SYSTEM AND METHOD FOR QUENCH PROTECTION OF A
SUPERCONDUCTOR·
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH &
DEVELOPMENT
This invention was made with Government support under contract number DE-FC3602GOIIIOO
awarded by U.S.Dept.of Energy. The Govenunent has certain rights in
the invention.
BACKGROUND
The present invention relates generally to a superconductor, and in particular to a
system and method for quench protection ofa superconductor.
A superconductor is an element, inter-metallic alloy, or compound that will conduct
electricity without resistance when cooled below a critical temperature.
Superconductivity occurs in a wide variety of materials, including elements such as
tin and aluminum, various metallic alloys, some heavily doped semiconductors, and
certain ceramic compounds. In conventional superconductors, ~uperconductivity is
caused by a force of attraction between certain conduction electrons arising from the
exchange of phonons, which causes the fluid of conduction electrons to exhibit a
super fluid phase composed ofcorrelated pairs of electrons.
Superconductors are useful in a variety of applications including magnetic resonance
imaging systems and power generation systems, such as motors and generators. The
loss of electrical resistance in the superconductor enables these devices to be operated
with a much greater efficiency. However, a portion of the superconductor undergoes
a transition from the superconducting state to a normal resistive state when the current
in the superconductor is driven beyond a critical current limit. This causes the
temperature ofthe superconductor to rise due to heat produced by the resistive beating
occurring in the superconductor. If this resistive heating loss continues, the
superconductor may enter a state of irreversible thermal runaway, known as a quench.
Damage may be caused to the superconductor due to the thermal runaway. For
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example, in a superconductive rotor coil, an excessive temperature gradient may be
generated as a result of resistive heating. The temperature gradient may cause
differential thennal expansion across the superconductive coil that damages the
superconductive coil. The superconductor and/or insulation may also be damaged by
excessive temperature reached during a quench.
Accordingly, a technique that enables a quench condition of the superconductor to be
detected is desirable. In addition, a technique that enables the superconductor to be
protected from damage that may be caused by quenching is also desirable.
BRIEF DESCRIPTION
In accordance with one" aspect of the present technique, a rotating electric machine
having a superconductive rotor coil is provided. The rotating electrical machine also
comprises a quench protection system. T~e quench protection system is operable to
protect the superconductive rotor coil from damage due a quench condition. The
quench protection system may comprise a voltage detector that is operable to produce
a signal representative of superconductive rotor coil voltage. The voltage detector is
communicatively coupled to a circuit that is operable to cancel electrical noise from
the signal representative of superconductive rotor coil voltage. The resulting signal is
representative of a rise in superconductor rotor voltage over time. The system may
comprise a control circuit that is also communicatively coupled to the circuit. The
control circuit is configured to receive the signal representative of a rise in
superconductor rotor voltage over time. The control circuit also is operable to initiate
a corrective action to protcct the superconductive rotor coil from the quench condition
when the signal representative ofa rise in superconductor rotor voltage over time rises
to a defined voltage.
In accordance with another aspect of the present technique, a method of detecting a
quench condition in a superconductor is provided. The method may comprise
detecting voltage across the superconductor and removing electrical noise from a
signal representative of voltage across the superconductor due to electrical resistance
in the superconductor. The method may also comprise comparing the signal
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representative of voltage across the superconductor due to electrical resistance in the
superconductor to a reference voltage representative of voltage across the
superconductor due to a defined electrical resistance in the superconductor.
DRAWINGS
These and other features, aspects, and advantages of the present invention will
become better understood when the following detailed description is read with
reference to the accompanying drawings in which like characters represent like parts
throughout the drawings, wherein:
FIG. 1 is a schematic view of a generator having a superconductive rotor coil and a
quench protection system for protecting the superconductive rotor coil from damage
due to quenching in the superconductive rotor coil, in accordance with an exemplary
embodiment ofthe present technique;
FIG. 2 is a flow chart illustrating a method of quench protection for the
superconductive rotor coil ofFIG. 1;
FIG 3 is a gmph of superconductive rotor coil voltage versus time, in accordance with
an exemplary embodiment ofthe present technique;
FIG. 4 is a gmph of superconductive rotor coil voltage filtered to block
superconductive rotor coil voltage signals that are not representative of a change in
coil voltage caused by quenching; and
FIG. 5 is a detailed view of a quench protection system for the superconductive rotor
coil in accordance with another aspect of the present technique.
DETAILED DESCRIPTION
Referring now to FIG 1, a rotating electrical machine is illustrated, and represented
generally by reference numeral 10. In this embodiment, the rotating electrical
machine lOis a genemtor. However, the techniques described below are applicable to
motors, as well as generators. In addition, the techniques may be used in other
systems that utilize superconductors, such as medical imaging systems. In this
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embodiment, the generator 10 has a high temperature superconductive rotor coil 12
that is disposed on a rotor 14. The superconductive rotor coil 12 produces a magnetic
field from electricity that it receives from an exciter circuit 16. Examples of exciter
circuits may include DC exciter circuit. AC exciter circuit, static exciter circuit or the
like. The exciter circuit 16 is explained in more detail below. When the electrical
. current flowing through the superconductive rotor coil 12 exceeds the critical current,
a portion ofthe superconductive rotor coil 12 loses its superconductivity and a quench
condition in the superconductive rotor coil 12 may result.
A quench protection system 18 is provided to protect the superconductive rotor coil
12 from damage due to a quench condition. The quench protection system 18 is
operable to detect an increase in the resistance of the superconductive rotor coil 12 as
a result of resistive heating before a quench condition exists in the rotor coil 12. This
enables the system 18 to act to prevent damage to the rotor coil 12 before the quench
condition exists. The quench condition may be detected based on an increase in
voltage across the coil 12 caused by the increase in electrical resistance ofthe portions
of the coil 12 experiencing resistive heating. As the resistive heating propagates
through the superconductive rotor coil 12, the volume of the coil 12 contributing to
the increase in resistance of tbe coil 12 increases, increasing the voltage across the
coil 12. However, voltages also are induced in the superconductive rotor coil 12
during normal operation. For example, voltage changes may be induced in the rotor
coil 12 by external magnetic fields and by changes in the load on the generator. It is
therefore difficult to distinguish voltage increase due to resistive heating or a quench
from the relatively large voltages induced in the coil 12. However, the present system
is operable to determine whether the increase in voltage across the rotor coil is due to
nonnal operation or a quench condition.
In the illustrated embodiment, the quench protection system 18 comprises a voltage
detector 20, a frequency filter 22, a telemetry system 24, a control circuit 26, and a
dump circuit 28. A disadvantage of prior techniques is that it was difficult to detect
the superconductive rotor coil voltage accurately across the rotating superconducting
coil because the quench detection circuit was typically located on a stationary portion
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of the generator away· from the rotor. However, in this embodiment, the voltage
detector 20 is disposed on the rotor 14 and is coupled directly across the coil 12.
The voltage detector 20 is operable to transmit a signal representative of the voltage
across the rotor coil 12 to the control circuit 26 via the frequency filter 22. In this
embodiment, the frequency filter 22 is a low pass filter. The low pass filter is selected
to block voltage signals that are not representative ofan increase in voltage across the
rotor coil 12 caused by a quench condition. Upon receipt of the signal from the
frequency filter 22, the control circuit 26 detennines if a quench condition exists in
the rotor coil 12. If a quench condition exists, the control circuit may remove or
reduce power to the rotor coil 12, or direct the quench protection system 18 to take
other actions to protect the superconductive rotor coil 12 from .any damage that may
be caused by the quench condition. In addition, the control circuit 26 is operable to
direct the dump circuit 28 to discharge the magnetic energy stored in the coil 12 when
a quench condition is detected.
In this embodiment, the voltage detector 20 is disposed on the rotor 14. Furthermore,
in accordance with the embodiment illustrated in FIG 1, the frequency filter 22, the
control circuit 26, and the dump circuit 28 are displaced away from the rotor 14.
However, one or more of these devices may be disposed on the rotor 14. An
electromagnetic induction shield 30 is disposed around the superconductive rotor coil
12. The voltage detector 20 has conductive leads 32 and 34 that are routed tlrrough
the electromagnetic induction shield 30 to the superconductive rotor coil 12.
The illustrated embodiment utilizes the telemetry system 24 to transmit the signal
representative of superconductive rotor coil voltage from the voltage detector 20 to
the portions of the quench protection system 18 located externally of the rotor 14,
such as the control circuit 26. The frequency filter 22 receives the signal
representative of voltage across the superconductive rotor coil 12 ii'om tIle voltage
detector 20 and blocks signals that are not related to the detection of a quench
condition in the superconductive rotor coil 12, such as electrical noise and voltages
induced in the rotor coil 12 by external magnetic fields. For example, the :frequency
filter 22 may block signals having a frequency outside an expected frequency range
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for signals representative of an increase in voltage due to an increase in resistive
heating in the coil 12. Conversely, the filter 22 enables signals having a frequency
within the expected frequency range to pass to the control circuit 26. The control
circuit 26 receives the output of the frequency filter 22 and uses the output to
establishes whether or not resistive heating or a quench condition is occurring in the
superconductive rotor coil 12.
As noted above, the superconductive rotor coil 12 receives electricity from the exciter
circuit 16. The exciter circuit 16 comprises an exciter 36, a switch 38, and slip rings
40 and 42. The exciter 36 produces an electric current. The slip rings 40 and 42
couple the electrical current from the exciter 36, which is stationary, to the rotor 14,
which is rotating. The switch 38 is controlled by the control circuit 26. The switch
38 may be opened by the control circuit 26 to prevent current from flowing from the
exciter to the 'superconductive rotor coil 12. The exciter 36 may also be used to
provide power to the voltage detector 20, the frequency filter 22, the control circuit
26, and the dump circuit 28.
The dump circuit 28 comprises a dump resistor 44 and a switch 46. The switch 46 is
controlled by the control circuit 26. As discussed above, the control circuit 26 opens
the exciter circuit switch 38 when a rise in voltage due to a quench condition is
detected across the rotor coil 12. In addition, the control circuit 26 closes the dump
circuit switch 46. When the dump circuit switch 46 is closed, a' circuit is completed
between the superconductive rotor coil 12 and the dump resistor 44. This provides a
path for the superconductive rotor coil 12 to discharge the magnetic energy stored in
the coil 12 through the dump resistor 44. The magnetic energy stored in the coil 12 is
converted into electricity and discharged through the dump resistor 44. Thus, in this
embodiment, the quench protection system 18 is operable to remove the supply of
current to the coil 12 from the exciter 36 and to enable the energy stored in the coi112
to be discharged. This prevents further resistive heating of the superconductive rotor
coil 12.
The voltage across the superconductive rotor coil 12 may be influenced by external
transient faults such as a grid transmission breaker trip or a lightning strike. This may
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lead the quench protection system 18 to believe that a quench condition exists, when
in actuality it does not. In the illustrated embodiment, an external control system 48
is provided to detect external transient faults. When an external transient fault is
detected, the external control system 48 may transmit a signal to the control circuit 26
to infonn the control circuit 26 of the transient fault. The control circuit 26 may then
disregard, or compensate for, the signal from the voltage detector 20 until the
transient fault h~s passed to prevent an inadvertent loss ofoperation.
Referring generally to FIG 2, a method of utilizing the quench protection system 18 to
protect the superconductive rotor coil 12 from a quench condition is illustrated, and
represented generally by reference numeral 50. The method 50 comprises providing a
signal representative of voltage across the superconductive rotor coil 12, as
represented by block 52. The method also comprises utilizing the frequency filter 22
to filter the signal to remove voltage signals that are not related to establishing
whether a quench condition exists in the superconductive rotor coil 12, as represented
by block 54. For example, the frequency filter 22 may be used to block higher
frequencies representative of electrical noise from passing through the filter 22, while
enabling signals having lower frequencies that are representative of an increase in
voltage across the coil 12 due to resistive heating to pass.
The method also comprises providing a reference voltage (V0) and comparing the
filtered superconductive rotor coil voltage signal (Vr) to the reference voltage (Vo), as
represented by block 56. The reference voltage (Vo) is a threshold voltage that
represents a voltage increase across the superconductive rotor coil 12 due to a defined
amount of resistive heating in the coil 12. The reference voltage (Vo) is utilized to
prompt action by the control circuit 26 of quench protection system 18. The control
circuit 26 may be used to provide the reference voltage (Vo) and to compare the
filtered superconductive rotor coil voltage signal (Vr) to the reference voltage {Vol.
The value of the reference voltage (Va) may be adjusted to produce the desired
response. Several factors may be considered in establishing the value of the reference
voltage. For example, a low reference voltage may result in spurious indications of a
quench condition, causing the quench protection system to operate inadvertently.
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Conversely, a high reference voltage may result in damage to the coil 12 from
resistive heating before the quench protection system 18 can act to protect the coil 12.
The method also comprises determining whether the filtered superconductive rotor
coil voltage signal (Vr) is greater than the reference voltage (Vo) or not, as
represented by block 58. If the filtered superconductive rotor coil voltage (Vr) does
not exceed the reference voltage (Vo), the operation of the system continues as
normal. However, if the filtered superconductive rotor coil voltage (Vr) exceeds the
reference voltage (Vo), the control circuit 26 directs the exciter circuit 16 to remove
power to the superconductive rotor coil 12, as represented by block 60. In addition,
once the exciter circuit 16 is open, the control circuit 26 activates the dump circuit 28
to discharge the magnetic field in the superconductive rotor coil ~2, as represented by
block 62. With no current flowing through the coil 12, no resistive heating can occur
in the coil 12. Thus, the superconductive coil 12 is protected from the harmful effects
of quenching.
Referring generally to FIG 3 and 4, an example ofthe operation ofthe fi.-equency filter
in· filtering the signal representative of the superconductive rotor coil voltage is
illustrated. FIG 3 illustrates a graph of the superconductive rotor coil voltage (V)
over time (T) prior to filtering, represented generally by reference numeral 64. The
illustrated example of unfiltered superconductive rotor coil 'Voltage (V) includes an
electrical noise component, giving the voltage (V) a generally sinusoidal shape.
However, the superconductive rotor coil 12 is experiencing resistive heating in this
example. As a result, the voltage (V) is gradually increasing. FIG 4 is a graph ofthe
superconductive rotor coil voltage (Vr) filtered by the frequency filter 22. The
filtered superconductive rotor coil voltage (Vr), represented generally by reference
numeral 66, reflects the gradual increase in superconductive rotor coil voltage caused
by the resistive heating in the superconductive rotor coil 12, and not electrical noise.
Referring generally to FlO 5, an alternative embodiment of a generator having a
superconductive rotor coil 12 is illustrated. In this alternative embodiment, the
quench protection system 18 functions in a similar manner to the previous
embodiment. However, the frequency filter 22, the control circuit 26, and the dump
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circuit 28. are disposed on or within the rotor 14, rather than a stationary portion of
the generator. Thus. no telemetry circuit is provided in this embodiment. A
rechargeable battery 68 is provided inside the rotor 14. The rechargeable battery 68 is
operable to power the voltage detector 20. the frequency filter 22. the control circuit
26, and the dump circuit 28. The battery 68 is charged by a charging cu'Cuit 70
operable to remove a predetermined amount of current fed to the rotor coil 12 via the
exciter circuit 16. Charging of the battery may occur during periods when
substantially sufficient voltage is generated across the.coiI12.
While only certain features of the invention have been illustrated and described
herein. many modifications and changes will occur to those skilled in the art. It is,
therefore, to be understood that the appended claims are intended to cover all such
modifications and changes as fall within the true spirit ofthe invention.

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We Claims:
1. A method of detecting a quench condition in superconductor (12), comprising:
detecting voltage across the superconductor;
producing a signal representative of voltage across the superconductor (52);
removing electrical noise from the signal representative of voltage across the superconductor to
produce a signal representative of a voltage across the superconductor due to electrical resistance in
the superconductor (54); and
comparing the signal representative of voltage across the superconductor due to electrical resistance
in the superconductor to a reference voltage representative of voltage across the superconductor due
to a defined electrical resistance in the superconductor (56).
2. The method of claim 1, wherein comparing (56) comprises coupling the s~gnal representative
of voltage across the superconductor (12) due to electrical resistance in the superconductor (12) to a
control circuit (26) operable to produce a signal when the signal representative of voltage across the
superconductor (12) due to electrical resistance in the superconductor (12) rises to the reference
voltage representative of voltage across the superconductor (12) due to a defined electrical resistance
in the superconductor.
3. The method of claim 1, wherein removing electrical noise from the signal representative of
superconductor voltage (54) comprises coupling the signal representative of voltage across the
superconductor (12) through a filter (22) operable to block electrical noise from passing through the
filter (22).
4. The method of claim 1, further comprising detecting transient conditions on an external
circuit coupled to the superconductor (12) and providing a signal representative of the transient
condition to the control circuit (26) to direct the control circuit (26) to stand-by until the transient
condition passes.
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5. The method of claim 1, wherein the superconductor (12) is diposed on a rotor (14) of a
rotating electrical machine (10), and wherein detecting voltage across the superconductor (12)
comprises disposing a voltage detector (20) on the rotor (14) to detect voltage across the
superconductor (12).
Dated this 26th day of November 2012
Agents for the Applicants