/01069
6
by T2 is preferably in the range of 0 - 20° C lower than T1, and with
advantage in
the range of 0 - 10° C lower than T1, or in the order of 10 - 15% lower
than T1.
The insulation of the XLPE-conductor remains at a constant volume be-
cause the temperature is constant or close to constant, which simplifies
securing
and assembly of the winding in addition to simplifying and making the whole
prin-
ciple of construction trustworthy. No noticeable relative movements are
obtained
as a result of differences in the coefficients of expansion between the
different
parts of the stator and the winding.
According to an advantageous embodiment, the winding is installed, dur-
ing assembly of the machine, in the slots of the stator with play, which
essentially
corresponds to the expected expansion of the winding during the operating tem-
perature of the machine. The winding, before installation in the slots of the
stator,
can alternatively be deformed mechanically in such a way that the winding,
which
is installed thereafter in the slots, returns to its non-deformed state and
bears on
the walls of the slot. According to another alternative, the winding may be
cooled
down before installation in the slots of the stator, undergoing thereby
thermal
shrinkage, after which the winding is installed in the slots and regains its
original
state as a result of heating whereby the winding bears on the walls of the
slot. In
all cases the stator is heated, after the winding has been installed but
before oper-
sting, with advantage to a temperature T3, which preferably essentially corre-
sponds to the expected operating temperature T0.
Thus, the system for supervision of the machine comprises with advantage
also means for measuring the temperature of the stator before being taken into
operation for the first time, means controlling the cooling and heating system
such
?5 that the stator, before operating for the first time, is heated to a
temperature T3,
and means which control the machine such that it is taken into operation only
when the temperature T3 has been reached.
The temperature T3 may even be somewhat lower than the expected op-
erating temperature of the stator T0, whereby T3 is preferably in the range of
0 -
20°C lower than T0, and with advantage in the range of 0 - 10°C
lower than T0.
The advantage of having a winding which is not fastened within the slot of
the stator until the operating temperature has essentially been reached is
obtained
by utilising, among other things, the "memory effect" of an XLPE-insulated
conduc-
CA 02373784 2001-11-09
WO 00/77913 PCT/SE00/01069
7
for or similar conductor which is released by heat or time. This also improves
the
possibilities of replacing a damaged part of the winding.
According to an preferred embodiment, the cooling and heating system of
the rotating machine comprises at least one expandable conducting means for
transportation of a cooling and/or a heating medium, which is inserted into
ducts in
the stator core, which are adapted for this purpose, in addition to means for
the ex-
pansion of said conducting means, whereby said conducting means after having
expanded, presses against the inside of the duct in order to obtain good
contact
and heat transfer.
Said conducting means, which is preferably a proportionately rigid tube
made of XLPE material or the like, has with advantage, before being inserted,
been coated with a layer of fusible adhesive, in the form of glue film for
example,
which is wound onto the tube or extruded on the outside of the tube. The
thickness
of the layer may be in the range of one to some tenths of a mm. The fusible
adhe-
sive may contain a filler having good thermal conductivity such as aluminium
oxide
or boron nitride.
Additionally, the machine comprises with advantage means for the expan-
sion of the conducting means, comprising means for simultaneously subjecting
the
conducting means to overpressure and heating, so that the conducting means
bears on the walls of the duct and so that said fusible adhesive melts and sub-
stantially fills all cavities between the conducting means and the walls of
the duct,
whereby the conducting means is secured against the walls of the duct.
Pressuri-
sation and heating can take place by, for example, warm glycol being
circulated
inside the conducting means. Heating up to approx. 150° is required for
the XLPE
material and the conducting means to expand, and the medium used for pressuri-
sation and heating must therefore be able to withstand this temperature.
Besides,
the medium can consist of the cooling and /or heating medium, which is used
later
for cooling and heating the stator respectively. The conducting means becomes
supple and can be reshaped during heating and the glue, which melts and fills
up
possible cavities between the conducting means and the stator core, then
hardens
and secures the conducting means when cooling down. This arrangement has the
advantage of being able to substitute the injection of silicon rubber, which
is other-
wise used for securing the conducting means and as a "sealing" between the con-
ducting means and the stator ducts. The invention therefore shortens the
distance
CA 02373784 2001-11-09
WO 00/77913 PCT/SE00/01069
8
that the heat travels between the stator core and the conducting means by
approx.
2 mm.
According to a variant, the conducting means can be deformed radially,
before being inserted into the duct, so as to correspond to a smaller diameter
than
that of the duct.
According to another preferred embodiment, the cooling and heating sys-
tem of the rotating machine comprises at least one expandable conducting means
for transportation of a cooling and/or a heating medium, which conducting
means
is inserted into the slots of the stator core, in the cavities that are formed
between
the turns of the winding lying adjacent to each other, in addition to means
for the
expansion of said conducting means, whereby said conducting means, after hav-
ing expanded, clamps the winding firmly within the stator slots. This
conducting
means has with advantage a profile, which principally corresponds to the
geomet-
rical cross-section of said cavities and which is preferably an essentially
triangular
profile.
According to a variant, the expandable conducting means is inserted into
the slots of the stator core in an evacuated condition. The conducting means
may,
for example, be made of reinforced hose and said means for the expansion of
the
conducting means, being inserted into the slots of the stator core, preferably
com-
prise means for feeding a pressurised fluid into the conducting means. The con-
ducting means can, for example, be pressurised by means of a static water pres-
sure, whereby the winding is clamped firmly within the stator slot. The water
can
be circulated thereafter in order to heat and cool the slot/stator and winding
re-
spectively.
According to another variant, said means for the expansion of the con-
ducting means comprise means for simultaneously subjecting the conducting
means to overpressure and heating, and the machine also comprises means for
cooling the conducting means while retaining an overpressure, whereby the con-
ducting means retains its expanded form. The conducting means then preferably
constitutes an XLPE-tube or is made of a similar material, which can be made
to
expand in a corresponding manner to the above-mentioned description of the con-
ducting means within the ducts of the stator core, and which has the
correspond-
ing advantages.
CA 02373784 2001-11-09
WO 00/77913 PCT/SE00/01069
9
Additional features and advantages of the present invention will be made
evident in the remaining dependent claims.
Thus, not only has a solution been found for the problem of avoiding tem-
perature changes in the stator and other problems in this connection, improved
ways and devices have also been found for securing the winding in the stator
slots
and fastening the conducting means for heating and cooling purposes within the
ducts in the stator core and the slots, in addition to finding a solution for
improved
heat transmission.
Embodiments of the present invention will now be described, by way of
example only, with particular reference to the accompanying drawings in which:
Figure 1 shows a schematic sketch of the supervision system, which is part of
the invention;
Figure 2 shows a schematic sketch illustrating the installation of the winding
in
the stator slots;
Figure 3 shows a variant of the installation of the winding;
Figure 4 shows ducts in the stator core, into which a conducting means for
transportation of a cooling and/or heating medium has been inserted;
Figure 5 shows stator slots with winding and conducting means for transpor-
tation of the cooling and/or heating medium; and
Figure 6 shows an example of an insulated electric conductor suitable for use
as winding.
Figure 1 shows schematically, in accordance with the invention, the speci-
fications that a supervision system for a rotating electric machine will
perform. As
mentioned above, it is desirable that the temperature of the stator, when it
has
been put into operation, remains relatively constant. In order to achieve
this, a
system cooling the stator in operation and heating the stator when it is out
of op-
eration is required as well as a supervision system. Examples of different
embodi-
ments constituting cooling and heating systems will be shown below, not
excluding
other possible embodiments. Measurement of the temperature of the stator is an
important part of the supervision system. The supervision system can naturally
al-
so be used to control the temperature that the conducting means for cooling
and
heating the stator and winding respectively are heated up to and cooled down
to
respectively during installation, as well as controlling the pressure they are
sub-
jected to, or anything else that is serviceable in this connection. The
supervision
CA 02373784 2001-11-09
WO 00/77913 PCT/SE00/01069
system and controlling system inclusively, for the cooling and heating system
and
the measurement of the temperature of the stator are preferably computerised.
Such a system can be designed with the help of known technique and will there-
fore not be described in detail herewith.
As in the above-mentioned, it is especially desirable to use a type of insu-
lated electric conductor or cable, which is a so-called XLPE-cable, as winding
which in the present connection is also termed an XLPE-insulated conductor.
This
cable expands when the temperature rises, i. e. when the machine is in
operation
and this condition can be exploited during installation of the cable. As
illustrated in
10 Figure 2, on the right hand side of the illustration, the cable 8 is
installed in the sta-
for slots with play between the outside of the cable and the inside of the
slot.
When the stator is heating up, which must take place before it is put into
operation
because the winding, shown on the right hand side of Figure 2, has not been
fas-
tened in the slots yet, the cable expands thermally so that it bears on the
slot and
is thus secured in the slot. The machine is then ready for service. The
purpose of
the supervision system is to control the temperature of the stator so that it
reaches
a temperature that approximately corresponds to the operating temperature, of
which temperature the cable is presumed to be secured in the slot, before the
ma-
chine starts operating. A stator slot 9' is illustrated on the left-hand side
of Figure 2
where the winding/cable 8' has expanded to such an extent that it is secured
adja-
cent to the inner wall of the slot and the machine is then ready for service.
Alternatively, the "memory effect" of the cable can be utilised in order to
cool down the cable before installation in the slot. When the cable is heated
the
cable regains its original dimension and then bears on the walls of the slot
and the
iron core.
It should be noted that it is important that, when the operating temperature
has been reached, it be kept at a fairly constant level, i. e. when the
machine is out
of operation, a temperature is maintained, which is approximately equal to the
op-
erating temperature derived from heating, so that the winding will not loosen
its
adjacent hold on the inside of the stator slots as a result of shrinkage when
cooling
down too much.
The variant illustrated in figure 3, for installation of the winding, shows a
stator having especially designed slots 19 for the winding, i.e. slots, which
are oval
in the radial direction. Two electric conductors 18 are installed in each such
oval
CA 02373784 2001-11-09
WO 00/77913 PCT/SE00/01069
11
slot 19, i.e. corresponding to two winding turns. Thus, the conductors of the
wind-
ing can be wound two at a time. This variant is especially advantageous for
air-
cooling but can naturally also be used in other types of cooling. More space
is
usually needed in the slots in order to utilise air-cooling and by winding the
con-
s ductors of the winding two at a time, in the oval slots, a duct shaped space
is
formed between the two conductors in a slot, which duct may be utilised for
cool-
ing. Thus, this invention makes it possible to cool both the winding and the
stator
teeth by means of air.
The conductors 18 may suitably be treated and installed in accordance
with the method described above, in connection with figure 2. Alternatively or
complementary thereto, combined means 17, which are of a corresponding type
and can be arranged in a corresponding way to the above, may be applied be-
tween the two conductors 18 in a slot 19 in order to clamp the conductors and
which means can be utilised for cooling/heating the stator and the winding,
which
1 S will be described further below.
It should be noted that the invention illustrated in figure 3 can also consti-
tute a separate invention as regards a stator having oval slots in which the
con-
ductors of the winding are installed two at a time, which invention is not
solely
confined to a rotating electric machine of the type referred to in claim 26 or
to the
method defined in claim 1. It should also be noted that the means 17 could be
constituted of winding clamping means of any suitable type, i. e. without the
com-
bined cooling and/or heating function.
Figure 4 shows how the cooling/heating of the stator may be carried out.
The stator core 20 is provided with a plurality of ducts 21 for
cooling/heating. A
conducting means 22 is inserted into these ducts in order to transport a
cooling
and/or heating medium, which conducting means preferably constitutes an XLPE-
tube or the like. The tube has preferably been deformed radially in advance so
that
it corresponds to a diameter that is smaller than the diameter of the duct in
the sta-
tor steel sheets. In any case, the tube 22 has a diameter of d,, which is
smaller
than the diameter of the duct dk. This considerably simplifies the process of
lead-
ing the tube into the duct. The tube 22 is then allowed to expand in the duct
until it
attains a diameter d2, which preferably corresponds to or is somewhat bigger
than
the smallest diameter dk of the duct in order to ensure that the conducting
means/-
tube is secured and bears on the inside of the duct, and thereby to the stator
steel
CA 02373784 2001-11-09
WO 00/77913 PCT/SE00/01069
12
sheets, in such a way that it obtains good contact and good heat transfer. The
ex-
pansion of the tube is preferably achieved through a combination of heating
and
pressure, caused by a heated, compressed fluid circulating through the
conducting
means.
The conducting means/tube can be coated on its outside with a layer of
fusible glue film, which melts when the tube is heated in order to further
improve
the process of fastening the conducting means so that it bears on the inside
of the
duct. The glue then fills all possible cavities between the conducting means
and
the stator sheet layers in the core duct. The glue film can also contain a
filler hav-
ing good thermal conductivity such as aluminium oxide or boron nitride, which
fur-
ther improves the melting of the glue and the contact of the conducting means
to
the stator sheets as well as the heat transfer between the conducting means
and
the stator.
Figure 5 shows how a conducting means 27 for transportation of a cooling
and/or heating medium can be located in the space between the winding 28 and
the stator slot 29. This conducting means can have the same characteristics as
the conducting means, being used in the ducts in the stator core. Thus, this
con-
ducting means can be constituted of an XLPE-tube which has advantageously
been given a triangular profile, which corresponds to the form of the
accessible
space between the inside of the stator slot and two winding turns 28 lying
adjacent
to each other. This could have taken place through deforming the conducting
means. The conducting means can expand in a corresponding way to the above-
mentioned after having been inserted, i. a preferably by means of a
combination of
heating and pressurising a medium circulating through the conducting means.
Thus, the conducting means hereby bears on both the insides of the slot and
the
winding whereby the conducting means thus fixes the winding within the stator
slot. Good contact is established at the same time between the conducting
means
and the winding as well as the stator sheets, which is favourable for heat
transfer
and can thus be utilised for cooling and heating of the stator (and the
winding) re-
spectively. The conducting means in this figure 5 is only arranged on the one
side
of the winding but can naturally be arranged on both sides of the winding,
such as
shown previously in figure 3. It is also noteworthy that figure 3 illustrates
a corre-
sponding conducting means in a non-expanded condition.
CA 02373784 2001-11-09
WO 00/77913 PCT/SE00/01069
13
A reinforced hose can alternatively also be used as a conducting means
instead of the XLPE-tube, which is preferably threaded in an evacuated
condition
into the accessible spaces. This is thereafter pressurised by means of static
water
pressure and clamps the winding cable in this manner within the slot.
Finally, a cross-section of an insulated electric conductor/cable is shown in
figure 6, which is especially suitable for use as winding in the stator
according to
the invention. The cable 30 comprises at least one current carrying conductor
31
surrounded by a first semiconductive layer 32. An insulation layer 33 is
arranged
around this first semiconductive layer, which layer is surrounded in turn by a
sec-
and semiconductive layer 34. The electric conductor 31 can consist of a
plurality of
strands 35. The three layers are designed in such a way that they adhere to
each
other even when the cable is bent. The flexibility of the shown cable is a
life-long
characteristic. The illustrated cable also differs from conventional high
voltage ca-
bles because the outer, mechanical protective sheathing and the metal screen,
which normally surrounds such a cable, are eliminated.
The present invention should not be considered limited to the shown em-
bodiments, but can be varied by a person skilled in the art in numerous ways
with-
in the frame of the invention as defined in the attached patent claims.
