Electricity: electrical systems and devices – Safety and protection of systems and devices – Superconductor protective circuits
Reexamination Certificate
1997-09-16
2001-05-29
Barrera, Ramon M. (Department: 2832)
Electricity: electrical systems and devices
Safety and protection of systems and devices
Superconductor protective circuits
C335S216000, C505S885000, C174S015400
Reexamination Certificate
active
06239957
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a current limiting device.
2. Description of the Related Art
Current limiting devices are used in a wide variety of applications to handle fault conditions when a current surges above a safe limit. In high current applications such as power supply lines and the like, it has been known in the past to pass the current through a coil which is provided about a leg of an iron former. Another leg of the former provides the core of a superconducting coil which is activated to hold the iron former in a saturated condition. Thus, under normal conditions, the iron is saturated and so effectively the coil carrying the current sees an air core. When a fault occurs, the current rises causing a consequent increase in the magnetic field generated by the coil which opposes the field due to the superconducting coil. This causes an increase in the permeability of the iron core and this increases the voltage across the coil carrying the current which limits the current being carried.
Although this current fault limiter is effective, it is very expensive due to the need to provide the iron core and the complexities due to the need to cool the superconducting coil to liquid helium temperatures.
More recently, it has been proposed to use a superconducting switch. In this case, a length of high temperature (HTc) superconductor is placed into the circuit carrying the current. HTc materials have a critical temperature which is relatively high (typically equivalent to a liquid nitrogen temperature) and have a critical current (strictly current density) which varies inversely with an applied magnetic field. If the current carried by the superconductor exceeds the critical current then the material of the conductor makes a transition to a resistive state which acts to limit the current being carried. The critical current value at which this transition occurs can be changed by changing the applied magnetic field.
In U.S. patent application Ser. No. 08/737,080 we describe a current limiting device which can be controlled to recover from a resistive state without terminating the flow of current.
In all these devices, in order to achieve the superconducting condition, the superconductor must be cooled to or below its critical temperature. Conventionally, this is achieved by immersing the superconductor in a boiling liquid coolant, or by passing such a coolant past the superconductor. In the case of high temperature superconductors, the liquid is typically nitrogen which boils at 77K.
One of the problems with this approach to cooling the superconductor is that the rate of heat transfer varies significantly depending upon the temperature difference between the liquid nitrogen and the superconductor. As can be seen in
FIG. 1
, when the superconductor has a temperature in the region of 77K, the boiling point of liquid nitrogen, the rate of heat transfer to the boiling liquid is high as indicated in a region
21
in FIG.
1
. However, at higher temperatures as indicated in the region
22
, the rate of heat transfer suddenly drops to much lower values as the cooled surface becomes occluded by a film of gas (Monroe et al, J. Applied Physics, p619 (1952)). This can lead to a risk of break down of the device since heat cannot be transferred away sufficiently quickly. The result is that the temperature of the superconductor must be very carefully controlled to stay within the region
21
, which is undesirable.
SUMMARY OF THE INVENTION
In accordance with the present invention, an electrical current limiting device comprises an electrical superconductor for attachment in an electrical circuit, the superconductor achieving a superconducting condition at a relatively high temperature; and a cooling system including means for flowing a cooled gas past the superconductor so that heat can be removed from the superconductor by a heat transfer process with the cooled gas.
In contrast to the known arrangements described above, the invention makes use of the forced flow of a gas. As we will show below, heat transfer to a moving gas does not exhibit very different transfer regimes in contrast to the regimes
21
,
22
of a boiling liquid and this has the advantage that the temperature of the superconductor can be controlled over a much wider range than has been possible previously. Furthermore, the apparatus needed to provide the cooling gas is much simpler and thus cheaper, while the temperature to which the superconductor is cooled can be controlled by varying the gas flow rate. Typically, the system will stabilize at a temperature differential of about 20K.
The use of forced gas flow allows greater temperature excursions when the superconductor becomes resistive, without sacrificing heat transfer, so as to prevent burn out, and allow recovery. Furthermore, operating temperatures can be used which are inaccessible using boiling liquids. This broadens the choice of superconducting materials, and frees the designer to choose critical current densities which would not otherwise be possible.
Heat transfer to moving gas can be approximately described by:
Nu
=
0.023
Re
0.8
Pr
0.4
Nu
=
Nusselt
number
=
hD
k
Re
=
Reynold
'
s
number
=
ρ
vD
η
Pr
=
Prandtl
number
=
C
p
η
K
where
h=heat transfer coefficient
D=Hydraulic diameter of duct
K=thermal conductivity
&rgr;=density
v=velocity
&eegr;=viscosity
C
p
=specific heat.
Some representative values are tabulated below for helium and neon at 25 bar:
TABLE 1
D
K
dens
T pres
V
viscos
h
m
W/m{circumflex over ( )}2/K
kg/m{circumflex over ( )}3
K bar
m/s
Ns/m{circumflex over ( )}2
Re
Pr
w/m{circumflex over ( )}2/K
Helium
1.00E−02
6.00E−02
1.96E+01
60
25
1.00E+01
7.00E−06
2.80E+05
7.50E−01
2.80E+03
Neon
1.00E−02
1.70E−02
9.79E+01
60
25
1.00E+01
1.10E−05
8.90E+05
7.80E−01
2.04E+03
Although the heat transfer coefficients h quoted above are lower than those achievable with a boiling liquid, this is compensated for by the fact that a much larger temperature difference is possible between the cooling gas and the superconductor with the result that heat transfer coefficients in the region of 50000 W/m
2
could be achieved.
The cooling system could comprise any conventional system such as a refrigeration system or the like. Preferably, however, the cooling system comprises a closed loop around which gas flows and having compression and expansion means for cooling the gas, the loop extending through or past the superconductor.
With this arrangement, effectively the process gas of a refrigerator is used to cool the superconductor. There is no need for a separate refrigeration system to cool the gas which is then subsequently flowed past the superconductor. This again simplifies construction and complexity and thus reduces cost.
Typically, the compression and expansion means are controllable to adjust the temperature of the gas and hence the temperature to which the superconductor is cooled.
The gas can be any conventional refrigerant but will typically be an inert gas such as helium or neon.
The superconductor can comprise any conventional superconductor which superconducts at a relatively high temperature. By “relatively high temperature” we mean temperatures above those of liquid helium (4K), currently known high temperature superconduct superconducting at temperatures at or below 77K. A preferred example is YBa
2
Cu
3
O
7
although other materials such as Nb
3
Sn (Tc≈18K) operating at 10K could be used.
In some applications the electrical superconductor will be connected in series in the electrical circuit. In other applications, the electrical superconductor forms the secondary of a transformer and is short-circuited, the primary being connected in the electrical circuit.
REFERENCES:
patent: 3141979 (1964-07-01), Rinia et al.
patent: 3308310 (1967-03-01), Burnett
patent: 332
Hanley Peter
McDougall Ian Leitch
Barrera Ramon M.
Oxford Instruments (UK) Ltd.
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