Note: Descriptions are shown in the official language in which they were submitted.
CA 0225~725 1998-11-20
WO 97/45937 PCT/SE97/00904
A DEVIC~ rN THE STATOR OF A ROT~T~G ELECTRIC MACHINE
The present invention relates to a device for increasing the mechanical
rigidity and natural frequency of the stator in a rotating electric machine
5 and prevent damaging oscillations occurring between the stator teeth.
Certain attempts at a new approach as regards the design of synchronous
machines are described, inter alia, in an article entitled 'Water-and-oil-
cooled Turbogenerator TVM-300" in J. Elektrotechnika, No. 1, 1970, pp 6-
l0 8, in US 4,429,244 "Stator of Generator" and in Russian patent documentCCCP Patent 955369.
The water- and oil-cooled synchronous machine described in J.
Elektrotechnika is intended for voltages up to 20 kV. The article describes
15 a new insulating system consisting of oil/paper insulation, which makes it
possible to immerse the stator completely in oil. The oil can then be used
as a coolant while at the same time using it as insulation. To prevent oil in
the stator from leaking out towards the rotor, a dielectric oil-separating
ring is provided at the internal surface of the core. The stator winding is
20 made from conductors with an oval hollow shape provided with oil and
paper insulation. The coil sides with their insulation are secured to the
slots made with rectangular cross section by means of wedges. As coolant,
oil is used both in the hollow conductors and in holes in the stator walls.
Such cooling systems, however, entail a large number of connections of
25 both oil and electricity at the coil ends. The thick insulation also entails an
increased radius of curvature of the conductors, which in turn results in
an increased size of the winding overhang.
The above mentioned US patent relates to the stator part of a synchronous
30 machine which comprises a laminated magnetic core of electrical steel
with trapezoidal slots for the stator winding. The slots are tapered since
~e need of insulation of the stator winding is less towards the interior of
the rotor where that part of the winding which is located nearest the
neutral point is located. In addition, the stator part comprises a dielectric
35 oil-separating cylinder nearest the inner surface of the core which may
increase the magnetization requirement relative to a machine without this
ring. The stator winding is made of oil-immersed cables with the same
diameter for each winding layer. The layers are separated from each other
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by means of spacers in the slots and secured by wedges. What is special
regarding the winding is that it comprises two so-called half-windings
connected in series. One of the two half-windings is located, centred,
inside an insulation sleeve. The conductors of the stator winding are
5 cooled by surrounding oil. The disadvantages with such a large quantity
of oil in the system are the risk of leakage and the considerable amount of
cleaning work which may result from a fault condition. Those parts of the
insulation sleeve which are located outside the slots have a cylindrical
part and a conical termination reinforced with current-carrying layers, the
10 duty of which is to control the electric field strength in the region where
the cable enters the end winding.
From CCCP 955369 it is clear, in another attempt to raise the rated voltage
of the synchronous machine, that the oil-cooled stator winding comprises
15 a conventional high-voltage cable with the same dimension for all the
layers. The cable is placed in stator slots formed as circular, radially
disposed openings corresponding to the cross-section area of the cable and
the necessary space for fixing and for coolant. The different radially
located layers of the winding are surrounded by and fixed in insulated
20 tubes. Insulating spacers fix the tubes in the stator slot. Because of the oil
cooling, an internal dielectric ring is also needed here for sealing the
coolant against the internal air gap. The design shows no tapering of the
insulation or of the stator slots. The design exhibits a very narrow radial
waist between the different stator slots, which implies a large slot leakage
25 flux which significantly influences the magnetization requirement of the
machine.
The problem addressed by the invention appears in connection with a
high-voltage electric alternating current machine, primarily intended as a
30 generator in a power station for generating electric power. Such machines
have conventionally been designed for voltages in the range 15-30 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 network via a
transformer which steps up the voltage to the level of the power network,
3~ i.e. in the range of 130-400 kV.
By using high-voltage insulated electric conductors, in the following
termed cables, in the stator winding, with permanent insulation similar to
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that used in cables for transmitting electric power, e.g., crosslinked
polyethylene (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 step-up transformer is thus eliminated.
This concept generally requires that the slots in which the cables are
placed in the stator to be deeper than with conventional technology
(thicker insulation due to higher voltage and more turns in the winding).
This entails new problems with regard to mechanical natural frequencies
10 in the stator teeth between the stator slots. A stator with deep slots may be subjected to damaging vibrations at the air gap due to resonance with
disturbing force, typically electromagnetic forces with a frequency of
100 Hz for a machine having a nominal output frequency of 50 Hz.
15 The object of the present invention is to solve this problem and ~us
prevent oscillations between the stator teeth. This object is achieved with
the method and the device defined in the appended claims.
The invention will now be described in more detail with reference to the
20 accompanying drawings in which
Figure 1 shows a cross section through the insulated electrical conductor
which is used in conjunction with the invention and is here termed a
cable,
Figure 2 shows an axial view of a sector in a stator core,
Figures 3 and 4 show axial views of the end of a slot situated at the air gap
in the stator core, according to two embodiments of the invention,
Figure 5 shows an axial view of a sector of a stator core according to a
third embodiment of the invention,
Figure 6 shows an axial view of a sector of a stator core with yet another
35 application of the device according to the invention,
Figure 7 shows an axial section through the stator part corresponding to
Figure 6, and
.. .. .
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Figures 8 and 9 show a radial and an axial view, respectively, partially in
section, of the end part of the stator core near the air gap.
S Figure 1 illustrates a cross-sectional view of an insulated electric
conductor or cable 1, used in conjunction with the present invention. The
cable 1 comprises a conductor 2 with circular cross section, consisting of a
number of strands and made of copper, for instance. This conductor 2 is
arranged in the middle of the cable 1. Around the conductor 2 is a first
10 semiconducting layer 3. Around the first semiconducting layer 3 is an
insulating layer 4, e.g., XLPE insulation. Around the layer of insulation 4
is a second semiconducting layer 5. In this context, therefore, the cable
does not indude the outer protective sleeve whidh normally surrounds a
cable for power distribution.
Figure 2 shows part of a stator lamination 6 intended for a new high-
voltage alternating current generator. These stator laminations 6, placed
one on top of the other, form the core of the stator. This is annular and
surrounds the rotor (not shown) with an air gap 7. Slots 8 to receive the
20 cables extending axially through the stator are deeper than in
conventional machines. This entails the above-mentioned drawbacks of
the stator having low natural frequencies and that oscillations easily occur
in the stator teeth 10.
25 In order to solve this problem it is proposed according to Figure 3 that a
spacer or a slot wedge 11 is inserted into the opening of the slot 8. The
wedge is made of a material which is electrically non-conducting and is
non-magnetic, rigid and strong, e.g., g~ fihre-reinforced plastic (epoxy
plastic), and extends across the entire axial length of the stator. This
30 wedge is inserted with radial force as indicated by the arrow 12 during
assembly, thus providing tangentially stiff connections between the stator
teeth at the air gap all round the stator. This stiff connection increases the
natural frequency and offers greatly increased rigidity in each individual
tooth, and even increased flexural rigidity in the whole stator core.
35 Another important advantage is that the tangential electromagnetic forces
at the air gap, deriving from the rotor poles, are distributed more
uniformly between the teeth.
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As can be seen in Figure 3, the wedge 11 does not abut the cables 1 with
radial force, the nearest cable being shown in the drawing. As is clear
from Figure 2, unlike in conventional generators, the slots 8 are designed
in a shape similar to a bicycle chain, with recesses for each cable 1 which is
S thus radially fixed. In previously known generators the cable slots were of
uniform width and the cables were pressed in with a radial force achieved
by a slot wedge that gave no tangential loading. These previously known
slot wedges thus had a completely different function from the slot wedge
11 according to the present invention, the only function of which is to
lO achieve tangential pre-stressing F~ANt which permits sufficiently rigid and
strong connection between the free ends of the stator teeth.
Figure 4 shows another embodiment of the device according to the
invention. Here the wedge 11 has inverted wedge shape, as also the
1~ wedge surfaces cooperating therewith on the stator teeth 10. Upon being
placed under pressure the wedge is in this case pressed out towards the
air gap, making use of the cable 1 radially fixed innermost in its seat. It is
thus possible to utilize a tube, known per se which, upon being
pressurized expands between the cable 1 and wedge 11, a tube 14 which is
20 filled with, e.g., liquid epoxy compound which hardens under pressure.
Such a tube has been used previously in conventional generators in order
to press the conductors forming the winding into the slot outwardly
towards the bottom of the slot, a function not at all demanded in the
present case.
A ~ird embodiment is shown in Figure 5. Here the tangential
compression between the stator teeth is achieved via the wedges 11 by a
tensile force F being applied to the stator core 15 through an external
arrangement in the form of tie-rods, cords 16 or the outer stator frame 17.
30 The stator consisting of segments is joined together at final assembly so
that when tensile force is applied to the outer arrangement, a counter
compressive force is obtained in the stator teeth and wedges at the air gap.
In Figures 3 and 4 the spacers 11 are wedge-shaped, as described.
35 However, they may also be parallel-epipedic, in which case the
tangentially stiff connection can be achieved in accordance with Figure 5.
. .
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W O 97/45937 PCT/SE97/00904
Adhesive joints may also be arranged between the spacers 11 and stator
teeth 10, either as the sole fixing means or prior to fixing by means of
tangential clamping.
5 Figures 6-9 illustrate how the slot wedges according to the invention can
also be utilized to achieve axial compressive pre-stressing of the stator
core 15. The pressure fingers 18 are arranged on each side of the core 15,
immediately opposite the stator teeth 10, to act as a force-transmission
device to convert the tensile force in the wedges 11 to a uniformly
10 distributed compressive force in the stator core 15. To achieve this the
ends of the wedges are joined together by means of transverse pieces 19
which are able to cooperate with the pressure fingers 18. The transverse
pieces 19 in the embodiment shown are joined to the wedges 11 by means
of pins 20, slidable in the transverse pieces 19, which are loaded
15 outwardly by means of a compression spring 21. One end of the pressure
fingers 18 engages below the transverse piece 19, enabling it to load the
transverse piece and thus the wedges in a direction outwards from the
laminated core 15. The o~er end of the pressure fingers 18 is clamped
between two devices 22 and 23 connected to the stator frame 17.
Instead of the transverse pieces 19, according to the right side of Figures 8
and 9 the tensile force in the wedge 11 can be converted to a compressive
force in the stator core 15 via nuts 24 cooperating with screw threading 25
on the edges of the wedges 11. The compressive force from the nuts 24 is
25 transmitted to the stator core 15 via plates 26 of, e.g., laminated glassfibre.
As shown in Figures 6-9 the slot wedges used for tangential positioning of
the stator teeth are also utilized advantageously as tie-rods to achieve the
requisite compressive stress in the stator core.
The invention is also applicable to other electric machines such as double-
fed machines, applications in asynchronous static current converter
cascades, outer pole machines and synchronous flux machines,
particularly if their windings are manufactured with insulated electric
35 conductors of the type described in the introduction, and ~lefeldbly in the
voltage range 36-800 kV.
