Superconducting switch

This invention is a high voltage superconductor switch comprising: a length of superconductor having a switching portion located within an air gap; a magnetic circuit including at least one flux guide having ferrite pole pieces defining an air gap in which a switching portion of a superconductor can reside in use and at least one primary magnetic flux source located within the circuit so as to provide a quenching magnetic field across the air gap via the ferrite pole pieces.

TECHNICAL FIELD OF INVENTION

This invention relates to a superconducting switch. In some aspects, the superconducting switch is for use in a high voltage electrical system. In other aspects, the superconducting switch is used to provide an isolation switch. The isolating switch is particularly, though not exclusively, well suited to providing a failsafe isolation switch.

BACKGROUND OF INVENTION

Conventional state of the art propulsion systems for large civil aircraft typically include one or more gas turbine engines placed under the wings of the aircraft. However, some studies have indicated that so-called distributed propulsion, which involves having numerous smaller propulsion units preferentially arranged around an aircraft, may provide some significant benefits in terms of noise reduction and fuel efficiency when compared with the current state of the art technology.

One option for a distributed propulsion system is to have numerous electrically powered fan units located around the aircraft. However, early studies by the applicant have indicated that novel electrical technology will be required to implement such a distributed electrical system.

One such technology is the creation of a superconducting system to provide the electrical power to the fan units so as to try and reduce the weight of the electrical system.

The concept of using a superconductor for providing electrical power is well known. A superconductor conducts electricity without loss, that is, with zero electrical resistance. In order to be superconducting, current state of the art superconductor materials must be maintained below a critical temperature, current density and magnetic field. If any of the critical limits are exceeded then the superconductor is said to “quench”, at which point it reverts to its “normal” electrical (and magnetic) properties.

For example, in the case of Yttrium Barium Copper Oxide, YBCO, the critical temperature is 93K; the upper critical magnetic flux density field is 120 T for a field perpendicular and 250 T for a field parallel to the copper oxide planes, and the critical current density is 30 GA m−2. The so-called “supercurrent”, that is the current that flows in the super conductor when in its superconducting state, flows in a very thin layer at the surface of the superconductor, typically 800 nm (the London Depth). However, the critical current density reduces with applied magnetic field and also will reduce as the temperature approaches the critical temperature.

In the case of ceramic superconductors the quenched electrical resistance can be very high. Hence, it is possible, and known, to provide a switch where an applied magnetic field is used to control the superconducting state of a superconductor and thus switch it between operating points having high and low (zero) resistance.

FIG. 1shows the basic concept for a cryotron10which uses an electrical coil12wrapped around a length of superconductor14. The superconductor current, Ig, flows until a direct current, Ic, of sufficient magnitude to produce a quenching magnetic field flows through the electrical coil10. Once this occurs, the resistance increases until there is negligible current flow, thereby providing a switch.

The present invention seeks to provide a superconducting switch of general application but which may preferably be used in a distributed propulsion system of an aircraft.

STATEMENTS OF INVENTION

In a first aspect there is provided a high voltage superconductor switch comprising: a length of superconductor having a switching portion located within an air gap; a magnetic circuit including at least one flux guide having ferrite pole pieces defining an air gap in which a switching portion of a superconductor can reside in use and at least one primary magnetic flux source located within the circuit so as to provide a quenching magnetic field across the air gap via the ferrite pole pieces.

The magnetic circuit can include one or more ferromagnetic or ferrimagnetic portions. In one embodiment, substantially all of the magnetic flux guide is made from ferrite.

The superconductor can have a circular cross-section. The cross-section may be polygonal. The cross-section may be rectangular. The superconductor can be surrounded by electrical insulation taken from the non-exclusive group comprising ceramics or plastics or glass cloth. The superconductor can include thermal insulation taken from the non-exclusive group comprising ceramics or plastics or glass cloth.

The magnetic flux guide can include a U-shaped core or a C-shaped core.

For the purpose of the invention, high voltage is taken to mean a voltage above 1500V DC or 1000 V AC rms between electrical conductors or between one or more electrical conductors and Earth (ground).

The superconductor may be located within a cryostat having at least one wall. The ferrite pole pieces may pass through the at least one wall.

The magnetic flux source may be located outside cryostat. In another embodiment, the magnetic flux source may be located within the cryostat.

The magnetic flux guide may be an electromagnet.

The magnetic flux source may be a permanent magnet. The electromagnet may be selectively energised. The electromagnet may comprise a superconductor. The permanent magnet may be a superconductor. The permanent magnet may be magnetised by one of the known flux pumping techniques.

The high voltage superconductor switch may further comprise a selectively operable secondary magnetic flux source positioned to induce magnetic flux within the magnetic flux guide so as to disrupt or divert the magnetic flux generated by the primary magnetic source thereby reducing or removing the magnetic field produced across the air gap.

The selectively operable secondary magnetic flux source may be an electromagnet. The electromagnet may be coupled to a direct current electrical supply. The coupling to the direct current electrical supply may be via a switch. The electromagnet may include a superconductor. The electromagnet may be configured to provide a magnetic field within the magnetic flux guide which opposes and cancels the magnetic field produced by the primary magnetic flux source. The secondary magnetic flux source may be configured to saturate the magnetic material thereby increasing the magnetic reluctance of a portion of the magnetic circuit.

The secondary magnetic flux source may be in magnetic series with the primary magnetic flux source.

The magnetic flux guide may include a diverting flux guide path in parallel to the primary magnetic flux source.

The secondary magnetic flux source may be located along the diverting magnetic flux guide path.

The diverting magnetic flux guide path may include a diverting air gap.

The diverting air gap may have a larger reluctance than the air gap between the ferrite pole pieces. Thus, when the secondary magnetic flux source is not energised, a magnetic field is preferentially set up across the superconductor rather than the diverting air gap.

The secondary magnetic flux source may be arranged so as to saturate the magnetic flux guide when energised.

The magnetic flux guide may include a reluctance switch which is operable to increase the reluctance of at least a portion of the magnetic flux path.

The reluctance switch may include a mechanically removable portion of the magnetic circuit so as to increase the reluctance of the magnetic circuit beyond the reluctance of the air gap between the ferrite poles.

The mechanically removable portion may be connected to the magnetic circuit via a hinge, a slide or a two part coupling.

The reluctance switch may include a rotatable portion of the magnetic flux guide.

In a second aspect, the present invention provides a high voltage superconducting system comprising: the high voltage superconducting switch of the first aspect; a sensor for detecting the electrical condition of the superconductor; a switching system for operating the high voltage superconductor switch from a first, superconducting, state to a second, quenched, state.

The system may have a low electrical inertia. The system can be all or part of an isolated network. The isolated may have more than one electrical generator. The system may have less than ten electrical generators. The high voltage switch can be for the use of providing isolation within the system. The isolation may be for part of the system or a dedicated switch for a piece of electrical equipment. The system may be part of an electrical system in a vehicle, vessel or aircraft. The aircraft may include a distributed propulsion system. The distributed power system may include one or more electrically driven fan units located on and around the aircraft.

The electrical condition can include one from the group comprising a current or power flow or voltage above or below a predetermined amount and a loss of superconduction in the superconductor. The predetermined amount of current or power flow or the voltage can be based on an average or instantaneous rated value for the system or for a particular equipment. The skilled person will appreciate that other electrical characteristics may be used as the electrical condition.

The switching system may be operable to switch the reluctance switch or secondary magnetic flux source.

In a third aspect, the present invention provides a superconducting switch comprising: a length of superconductor having a switching portion located within an air gap; a magnetic circuit including: at least one flux guide having pole pieces defining the air gap in which the switching portion of the superconductor is located; at least one primary magnetic flux source located within the circuit so as to provide a quenching magnetic field across the air gap via the pole pieces; and, a selectively operable secondary magnetic flux source positioned to induce magnetic flux within the magnetic flux guide so as to disrupt or divert the magnetic flux generated by the primary magnetic source, thereby reducing or removing the magnetic field produced across the switching portion air gap in use.

In a fourth aspect, the present invention provides a superconducting switch comprising: a length of superconductor having a switching portion located within an air gap; a magnetic circuit including: at least one flux guide having pole pieces defining the air gap in which the switching portion of the superconductor is located; at least one primary magnetic flux source located within the circuit so as to provide a quenching magnetic field across the air gap via the pole pieces; and, a reluctance switch which increases the reluctance of at least a portion of the magnetic flux path, thereby reducing or removing the magnetic field produced across the switching portion air gap.

It will be appreciated that the various exemplary features relating to the ferrite pole pieces, secondary magnetic flux source and the reluctance switch described in relation to the first and second aspects are applicable to the third and fourth aspects.

DETAILED DESCRIPTION OF INVENTION

FIG. 2shows a high voltage superconductor switch210. The switch includes a length of superconductor212having a switching portion located within an air gap214, and a magnetic circuit generally shown by numeral216. The switching portion is that defined as the length of superconductor212which experiences the quenching conditions when the switch210is operated.

The magnetic circuit216includes a magnetic flux guide218in the form of a ferromagnetic C-shaped core which has ferrite pole pieces220which define the air gap214in which a superconductor resides in use. The magnetic circuit216includes a magnetic flux source222in the form of an electromagnetic coil which is wound around a portion of the ferromagnetic core218. The electromagnetic coil222and core218are such that a magnetic field sufficient to quench the superconductor212is placed across the air gap214via the ferrite pole pieces220when the coil222is energised with its rated current.

The superconductor is high voltage in that it is part of a network which operates at a voltage in excess of 1000 Vrms or 1500V DC. Hence, although not shown, the superconductor forms part of a larger electrical system. Although the invention has been conceived with aero applications in mind, specifically distributed propulsion, the system is not considered to be a limitation of the invention.

The superconductor212can be any suitable material such as Bismuth Strontium Calcium Copper Oxide (BSCCO), Yttrium Barium Copper Oxide (YBCO) or Magnesium Diboride (Mg2B). As is well known, in the case of YBCO, the critical temperature is 93K; the upper critical magnetic flux density field is 120 T for a field perpendicular and 250 T for a field parallel to the copper oxide planes, and the critical current density is 30 GA m−2. The so-called “supercurrent” , that is the current that flows in the super conductor when in its superconducting state, flows in a very thin layer at the surface of the superconductor, typically 800 nm (the London Depth). However, the critical current density will reduce with applied magnetic field and also will reduce as the temperature approaches the critical temperature.

In order for the superconductors to be in a superconducting state they must be cooled at or preferably below the critical temperature of the respective material. Hence, the superconductor212shown inFIG. 1includes a cryostat as indicated schematically by the dashed line224and is surrounded by electrical and thermal insulation226as required. Typical materials for the electrical insulation and thermal insulation are ceramics or plastics or glass cloth. These materials have both the required electrical insulating properties and the required thermal properties.

The use of ferrite pole pieces220is particularly advantageous for application to a high voltage superconductive switch for numerous reasons. The first of these is that material is a ceramic and thus has a high dielectric strength which provides a high degree of electrical insulation for the superconductor212. Further, ferrites generally have a lower thermal conductivity than their ferromagnetic counterparts. Thus, as shown inFIG. 1, the ferrite pole pieces220can extend through a wall of the cryostat224whilst minimising the thermal conduction and consequential leakage of heat into the cryostat224. This allows for a more efficient design of switch210. Further, ferrites can be of a lower density than many ferromagnetic materials which is preferable for aero applications which is a considered application of this technology.

The ferrite pole pieces220can, for example, be taken from the groups comprising iron oxide, manganese zinc or nickel zinc ferrites as known in the art.

It will be appreciated that, although the embodiment described in relation toFIG. 2has only portions of the ferrite poles220and superconductor212located within the cryostat224, it is possible to place the whole of the switch210within the cryostat224should this be considered a preferable option. Also, although the magnetic flux guide218is shown as having only ferrite pole pieces220, it is possible for any part, or even the entirety of the magnetic flux guide218, to be made from ferrite.

The ferrite pole pieces220taper along their length from junction with the body of the magnetic flux guide218to the pole faces228so as to reduce the pole face area and maximise the magnetic flux density within the air gap214. This allows a concentration of the magnetic field within the air gap to aid the quenching of the superconductor212in the switching portion and also reduces the likelihood of an electrical discharge between the high voltage superconductor212and magnetic flux guide214. The latter advantage meaning that the electrical insulation226around the superconductor212can be reduced and the pole pieces220placed closer to the superconductor212, thereby reducing the energy required to set up the magnetic field.

An example of a working system includes a stainless steel strip wire that is 6 mm thick with a 1 mm thick coating of superconductor YBCO on each side, with the coatings on each side connected in parallel. This forms a superconducting arrangement212approximately 8 mm thick. The reason that the superconductor is not made entirely of superconducting material is because supercurrents flow in a very small layer at the surfaces of the superconductor. This layer is known as the London depth and is very shallow, typically 120 to 800 nm depending on the orientation of the supercurrent with respect to the crystal lattice.

The superconductor is cooled to an operating temperature of 75K and carries a full load current of 600 A with no applied magnetic field. The magnetic field produced by the current, 0.03 T, may be neglected when considering quenching. The operating temperature, 75K is well below the critical temperature of 93 K and is also below the boiling point of nitrogen, 77K and so liquid nitrogen may be used as a coolant.

An applied magnetic field of 0.25 T reduces the critical current density to 800 MA m−2reducing the critical current of the conductor to 16 A, well below the 600 A full load current. The superconductor would quench, that is, change to a high resistance state if it were carrying more than 16 A.

The ferrite pole pieces220are arranged to taper from 24×24 mm to 12×12 mm so that the flux density in the pole pieces falls from 0.25 T to 0.0625 T. The gap between the ferrite pole pieces222is 12 mm which equals the sum of the thicknesses of the steel (6 mm), the superconducting coatings (1 mm each) and the electrical and thermal insulation (2 mm each side of the steel) of the superconductor212, 8 mm and 2 mm thick electrical insulation.

The magnetic circuit216is not worked close to magnetic saturation and so most of the magneto motive force produced by the electromagnetic coil222appear across the gap between the ferrite pole-pieces220and in particular the high reluctance paths presented by the electrical and thermal insulation (2 mm each side of the steel) and the superconducting coatings (1 mm each side). The magneto motive force across the 12 mm gap will therefore be developed across the 6 mm high reluctance path presented by the superconducting coatings and the electrical and thermal insulation between the ferrite pole-pieces220. The magneto-motive-force required to produce a flux density of 0.25 T across the 6 mm high reluctance path is 1194 Am−1. Hence, having an electromagnetic coil which can produce 2000 A turns allows for some magneto motive force to be used to magnetise the magnetic flux guide and the pole pieces and also to allow for leakage flux.

A possible rating of wire used for the electromagnetic coil of the magnetic flux generator222would be 1 A, and so the coil would have 2000 turns of such wire, each carrying 1 A. The coil would typically require 12 V DC to circulate 1 A DC.

In the above embodiment, DC excitation is preferred because the magnetic flux guide218and the ferrite pole-pieces220can be made from solid materials instead of laminated materials, simplifying construction. However, in another embodiment, laminated construction would result in a faster operating time of the switch because there would be less opposition from eddy currents to changes in magnetic fields. Also laminated construction would allow AC to be used which may be advantageous if AC were more easily provided than DC.

The magnetic flux guide has a cross sectional area of 24×24 mm. The limb carrying the electromagnetic coil would be typically 50 mm long and the limbs connecting the ferrite pole pieces220would typically be 80 mm long.

FIG. 3shows a superconducting switch310having a magnetic circuit316and superconductor312as described for the embodiment shown inFIG. 2and having corresponding numerals (3XX) for each of the constituent parts. However, the primary the magnetic flux source in this embodiment is a permanent magnet322.

An additional magnetic flux path330is provided as part of the magnetic circuit316in this embodiment. The additional magnetic flux path330extends between the arms332a,332bof the C-shaped core318so as to be parallel to, and provide a diversionary path for the magnetic flux created by, the permanent magnet322. The diversionary path330includes a secondary magnetic flux source in the form of an electromagnetic coil334which is configured to be selectively energised via a power source (not shown). An air-gap336is also provided at the mid-point of the diversionary path330.

In operation, the electromagnetic winding is energised with a direct current so as to set up a magnetic field which opposes that of the primary magnetic flux source322. Hence, the flux path extends from the North pole, N, of the permanent magnet to the South pole, S, of the electromagnetic winding334with a corresponding relationship between the south and north poles of the permanent and electromagnets, respectively. When in this configuration, there is insufficient magnetic field placed across the air-gap314and so the superconductor312remains in a superconducting state.

When isolation is required, the direct current in the electromagnetic winding334is switched off and the magnetic field created by the permanent magnet is then placed across the superconductor312which has a lower reluctance than the diversionary path330. As will be appreciated, the ratio of the air gaps314and336will be dependent on the geometry of the magnetic circuit316and the magnetic field required to quench the superconductor312.

The embodiment ofFIG. 3is particularly advantageous as it allows a failsafe mechanism to be implemented for the switch310. Hence, if the D.C. supply is removed from the electromagnetic winding334, for example, if it fails for some reason, then the load supplied by the superconductor312is automatically isolated. In this way, the electromagnetic winding334on the diversionary path330can be configured to sense and detect an undesirable electrical condition. As will be appreciated by the skilled person, other sensors may be employed as part of a larger system and the sensors may monitor various different parameters such as a maximum or minimum predetermined current flow or the superconducting state of the system. These sensors could be used by a generic control system to remove the D.C. supply to the winding334.

FIG. 4shows another embodiment of a switch410in which the diversionary path430is provided with a mechanical reluctance switch438. The reluctance switch438is in the form of a rotatable ferromagnetic member440which resides in series within the diversionary flux guide path430so as to provide the magnetic flux path with a rotatable portion or gate. The rotatable member440is centrally mounted on a lever arm442which is rotated about a rotational axis by an acutator (not shown). The actuator is configured to move the rotatable member440between a first, low reluctance, position and a second, high reluctance, position.

The rotatable member440is substantially rectangular in shape having perpendicular major and minor axes, wherein the length of the rotatable member is substantially greater along the major axis relative to the minor axis. The faces444at the opposing ends of the major axis of the rotatable member440are curved with a radius having a centre which is coaxial with the rotational axis of the member as defined by the lever arm442. The curved surfaces form uniform air gaps446with the corresponding faces of the diversionary path430when the major axis and longitudinal axis of the diversionary path430are aligned.

In the first position, the major axis is aligned with the longitudinal axis of the diversionary path body and the reluctance of the branch is at a minimum, thereby causing the majority of the flux to flow from the north pole of primary magnetic flux source, permanent magnet422to the south pole of the same via the diversionary path430and rotatable member440.

To place the rotatable member440in the second position, the lever arm442is rotated through ninety degrees such that the minor axis of the rotatable member440is aligned with the longitudinal axis of the diversionary path430. When in this configuration, the diversionary air gap444is formed by the curved surfaces of the diversionary path430and the flat sides448of the rotatable member440. Thus, in the second position the air gaps444at either end of the rotatable member are increased relative to the first position and the reluctance of the diversionary path430is increased. Hence, in the second position the magnetic field created by the permanent magnet is set up across the air gap in which the superconductor412sits, thereby causing it to quench.

The operation of the lever arm442can be in response to any desired signal. So, for example, the actuator which drives the lever arm442could be activated in the event of a monitored electrical condition which falls outside of predetermined limits. If the lever arm is provided with a mechanical bias so as to be normally in the open position, the switch can be provided with a failsafe function as described in the previous embodiment.

In the alternative embodiment shown inFIG. 5, the switch510is provided with a rotatable member540in magnetic series with the primary magnetic flux source522and the diversionary path shown inFIG. 4is no longer required. Thus, the rotatable member540can be thought of as being open when isolation of the current flow in the superconductor and is not required, with the minor axis been aligned to the longitudinal axis of the magnetic flux guide518. When a separation is required, the rotatable member540is rotated through ninety degrees by the lever arm542such that the major axis of the rotating member540aligns with the longitudinal axis of the magnetic flux guide518thereby reducing the reluctance and greatly increasing the magnetic field across the air gap514in which the superconductor512sits.

In yet another embodiment, the magnetic circuit616of the switch is provided with moveable portions650as shown inFIG. 6. It be appreciated that the mechanism by which the moving portions650are provided can vary and include but are not limited to the hinges, slides and two-part fasteners; all of which can be operated using actuators.

Another option to provide magnetic isolation of the superconductor air gap is to saturate the magnetic material in a portion of the magnetic circuit. Thus,FIGS. 7 and 8show embodiments of the invention which include electrical coils752,852or other means which introduce a perpendicular magnetic field into the magnetic flux guide718,818in the diversionary path (FIG. 7), and in magnetic series with the primary magnetic flux source (FIG. 8).

Energising the saturation coils752,852results in a decrease in the relative permeability of the material local to the coils752,852which causes the flux generated by the primary magnetic flux source722,822to be diverted away from or towards the superconductor air gap as required. As described in the previous embodiments, the operation of the saturating coils752,852can be on demand and as a result of monitoring an electrical condition.

Although the invention is described above with specific embodiments, these are not intended to restrict the scope of the invention which is defined by the claims.

Hence, for example, although the embodiments described above relate generally to a high voltage superconductor switch, it will be appreciated that this can be incorporated within a larger system. That system may be any electrical system which utilises superconductors.

In one embodiment, the system may be included in an electrical system of a vehicle, vessel or aircraft. The aircraft may include a distributed propulsion system. The distributed power system may include one or more electrically driven fan units located on and around the aircraft. For example, the system can be all or part of an isolated network. The isolated system may have more than one electrical generator. The system may have less than ten electrical generators. The high voltage switch can be for the use of providing isolation within the system. The isolation may be for part of the system or a dedicated switch for a piece of electrical equipment.

As will be appreciated, although the invention is well suited to high voltage applications, there are aspects of the invention which can be utilised in a system or network operating at any voltage. Thus, although the ferrite pole pieces are particularly advantageous to high voltage applications, this should not be seen as limiting. Further, it is possible for many aspects of the invention to be used without the ferrite pole pieces.

It will also be appreciated that combinations of features not specifically described above may be incorporated in a single device. For example, one embodiment may include both a reluctance switch and a secondary magnetic coil. Further, the superconductor may comprise magnetic stainless steel wire coated with superconducting material. This arrangement would improve the magnetic reluctance between the ferrite pole pieces.