U.S. patent application number 11/673281 was filed with the patent office on 2008-08-14 for parallel connected hts utility device and method of using same.
Invention is credited to DOUGLAS C. FOLTS, James Maguire, Alexis P. Malozemoff, Jie Yuan.
Application Number | 20080191561 11/673281 |
Document ID | / |
Family ID | 41010797 |
Filed Date | 2008-08-14 |
United States Patent
Application |
20080191561 |
Kind Code |
A1 |
FOLTS; DOUGLAS C. ; et
al. |
August 14, 2008 |
PARALLEL CONNECTED HTS UTILITY DEVICE AND METHOD OF USING SAME
Abstract
A superconducting electrical cable system is configured to be
included within a utility power grid. The superconducting
electrical cable system includes a superconducting electrical path
interconnected between a first and a second node within the utility
power grid. A non-superconducting electrical path is interconnected
between the first and second nodes within the utility power grid.
The superconducting electrical path and the non-superconducting
electrical path are electrically connected in parallel. The
superconducting electrical path has a lower series impedance, when
operated below a critical current level, than the
non-superconducting electrical path. The superconducting electrical
path has a higher series impedance, when operated at or above the
critical current level, than the non-superconductor electrical
path.
Inventors: |
FOLTS; DOUGLAS C.; (Baraboo,
WI) ; Maguire; James; (Andover, MA) ; Yuan;
Jie; (South Grafton, MA) ; Malozemoff; Alexis P.;
(Lexington, MA) |
Correspondence
Address: |
HOLLAND & KNIGHT LLP
10 ST. JAMES AVENUE, 11th Floor
BOSTON
MA
02116-3889
US
|
Family ID: |
41010797 |
Appl. No.: |
11/673281 |
Filed: |
February 9, 2007 |
Current U.S.
Class: |
307/147 ;
174/125.1; 505/120; 505/121; 505/123; 505/230 |
Current CPC
Class: |
Y02E 40/60 20130101;
H01L 39/16 20130101; Y02E 40/647 20130101; Y02E 40/68 20130101;
Y02E 40/69 20130101; H01B 12/16 20130101; H02H 9/023 20130101; H02H
7/001 20130101; H01B 12/02 20130101; Y02E 40/641 20130101 |
Class at
Publication: |
307/147 ;
505/120; 505/121; 505/123; 505/230; 174/125.1 |
International
Class: |
H02G 7/00 20060101
H02G007/00; C04B 35/45 20060101 C04B035/45; H01B 12/00 20060101
H01B012/00 |
Claims
1. A superconducting electrical cable system configured to be
included within a utility power grid, the superconducting
electrical cable system comprising: a superconducting electrical
path interconnected between a first and a second node within the
utility power grid; and a non-superconducting electrical path
interconnected between the first and second nodes within the
utility power grid; wherein the superconducting electrical path and
the non-superconducting electrical path are electrically connected
in parallel; wherein the superconducting electrical path has a
lower series impedance, when operated below a critical current
level, than the non-superconducting electrical path, and wherein
the superconducting electrical path has a higher series impedance,
when operated at or above the critical current level, than the
non-superconductor electrical path.
2. The superconducting electrical cable system of claim 1 wherein
the series impedance of the superconducting electrical path, when
operating in a non-superconducting mode, is at least N times the
series impedance of the non-superconducting electrical path, and
wherein N is greater than 1.
3. The superconducting electrical cable system of claim 2 wherein N
is greater than or equal to 3.
4. The superconducting electrical cable system of claim 2 wherein N
is greater than or equal to 5.
5. The superconducting electrical cable system of claim 1 wherein
the superconducting electrical path includes one superconducting
electrical cable, whereby the non-superconducting electrical path
is external of the superconducting electrical cable.
6. The superconducting electrical cable system of claim 1 wherein
the superconducting electrical path includes two or more
superconducting electrical cables.
7. The superconducting electrical cable system of claim 1 wherein
the non-superconducting electrical path includes at least one
non-superconducting electrical cable.
8. The superconducting electrical cable system of claim 1 wherein
the non-superconducting electrical path includes at least one
non-superconducting electrical overhead line.
9. The superconducting electrical cable system of claim 1 wherein
the non-superconducting electrical path includes at least one of:
one or more buses; one or more substations; one or more reactor
assemblies; and one or more fast switch assemblies.
10. The superconducting electrical cable system of claim 5 wherein
the superconducting electrical cable includes a centrally-located
axial coolant passage configured to allow for axial distribution of
a refrigerant through the centrally-located axial coolant
passage.
11. The superconducting electrical cable system of claim 5 wherein
the superconducting electrical cable includes one or more HTS
conductors.
12. The superconducting electrical cable system of claim 11 wherein
at least one of the HTS conductors is constructed of a material
chosen from the group consisting of:
rare-earth-barium-copper-oxide;
thallium-barium-calcium-copper-oxide;
bismuth-strontium-calcium-copper-oxide;
mercury-barium-calcium-copper-oxide; and any of the MgB.sub.2
magnesium diboride compounds.
13. The superconducting electrical cable system of claim 11 wherein
at least one of the one or more HTS conductors includes a
stabilizer layer having a total thickness within a range of 200-400
microns and a resistivity within a range of 3-10 microOhm-cm at 90
K.
14. The superconducting electrical cable system of claim 13 wherein
the stabilizer layer is constructed, at least in part, of a brass
material.
15. The superconducting electrical cable system of claim 11 wherein
at least one of the one or more HTS conductors is configured to
operate in a superconducting mode below a critical current
level.
16. The superconducting electrical cable system of claim 15 wherein
at least one of the one or more HTS conductors is configured to
operate in a non-superconducting mode at or above the critical
current level.
17. The superconducting electrical cable system of claim 1 wherein
the superconducting electrical path includes a fast switch
assembly.
18. The superconducting electrical cable system of claim 1 wherein
the non-superconducting electrical path includes a reactor
assembly.
19. A method of controlling fault currents within a utility power
grid comprising: coupling a superconducting electrical path between
a first and a second node within the utility power grid; and
coupling a non-superconducting electrical path between the first
and second nodes within the utility power grid; wherein the
superconducting electrical path and the non-superconducting
electrical path are electrically connected in parallel; wherein the
superconducting electrical path has a lower series impedance, when
operated below a critical current level, than the
non-superconducting electrical path, and wherein the
superconducting electrical path has a higher series impedance, when
operated at or above the critical current level, than the
non-superconductor electrical path.
20. The method of claim 19 wherein the series impedance of the
superconducting electrical path, when operating in a
non-superconducting mode, is at least N times the series impedance
of the non-superconducting electrical path, and wherein N is
greater than 1.
21. The method of claim 20 wherein N is greater than or equal to
3.
22. The method of claim 20 wherein N is greater than or equal to
5.
23. The method of claim 19 wherein the superconducting electrical
path includes one superconducting electrical cable, whereby the
non-superconducting electrical path is external of the
superconducting electrical cable.
24. The method of claim 19 wherein the superconducting electrical
path includes two or more superconducting electrical cables.
25. The method of claim 19 wherein the non-superconducting
electrical path includes at least one non-superconducting
electrical cable.
26. The method of claim 19 wherein the non-superconducting
electrical path includes at least one non-superconducting
electrical overhead line.
27. The method of claim 1 wherein the non-superconducting
electrical path includes at least one of: one or more buses; one or
more substations; one or more reactor assemblies; and one or more
fast switch assemblies.
28. The method of claim 23 wherein the superconducting electrical
cable includes one or more HTS conductors, the method further
comprising: configuring at least one of the one or more HTS
conductors to operate in a superconducting mode below a critical
current level.
29. The method of claim 28 further comprising: configuring at least
one of the one or more HTS conductors to operate in a
non-superconducting mode at or above the critical current
level.
30. The method of claim 23 wherein the superconducting electrical
cable includes one or more HTS conductors and at least one of the
HTS conductors is constructed of a material chosen from the group
consisting of: rare-earth-barium-copper-oxide;
thallium-barium-calcium-copper-oxide;
bismuth-strontium-calcium-copper-oxide;
mercury-barium-calcium-copper-oxide; and any of the MgB.sub.2
magnesium diboride compounds.
31. The method of claim 23 wherein the superconducting electrical
path includes a fast switch assembly.
32. The method of claim 23 wherein the non-superconducting
electrical path includes a reactor assembly.
33. A superconducting electrical cable configured to be included
within a utility power grid, the superconducting electrical cable
comprising: a hollow axial core: one or more conductive layers of
superconducting material positioned coaxially with respect to the
hollow axial core; a shield layer positioned coaxially with respect
to the hollow axial core; and an insulation layer positioned
coaxially with respect to the hollow axial core and positioned
between the one or more conductive layers and the shield layer.
34. The superconducting electrical cable of claim 33 wherein the
superconducting material includes an HTS material.
35. The superconducting electrical cable of claim 33 wherein the
HTS material includes a stabilizer layer having a total thickness
within a range of 200-400 microns and a resistivity within a range
of 3-10 microOhm-cm at 90 K.
36. The superconducting electrical cable of claim 33 wherein the
stabilizer layer is constructed, at least in part, of a brass
material.
37. The superconducting electrical cable of claim 33 wherein the
HTS material is chosen from the group consisting of:
rare-earth-barium-copper-oxide;
thallium-barium-calcium-copper-oxide;
bismuth-strontium-calcium-copper-oxide;
mercury-barium-calcium-copper-oxide; and any of the MgB.sub.2
magnesium diboride compounds.
38. The superconducting electrical cable of claim 33 wherein the
hollow axial core is configured to allow for axial distribution of
a refrigerant within the superconducting electrical cable.
39. The superconducting electrical cable of claim 38 wherein the
hollow axial core forms, at least in part, a coolant passage.
40. The superconducting electrical cable of claim 33 wherein the
superconducting material is configured to operate in a
superconducting mode below a critical current level.
41. The superconducting electrical cable of claim 33 wherein the
superconducting material is configured to operate in a
non-superconducting mode at or above the critical current level.
Description
TECHNICAL FIELD
[0001] This disclosure relates to HTS devices and, more
particularly, to HTS devices configured to operate as fault current
limiting devices.
BACKGROUND
[0002] As worldwide electric power demands continue to increase
significantly, utilities have struggled to meet these increasing
demands both from a power generation standpoint as well as from a
power delivery standpoint. Delivery of power to users via
transmission and distribution networks remains a significant
challenge to utilities due to the limited capacity of the existing
installed transmission and distribution infrastructure, as well as
the limited space available to add additional conventional
transmission and distribution lines and cables. This is
particularly pertinent in congested urban and metropolitan areas,
where there is very limited existing space available to expand
capacity.
[0003] Power cables using high temperature superconductor (HTS)
wire are being developed to increase the power capacity in utility
power transmission and distribution networks, while maintaining a
relatively small footprint. For this disclosure, HTS is defined as
a superconductor with a critical temperature at or above 30.degree.
Kelvin, which includes materials such as rare-earth- or
yttrium-barium-copper-oxide; thallium-barium-calcium-copper-oxide;
bismuth-strontium-calcium-copper-oxide;
mercury-barium-calcium-copper-oxide; and magnesium diboride. Such
HTS cables allow for increased amounts of power to be economically
and reliably provided within congested areas of a utility power
network, thus relieving congestion and allowing utilities to
address their problems of transmission and distribution
capacity.
[0004] An HTS power cable uses HTS wire as the primary conductor of
the cable (i.e., instead of traditional copper conductors) for the
transmission and distribution of electricity. The design of HTS
cables results in significantly lower series impedance, when
compared to conventional overhead lines and underground cables.
Here the series impedance of a cable or line refers to the
combination of resistive impedance of the conductors carrying the
power, and the inductive impedance associated with the cable
architecture or overhead line. For the same cross-sectional area of
the cable, HTS wire enables a three to five times increase in
current-carrying capacity when compared to conventional alternating
current (AC) cables; and up to a ten times increase in
current-carrying capacity when compared to conventional direct
current (DC) cables.
[0005] In addition to capacity problems, another significant
problem for utilities resulting from increasing power demand (and
hence increased levels of power being generated and transferred
through the transmission and distribution networks) are increased
"fault currents" resulting from "faults". Faults may result from
network device failures, acts of nature (e.g. lightning), acts of
man (e.g. an auto accident breaking a power pole), or any other
network problem causing a short circuit to ground or from one phase
of the utility network to another phase. In general, such a fault
appears as an extremely large load materializing instantly on the
utility network. In response to the appearance of this load, the
network attempts to deliver a large amount of current to the load
(i.e., the fault).
[0006] Detector circuits associated with circuit breakers monitor
the network to detect the presence of a fault (or over-current)
situation. Within a few milliseconds of detection, activation
signals from the detector circuits may initiate the opening of
circuit breakers to prevent destruction of various network
components. Currently, the maximum capability of existing circuit
breaker devices is 80,000 amps. Many sections of the utility
network built over the previous century were built with network
devices capable of withstanding only 40,000 to 63,000 amps of fault
current. Unfortunately, with increased levels of power generation
and transmission on utility networks, fault current levels are
increasing to the point where they will exceed the capabilities of
presently installed or state-of-the-art circuit breaker devices
(i.e. be greater than 80,000 amps). Even at lower fault current
levels, the costs of upgrading circuit breakers from one level to a
higher one across an entire grid can be very high. Accordingly,
utilities are looking for new solutions to deal with the increasing
level of fault currents. One such solution in development is a
device called an HTS fault current limiter (FCL).
[0007] An HTS FCL is a dedicated device interconnected to a utility
network that reduces the amplitude of the fault currents to levels
that conventional, readily available or already installed circuit
breakers may handle. Unfortunately, such standalone HTS FCLs are
currently quite large and expensive. Utilities may also use large
inductors, but they may cause extra losses, voltage sag and grid
stability problems. And, unfortunately, pyrotechnic current
limiters (e.g., fuses) need replacement after every fault event.
Further, while new power electronic FCLs are under development,
they are also expected to be large and expensive.
[0008] To protect HTS cables against fault currents, a significant
amount of copper is introduced in conjunction with the HTS wire,
but this adds to the weight and size of the cable. Often, copper
fills the central former in the core of the HTS cable around which
the HTS wire is spirally wound, and this prevents the core from
being used as a passage for the flow of liquid nitrogen.
[0009] It is desirable to improve the way HTS cables handle fault
currents and to provide an improved alternative to the use of
standalone FCLs or other fault current limiting devices.
SUMMARY OF DISCLOSURE
[0010] In a first implementation of this disclosure, a
superconducting electrical cable system is configured to be
included within a utility power grid. The superconducting
electrical cable system includes a superconducting electrical path
interconnected between a first and a second node within the utility
power grid. A non-superconducting electrical path is interconnected
between the first and second nodes within the utility power grid.
The superconducting electrical path and the non-superconducting
electrical path are electrically connected in parallel. The
superconducting electrical path has a lower series impedance, when
operated below a critical current level, than the
non-superconducting electrical path. The superconducting electrical
path has a higher series impedance, when operated at or above the
critical current level, than the non-superconductor electrical
path.
[0011] One or more of the following features may be included. The
series impedance of the superconducting electrical path, when
operating in a non-superconducting mode, may be at least N times
the series impedance of the non-superconducting electrical path. N
may be greater than 1. N may be greater than or equal to 3. N may
be greater than or equal to 5.
[0012] The superconducting electrical path may include one
superconducting electrical cable, whereby the non-superconducting
electrical path is external of the superconducting electrical
cable. The superconducting electrical path may include two or more
superconducting electrical cables. The non-superconducting
electrical path may include at least one non-superconducting
electrical cable. The non-superconducting electrical path may
include at least one non-superconducting electrical overhead line.
The non-superconducting electrical path may include at least one
of: one or more buses; one or more substations; one or more reactor
assemblies; and one or more fast switch assemblies.
[0013] The superconducting electrical cable may include a
centrally-located axial coolant passage configured to allow for
axial distribution of a refrigerant through the centrally-located
axial coolant passage. The superconducting electrical cable may
include one or more HTS conductors. At least one of the HTS
conductors may be constructed of a material chosen from the group
consisting of: rare-earth-barium-copper-oxide;
thallium-barium-calcium-copper-oxide;
bismuth-strontium-calcium-copper-oxide;
mercury-barium-calcium-copper-oxide; and any of the MgB.sub.2
magnesium diboride compounds. At least one of the one or more HTS
conductors may include a stabilizer layer having a total thickness
within a range of 200-400 microns and a resistivity within a range
of 3-10 microOhm-cm at 90 K. The stabilizer layer may be
constructed, at least in part, of a brass material. At least one of
the one or more HTS conductors may be configured to operate in a
superconducting mode below a critical current level.
[0014] At least one of the one or more HTS conductors may be
configured to operate in a non-superconducting mode at or above the
critical current level. The superconducting electrical path may
include a fast switch assembly. The non-superconducting electrical
path may include a reactor assembly.
[0015] In another implementation of this disclosure, a method of
controlling fault currents within a utility power grid includes
coupling a superconducting electrical path between a first and a
second node within the utility power grid. A non-superconducting
electrical path is coupled between the first and second nodes
within the utility power grid. The superconducting electrical path
and the non-superconducting electrical path are electrically
connected in parallel. The superconducting electrical path has a
lower series impedance, when operated below a critical current
level, than the non-superconducting electrical path. The
superconducting electrical path has a higher series impedance, when
operated at or above the critical current level, than the
non-superconductor electrical path.
[0016] One or more of the following features may be included. The
series impedance of the superconducting electrical path, when
operating in a non-superconducting mode, may be at least N times
the series impedance of the non-superconducting electrical path. N
may be greater than 1. N may be greater than or equal to 3. N may
be greater than or equal to 5.
[0017] The superconducting electrical path may include one
superconducting electrical cable, whereby the non-superconducting
electrical path is external of the superconducting electrical
cable. The superconducting electrical path may include two or more
superconducting electrical cables. The non-superconducting
electrical path may include at least one non-superconducting
electrical cable. The non-superconducting electrical path may
include at least one non-superconducting electrical overhead line.
The non-superconducting electrical path may include at least one
of: one or more buses; one or more substations; one or more reactor
assemblies; and one or more fast switch assemblies.
[0018] The superconducting electrical cable may include one or more
HTS conductors. At least one of the one or more HTS conductors may
be configured to operate in a superconducting mode below a critical
current level. At least one of the one or more HTS conductors may
be configured to operate in a non-superconducting mode at or above
the critical current level. The superconducting electrical cable
may include one or more HTS conductors and at least one of the HTS
conductors may be constructed of a material chosen from the group
consisting of: rare-earth-barium-copper-oxide;
thallium-barium-calcium-copper-oxide;
bismuth-strontium-calcium-copper-oxide;
mercury-barium-calcium-copper-oxide; and any of the MgB.sub.2
magnesium diboride compounds. The superconducting electrical path
may include a fast switch assembly. The non-superconducting
electrical path may include a reactor assembly.
[0019] In another implementation of this disclosure, a
superconducting electrical cable is configured to be included
within a utility power grid. The superconducting electrical cable
includes a hollow axial core. One or more conductive layers of
superconducting material are positioned coaxially with respect to
the hollow axial core. A shield layer is positioned coaxially with
respect to the hollow axial core. An insulation layer is positioned
coaxially with respect to the hollow axial core and positioned
between the one or more conductive layers and the shield layer.
[0020] One or more of the following features may be included. The
superconducting material may include an HTS material. The HTS
material may include a stabilizer layer having a total thickness
within a range of 200-400 microns and a resistivity within a range
of 3-10 microOhm-cm at 90 K. The stabilizer layer may be
constructed, at least in part, of a brass material. The HTS
material may be chosen from the group consisting of:
rare-earth-barium-copper-oxide;
thallium-barium-calcium-copper-oxide;
bismuth-strontium-calcium-copper-oxide;
mercury-barium-calcium-copper-oxide; and any of the MgB.sub.2
magnesium diboride compounds.
[0021] The hollow axial core may be configured to allow for axial
distribution of a refrigerant within the superconducting electrical
cable. The hollow axial core may form, at least in part, a coolant
passage. The coolant passage may be configured to allow for axial
distribution of a refrigerant within the superconducting electrical
cable. The superconducting material may be configured to operate in
a superconducting mode below a critical current level. The
superconducting material may be configured to operate in a
non-superconducting mode at or above the critical current
level.
[0022] The details of one or more implementations are set forth in
the accompanying drawings and the description below. Other features
and advantages will become apparent from the description, the
drawings, and the claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0023] FIG. 1 is a schematic diagram of a copper-cored HTS cable
system installed within a utility power grid;
[0024] FIG. 2 is an isometric view of the copper-cored HTS cable of
FIG. 1;
[0025] FIG. 3 is an isometric view of a hollow-core HTS cable;
[0026] FIG. 4 is a schematic diagram of a hollow-core HTS cable
system installed within a utility power grid;
[0027] FIG. 5 is a schematic diagram of a utility power grid;
[0028] FIG. 6 is a model of a superconducting/conventional cable
pair;
[0029] FIG. 7 is a cross-sectional view of an HTS wire;
[0030] FIG. 8 is a model of the HTS wire of FIG. 7;
[0031] FIG. 9 is an alternative model of the
superconducting/conventional cable pair of FIG. 6;
[0032] FIG. 10 is a model of the superconducting/conventional cable
pair of FIG. 9 during superconducting mode; and
[0033] FIG. 11 is a model of the superconducting/conventional cable
pair of FIG. 9 during non-superconducting mode.
[0034] Like reference symbols in the various drawings indicate like
elements.
DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS
Overview
[0035] Referring to FIG. 1, a portion of a utility power grid 10
may include a high temperature superconductor (HTS) cable 12. HTS
cable 12 may be hundreds or thousands of meters in length and may
provide a relatively high current/low resistance electrical path
for the delivery of electrical power from generation stations (not
shown) or imported from remote utilities (not shown).
[0036] The cross-sectional area of HTS cable 12 may be only a
fraction of the cross-sectional area of a conventional copper core
cable and may be capable of carrying the same amount of electrical
current. As discussed above, within the same cross-sectional area,
an HTS cable may provide three to five times the current-carrying
capacity of a conventional AC cable; and up to ten times the
current-carrying capacity of a conventional DC cable. As HTS
technology matures, these ratios may increase.
[0037] As will be discussed below in greater detail, HTS cable 12
may include HTS wire, which may be capable of handling as much as
one-hundred-fifty times the electrical current of similarly-sized
copper wire. Accordingly, by using a relatively small quantity of
HTS wire (as opposed to a large quantity of copper conductors
stranded within the core of a traditional AC cable), an HTS power
cable may be constructed that is capable of providing three to five
times as much electrical power as an equivalently-sized traditional
copper-conductor power cable.
[0038] HTS cable 12 may be connected within a transmission grid
segment 14 that carries voltages at a level of e.g., 138 kV and
extends from grid segment 14 to grid segment 16, which receives
this voltage and transforms it to a lower level of e.g., 69 kV For
example, transmission grid segment 14 may receive power at 765 kV
(via overhead line or cable 18) and may include a 138 kV substation
20. 138 kV substation 20 may include a 765 kV/138 kV transformer
(not shown) for stepping down the 765 kV power received on cable 18
to 138 kV. This "stepped-down" 138 kV power may then be provided
via e.g., HTS cable 12 to transmission grid segment 16.
Transmission grid segment 16 may include 69 kV substation 24, which
may include a 138 kV/69 kV transformer (not shown) for stepping
down the 138 kV power received via HTS cable 12 to 69 kV power,
which may be distributed to e.g., devices 26, 28, 30, 32. Examples
of devices 26, 28, 30, 32 may include, but are not limited to 34.5
kV substations.
[0039] The voltage levels discussed above are for illustrative
purposes only and are not intended to be a limitation of this
disclosure. Accordingly, this disclosure is equally applicable to
various voltage and current levels in both transmission and
distribution systems. Likewise, this disclosure is equally
applicable to non-utility applications such as industrial power
distribution or vehicle power distribution (e.g. ships, trains,
aircraft, and spacecraft).
[0040] One or more circuit breakers 34, 36 may be connected on
e.g., each end of HTS cable 12 and may allow HTS cable 12 to be
quickly disconnected from utility power grid 10. Fault management
system 38 may provide over-current protection for HTS cable 12 to
ensure that HTS cable 12 is maintained at a temperature that is
below the point at which HTS cable 12 may be damaged.
[0041] Fault management system 38 may provide such over-current
protection by monitoring the current flowing in the segment of the
utility grid to which HTS cable 12 is coupled. For example, fault
management system 38 may sense the current passing through 138 kV
substation 20 (using e.g., current sensor 40) and may control the
operation of breakers 34, 36 based, at least in part, on the signal
provided by current sensor 40.
[0042] In this example, HTS cable 12 may be designed to withstand a
fault current as high as 51 kA with a duration of 200 ms (i.e., 12
cycles of 60 Hz power). The details of fault management system 38
are described in co-pending U.S. patent application Ser. No.
11/459,167, which was filed on 21 Jul. 2006, and is entitled Fault
Management of HTS Power Cable. Typically, this requires the HTS
cable to contain a significant amount of copper, which helps to
carry the high fault current and thus to protect the HTS wires.
[0043] Referring also to FIG. 2, there is shown a typical
embodiment of copper-cored HTS cable 12 that may include stranded
copper core 100 surrounded in radial succession by first HTS layer
102, second HTS layer 104, high voltage dielectric insulation layer
106, HTS shield layer 108, copper shield layer 110, coolant passage
112, inner cryostat wall 114, thermal insulation 116, vacuum space
118, outer cryostat wall 120 and an outer cable sheath 122. During
operation, a refrigerant (e.g., liquid nitrogen, not shown) may be
supplied from an external coolant source (not shown) and may be
circulated within and along the length of coolant passage 112.
[0044] Additionally/alternatively, additional coaxial HTS layers
may be utilized. For example, a third HTS layer (not shown) may be
utilized to carry a third phase of current. An example of such a
cable arrangement is the Triax HTS Cable arrangement proposed by
Ultera (i.e., a joint venture of Southwire Company of Carrollton,
Ga. and nkt cables of Cologne, Germany). Other embodiments of HTS
cable 12 may include, but are not limited to: warm and/or cold
dielectric configurations; single-phase vs. three-phase
configurations; and various different shielding configurations
(e.g., no shield and cryostat-based shielding).
[0045] Copper core 100 and copper shield layer 110 may be
configured to carry fault currents (e.g., fault current 124) that
may appear within cable 12. For example, when fault current 124
appears within cable 12, the current within HTS layers 102, 104 may
dramatically increase to a level that exceeds the critical current
level (i.e., I.sub.c) of HTS layers 102, 104, which may cause the
superconductors to lose their superconducting characteristics
(i.e., HTS layers 102, 104 may go "normal"). A typical value for
critical current level I.sub.c is 4242 A peak for a cable rated at
3000 A.sub.rms.
[0046] The critical current level in HTS materials may depend upon
the choice of electric field level. Conventionally, the critical
current level is defined at an electric field level of 1
microvolt/cm, though lower values are also used. However, typically
superconductors exhibit a transition region between the
zero-resistance (i.e., superconducting) and fully-resistive (i.e.,
non-superconducting) states as a function of current level.
Accordingly, if conductor losses resulting from operation in a
state are below those of the fully-resistive state, it may be
understood that the superconductor is operating "below the critical
current". When current is increased to the point that the conductor
losses are essentially those of the fully resistive state, one can
understand this to mean the conductor is "above the critical
current," and this meaning is used herein. Therefore, in practice,
the HTS cable may switch to fully resistive state at a critical
current level somewhat above the conventional critical current
level defined by the 1 microvolt/cm criterion.
[0047] Accordingly, when the critical current level (as defined
above) is exceeded, the resistance of HTS layers 102, 104 may
increase significantly and may become comparatively very high
(i.e., when compared to copper core 100). As the current passing
through a plurality of parallel conductors is distributed inversely
with respect to the resistance of the individual conductors, the
majority of fault current 124 may be diverted to copper core 100,
which is connected in parallel with HTS layers 102, 104. This
transmission of fault current 124 through copper core 100 may
continue until: fault current 124 subsides; or the appropriate
circuit breakers (e.g., circuit breakers 34, 36) interrupt the
transmission of fault current 124 through HTS cable 12.
[0048] By redirecting fault current 124 (or at least a portion
thereof) from HTS layers 102, 104 to copper core 100, the
overheating of the HTS conductors in HTS cable 12 may be avoided.
In the event that fault current 124 (or at least a portion thereof)
was not redirected from HTS layers 102, 104 to copper core 100,
fault current 124 may heat the HTS conductors in HTS cable 12
significantly due to the high resistance of HTS layers 102, 104,
which may result in the formation of gaseous "bubbles" of liquid
nitrogen (i.e., due to liquid nitrogen being converted from a
liquid state to a gaseous state within coolant passage 112).
Unfortunately, the formation of gaseous "bubbles" of liquid
nitrogen may reduce the dielectric strength of the dielectric layer
and may result in voltage breakdown and the destruction of HTS
cable 12. For warm dielectric cable configurations (not shown),
fault current not redirected away from HTS layers 102, 104 may
simply overheat and destroy HTS layers 102, 104.
[0049] Examples of HTS cable 12 may include but are not limited to
HTS cables available from Nexans of Paris France; Sumitomo Electric
Industries, Ltd., of Osaka, Japan; and Ultera (i.e., a joint
venture of Southwire Company of Carrollton, Ga. and nkt cables of
Cologne, Germany).
[0050] While copper core 100 redirects fault currents (or portions
thereof) around HTS layers 102, 104, there are disadvantages to
utilizing such an "internal" copper core. For example, copper core
100 may require HTS cable 12 to be physically larger and heavier,
which may result in increased cost and greater heat retention
within HTS cable 12. Accordingly, more refrigeration may be
required to compensate for the additional heat retention, resulting
in higher overall system and operating costs. Moreover, the
increased heat capacity of copper core 100 and thermal resistance
between HTS layers 102, 104 and the coolant due to the dielectric
layer may greatly increase recovery times should the energy of a
fault current increase the temperature beyond the point where
superconductivity can be maintained in HTS layers 102, 104. For
example, in the event that a fault current is redirected through
copper core 100, it may take several hours for the refrigeration
system (not shown) to cool down HTS cable 12 to within the
appropriate operating temperature range (e.g., 65-77.degree.
Kelvin). The time required to cool down HTS cable 12 to within the
operating range of the cable is commonly referred to as the
"recovery time", which may be required by utilities to be a few
seconds (or less) for transmission devices or a few tenths of a
second (or less) for distribution devices. Alternatively, a
standalone fault current limiter may be used with HTS cable 12 to
limit fault currents; however this has the disadvantage of
requiring another large and costly piece of electrical equipment to
be installed in the substation linked to HTS cable 12.
[0051] Referring to FIG. 3, there is shown a hollow-core HTS cable
150, according to this disclosure. While HTS cable 150 may include
various components of prior art copper-cored HTS cable 12, HTS
cable 150 does not include stranded copper core 100 (FIG. 2) in the
cable core, but rather it was replaced with a hollow core, such as
inner coolant passage 152, through which a refrigerant (e.g.,
liquid nitrogen) may flow.
[0052] In a fashion similar to that of copper-cored HTS cable 12,
inner coolant passage 152 may be surrounded in radial succession by
first HTS layer 102, second HTS layer 104, high voltage dielectric
insulation layer 106, HTS shield layer 108, copper shield layer
110, coolant passage 112, inner cryostat wall 114, thermal
insulation 116, vacuum space 118, outer cryostat wall 120 and an
outer cable sheath 122. During operation, a refrigerant (e.g.,
liquid nitrogen, not shown) may be supplied from an external
coolant source (not shown) and may be circulated within and along
the length of coolant passage 114 and inner coolant passage
152.
[0053] Referring also to FIG. 4, utility power grid portion 10' may
include a conventional (i.e. non-superconducting cable) 200,
connected in parallel with HTS cable 150. An example of
conventional cable 200 includes but is not limited to a 500 kcmil,
138 kV Shielded Triple Permashield (TPS) power cable available from
The Kerite Company of Seymour, Conn. Conventional cable 200 may be
an existing cable in a retrofit application where HTS cable 150 is
being added to replace one or more conventional cables to e.g.,
increase the power capacity of an electrical grid. Alternatively,
conventional cable 200 may be a new conventional cable that is
installed concurrently with HTS cable 150 and interconnected with
appropriate bus work and circuit breakers.
[0054] HTS cable 150 and/or additional HTS cables (not shown) may
be included within superconducting electrical path 202, which may
include a portion of a power utility grid. Further, superconducting
electrical path 202 may include other superconducting power
distribution devices, such as buses (not shown), transformers (not
shown), fault current limiters (not shown), and substations (not
shown).
[0055] Conventional cable 200 and/or additional conventional cables
(not shown) may be included within non-superconducting electrical
path 204, which may include a portion of a power utility grid.
Further, non-superconducting electrical path 204 may include other
power distribution devices, such as buses (not shown), transformers
(not shown), fault current limiters (not shown), and substations
(not shown).
[0056] By removing copper core 100 (FIG. 2) from the inside of HTS
cable 150 and utilizing an external (i.e., with respect to HTS
cable 150) parallel-connected conventional cable 200 to carry e.g.,
fault current 124, HTS cable 150 may be physically smaller, which
may result in decreased fabrication cost and lower heat loss from
HTS cable 150. Accordingly, HTS cable 150 may require less
refrigeration (when compared to HTS cable 12, which has greater
heat retention) and may result in lower overall system and
operating costs. Further, by moving copper core 100 from the inside
of HTS cable 12 to the outside of HTS cable 150 (in the form of
conventional cable 200), the heat capacity of HTS cable 150 and the
thermal resistance between HTS layers 102, 104 and the coolant are
both reduced, thus allowing for quicker recovery times in the event
that fault current 124 increase the temperature of HTS cable 150
beyond the point where superconductivity can be maintained in HTS
layers 102, 104. By removing the copper core 100 from the inside of
the HTS cable 12 and by using an appropriately optimized HTS wire,
one can incorporate fault current limiting functionality directly
into HTS cable 150, thus removing the need for a separate
standalone fault current limiter if one wants to protect the HTS
cable or downstream utility equipment from fault currents.
HTS Cable and Fault Current Limiters
[0057] Referring again to FIG. 1, if a fault current within grid
section 10 causes the current flowing through HTS cable 12 to rise
beyond the limits of conventional circuit breakers 34, 36, an HTS
FCL device 42 (shown in phantom) or conventional reactor technology
(not shown) may be incorporated within grid section 10 to limit the
amplitude of the fault current flowing through HTS cable 12 to a
level that conventional circuit breakers 34, 36 can interrupt.
Under normal conditions, when nominal current levels are flowing in
grid section 10, HTS FCL introduces no impedance into the grid.
However, when a fault current appears in grid section 10, the
current causes the superconductor in HTS FCL 42 to instantaneously
go "normal" or non-superconducting and this adds a very large
impedance into grid section 10. The HTS FCL 42 is designed to limit
the fault current to a predetermined level which is within the
interrupting capability of convention circuit breakers.
[0058] Standalone HTS FCL devices 42 are being developed by various
companies, including American Superconductor Corporation (of
Westboro, Mass.) in conjunction with Siemens AG (of Germany).
Unfortunately, adding HTS FCL device 42 to grid section 10 may be
very costly and may require a significant amount of space to
accommodate device 42, which may be difficult to accommodate
(especially in urban areas).
[0059] According to the present disclosure, an HTS device, e.g. HTS
cable 150 (FIG. 4), may be used as a fault current limiter itself
without the need to incorporate a separate HTS FCL, such as HTS FCL
42 (FIG. 1). Therefore, the HTS cable itself may be utilized to
obtain the desirable effects (e.g., attenuation of fault currents)
of a typical standalone HTS FCL while avoiding the undesirable
effects (e.g., cost and size) of the typical standalone HTS FCL. In
order to achieve the fault current limiting effects, the HTS cable
may be placed in parallel with a conventional (i.e.,
non-superconducting) cable. For example, if superconducting cable
150, and conventional cable 200 are placed in parallel, this
combination may be designed and operated to act as a fault current
limiting cable system that is described in more detail below.
[0060] This disclosure may be applied to other HTS devices as well.
For example, if another type of the superconducting device, such as
a superconducting transformer (not shown) is placed in parallel
with a conventional transformer (not shown), this combination of
devices may be designed and operated to act as a fault current
limiting system. Alternatively, where fault current attenuation is
not required, this arrangement may allow the superconducting
transformer to be smaller because not all of the fault current will
flow through the superconducting transformer, preferring instead to
flow through the conventional transformer. Accordingly, by placing
a conventional device in parallel with a superconducting device
according to this disclosure, the amplitude of the fault current on
the grid may be limited to the desired level (by properly sizing
the conventional parallel device and/or the superconducting
device), thereby allowing the use of readily available circuit
breakers.
[0061] During normal operation of the HTS device (e.g., HTS cable
150), the impedance (i.e., both real and reactive) of the HTS
device will be significantly lower than that of the conventional
device (e.g., conventional cable 200). For example, the typical
impedance of HTS cable 150 is essentially 0.00+j0.007 ohms per
kilometer (when superconducting) and 1.35+j0.007 ohms per kilometer
(when not superconducting), and the typical impedance of
conventional cable 200 is 0.095+j0.171 ohms per kilometer. Note
that HTS cable 150 has essentially zero resistance when
superconducting. Accordingly, when HTS cable 150 is
superconducting, the majority of the current passing through
breakers 34, 36 will flow through HTS cable 150 (with very little
or zero current passing through conventional cable 200). However,
when not superconducting, the vast majority of the current will
flow through conventional cable 200 (with only a small fraction
flowing through HTS cable 150).
[0062] A transient-rated (or fully-rated) reactor assembly 206 may
be coupled in series with conventional cable 200. An example of
reactor assembly 206 is an air-core dry-type power reactor
manufactured by Trench.RTM. Limited of Scarborough, Ontario,
Canada. Further, a fast switch assembly 208 may be coupled in
series with HTS cable 150. An example of fast switch assembly 208
is a 138 kV Type PM Power Circuit Breaker manufactured by ABB Inc.
of Greensburg, Pa. One or both of reactor assembly 206 and/or fast
switch assembly 208 (e.g., a switch capable of opening in 4 cycles)
may be controllable by fault management system 38. For example,
upon sensing fault current 124, fault management system 38 may open
fast switch assembly 208, resulting in reactor assembly 206 along
with the conventional cable absorbing a portion of the power of
fault current 124 and effectively isolating HTS cable 150 from
fault current 124. The fast switch is also protected by the current
limitation from the rapidly switching HTS cable. For multiphase
power, a plurality of reactor assemblies 206 and/or fast switch
assemblies 208 may be utilized. The fast switch may be reclosed
after some minutes when the HTS cable has recovered to its
superconducting state.
[0063] Referring also to FIG. 5, the operation of HTS cable 150 as
an FCL within the context of utility power grid 250 is shown.
Utility power grid 250 is shown to include 765 kV bus 252, 69 kV
bus 254, and 34.5 kV bus 256. Further, utility power grid 250 is
shown to include three 138 kV substations 20, 258, 260, each of
which provides power to 69 kV bus 254 through three 69 kV
substations 24, 262, 264. Three 34.5 kV substations 266, 268, 270
may provide power from 69 kV bus 254 to 34.5 kV bus 256. The HTS
cable and FCL system, 150, 200, is shown between substations 20 and
24
[0064] When a fault current (e.g., fault current 124) is present
within utility power grid 250, current may flow from all
interconnected substations through all available paths to feed the
fault, which may appear as a very large load placed on utility
power grid 250. When calculating the fault currents realizable
during a fault condition, the fault may be modeled as a
short-circuit to ground.
[0065] Referring also to FIG. 6, when determining how much fault
current a particular substation (e.g., 138 kV substation 20)
contributes to e.g., fault current 124, the open circuit generation
voltage may be modeled as ideal voltage source 300. Further, the
impedance of cables 150, 200 may be modeled as their resistive and
reactive equivalent circuit elements and upstream impedance may be
combined with the transformer impedances and represented as source
impedance 302. Impedance in this context may be a complex vector
quantity consisting of a real and a reactive component.
Mathematically, impedance Z=R+jX, where R is the real (i.e.,
resistive) component and X is the reactive component. In this
example, the reactive component is inductive and equal to
j.omega.L, where .omega.=2.pi.f and f is the frequency of the
current flow (e.g. 60 Hz in North America).
[0066] The cables may likewise be modeled as a complex impedance.
For example, cables 150, 200 are shown terminated to ground
because, as discussed above, the fault is modeled as a short
circuit to ground. Ohm's Law may be used to determine the expected
level of fault current provided by 138 kV substation 20. Using this
approach with respect to the other substations with in grid 250,
the overall fault current contributions may be calculated and the
fault current expected to pass through cable 150 may be determined.
The HTS cable 150 and conventional cable 200 may then be designed
to limit this otherwise expected fault current 124 to a lower,
predetermined level which the conventional circuit breakers are
capable of handling.
[0067] In designing the HTS device and conventional device to work
properly as an FCL, certain criteria must be considered. For
example, during a fault condition, HTS cable 150 must be configured
to achieve resistance high enough to provide the grid with a
sufficient impedance to lower the fault current to the desired
level. It also must be high enough relative to the impedance of the
conventional cable 200 for the majority of the fault current 124 to
flow through conventional cable 200. The design of this voltage
divider must be such that the voltage drop across the HTS cable 150
during a fault does not cause the cable temperature to rise to a
point where the refrigerant (e.g., liquid nitrogen) changes from a
liquid state to a gaseous state. If this were to occur, the
dielectric strength of the liquid nitrogen between the high voltage
cable core (e.g., HTS layers 102, 104) and the shield (e.g., HTS
shield layer 108), would not be maintained and voltage breakdown
within HTS cable 150 could occur potentially resulting in
destruction of the cable.
[0068] The criterion that the HTS device achieve a resistance high
enough for the majority of the fault current to be directed through
the conventional device is achievable due to the high resistivity
of HTS material after transition from superconducting state to
normal (i.e., non-superconducting) state. As with all
superconductors, as long as temperature, current density, and
magnetic field strength remain below certain critical values,
current will flow in the superconductor with essentially zero
resistance.
[0069] Assume that HTS cable 150 is a 600 m long HTS cable rated
for 2400 A of continuous current at 138 kV The HTS conductors
(e.g., HTS layers 102, 104) of HTS cable 150 may include
twenty-eight strands of HTS wire in parallel. Further, assume that
cable 150 is constructed using an HTS conductor 0.44 cm wide,
laminated with 300 microns of brass with a resistivity at 90 K of
5.8 microOhm-cm. Then one strand of this conductor has a 90 K
resistance of approximately 37.9 Ohms per kilometer. Such an HTS
conductor is available from American Superconductor Corporation.
Accordingly, the cable resistance per phase would be
37.9.OMEGA./km*2.6 km*1.08/28 strands=3.80.OMEGA.. The 1.08 factor
results from the spiral cabling process which requires each strand
be longer than the length of HTS cable 150. For conventional cable
200, the impedance is 2.6
km*(0.095+j0.17).OMEGA./km=0.25+j0.44.OMEGA.. Accordingly, while
HTS cable 150 has an impedance value when superconducting that is
substantially lower (i.e., 1.35+j0.007.OMEGA./km) than conventional
cable 200, when HTS cable 150 is not superconducting (e.g., when a
high temperature condition occurs), the impedance of HTS cable 150
is substantially higher (having an impedance of
1.35+j0.007.OMEGA./km) than conventional cable 200 (having an
inductive impedance of 0.095+j0.17.OMEGA./km).
[0070] Referring also to FIG. 7, there is shown a cross-sectional
view of one HTS conductor 350 used to construct HTS layers 102,
104. In this example, HTS conductor 350 used in HTS layers 102, 104
is shown to include a stabilizer layer 352 and substrate 354. An
example of stabilizer layer 352 may include but is not limited to
brass, copper, stainless steel. An example of substrate 354 may
include but is not limited to nickel-tungsten, stainless steel and
Hastelloy. Positioned between stabilizer layer 352 and substrate
354 may be buffer layer 356, HTS layer 358 (e.g., a
yttrium-barium-copper-oxide layer), and cap layer 360. An example
of buffer layer 356 is the combination of yttria, yttria-stabilized
zirconia, and cerium oxide (CeO.sub.2), and an example of cap layer
360 is silver. A solder such as SnPbAg may be used to bond the
stabilizer layer to the substrate on one side and the silver cap
layer on the other.
[0071] Referring also to FIG. 8, an equivalent electrical model 400
of HTS conductor 350 is shown. For illustrative purpose, equivalent
electrical model 400 illustrates HTS conductor 350 as a
superconducting layer 402 on the lower half of model 400 and all
other wire structures combined to form resistive metallic layer 404
on the upper half of model 400. When HTS conductor 350 is in
superconducting mode, all current flows within the essentially zero
resistance superconducting layer 402. When in non-superconducting
mode, current flows within resistive metallic layer 404, consisting
principally of the stabilizer.
[0072] Referring also to FIG. 9 and as discussed above, exceeding a
critical current level is what differentiates between HTS conductor
350 functioning in a superconducting mode or in a
non-superconducting mode. HTS conductor 350 may be modeled to
include a switch 406 that, for low currents (i.e., below the
critical current level), is closed and shunts the resistance 408 of
metallic layer 404. Accordingly, when switch 406 is closed, all
current flows through superconducting layer 402, which is modeled
as zero resistance. When the critical current level is exceeded,
superconducting layer 402 may become highly resistive and switch
406 may be opened, resulting in all current flowing through
resistive metallic layer 404.
[0073] Referring also to FIG. 10, there is shown a model of the
combination of HTS cable 150 and conventional cable 200 during
superconducting operation mode. For this model, assume a typical
source voltage of 79.7 kV line-to-ground and a source impedance of
0.155+j1.55 Ohms (Vs, Ls, and Rs in FIG. 6). These values result in
a fault current of 51 kA for a fault in substation 20 ahead of the
cables 150 and 200. Inserting typical real and reactive impedance
values for e.g., a 2600 meter cable, during normal operation where
current (i.e., below the critical current level) flows from one
substation to another substation, the switch is closed and 96% of
the current flows within HTS cable 150.
[0074] Referring also to FIG. 11, during a fault condition, the
critical current level is met or exceeded, causing switch 410 (FIG.
9) to open. The additional resistance of metallic layer 402 (FIG.
8) of HTS cable 150 may cause the majority of the fault current to
flow within conventional cable 200. Specifically, for the values
shown, 88% of the fault current flows within conventional cable 200
and 12% within HTS cable 150. Total fault current flowing in cables
150 and 200 is 40 kA which is significantly reduced from the 51 kA
available. This 20% reduction in available fault current is typical
of what may be required of a fault current limiter.
[0075] To prevent HTS cable 150 from heating excessively during a
fault, several measures may be taken. Typically, fast switch
assembly 208 (FIG. 4), which is in series with the HTS cable 150,
is opened after e.g., 4 cycles, and is only closed after HTS cable
150 cools to an acceptable starting temperature. Alternatively,
circuit breakers 34 and/or circuit breaker 36 may be opened.
[0076] To further minimize the temperature rise, stabilizer layer
352 (FIG. 7) may be quite thick (e.g., 300 microns) to increase the
heat capacity. At the same time, the resistivity of stabilizer
layer 352 maybe chosen at a value to minimize the temperature rise
due to resistive heating, while at the same time being high enough
to insure that in its switched state, HTS cable 150 has a high
enough resistance to insure transfer of a majority of fault current
124 (FIG. 5) to conventional cable 200. Typically, values in the
range of 3-10 microOhm-cm at around 90 K fulfill these requirements
for typical applications. A convenient material family to achieve
such values is brass (Cu--Zn alloy), but many other alloys such as
CuNi and CuMn are also possible. These values are provided for
illustrative purposes only and are not intended to be a limitation
of this disclosure. For example, in the case described above, with
28 parallel HTS wires, each 0.44 cm in width, and with a total of
300 microns of stabilizer with 5 microOhm-cm resistivity provides a
resistance of 13 Ohm/km, while the temperature rise during a
4-cycle (0.067 sec) hold time prior to the opening of the fast
switch, for an effective critical current of 350 A/cm is about 11
K. For a pressurized cable system with pressures in the range of
15-20 bar, nitrogen bubbles above about 110 K; so this temperature
rise is acceptable for operation in the 70-80K temperature
range.
[0077] Now consider the example of a 600 m length of the same cable
(i.e. 138 kV, 2400 A and constructed the same way with the same
wire characteristics). The source voltage and impedance values of
FIG. 9 remain unchanged but now conventional cable impedance is
0.57+j0.10 Ohms and HTS cable impedance is 0.88+j0.005 Ohms in the
non-superconducting state. Now fault current is only reduced to 48
kA from 51 kA. To reduce the fault current further, a reactor may
be inserted in series with the conventional cable. For example, a
1.4 mH reactor has a impedance of 0+j0.53 Ohms and when that
impedance is added to the conventional cable impedance (because
they are connected in series), the total fault current flowing in
the cables is reduced to 40 kA.
[0078] The net effect of the fault-current limiting cable is to
limit current in its branch of the cable system to a level no
larger than the critical current, protecting the fast switch and
diverting the remaining current to the non-superconducting cable
with its series reactor. In the above example, without the use of
the fault current limiting HTS cable design according to this
disclosure, the fault current in the branch of the cable system may
be significantly higher (e.g., an order of magnitude greater).
However, precise current levels depend on the impedances and power
levels within the electrical path. After the fast switch opens, the
non-superconducting cable with its reactor limits the fault current
until the circuit breaker opens. Through proper choice of the
impedance of the non-superconducting cable and its reactor, the
current can be limited to the desired level. After the
superconducting cable recovers to its superconducting state after a
few minutes, the fast switch can be closed, allowing the system to
resume its original operation.
[0079] A number of implementations have been described.
Nevertheless, it will be understood that various modifications may
be made. Accordingly, other implementations are within the scope of
the following claims.
* * * * *