GB2540623A - Superconducting winding arrangements - Google Patents
Superconducting winding arrangements Download PDFInfo
- Publication number
- GB2540623A GB2540623A GB1513090.9A GB201513090A GB2540623A GB 2540623 A GB2540623 A GB 2540623A GB 201513090 A GB201513090 A GB 201513090A GB 2540623 A GB2540623 A GB 2540623A
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- United Kingdom
- Prior art keywords
- superconducting
- winding
- winding portion
- superconducting winding
- normally conducting
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01F—MAGNETS; INDUCTANCES; TRANSFORMERS; SELECTION OF MATERIALS FOR THEIR MAGNETIC PROPERTIES
- H01F6/00—Superconducting magnets; Superconducting coils
- H01F6/02—Quenching; Protection arrangements during quenching
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01F—MAGNETS; INDUCTANCES; TRANSFORMERS; SELECTION OF MATERIALS FOR THEIR MAGNETIC PROPERTIES
- H01F6/00—Superconducting magnets; Superconducting coils
- H01F6/06—Coils, e.g. winding, insulating, terminating or casing arrangements therefor
Abstract
A winding arrangement for mitigating the effect of a quench of a superconducting coil has, a first electrical circuit A with a switch 41 and a persistent or non-persistent superconducting winding portion 31, and a second electrical circuit B with a normally conducting winding portion 32. The winding portions are magnetically coupled. When a quench is detected, the switch 41 is opened, current in the superconducting portion 31 is interrupted and inductively transferred to the ordinary conducting portion 32. The superconducting and normally conducting winding portions may be co-wound in adjacent, alternating layers, or a compound winding material may be used. A circuit dump resistor 44, 52 or diode 51 may be provided in the first or second circuits, to control and dissipate current. A switch may be provided in the second circuit B, to cause a forced quench of the superconducting winding portion.
Description
Superconducting Winding Arrangements
This invention relates to superconducting winding arrangements which are arranged for providing quench protection and also to methods of quench protection.
Electrical apparatus including superconducting materials provide tremendous advantages. Examples of such apparatus are superconducting magnets as well as electrical motors, electrical generators and electrical transformers including superconducting windings.
However, superconducting windings can sometimes cease to be superconducting. This is usually due to a local event that causes a small section of winding to heat up exceeding its supercurrent carrying capacity (or critical current) and becoming “normal” (ie with the resistive properties of a normal resistive metal). If the event is very small it may self-heal provided there is sufficiently good low-resistance shunt through a matrix material surrounding the superconductor. However, if the disturbance exceeds a certain energy this is no longer possible and a “normal zone” becomes established. Dissipation in the normal zone generates local heating causing the initiation point to increase in temperature while thermal conduction along and perpendicular to the conductor causes neighbouring parts of the coil to become normal.
This event is called a “quench” and once started it proceeds to completion causing all of the superconducting winding’s stored magnetic energy to be converted into heat. Quenches are generally spontaneous and undesirable and they can arise from a number of random and uncontrollable causes. Thus it is desirable that superconducting windings are designed to withstand them without damage.
There are two main causes of damage during a quench. Firstly large internal voltages may develop as the current decays through the windings causing electrical break down and damage to insulation. Secondly the point of quench initiation and the zone around it may reach unacceptably high temperature causing damage to the local insulation or to the conductor itself.
The risk of over-temperature is greatest if the quench only propagates slowly to adjacent conductors. The main factors determining this rate are (a) the critical temperature Teat which the conductor becomes normal (which depends on the local magnetic field) and (b) the specific heat capacity of the conductor.
For metals specific heat capacity increases rapidly as the temperature rises above absolute zero. Consequently if Tc is high (> 50K), as in many high temperature superconductors (HTS), much more local dissipation is necessary to turn nearby material normal compared to low temperature superconductors (Tc < 25K).
Therefore quench propagation is slow in HTS materials and they are more prone to damage from overheating.
It is generally desirable to provide methods of quench protection for use in superconducting windings such as the windings of the superconducting magnets. Quench protection can be all the more necessary and difficult to achieve where the superconducting material concerned is a high temperature superconductor.
According to a first aspect of the present invention there is provided a superconducting winding arrangement comprising a first electrical circuit including a superconducting winding portion and a second electrical circuit including a normally conducting winding portion, wherein the first circuit comprises switch means and the superconducting winding arrangement comprises a quench detection system for detecting quench events in the superconducting winding portion and causing opening of the switch means upon quench detection to interrupt current flow around the superconducting winding portion, wherein the winding portions are arranged and wound with one another in a winding module such as to provide magnetic coupling therebetween such that upon opening of the switch means, current in the superconducting winding portion is inductively transferred to the normally conducting winding portion for mitigating the effect of the quench.
According to a second aspect of the present invention there is provided a superconducting winding arrangement comprising a first electrical circuit including a superconducting winding portion and a second electrical circuit including a normally conducting winding portion, the first circuit comprising switch means arranged such that opening of the switch means will interrupt current flow around the superconducting winding portion, wherein the winding portions are arranged and wound with one another in a winding module such as to provide magnetic coupling therebetween such that upon opening of the switch means, current in the superconducting winding portion is inductively transferred to the normally conducting winding portion for mitigating the effect of a quench in the superconducting winding portion.
The superconducting winding arrangement may comprise a quench detection system for detecting quench events in the superconducting winding portion and causing opening of the switch means upon quench detection.
Such an arrangement can allow the current in a superconducting winding to be quickly and safely extracted from the superconducting winding. Moreover this can be achieved without a significant immediate change to the magnetic field distribution or stored magnetic energy. Thus the arrangement can help avoid excessive overheating in any region in the superconducting winding whilst avoiding potentially dangerous sparking or arcing between terminals on the superconducting winding. A magnetic coupling constant K between two windings may be defined as the square of their mutual inductance (M12) divided by the product of their self inductances (Li an L2), thus. Κ= Μι2^/( Li X Ls).
The winding portions may be arranged so that the magnetic coupling constant between the two winding portions is at least 0.8. This helps ensure that that there is swift and effective current transfer from the superconducting winding portion to normally conducting winding portion.
The winding portions may be arranged in the winding module in a number of different ways that provide suitable magnetic coupling.
Turns of the superconducting winding and normally conducting winding may be wound in the winding module in layers.
The superconducting winding portion and the normally conducting winding portion may be co-wound such that each turn of the superconducting winding is adjacent a corresponding turn of the normally conducting winding in the or each layer of the winding module.
The superconducting winding portion and the normally conducting winding portion may be wound in respective layers such that turns of the superconducting winding occupy one or more superconducting layers and turns of the normally conducting winding occupy one or more normally conducting layers.
The superconducting layers and normally conducting layers may be arranged alternately in the winding module.
There may be a larger number of superconducting layers than there are of normally conducting layers. As an example, every fourth layer may be a normally conducting layer.
The number of turns in the superconducting winding portion may be the same as or different to the number of turns in the normally conducting winding portion.
In some cases the winding module may be wound using a compound winding material comprising both normally conducting material and superconducting material which is insulated from the normally conducting material such that the normally conducting winding portion comprises the normally conducting material of the compound winding material and the superconducting winding portion comprises the superconducting material of the compound winding material.
The first electrical circuit may comprise a first circuit dump resistor provided across terminals of the superconducting winding portion such that when the switch means is opened current may flow through the dump resistor. Note that the first electrical circuit dump resistor, where present, is provided not to dissipate a large proportion of the stored magnetic field, but rather to provide a path for residual currents in the superconducting winding portion caused by imperfect coupling between the normally conducting winding and the superconducting winding.
At least one diode arrangement may be provided in series with the first circuit dump resistor with a forward bias voltage that is greater than a normal energisation voltage of the superconducting winding portion such that current will not flow through the first circuit dump resistor during energisation of the superconducting winding portion. A crossed diode arrangement may be provided in series with the first circuit dump resistor the crossed diode arrangement having respective forward bias voltages that are greater than a normal energisation voltage of the superconducting winding portion such that current will not flow through the first circuit dump resistor during energisation of the superconducting winding irrespective of the polarity of the energisation current.
The second electrical circuit may comprise a second circuit dump resistor provided across terminals of the normally conducting winding portion such that current flowing in the second electrical circuit may decay in the second circuit dump resistor.
At least one diode arrangement may be provided in series with the second circuit dump resistor with a forward bias voltage that is greater than a normal voltage which would be seen in the normally conducting winding portion during energisation of the superconducting winding portion such that current will not flow through the second circuit dump resistor during energisation of the superconducting winding. A crossed diode arrangement may be provided in series with the second circuit dump resistor the crossed diode arrangement having respective forward bias voltages that are greater than a normal voltage which would be seen in the normally conducting winding portion during energisation of the superconducting winding portion such that current will not flow through the second circuit dump resistor during energisation of the superconducting winding irrespective of the polarity of the energisation current.
The second electrical circuit may comprise a switch means connected in series with the normally conducting winding portion, such that the second electrical circuit may be rendered open circuit by the switch being open. This can be used to ensure that the second electrical circuit does not extract energy from the superconducting winding portion, unless and until the switch is closed.
The quench detection system may be arranged to close the switch in the second electrical circuit upon detection of a quench event.
The quench detection system may be arranged to detect the occurrence of a quench event on the basis of one of a number of different factors, for example: unexpected changes in terminal voltage; an amplified voltage difference between two matched coils where such coils are present in the superconducting winding arrangement; deviation from expected heat budget; a spike in cryogen boiloff.
In a specific example the quench detection system may be arranged for monitoring the voltage across the dump resistor provided in the first electrical circuit.
In some embodiments, the superconducting winding portion is a persistent superconducting winding. In a persistent superconducting winding an external power source is used in energising the winding at start up but after start up the external power source is not required and can be disconnected.
Where the superconducting winding portion is a persistent superconducting winding, the switch means may comprise a superconducting switch. The superconducting switch will be open during energisation using the external power source, closed in normal operation and opened when it is desired to extract stored energy into the normally conducting winding.
The superconducting switch will generally comprise a piece of superconducting material and a heater for locally heating the piece of superconducting material to drive it normaliy conducting to open the switch.
In other embodiments, the superconducting winding portion is a non-persistent superconducting winding. In a non-persistent superconducting winding, an external power source is used in energising the winding at start up and to drive the winding in normal operation.
In some cases the first electrical circuit may comprise a plurality of superconducting winding portions. In such a case the second electrical circuit may comprise a plurality of normally conducting winding portions, each wound with a respective one of the superconducting winding portions. In an alternative a plurality of second electrical circuits may be provided each comprising a respective normally conducting winding portion wound with a respective one of the superconducting winding portions.
The or each superconducting winding portion may comprise high temperature superconductor material and/or low temperature superconductor material. That said, the present ideas are most likely to be most useful/important in the case of high temperature superconductor materials.
It may be most readily apparent that the present techniques may be used with direct current (dc) windings and drive sources. However the present techniques may also be used with alternating current (ac) systems, for example, low frequency ac applications, say operating in the range upto 100Hz.
In an ac system the second electrical circuit may comprise a crossed diode arrangement and/or a switch. This can help ensure that energy is only absorbed in the second electrical circuit when desired.
Typically the superconducting winding portion and the winding module in general will be provided within a cryostat.
The dump resistor in the first circuit may be disposed within the cryostat or externally of the cryostat.
The dump resistor in the second circuit may be disposed within the cryostat or externally of the cryostat.
If within the cryostat, the dump resistor can provide controlled heating of the superconducting winding portion which may be desirable to avoid hotspots and allow a controlled quench, if external this can help minimise heat generation within the cryostat which can be desirable to minimise temperature gain within the system in general. What is preferable depends on circumstances.
The superconducting winding arrangement as defined above may be used as part of any one of a superconducting magnet, an electrical transformer, an electrical motor, and an electrical generator.
According to a further aspect of the invention there is provided a superconducting magnet comprising superconducting winding arrangement as defined above.
According to a further aspect of the invention there is provided an electrical transformer comprising superconducting winding arrangement as defined above.
According to a further aspect of the invention there is provided an electrical motor comprising superconducting winding arrangement as defined above.
According to a further aspect of the invention there is provided an electrical generator comprising superconducting winding arrangement as defined above.
According to another aspect of the present invention there is provided a quench protection method for protecting a first electrical circuit including a superconducting winding portion, the method comprising the steps of: providing a switch means in the first circuit; winding a normally conducting winding portion of a second electrical circuit with the superconducting winding portion so as to provide magnetic coupling therebetween; detecting quench events in the superconducting winding portion; and opening the switch means upon detection of a quench event to interrupt current flow around the superconducting winding portion so allowing current stored in the superconducting winding portion to be inductively transferred to the normally conducting winding portion for mitigating the effect of the quench.
The method may comprise the further step of allowing current to flow in the normally conducting portion for a selected period so heating the superconducting winding portion.
This can force a significant portion of the superconducting winding portion to become normally conducting.
The method may comprise the further step of re-closing the switch means in the first circuit and opening a switch in the second circuit after the selected period to allow a widespread quench to occur in the superconducting winding portion.
According to yet another aspect of the invention there is provided a superconducting winding module comprising a first superconducting winding portion and a second normally conducting winding portion, the winding portions being wound with one another to provide magnetic coupling therebetween.
The optional features defined above following the first and other aspects of the invention are where appropriate and with any necessary changes in wording also optional features for each of the other aspects of the invention. They are not rewritten in full after each aspect of the invention in the interests of brevity.
Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings in which:
Figure 1 schematically shows an electrical apparatus including a superconducting winding arrangement;
Figure 2 shows an equivalent circuit diagram in respect of the superconductor winding arrangement of Figure 1;
Figure 3 shows an equivalent circuit diagram for a modified version of the superconducting winding arrangement shown in Figure 2;
Figure 4 shows an equivalent circuit for another modified version of the superconducting winding arrangement shown in Figure 2;
Figure 5 shows another alternative superconducting winding arrangement;
Figure 6 shows yet another alternative superconducting winding arrangement;
Figure 7 shows another superconducting winding arrangement which is similar to that shown in Figures 2-6 but includes a persistent superconducting winding rather than a non-persistent superconducting winding;
Figure 8 schematically shows a first possible winding pattern for a winding module which may be used in the superconducting winding arrangements of the types shown in Figures 1-7;
Figure 9 shows a second possible winding pattern which may be used in a winding module of winding arrangements of the types shown in Figures 1-7; and
Figure 10 shows a third possible winding pattern which may be used in a winding module of winding arrangements of the type shown in Figures 1-7.
Figure 1 shows, in highly schematic form, an electrical apparatus including a superconducting winding arrangement. The electrical apparatus might, for example, be a superconducting magnet, an electrical motor, an electrical generator or an electrical transformer. The electrical apparatus comprises a cryostat 1 and a superconducting winding arrangement 2. As will be appreciated, parts of the superconducting winding arrangement 2 which need to be kept at a low temperature will be provided within the cryostat 1 whereas other parts may be provided outside of the cryostat 1.
Thus, at least a winding module 3 of the superconducting winding arrangement 2 which comprises a superconducting winding portion 31 wound together with a normally conducting winding portion 32 is provided in the cryostat 1. On the other hand, control circuitry 4 of the superconducting winding arrangement 2 might be provided outside the cryostat 1 as shown in Figure 1, inside the cryostat 1 or partly inside the cryostat 1 and partly outside of the cryostat 1 depending on particular circumstances and design choice.
The control circuitry 4 comprises switch means 41 and a quench detection system 42 for controlling operation of the switch means 41. Further, in the present embodiment, the control circuitry 4 also comprises an electrical energy source 43, for example a current source, for energising the superconducting winding portion 31.
As will be explained in more detail below, superconducting winding arrangements embodying the present invention may be persistent or non-persistent. With a persistent superconducting winding, an external electrical power source is used to energise the superconducting winding 31 at start up but after this may be disconnected and/or removed. On the other hand, with a non-persistent superconducting winding then an external electrical power source is used during start up and normal operation of the superconducting winding.
Thus, as the present embodiment is directed to a non-persistent arrangement, the control circuitry 4 includes the current source 43 for driving the superconducting winding 31.
Figure 2 shows an equivalent circuit diagram for the superconducting winding arrangement shown in Figure 1. The superconducting winding portion 31, which has a plurality of turns, forms part of a first electrical circuit A and is connected across the power source 43. The switch means 41 is provided so as to control whether the power source 43 provides current to the superconducting winding portion 31. A dump resistor 44 is connected across the superconducting winding portion 31. In normal operation of the superconducting winding 31, the switch means 41 will be closed and the superconducting winding 31 is driven by the power source 43. In these circumstances the dump resistor 44 has little effect as its resistance is significantly higher than that of the superconducting winding portion 31. The quench detection system 42 (not shown in Figure 2) is arranged to monitor the voltage across the first circuit dump resistor 44 (or across the power source 43) to detect quench events. A quench in the superconducting winding portion 31 will cause an increase in detected voltage.
The normally conducting winding portion 32, which has a plurality of turns wound together with the turns of the superconducting winding portion 31, forms part of a second electrical circuit B. In the present embodiment the ends of the normally conducting winding 32 are connected together via a crossed diode arrangement 51 and a second circuit dump resistor 52. It will of course be appreciated that the superconducting winding portion 31 is electrically insulated from the normally conducting winding portion.
In normal operation where the superconducting winding portion 31 is driven by a (constant) dc power source, little or no current will be induced in the normally conducting winding portion 32 and hence little or no current will flow in the second electrical circuit B.
On the other hand however, the superconducting winding arrangement is arranged so that if the quench detection system 42 (see Figure 1) detects a quench event in the superconducting winding 31, the quench detection system 42 causes opening of the switch means 41 so interrupting the supply of current to the superconducting winding portion 31. As this occurs, the current in the superconducting winding portion 31 quickly reduces. As this change in current is occurring in the superconducting winding portion 31, it induces a corresponding current in the normally conducting winding portion 32 due to the magnetic coupling therebetween. This current may then flow around the second electrical circuit B and be dissipated in the second electrical circuit dump resistor 52. This allows most, if not all, of the energy in the superconducting winding 31 to be extracted from the superconducting winding 31 quickly so as to prevent further uncontrolled spreading of the quench event, prevent undesirable heating in the superconducting winding portion 31, and prevent the production of electrical arcing between the contacts of the switch means 41 or other terminals. Remaining current in the first electrical circuit A, due to non-perfect magnetic coupling, is dissipated in the first electrical circuit dump resistor 44. Note that in this embodiment, the first electrical circuit dump resistor 44 has a much higher value than a conventional dump resistor.
The provision of the crossed diode arrangement 51 helps ensure that current does not flow in the second electrical circuit B during normal operation of the superconducting winding portion 31. The diode arrangement 51 is chosen such that the respective forward bias voltages of the diode arrangement 51 are greater than any voltage which is likely to occur in the second electrical circuit B either during start up energisation of the superconducting winding portion 31 or in normal operation.
In a modification, a similar crossed diode arrangement 51 might be provided in series with the first electrical circuit dump resistor 44. However, these are considered to be unnecessary in normal circumstances.
The values chosen for the dump resistors 44, 52 and the necessary respective forward bias voltages of the diode arrangement 51 will depend on particular implementations.
As an example, the resistance of first circuit dump resistor 44 might be in the order of lOOOhms, while the resistance of the second circuit dump resistor 52 might be in the order of a few Ohms or less. A typical energisation voltage of a superconducting winding portion 31 in a particular implementation might be in the order of 5-10 volts. A proportional voltage is induced in the second electrical circuit B, scaled by the ratio of turns in the second electrical circuit B to the first electrical circuit A. Thus, as will be appreciated, the selected forward bias voltage may be of such an order (obviously higher than the actual voltage energisation voltage expected in the second electrical circuit B) and each diode arrangement 51 may in fact comprise a number of series connected diodes.
Note that the provision of a crossed diode arrangement rather than a single polarity diode arrangement merely allows for the fact that the superconducting winding portion 31 might be connected with either polarity.
Figure 3 shows an equivalent circuit diagram for a modified version of the superconducting winding arrangement shown in Figures 1 and 2. Here the first circuit dump resistor 44 is omitted. This is possible in at least some embodiments of the present invention because upon quench, the energy stored in the superconducting winding portion 31 can be primarily extracted into the normally conducting winding portion 32. Thus a first circuit dump resistor 44 of the type shown in the arrangement of Figure 2 is used only to limit voltages which may occur in the first electrical circuit A due to imperfect coupling between the winding portions 31,32.
Figure 4 shows an equivalent circuit diagram for another modified version of the superconducting winding arrangement shown in Figures 1 and 2. Here both the first circuit dump resistor 44 and the crossed diode arrangement 51 of the second electrical circuit B are omitted. This represents a simpler implementation which can work satisfactorily in at least some circumstances. The removal of the crossed diode arrangement 51 means that there is less protection against dissipation during start up of the superconducting winding portion 31 or due to any minor fluctuations in current in the superconducting winding portion 31 during use. However, these considerations may not be important in at least some practical implementations. In a further alternative in some cases the second circuit dump resistor 52 may also be omitted such that the ends of the normally conducting winding 51 are short circuited together. This then relies on the resistance in the winding itself to dissipate the energy.
Figure 5 shows yet a further alternative superconducting winding arrangement similar to those shown in Figures 1-4. However in this case, the first electrical circuit A comprises a plurality of superconducting winding portions 31, whilst the second electrical circuit B comprises a corresponding plurality of normally conducting winding portions 32 each of which is wound together with a respective one of the superconducting winding portions 31.
Otherwise, the structure and operation of such an embodiment will be the same as that described with reference to Figures 1 and 2. Note that whilst omitted from the embodiment shown in Figure 5, a crossed diode arrangement may be provided in series with the dump resistor 52 in the second electrical circuit B in an arrangement such as that shown in Figure 5 and also a first circuit dump resistor with or without a crossed diode arrangement may be provided in the first electrical circuit A.
Figure 6 shows an equivalent circuit for another alternative superconducting winding arrangement which is similar to that shown in Figure 5. Here again there is a plurality of superconducting winding portions 31 in the first electrical circuit A. However, in this case a plurality of second electrical circuits B is provided with each one having a respective normally conducting winding portion 32 with its own dump resistor 52 connected thereacross. Again a first circuit dump resistor and/or crossed diode arrangements may be provided.
Currently, it is considered that the arrangement shown in Figure 6 is less desirable than that shown in Figure 5 but again there may be circumstances where such an arrangement would be chosen. Potential difficulties with the arrangement shown in
Figure 6 compared with that shown in Figure 5 are as follows. Although on opening the switch 41 in the first electrical circuit, each of the normally conducting winding portions 32 would carry the same ampere-turns as the corresponding superconducting winding portion 31, the currents might evolve differently with time due to differing resistances and mutual couplings.
Note that whilst each of the equivalent circuits above shows a switch means in series with the power source 43 in the first electrical circuit A, other physical implementations are possible provided that the switch means serves to interrupt the current flowing in the superconducting winding portion 31. Thus, for example, a switch means may be arranged to shut down the power source 43 or disconnect the power source 43 from its mains supply or similar.
Each of the embodiments described above with reference to Figures 1 -6 relate to situations where the superconducting winding portion is a non-persistent superconducting winding portion such that the power source 43 is connected to the superconducting winding portion 31 during normal operation. However it is relatively common to have persistent superconducting winding portions, that is to say apparatus including persistent superconducting winding portions. As an example, many superconducting magnets are persistent, that is, they have a persistent superconducting winding portion.
Figure 7 shows an equivalent circuit for embodiment where the superconducting winding portion 31 is persistent.
The majority of the structure, functioning and operation of the superconducting winding arrangement shown in Figure 7 is the same as that shown in and described in relation to Figures 1 and 2. Thus there is a first electrical circuit A comprising a superconducting winding portion 31 and connected across this a first circuit dump resistor 44. In this case the dump resistor 44 is provided in series with a crossed diode arrangement 51.
Similarly, there is a second electrical circuit B comprising a normally conducting winding portion 32 which is wound together with the superconducting winding portion 31 to provide magnetic coupling therebetween. Further, connected across the normally conducting winding portion 32 is a second circuit dump resistor 52 and a crossed diode arrangement 51 in series therewith.
However, here the power source 43 is separable from the first electrical circuit A.
The switch means 41 is provided across the terminals of the superconducting winding portion 31 and, in normal operation, the switch means 41 is closed such that current may flow around the closed first electrical circuit. The current path is through the superconducting winding portion 31, through the closed switch 41 and then back into the superconducting winding portion 31.
In normal operation, the external power supply 43 is disconnected from the first electrical circuit A. On the other hand, during initial energisation of the superconducting winding portion 31, the external power source 43 is connected to the superconducting winding portion 31 by, for example, closure of a pair of switch means 46 whilst the switch means 41 of the first electrical circuit A is opened.
In this embodiment, the switch means 41 of the first electrical circuit A is a superconducting switch. Typically, this comprises a portion of superconducting material with a controllable heater 47 provided for locally heating the superconducting material to force it normal and thereby open the switch 41 when desired.
Thus, it will be appreciated that in a persistent embodiment, such as that shown in Figure 7, the switch means 41 as well as the remainder of the first electrical circuit A will be provided within the cryostat 1 or otherwise held at low temperature.
On the other hand, the external power supply and the power supply for driving the controllable heater 47 may be provided outside of the cryostat and at room temperature.
As mentioned above, during initial energisation of the superconducting winding portion 31, the switch means 41 will be open and the switch means of the power supply 46 will be closed whereas in normal operation the switch means of the power supply 46 will be open (or the power supply otherwise disconnected) whilst the switch means 41 of the first electrical circuit A will be closed. Then if a quench event is detected by the quench detection system 42, this will cause opening of the switch means 41 causing a collapse in current in the superconducting winding portion 31 as described above which, again as described above, causes a corresponding induced current in the normally conducting winding portion 32 of the second electrical circuit B.
Note that typical operating currents of large superconducting windings might be in the range of 250A - 5000A or more. Thus, the amounts of stored energy can be extremely large. In one particular example, where the electrical apparatus is a large superconducting magnet, there might be two pairs of solenoidal coils and a solenoidal shielding coil giving five coils in all. The coils might range in diameter from 1000mm to 1900mm. Further, the coils may vary from 42 layers with 20 turns per layer to 16 layers with 87 turns per layer. The operating current can be over 2400A and the stored energy can be over 17MJ.
Such a magnet might be provided with quench protection using the ideas of the present application by providing suitable normally conducting winding portions wound in each of the coils with the same number of layers and turns in the normally conducting winding portions as in the superconducting winding portions and using dump resistors having the types of values discussed above. However, in some circumstances, fewer turns and/or layers of normally conducting winding may be provided.
In the present techniques what provides good performance is ensuring that a high coupling constant is achieved between a superconducting winding portion 31 and the respective normally conducting winding portion 32. For good operation of the present systems, it is desired that this coupling constant K (the square of the mutual inductance of the two winding portions divided by the product of the self inductances of the two winding portions) should be 0.8 or above. However, in many practical circumstances, it will be possible to achieve coupling constants which are much greater than this and up to 0.98 or 0.99 provided that the inter-winding is fine enough.
Figures 8, 9 and 10 show possible winding patterns for winding modules 3 for use in apparatus of the type described above with reference to Figures 1-7.
Figure 8 is a cross sectional view schematically illustrating a co-wound “two in hand” solenoidal winding which may be used to provide a winding module 3. Here the winding module has 8 layers and in each layer alternate turns, as one progresses along each layer, are from the superconducting winding portion 31 and the normally conducting winding portion 32. Each light rectangle represents a cross section through a respective turn of the superconducting winding portion 31 and each dark rectangle represents a cross section through a turn of the normally conducting winding portion 32.
Figure 9 shows an alternative winding pattern where there are superconducting layers and normally conducting layers. That is to say the solenoidal winding is built up by say a first layer comprising turns of the superconducting winding portion 31 and a second iayer comprising turns of the normally conducting winding portion 32 and so on.
Figure 10 shows a third possible winding pattern. Here there are more turns in the superconducting winding portion 31 than in the normally conducting winding portion 32. In particular, this winding pattern represents a 3:1 arrangement such that there are three times as many turns in the superconducting winding portion than in the normally conducting winding portion 32. Again the turns are provided in layers where each layer is either a normally conducting layer or a superconducting layer.
Of course, other winding patterns may be used. In one particular alternative a compound winding material comprising both normally conducting material and superconducting material may be used to build up a winding module.
The above embodiments have been described in relation to use with dc currents and dc power supplies.
However, the present techniques are also applicable for use in ac systems. In particular, the techniques may be used, for example, in low frequency ac applications, say where the ac frequency is upto 10OHz. In such a case switch means may be provided in the second electrical circuit B and the switch means retained open unless and until a quench event is detected. Upon quench detection the switch means may be closed so as to put the second electrical circuit into effect. Alternatively, the provision of a crossed diode arrangement 51 in the second electrical circuit B (as for example, shown in Figure 2) may be sufficient to allow the use of an ac signal in the superconducting winding portion 31. Providing that the resulting voltage induced in the normally conducting winding portion 32 does not exceed the forward bias voltages of the diode arrangement 51, this will prevent any excessive dissipation of energy from the superconducting coil portion 31 in normal operation even when ac signals are used. Different design choices may be made depending on the frequency of the ac system as well as other factors.
When implementing one of the above embodiments in a particular system, various design choices may be made depending on the circumstances including the physical scale of the superconducting winding portion and depending on the properties of the superconductor material from which the superconductor winding portion 31 is made.
Thus, the designer may choose the design of the winding module 3 as well as the values for the dump resistors, diodes and so on with one or more of the following aims: 1) to extract as much energy stored from the magnetic field as possible and dissipate it outside of the cryostat keeping the superconducting winding portion as cold as possible. 2) to dissipate the stored energy inside the cryostat causing the superconducting winding portion and possibly other superconducting winding portions within the cryostat to heat up uniformly and avoid hotspots. 3) to select a material for the normally conducting winding portion for its high tensile strength as well as for its electrical properties. This might be particularly useful in large coils where the primary limitation in constructing the coil is mechanical strength rather than current density. As an example, the normally conducting winding portion might be formed of stainless steel or a precipitation hardened copper alloy for example “Glidcop” (Trademark). 4) to allow current to flow in the second electrical circuit B for a limited period so that it acts as a heater forcing a significant part of the superconducting winding portion to become normal and then reclose the switch means in the first electrical circuit A to remake the superconducting circuit and open a switch means provided in the second electrical circuit B so causing the energy in the normally conducting winding portion 32 to be inductively transferred back into the superconducting winding portion 31, such that a widespread quench can occur in the superconducting winding portion 31.
Claims (21)
1. A superconducting winding arrangement comprising a first electrical circuit including a superconducting winding portion and a second electrical circuit including a normally conducting winding portion, wherein the first circuit comprises switch means and the superconducting winding arrangement comprises a quench detection system for detecting quench events in the superconducting winding portion and causing opening of the switch means upon quench detection to interrupt current flow around the superconducting winding portion, wherein the winding portions are arranged and wound with one another in a winding module such as to provide magnetic coupling therebetween such that upon opening of the switch means, current in the superconducting winding portion is inductively transferred to the normally conducting winding portion for mitigating the effect of the quench.
2. A superconducting winding arrangement according to claim 1 in which winding portions are arranged so that the magnetic coupling constant between the two winding portions is at least 0.8.
3. A superconducting winding arrangement according to claim 1 or claim 2 in which the superconducting winding portion and the normally conducting winding portion are co-wound such that each turn of the superconducting winding is adjacent a corresponding turn of the normally conducting winding in the or each layer of the winding module.
4. A superconducting winding arrangement according to claim 1 or claim 2 in which the superconducting winding portion and the normally conducting winding portion are wound in respective layers such that turns of the superconducting winding occupy one or more superconducting layers and turns of the normally conducting winding occupy one or more normally conducting layers.
5. A superconducting winding arrangement according to claim 4 in which the superconducting layers and normally conducting layers are arranged alternately in the winding module.
6. A superconducting winding arrangement according to claim 4 in which there are a larger number of superconducting layers than there are of normally conducting layers.
7. A superconducting winding arrangement according to any one of claims 1 to 3 in which the winding module is wound using a compound winding material comprising both normally conducting material and superconducting material which is insulated from the normally conducting material such that the normally conducting winding portion comprises the normally conducting material of the compound winding material and the superconducting winding portion comprises the superconducting material of the compound winding material.
8. A superconducting winding arrangement according to any preceding claim in which the second electrical circuit comprises a second circuit dump resistor provided across terminals of the normally conducting winding portion such that current flowing in the second electrical circuit may decay in the second circuit dump resistor.
9. A superconducting winding arrangement according to claim 8 in which at least one diode arrangement is provided in series with the second circuit dump resistor with a forward bias voltage that is greater than a normal voltage which would be seen in the normally conducting winding portion during energisation of the superconducting winding portion such that current will not flow through the second circuit dump resistor during energisation of the superconducting winding.
10. A superconducting winding arrangement according to claim 9 in which a crossed diode arrangement is provided in series with the second circuit dump resistor, the crossed diode arrangement having respective forward bias voltages that are greater than a normal voltage which would be seen in the normally conducting winding portion during energisation of the superconducting winding portion such that current will not flow through the second circuit dump resistor during energisation of the superconducting winding irrespective of the polarity of the energisation current.
11. A superconducting winding arrangement according to any preceding claim in which the first electrical circuit comprises a first circuit dump resistor provided across terminals of the superconducting winding portion such that when the switch means is opened current may flow through the dump resistor.
12. A superconducting winding arrangement according to claim 11 in which at least one diode arrangement is provided in series with the first circuit dump resistor with a forward bias voltage that is greater than a normal energisation voltage of the superconducting winding portion such that current will not flow through the first circuit dump resistor during energisation of the superconducting winding portion.
13. A superconducting winding arrangement according to claim 12 in which a crossed diode arrangement is provided in series with the first circuit dump resistor the crossed diode arrangement having respective forward bias voltages that are greater than a normal energisation voltage of the superconducting winding portion such that current will not flow through the first circuit dump resistor during energisation of the superconducting winding irrespective of the polarity of the energisation current.
14. A superconducting winding arrangement according to any preceding claim in which the second electrical circuit comprises a switch means connected in series with the normally conducting winding portion, such that the second electrical circuit may be rendered open circuit by the switch being open.
15. A superconducting winding arrangement according to any preceding claim in which the quench detection system is arranged to close the switch in the second electrical circuit upon detection of a quench event.
16. A superconducting winding arrangement according to any preceding claim in which the superconducting winding portion is a persistent superconducting winding.
17. A superconducting winding arrangement according to any one of claims 1 to 15 in which the superconducting winding portion is a non-persistent superconducting winding.
18. A superconducting winding arrangement according to any preceding claim in which the superconducting winding arrangement is used as part of any one of a superconducting magnet, an electrical transformer, an electrical motor, and an electrical generator.
19. A quench protection method for protecting a first electrical circuit including a superconducting winding portion, the method comprising the steps of: providing a switch means in the first circuit; winding a normally conducting winding portion of a second electrical circuit with the superconducting winding portion so as to provide magnetic coupling therebetween; detecting quench events in the superconducting winding portion; and opening the switch means upon detection of a quench event to interrupt current flow around the superconducting winding portion so allowing current in the superconducting winding portion to be inductively transferred to the normally conducting winding portion for mitigating the effect of the quench.
20. A quench protection method according to claim 19 in which the method comprises the further steps of allowing current to flow in the normally conducting portion for a selected period so heating the superconducting winding portion; and re-closing the switch means in the first circuit and opening a switch in the second circuit after the selected period to allow a widespread quench to occur in the superconducting winding portion.
21. A superconducting winding arrangement comprising a first electrical circuit including a superconducting winding portion and a second electrical circuit including a normally conducting winding portion, the first circuit comprising switch means arranged such that opening of the switch means will interrupt current flow around the superconducting winding portion, wherein the winding portions are arranged and wound with one another in a winding module such as to provide magnetic coupling therebetween such that upon opening of the switch means, current in the superconducting winding portion is inductively transferred to the normally conducting winding portion for mitigating the effect of a quench in the superconducting winding portion.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB1513090.9A GB2540623A (en) | 2015-07-24 | 2015-07-24 | Superconducting winding arrangements |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB1513090.9A GB2540623A (en) | 2015-07-24 | 2015-07-24 | Superconducting winding arrangements |
Publications (2)
| Publication Number | Publication Date |
|---|---|
| GB201513090D0 GB201513090D0 (en) | 2015-09-09 |
| GB2540623A true GB2540623A (en) | 2017-01-25 |
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Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| GB1513090.9A Withdrawn GB2540623A (en) | 2015-07-24 | 2015-07-24 | Superconducting winding arrangements |
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| Country | Link |
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| GB (1) | GB2540623A (en) |
Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US10998730B1 (en) | 2019-04-26 | 2021-05-04 | NeoVolta, Inc. | Adaptive solar power battery storage system |
| US11521771B2 (en) | 2019-04-03 | 2022-12-06 | General Electric Company | System for quench protection of superconducting machines, such as a superconducting wind turbine generator |
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| US4727346A (en) * | 1985-09-11 | 1988-02-23 | Bruker Analytische Mebtechnik Gmbh | Superconductor and normally conductive spaced parallel connected windings |
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| US11521771B2 (en) | 2019-04-03 | 2022-12-06 | General Electric Company | System for quench protection of superconducting machines, such as a superconducting wind turbine generator |
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| US11605952B1 (en) | 2019-04-26 | 2023-03-14 | NeoVolta, Inc. | Adaptive solar power battery storage system |
Also Published As
| Publication number | Publication date |
|---|---|
| GB201513090D0 (en) | 2015-09-09 |
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