Note: Descriptions are shown in the official language in which they were submitted.
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A WINDING IN AN ELECTRIC MACHINE WITH STATIONARY PARTS
The present invention relates to a winding as stated in
the preamble of claim 1 in an electric machine with
stationary parts, e.g. a power transformer, intended
for use at high voltages, by which is meant primarily
electric voltages exceeding 10 kV. A typical working
range for a transformer according to the invention may
be 36-800 kV.
Conventional power transformers have, as disclosed e.g.
in the book "Elektriska Maskiner" by Fredrik Gustavson,
Page 3.6 - 3.12, Kungliga Tekniska Hogskolan, 1996,
usually been cooled and insulated by oil. However, a
number of problems are inherent in such oil-filled
power transformers. An outer housing is required for
the transformer with a transformer core with windings,
oil for insulation and cooling, and mechanical bracing
means of various types. The mechanical demands placed
on this housing are considerable and the manufacturing
and assembly processes are extremely time-consuming.
Finally, the external dimensions of the housing are
large, thus entailing transport problems. Oil-cooling,
particularly pressurized oil-cooling, also requires ac-
cess to oil pumps, external cooling elements and expan-
sion vessels, etc. The insulating material must also
be extremely pure and freely from conducting particles.
The moisture content in both the oil and other insula-
ting material must also be far below that of the at-
mosphere. In normal production the moisture content in
separate processes is reduced to values below 1 o for
paper and other cellulose materials and a few micro-
parts in the oil. The whole insulation system must be
carefully dried at the end of the manufacturing pro-
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cess. This high degree of purity and low moisture con-
tent must be maintained during transport and operation
of the transformer.
Through e. g. JP 9 179 107, JP 6 196 343, and JP 7 057
951 a winding in the form of prefabricated drums is
previously known. However, such a winding has not been
used with high-voltage cables.
A conductor is known through US 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 ma-
chine with such an insulation, as described in US
5,066,881 for instance, where a semiconducting pyrol-
ized glassfiber layer is in contact with the two paral-
lel 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.
The object of the present invention is to solve the
above problems and further improve such machines by
simplifying manufacture, facilitating transport and re-
ducing manufacturing and assembly costs. This object
is achieved in that the machine according to the inven-
tion is given the features defined in the characteri-
zing portion of claim 1.
The invention is primarily intended for use, and its
advantages are particularly apparent, with a high-
voltage cable of the type built up of a core having a
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3
plurality of strands, an inner semi-conducting layer
surrounding the core, an insulating layer surrounding
' the inner semi-conducting layer and an outer semi
conducting layer surrounding the insulating layer. Mo
re particularly it relates to such a cable with a dia
meter in the range of 20-200 mm and a conducting area
in the range of 80-3000 mm2. Such applications of the
invention thus constitute preferred embodiments.
In the arrangement according to the invention the win-
dings are preferably of a type corresponding to cables
with solid, extruded insulation, such as those used
nowadays for power distribution, e.g. XLPE-cables or
cables with EPR-insulation. Such a cable comprises an
inner conductor composed of one or more strand parts,
an inner semiconducting layer surrounding the conduc-
tor, a solid insulating layer surrounding this and an
outer semiconducting layer surrounding the insulating
layer. Such cables are flexible, which is an important
property in this context since the technology for the
device according 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 cable 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 curva-
ture 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
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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, radial expansion can ta
ke 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
the range of 10-1-106 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, f.or example, of a so-
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.
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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-
S 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 achieve 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 polyethylene, 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.
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
elastic, and the adhesion between the layers is at le-
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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 great 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
electrical 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 schematic section through one phase
of a power transformer according to the in
vention and
Figure 2 shows a cross section through a winding cable
used in the transformer according to the in-
vention.
Figure 1 shows a part of a power transformer in sec-
tion, having a transformer core 11, a low-voltage win-
ding 12 and a high-voltage winding 13. According to
the invention the windings are wound onto prefabricated
drums 14 and 15. These drums are completely wound at
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the factory and then transported to the site where the
transformer is to be used, where they are a mounted on
respective phases of the core (only one phase of the
transformer is shown in Figure 1).
In the example shown in Figure 1 the high-voltage win-
ding 13 is divided into two drums 15 for manufacturing
and transport reasons. When the winding is divided in-
to several drums the cables in the individual windings
are connected by a cable joint 16 on site.
Figure 2 shows a section through a power cable 1 for
use in a dry power transformer according to the present
invention. The cable 1 comprises a number of strands 2
consisting of a conductor made of copper, for instance,
having circular cross section. This conductor is ar-
ranged in the middle of the cable 1. Around the cable
is a first semi-conducting layer 3. Around the first
semi-conducting layer 3 is an insulating layer 4, e.g.
XLPE insulation. Around the insulating layer 4 is a
second semi-conducting layer 5. In this case, therefo-
re the cable does not include the outer sheath that
normally surrounds such cables for power distribution.
The cable may be of the size stated in the introduc-
tion.
Tubes or ducts for cooling air are arranged between the
winding cables to cool the winding in the transformer
according to the present invention. These tubes or
ducts are suitably arranged in the drums 14 and 15 at
manufacture before the transformer is transported to
where it is to be used.
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_.
Thanks to the invention a dry power transformer is
achieved which is simpler to manufacture than conven-
tional transformers. The transformer need not be
transported as a unit from factory to site, and both
transport and assembly become less expensive.
The invention is of course not limited to a power
transformer but is also applicable to other electrical
machines with stationary parts, such as inductive reac-
tors.
