Note: Descriptions are shown in the official language in which they were submitted.
CA 02403666 2002-09-19
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SUPERCONDUCTIVE ARMATURE WINDING FOR AN
ELECTRICAL MACHINE
BACKGROUND OF THE INVENTION
This invention relates to tuxbomachinery and, more particularly, to stator
armature
windings for an electrical machine using superconductive materials.
In conventional generators, a significant portion of losses are attributed to
I2R losses
in the two main generator windings, namely the field winding on the rotor and
the
armature winding on the stator. The development of superconductor (SC)
technology,
in particular of high temperature superconductors (HTSC), has provided a
conductor
medium that, when implemented successfully, has the potential for
significantly
reducing, if not completely eliminating, the associated I2R losses in the main
generator windings.
The successful introduction of SC technology into generators hinges on the
solution
of issues of cooling the superconductors, providing adequate mechanical
support, and
shielding the superconducting wires from alternating magnetic fields to
minimize
parasitic eddy currents. The critical current density (Jc) below which the
superconducting materials retain their superconducting capability is strongly
reduced
if the superconductor is placed into a large magnetic field. Since the current
density
(Jc) decreases with increasing magnetic flux density, it becomes important to
shield
the superconductor effectively from magnetic fields.
hi conventional generators, the armature winding is located in stator slots,
and steel
teeth guide the main magnetic flux from the airgap to the stator yoke. In this
configuration, the armature conductors are not exposed to the main magnetic
flux but
only to the significantly smaller slot leakage flux. The magnetic torque acts
on the
stator teeth that transfer it to the core and the stator frame. The small slot
leakage
field causes eddy current losses in the conductors of the armature winding and
gives
rise to forces acting on the slot-embedded conductors, which are manageable
with
present slot-support methods.
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Several concepts for superconducting s-ynchrotious generators have been
proposed
and implemented to date, an example of which is disclosed in U.S. Patent No.
5,548,168, the contents of which are incorporated by reference herein.
During steady state operation, the rotor field winding of a synchronous
generator
carries DC current and is exposed only to the relatively low magnetic leakage
field.
Therefore, the field winding has been traditionally the first focus for
applying SC
technology to generators. The field winding is assembled from superconductors
to
eliminate excitation I2R losses and to provide a source for magnetic airgap
fields that
are, in all concepts fox SC generators presented to date, considerably higher
than in
conventional generators. The winding is cooled by liquid helium in the case of
low
temperature superconductors (LTSC) and liquid nitrogen in the case of a HTSC.
Time-varying fields during load imbalance or transients such as during load
shedding
are shielded from the SC rotor winding by an electrically conductive shield
around the
rotor.
Most existing concepts for superconducting generators proposed in literature
and
patents to date are based on a tooth-less stator core that consists of a steel
yoke or flux
shield and an "airgap" armature winding (see e.g., "'Panel Discussion on the
Impact of
Superconducting Technologies on Future Power Systems and Equipment -
Superconducting Generators" by D. Lambrecht, Study Committee 11, CIGRE, 1990
Session). With this configuration, the armature winding is located in the main
magnetic flux path and exposed to magnetic fields of full airgap flux density
levels of
2 Tesla or more. The large magnetic airgap is magnetized by the high ampere-
turn
capability of the superconducting field winding. In addition, the magnitudes
of airgap
flux density levels are further increased above the ones used in conventional
generators to achieve higher power densities and reduced overall generator
size.
The airgap armature winding is typically assembled from copper conductors that
are
supported by a nonmagnetic structure. These concepts have several inherent
problems. The armature winding is exposed to the full airgap flux densities
resulting
in large AC losses in the copper conductors. Since the armature is located in
the main
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airgap field, the full rated magnetic torque is acting directly on the
armature winding,
and radial forces are also significantly larger than in conventional
generators. This
requires that the nonmagnetic supporting structure of the armature winding be
designed for both rated torque and large radial forces. These problems that
are
associated with the High Power Density superconducting generator even when a
conventional copper armature winding is employed have been addressed by the
Low
Power Density concept in the noted U.S. patent. Taking the next step and
replacing
the conventional copper conductor with a superconducting wire in the airgap
armature
winding is complicated by the fact that superconductors are not yet capable of
carrying AC currents in strong magnetic fields without incurring high AC
losses,
leading to a loss in superconductivity. Therefore, following this paradigm of
an
airgap armature winding there has been limited success to date to use
superconductors
in the armature winding. '
In recent years, SC wires have been implemented in AC power cable prototypes
by
various cable manufacturers. In these cables, the electrical line-ground
insulation is
either at room temperature (warm dielectric) or cryogenic temperatures (cold
dielectric), and the conductor is assembled from HTSC wire. The conductors in
these
AC power cables are exposed only to the small self field, which is
sufficiently small
for today's superconducting materials.
Several concepts also exist for cable-wound generators in which the stator
winding is
assembled of low- or high- voltage cables with conventional copper conductors.
It is thus desirable to provide a superconducting armature winding that is
assembled
by placement into stator slots much like in conventional generators. The
stator teeth
serve to shield the SC winding from magnetic AC fields resulting in
minimization of
AC losses, forces and torques acting on the supeconducting wires. It would
also be
beneficial to manufacture the winding from continuous cables of
superconducting
wires or alternatively from multi-filamentary wires of aspect ratios close to
unity. It
may further be beneficial to employ magnetic wedges to further shield the SC
conductors from AC magnetic fields.
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BRIEF SUMMARY OF THE INVENTION
In an exemplary embodiment of the invention, a superconducting synchronous
generator includes a rotor and a stator. The stator comprises a plurality of
stator slots
and armature windings respectively disposed in the stator slots. The armature
windings are formed of superconductive cable. The superconductive cable may
comprise multi-filarnentary superconductive wire tape with an aspect ratio
greater
than one or alternatively with an aspect ratio of about one. In an alternative
arrangement, the superconductive cable comprises continuous cables of
superconducting wire.
The superconductive cable may include a substantially concentrically layered
construction including a cryo-refrigeration coolant passage, a superconductive
material and insulation. The insulation may be thermal insulation disposed
over
electrical insulation or the opposite with electrical insulation disposed over
thermal
insulation. The stator may further include stator teeth defining the stator
slots, where
the stator teeth shield the superconductive cable from a majority of magnetic
fields
generated during operation of the generator.
The armature windings of superconductive cable may be toroidal windings, and
the
stator may further include magnetic slot wedges respectively disposed in
openings of
the slots.
In another exemplary embodiment of the invention, an armature winding for an
electrical machine is formed of superconductive cable.
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BRIEF DESCRIPTION OF THE DRAWINGS
FIGURE 1 is a schematic illustration of a slot-embedded superconducting
armature
winding with a cold dielectric;
FIGURE 2 is a schematic illustration of a slot-embedded superconducting
armature
winding with a warm dielectric;
FIGURE 3 illustrates a toroidial slot-embedded SC armature winding for an
example
of a two-pole generator;
FIGURE 4 shows a slot-embedded superconducting cable made from
superconducting tape of large aspect ratio;
FIGURE 5 shows slot-embedded superconducting cable made from superconducting
tape of an aspect ratio close to unity; and
FIGURE 6 shows a magnetic slot wedge disposed in the opening of the stator
slot.
DETAILED DESCRIPTION OF THE INVENTION
Referring to FIGURES 1 and 2, an SC armature winding 10 is shown disposed in a
conventional stator slot 12. The SC armature winding I O is formed in a
substantially
concentric layered construction including a cryo-refrigeration coolant passage
14 for
receiving coolant, a superconductor 16 and insulation 18, 20. The conductors
16 of
the SC stator winding are placed in the stator slots 12 similar to the winding
arrangement of conventional generators. The main magnetic flux is guided
through a
toothed stator core 22 that shields the SC wire from large AC flux densities.
Since the
main magnetic field is guided through the laminated core structure, the
magnetic
forces, torques, and additional AC losses are limited to values that are due
to only the
slot leakage field, but not the main magnetic field. Therefore, the forces and
torques
acting on the SC wires are comparable to the ones in conventional machines,
and the
SC conductors can be supported by conventional structures. With this
structure, SC
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wire is exposed to an AC field that is limited to the slot leakage field.
Moreover,
since the AC field is small, the critical current density of the
superconducting wire has
to be reduced only modestly. Still fiuther, the AC losses induced in the SC
wire by
the slot leakage field are minimal, and the full rated torque is transmitted
to the
magnetic yoke not by the armature winding, but rather by the magnetic teeth
for better
reliability.
The superconductor may be arranged in several different configurations in the
slot,
either with a cold dielectric (thermal insulation 20 around the electrical
insulation 18,
as shown in FIGURE 1), or a warm dielectric (electrical insulation 18 around
the
thermal insulation 20, as shown in FIGURE 2), and the conductors may have
either
rectangular or round or other shaped configurations.
It is preferable that the SC wires within a turn or coil will be arranged in
accordance
with any of the well-known techniques that reduce or eliminate circulating
currents
among the wires. One such technique uses the "Roebel" arrangement, for which
long-
standing patents by Ringland (Allis Chalmers) and Willyoung (General Electric)
are
typical. In the specific application to SC cables, it is preferable that the
Wires will be
wound in a spiral fashion to accomplish the cancellation of circulating
currents
The SC slot-embedded conductors may be connected in any of the typical
connection
schemes, such as individual bars or single- or multi-turn coils connected into
a
toroidal winding or a single- and multi-layer winding that is assembled from
equal or
concentric coils connected in a wave- or lap-winding pattern. The concept of
SC slot
embedded conductors also applies to salient pole stator windings and helical
armature
windings.
With reference to FIGURE 3, a toroidal winding 23 consists of turns that
extend
around the yoke 24 of the stator core 22. The drawback of a toroidal winding
in
conventional generators is that they need approximately twice the coil length
per
induced voltage, resulting in twice the I2R losses of coils. This drawback of
twice the
I2R losses is eliminated by the use of SC wires as shown in FIGURE 3, and the
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advantage is compactness of the toroid~al winning placed in a few
slots/pole/phase:
The slot-content of such a winding may implement the concepts of FIGURES 1 and
2.
The cryogenic cooling paths 14 for the superconductor may be implemented in
several configurations. In one configuration, each circuit of the armature
winding
forms a continuous cryogenic loop. In this case, each cryogenic circuit
contains the
same conductors as each electrical circuit. In another configuration, the
cryogenic
and electrical circuits may consist of different connection schemes. The
cryogenic
circuit may consist of a parallel connection of either individual coils or
bars or groups
of these.
The superconducting armature winding is assembled from a continuous
superconducting cable. The cable is assembled from layers of superconducting
wire
surrounded by a continuously extruded insulation system. In this concept, the
superconducting wire extends continuously between the two terminals of each
phase,
or sections thereof This approach minimizes the splices of superconducting
wire that
are required compared to a winding assembled from individual bars or coils. In
different embodiments of the superconducting cable winding, the coolant may
circulate either as an integral component of the continuous cable or around
the
extruded cable as part of the slot-containment of the cable. In the latter
case, one or
several cables can be immersed in the same coolant circuit within a slot.
The concept of a SC cable winding applies to all winding configurations and
connection schemes, including single- and multi-layer windings, wave- and lap-
windings, toroidal windings, salient-pole windings, helical windings.
Present prototypes of SC cables are built from multifilamentary SC wire tape
with a
high aspect ratio, i.e., a tape width that is several times the tape
thickness. The
conductor section 16 is preferably wound from such SC tape as indicated in
FIGURE
4, where the individual tapes are twisted in the axial direction of the cable.
The
resulting self field of the cable in air is indicated by the arrow A, and the
self field of
slot-embedded cable is indicated by the arrow B. In this configuration, the
magnetic
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leakage field A extends in peripheral direction of the cable and intersects
the SC tape
only over its thickness. Thereby, parasitic eddy currents are minimized.
In the slot-embedded armature winding, the magnetic leakage field B is
perpendicular
to the axis of the cable. If a conventional cable of FIGURE 4 is used, the
magnetic
leakage field would be perpendicular to the width of the SC wire tapes in a
large
section of the SC region. This would result in excessive eddy current losses
that are
proportional to the square of the tape dimension that is perpendicular to the
magnetic
field. To minimize these eddy currents in a slot-embedded cable, a new
configuration
of wire tape is proposed, wherein the mufti-filamentary SC wire has a cross
section
with an aspect ratio close to unity, such as wire strands of square or round
cross
sections as shown in FIGURE 5.
When extending SC technology to AC applications, it is important to shield the
SC
wires from exposure to AC magnetic fields. In conventional machines, magnetic
slot
wedges are employed to reduce the stator slotting permeances. In the SC
generator
construction of the invention, referring to FIGURE 6, magnetic slot wedges 26
are
disposed in openings of the slots 12. The magnetic wedges 26 serve the
additional
purpose of shielding the SC wire embedded in the stator slot from airgap field
harmonics due to rotor MMF and permeance harmonics. Anisotropic wedges may be
employed to reduce the slot leakage field passing through the wedge by
aligning the
magnetic preferential direction of the anisotropic wedge with the radial
direction of
the slot. Magnetic flux lines are shown in FIGURE 6.
With the structure of the present invention, a superconducting stator armature
winding
can be assembled into conventional stator slots. The stator teeth serve to
shield the
SC winding from magnetic AC fields, resulting in minimization of AC losses,
forces
and torques acting on the superconducting wires. The winding is manufactured
from
continuous cables of superconducting wires or alternatively from mufti-
filamentary
wires of aspect ratios close to unity. Magnetic wedges further shield the SC
conductors from AC magnetic fields.
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While the invention has been described in cormection with what is presently
considered to be the most practical and preferred embodiments, it is to be
understood
that the invention is not to be limited to the disclosed embodiments, but on
the
contrary, is intended to cover various modifications and equivalent
arrangements
included within the spirit and scope of the appended claims. For example, the
invention is applicable to various types of electrical machines beyond the
synchronous type, including, but not limited to, DC motors and generators and
induction motors, etc.
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