GB2436648A

GB2436648A - Superconducting earth connection

Info

Publication number: GB2436648A
Application number: GB0606160A
Authority: GB
Inventors: Stephen Mark Husband
Original assignee: Rolls Royce PLC
Current assignee: Rolls Royce PLC
Priority date: 2006-03-28
Filing date: 2006-03-28
Publication date: 2007-10-03
Also published as: GB0606160D0

Abstract

A reference point of an electrical system is connected to earth 24 through a series fault current limiter 22 comprising an element 26 that is superconducting during normal use and is rendered non-superconducting by the occurrence of excessive fault current. Super conducting element 26 may remain superconducting while conducting normal earth leakage current, and may be a material which quenches when conducting current that exceeds its critical current. A shunt 30 may be connected in parallel with the superconductor 26 to reduce the heat dissipated by the superconductor 26 to cryogenic arrangements 32 when in fault conditions and assist recovery from a quenched condition. Shunt 30 may be resistive, and may have lower impedance than that of superconductor 26. Monitor 34 may detect a voltage difference across superconducting element 26 to indicate the onset of fault conditions by a signal 36. The reference point (fig 1, 20) may be a neutral point and the electrical system may comprise a plurality of aforesaid connections.

Description

EARTHING ARRANGEMENTs Embodiments of the present invention relate to

earthing arrangements for electrical systems.

There are three broad classes of earthing arrangements for electrical systems. For some, the system is unearthed, having no conductive path from the system to earth. This has advantages of continuity of service during fault conditions. However, an unearthed system is, in reality, capacitively coupled to earth, which can give rise to EMC problems and safety problems. An alternative is to provide a very low impedance path from an appropriate point of the system to earth, which is known as a solidly earthed'' system. This improves safety and EMC performance and reduces the incidence of protection circuitry being unnecessarily tripped, particularly over long transmission lengths or during high frequency transients. Earth fault detection is also simplified. However, solidly earthed systems give rise to very high fault currents to earth, on the occurrence of an earth fault, because of the low impedance in the connection. Fault currents can be reduced by using high impedance'' earthing, with a high impedance route from an appropriate point of the system to the earth.

High impedance earthing is very commonly used, but lacks some of the advantages of a solidly earthed system.

Embodiments of the present invention provide an electrical system having an earth connection between a reference point of the system and earth, the earth connection comprising a fault current limiter element connected in series between the reference point and earth to provide a path for fault current from the reference point to earth, the element being superconducting, during normal use, and being rendered non-superconducting by the occurrence of an excessive fault current.

Preferably the fault current limiter element is arranged to remain superconducting while conducting earth leakage current arising during normal use.

The earth connection preferably includes a shunt in parallel with the fault current limiting element. The shunt may have lower impedance than the fault current limiting element when non-superconducting. The shunt may be resistive.

Monitoring means may be provided for monitoring, in use, the voltage across the fault current limiting element for detecting and/or responding to the onset of fault conditions.

The reference point may be a neutral conductor of the system.

The fault current limiting element preferably includes a conducting material which quenches from a superconducting to a non-superconducting state when conducting current in excess of the critical current.

The electrical system may comprise a plurality of earth connections, as aforesaid.

Embodiments of the present invention also provide an earth connection for an electrical system, for providing a connection between the system and earth, the earth connection comprising a fault current limiter element for connection in series between the system and earth to provide a path for fault current from the system to earth, the element being superconducting, during normal use, and being rendered non-superconducting by the occurrence of an excessive fault current.

Preferably the earth connection includes a shunt in parallel with the fault current limiting element. The shunt may have a lower impedance than the fault current limiting element when non-superconducting. The shunt may be resistive.

Preferably, monitoring means are provided for monitoring the voltage across the fault current limiting element, in use, for detecting and/or responding to the onset of excessive fault current.

Preferably the fault current limiting element is a material which quenches from a superconducting to a non-superconducting state when conducting current in excess of a critical current.

Example embodiments of the present invention will now be described in more detail, by way of example only, and with reference to the accompanying drawings, in which: Fig. 1 is a schematic and simplified diagram of an electrical system having an earth connection; and Fig. 2 is a schematic diagram of the earth connection used in the circuit of Fig. 1.

Fig. 1 illustrates an electrical system 10. The electrical system is illustrated in Fig. 1 in highly schematic form, having a generator arrangement 12 illustrated schematically as three coils 14 each generating a respective phase 16 for supplying respective parts 18 of the system 10. A common neutral conductor 20 is also provided, to which the parts 18 are also connected. To the extent just described, the system 10 is generally conventional and typical of very many circuit arrangements.

Earthing arrangements for the system 10 are by provision of an earth connection 22 between a reference point of the system, in this case the neutral conductor 20, and earth 24. The earth connection 22 comprises a fault current limiter element 26 (Fig. 2) connected in series between the neutral conductor 20 and earth 24 to provide a path 28 for fault current from the neutral conductor 20 to earth 24. The element 26 is superconducting, during normal use and is rendered non-superconducting by the occurrence of an excessive fault current, as will be described.

In more detail, the connection 22 includes the element 26 and a shunt 30. The element 26 and the shunt 30 are connected in parallel with each other, between the neutral conductor 20 and earth 24.

The fault current limiting element 26 is arranged to remain superconducting during normal use, including the situation in which typical earth leakage current is being conducted from the neutral conductor 20 to earth 24.

Various superconducting materials will be known to the skilled reader. Examples include Nb2Sn and Nb-TL. Bodies of these materials may remain in a superconducting condition, according to their temperature, the magnetic field applied to the element, and the current being conducted. Thus, for a particular magnetic field strength and temperature the element will cease to be superconducting when the current exceeds a critical current, the value of which will be different for different combinations of temperature and external magnetic field.

The process of changing from a superconducting to a non-superconducting condition is generally known as quenching1!. Within the connection 22, the temperature of

the element 26 and the applied magnetic field are

preferably maintained by appropriate arrangements at 32, so that quenching of the element 26 is controlled substantially solely by the current being conducted. The arrangements 32 include cryogenic systems for maintaining very low temperatures of the element 26.

The voltage across the element 26 is monitored at 34.

When the element 26 is superconducting, there will be no voltage difference across the element. When the element 26 quenches, any current conducted will give rise to a voltage difference, detected by the monitor 34. Accordingly, the voltage across the element 26 can be used as an output 36, indicating the onset of fault conditions which have caused the element 26 to quench. The output 36 can be passed to protection systems to initiate an appropriate response to the fault condition detected.

The shunt 30 is preferably a lower impedance than the element 26 when not superconducting, and is preferably resistive. The significance of these factors will become apparent from the following description of the manner of operation of the connection 22.

During normal use, with the electrical system 10 operating normally, a small amount of earth leakage current may be passed by the connection 22, between the neutral conductor 20 and earth 24. This current will be too small to quench the element 26. Accordingly, the earth leakage current will pass along the path 28, through the element 26. No current is expected to pass along the path 38, through the shunt 30, because the shunt 30 is shorted by the zero impedance of the superconducting element 26.

However, if an earth fault now arises within the system 10, fault current will begin to flow between the neutral conductor 20 and earth 24, with a magnitude greater than the earth leakage current previously described. When the fault current has reached a critical value determined by the temperature and magnetic conditions set by the arrangement 32, the element 26 will quench to a non-superconducting state. The connection 22 now presents to the fault current a non-zero impedance provided by the shunt 30 and the quenched element 26, in parallel.

Accordingly, it can be understood from the above description that while the element 26 is superconducting, prior to the onset of fault conditions, the connection 22 presents zero impedance between the neutral conductor 20 and earth 24, resulting in the system 10 operating as a solidly earthed system. Immediately the fault conditions cause the element 26 to quench, the connection 22 provides an impedance from the neutral conductor to the earth 24 and, by appropriate setting of component parameters, the electrical system 10 then performs as a system with high impedance earthing with an earth fault with limited earth fault currents.

This allows the fault to be tolerated until other systems can locate and/or isolate the fault. In particular, quenching of the element 26 will result in a voltage across the element 26 and a change in the output 36, allowing these other systems to be initiated.

In the example being described, the impedance of the element 26, once quenched, is higher than the impedance of the shunt 30. Accordingly, once the element 26 has quenched, more fault current will pass through the shunt 30 than through the element 26. This helps reduce the load on the arrangements 32 arising from the current being passed, particularly the requirement to dissipate heat generated within the element 26, when quenched. Once the fault has been corrected or isolated, current through the connection 22 will fall back to the normal earth leakage current level, allowing the element 26 to recover to its superconducting state at which point, the shunt 30 is again shorted out of circuit by the element 26.

In addition to the shunt 30 providing support for the element 26 when recovering to its superconducting state, the shunt 30 also provides a fail-safe facility in the event that a fault within the element 26 or its associated arrangements 32, 34 results in the element 26 being taken out of circuit. In that event, the shunt 30 will provide high impedance earthing from the system 10 to earth 24.

It can be seen from the above description that the

arrangements described are able to act in direct response to the earth fault current, with no control circuitry being required to detect the fault and initiate the response. In addition, monitoring of the voltage across the element 36 provides a stable and reliable earth fault detection monitor.

Many variations and modifications can be made to the apparatus described above, without departing from the scope of the present invention. In particular, other quenchable superconducting elements can be used in place of the element 26 and many arrangements can be envisaged for controlling the environment of the superconducting element, in order to set the current level at which the element is quenched. The shunt 30 may be omitted but is preferred for the reasons set out above.

Whilst endeavouring in the foregoing specification to draw attention to those features of the invention believed to be of particular importance it should be understood that the Applicant claims protection in respect of any patentable feature or combination of features hereinbefore referred to and/or shown in the drawings whether or not particular emphasis has been placed thereon.

Claims

CLAIMS

1. An electrical system having an earth connection between a reference point of the system and earth, the earth connection comprising a fault current limiter element connected in series between the reference point and earth to provide a path for fault current from the reference point to earth, the element being superconducting, during normal use, and being rendered non-superconducting by the occurrence of an excessive fault current.

2. A system according to claim 1, wherein the fault current limiter element is arranged to remain superconducting while conducting earth leakage current arising during normal use.

3. A system according to claim 1 or 2, wherein the earth connection includes a shunt in parallel with the fault current limiting element.

4. A system according to claim 3, wherein the shunt has lower impedance than the fault current limiting element when non-superconducting.

5. A system according to claim 3 or 4, wherein the shunt is resistive.

6. A system according to any preceding claim, wherein monitoring means are provided for monitoring, in use, the voltage across the fault current limiting element for detecting and/or responding to the onset of fault conditions.

7. A system according to any preceding claim, wherein the reference point is a neutral conductor of the system.

8. A system according to any preceding claim, wherein the fault current limiting element includes a conducting material which quenches from a superconducting to a non-superconducting state when conducting current in excess of the critical current.

9. A system according to any preceding claim, wherein the electrical system may comprise a plurality of earth connections, as aforesaid.

10. An earth connection for an electrical system, for providing a connection between the system and earth, the earth connection comprising a fault current limiter element for connection in series between the system and earth to provide a path for fault current from the system to earth, the element being superconducting, during normal use, and being rendered non-superconducting by the occurrence of an excessive fault current.

11. A connection according to claim 10, including a shunt in parallel with the fault current limiting element.

12. A connection according to claim 11, wherein the shunt has a lower impedance than the fault current limiting element when non-superconducting.

13. A connection according to claim 11 or 12, wherein the shunt is resistive.

14. A connection according to any of claims 10 to 13, wherein monitoring means are provided for monitoring the voltage across the fault current limiting element, in use, for detecting and/or responding to the onset of excessive fault current.

15. A connection according to any of claims 10 to 14, wherein the fault current limiting element is a material which quenches from a superconducting to a non-superconducting state when conducting current in excess of a critical current.

16. An electrical system substantially as described above, with reference to the accompanying drawings.

17. An earth connection for an electrical system, substantially as described above, with reference to the accompanying drawings.

18. Any novel subject matter or combination including novel subject matter disclosed herein, whether or not within the scope of or relating to the same invention as any of the preceding claims.