Note: Descriptions are shown in the official language in which they were submitted.
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WO 98/34326 ~ PCT/SE98/00166
A ROTATING ELECTRIC: MACHINE
The present invention refers to rotating electric ma-
chines such as synchronous machines, as well as dual-
fed machines, applications in asynchronous static cur-
s rent converter cascades, outerpole machines and syn-
chronous flow ::nach.ines. The invention relates to such
a machine accordinc; to the preamble of claim 1. The ma-
chine is provided with a device for avoiding wear bet-
ween conductors in the coil-end package of the stator
in the machine. The machine according to the invention
is intended fo:~ use with high voltages, by which is me-
ant electric voltages in excess of 10 kV. A typical
working range :Eor 1=he device according to the invention
may be of 36 kV-800 kV.
The problem addressed by the invention has arisen in
connection with high-voltage electric alternating cur-
rent machines, in the first place intended as generator
in a power station for generating electric power. Such
machines have ~~onventionally been designed for voltages
in the range 15-30 kV, and 30 kV has normally been con
sidered to be an upper limit. This generally means
that a generator must be connected to the power network
via a transformer which steps up the voltage to the le
vel of the power network, i.e. in the range of approxi
mately 130-400 kV.
In US-A 2,885,581 the problem is to prevent movements
in the coil-ends when the magnetic forces increase with
increasing machine-sizes. A rigid joint is accomplished
by means of impregnation. In US-A 2, 959, 699 a layer of
stiff mat.eria~_ is formed around between overlapping
parts of the coil-ends so that a cylinder of stiff ma-
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2
terial is formed around the core to reduce the move-
ments.
A conductor is known through US-A 5,036,165, in which
the insulation is provided with an inner and an outer
layer of semiconducting pyrolized glassfiber. It is
also known to provide conductors in a dynamo-electric
machine with such an insulation, as described in
US-A 5,066,881 for instance, where a semiconducting py-
rolized glassfiber layer is in contact with the two pa-
rallel rods forming the conductor, and the insulation
in the stator slots is surrounded by an outer layer of
semiconducting pyrolized glassfiber. The pyrolized
glassfiber material is described as suitable since it
retains its resistivity even after the impregnation
treatment.
By using high-voltage insulated electric conductors, in
the following termed cables, in the stator winding of
the machine, with solid insulation similar to that used
in cables for transmitting electric power (e. g. XLPE
cables) the voltage of the machine can be increased to
such levels that it can be connected directly to the
power network without an intermediate transformer. The
conventional transformer can thus be eliminated.
With these high-voltage electric machines problems also
arise in that the cables have a tendency to vibrate,
causing the large coil-end packages to vibrate in rela-
tion to each other at frequencies double the network
frequency, i . a . 100 Hz in a power network with a net-
work frequency of 50 Hz, and 120 Hz in a power network
with a nominal network frequency of 60 Hz, and at amp-
litudes of approximately 0.1 mm. The cables, provided
with an outer semiconducting layer by means of which
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3
their potential in relation to the surroundings shall
be defined, are thus easily damaged due to wear against
adjacent cables in the coil-end package.
The object of the present invention is to solve the
above mentioned problem, and this is achieved by the
machine described in the preamble of claim 1 being gi-
ven the features specified in the characterizing por-
tion of claim 1. This solution means that the cables
are secured relative eah other permitting a relative
movement in which the cables are not rubbing against
each other
The invention is in the first place intended for use
with a high-voltage cable of the type built up of a co-
re composed of a number of strand parts, a semiconduc-
ting layer surrounding the core, an insulating layer
surrounding the inner semiconducting layer, and an ou-
ter sem.iconducti:ng layer surrounding the insulating
layer, and its advantages are particularly prominent
here. The invention refers particularly to such a cab-
le having a diameter within the interval 20-200 mm and
a conducting area within the interval 80-3000 mm2.
Such applications of the invention thus constitute pre-
ferred embodiments thereof.
In the machine according to the invention the windings
are pref:erab:~y o:E a type corresponding to cables with
solid, extruded insulation, such as those now used for
power distribution, e.g. XLPE-cables or cables with
EPR-insulation. Such a cable .comprises an inner con-
ductor c:ompo:~ed of one or more strand parts, an inner
semiconducting layer surrounding the conductor, a solid
insulating layer surrounding this and an outer semicon-
ducting layer surrounding the insulating layer. Such
cables are f:Lexible, which is an important property in
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this context since the technology for the device accor-
ding to the invention is based primarily on winding
systems in which the winding is formed from cable which
is bent during assembly. The flexibility of a XLPE-
cable normally corresponds to a radius of curvature of
approximately 20 cm for a cable 30 mm in diameter, and
a radius of curvature of approximately 65 cm for a cab-
le 80 mm in diameter. In the present application the
term "flexible" is used to indicate that the winding is
flexible down to a radius of curvature in the order of
four times the cable diameter, preferably eight to
twelve times the cable diameter.
The winding should be constructed to retain its proper-
ties even when it is bent and when it is subjected to
thermal stress during operation. It is vital that the
layers retain their adhesion to each other in this con-
text. The material properties of the layers are deci-
sive here, particularly their elasticity and relative
coefficients of thermal expansion. In a XLPE-cable,
for instance, the insulating layer consists of cross-
linked, low-density polyethylene, and the semiconduc-
ting layers consist of polyethylene with soot and metal
particles mixed in. Changes in volume as a result of
temperature fluctuations are completely absorbed as
changes in radius in the cable and, thanks to the com
paratively 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 consi-
dered only as examples. Other combinations fulfilling
the conditions specified and also the condition of
being semiconducting, i.e. having resistivity within
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the range of lOwl-106 ohm-cm, e.g. 1-500 ohm-cm, or
10-200 ohm-cm, naturally also fall within the scope of
the invention.
The insulating layer may consist, for example, of a so-
y lid thermoplastic material such as low-density polyet-
hylene (LDPE), high-density polyethylene (HDPE), po-
lypropylene (PP), polybutylene (PB), polymethyl pentene
(PMP), cross-linked materials such as cross-linked po-
lyethylene (XLPE), or rubber such as ethylene propylene
rubber (EPR) or silicon rubber.
The inner and outer semiconducting layers may be of the
same basic material but with particles of conducting
material such as soot or metal powder mixed in.
The mechanical properties of these materials, particu-
larly 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 achiE~ve the conductivity necessary accor-
ding to the invention. The insulating layer and the
semiconducting layers thus have substantially the same
coefficients of thermal expansion.
Ethylene-vinyl-acetate copolymers/nitrile rubber, butyl
graft polyet)zylene, ethylene-butyl-acrylate-copolymers
and ethylene-ethyl-acrylate copolymers may also consti-
tute suitable polymers for the semiconducting 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 combination of the materi
als listed above.
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The materials listed above have relatively good elasti-
city, with an E-modulus of E<500 MPa, preferably
<200 MPa.
The elasticity is sufficient for any minor differences
between the coefficients of thermal expansion for the
materials in the layers to be absorbed in the radial
direction of the elasticity so that no cracks appear,
or any other damage, and so that the layers are not re-
leased from each other. The material in the layers is
t0 elastic, and the adhesion between the layers is at le-
ast of the same magnitude as the weakest of the materi-
als.
The conductivity of the two semiconducting layers is
sufficient to substantially equalize the potential
along each layer. The conductivity of the outer semi-
conducting layer is sufficiently large to enclose the
electrical field in the cable, but sufficiently small
not to give rise to significant losses due to currents
induced in the longitudinal direction of the layer.
Thus, each of the two semiconducting layers essentially
constitutes one equipotential surface and the winding,
with these layers, will substantially enclose the elec-
trical field within it.
There is, of course, nothing to prevent one or more ad
ditional semiconducting layers being arranged in the
insulating layer.
The invention will now be described in more detail with
reference to the accompanying drawings in which:
Figure 1 shows a view in perspective of a part of the
coil-end package at one end of the stator in an
electric alternating current generator,
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Figure 2 shows a cross section through a cable of the
type used in the stator winding,
Figure 3 shows a cross section through a cable in the
coil-end package incorporating a device accor-
ding i=o the invention,
Figure 9 shows the contact area between two cables in
the coil-and package, and
Figure 5 shows the manufacture of a device according
t:o the invention.
Figure 1 illustrates a section of the coil-end package
in an altern,~tinc~ current generator. With its inner
vertical surface 2, the stator 1 surrounds the rotor of
the generator with an air gap. Cables 4 forming the
winding protrude in an arc from one slot in the upper
IS surface 3 of the stator 1 and enter another slot in the
stator. There arcs of cables or coils form coil ends
which come into contact with each other. One such con-
tact point is designated 5 in Figure 1.
The arc--shaped coil ends become relatively loose and
slippery and the vibration level reached by the cables
during operation at a frequency of approximately 100 Hz
causes relative n.ovement between the cables in the con-
tact area, a relative movement with an amplitude of ap-
proximately 0.1 mm. Such movement would cause damaging
wear be~ween the cables which in this case have no
sheath.
Figure 2 shows a cross section through a cable 4 which
is used in cc>njun.ction with the present invention. The
cable 4 consists of a core 6 composed of a number of
strand parts made of copper, for instance, and having
circular cross s.°ction. This conductor 6 is arranged
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in the middle of the cable 4. Around the conductor 6
is a first semiconducting layer 7. Around the first
semiconducting layer 7 is an insulating layer 8, e.g.
XLPE-insulation, and around the insulating layer 8 is a
second semiconducting layer 9. In this context, there-
fore, a cable does not include the outer protective
sheath that normally surrounds a cable for power dist-
ribution. The cable may be of the size specified in
the introduction.
Figure 3 shows a cross section through such a cable,
incorporating a device according to the invention. To
avoid wear in the contact areas between the cables, the
cables must be secured in relation to each other while
permitting a relative movement that does not entail the
cables rubbing against each other and thereby becoming
worn. The cables 4 are therefore provided with a slee-
ve 10 in the contacts.
The sleeve 10 consists of a helically shaped wound tape
11 (Fig. 4). The material in the sleeve 10 is not li-
mited to any specific material but may be any type of
material with a certain elasticity. Neither need the
material be electrically insulating. It may be elect-
rically conducting, although the former may be prefe-
rable in certain machine designs.
Figure 4 shows how the cables are secured to each other
at the contact point 5 by means of a securing device in
the form of a bundle binder 12. It is also advisable
for the cables 4 to be similarly secured and clad in
resilient material at outer fixed points in the stator
as well.
Figure 5 shows a suitable method of producing the tape
11 which is to form the sleeve 10 on the cables. A
work piece 13 in the form of a tube or hose of material
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suitable for the purpose is slit along a helical line
14. A helically shaped tape 11 is thus formed to pro-
duce the sleeve 1.0 to cover the cable 9 in the contact
area 5. Contrary to a normal, straight tape wound aro-
and the cable, t:he helically shaped tape is held in
place by it:>elf. No binder is therefore required,
which would make the coil-end package thicker.
Using this method, tape can easily be produced which,
despite being relatively thick, can easily be given the
necessary he~_ical. shape. The layer in the sleeve 10
must be sufficiently thick to permit relative movement
between the cables through shearing in the material,
without slipping between the surfaces. The thickness
of the layer may vary between 0.5 and 5 mm depending on
the cable diameter which may vary between 10 and
150 mm. The device according to the invention prevents
wear on the cables which would rapidly damage the outer
semiconductor on the XLPE insulation.
