superconducting switch

Abstract

The invention relates to a superconducting electric switch. The switch comprises two parallel branches of a superconducting material in a loop and a magnetic field generator that generates a time-varying magnetic field through the loop in a direction generally parallel to the axis of the loop. The magnetic field generator is selectively activated and deactivated to switch the electrical switch between a low resistance state and a higher resistance state. In the low resistance state, there is no magnetic field through the loop, and a transport current flows through the loop. In the higher resistance state, the magnetic field through the loop induces a screening current, such that the sum of the transport current and the screening current is greater than or substantially equal to the critical current of the superconducting material. The switch can be used, for example, in a rectifier or a fault current limiter.

Description

Technology Field

[0001] The present invention relates to an electric switch. A specific application of the present invention is a superconducting switch, but this should not be construed as a limitation to the invention. Background Technology

[0002] Introducing components with similarly low or zero resistance conductive paths into superconducting circuits is typically difficult. Introducing non-superconducting components can cause a degradation in circuit performance, which reduces the overall efficiency of the circuit. Furthermore, when resistance is added to high-current superconducting circuits, additional cooling is required to counteract the heat generated by the resistive components.

[0003] An additional problem associated with introducing resistive components into a superconducting circuit is that any DC current flowing through the circuit will attenuate immediately when the current supply is removed. In contrast, in a superconducting circuit, the current attenuates according to the circuit's time constant (L / R). In other words, a superconducting circuit has the advantage of being able to support the flow of DC current for a long time after the supply has been turned off. The rate of current attenuation in such a superconducting circuit is determined by any non-zero resistance associated with the normal-conducting joints within the circuit.

[0004] Therefore, it is necessary to provide components or arrangements that can operate efficiently in superconducting circuits without significantly degrading their performance.

[0005] One type of component used in these superconducting circuits is the variable resistance current switch. However, there are several known disadvantages regarding conventional current switch devices. For example, thermally controlled switches are generally slow to respond and are unsuitable for high-speed switching applications. Current-controlled switches typically apply current pulses exceeding the critical current of the superconductor, and as a result, this type of switch device is prone to performance degradation. Conventional magnetic field-controlled switches are controlled by high-intensity magnetic fields, but when active, the resistance per unit length of these magnetic field-controlled switches is small, and the switching speed is typically limited by the large inductance of the applied field coil.

[0006] Purpose of the invention

[0007] The present invention aims to provide an electric switch. Alternatively, the present invention aims to provide an electric switch that can be controllably switched between a higher resistance state and a lower resistance state.

[0008] Alternatively, the present invention aims to provide a superconducting electric switch.

[0009] Alternatively, the present invention aims to provide a superconducting electric switch that is smaller than / or more efficient than at least some existing superconducting electric switches, has lower cost / or more fast switching, or has higher switching resistance.

[0010] Alternatively, the present invention aims to provide a rectifier comprising a superconducting electric switch.

[0011] Alternatively, the present invention aims to provide a fault current limiter utilizing a superconducting electric switch.

[0012] Alternatively, the present invention aims to provide at least a useful option to the public.

[0013] Summary of the Invention

[0014] An electric switch is provided according to the first aspect of the present invention.

[0015] According to a second aspect of the present invention, an electric switch is provided configured to switch the resistance experienced by the current flowing between the terminals from a low resistance state to a higher resistance state.

[0016] According to a further aspect of the present invention,

[0017] A loop of a superconducting material, wherein the loop comprises a first branch and a second branch, the first branch and the second branch are electrically connected in parallel between a first terminal and a second terminal, and the loop has an axis substantially perpendicular to the plane of the loop; and

[0018] An electric switch is provided comprising a magnetic field generator configured to generate a time-varying magnetic field through a loop in which the direction of the magnetic field through the loop is generally parallel to the axis of the loop or has a generally parallel component.

[0019] The loop is configured to carry transport current between the first terminal and the second terminal, and

[0020] The magnetic field generator is configured to be selectively enabled and disabled to switch an electrical switch between a low resistance state and a high resistance state, and

[0021] In a low resistance state, the magnetic field generator does not generate a time-varying magnetic field through the loop, and a transport current flows through the loop between the two terminals, and in a higher resistance state, the magnetic field generator generates a time-varying magnetic field through the loop, inducing a screening current in the loop, so that the sum of the transport current and the screening current in one or more branches approaches the critical current of the superconducting material, or otherwise is greater than or substantially equal to the critical current.

[0022] Preferably, the superconducting material is a high-temperature superconductor. The use of a high-temperature superconductor can advantageously enable the present invention to be used at higher temperatures, which can reduce the cost of the electric switch and increase the range of feasible operating temperatures. For example, the superconducting material may include rare-earth barium copper oxide (ReBCO) or any other suitable high-temperature superconductor. For example, the rare-earth component may include yttrium (Y), gadolinium (Gd), samarium (Sm), neodymium (Nd), or any other suitable element. In alternative embodiments of the present invention, the superconducting material may be a traditional low-temperature superconductor such as magnesium diboride (MgB2), bismuth strontium calcium copper oxide (BSCCO), or niobium-titanium alloy (NbTi).

[0023] Preferably, a superconducting material may be included as part of a tape, for example, the tape may comprise a thin film of a superconductor deposited on a metal substrate and / or ceramic layers. In some cases, such a thin film is then coated with an additional outer metal protective layer known to those skilled in the art. However, this should not be construed as a limitation to the invention. For example, the superconducting material may comprise one or more filaments or wires of the superconducting material embedded within a metal matrix.

[0024] In preferred embodiments, the electric switch may include one or more joints between the superconducting elements of the switch, such as joining branches of superconducting material at each terminal. Additionally, joints may be provided between branches of superconducting material. These joints may be composed of non-superconducting material or otherwise have non-zero resistance; for example, the joints may be formed by a superconducting metal solder having non-zero resistance at the operating temperature of the circuit, such as a lead-tin (PbSn) alloy or an indium-silver (InAg) alloy. In some embodiments of the invention, the switch may be configured so as not to have joints between the branches; for example, the branches may be formed by splitting a continuous strip of superconducting material or tape to create two fully joined branches.

[0025] Preferably, each branch of the superconducting material may also include one or more coils of the superconducting material. For example, the first branch may include at least one coil, and the second branch may include at least one coil. The use of superconducting coils can advantageously enable better coupling between the magnetic field generator and the loop. In alternative embodiments, coils may not be used.

[0026] It will be understood that each coil of a superconducting material may include one or more turns. These turns may be arranged around a common axis. Providing one or more turns around a common axis can advantageously enable a simpler configuration and / or better coupling between the time-varying magnetic field generator and the superconducting material.

[0027] When coils are used, it may be advantageous for the coil(s) of each branch of the loop to be wound in a reverse direction (i.e., one coil taking the form of a right-handed helix and another coil taking the form of a left-handed helix). For example, the coil of the first branch of the two branches may be wound clockwise, while the coil of the second branch of the two branches may be wound counterclockwise. In other words, the coils of the first branch may be wound in a first rotational direction, and the coils of the second branch may be wound in a second rotational direction, where the first rotational direction is different from the second rotational direction. This arrangement can advantageously further promote the generation of screening current within the loop. Additionally, by counter-winding the coils, it may be possible to correspond to or minimize the total inductance between the terminals of the switch.

[0028] Preferably, the magnetic field generator may include a supply in the form of an alternating current power source, such as an AC current or voltage source. For example, the magnetic field generator may include an alternating current power source magnetically coupled to a loop of a superconducting material through one or more turns of a conductor.

[0029] Preferably, the magnetic field generator can be magnetically coupled to a loop of superconducting material by a core. In preferred embodiments, the core is a magnetic or ferrous core comprising iron or ferrite, but in alternative embodiments, the core may be an air core or a non-magnetic core known to those skilled in the art. The use of a ferrite core may be particularly advantageous when the switch is used at high frequencies.

[0030] In embodiments where a magnetic core is used, it may be advantageous for the magnetic core to be composed of a closed loop of magnetic material. The use of a closed loop may make it more possible for the magnetic field generated by the magnetic field generator to be magnetically coupled to the branches of the superconducting material.

[0031] Preferably, the magnetic field generator may include a magnetic field generator coil composed of one or more turns of a conductor. Such generator coils may be arranged to generate a magnetic field in the core by coupling current from an AC power source to the core through one or more turns. For example, the generator coil may include one or more turns wrapped around the core. The magnetic field generated in the core may be magnetically coupled to a loop of superconducting material by looping the superconducting material around at least a portion of the core. Additionally, when coils are added to the loop of the superconducting material as described herein, these coils may be used to couple the magnetic field to the superconducting loop. In other words, coils may be wound around the magnetic core to increase magnetic coupling between the magnetic field generator and the superconducting loop.

[0032] In other embodiments of the present invention, the magnetic field generator coil may be placed inside the loops and / or coils of the superconducting material. Alternatively, the generator coil may be wound around the loops and / or coils of the superconducting material. As a further alternative, the generator coil may be placed along a common axis for the loops and / or coils. In each of the arrangements described above, it may be desirable for the generator coil and the superconducting loops and / or coils to have the same axis. That is, the generator coil is arranged such that the axis of the generator coil is generally parallel to the axis of the superconducting loops and / or coils, and the direction of the magnetic field generated by the generator coil through the loops and / or coils is generally parallel to the axis of the loops. It will also be understood that in embodiments of the present invention where coils are not used, the arrangements described above may be applied only to the loops.

[0033] Preferably, the conductor used in the generator coil may be a non-superconducting material such as copper or aluminum. The use of a non-superconducting material can advantageously enable a low-cost magnetic field generator. However, this should not be considered a limitation to the invention, and in alternative embodiments, the generator coil may comprise one or more superconductors such as yttrium barium copper oxide (YBCO).

[0034] Preferably, the magnetic field generator is configured to generate a time-varying magnetic field through the loop in a direction substantially parallel to the axis of the loop. Passing the time-varying magnetic field through the loop in a direction having a substantially parallel component can advantageously be more efficient than passing the magnetic field through one or more conductors of the loop.

[0035] In one or more embodiments of the present invention, the transport current passing between the terminals of the electric switch may be direct current (DC). In alternative embodiments, alternating current (AC) may be used.

[0036] Preferably, the magnetic field generator may be configured to induce a screening current in branches sufficient to cause resistive dissipation in the superconducting material without quenching when activated. It will be understood that quenching of a superconducting material occurs when the temperature exceeds a critical temperature. Therefore, if there is sufficient cooling to keep the superconducting material below the critical temperature, it is possible to allow the total current of the superconducting material to exceed the critical current for the superconductor without quenching. In other words, if local heat dissipation in the superconducting material exceeds the rate at which heat can be removed, there is a high probability of quenching. For example, in one or more applications of the present invention, operating the superconductor in an unquenched state may be beneficial, particularly in applications such as the switch component of a rectifier requiring a fast switching speed.

[0037] In an alternative embodiment of the present invention, the magnetic field generator may be configured to induce a screening current sufficiently large in the branches such that, when activated, the total energy dissipation in the branches is sufficient to quench the superconducting material. While the quenching of the superconducting material is a function of the cooling of the system, it will be understood that in one or more embodiments, the quenching of the superconducting material may be configured to occur when the sum of the transport current and the screening current is equal to or greater than the critical current of the superconducting material. This quenched state may be particularly advantageous because it may be possible to generate a higher current limiting resistance when an electric switch is used in the fault current limiter.

[0038] Preferably, when the magnetic field generator is deactivated, the magnetic field generator may substantially not generate a magnetic field, generate a constant magnetic field, or otherwise generate a magnetic field insufficient to induce a screening current that increases the resistance of the branches.

[0039] Preferably, the branches of the superconducting material may be superconductive when in a low resistance state. For example, it may be advantageous for the branches of the superconducting material to have substantially zero resistance in a low resistance state. It will be understood that the joints between the branches of the superconducting material may have some non-zero resistance as described herein, while still being included within the form of the present invention.

[0040] Preferably, when the magnetic field generator is active, the branches may be in a higher resistance state where they are still superconducting. For example, it may be advantageous for the branches of the superconducting material to remain superconducting while still having a resistance higher than that of the superconducting material in a low resistance state. However, this should not be considered a limitation to the invention, such as the branches of the superconducting material being non-superconducting in the higher resistance state, for example, the superconducting material being quenched, or the current otherwise exceeding the critical current of the superconductor.

[0041] According to a further aspect of the present invention, a control mechanism is provided comprising a rectifier including an electric switch as described herein and a control mechanism configured to control each of at least one electric switch between a low resistance state and a higher resistance state to rectify current from an alternating current source.

[0042] Preferably, the rectifier includes a transformer comprising a primary side and a secondary side, wherein at least one electric switch is connected to the secondary side of the transformer. A control mechanism can control each of the at least one electric switch between a low resistance state and a higher resistance state based on the direction of flow of alternating current in the transformer. For example, the control mechanism may be configured to control at least one electric switch based on the direction of flow of alternating current in the primary side of the transformer.

[0043] In one embodiment, the rectifier may be a half-wave rectifier. In alternative embodiments, the rectifier may be a full-wave rectifier.

[0044] Preferably, the rectifier may include one or more switches according to the present invention.

[0045] According to a further aspect of the present invention, a fault current limiter comprising an electric switch as described herein and a control mechanism configured to place at least one electric switch in a higher resistance state when a fault is detected are provided.

[0046] Preferably, the fault current limiter may include one or more switches according to the present invention.

[0047] From the foregoing description, it becomes apparent that the present invention can provide one or more advantages compared to conventional switches, including the following.

[0048] Higher switching resistance, that is, maintaining superconductivity in a low resistance state while having higher resistance (superconductivity or not) in a higher resistance state;

[0049] A more compact switch, particularly in embodiments where the generator coil and the superconducting coil are coaxially aligned;

[0050] A switch that can operate using a smaller magnetic core compared to previous switches, as resistance is generated in the superconducting loop;

[0051] A more efficient switch that can also have reduced total loss in cryogenic systems

[0052] A simpler structure with lower cost compared to existing superconducting switches;

[0053] A switch capable of transitioning more quickly between a low resistance state and a high resistance state compared to existing switches;

[0054] In particular, in embodiments where the switch is non-inductive or otherwise has low inductance, a switch that takes into account smaller magnetic field generator current and voltage compared to conventional switches;

[0055] A switch structure operable using a smaller magnetic field generator coil size, as there is no need to apply a magnetic field perpendicular to the surface of the superconductor;

[0056] A switch operable using a smaller applied magnetic field, as the magnetic field does not need to penetrate the superconducting material; and

[0057] A switch capable of mitigating or otherwise reducing the occurrence of localized hot spots in a superconductor.

[0058] Additional aspects of the present invention that should be considered in all novel aspects will become apparent to those skilled in the art by reading the following description, which provides at least one example of an actual application of the present invention. Brief explanation of the drawing

[0059] With reference to the drawings below, one or more embodiments of the present invention will be described below without the intention of being limited merely by example. Figure 1 shows an exemplary electric field versus current graph for a high-temperature superconductor. FIG. 2a shows a first embodiment of a switch according to the present invention. Figure 2b shows the switch of Figure 2a in a low resistance state. Figure 2c shows the switch of Figure 2a in a higher resistance state. FIG. 3 shows an additional embodiment of a switch according to the present invention. FIG. 4 illustrates a method of coupling a magnetic field generator to a switch according to the present invention. Figure 5 shows a graph comparing the voltage measured across the terminals of a superconducting switch in the presence and absence of a time-varying magnetic field. FIG. 6 illustrates an alternative method of coupling a magnetic field generator to a switch according to the present invention. FIG. 7 shows a simplified schematic of a full-wave rectifier adopting switches according to the present invention. FIG. 8 shows a simplified schematic of a half-wave rectifier adopting switches according to the present invention. FIG. 9 illustrates one method of configuring a full-wave rectifier employing switches according to the present invention. FIG. 10 shows exemplary current and voltage waveforms for a full-wave rectifier configured according to the embodiment of FIG. 9. FIG. 11 shows a simplified schematic of a fault current limiter employing switches according to the present invention. Specific details for implementing the invention

[0060] Background Technology

[0061] To aid in understanding the present invention, the reader should be familiar with superconducting terms including the following.

[0062] Critical temperature for superconductors; and

[0063] Critical current for superconductors

[0064] However, for the reader, these concepts are briefly discussed below.

[0065] The critical temperature for a superconductor is conventionally defined as the temperature below which the superconductor's conductivity drops to zero or near zero. In other words, a superconductor is said to be in a superconducting state when its temperature is below the critical temperature and in a non-superconducting state when it is above the critical temperature. Many superconductors have critical temperatures close to absolute zero; for example, mercury is known to have a critical temperature of 4.1 K. However, some materials are also known to have much higher critical temperatures, such as 30 to 125 K; for example, magnesium diboride has a critical temperature of about 39 K, while yttrium barium copper oxide (YBCO) has a critical temperature of about 92 K. These superconductors are often generally referred to as high-temperature superconductors (HTS).

[0066] The critical current for a high-temperature superconducting wire or tape is conventionally defined as the current flowing through the high-temperature superconducting wire / tape that causes an electric field drop along a wire of 100 μV / m (= 1 μV / cm). It will be understood that the critical current is a function of both the superconducting material used and the physical arrangement of the superconducting material. For example, a wider tape / wire may have a higher critical current than a thinner tape / wire composed of the same material. However, it will be understood that references to the critical current of superconductors / superconducting materials throughout this specification are made for the sake of simplicity of discussion.

[0067] In superconductors / superconducting materials, current I critical current I c If it is approximately the same, the resistance of the superconductor is non-zero but small. However, I critical current I c If the resistance is much larger, the resistance of the superconductor becomes large enough to cause heat dissipation that can heat the superconductor above its critical temperature, eventually rendering it no longer superconductive. This state is sometimes referred to as "quenching" and can damage the superconductor itself. Some breakdown current limiters utilize the quenching effect of superconductors to achieve a rapid, single switching of high resistance into a power circuit. These breakdown current limiters can generally be reset and reused, but this is usually a slow process due to the need to recool the semiconductor material below its critical temperature.

[0068] Figure 1 shows a representative graph plotting the internal electric field versus current curve for a high-temperature superconductor. You will understand that the electric field shown in this graph is related to resistance through the equation below.

[0069]

[0070] Here,

[0071] E is the electric field, and

[0072] I is the current passing through the superconductor, and

[0073] R is the resistance of the wire,

[0074] L is the length of the wire.

[0075] Therefore, the diagram in Figure 1 relates to the resistance per unit length for a superconductor, and since the curve shown is nonlinear, the resulting resistance for the superconductor is nonlinear with respect to current.

[0076] In this figure, the electric field strength in the superconductor is the critical current for the superconductor ( I c It can be seen that it is practically zero from the following. As the current in a superconductor approaches the critical current, the electric field in the superconductor begins to increase. At the critical current, the electric field in the superconductor is 100 μV / m. Increasing the current in the superconductor further beyond the critical current causes a rapid increase in the electric field strength in the superconductor.

[0077] It will be understood that throughout this specification, the relative resistance of superconducting switches and their components is referred to. It will be understood by those skilled in the art that a superconducting material may have zero or substantially zero resistance when in a superconducting state, and that, accordingly, these resistances are more commonly expressed in terms of the electric field existing across the superconducting material for a given current. However, for the sake of simplicity, relative resistance, low resistance, and higher resistance states are referred to throughout this specification.

[0078] Forms of invention

[0079] FIG. 2a is a schematic diagram relating to a switch (200) according to the present invention.

[0080] The switch (200) includes a superconducting material (202) connected between a first terminal (204) and a second terminal (206). In describing the invention herein, the switch is referred to as "superconducting" including its components. It should be clear to those skilled in the art that this term is not intended to limit the scope of the invention, and it will be understood that under certain conditions the switch and its components may not be in a superconducting state, that is, the switch may be described as superconductive but not superconducting. However, for consistency, the switch and its components may be described as "superconducting" throughout this specification.

[0081] In a preferred embodiment of the present invention, the superconducting material (202) comprises a rare earth barium copper oxide (ReBCO) tape, such as a yttrium barium copper oxide tape or a gadolinium barium copper oxide tape. However, it should be understood that this should not be considered a limitation to the present invention, that the present invention is compatible with other superconducting materials including high-temperature superconductors, and that the use of the tape should not be considered a limitation.

[0082] Transport current in reality I t It is optionally supplied to flow between the first terminal (204) and the second terminal (206). In other words, transport current I t It can be turned off and turned on as needed.

[0083] In FIG. 2a, it will be understood that for the sake of simplicity of the preceding discussion, the first terminal (204) is depicted as the positive terminal and the second terminal (206) is depicted as the negative terminal. Similarly, transport current I t It is illustrated as flowing from the first (positive) terminal (204) to the second (negative) terminal (206) as expected for a previously defined direct current (DC) voltage. However, the use of DC voltage should not be considered a limitation to the invention, as is evident from the preceding discussion. It will also be understood that when DC current is used, the superconducting material (202) suffers negligible losses due to its low, zero or near-zero resistance. When AC current is used, although the total loss is significantly less than that of an equivalent non-superconducting material, the loss will generally be greater due to parasitic effects.

[0084] The superconducting material (202) between the two terminals (204, 206) is formed into a loop (210) containing two electrically parallel superconducting branches (212a, 212b).

[0085] Superconducting branches (212a, 212b) may be formed by combining two or more superconducting materials using a method known to a person skilled in the art, including a method of providing non-zero joint resistance. Alternatively, branches (212a, 212b) may be formed by dividing a superconducting tape into two parallel branches, for example, using any method that provides a substantially zero resistance joint.

[0086] In practice, the transport current is applied between the first terminal (204) and the second terminal (206) using an electric circuit (not shown) connected between the first terminal (204) and the second terminal (206). This electric circuit may include a power source such as a voltage source or a current source, a transformer, or any other suitable electronic circuit known to a person skilled in the art. Time-varying magnetic field B app (t)The magnetic field is then optionally applied to or in the loop (210) in a direction parallel to the axis of the loop (210), that is, when the axis of the loop (210) is perpendicular to the plane of the loop (210). In FIG. 2a, a time-varying magnetic field is applied to the loop (210) so that the magnetic field passes through the loop (210). It may be advantageous to provide a time-varying magnetic field in a direction substantially or at least partially perpendicular to the plane of the loop to reduce losses otherwise caused by a magnetic field penetrating or attempting to penetrate the superconducting material (202).

[0087] These time-varying magnetic fields B app (t) Screening current to counteract flux change in the loop (210) I s Allows the screening current to flow around between the branches (212a, 212b) of the loop (210). I s This is added to the transport current flowing around the loop (210), and as a result, the total current flow increases. This increase in current (e.g., as shown in FIG. 1, the current is a critical current) I c A slight increase in the resistance of the superconducting material when smaller than) or (e.g., current as shown in Fig. 1) I c (When the current is close to, greater than, or equal to the critical current for a superconductor) it can cause a significant increase in the resistance of the superconducting material.

[0088] Refer now to FIG. 2b, which shows how the switch (200) of FIG. 2a operates when there is no time-varying magnetic field, or when there is a non-time-varying magnetic field or a weak time-varying magnetic field. In FIG. 2b, the current flowing through the loop (210) is a transport current. I t It can be seen that it is substantially identical to. As a time-varying magnetic field is applied, or as the magnitude of the magnetic field increases or decreases, the screening current as shown in Fig. 2c I s A flows around the loop (210). Transport current in either of the branches (212a, 212b) I t and screening current I s When the sum approaches or exceeds the critical current of the superconductor, the effective resistance of the branches (212a, 212b) increases. As a result, a time-varying magnetic field is applied to transition the switch between a low resistance state and a higher resistance state.

[0089] The low resistance state of the switch (200) can be considered the same as the closed state of the switch (200), while the higher resistance state is similar to the open state of the switch (200). However, the higher resistance state is not an electrical open circuit as is typical for mechanical switches, T c If further heating does not occur, the loop is maintained in a superconducting state, indicating a higher resistance conductive state.

[0090] In some embodiments, the low-resistance state may be a superconducting state in which the resistance of the branches (212a, 212b) of the loop (210) is close to zero or substantially zero. In other words, in a preferred embodiment, at least a portion of the loop (210) is in a superconducting state during the low-resistance state of the switch. However, this should not be considered a limitation to the invention, and in alternative embodiments, the low-resistance state may be partially superconducting. For example, one or more low-resistance joints are formed between the superconducting elements of the loop by means of a normal conducting metal soldering joint.

[0091] Similarly, the high resistance state may be a superconducting state in which the resistance of the branches (212a, 212b) of the loop (210) is close to zero but greater than the resistance in the low resistance state. In one application of the invention, the resistance of the branches (212a, 212b) of the loop (210) in the high resistance state may be substantially non-zero. For example, the loop (210) may be in a non-superconducting state or partially superconducting state.

[0092] Accordingly, it will be understood that the present invention provides a switch capable of transitioning between a low resistance state and a high resistance state when exposed to a time-varying magnetic field or when the magnitude of the time-varying magnetic field changes. This advantageously enables the configuration of a contactless switch with faster switching and more reliable performance compared to conventional current switches. Other potential advantages include the ability to generate the following switching elements:

[0093] As is evident from the preceding discussion, it is more compact compared to conventional switches;

[0094] Faster switching than thermal switches, especially in applications where the higher resistance state of the switch is a superconducting state;

[0095] Higher off-resistance can be achieved compared to conventional AC field control switches;

[0096] It is more efficient than conventional switches because the loss in the superconductor is reduced due to the extraction of the magnetic field in the superconductor;

[0097] The switch is more reliable than conventional switches because it is contactless and has lower loss; and

[0098] Lower inductance considering a lower power driving source for the magnetic field generator.

[0099] Further embodiments relating to a switch (300) according to the present invention are illustrated in FIG. 3. As in the previously described embodiment, the superconducting material (202) is arranged in a loop (310) comprising two electrically parallel branches (312a, 312b). However, in this embodiment, each branch (312a, 312b) comprises a coil (314a, 314b) of the superconducting material, and each coil comprises one or more turns.

[0100] It will be understood from reference to the drawings and the foregoing description that the loop of the present invention comprises two or more electrically parallel conductive paths. On the other hand, each of the coils of the present invention has a single conductive path composed of one or more turns.

[0101] In reality, time-varying magnetic field B app It is optionally applied through the loop (310) across both (314a, 314b) in a direction perpendicular to (or having a perpendicular component to) the plane of the loop (310) and / or coils (314a, 314b). This is a screening current I s The current is allowed to flow around a closed loop (310) formed by two parallel-connected branches (312a, 312b). This screening current can be used to selectively transition the switch (300) between a low resistance state and a higher resistance state, according to the embodiments described above.

[0102] While it may be preferable for the first branch (312a) to include one or more coils (314a) wound clockwise, the second branch (312b) to include one or more coils (314b) wound counterclockwise, and vice versa. This arrangement may be useful for causing a screening current to flow around the loop (310) by ensuring that the screening current of the first coil (314a) is induced in the same direction as the screening current of the second coil (314b). Additionally, the use of coils (314a, 314b) can advantageously make it easier to make the inductances of each branch (312a, 312b) actually equal. For example, by using coils (314a, 314b) having one or more turns, the geometric difference between each branch (312a, 312b) becomes a smaller part of the number of current turns of each branch.

[0103] An additional potential advantage of this arrangement is that higher switching resistances can be considered compared to the previous embodiment due to the longer switching lengths in each branch.

[0104] FIG. 4 is a schematic diagram illustrating one actual application of the switch (300) of FIG. 3. In this embodiment, the coupling between the time-varying magnetic field generator (402) and the switch (404) can be improved by directing the magnetic flux to pass through the core (406). In this way, the arrangement of FIG. 4 becomes similar to the arrangement of a transformer as well known to a person skilled in the art.

[0105] The core (406) is preferably a high-permeability magnetic core such as an iron core, but this should not be considered a limitation to the invention, as is evident from the preceding discussion.

[0106] For simplicity, the magnetic field generator (402) is depicted as an AC source (408) magnetically coupled to a core through a coil (410) comprising one or more turns. However, this should not be considered a limitation to the invention, and alternative methods for generating a time-varying magnetic field may be used, for example, by switching a DC current source.

[0107] To simplify the preceding discussion and to distinguish the coil (410) of the magnetic field generator from the coils (414a, 414b) of the switch (402), the coil (410) of the magnetic field generator will be referred to in this specification as the magnetic field generator coil (410) or simply the generator coil.

[0108] It will also be understood that the magnetic field generator coil (410) may be composed of any suitable conductive material, including superconducting materials such as YBCO or non-superconducting materials such as copper and aluminum. In a preferred embodiment, the generator coil (410) is composed of copper. This can advantageously provide a switch at a lower cost compared to similar switches using superconducting generator coils (410).

[0109] Continuing the transformer analogy, the generator coil (410) can be thought of as the primary side of the transformer, while the coils (414a, 414b) can be thought of as the secondary side of the transformer.

[0110] However, unlike traditional transformers, the superconducting coils (414a, 414b) of the switch (404) are wound around the core (406) in a counter-rotating direction and are electrically connected in parallel with each other. As a result, the secondary coils are shortened, and the net flux surrounded by the coils (414a, 414b) is approximately zero when each of the coils (414a, 414b) has the same number of turns. The purpose of the core (406) is to couple the magnetic field applied to the pair of coils (414a, 414b) forming the loop (416). As a result, the screening current generated in the loop (416) resists the flux change from the magnetic field generator (402).

[0111] In practice, the magnetic field generator (402) may be selectively enabled as needed to transition the switch (404) between a low resistance state and a higher resistance state, or its size may be varied in other ways.

[0112] In a low resistance state where the magnetic field generator (402) does not generate a time-varying magnetic field in the core (406) or otherwise generates a weak magnetic field in the core (406), the transport current I t It tends to be shared evenly between the two parallel branches (412a, 412b). In this state, these branches (412a, 412b) have zero or near-zero resistance, and also, because the coils (414a, 414b) are wound in the reverse direction, there is almost no inductance across the terminals (204 and 206). Also, because the coils (414a, 414b) are wound in the reverse direction and connected in parallel, the flux induced from the coils (414a, 414b) to the core (406) is also substantially zero or otherwise very small.

[0113] When the magnetic field generator (402) is activated, or when the magnitude of the applied magnetic field increases or decreases, a screening current in the loop (416) of the superconducting wire as described above I s A transport current is formed in one or both of the branches (412a, 412b). I t and screening current I s If the sum is close to, equal to, or exceeds the critical current of the superconducting material, there will be non-zero resistance in the branch(s). Therefore, to switch the switch (404) between a low resistance state and a higher resistance state, the magnetic field generator (402) may be activated, or alternatively, the size of the magnetic field generator may be changed.

[0114] FIG. 5 illustrates an example of a voltage measured across the terminals (204, 206) of a switch according to one embodiment of the present invention. In this figure, the voltage across the switch is the voltage measured between the terminals (204, 206) of the switch. The magnetic field generator current reflects the magnitude of the current applied to the generator coil (410) by the magnetic field generator (402).

[0115] For completeness, note that a DC current of about 80 Amps is applied across the terminals (204, 206) using a current source. Each of the superconducting coils (414a, 414b) consists of a single turn of 4 mm wide YBCO tape, and the generator coil uses 200 turns of copper wire.

[0116] It can be seen that when there is no time-varying magnetic field (after about 80.5 seconds), the voltage measured across the terminals (204, 206) is practically zero. In other words, when there is no time-varying magnetic field, the superconducting material is superconducting. This indicates a low resistance state of the switch. In contrast, when a time-varying magnetic field is applied (before 80.5 seconds), the voltage measured across the terminals fluctuates, which indicates a change in the resistance of the superconducting material. Additionally, a peak voltage is observed across the terminals when this time-varying magnetic field is applied. This indicates a higher resistance state of the switch. It will be understood that this higher resistance state of the switch can indicate a superconducting or non-superconducting state depending on the application of the switch.

[0117] FIG. 6 illustrates another additional embodiment of the switch (600) according to the present invention. In this embodiment, the generator coil (410) of the time-varying magnetic field generator (402) is coaxially aligned with the coils (414a, 414b) of the switch (600). This can advantageously improve the magnetic field coupling between the superconducting coils (414a, 414b) of the switch (600) and the generator coil (410).

[0118] In the illustrated embodiment, the turns of the superconducting coils (414a, 414b) of the switch (600) are interlaced with the turns of the generator coil (410). However, this should by no means be considered a limitation to the invention. For example, the superconducting coils (414a, 414b) may be axially positioned inside the generator coil (410). Alternatively, the generator coil (410) may be axially positioned inside the superconducting coils (414a, 414b). As another additional alternative, the generator coil (410) may be spaced longitudinally from the coils (414a, 414b). It can also be assumed that the first superconducting coil (414a) may be placed in an alternative position to the second superconducting coil (414b), for example, the first superconducting coil (414a) may be placed below the generator coil (410) and the second superconducting coil (414b) may be placed above the generator coil (410).

[0119] An additional advantage of coaxially aligning the generator coil (410) and the superconducting coils (414a, 414b) is that a non-magnetic core or an air core (602) can be used. This can advantageously reduce the size, weight, and cost of the switch. Additional advantages regarding the use of an air core, such as the ability to induce a higher screening current without saturating the core, will also be apparent to a person skilled in the art.

[0120] Applications of the invention

[0121] FIGS. 7 and FIGS. 8 respectively illustrate representative circuit diagrams relating to a full-wave rectifier (700) and a half-wave rectifier (800) according to embodiments of the present invention. These examples illustrate applications for the switch of the present invention and should not be construed as a limitation on the scope of the present invention. Further applications for the switch will become apparent to those skilled in the art, and in particular, the superconducting switch of the present invention appears to be an improvement over conventional current switches and may be applied to superconducting magnets, magnetic resonance imaging (MRI), nuclear magnetic resonance (NMR), flux pumps, fault current limiters, and magnetic energy storage systems. Alternatively, the switch of the present invention may be used to replace traditional switches, such as AC field switches, semiconductor switches, and / or mechanical switches in a wide range of applications.

[0122] In these embodiments, switches to simplify the preceding discussion S 1 , S 2 It was expressed symbolically. However, these switches S 1 , S 2 It will be understood that this can be constructed using any one of the methods described in this specification. Additionally, for the sake of simplicity in the foregoing discussion, it is assumed that both switch symbols represent a superconducting switch according to the present invention. However, it will be understood that one or more of the illustrated switches may instead be traditional switches such as AC field switches, semiconductor switches such as transistors, or mechanical switches such as relays or resistors.

[0123] Referring first to FIG. 7, it can be seen that the overall circuit topology relates to a known full-wave rectifier (700). The transformer (704) has a primary side (702a) comprising at least one coil and a secondary side (702b) comprising at least one coil. An AC source is provided to the primary side (702a). Two switches on the secondary side (702b) of the transformer (704) S 1 , S 2 This is connected. Switches between the first terminal (708) and the second terminal (710) S 1 , S 2 A load (706) is connected in parallel with it. The first terminal (708) is connected to the intermediate winding of the secondary side (702b) of the transformer (704), for example, through a common central branched connection.

[0124] The general principles regarding such circuits will be well known to those skilled in the art, but for the sake of clarity, their operation is briefly summarized.

[0125] In reality, exchange I 1 The current is applied to the primary side (702a) of the transformer (704). This current is the current flow on the secondary side (702b) of the transformer. I 2 Induces. As is known to a person skilled in the art, the ratio of the primary side (702a) current to the secondary side (702b) current is a function of the ratio of the number of turns on the primary side (702a) of the transformer (704) to the number of turns on the secondary side (702b) of the transformer (704).

[0126] The rectifier (700) includes a control mechanism (not shown). The control mechanism includes switches to rectify the AC source. S 1 , S 2Each state is configured to control. For example, the control mechanism is configured such that the state of each switch is based on the direction of alternating current flow on the primary side (702a) or the secondary side (702b) of the transformer (704). S 1 , S 2 It controls each one. Because the direction of AC flow depends on the phase of the AC, the control mechanism is switches S 1 , S 2 Each state is controlled in a manner configured to operate after a certain period of time.

[0127] When the current on the secondary side (702b) of the transformer (704) flows in the first direction (e.g., when the current is positive), the first switch S 1 While is placed in its low resistance state, the second switch S 2 It is placed in a higher resistance state. This causes a lower impedance path around the upper half of the circuit and causes current flow through the load (706) from the first terminal (708) to the second terminal (710). In this state, the first switch S 1 While is in series with the load, the second switch S 2 is in parallel with a load. This load may be any suitable load known to a person skilled in the art. For example, it may be a load coil or a resistive element. In other additional embodiments, the load may be substantially open circuit or large simply to provide an open circuit voltage.

[0128] One application of the present technology is to attach a superconducting coil as a load. Such a superconducting coil can be attached using normal conducting solder joints, which are incorporated within the embodiments of the present invention.

[0129] When the current on the secondary side (702b) of the transformer (704) flows in the second direction (e.g., when the current is negative), the first switch S 1 While is placed in its higher resistance state, the second switch S 2 is placed in a low resistance state. In this state, the first switch S 1 While is in parallel with the load, the second switch S 2 It is in series with the load. This causes a lower impedance path around the lower half of the circuit and causes current flow through the load (706) from the first terminal (708) to the second terminal (710).

[0130] In this way, regardless of the direction of the current induced in the secondary side (702b) of the transformer (704), the current always flows through the load in a single direction from the positive terminal to the negative terminal. Thus, the alternating current of the primary side (702a) of the transformer (704) is fully rectified into direct current through the load (706).

[0131] In the examples, switches S 1 , S 2 When transitioning from a higher resistance state to a lower resistance state or vice versa, a control mechanism switches to reduce any short circuit or "crowbar" current that may otherwise exist during switching. S 1 , S 2 A control mechanism can be configured to first temporarily transition both of them to a higher resistance state.

[0132] It will be understood that a full-wave rectifier can be configured such that the current path on the secondary side (702b) is substantially superconducting. This can advantageously provide a full-wave rectifier that is more compact or otherwise more efficient than existing designs. Furthermore, it will be understood that the rectifier can maintain a substantially superconducting secondary side current path while also including non-superconducting components. For example, the primary side (702a) of the transformer can be configured using non-superconducting materials such as copper or aluminum, and switches S 1 , S 2 The generator coils (not shown) may also adopt non-superconducting components as described in this specification. Additionally, joints between any components of the present invention, including joints of superconducting elements, may be formed using non-superconducting materials such as metal solder while still being included within the forms relating to the present invention.

[0133] You will understand that in rectifier applications, a short switching time between the high resistance state and the low resistance state can be important. In these applications, since quenching of the superconducting material can introduce a thermal time lag after switching during which the superconductor must be cooled back to its operating temperature, it may be desirable for the superconducting material to be kept below its critical temperature.

[0134] Now, referring to FIG. 8, it can be seen that the general circuit topology relates to a known half-wave rectifier (800). The transformer (704) has a primary side (702a) comprising at least one coil and a secondary side (702b) comprising at least one coil. An AC source is provided to the primary side (702a). As shown, two switches on the secondary side (702b) of the transformer (704) S 1 , S 2This is connected. One of the switches, in the example of FIG. 8 S 2 A load (706) is connected in parallel across the. The load (706) is connected between the first terminal (708) and the second terminal (710).

[0135] The general principles regarding such circuits will be well known to those skilled in the art, but for the sake of clarity, their operation is briefly summarized.

[0136] As in the previous embodiment, AC to the secondary winding (702b) of the transformer on the primary side (702a) of the transformer (704). I 2 Exchange that induces I 1 is supplied.

[0137] The rectifier (800) includes a control mechanism (not shown). The control mechanism includes switches to rectify the AC source. S 1 , S 2 Each state is configured to control. For example, the control mechanism is configured such that the state of each switch is based on the direction of alternating current flow on the primary side (702a) or the secondary side (702b) of the transformer (704). S 1 , S 2 It controls each one. Because the direction of AC flow depends on the phase of the AC, the control mechanism is switches S 1 , S 2 Each state is controlled in a manner configured to operate after a certain period of time.

[0138] Secondary side current I 2 When the current flows in the first direction (i.e., when the current is positive), the first switch S 1 is placed in its low resistance state, and the second switch S 2It is placed in a higher resistance state. In this way, from the transformer secondary side (702b), the switch S 1 A low-resistance path is formed around the outer circumference of the loop, passing through and across the load (706) from the first terminal (708) to the second terminal (710). As the polarity of the current changes (e.g., from positive to negative), the switch S 1 It transitions to its high resistance state, and the switch S 2 It transitions to a low resistance state. S 1 The higher resistance state of obstructs the current flow from the transformer, providing a certain degree of blockage for negative polarity current flow. At the same time, S 2 The low resistance state of provides a path that allows current flow through the load to continue, even while attenuating. Therefore, the current flow through the load is half-wave rectified.

[0139] A person skilled in the art can see the switches of the form relating to the present invention shown in FIGS. 7 and 8 S 1 , S 2 You will understand an appropriate control mechanism that ensures the state of is controlled as described. For example, switches S 1 , S 2It may have individual time-varying magnetic field generators that are activated in a state synchronized with the polarity of the transformer current (either primary or secondary) to achieve a low resistance state and a higher resistance state. This can be achieved using any known control mechanism, including using a half-wave rectified current signal as a means to activate the magnetic field generators. Such a half-wave rectified current signal may be provided as an additional secondary winding for the transformer (704) or by any other method known to a person skilled in the art.

[0140] However, it will be understood that the magnetic field generators do not need to be activated in synchronization with the transformer current. For example, around the zero crossing of the transformer current ( S 1 or S 2 It can be advantageous to have a bit of dead time (where none of them are activated or in a low resistance state).

[0141] In embodiments provided with multiple switches, such as the half-wave rectifier (800) and full-wave rectifier (700) described herein, it will also be understood that a single magnetic field generator may be configured to generate a screening current to one or more switches. For example, the magnetic field generator may have two or more generator coils, each configured to generate a time-varying magnetic field across each of the individual switches. The magnetic field generator may then be configured to alternate which of the generator coils is driven based on the polarity of the current to be regulated. In one such embodiment, the control mechanism of the rectifier includes a mechanism that controls the magnetic field generator to operate in this manner. The use of a single magnetic field generator in rectifier applications can advantageously reduce the size, cost, and / or complexity of the resulting rectifier.

[0142] Refer now to FIG. 9, which illustrates an example of how the full-wave rectifier (900) of FIG. 7 can be configured according to the present invention. In this embodiment, the driving current or transport current for the circuit I t / I 2 A driving transformer (902) is provided to generate. In a preferred embodiment, the driving transformer (902) is a step-down transformer; that is, it may be desirable for the driving transformer (902) to have a greater number of turns on its primary side (904a) compared to its secondary side (904b) to achieve the high current used in a superconducting circuit. For example, this driving transformer (902) may have a ratio of turns between 300:1 and 600:1. In the illustrated embodiment, the transformer has about 900 primary turns and two secondary turns for a ratio of 450:1. Additionally, reducing the number of turns on the secondary side (904b) of the driving transformer (902) can reduce costs as it requires relatively fewer turns of the expensive superconducting material. Using a large ratio of turns can also be useful for generating a large superconducting AC secondary current using a normal conducting primary current. To achieve this, using a smaller number of primary turns reduces resistive losses on the primary side while keeping the size small. In alternative applications where reducing flux leakage is advantageous, increasing the number of turns on the secondary side may be beneficial.

[0143] It will be understood that this driving transformer (902) substantially reflects the transformer (704) of FIG. 7.

[0144] In this drawing, a first switch (906) and a second switch (908) are also shown. Each switch includes a coupling core (910a, 910b) and a time-varying magnetic field generator (912a, 912b). The coupling cores (910a, 910b) are the switches of FIG. 7 S 1 and S 2 To activate it, the magnetic field generated in the generator coils (914a, 914b) is used to couple to the respective coils (916a, 916b, 916c, 916d). The illustrated cores (910a, 910b) are preferably ferrite cores or laminated steel / iron cores. However, it will be understood that alternative cores having high relative permeability at the operating frequency may also be used. In alternative embodiments of the invention, the cores may instead comprise an air core as described with respect to FIG. 6 or a substantially non-magnetic material.

[0145] In a preferred embodiment, the switches (906, 908) include a greater number of turns in the generator coils (914a, 914b) compared to the number of turns in the superconducting coils (916a, 916b, 916c, 916d). This ratio can advantageously enable a greater flux density in the superconducting coils without requiring a large number of turns of the relatively expensive superconducting material. For example, each switch (906, 908) may have a ratio of turns between 50:1 and 150:1. In the illustrated embodiment, each of the switches (906 and 908) has about 200 primary (generator coil) turns and 2 secondary (superconducting coil) turns for a ratio of 100:1.

[0146] One advantage of the structure illustrated in FIG. 9 is that the winding on the primary side (904a) of the drive transformer is electrically insulated from the superconductor winding on the secondary side (904b) of the drive transformer (902). Additionally, the generator coils (914a, 914b) of each switch (906, 908) are electrically insulated from each coil (916a, 916b, 916c, 916d) of the superconducting material. This can advantageously result in a safer structure compared to conventional rectifier circuits and can also enable DC magnets to be charged while operating at high current.

[0147] It will be obvious to a person skilled in the art that this topology reflects the topology of FIG. 7 exactly as it is. Accordingly, the discussion regarding the operation of FIG. 7 also applies to the embodiment of FIG. 9.

[0148] A suitable control mechanism for controlling the timing of changing the state of the switches (906 and 908) between a low resistance state and a higher resistance state in a working form is the primary current on the primary side (904a) of the driving transformer (902). I 1 and currents in the generator coils (914a and 914b) of the switches (906 and 908) I s1 and I s2 It may include connections between. Alternatively, currents I s1 and I s2 2nd current I 2 It can be controlled based on or by a third coil (not shown) coupled to the drive transformer (902). For example, the primary current on the primary side (904a) of the drive transformer (902). I 1 When it flows in the first direction (e.g., when it is positive) I s1 A flows, and the primary current at the primary side (904a) of the driving transformer (902) flows. I 1 When it flows in the second direction (e.g., when it is negative) I s1 To prevent flow, and primary current at the primary side (904a) of the driving transformer (902) I 1 When it flows in the second direction (e.g., when it is negative) I s2 A flows, and the primary current at the primary side (904a) of the driving transformer (902) flows. I 1 When it flows in the first direction (e.g., when it is positive) I s2 The connection can be configured so that no current flows. As previously mentioned, it may be advantageous for the control mechanism to include a dead-time period around the zero crossing for the transformer current (where neither of the switches (906, 908) is in a low resistance state).

[0149] In one example relating to the present invention, the control mechanism is currents I s1 and I s2 It includes one or more windings connected to a drive transformer (902) that supplies currents. As previously mentioned, currents I s1 and I s2 It may be advantageous for it to be activated in a manner synchronized with the current of the drive transformer (902). For example, when the drive current is positive, the current I s1 It can be activated, and current I s2 is deactivated. Similarly, when the driving current is negative, the current I s1 It can be activated, and current I s2 It is activated (or vice versa). This can be achieved using any control method known to a person skilled in the art, including semiconductor switches such as diodes and transistors. For example, a diode may be provided to activate the time-varying magnetic field generators (912a, 912b) in a state synchronized with each phase of the driving current. In some examples, the forward voltage drop of the semiconductor switch (e.g., diode) may provide a dead time period to the circuit when neither of the switches is conducting.

[0150] In other examples relating to the invention described in this specification, currents I s1 and I s2 It may be advantageous to operate at a frequency higher than the driving current. In these examples, the control mechanism based on the polarity of the driving current, the currents I s1 and I s2 It may be configured to activate. For example, the driving current may be configured to enable the time-varying magnetic field generators (912a, 912b) based on the polarity of the driving current. For example, a semiconductor switch, such as a transistor, may be used to turn on the time-varying magnetic field generator (912a) when the driving current is positive and to turn on the time-varying magnetic field generator (912b) when the driving current is negative. Alternatively, a single time-varying magnetic field generator may be used to switch the output of the time-varying magnetic field generator between the switches (906 and 908) using methods obvious to a person skilled in the art, and the phase of the driving current may be used.

[0151] FIG. 10 shows the resulting current and voltage waveforms measured on a circuit configured according to the embodiment of FIG. 9. This figure includes the following five graphs shown from top to bottom.

[0152] Current applied to the primary coil (904a) of the driving transformer (902);

[0153] Current measured in the secondary (superconducting) coil (904b) of the driving transformer (902);

[0154] Current applied to the generator coil (914a) of the first time-varying magnetic field generator (912a);

[0155] Current applied to the generator coil (914b) of the second time-varying magnetic field generator (912b); and

[0156] The resulting open circuit voltage measured across the terminals (204, 206).

[0157] From these graphs, it can be seen that the frequency of the current applied to the drive transformer (902) is lower than the frequency of the current generated in the generator coils (914a, 914b) of the switches (906, 908). In reality, the drive frequency is about 4 Hz and the screening current is about 200 Hz. By using a higher frequency in the generator coils (914a, 914b), it is possible to ensure that at least one cycle of the screening current is completed during the time the switches (906, 908) are powered. As illustrated in FIG. 10, while the switch is powered (i.e., in its higher resistance state), it may also be advantageous for the switches to complete two or more cycles of the screening current, as this can provide a more stable higher resistance state.

[0158] The resulting open-circuit voltage waveform across the terminals (204, 206) is shown as full-wave rectified. In other words, regardless of the polarity of the current of the drive transformer (902), the voltage across the terminals (204, 206) has negative voltage peaks corresponding to the high resistance states of each of the switches (906, 908). As the voltage across the terminals (204, 206) has a consistent polarity (negative) during regulation, the resulting current flowing from the terminals (204, 206) to the load (706) will be similarly consistent, thereby full-wave rectifying during the period when one or more of the switches are in a higher resistance state. In other words, the circuit of FIG. 9 can convert AC current to DC under superconducting conditions.

[0159] It can be seen that when both switches (906, 908) are inactive, the output voltage across the terminals (204, 206) of the rectifier is substantially zero. One application for this rectified DC voltage would be the charging of a superconducting magnet. However, this is not to be considered a limitation to the invention, and other applications will be apparent to those skilled in the art.

[0160] FIG. 11 shows an active fault current limiter (1100) that adopts a switch (1102) according to one embodiment of the present invention.

[0161] In this embodiment, the AC power source (1104) is connected in series with the switch (1102) and the load (1106). In practice, the time-varying magnetic field generator (1108) of the switch (1102) is inactive or generates a time-varying magnetic field of sufficiently small magnitude so that the current passing through each of the branches (1110a, 1110b) of the loop (1102) does not exceed the critical current of the superconducting material (1114). This results in a low resistance state in the switch (1102) and a minimum voltage drop across the superconducting material (1114). When a fault (1116), such as an overload or short circuit, is applied to the circuit, an increase in current can be detected using any method known to a person skilled in the art. In order to limit the total power to be delivered to the fault (1116) by transitioning the switch (1102) to a higher resistance state upon such detection, the control mechanism may be configured to activate the time-varying magnetic field generator (1108) or increase its driving current.

[0162] When the fault current limiter (1100) is adopted in a domestic AC or grid power application (typically 50 to 60 Hz), it may be advantageous to drive the time-varying magnetic field generator (1108) at a frequency of several kilohertz or higher, e.g., 2 kHz to 100 kHz. This can ensure that multiple screening cycles are provided for each cycle of the source voltage, which can ultimately provide a more stable current limit in the circuit.

[0163] The fault current limiter (1100) of the present invention may have several advantages over existing fault current limiters, such as the following.

[0164] The ability to rapidly transition the switch (1102) to a higher resistance state; for example, the fault current limiter (1100) can be activated within milliseconds after detecting the fault state.

[0165] The ability to actively enable the fault current limiter (1100) based on arbitrary criteria; for example, to select the current at which the fault current limiter (1100) is activated, to determine when the fault current limiter (1100) is deactivated, or to dynamically control the triggering fault current based on the current load in the circuit.

[0166] Providing a safe, low rebound voltage to the AC power source (1104). In other words, since the switch (1102) can be configured with a low or substantially zero inductance, the spikes that may occur due to the switching of inductive elements can be reduced by limiting the current flow using a substantially resistive load.

[0167] It will also be understood that the present invention can enable greater resilience to damage caused by hot spots. Generally speaking, hot spots are the critical current of a superconducting wire I c It describes a phenomenon in which superconducting materials are locally heated by the transport current due to a local reduction of [value]. This I c Local reduction of can occur due to defects in the superconducting material, or due to localized thermal or magnetic states that can eventually cause localized heating and lead to a significant local temperature rise. This temperature rise can eventually damage the superconductor. This situation can be particularly dangerous in fault current limiters.

[0168] Because the present invention can generate a screening current flowing around a full loop of a superconducting material, a significant portion, even if not the entire superconductor, can be induced into a higher resistance state. As such, the switch according to the present invention can reduce localized heating effects by quenching more homogeneously along a longer length of the superconducting wire compared to conventional switches.

[0169] Accordingly, applications of the present invention include superconducting magnets, magnetic resonance imaging (MRI), nuclear magnetic resonance (NMR), flux pumps, fault current limiters, and magnetic energy storage systems.

[0170] Unless clearly required otherwise in the context, throughout the detailed description and claims, words such as “comprise” and “comprising” must be interpreted in an inclusive sense, as opposed to an exclusive or exhaustive sense—that is, in the sense of “including, but not limited to”.

[0171] If any, all disclosures of all applications, patents, and published documents cited above and below are incorporated herein by reference.

[0172] Any reference to any prior art in this specification is not intended to acknowledge, nor should it be construed in any form, that such prior art forms part of the common general knowledge of the field in which efforts are being made in any country worldwide.

[0173] The features of the present invention are also generally in the parts, elements, and features mentioned or shown in the specification of this application, and may be in any or all combinations of two or more of said parts, elements, or features.

[0174] Where components having integers or known equivalents thereof are referred to in the preceding description, such integers are included in this specification as if presented individually.

[0175] It will be understood by those skilled in the art that various changes and modifications to the preferred embodiments described herein will be apparent. Such changes and modifications may be made without departing from the spirit and scope of the invention and without diminishing the accompanying advantages. Accordingly, such changes and modifications are intended to be included in the invention.

Claims

Claim 1 In an electric switch, the loop of the superconducting material comprises a first branch and a second branch, wherein the first branch and the second branch are electrically connected in parallel between a first terminal and a second terminal, the loop has an axis perpendicular to the plane of the loop, and the loop is configured to carry a transport current between the first terminal and the second terminal; and a magnetic field generator, wherein the magnetic field generator is configured to generate a time-varying magnetic field through the loop such that the direction of the time-varying magnetic field through the loop has a component parallel to at least the axis of the loop, and the magnetic field generator is configured to be selectively controlled to switch the electric switch between a low resistance state and a higher resistance state, wherein in the low resistance state, the magnetic field generator does not generate a time-varying magnetic field through the loop, or the time-varying magnetic field through the loop is relatively weak, and the transport current flows through the loop between the first terminal and the second terminal, and in the higher resistance state, the magnetic field generator generates a time-varying magnetic field through the loop, or the time-varying magnetic field through the loop is increased to induce a screening current in the loop, such that the sum of the transport current and the screening current at one or more of the first branch and the second branch approaches the critical current of the superconducting material, or the critical current and An electric switch that is equal to or greater than the above threshold current. Claim 2 An electric switch according to claim 1, wherein the superconducting material comprises a high-temperature superconductor. Claim 3 An electric switch according to claim 1, wherein the superconducting material comprises rare-earth barium copper oxide (ReBCO). Claim 4 An electric switch according to claim 1, wherein the superconducting material is included as part of a superconducting tape. Claim 5 An electric switch according to claim 1, wherein the electric switch further comprises a first joint connecting the first branch and the second branch to the first terminal, and a second joint connecting the first branch and the second branch to the second terminal. Claim 6 An electric switch according to claim 5, wherein the joints comprise a non-superconducting material. Claim 7 An electric switch according to claim 1, wherein the first branch comprises one or more coils of the superconducting material. Claim 8 An electric switch according to claim 7, wherein the second branch comprises one or more coils of the superconducting material. Claim 9 An electric switch according to claim 8, wherein the coils of the first branch are wound around the same axis as the coils of the second branch. Claim 10 An electric switch according to claim 8, wherein the coils of the first branch are wound in a first rotational direction and the coils of the second branch are wound in a second rotational direction, and the first rotational direction is different from the second rotational direction. Claim 11 An electric switch according to claim 1, wherein the magnetic field generator includes an alternating current power source. Claim 12 An electric switch according to claim 1, wherein the magnetic field generator is magnetically coupled to a loop of the superconducting material by a core. Claim 13 An electric switch according to claim 12, wherein the core is a magnetic core comprising iron or ferrite. Claim 14 An electric switch according to claim 13, wherein the core comprises a closed loop of magnetic material. Claim 15 An electric switch according to claim 1, wherein the magnetic field generator comprises a magnetic field generator coil comprising one or more turns of a conductor. Claim 16 An electric switch according to claim 15, wherein the conductor comprises a non-superconducting material. Claim 17 An electric switch according to claim 1, wherein when the magnetic field generator is deactivated, the magnetic field generator does not generate a magnetic field or generates a constant magnetic field. Claim 18 An electric switch according to claim 1, wherein the branches of the superconducting material are superconducting in the higher resistance state. Claim 19 A rectifier comprising at least one electric switch according to any one of claims 1 to 18 and a control mechanism configured to control each of the at least one electric switch between the lower resistance state and the higher resistance state to rectify current from an alternating current source. Claim 20 In claim 19, the rectifier comprises a transformer including a primary side and a secondary side, and the at least one electric switch is connected to the secondary side of the transformer. Claim 21 A rectifier according to claim 20, wherein the control mechanism controls each of the at least one electric switch between the low resistance state and the higher resistance state based on the direction of flow of alternating current in the transformer. Claim 22 In paragraph 19, the rectifier is a half-wave rectifier. Claim 23 In paragraph 19, the rectifier is a full-wave rectifier. Claim 24 A rectifier according to claim 19, wherein the activation of the magnetic field generator induces the screening current in branches that causes increased resistive dissipation in the superconducting material without quenching of the superconducting material. Claim 25 A fault current limiter comprising at least one electric switch according to any one of claims 1 to 18 and a control mechanism configured to place the at least one electric switch in the higher resistance state when a fault is detected. Claim 26 A fault current limiter according to paragraph 25, wherein the activation of the magnetic field generator induces a screening current in branches that causes the superconducting material to quench.

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