GB2439764A

GB2439764A - Fault current limiting

Info

Publication number: GB2439764A
Application number: GB0606013A
Authority: GB
Inventors: Stephen Mark Husband; David Reginald Trainer
Original assignee: Rolls Royce PLC
Current assignee: Rolls Royce PLC
Priority date: 2006-03-25
Filing date: 2006-03-25
Publication date: 2008-01-09
Also published as: GB0606013D0

Abstract

A superconducting element 18a quenches into a non-superconducting state when a fault current flows through it, from a generator 12 to a load 14, 22. A bleed circuit 32, in parallel with the superconducting element 18a, is switched to a low impedance state in response to the removal of the fault condition, thereby reducing the current through superconductor 18a, removing resistive heating effects and allowing it to recover its superconducting state. Bleed circuit 32 can then be returned to a high impedance, or open circuit condition. Bleed circuit 32 may comprise a switch 34 that may take the form of a semiconductor switch, a circuit breaker or a spark gap (fig 3, 34a) which may have a trigger element. Bleed circuit 32 may also comprise a series inductive element 40 and a series fuse element 42 that may itself be a quenchable superconducting fuse. A control element 36 may detect the removal of the fault condition and may monitor the voltage across the superconducting element 18a to detect a change caused by removal of the fault condition. Control element may be a passive arrangement 36a, and receive a fault removal signal from a network protection system 26 causing bleed circuit 32 to switch to the low impedance state. The invention is intended to limit fault current within an electrical distribution system.

Description

FAULT CURRENT LIMITING

The present invention relates to fault current limiting. In particular, but not exclusively, the invention relates to limiting fault currents within electrical distribution systems.

Fault conditions with electrical distribution systems have the potential for creating high fault current levels.

It is a common condition to require the electrical supply to be maintained in the event of a fault occurring, until isolators act to switch a fault out of circuit. Fault current will flow until the isolators act. Consequently, electrical generators may be subjected to the full fault current level arising from the fault condition and the generators must therefore be rated to avoid damage in this circumstance. High fault currents may place mechanical demands on generators, such as high torques, or significant speed variations, and electrical demands, particularly high currents. Inductors have been proposed in series with the generators, to limit fault current levels, but these result in power losses during normal operation and can give rise to difficulties in voltage regulation in a system, particularly if varying current levels are normal.

Superconducting elements have also been proposed for fault current limitation, connected in series to quench to a non- superconducting state in the event of excessive fault current being experienced. After quenching to the non-superconducting state, the element presents an impedance and thus contributes to a reduction in the fault current.

Embodiments of the present invention provide a fault current limiting arrangement having: a superconducting element quenchable from a superconducting state to a non-superconducting state by a fault current flowing through the element, a bleed circuit in parallel with the superconducting element and operable selectively to change between a high and a low impedance state, and; a control arrangement operable in response to the removal of a fault condition to cause the bleed circuit to change to the low impedance state, to reduce current through the superconducting element.

The bleed circuit may include a switchable element in series in the bleed circuit and having a high impedance state and being switchable to a low impedance state. The switchable element may include a spark gap. The switchable element may include a trigger arrangement operable to trigger a spark in the spark gap. The switchable element may be a solid state switch or circuit breaker.

The bleed circuit may include a fuse element in series in the bleed circuit. The fuse element may be a quenchable superconducting element. The bleed circuit may include an inductive element in series in the bleed circuit.

The control arrangement may be operable to detect the removal of a fault condition. The control arrangement may monitor the voltage across the superconducting element to detect a change caused by removal of a fault condition.

Alternatively the control arrangement may be operable to receive a fault removal signal from a network protection system, and to operate in response thereto to cause the bleed circuit to change to the low impedance state.

The bleed circuit may provide an open circuit across the superconducting element, when not in the low impedance state.

Examples 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 diagram of a fault current limiting arrangement according to a previous proposal; Fig. 2 is a schematic diagram of a fault current limiting arrangement according to an embodiment of the invention; and Fig. 3 corresponds with part of Fig. 2, showing an alternative embodiment of the invention.

Fig. 1 shows a fault current limiting arrangement indicated generally at 10 for a generator 12 providing power for a load 14, illustrated as an impedance ZL.

The generator 12 is illustrated as having a source impedance 16 of size Z, in series with the load 14 and generator 12.

The fault current limiter arrangement io includes a superconducting element 18 in series between the source impedance 16 and the load 14.

During normal conditions, current from the generator 12 passes through the element 18 to the load 14.

Fault conditions are illustrated schematically by a switch 20 and a fault load 22, having an impedance ZF. The occurrence of a fault condition is equivalent to the switch changing state to switch the load 14 out of circuit, and to switch the fault load 22 into circuit, so that the generator 12 is supplying the impedance Z. It is to be understood that the value of ZF would depend on the nature of the fault, but may be very low, giving rise to high fault currents.

In the event that the type of fault results in ZF being lower than ZL, the occurrence of the fault will result in additional current demands on the generator 12. Limitation of this fault current is provided by the element 18 which will not remain in its superconducting state when the current it carries rises above a threshold value. The threshold value depends on its working conditions, particularly temperature and magnetic field. In this example, the temperature of the element 18 and the magnetic field experienced by it are maintained substantially constant by arrangements at 24, so that the superconducting state of the element 18 is determined by the current being carried.

Accordingly, when the fault load 22 is switched into circuit, increasing the current from the generator 12 to a fault current level, the element 18 is quenched to a non-superconducting state in the event that the fault current level is above a level set by the working conditions of the element 18. The element 18 becomes resistive and limits or reduces the current flowing. This reduces the demand on the generator 12 during fault conditions.

Network protection systems 26 will detect the onset of fault conditions. This may be by monitoring current levels, for example. When a fault condition is detected, the system 26 identifies the location and operates appropriate trip gear to switch the faulted part of the circuit out of circuit. This restores the circuit to normal conditions and is therefore equivalent to the system 26 restoring the switch 20 to the original state shown in Fig. 1, connecting the normal load ZL into the circuit.

At the time the switch 20 returns to the original state, the element 18 will be non-superconducting and will thereafter be required to carry the normal non-fault current. We have realised that in some conditions, normal circuit current and the resistance of the quenched element 18 can result in sufficient resistive power loss within the element 18 to heat the element 18 to a temperature which maintains the element 18 in the quenched non-superconducting state, even after the fault has been isolated. Accordingly, we have realised that it is necessary to remove the element 18 from the circuit, when normal conditions are restored, to allow the element 18 to recover to its superconducting state, before being connected back into circuit. This delays recovery of the required protection for the generator 12.

Fig. 2 illustrates an embodiment of a fault current limiting arrangement 30. Some elements illustrated in Fig. 2 correspond with elements of Fig. 1 and are given the same

numerals for simplicity of description.

The arrangement 30 has a superconducting element 18a which is quenchable from a superconducting state to a non-superconducting state by a fault current flowing through the element. Temperature and magnetic control is provided by an arrangement 24a.

The element 18a is in parallel with a bleed circuit 32. The principal element of the bleed circuit 32 is a switchable element 34, in series in the bleed circuit 32.

The element 34 has a first, high impedance state (illustrated as open circuit) and is switchable to a low impedance state (illustrated as a short) . Accordingly, setting the state of the element 34 causes the state of the bleed circuit 32 to be selected as a high impedance state or a low impedance state. A control arrangement 36, 36a, to be described in more detail below, is operable in response to the removal of a fault condition, as will be described, to cause the bleed circuit 32 to change to the low impedance state. This reduces current through the superconducting element 18a, as will be described.

In more detail, the bleed circuit 32 is illustrated, for example, as three components in series, across the element 18a. These elements are a series inductive element 40, the switchable element 34 and a fuse element 42. In a first alternative, the control arrangement 36 is operable to detect the removal of a fault condition. In this example, the removal of a fault condition is detected by monitoring the voltage at 44, that is, by monitoring the voltage across the superconducting element 18a, as follows.

During normal conditions, prior to the onset of fault conditions, the element 18a will be in the superconducting state and thus the voltage drop across the element 18a will nominally be zero volts. At the onset of fault conditions, the element 18a quenches to a non-superconducting state as the fault load ZF is connected into the circuit, as described above in relation to Fig. 1. Accordingly, the voltage across the element 18a is now determined by the ratio of the source impedance Z, fault load impedance ZF and the quenched impedance of the element 18a.

When the fault is isolated, and the network protection systems 26 switch the fault load ZF out of circuit, and the normal load ZL back into circuit, the voltage across the element 18a then changes to a voltage determined by the ratio of the source impedance Z, the normal load ZL and the quenched impedance of the element 18a. Thus, a step change in the voltage across the element 18a occurs when the fault load ZF is switched out of circuit, and the normal load ZL is switched back into circuit. This step change in the voltage is monitored at 36 by a voltage monitor circuit.

When the monitor circuit 36 senses that the fault condition has been removed by operation of the network protection systems 26, the monitor circuit 36 controls the switchable element 34 to switch from the open circuit condition illustrated in Fig. 2, to the alternative condition connecting the inductive element 40 and fuse 42 in series across the element 18. This results in a low impedance path in parallel with the element l8a, which may be arranged to be nominally a short across the element 18a.

The low impedance path provided by the bleed circuit 32 thus results in at least some (which may be most or all) of the current from the generator 12 flowing through the bleed circuit 32 rather than the element l8a. This reduction or removal of current flow from the element 18a reduces or removes resistive heating effects within the element 18a, allowing the element l8a to cool under the control of the arrangement 24a to the desired operating conditions, thereby re-establishing the element l8a in its superconducting state. This re-establishes the element 18a as a zero impedance path for current from the generator 12.

The switchable element 34 can then be returned to its open circuit condition.

It is to be noted that this sequence restores the superconducting element 18a to its superconducting state without requiring the element l8a to be taken out of circuit for cooling. Moreover, the provision of a low impedance bypass of the element 18a, through the bleed circuit 32, allows normal or nearly normal network voltage regulation to be restored as soon as the switchable element 34 is switched to its low impedance state.

In the embodiment which has been described, the control arrangement for the switchable element 34 is active to detect the removal of a fault condition. In an alternative embodiment, the control arrangement may be a passive arrangement 36a which receives a signal from the network protection systems 26, at 48. This signal indicating that the network protection systems 26 have removed the fault. In response to the fault removal signal 48, the control arrangement 36a switches the switchable element 34 to the low impedance state, to switch the bleed circuit 32 into circuit across the element 18a, with subsequent operation of the circuit being as has just been described in relation to Fig. 2.

The inductive element 40 is provided in series in the bleed circuit 32 to limit any current surges, particularly at the point that the bleed circuit 32 is switched to the low impedance state. The fuse 42 is similarly present to provide protection against excessive current and may itself be a quenchable superconducting element. Either or both of the elements 40, 42 may be omitted.

Various different arrangements can be used to provide the switchable element 34. For example, the element 34 may be an appropriate solid state switch or a circuit breaker.

In a preferred arrangement, which can now be described with reference to Fig. 3, a triggerable spark gap 34a is used as the switchable element.

In Fig. 3, components which are common to the arrangements of Fig. 2 and Fig. 3 are given the same numerals and will not be described again, in the interests of brevity.

The triggerable spark gap 34a is connected in series between the inductive element 40 and the fuse 42. The spark gap 34a has two electrodes 50a, 50b separated by a gap 52 in an evacuated chamber 54. The dimensions and pressure within the chamber 54 set a voltage at which breakdown will occur, resulting in an arc across the gap 52, between the electrodes 50a, 50b. This changes the gap 52 from a high impedance state (open circuit) to a low impedance path, while an arc exists across the gap 52. The arc will be maintained across the gap 52, while sufficient current flows.

Accordingly, an arc can form across the gap 52 in response to a voltage across the gap 52, arising from voltages within the bleed circuit 32. Alternatively, the arc can be triggered by operation of a trigger circuit 56.

The trigger circuit 56 is operable to inject a high voltage into the gap 52, by means of a needle electrode 58, resulting in the creation of an arc across the gap 52.

Once the arc has formed, it will be self-sustaining if sufficient current flows between the electrodes 50a, 50b, and the voltage on the needle electrode 58 can therefore be removed.

The trigger circuit 56 is controlled by an input 60, which may be from the voltage monitor circuit 36, or from the network protection system 26.

The alternative illustrated in Fig. 3 will function in the following manner. During normal conditions, no arc exists in the gap 52. The electrodes 50a, 50b are open circuit. All current from the generator 12 to the load 14 passes through the element 18a, which is in its superconducting state.

No current will normally flow through the bleed circuit 32. However, the element 34a provides over voltage protection for the element 18a, so that in the event of an over voltage occurring across the element l8a, sufficient to cause the spark gap 52 to break down, the over voltage is removed from the element 18a.

The breakdown setting for the spark gap 34a is set to be above the voltage which will arise across the element l8a when the element l8a has quenched during fault conditions. That is, the breakdown voltage for the gap 52 is above the voltage set by the ratio of the source impedance Z, fault load ZF and the element l8a when quenched. Accordingly, during fault conditions, the spark gap 34a will remain open circuit. In this condition, the element l8a is non-superconducting and provides fault current limiting, as described above.

When the fault condition is removed, restoring the normal load ZL to the circuit, the element 18a will initially remain non-superconducting, as described. At this point, the input 60 causes the trigger circuit 56 to trigger the spark gap 34a, creating an arc in the gap 52 and switching the element 34a to its low impedance state.

This connects the bleed circuit 32 into circuit across the element l8a, resulting in current from the generator 12 passing through the bleed circuit 32, reducing or eliminating the current carried by the element 18a, allowing the element 18a to recover its superconducting state more readily, as has been described above in relation to Fig. 2.

The arrangements which have been described in relation to Figs. 2 and 3 allow the generator 12 to remain on-line throughout the fault event, but fault current levels are limited at all times, thus reducing the mechanical and electrical ratings required of the generator 12. This, in turn, is expected to result in reduced electro-dynamic effects, leading to reduced mechanical stresses within the generator 12, reduced thermal effects leading to increased component lifetime, reduced fault currents leading to reduced damage to insulation and conductors, and reduced arc energy, leading to reduced damage during fault conditions, improved safety and reduced fire risks. The reduction in required ratings is expected to result in a reduction in weight, space and cost of components, particularly conductors.

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. A fault current limiting arrangement having: a superconducting element quenchable from a superconducting state to a non-superconducting state by a fault current flowing through the element, a bleed circuit in parallel with the superconducting element and operable selectively to change between a high and a low impedance state, and; a control arrangement operable in response to the removal of a fault condition to cause the bleed circuit to change to the low impedance state, to reduce current through the superconducting element.

2. An arrangement according to claim 1, wherein the bleed circuit includes a switchable element in series in the bleed circuit and having a high impedance state and being switchable to a low impedance state.

3. An arrangement according to claim 2, wherein the switchable element includes a spark gap.

4. An arrangement according to claim 3, wherein the switchable element includes a trigger arrangement operable to trigger a spark in the spark gap.

5. An arrangement according to claim 2, wherein the switchable element is a solid state switch or circuit breaker.

6. An arrangement according to any preceding claim, wherein the bleed circuit includes a fuse element in series in the bleed circuit.

7. An arrangement according to claim 6, wherein the fuse element is a quenchable superconducting element.

8. An arrangement according to any preceding claim, including an inductive element in series in the bleed circuit.

9. An arrangement according to any preceding claim, wherein the control arrangement is operable to detect the removal of a fault condition.

10. An arrangement according to claim 9, wherein the control arrangement is operable to monitor the voltage across the superconducting element to detect a change caused by removal of a fault condition.

11. An arrangement according to any of claims 1 to 8, wherein the control arrangement is operable to receive a fault removal signal from a network protection system, and to operate in response thereto to cause the bleed circuit to change to the low impedance state.

12. An arrangement according to any preceding claim, wherein the bleed circuit provides an open circuit across the superconducting element, when not in the low impedance state.

13. A fault current limiting arrangement, substantially as described above, with reference to the accompanying drawings.

14. 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.