QUENCH PROTECTION ARRANGEMENT

Abstract

A quench protection arrangement for a superconducting magnet is disclosed. The arrangement comprises: a superconducting magnet comprising a plurality of magnet sections; a plurality of varistors, wherein each of the plurality of varistors is electrically connected in parallel across a respective one of the plurality of magnet sections; and a heater arrangement electrically connected to the plurality of varistors and configured to apply heat to each of the plurality of magnet sections in response to a change in a voltage across any one or more of the plurality of varistors. A method of protecting a superconducting magnet is also disclosed.

Claims

1. A quench protection arrangement for a superconducting magnet, the arrangement comprising: a superconducting magnet comprising a plurality of magnet sections; a plurality of varistors, wherein each of the plurality of varistors is electrically connected in parallel across a respective one of the plurality of magnet sections; and a heater arrangement electrically connected to the plurality of varistors and configured to apply heat to each of the plurality of magnet sections in response to a change in a voltage across any one or more of the plurality of varistors.

2. An arrangement according to claim 1, wherein the heater arrangement is configured to apply heat in response to a voltage across any one or more of the plurality of varistors reaching a threshold voltage value.

3. An arrangement according to claim 1, wherein the heater arrangement comprises a plurality of heaters, wherein each heater of the plurality of heaters is positioned for applying heat to a respective one of the plurality of magnet sections.

4. An arrangement according to claim 1, wherein the heater arrangement is configured to generate heat to be applied to each of the plurality of magnet sections by way of Joule heating as a result of the voltage change across the said one or more of the plurality of varistors.

5. An arrangement according to claim 1, wherein one or more of the plurality of varistors each comprises two or more varistor components.

6. An arrangement according to claim 5, wherein each varistor component is formed as a disc, and for each of one or more varistors that comprise two or more of the said varistor components, the varistor components are arranged in a stack.

7. An arrangement according to claim 1, further comprising an active quench protection system that comprises: a quench detector module configured to detect a quench occurring in one or more of the plurality of magnet sections based upon a voltage across one or more of the plurality of varistors reaching a predetermined quench threshold voltage value; and a quench inducer system configured to effect a quench condition in one or more of the plurality of magnet sections in response to the quench detector module detecting a quench occurring.

8. An arrangement according to claim 7, configured such that a quench occurring in one or more of the plurality of magnet sections while the magnitude of the electrical current through the magnet sections is less than 50% of a predetermined current value corresponding to an operating current of the magnet causes the voltage across the one or more of the plurality of varistors to reach the predetermined quench threshold voltage value.

9. An arrangement according to claim 1, wherein the superconducting magnet sections comprise at least one Low Temperature Superconductor, LTS, magnet section and at least one High Temperature Superconductor, HTS, magnet section, and wherein the quench protection arrangement comprises an HTS quench protection system adapted to apply quench protection to each of the at least one HTS magnet section in response to a voltage across any one or more of the plurality of varistors reaching a threshold voltage value.

10. An arrangement according to claim 9, wherein HTS quench protection system is adapted to apply the quench protection by way of any one or more of: causing energy to be dissipated from the at least one HTS magnet section to a body external to the magnet; and applying an alternating current having a predetermined frequency and magnitude configured to cause Joule heating within a coil of the at least one HTS magnet section.

11. An arrangement according to claim 1, wherein each of the plurality of varistors comprises silicon carbide.

12. An arrangement according to claim 1, wherein the heater arrangement is associated with the plurality of varistors such that, when a quench occurs in one of the plurality of magnet sections, a resulting voltage across the said varistor causes the heating arrangement to provide heat to at least one further magnet section of the plurality of magnet sections.

13. A method of protecting a superconducting magnet comprising a plurality of magnet sections in which each of a plurality of varistors is electrically connected in parallel across a respective one of the plurality of magnet sections, and a heater arrangement is electrically connected to the plurality of varistors, the method comprising applying heat to each of the plurality of magnet sections in response to a change in a voltage across any one or more of the plurality of varistors.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0046] Examples of the present invention will now be described, with reference to the accompanying drawings, in which:

[0047] FIGS. 1A and 1B are line graphs respectively illustrating the properties of a typical varistor in comparison with a typical linear resistor, and the characteristics of an exemplary varistor compared to those of a resistor over a range of interest;

[0048] FIG. 2 is a line graph comparing varistor and linear resistor characteristics at lower currents;

[0049] FIG. 3 is a schematic illustration of a cross-section through a magnet test coil used to model quench behaviour with varistors being used in place of resistors in a magnet quench protection arrangement;

[0050] FIG. 4 is a line graph showing a comparison of modelled coil voltages across a two-section magnet with a quenching section being protected with a varistor as well as it being protected by a fixed resistor;

[0051] FIG. 5 is a line graph plotting modelled coil currents through a magnet in which both coils are quenching, and showing the currents when the magnet is protected with varistors and when it is protected with linear resistors;

[0052] FIG. 6 is a schematic diagram of an example protection arrangement, illustrating the protection circuit in relation to the superconducting magnet circuit, wherein the magnet shown is a simplified example having only two coil sections;

[0053] FIG. 7 is a schematic drawing including a comparison of a protection circuit for a protection arrangement according to the invention, across one protected coil section together with an equivalent circuit according to the prior art; and

[0054] FIG. 8 is a schematic diagram showing an example arrangement and protection circuit according to the invention wherein internally activated quench heaters on an LTS magnet are combined with a trigger for externally activated quench protection for an HTS insert in the magnet.

DESCRIPTION OF EMBODIMENTS

[0055] Quench modelling has been performed in order to demonstrate the capabilities of varistors in quench protection arrangements. For simplicity, and in view of the propensity for more complex modelled scenarios to render less clear the effect of the varistors or lead to systematic modelling errors, a simple, two-coil design has been used to test the concept. This is for illustrative purposes only, and it is envisaged that the quench protection arrangement may be implemented with any number of magnet coils. Computer software has been used to model the coil configuration and quench modelling.

[0056] FIG. 3 shows a cross-section through one half of the cylindrical coils as modelled in this general example. The coils are shown as cylindrical blocks centred on R=0 (corresponding to the y axis in the figure) and extending in the z direction (parallel with the x axis in the figure). Both coil 1 and coil 2 consist of NbTi wire of the same size and the coils have been chosen to have comparable resistance and inductance for simplicity and for comparison of the quench modelling results with approximate theoretically-derived analytical calculations. The varistor parameters used have been derived from experimental measurements of their voltage-current characteristics at cryogenic temperatures. The superconducting wire and other material properties as well as the other internal model parameters used have been long established and used reliably in the quench model computer program during magnet design and for analysis of magnet test results.

[0057] The effect of varistor-based quench protection on the derived voltage across the quenching section has been demonstrated by way of modelling a quench starting in coil 2. The case where the magnet is protected using varistors in place of resistors has been found to result in a significantly faster rise in voltage compared to the case where the quenching section is protected with a conventional resistor. Based on the data visualised in FIG. 4, it can be seen that a quench heater may be triggered approximately 50 ms earlier by using the appropriate varistors in the protection circuit, rather than resistors.

[0058] In order to demonstrate the effect of using varistors on the maximum over-currents induced in coils in a quench, a model has been run in which both of the modelled coils are quenching. The results are thus illustrative of the case where quench heaters have been triggered across all coils. This would also approximate the case if the effects from the coil in which the quench initiated could be used to induce a quench in the other coils very soon after the first coil starts to quench.

[0059] The model showed that the peak current in the slowest quenching section was much reduced when the magnet was protected with varistors, compared to the magnet protected with conventional resistors. Based on the data visualised in FIG. 5, the peak over-current could be reduced by more than half by using the appropriate varistors in the protection circuit instead of the conventional resistors.

[0060] The illustrative output from the quench modelling is shown in FIGS. 4 and 5.

[0061] Examples of quench protection arrangement employing the principles demonstrated by the quench modelling are now described.

[0062] In some examples, the varistors are preferably connected in series with “back-to-back” diodes across the protected magnet sections, with the derived voltage used to activate heaters in the ‘passive with heaters’ scenario described above. Protection schemes such as those described in GB 2514372 B are suitable for being implemented as a means of protecting magnet coils in response to a voltage change across a varistor.

[0063] FIG. 6 is a schematic representation of a first example arrangement, in which, for simplicity, a magnet having only two coil sections is shown. Applications of the arrangement are envisaged to include significantly more protected magnet coil sections than this simplified example.

[0064] FIG. 7 shows a schematic representation of an example protection circuit comprising varistors at the right of the figure, together with a circuit according to the prior art comprising linear resistors at the left of the figure.

[0065] In a further example, which is particularly suited to protecting magnets including an LTS magnet with an HTS insert, different protection schemes are used for the LTS coils and HTS coils respectively. These two magnet parts may run in parallel, using separate power supplies, or may run in series. This example employs the same ‘passive with heaters’ scheme for protecting the LTS section as the previous example arrangement. In addition, the arrangement is configured to detect, more rapidly than would be possible with prior art arrangements, the voltage change across the LTS magnet protection elements in order to activate a quench protection scheme externally for the HTS magnet. Such a combination of protection schemes for these hybrid-type magnets is alluded to above. FIG. 8 shows schematically the additional elements involved in this example.

[0066] Details of example SiC varistors and superconducting magnet quench protection arrangements comprising them are now described.

[0067] As noted above, varistor properties can be characterized by the relation V=CI.sup.β. The “beta value”, β, defining the degree of nonlinearity of the varistor affects the speed of discharge. This can be modified by way of dopants in the material of the varistor, and process adjustments.

[0068] Some example varistors are semiconductor devices manufactured from SiC particles in a clay matrix, to produce a disc or tile.

[0069] Failure-mode testing of an example arrangement has been carried out, in liquid nitrogen and involving increasing the energy to more than five times that for which the device was at room temperature. It was found that a non-propagating hotspot formed on the varistor disc, which failed via short circuit and continued to pass current. This is a particularly useful attribute for magnet protection. The SiC material of the example varistor maintained its integrity in cryogenic conditions, and with material optimisation it is envisaged that further examples may provide effective replacements for linear dump resistors.