Note: Descriptions are shown in the official language in which they were submitted.
CA 02261638 1998-11-20
W O 97/45915 PCT/SE97100894
I
ROTARY ELECTRIC MACHINE WITH RADIAL COOLING
TECHNICAL FIELD:
The present invention relates to rotating electric
machines such as synchronous machines, but also double-
fed machines, applications in asynchronous static
current converter cascades, outer pole machines and
synchronous flux machines, as well as alternating
current machines intended primarily as generators in a
power station for generating electric power. The
invention relates particularly to the stator of such
machines and to an embodiment for cooling the stator
teeth and thus indirectly also to the insulated
electric conductors constituting the stator winding.
BACKGROUN~ ART:
Similar 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. In the case of
generators, this usually 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
- in the range of approximately 130-400 kV. The present
invention is intended for use with high voltages, by
which implies in the first place voltages exceeding
10 kV. A typical working range for a device according
to the invention may be 36-800 kV.
By using high-voltage insulated electric conductors, in
the following termed cables, in the stator winding,
with solid insulation similar to 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 conventional transformer
can thus be eliminated. The concept generally requires
that the slots in which the cables are placed in the
CA 02261638 1998-11-20
WO97/45915 PCT/SE97/00894
stator be deeper than regards conventional technology
(thicker insulation due to higher voltage and more
turns in the winding). The loss distribution will
therefore differ to that of a conventional machine,
which in turn entails new problems with regard to
cooling the stator teeth, for instance.
Two different air-cooled systems exist in conventional
cooling: radial cooling where the air passes the rotor
through the hub and radial ducts in the rotor, and
axial cooling where the air is blown into the air gap
with the aid of axial fans.
~he stator is then divided into radial air ducts formed
by (usually straight) spacers which are welded in
place. Due to the poor thermal conductivity axially
through the stator laminations, the air ducts must be
frequently repeated. The drawback with air cooling is
that the ventilation losses are considerable and that
the stator must be longer due to the ventilation ducts.
The ventilation ducts may also result in a weak
mechanical structure, especially in said high-voltage
generators with long teeth.
2~ Water-cooled systems, e.g., instead of air-cooled
systems for high-voltage generators have the advantage
that the radial ventilation ducts can be eliminated,
resulting in shorter machines while at the same time
increasing the efficiency. Water-cooled systems for
stators in large alternating current machines are often
based on hollow winding parts i.e. the electric
conductors are hollow with longitudinal ducts for the
coolant, in certain cases combined with cooling tubes
inserted axially in the stator yoke. Constructions are
known in which the stator yoke is cooled using
aluminium blocks inserted at regular intervals along
the axial extension of the stator. However, there is no
example of direct cooling of the stator teeth with such
CA 02261638 1998-11-20
WO97/45915 PCT/S~97/00894
cooling clamps since these are cooled indirectly
through the water-cooling of the stator winding.
It is considered that coils for rotating generators can
be manufactured with good results within a voltage
range of 3 - 25 kV.
Attempts to develop generators for higher voltages,
however, have been in progress for a long time. This is
obvious, e.g., from ~Electrical World", October 15,
1932, pages 524-525. This paper describes how a
generator designed by Parson 1929 was arranged for
33 kV. It also describes a generator in Langerbrugge,
Belgium, which produced a voltage of 36 kV. Although
the article also speculates on the possibility of
lS increasing voltage levels still further, the
development was curtailed by the concepts upon which
these generators were based. This was primarily because
of the shortcomings of the insulation system where
varnish-impregnated layers of mica oil and paper were
used in several separate layers.
With reference to a report from the Electric Power
Research Institute, EPRI, EL-3391 from April 1984, an
account is given of generator concepts for achieving
higher voltage in an electric generator with the object
of connecting such a generator to a power network
without intermediate transformers. Such a solution is
assessed in the report as offering good gains in
efficiency and considerable financial advantages. The
main reason that it was deemed possible in 1984 to
start developing generators for direct connection to
power networks was that a superconducting rotor had
been developed at the time. The considerable excitation
capacity of the superconducting field winding allows
the use of airgap-winding with sufficient thickness to
withstand the electrical stresses.
CA 02261638 1998-11-20
W O 97/45915 PCT/SE97/00894
By combining the concept deemed most promising
according to the project, that of designing a magnetic
circuit with winding, known as "monolithe cylinder
armature", a concept in which two cylinders of
conductors are enclosed in three cylinders of
insulation and the whole structure is attached to an
iron core without teeth, it was assessed that a
rotating electric machine for high voltage could be
directly connected to a power network. The solution
entailed the main insulation having to be made
sufficiently thick to withstand network-to-network and
network-to-earth potentials. Obvious drawbacks
regarding the proposed solution, besides its demanding
a superconducting rotor, are that it also requires
extremely thick insulation, which increases the machine
size. The end windings must be insulated and cooled
with oil or freones in order to control the large
electric fields at the ends. The whole machine must be
hermetically sealed in order to prevent the liquid
dielectric medium from absorbing moisture from the
atmosphere.
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. 6-8, in US 4,429,244 "Stator of Generator~
and in Russian patent document CCCP Patent 955369.
The water- and oil-cooled synchronous machine described
in J. Elektrotechnika is intended for voltages up to 20
kV. The article describes a new insu~ation 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
.. . . ....... .... ...... ...
CA 02261638 1998-11-20
W O 9714~915 PCT/SE97tOO894
oil-separating ring is provided at the internal surface
of the core. The stator winding is made from conductors
with an oval hollow shape provided with oil and paper
insulation. The coil sides with their insulation are
S 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 both oil and electricity
}0 at the end windings. 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 machine which comprises a
magnetic core of laminated sheet with trapezoidal slots
for the stator winding. The slots are tapered since the
need for insulation of the stator winding is less
towards the interior of the rotor where the part of the
winding which is located nearest the neutral point is
disposed. In addition, the stator part comprises a
dielectric 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 coil layer. The
layers are separated from each other 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 cooled by surrounding oil. The disadvantages with
such a large ~uantity 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 s1eeve which are located outside the
-
CA 02261638 1998-11-20
W O 97/4591~ PCT/SE97/00894
slots have a cylindrical part and a conical termination
reinforced with current-carrying layers, the purpose 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 a
conventional high-voltage cable with the same dimension
for all the layers. The cable is placed in stator slots
formed as circular, radially located 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 tubes. Insulating
spacers fix the tubes in the stator slot. Because of
the oil cooling, an internal dielectric ring is also
needed for sealing the coolant against the internal air
gap. The design shown has 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 flux which
significantly influences the magnetization requirement
of the machine.
DE 2917717 shows a cooling segment for cooling medium
in an electric machine. The segment comprises internal
cooling ducts disposed in the segment.
US 3,447,002 shows a stator core provided with a
plurality of annular grooves, in which heat conducting
bodies are located, arranged tangentially one after the
other in each groove with cooling tubes embedded in the
cooling bodies.
3~
US 2,217,430 shows a dynamo electric machine with means
for cooling the stator for such a machine by the
circulation of water through the stator core.
CA 02261638 1998-11-20
rCT/SE97100894
WO97/45915
According to the present invention direct cooling of
the teeth is a necessity and the stator winding is
therefore cooled indirectly. The teeth are also
exceptionally long in comparison with conventional
generators and this also necessitates direct cooling of
the teeth.
OBJECT OF THE I ~ ENTION:
The object of the present invention is to provide an
arrangement of the type described in the introduction
which will permit direct cooling of the stator teeth
while cooling the cables constituting the stator
winding indirectly. Advantageous further developments
lS of the invention are indicated in the following
description.
SU~ARY OF THE I~r~ENTION:
The present invention relates to an arrangement for
cooling the stator teeth, and indirectly the stator
winding, in a high-voltage electric machine such as a
high-voltage alternating curre~t generator.
The arrangement comprises radial~y-running tubes,
electrically insulated, and placed in loops through the
stator teeth at a certain axial distance from adjacent
loops. The arrangement also comprises radially
extending cooling clamps containing cooling tubes in
which coolant circulates. The cooling clamps are
inserted in the stator at approximately the same axial
distance as conventional air-ventilation ducts. The
tubes run along the entire radial length of the stator
teeth.
According to a particularly preferred embodiment of the
invention, at least one of the semiconducting layers,
preferably both, have the same coefficient of thermal
expansion as the solid insulation. The decisive benefit
CA 02261638 1998-11-20
W O 97145915 PCT/SE97100894
is thus achieved that defects, cracks or the like are
avoided at thermal movement in the winding.
BRIEF DESCRIPTION OF THE DRAWINGS:
The invention will be described in more detail with
reference to the accompanying drawings.
Figure lshows schematically a perspective view of a
section taken diametrically through the stator
of a rotating electrical machine.
Figure 2shows a cross sectional view of a high-voltage
cable according to the present invention,
Figure 3shows schematically a sector of a rotating
electric machine,
Figure 4shows a first embodiment according to the
present invention,
Figure 5shows schematically a second embodiment
according to the present invention
Figure 6a-6d show sections through one of each of four
embodiments of cooling-tube teeth according to
the invention,
Figure 7shows a coo~ing circuit according to the
present invention.
DESCRIP~ION OF THE INrVENTION:
Figure l shows part of an electric machine in which the
rotor has been removed to show more clearly the
arrangement of a stator l. The main parts of the stator
l constitute a stator frame 2, a stator core 3
comprising stator teeth 4 and an outer yoke portion
defining a stator yoke. The stator also comprises a
stator winding 6 composed of high-voltage cable
situated in a space 7 shaped like a bicycle chain, see
Figure 3, formed between each individual stator tooth
4. In Figure 3 the stator winding 6 is only indicated
by its electric conductors. As can be seen in Figure l,
the stator winding 6 forms a end-winding package ~ on
both sides of the stator l. It is also clear from
CA 02261638 1998-11-20
PCT/SE97100894
W O 97/45915
Figure 3 that the insulation of the high-voltage cables
has several dimensions, arranged in groups depending on
the radial position of the cables in the stator 1.
S In larger conventional machines the stator frame 2
often consists of a welded sheet steel construction. In
large machines the stator core 3, is generally formed
of 0.35 mm sheet of electrical steel divided into
stacks with an axial length of approximately 50 mm,
separated from each other by 5 mm ventilation ducts
forming partitions. In a machine according to the
present invention, however, the ventilation ducts are
eliminated. In large machines each stack of laminations
is formed by fitting punched segments 9 of suitable
size together to form a first layer, after which each
subsequent layer is placed at right angles to produce a
complete plate-shaped part of a stator core 3. The
parts and the partitions are held together by pressure
legs 10 pressing against pressure rings, fingers or
segments, not shown. Only two pressure legs are shown
in Figure 1.
Figure 2 shows a cross-sectional view of a high-voltage
cable 11 according to the invention. The high-voltage
cable 11 comprises a number of strands 12 o~ copper
(Cu), for instance, having circular cross section.
These strands 12 are arranged in the middle of the
high-voltage cable 11. Around the strands 12 is a first
semiconducting layer 13, and around the first
semiconducting layer 13 is an insulating layer 14, e.g.
crosslinked polyethylene (XLPE) insulation. Around the
~ insulating layer 14 is a second semiconducting layer
15. Thus, the concept "high-voltage cable" in the
present application does not include the outer
protective sheath that normally surrounds such cables
for power distribution.
CA 02261638 1998-11-20 PC~/S. 9 7 / C C 8 9 4
Tho Sw-dish P~ten~ C~c
PCrl~ttomo~ na!Appl;:~X3n 1997-12- ~ 9
Figure 3 shows schematically a radial sector of a
machine with a segment 9 of the stator 1 and with a
rotor pole 16 on the rotor 17 of the machine. As can be
seen the stator winding 6 is arranged in the space 7 in
the shape of a bicycle chain, formed between each
stator tooth 4. Each stator tooth 4 extends radially
inwards from the outer yoke portion 5.
Figure 4 shows a simplified view of a first embodiment
of the invention with a cooling tube 18 forming a
cooling tube loop in a cooling clamp 19 having
substantially the same shape as the segment 9, with
tooth parts 20 between which the characteristic slot
resembling a bicycle chain is formed. Figures 4 and 5
lS have been simplified to rectangular shape in order to
simply illustrate the principles of the embodiments.
According to Figure 3 a cooling tube loop 21 is formed
by one end of the cooling tube 18 being connected to an
inlet loop 22 and its other end being connected to an
outlet loop 23.
A coolant thus flows in the cooling tube 1~ from the
inlet loop 22 at the outer side 24 of the cooling
clamp, into the cooling clamp 19, and into a cooling
clamp tooth 25 towards its tip, whence the cooling tube
18 passes from tooth to tooth in a space 26 formed
between the air gap and an uppermost high-voltage cable
27. This space is taken up by a slot wedge, not shown,
which can be visualized cut at the transition of the
tube, allowing passage for said tube. This slot wedge
can also be divided into about thirty small wedges to
allow place for the tube bends. An advantageous
embodiment of a cooling clamp in the present invention
may be a tube cross section formed by bending the tube
to a rectangular shape which is then formed into loops,
the cooling tube loops being permanently cast in
aluminium on a lid.
AMENDED SHEET
CA 02261638 1998-11-20 PCr/S~ 97/00~94
1997 -12- l 9
11
Figure 5 shows a simplified view of a second embodiment
of the invention with a cooling tube 18 forming a
cooling tube loop in a cooling clamp 19 of
substantially the same design as the segment 9 with
S tooth parts 20 between which the characteristic slot
resembling a bicycle chain is formed. According to
Figure 5 a cooling tube loop 21 is formed in this
embodiment by one end of the cooling tube 18 being
connected to an inlet loop 22 and its other end being
connected to an outlet loop 23.
A coolant thus flows in the cooling tube 18 from the
inlet loop 22 at the outer side 24 of the cooling
clamp, into the cooling clamp 19, and into a cooling
clamp tooth 25 towards its tip, whence the cooling tube
turns at the tip, see the arrow, and extends back
outwardly in the same cooling clamp tooth to once again
form a similar tooth loop in the next tooth.
In this embodiment also a cooling clamp may be produced
with rectangular cross section of the tube by bending.
The tube is then formed into loops, the cooling tube
loops being permanently cast in aluminium on a lid or
attached to an intermediate steel beam with casting
compound. XLPE-tubes with intermediate steel beams can
also be embedded and formed suitably as cooling clamps
consisting of stator profiles separated by steel
spaces, partially filled with filler compound, e.g.
cured plastic.
The advantage of the embodiment according to Figure 4
is that the tube acquires a larger bending radius than
if it was to return to the same tooth as in Figure 5.
As indicated in Figures 4 and 5, adjacent cooling
clamps are shown by dotted arrows as connected in
parallel to inlet and outlet loops.
AME~ u ~n~tr
, CA 02261638 1998-11-20
PCI/5.97 /Ov~4
1997-12- 19
12
Figures 6a-6d illustrate different embodiments
according to Figures 4-5, in sections through the
cooling clamp teeth. Figure 6a shows a section through
a cooling clamp tooth according to Figure 4 showing an
S advantageous embodiment in which the cooling tube 18 of
steel has been bent to substantially rectangular cross
section, and the cooling tube has subsequently been
embedded in an aluminium block 28 provided with a cover
plate 29. The aluminium block may also be manufactured
in two halves which are fitted together around the
cooling tube. Figure 6b shows a section through a
cooling clamp tooth according to Figure 4 the cooling
tube 18 of the tooth running between two beams 30,
preferably of steel, acting as spacing and reinforcing
beams during assembly of the cooling tube. After
assembly of the cooling tube with the steel beams,
spaces are formed which are then filled with a casting
compound 31. Figure 6c shows a section through a
cooling clamp tooth according to Figure 5. In this
embodiment the cooling clamp has been produced by a
flexible hollow spacer around a beam 30, preferably of
steel, being placed in a loop and the intermediate
space filled with a casting compound 31, after which
the hollow spacer is removed thus forming a tubular
duct 32. Figure 6d also shows a section through a
cooling clamp tooth according to Figure 5 in which a
flexible cooling tube 33 of XLPE-tube type, is placed
in a cooling tube loop around a beam 30, preferably of
steel.
The embodiments of cooling media ducts in a cooling
clamp tooth shown here can be varied in many ways
within the scope of the appended claims. A cast
aluminium block, for instance, can be made in two
pieces with ducts for insertion of cooling tubes of
steel or of XLPE-tube type. The cross-section of the
cooling tube may vary from circular to oval or be
substantially rectangular.
AMENDED SHEET
