Electricity: electrical systems and devices – Safety and protection of systems and devices – Superconductor protective circuits
Reexamination Certificate
2000-01-31
2002-01-08
Tso, Edward H. (Department: 2838)
Electricity: electrical systems and devices
Safety and protection of systems and devices
Superconductor protective circuits
C505S850000
Reexamination Certificate
active
06337785
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to fault current limiters, and more particularly to a fault current limiter using a superconducting coil.
2. Description of the Background Art
A fault current limiter is used in an electric power circuit for instantly restricting excessive current flow caused by a fault or accident such as a short-circuit. Recently, a fault current limiter using a superconductor has been developed. The fault current limiter using the superconductor utilizes a mechanism for bringing the superconductor from a superconducting state to a normal conducting state in case of an accident such as a short-circuit.
A structure of a superconducting fault current limiter of an induction type is disclosed for example in T. Okazaki, P. D. Evans, “A Fault Current Limiter in Toroidal Form to Maximize Effective Jc”,
Proceedings of the
1998
Applied Superconductivity Conference
, California, Sep. 13-18, 1998. The superconducting fault current limiter of the induction type includes a primary coil having a normal conductor connected to an electric power system and a secondary coil having a superconductor with both ends short-circuited. The secondary coil is designed to be in the superconducting state during normal operation. In this state, a magnetic flux generated by the primary coil is cancelled by that generated by an induced current flowing through the secondary coil. If excessive current flows through the primary coil due to accidental short circuit or the like, the current flowing through the secondary coil also increases. When the superconductor of the secondary coil quenches due to the excessive current flow, a quenching resistance (a resistance after transition to the normal conducting state) is generated in the secondary coil. Thus, an induced current flowing through the secondary coil decreases. Accordingly, the magnetic flux generated by the primary coil is not sufficiently cancelled, and an impedance of the fault current limiter increases. The increased impedance restricts current flow generated in case of the accident.
The superconductor used for the superconducting fault current limiter operating as described above is required to have a high critical current value during normal operation and a high quenching resistance value in case of accidental short circuit or the like (when excessive current flow is caused). For the superconducting fault current limiter of the induction type which is disclosed in the above mentioned reference, it is proposed that a plain coil is arranged in a toroidal form. A primary coil is formed of a pancake coil including a copper wire of a normal conductor, whereas a secondary coil is formed of a ring including oxide superconductors of thin films provided on either side of a zirconia substrate. The primary and secondary coils are alternately arranged such that a coil axis is circular. Thus, the coils are arranged in the toroidal form. Such structure allows a magnetic field to be parallel with a surface of the superconductor and a critical current value of the superconductor to be maximum during normal operation. When excessive current flow is caused due to an accident or the like, the generated magnetic field is perpendicular to the surface of the superconductor and a resistance value of the superconductor is maximum. Thus, an amount of the superconductor required for a designed current value during normal operation becomes minimum, and the resistance value of the superconductor in the case of the accident increases. As a result, excessive current flow due to the accident in the fault current limiter of the induction type is effectively prevented.
In Japanese Patent Laying-Open No. 2-101926, a structure of a superconducting fault current limiter of a noninduction type is disclosed which has two superconducting coils connected in parallel. In the superconducting fault current limiter, one superconducting coil is brought into a normal conducting state, so that the other superconducting coil generates an inductance for restricting excessive current flow in the case of the accident.
In the conventional superconducting fault current limiter of the induction type as described in the above mentioned reference, however, an iron core must be positioned at axes of the primary and secondary coils arranged in the toroidal form. This is because it is difficult to ensure sufficient impedance in the case of the accident if the iron core is not used in the fault current limiter of the induction type. In the conventional superconducting fault current limiter disclosed in the above mentioned reference, after the secondary coil is brought from the superconducting state into the normal conducting state, a resistance value of the secondary coil increases in accordance with a current value of the primary coil. Assuming that the resistance value of the secondary coil is infinite, the fault current limiter disclosed in the reference can be considered to be a simple inductor. Thus, to ensure moderate impedance in the case of the accident in accordance with Faraday's law, the inductor must suitably be designed only on the side of the primary coil. As a result, to ensure a prescribed impedance required for restricting excessive current flow in the case of accident, the size of the iron core positioned at the axis of the coil must be increased. This disadvantageously increases a weight of the fault current limiter per se. In addition, an iron core in a prescribed shape must be inserted as axes of a plurality of coils arranged in the toroidal form. This results in a complicated structure of the fault current limiter.
Further, in the superconducting fault current limiter of the induction type disclosed in the reference, the primary and secondary coils are inductively coupled. As a result, ampere turns of the primary and secondary coils do not completely match because of an exciting current or leakage flux. Thus, a magnetic field cannot completely be cancelled.
On the other hand, in a superconducting fault current limiter of a noninduction type disclosed in Japanese Patent Laying-Open No. 2-101926, one of two superconducting coils connected in parallel must always be maintained in the superconducting state. Thus, the fault current limiter must securely be designed to allow a sufficient margin to always maintain one superconducting coil in the superconducting state. In addition, to restrict current flow in the case of the accident by the inductance, a suitable approach must be taken such as to increase the number of turns of the coil such that the inductance attains to an optimum value.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a fault current limiter capable of ensuring a sufficient impedance for restricting excessive current flow caused by an accident without using an iron core.
Another object of the present invention is to provide a compact fault current limiter capable of satisfying a canceling condition of a magnetic field as well as possible during normal operation and allowing a small residual impedance during normal operation.
The fault current limiter according to one aspect of the present invention includes first and second superconducting coils including windings of superconducting lines. The first superconducting coil is electrically connected in series with the second superconducting coil. The first and second superconducting coils generate magnetic fields in opposite directions by current flow. The fault current limiter of the present invention further includes a component selected from a group consisting of a resistor and an inductor, the component being electrically connected in parallel with the second superconducting coil.
In the fault current limiter of the present invention, as the first and second superconducting coils generate magnetic fields in opposite directions during normal operation, so that the magnetic field in the direction of a coil axis is cancelled. If excessive current flow is caused by an accident, one of the first and second superconducting coils
Sumitomo Electric Industries Ltd.
Tso Edward H.
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