Resistive fault current limiter

Details Resistive fault current limiter Resistive fault current limiter

1998-02-09

2001-08-14

Leja, Ronald W. (Department: 2836)

Electricity: electrical systems and devices

Safety and protection of systems and devices

Superconductor protective circuits

C361S058000, C174S125100

Reexamination Certificate

active

06275365

ABSTRACT:

BACKGROUND OF THE INVENTION
This invention relates to fault current limiters.
Fault currents are large (usually temporary) increases in the normal current flowing in a power transmission system. A fault current can occur from any number of different events including lightning strikes or catastrophic failure of electrical equipment which can cause short circuits. A short circuit, for example, can cause a twenty-fold or more increase in current flow to the circuit.
Conventional circuit breakers are used in virtually every power transmission and distribution system to “open” the circuit and interrupt current flow in the event of a fault. The fault current level grows as new equipment is added over time. However, with an increase in the magnitude of fault current comes an increase in the size and expense of the circuit breaker. Moreover, conventional circuit breakers do not open instantaneously. The fault current is generally first detected by a current sensor which generates a signal to a control circuit. The control circuit processes the signal and then generates a control signal to open the circuit breaker. During these steps (which may have a duration as long as 100 msec), the circuit breaker, as well as other parts of the transmission system are subjected to the higher fault current level. Thus, the circuit breaker, transformers, as well as other components of the system are often rated to withstand, for a period of time,the higher current levels.
Fault current limiters were developed to insert impedance in a connection quickly so as limit the magnitude of the fault current to a reduced level, thereby protecting the circuit breaker and the power transmission system. Many fault current limiters include tuned reactance circuits which store energy in proportion to the circuit inductance.
The transition characteristics of superconducting materials have been used advantageously to develop superconducting fault current limiters. For example, in one conventional approach, a superconducting current limiting device is constructed using a heat dissipating wafer (e.g., sapphire) having a thin coating of superconducting material deposited onto a surface of the wafer. When a fault is detected, the coating transitions into its normal state and becomes resistive, thereby limiting the flow of current until a circuit breaker, in parallel with the device, interrupts the current flow.
In another conventional approach, a resistive superconducting solenoid is connected within a bridge circuit of diodes or thyristors. In normal operation, current flows through forward-biased pairs of the diodes, bypassing the superconducting solenoid. On the other hand, when the level of current increases above a threshold fault level, the diodes or thyristors become reversed-biased causing the current to flow through the resistive superconducting solenoid which becomes resistive due to the high level of current. In still other approaches, bulk superconducting rods or rings are used to limit the level of fault currents.
SUMMARY OF THE INVENTION
The invention features a superconducting fault current limiter configured as a coil assembly to provide a minimized inductance and low resistance in a normal state of operation. When a fault current exceeds a predetermined threshold, the resistance increases to a level sufficient for limiting the flow of fault current. These electrical characteristics are achieved within a relatively small, compact coil arrangement and provide protection to electrical utility systems or other transmission systems on-demand and in a reliable manner.
In a general aspect of the invention, the coil assembly includes adjacent bifilar pancake coils electrically connected so that current in adjacent turns of the adjacent pancake coils flows in opposite directions. The bifilar pancake coils are disposed in a stack arrangement along the longitudinal axis. Each coil includes a pair of conductive winding sections joined along an innermost radial region of the coil, wound together, one over the other, radially outward and around the longitudinal axis.
Magnetic coils are generally constructed to provide a large inductive component in a relatively compact space. This coil arrangement, on the other hand, provides a coil which minimizes the total inductance while providing a required resistance when current flowing through the coil exceeds a threshold value, all within a compact space. The coil arrangement includes a pair of parallel wound, bifilar windings so that current in one segment of the winding flows in a direction opposite to that of an adjacent segment of the winding. Because the currents flow in opposite directions, magnetic flux and inductance induced by these currents are minimized. Thus, a relatively compact superconducting fault current limiter with minimum inductance and high resistance in its non-superconducting state is provided.
In another aspect of the invention, two or more of the coil assemblies are positioned on a support structure to provide a fault current limiter assembly.
In another aspect of the invention, a method of providing a coil includes electrically connecting adjacent pancake coils so that current flowing in adjacent turns of adjacent pancake coils flows in opposite directions.
Embodiments of the above aspects of the invention may include one or more of the following features.
Electrically connecting adjacent bifilar pancake coils in a series is one approach for providing an arrangement in which current flows in adjacent turns of adjacent pancake coils in opposite directions. The winding sections are each formed from an integral piece of conductive material provided in bifilar fashion. A second pair of conductive windings joined along an innermost radial region of the coil (i.e., a second bifilar winding) is wound over and connected in parallel to the first-mentioned conductive winding in order to increase the current-carrying capacity of the conductive winding sections.
The conductive winding is formed of a superconducting material (e.g., high or low temperature superconductor). In embodiments using an HTS superconductor, the material is anisotropic and is in the form of a superconducting tape. In certain applications, the anisotropic superconducting material is a multi-filament composite conductor including multiple superconducting filaments enclosed in matrix-forming material. In other embodiments, the superconducting material is not anisotropic and is a monolithic superconductor material.
The fault current limiters have numerous applications including bus-tie and co-generation applications. The superconducting fault current limiter also provides a level of isolation between neighboring power utility transmission and distribution networks, permitting them to be connected together while minimizing the risk that a fault occurring on one will affect the other.
Other advantages and features will become apparent from the following description and the claims.


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Underground Power Transmission by Superconducting Cable, chapter 5.3, pp. 37-38, Ed. E.B. Forsyth, Mar., 1972.
“The Development and Utilization of Superconducting Fault Current Limiters”, VDI Technology-Center Physical Technologies, Dusseldorf, Germany, pp. 44-47 and 57-61, 1995, No Month.

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