Note: Descriptions are shown in the official language in which they were submitted.
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A transformer/reactor and a method for manufacturing a
transformer/reactor
The present invention relates to a transformer/reactor
comprising a core and at least one winding.
The present invention also relates to a method for use in
the manufacturing of a corresponding transformer/reactor.
Transformers/reactors are available in all power ranges from
a few W up to the 1000 MW range. The term "power trans-
former/reactor" generally refers to transformers/reactors
having a rated output from a few hundred kW up to over
1000 MW and a rated voltage of from 3-4 kV up to extremely
high transmission voltages.
A conventional power transformer comprises a transformer
core, hereinafter called core, of laminated oriented sheet
metal, usually ferrosilicon. The core consists of a number
of core legs joined by a yoke. A number of windings are
placed around the core legs, generally termed primary,
secondary and regulating windings. In the case of power
transformers, these windings are almost always arranged
concentrically and distributed along the core legs. The
transformer core has a rectangular "window" through which
the windings pass: This rectangular window is primarily a
result of the production technique used when the core is
laminated.
The use of transformer cores of varying shape is known
through DE 40414, US 2 496 999, GB 2 025 150, US 3 792 399,
US 4 229 721, for instance. Some of these documents also
discloses cores made up of segments. However, none of these
documents pertain to high voltage power transformers, and
they would not be applicable to such transformers due to the
present technique of oil-cooling, discussed below.
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Conventional power transformers at the lower end of the
above-mentioned power range are sometimes provided with air
cooling in order to remove the unavoidable natural losses in
the form of heat. However, most conventional power trans-
formers are oil-cooled, generally by means of pressurized
oil cooling. This applies particularly to high-power trans-
formers. Oil-cooled transformers have a number of well known
drawbacks. They are, for instance, large, cumbersome and
heavy, thus entailing in particular considerable transport
problems, as well as the demands being extensive with regard
to safety and peripheral equipment.
However, it has been proved possible to replace oil-cooled
power transformers, to a great extent, with dry transformers
of a new type. This new dry transformer is provided with a
winding achieved by high-voltage cable, i.e. a high-voltage
insulated electric conductor. Dry transformers can thus be
used at considerably higher power rates than has previously
been possible. The expressions "dry transformer" and "dry
reactor" thus apply to a transformer/reactor which is not
oil-cooled, but preferably air-cooled.
With regard to reactors (inductors), these comprise a core
which mostly is provided with only one winding. In other
respects, what has been stated above concerning transformers
is substantially relevant also to reactors. It should be
particularly noted that also large reactors are oil-cooled.
The object of the present invention is to provide a trans-
former or a reactor enabling some of the drawbacks inherent
in the conventionally designed power transformers/reactors
described here, to be eliminated and also to provide a
method for use in manufacturing such a transformer/reactor.
The objects are achieved by means of a transformer/reactor
having the features defined in claim 1, and by means of a~
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method for manufacturing such a transformer/reactor in
accordance with the features defined in claim 25.
According to a first feature in claim 1, the core consists
of at least two segments. The corresponding method includes
the feature of manufacturing a core including at least two
segments. The expression "segment" or "segmented core" means
that the core of the transformer/reactor is built from
substantially identical segments or parts joined together
side by side to form the core.
Many advantages are gained with a core built from segments.
First of all, even relatively large cores can be made sub-
stantially annular in shape which offers significant advan-
tapes which will be explained below.
Secondly, simpler winding of the core is possible since each
segment can be wound separately.
A third advantage of segmented cores is that parts of the
core can be dismantled or assembled at any time during
manufacture.
Advantages are also obtained from the production point of
view since the core can be built in the form of modules,
each comprising one or more segments. This also offers
considerable advantages with regard to transport since the
core can be transported in segments and then assembled on
the site where it is to be used. If necessary, the winding
can also be wound on site.
According to a further feature in claim 1, the winding is
flexible and comprises an electrically conducting core
surrounded by an inner semiconducting layer, an insulating
layer surrounding the inner semiconducting layer and con-
sisting of solid material, and an outer semiconducting layer
surrounding the insulating layer, said layers adhering to
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each other. According to a further feature of the method,
said method comprises the step of installing a winding onto
the core which winding is defined in correspondence with
claim 1.
Thus, the windings in a transformer/reactor according to the
invention, are preferably of a type corresponding 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 com-
posed of one or more strand parts, an inner semiconducting
layer surrounding the conductor, a solid insulating layer
surrounding this and an outer semiconducting layer surround-
ing 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 (or curved) during assembly. The flexibility
of a XLPE-cable normally corresponds to a radius of curva-
ture of approximately 20 cm for a cable with a diameter of
mm, and a radius of curvature of approximately 65 cm for
a cable with a diameter of 80 mm. 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
25 times the cable diameter, preferably eight to twelve times
the cable diameter.
The winding should be constructed to retain its properties
even when it is bent and when it is subjected to thermal
30 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 coefficients of thermal expan-
sion. In a XLPE-cable, for instance, the insulating layer
consists of cross-linked, low-density polyethylene, and the
semiconducting layers consist of polyethylene with soot and
metal particles mixed in. Changes in volume as a result of
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temperature fluctuations are completely absorbed as changes
in radius in the cable and, thanks to the comparatively
slight difference between the coefficients of thermal expan-
sion 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 condi-
tions specified and also the condition of being semiconduct-
ing, 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, for example, of a solid
thermoplastic material such as low-density polyethylene
(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 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, particularly
their coefficients of thermal expansion, are affected rela-
tively 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 invention. 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-acrylate copolymers
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(EBA) and ethylene-ethyl-acrylate copolymers (EEA) may also
constitute 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 coefficients
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 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 released from each other. The
material in the layers is elastic, and the adhesion between
the layers is at least of the same magnitude as in the
weakest of the materials.
The conductivity of the two semiconducting layers is suffi-
cient 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 longitudinal direction
of the layer.
Thus, each of the two semiconducting layers essentially
constitutes one equipotential surface, and these layers will
substantially enclose the electrical field between them.
There is, of course, nothing to prevent one or more addi-
tional semiconducting layers being arranged in the insulat-
ing layer.
Other characteristics and advantages will become apparent-
from the remaining dependent claims.
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In addition to the above mentioned advantages obtained with
a winding consisting of a cable, less problems with magnetic
stray fields are encountered with the use of cable. This has
the advantage that a toroidal core can be used even in high-
voltage transformers, provided that the problem of arranging
a sufficiently large core is solved and this is done accord-
ing to the invention by using a segmented core. The impor-
tant advantage follows that technology can be used which is
previously known only from the low-voltage range and field
of electronics.
According to a particularly advantageous feature, it is
stated that the winding consists of high-voltage cable.
As another feature it is stated that the high-voltage cable
preferably has a diameter within the interval 20-250 mm and
a conductor area within the interval 80-3000 mm2.
According to a particularly advantageous characteristic, the
core is substantially annular. This design has the advantage
of providing a shorter magnetic path than a rectangular
core, and better flow distribution in the core. The advan-
tages of an annular core with a shorter magnetic path than a
conventional core include it requiring less material, it
will be less heavy and less expensive and result in less
power losses and is therefore more efficient.
According to another particularly favourable characteristic,
the core has a substantially toroidal shape. In a toroidal
core the coil can be distributed uniformly around the entire
core, thereby reducing the problems of undesired magnetic
fields. A high degree of symmetry is also favourable since
the magnetic field diminishes more quickly with the dis-
tance .
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According to one embodiment the core of the trans-
former/reactor has a window, which is substantially circular
in shape and the annular shape of the core is circular.
Alternatively the core may comprise a window which is sub-
s stantially elliptical and the annular shape of the core is
elliptical. The core may also be rectangular.
According to an advantageous embodiment the core is composed
of two segments. In many cases this is naturally the sim-
plest alternative, which per se constitutes an advantage.
According to another favourable embodiment the core is
composed of four segments, two straight segments and two
segments shaped as ring halves, the two segments shaped as
ring halves being joined together via the two straight
segments. This embodiment, as also the elliptical embodi-
ment, has the advantage that it can be used even in cramped
spaces.
That each segment comprises a plurality of plates and that
the core is constructed as a laminated core are also stated
to be advantageous features.
According to further advantageous features, the plates may
consist of magnetically oriented steel and the number of
segments is sufficiently large for the magnetic orientation
direction not to be lost. Alternatively, the plates may
consist of amorphous steel.
According to one embodiment, adjacent segments are held
together by one segment having at least one protruding plate
which is fitted into a corresponding gap, between plates,
arranged in the corresponding side of the nearest adjacent
segment, thereby forming an overlap joint. This results in
the advantage that no special attachment means are required
to keep the segments forming the core together. Alterna
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tively, or by way of supplement, however, the trans-
former/reactor may include attachment means.
According to yet another advantageous feature the segmented
core contains internal ducts which may be used for a cool-
ant. According to a particular embodiment of the cooling
ducts the core segments can be connected thereby.
Finally, the method according to the present invention is
characterized by the advantageous feature that the windings
of the core are wound onto the segment before the segment is
assembled to form the core.
The invention is preferably intended for single phase trans-
formers .
As a summary, it should be stressed that, through the combi-
nation of a winding as defined in claim 1 and a segmented
core, it is made possible by the present invention to pro-
vide dry transformers/reactors for high voltages, with large
cores of a substantially annular shape, and preferably
toroidal shape.
For a better understanding of the invention, four embodi-
ments will now be described in detail, by way of example,
with reference to the accompanying drawings in which:
Figure 1 shows a basic diagram in the form of a schematic
view in perspective of a first embodiment of the
invention,
Figure 2 shows a schematic view of a second embodiment of
the invention,
Figure 3 shows a schematic view of a third embodiment of the
invention,
Figure 4 shows a schematic view of a fourth embodiment of
the invention,
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Figure 5 shows a section through a segment of a core accord-
ing to the present invention, and
Figure 6 shows a cross-sectional view of a high-voltage
cable.
A basic diagram of the present invention, also constituting
a first embodiment, is shown schematically in Figure 1. The
figure illustrates a transformer core 1, which could equally
well be a reactor core, provided with a winding 2 passing
through a substantially circular window 5. The core is built
from a relatively large number of segments 4, for which only
one reference number is being used. The segments are pref-
erably identical since this is an advantage from the manu-
facturing point of view, but could be shaped with some
differences if suitable. The figure shows eighteen segments,
each segment consisting of a number of plates 3 which have
been stacked one on top of the other in a known manner. An
example of how these plates can be stacked on top of each
other is shown in Figure 5, illustrating a section through a
core segment. The plates are normally glued together. By
stacking the plates on top of each other a so called lami-
nated core is obtained. Different joining methods may be
used, of which only one possible method is being illustrated
in Figure 5. Another possible method is known as step lap,
for instance.
The individual plates illustrated in the embodiment in
Figure 1 have a shape corresponding to a parallel trapezium.
This means that the "annular" shape of the core is in fact a
polygon. However, with a relatively large number of seg-
ments, as in this case, an annular shape, or toroidal as is
the case with the cross section of the core, is approximated
with a polygonal shape.
It should be emphasized that the terms "annular, circular
window and toroidal", which comprises a circular cross
section, and all of which refer to the core, these terms
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refer in this context not only to a geometrically perfect
ring, torus or circle, but should also be considered as
including the approximate equivalents to these geometric
figures due to the fact that the core, because of the seg-
ments, may have a through-section both in transverse and
longitudinal direction that is in fact a polygon.
Figure 2 illustrates a second embodiment of the invention in
the form of a core 11 with segments 14, seen from above.
According to the embodiment in Figure 2, the segments have a
shape similar to a pie piece with a truncated tip so that
they may be combined to an approximate ring, preferably with
a toroidal shape. Each plate 3 in Figure 5 is thus cut to
fit the pie piece shape shown in Figure 2. In this case the
core 11 is composed of eight segments 14. The segments in
this core are built from plates of magnetically oriented
steel, as illustrated by arrows in the Figure. When magneti-
cally oriented steel is used it is important that the number
of segments is sufficient for the magnetic direction of
orientation not to be lost. Here too, the core has a circu-
lar window 15 through which the winding or windings are
intended to pass.
A third embodiment of the core is shown in Figure 3. The
segmented core 21 consists of only two segments in the form
of two ring halves 23, 24 which have been combined to a core
with a substantially circular window 25.
The fourth embodiment is illustrated in Figure 9, from which
is seen that the core 31 preferably comprises four segments:
two straight segments 36, 37 and two segments 33, 34 in the
form of half rings. The two segments 33, 34 in the form of
ring halves are connected via the two straight segments 36,
37. The core has a window 35.
The segments can be held together or combined in various
ways to form the annular core. It is thus feasible to con-
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figure the segments with some plates protruding outside the
actual side of the segment, i.e. the side facing an adjacent
segment, and which are inserted into corresponding gaps,
between plates, arranged in the corresponding side of the
nearest adjacent segment, and vice versa, so that plates in
adjacent segments overlap. A joint is thus obtained between
the plates in two adjacent segments, which is formed in an
equivalent way to the example of joints formed inside a
segment which is illustrated in Figure 5. Alternatively,
special attachment means may be used, such as clamps, yokes,
screws of the like.
One advantage of a segmented core is that it may include
internal ducts for a coolant. These ducts may consist of
interspaces 17, which have been provided between the plates
during lamination. Alternatively, tubes for a coolant may be
installed in the segments during lamination of the plates.
Another alternative is to subsequently drill ducts through
the segments. It would also be possible for the segments to
be held together by the internal cooling ducts, in such a
way that adjacent segments are being held together by at
least one segment being provided with a cooling duct termi-
nating in a protruding pipe end shaped to be fitted to a
corresponding pipe end terminating the cooling duct in an
adjacent segment.
Figure 6, finally, shows a section through a high-voltage
cable 6 particularly suitable for use in the invention. The
high-voltage cable 6 comprises a number of strands 7 made of
copper (Cu), for instance, and having circular cross sec-
tion. These strands are arranged in the middle of the high-
voltage cable. Surrounding the strands 7 is a first semi-
conducting layer 8. Surrounding this first semi-conducting
layer 8 is an insulating layer 9, e.g. XLPE insulation.
Surrounding the insulating layer 9 is a second semi-
conducting layer 10 provided. The cable illustrated differs
from conventional high-voltage cable in that the outer,
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mechanically protective sheath and the metal screen that
normally surround such cables are eliminated. Thus the
concept 'high-voltage cable" in the present application does
not necessarily include the metallic screen or the sheath
that normally surround such cables for power distribution.
The embodiments illustrated and described above shall be
considered only as examples and the invention shall not be
limited thereto, but can be varied within the scope of the
inventive concept as defined in the appended claims. Thus,
the window in the cores of three of the examples illustrated
has been shown only with substantially circular form, but
may of course also be elliptical or some other shape. Simi-
larly, the annular shape of the core may be elliptical
instead of circular. This may be preferable, for instance,
when the available space is limited widthwise. Furthermore,
there is naturally nothing to prevent a segmented core being
made rectangular, with a rectangular window.
The number of segments may also vary greatly depending on
many different considerations with regard to manufacturing
technique, winding technique, transport distance, etc. The
plates may also be made of steel other than magnetically
oriented steel, e.g. amorphous steel.
Finally, it should be mentioned that the invention is natu-
rally also applicable to a three-phase transformer/reactor
by combining three cores constructed in accordance with the
invention.
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