Field of the invention
[0001] The invention relates to a clamping bridge for a clamping system for an electric
transformer, said clamping bridge comprising a first mounting portion for mounting
the clamping bridge on a first clamping frame member of said clamping system, a second
mounting portion for mounting the clamping bridge on a second clamping frame member
of the clamping system, as well as a retaining portion adapted to be brought into
contact with a laminated yoke of a transformer core of said transformer.
[0002] Moreover, the invention relates to a clamping system for an electric transformer,
said clamping system comprising a first clamping frame with a first clamping frame
member and a second clamping frame member adapted to be arranged on opposing sides
of a laminated yoke of a transformer core of said transformer, at least one tension
element, preferably horizontal tie bars, connecting the first clamping frame member
to the second clamping frame member under tensile stress, as well as a clamping bridge
connecting the first clamping frame member to the second clamping frame member.
Background art
[0003] Electrical transformers and electrical power transformers in particular, usually
feature a transformer core consisting of multiple laminations. Windings of a particular
phase are arranged on each core column of this laminated core. In order to form a
closed magnetic circuit, the ends of said core columns are connected by yokes of the
laminated core; upper ends of the columns are connected to each other by an upper
yoke whereas lower ends of the columns are connected to each other by a lower yoke.
[0004] In order to guarantee stability of the laminated core, the laminations are pressed
together by means of a clamping system. To this end, clamping frames are employed
which are positioned around the upper and lower yokes of the laminated core. Respective
members of these clamping frames are positioned on opposing sides of the yoke and
are held together under tension, such that the respective yoke is wedged in between
said clamping frame members.
[0005] For the windings of an electric (power) transformer it is of utmost importance to
maintain mechanical stability. Therefore, each winding is pressed between two pressure
plates which are inserted between the upper yoke and the upper ending of the winding,
and between the lower yoke and the lower end of the winding respectively. In order
to maintain the required pressure on said pressure plates, the clamping frame assigned
to the upper yoke is connected to the clamping frame assigned to the lower yoke via
tension elements. After pressing the clamping frames assigned to the upper and lower
yoke towards each other, the clamping frames are firmly attached to the tension elements
such that said tension elements keep the distance between both clamping frames fixed.
[0006] In order to provide stability to the whole clamping system during this winding pressing
procedure, to absorb vertical forces produced during transportation, loading and unloading
of the transformer, and to provide support to the transformer cover under vacuum load,
it is known to employ clamping bridges. Such clamping bridges are connected to two
opposing clamping frame members of one clamping frame in such a way that they each
cover the respective yoke at least partially. When external pressure is applied on
one clamping frame, say of the one assigned to the upper yoke, in order to move it
closer to the clamping frame assigned to the lower yoke, said bridges will retain
the yoke thereby avoiding that the yoke is lifted from its position and/or that the
respective core laminations deform under pressure.
[0007] Known clamping bridges are made of magnetic steel grade (EN 10025 S235xx or S355xx)
and therefore constitute an electric connection between the clamping frame members
of one clamping frame. Electro-magnetic fields originating from the windings and connection
thus generate eddy currents that unhinderedly flow through the whole clamping frame.
This may result in the creation of hotspots within the active part of the transformer
and may also lead to gassing.
[0008] In order to avoid this and to achieve the required electrical isolation, additional
elements for electric isolation are usually positioned between the clamping bridge
and other electroconductive elements of the clamping frame, e.g. the clamping frame
members. However, it has been observed that the reliability of such isolation measures
depends crucially on manufacturing accuracy and that the employed isolation elements
can break due to stresses prevailing in the clamping system while winding pressing
and/or lifting of the active part of the transformer during the tank process.
Object of the invention
[0009] It is therefore an object of the invention to present a clamping bridge for a clamping
system, employment of which in a clamping system for an electric transformer leads
to the above-mentioned problems being avoided. In particular, the proposed clamping
bridge should minimize losses and heat or gassing problems stemming from eddy currents
in the transformer's laminated core, and at the same time feature an increased reliability
as compared to known isolation measures.
[0010] Moreover, it is an object of the present invention to present a clamping system featuring
the aforementioned advantages.
Description of the invention
[0011] A clamping bridge for a clamping system for an electric transformer, said clamping
bridge comprising a first mounting portion for mounting the clamping bridge on a first
clamping frame member of said clamping system, a second mounting portion for mounting
the clamping bridge on a second clamping frame member of the clamping system, as well
as a retaining portion adapted to be brought into contact with a laminated yoke of
a transformer core of said transformer, accomplishes an object of the invention in
that the clamping bridge is made of an epoxy resin laminate.
[0012] By employing epoxy resin (laminate) the clamping bridge itself can be used as a means
for electric isolation between the first clamping frame member and the second clamping
frame member. Thus, additional elements for electric isolation, as they are employed
in the prior art, become moot and effects attributed to eddy currents through the
clamping frame can be effectively minimized. At the same time, clamping bridges made
of epoxy resin laminate have structural strength and stability properties facilitating
to reliably keep the laminated yoke in shape and position when pressure is exerted
on the first and second clamping frame members.
[0013] In a preferred embodiment of clamping bridge according to the invention, the epoxy
resin laminate is realized as epoxy woven glass cloth.
[0014] Such a material provides the clamping bridge with optimal electric isolation and
stiffness properties and thereby greatly improves the overall stability of the corresponding
clamping frame and at the same time reduces stray losses in the clamping system.
[0015] In another preferred embodiment of the clamping bridge according to the invention,
the epoxy woven glass cloth is EP GC 203 or EP GC 205, as defined in the norm IEC
60893.
[0016] These materials have
- a minimum flexural strength of 340 MPa (measured at 150°C ± 3 K after 1 hour at 150°C
± 3 K not to be less than 50% of the specified value; test method in IEC 60893-2 Subclause
5.1; nominal thickness of sheet to which test is applicable is 1,5 mm or more),
- a minimum Charpy impact strength parallel to laminations of 33 kJ/m2 (EP GC 203) and 50 kJ/m2 (EP GC 205) (test method in IEC 60893-2 Subclause 5.4.2; nominal thickness of sheet
to which test is applicable is 5 mm or more),
- a minimum Izod impact strength parallel to laminations of 34 kJ/m2 (EP GC 203) and 54 kJ/m2 (EP GC 205) (test method in IEC 60893-2 Subclause 5.4.3; nominal thickness of sheet
to which test is applicable is 5 mm or more),
- a minimum breakdown voltage at 90°C in oil parallel to laminations of 35 kV (test
method in IEC 60893-2 Subclause 6.1; nominal thickness of sheet to which test is applicable
is 3 mm or more),
- and a minimum insulation resistance after immersion in water of 5 x 104 MΩ (EP GC 203) and 1 x 104 MΩ (EP GC 205) (test method in IEC 60893-2 Subclause 6.3).
[0017] Therefore, a clamping bridge made of this material is strong enough to maintain stability
of the clamping frame as well as of the transformer's laminated core under stress,
and in addition to that provides excellent isolation in order to reduce negative effects
associated with eddy currents through the clamping frame.
[0018] In another preferred embodiment of the clamping bridge according to the invention,
said retaining portion is arranged in a different plane than said first mounting portion
and/or said second mounting portion.
[0019] This lends more flexibility to the clamping bridge, enabling to adapt its form and
profile to the respective yoke of the laminated core. In particular, a surface of
the yoke may be uneven due to laminations having different lengths. The profile of
said surface, therefore, may have a maximum height in a central region with its height
falling off in outer regions of said surface. A shift of the clamping bridge's retaining
portion with respect to its first and/or second mounting region thus facilitates to,
at least approximately, align the shape of the clamping bridge with the shape of the
respective yoke or its outer surface facing the clamping bridge.
[0020] In another preferred embodiment of the clamping bridge according to the invention,
said retaining portion comprises a first planar section which is arranged substantially
parallel to the first mounting portion and/or the second mounting portion.
[0021] This design allows for an optimal approximation of generally encountered yoke (surface)
shapes.
[0022] In another preferred embodiment of the clamping bridge according to the invention,
the retaining portion comprises a second planar section protruding obliquely from
the first planar section and connecting the first planar section with the first mounting
portion, as well as a third planar section protruding obliquely from the first planar
section and connecting the first planar section with the second mounting portion.
[0023] By continuously connecting the first planar section with the first and second mounting
portion of the clamping bridge, said connection being established by the second and
third planar section, it is possible to retain the position of most or all of the
laminations and thereby the entire shape of the yoke when pressure is applied to the
clamping frame during winding pressing.
[0024] In another preferred embodiment of the clamping bridge according to the invention,
the clamping bridge has a first set of openings for receiving fastening elements,
preferably bolts or screws, for fastening the clamping bridge to the first clamping
frame member and/or to the second clamping frame member, and/or the clamping bridge
has a second set of openings for receiving tension elements, preferably vertical tie
bars, for connecting the first clamping frame member and/or the second clamping frame
member to a further clamping frame.
[0025] The first set of openings may be realized as elongated holes located in the first
and second mounting portion of the clamping bridge, each of which openings can be
open at one side. The first set of openings allows for a simple and reliable fastening
of the clamping bridge to the first and second clamping frame member.
[0026] The second set of openings for receiving the tension elements for connecting the
first clamping frame member and/or the second clamping frame member to a further clamping
frame may be positioned in the first and second mounting portion of the clamping bridge.
They can be realized as drilled holes which may be positioned between said first set
of openings of the respective mounting portion. Preferably, said second set of openings
can be supported by one or more support discs or support plates. Said second set of
openings allows for a particularly simple and reliable way of receiving the tension
elements for connecting the first clamping frame member and/or the second clamping
frame member to a further clamping frame. While pressure is applied to the clamping
frame (members) these tension elements may be loosely inserted in the respective second
set of openings, thereby allowing for a variation of the distance between the clamping
frame to which the first and second clamping frame members belong, and a further clamping
frame. As soon as the desired pressure (or distance between the clamping frames) has
been reached, the tension elements may be tightly fastened to the clamping bridge,
e.g. by means of threaded heads of the tension elements and corresponding nuts.
[0027] The feature 'first set of openings for receiving fastening elements' is independent
from the feature second set of openings for receiving tension elements'. In particular,
neither of these features is substantial for solving the object of the invention described
above. Moreover, neither of these features is essential for the functioning for the
clamping bridge with respect to the above-described object of the invention. Additionally,
as there exist various possibilities for fastening the clamping bridge to the first
clamping frame member and/or to the second clamping frame member - e.g. by gluing
the first and second mounting portion to the clamping frame members - as well as for
anchoring the tension elements within the clamping frame - e.g. by corresponding receiving
means located within the first clamping frame member and/or the second clamping frame
member, no significant adaption of the clamping bridge would be made necessary by
singling out either of these independent features.
[0028] In another preferred embodiment of the clamping bridge according to the invention,
the openings are lined with reinforcing members, preferably realized as glass fibre
tubes.
[0029] Due to material properties of the employed epoxy resin laminate, the clamping bridge
may have a tendency to delaminate due to threaded heads of the tension elements and/or
of the fastening elements. Under pressure the threads might eat into the inner walls
of the first and/or second set of openings of the clamping bridge. In order to prevent
this, the inner walls of the first set of openings and/or the inner walls of the second
set of openings may be reinforced by means of said reinforcing members.
[0030] An object of the invention is also achieved by means of a clamping system for an
electric transformer, said clamping system comprising a first clamping frame with
- a first clamping frame member and a second clamping frame member adapted to be arranged
on opposing sides of a laminated yoke of a transformer core of said transformer,
- at least one tension element, preferably realized as at least one horizontal tie bar,
connecting the first clamping frame member to the second clamping frame member under
tensile stress, as well as
- a clamping bridge connecting the first clamping frame member to the second clamping
frame member,
whereas the object is achieved in that the clamping bridge is realized according to
any of the embodiments described above.
[0031] By employing the clamping bridge according to the invention, additional elements
for facilitating electric isolation between the clamping bridge and the other elements
of the first clamping frame, in particular the first and second clamping frame members,
become unnecessary. Any effects attributed to eddy currents through the clamping frame
can thus be effectively minimized. At the same time, clamping bridges according to
the invention have structural strength and stability properties facilitating to reliably
keep the laminated yoke in shape and position when pressure is exerted on the first
clamping frame.
[0032] According to a preferred embodiment the clamping system comprises at least two tension
elements, preferably vertical tie bars, connecting the first clamping frame to a further
clamping frame of the clamping system, whereas these tension elements are received
in a second set of openings of the clamping bridge.
[0033] By having the tension elements for connecting the first clamping frame to a further
clamping frame of the clamping system received in the clamping bridge itself, preferably
in the first and second mounting portions of the clamping bridge, the winding pressing
procedure is significantly simplified. External pressure can be applied to the first
clamping frame member and to the second clamping frame member, as well as to those
parts of the first and second mounting portions of the clamping bridge which overlap
with the first and second clamping frame members respectively. As soon as the desired
external pressure, and therefore the desired distance between the first clamping frame
and a further, say second clamping frame has been reached, the pressure may be maintained
while the tension elements connecting the first clamping frame to the second clamping
frame can be tightly fixed to the clamping bridge. Only after said tight connection
has been established, thereby fixing the distance between the first and second clamping
frame, the first and second clamping frame members may be relieved from the external
pressure.
Brief description of the drawings
[0034] In what follows the invention is described further with regard to an example embodiment.
The drawings are, however, only exemplary and are not meant to restrict the scope
of the invention as described above.
- Fig. 1
- shows the clamping bridge according to the invention as a part of a clamping system
according to the invention
- Fig. 2
- shows a detail of the clamping system depicted in Fig. 1
- Fig. 3
- shows a known clamping system
Ways of carrying out the invention
[0035] Fig. 1 shows a clamping system according to the invention. As such it comprises a
first clamping frame 16 which is arranged on an upper yoke 7 of a laminated core of
an electric transformer, as well as a further clamping frame 17, which however is
not depicted in Fig. 1. The further clamping frame 17 is identical to the first clamping
frame 16 and is arranged on a lower yoke of the same laminated core.
[0036] The first clamping frame 16 comprises a first clamping frame member 5 arranged on
one side of the yoke 7, as well as a second clamping frame member 6 arranged on an
a side of the yoke 7 opposite of the first clamping frame member 5. The first clamping
frame member 5 and the second clamping frame member 6 are connected to each other
under tensile stress by means of one or more tension elements 15, which may be realized
as horizontal tie bars 19. Due to these tension elements 15, said first clamping frame
16 exerts a force on the yoke 7 pressing it laminations against each other thereby
holding the yoke 7 together.
[0037] A clamping bridge 1 is mounted on both clamping frame members 5, 6 such that it partially
covers the yoke 7. Since the clamping bridge 1 is made of epoxy resin laminate, it
facilitates on the one hand an electric isolation between the first clamping frame
member 5 and the second clamping frame member 6, and on the other hand it features
sufficient structural strength to fulfill its purpose, namely to retain the position
of most or all of the laminations and thereby the shape of the yoke 7 when pressure
is applied to the first clamping frame 16. Said clamping bridge 1 comprises a first
mounting portion 2 for mounting the clamping bridge 1 on the first clamping frame
member 5, as well as a second mounting portion 3 for mounting the clamping bridge
1 on the second clamping frame member 6. In order to mount the clamping bridge 1 on
the clamping frame members 5, 6, the clamping bridge 1 is placed on the clamping frame
members 5, 6 such that the fist mounting portion 2 at least partially overlaps with
the first clamping frame member 5 and that the second mounting portion 3 at least
partially overlaps with the second clamping frame member 6. By means of fastening
elements 14, which are left out in Fig. 1 for reasons of clarity but can be seen from
Fig. 2, the first mounting portion 2 is fixed to the first clamping frame member 5
and the second mounting portion 3 is fixed to the second clamping frame member 6.
In order to receive said fastening elements 14, which may be realized as screws or
bolts, the first mounting portion 2 and the second mounting portion 3 have a first
set of openings 12 which themselves are realized as elongated holes that are open
towards the sides of the clamping bridge 1.
[0038] The clamping bridge 1 also comprises a retaining portion 4 which is adapted to be
brought into contact with a laminated yoke 7. Said retaining portion 4 comprises a
first planar section 9, which is arranged in a plane different from a plane in which
the first mounting portion 2 and the second mounting portion 3 are positioned. Said
first planar section 9 is connected to the first mounting portion 2 by means of a
second planar section 10 protruding obliquely from the first planar section 9. The
transition from the first planar section 9 to the second planar section 10, as well
from the second planar section 10 to the first mounting portion 2 may be facilitated
by kinked or bent sections of the retaining portion 4.
[0039] Correspondingly, said first planar section 9 is connected to the second mounting
portion 3 by means of a third planar section 11 protruding obliquely from the first
planar section 9. The transition from the first planar section 9 to the third planar
section 11, as well from the third planar section 11 to the second mounting portion
3 may be facilitated by kinked or bent sections of the retaining portion 4.
[0040] Due to its structure the retaining portion 4 of the clamping bridge 1 resembles the
structure of the yoke 7 in good approximation. The yoke 7 depicted in Fig. 1 is assembled
out of multiple laminations having different height. As a result, a surface of the
yoke 7 facing the clamping bridge 1 is uneven due to laminations having different
height. The profile of the yoke 7 has a maximum height in a central region with its
height falling off in outer regions of said surface. Hence, the clamping bridge 1
allows to retain the position of most or all of the laminations and thereby the entire
shape of the yoke 7 when pressure is applied to the first clamping frame 16 during
the procedure of winding pressing.
[0041] During winding pressing an external force is exerted on the first clamping frame
16 which conveys said external pressure to windings of the transformer arranged on
core columns between the upper yoke 7 and the lower yoke. During this procedure the
first clamping frame 16 and the further clamping frame 17, which is attributed to
the lower yoke, are loosely connected such that their distance may still be varied.
To this end, tension elements 15 are foreseen which can be tightly fixed to the further
clamping frame 17 but are, at first, only loosely inserted into a corresponding second
set of openings 13 of the first clamping frame 16. In the embodiment depicted in Fig.
1 and 2, said second set of openings 13 is arranged in the clamping bridge 1, in particular
in the first mounting portion 2 and the second mounting portion 3 of the clamping
bridge 1. The openings of said second set of openings 13 are realized as bore holes
which in Fig. 1 and 2 are covered by support plates 8 aiding with the stability of
the mounting portions 2, 3 when the tension elements 15 are tightly fixed to the clamping
bridge 1. However, the positions of the second set of openings 13 may directly and
unambiguously be deferred from the position of the upper ends of the vertical tie
bars 20 serving as tension elements 15 for connecting the first clamping frame 16
to the further clamping frame 17.
[0042] As soon as the desired pressure on the windings has been achieved, the vertical tie
bars 20 are tightly fixed to the first clamping frame 16 by means of threaded nuts
21 tightened against the support plates 8. Thereby, the relative positions of the
first clamping frame 16 and the second clamping frame 17 are fixed and the distance
between the clamping frames 16, 17 cannot be varied any longer. Thus, the pressure
on the windings is maintained without the external pressure having to be exerted any
longer on the first clamping frame 16.
[0043] In order to avoid delamination of the clamping bridge 1, the inner walls of the first
set of openings 12 and/or of the second set of openings 13 are lined with reinforcing
members 18 which are preferably realized as glass fibre tubes.
[0044] Fig. 3 shows a clamping system according to the state of the art. It can be seen
that multiple elements for electric isolation are employed in order to facilitate
isolation between the clamping bridge, the clamping frame members, and the fastening
and tension elements.
Reference signs
[0045]
- 1
- clamping bridge
- 2
- first mounting portion
- 3
- second mounting portion
- 4
- retaining portion
- 5
- first clamping frame member
- 6
- second clamping frame member
- 7
- yoke
- 8
- core
- 9
- first planar section
- 10
- second planar section
- 11
- third planar section
- 12
- first set of openings
- 13
- second set of openings
- 14
- fastening elements
- 15
- tension elements
- 16
- first clamping frame
- 17
- further clamping frame
- 18
- reinforcing members
- 19
- horizontal tie bar
- 20
- vertical tie bar
1. Clamping bridge (1) for a clamping system for an electric transformer, said clamping
bridge (1) comprising
- a first mounting portion (2) for mounting the clamping bridge (1) on a first clamping
frame member (5) of said clamping system,
- a second mounting portion (3) for mounting the clamping bridge (1) on a second clamping
frame member (6) of the clamping system, as well as
- a retaining portion (4) adapted to be brought into contact with a laminated yoke
(7) of a transformer core (8) of said transformer,
characterized in that the clamping bridge (1) is made of an epoxy resin laminate.
2. The clamping bridge (1) according to claim 1, characterized in that the epoxy resin laminate is realized as epoxy woven glass cloth.
3. The clamping bridge (1) according to claim 2, characterized in that the epoxy woven glass cloth is EP GC 203 or EP GC 205, as defined in the norm IEC
60893.
4. The clamping bridge (1) according to any of the claims 1 to 3, characterized in that said retaining portion (4) is arranged in a different plane than said first mounting
portion (2) and/or said second mounting portion (3) .
5. The clamping bridge (1) according to any of the claims 1 to 4, characterised in that said retaining portion (4) comprises a first planar section (9) which is arranged
substantially parallel to the first mounting portion (2) and/or the second mounting
portion (3).
6. The clamping bridge (1) according to claim 5,
characterized in that the retaining portion (4) comprises
- a second planar section (10) protruding obliquely from the first planar section
(9) and connecting the first planar section (9) with the first mounting portion (2),
as well as
- a third planar section (11) protruding obliquely from the first planar section (9)
and connecting the first planar section (9) with the second mounting portion (3).
7. The clamping bridge (1) according to any of the claims 1 to 6, characterized in that the clamping bridge (1) has a first set of openings (12) for receiving fastening
elements (14), preferably bolts or screws, for fastening the clamping bridge (1) to
the first clamping frame member (5) and/or to the second clamping frame member (6),
and/or the clamping bridge (1) has a second set of openings (13) for receiving tension
elements (15), preferably vertical tie bars, for connecting the first clamping frame
member (5) and/or the second clamping frame member (6) to a further clamping frame
(17).
8. The clamping bridge (1) according to claim 7, characterized in that the openings are lined with reinforcing members (18), preferably realized as glass
fibre tubes.
9. Clamping system for an electric transformer, said clamping system comprising a first
clamping frame (16) with
- a first clamping frame member (5) and a second clamping frame member (6) adapted
to be arranged on opposing sides of a laminated yoke (7) of a transformer core (8)
of said transformer,
- at least one tension element (15), preferably at least one horizontal tie bar, connecting
the first clamping frame member (5) to the second clamping frame member (6) under
tensile stress, as well as
- a clamping bridge (1) connecting the first clamping frame member (5) to the second
clamping frame member (6),
characterized in that the clamping bridge (1) is realized according to any of the claims 1 to 8.
10. The clamping system according to claim 9, characterized in that it comprises at least two further tension elements (15), preferably vertical tie
bars, connecting the first clamping frame (16) to a further clamping frame (17) of
the clamping system, whereas these tension elements (15) are received in a second
set of openings (13) of the clamping bridge (1).