WO1999034489A1

WO1999034489A1 - An electric switching device

Info

Publication number: WO1999034489A1
Application number: PCT/SE1998/001192
Authority: WO
Inventors: Arne Gustafsson; Anders Sunesson; Mats Ekberg; Mikael Bergkvist; Per Skytt
Original assignee: Abb Ab
Priority date: 1997-12-17
Filing date: 1998-06-17
Publication date: 1999-07-08
Also published as: AU8051998A; EP1040540A1

Abstract

An electric switching device comprises a switch means (2) which presents an electrode gap (5) which is convertible between an insulating state and a conducting state, and members adapted to cause the electrode gap to assume conductivity. These members comprise a source for generally collimated radiation, and a system for directing the radiation towards the electrode gap in order to bring the gap or at least a part thereof to the form of a plasma. The radiation directing system comprises at least one directing element (15) for applying the radiation energy along a line (13) extending between the electrodes (3, 4). The directing element (15) is designed such that, and the generally collimated radiation inciding to the directing system is directed such that the radiation energy line (13) is at least partly laterally displaced in relation to the axis (20) of the inciding, generally collimated radiation.

Description

An electric switching device

FIELD OF THE INVENTION

This invention relates to an electric switching device. The device according to the invention may be used in any connection for switching purposes. Particularly preferred are applications where high power is to be switched. For example, it might refer to high-voltage connections and electric power transmission applications. A preferred but not de- limiting application of the device according to the invention is to protect an electric object in an electric power plant from the consequences of faults, primarily as far as current is concerned but also voltage.

The electric object in question may be of arbitrary nature as long as it is contained in an electric power network and requires protection against fault-related over-currents, i.e. in practice short-circuit currents. As an example, it may be mentioned that the object may be formed by an electric apparatus having a magnetic circuit, e.g. a generator, transformer or motor. Also other objects may be in question, e.g. power lines and cables, switch gear equipment etc. The present invention is intended to be applied in connection with medium and high voltage. Ac- cording to IEC Standards, medium voltage refers to 1-72.5 kV whereas high voltage is >72.5 kV. Thus, transmission, sub-transmission and distribution levels are included.

As pointed out hereinabove, the invention is, however, not only restricted to protection applications. In other switching situations it is a disadvantage to have to re- sort to rather costly and bulky switching devices when high power is involved, for instance banks of semiconductor components, in order to manage the switching function aimed at. Today's semi-conductor component, which preferably is produced in silicon even if other materials may be in question, has for practical reasons a restriction as to the maximum electric field strength which the component may withstand before an electrical breakthrough occurs in the semi-conductor material. This implicates im- mediately corresponding restrictions of the maximum electric voltage that the component may be subjected to. In particular in high voltage connections, one is therefore forced to couple in series (stack) a large number of semiconductor components in such a way that none of the compo- nents contained in the stack is subjected to a voltage which is above a safe level for the component.

OBJECT OF THE INVENTION

The object of the present invention is to provide an electric switching device, the properties of which should be such that the device can be brought into a closing state extremely rapidly, such that the switching device can be used in very qualified connections such as, for example, pure switching functions with reference to normally high voltages and effects, and also special protection situations where a rapid reaction and a high current diversion ability is desired in order to protect objects against loads due to faults, e.g. insulation system faults or other abnormal con- ditions as to the current and/or voltage.

SUMMARY OF THE INVENTION

According to the invention, the switch means of the electric switching device is designed in accordance with the characterising part of patent claim 1. Since the electrode gap of the switch means is brought to an electrically conducting state by supplying energy directly to the electrode gap itself in the form of radiation in order to establish ionisa- tion/plasma in the electrode gap, conditions are created for a very rapid operation of the switch means according to the invention. The ionisation/plasma in the electrode gap causes/initiates an electrically conducting plasma channel with a very high conductivity, such that very large currents can be conducted during relatively long periods without negative effects. In order to further develop the inventive switch means, the latter is provided with a radiation directing system comprising at least one directing element adapted to apply the radiation energy along a line extending between the electrodes, the directing element being designed and the substantially collimated radiation inciding onto the directing system being directed such that the radiation energy line is at least partly laterally displaced in relation to the centre axis of the inciding, substantially collimated radiation.

The device according to the invention creates conditions for a significantly more effective closure of the electrode gap, and, moreover, the directing element included in the radiation directing system will obtain a more protected position due to the lateral displacement of the radiation energy line than would be the case without any possibility of such a lateral displacement.

According to a preferred embodiment, at least one of the electrodes by the electrode gap has an opening through which the directing element is adapted to direct the radiation en^¬ ergy. Thereby, it is suitable to have the opening extending obliquely in relation to a symmetry axis of the electrodes.

Advantageously, the directing element is adapted to direct the radiation energy obliquely in relation to a symmetry axis of the electrodes, and if one of the electrodes has an opening that one should have a correspondingly oblique or in any other way adapted design.

Moreover, it is preferred that the opening is eccentric in relation to a symmetry axis of the electrodes. This means that the opening, and thus the connection to the directing element, will be positioned displaced in a lateral direction from the centre of the electrodes where, normally, the main part of the current conduction takes place through an electric arc present between the electrodes.

The solution according to the invention based upon a switch means implies a particularly advantageous fulfilling of de- mands which may be set up in order to achieve a satisfactory protection function. Thus, a very rapid triggering may be achieved by the switch means so that the current conduction, with a very small delay in time, can take place through the switch means as soon as the electrode gap has adopted an electrically conductive condition. It is pointed out that the term "triggering" in this connection means bringing the switch means into an electrically conducting state. By means of the arrangement of the switch means, said switch means may easily be dimensioned to be able to conduct very large currents. In order to obtain a satisfactory protection function it is, namely, desirable that the current conducting channel, which is established through the switch means, has a very low resistance. Besides, a switch means according to claim 1 may with a small effort be caused to function with a particularly high triggering safety. The switch means according to the invention gives on the other hand rise to the possibility to dimensioning in order to achieve a very high electric strength in a non-triggered condition. The probability for a spontaneous breakdown is thus to be at a mini- mum. It is especially preferred to thereby use at least one laser for triggering. Further advantages and features of the invention appear from the following description and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

With reference to the enclosed drawings, a more specific description of an embodiment example of the invention follows hereinafter.'

In the drawings:

fig. 1 is a schematic view illustrating the inventive switch means in a basic form,

fig. 2a and 2b are schematic views illustrating the principle of an axicone,

fig. 3 is a cross-section of a preferred embodiment accord- ing to the invention,

fig. 4 is a cross-section of an opening extending in one of the electrodes according to fig. 3,

fig. 5 is an enlarged, detailed view of the electrode design in connection to the opening,

fig. 6 is a schematic, partly cut side view of an alternative switch means embodiment,

fig. 7 is a view similar to fig. 6 of a variant,

fig. 8 is a view of another variant, with a directing element by the side of an electrode, fig. 9 is a variant of the embodiment according to fig. 5, and

fig. 10 is a view corresponding to fig. 9, but illustrating a condition with a stable electric arc instead of what is shown in fig. 9 which shows the situation during the initiation of an electric arc.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Fig. 1 represents a principle view of a switching device 1 in order to show the most important features thereof. It is specifically pointed out that the switch means in fig. 1 differs from the present invention in one important aspect which will be discussed later. The switch means 2 itself presents electrodes 3, 4 and a gap located between these electrodes. The switching device 1 comprises members for triggering the electrode gap 5 to form an electrically conducting path between the electrodes. A control unit 8 is adapted to control the operation of the members 6, 7. In the example, the members 6, 7 are adapted to cause or at least initiate the electrode gap to assume an electrical conductivity by making the gap or a part thereof form a plasma. Thereby, it is essential that the members β, 7 are capable of supplying triggering energy to the electrode gap with great rapidity. Thereby, it is preferred that triggering energy is supplied in the form of radiation energy capable of effecting ionisation/plasma initiation in the electrode gap.

According to an especially preferred embodiment of the invention, the members 6, 7 comprise at least one radiation source 6 which accomplishes ionisation/plasma formation in at least a part of the electrode gap through a supply of energy through the electrode gap, and a radiation directing system 7. According to the invention, it is preferred to supply energy to the electrode gap 5 by means of one or more lasers or other members 6, such that the whole electrode gap is ionised and brought to the form of a plasma almost momentarily, and such that the whole gap 5 is also immediately brought to electrical conductivity. According to the invention it is preferred that the radiation energy is applied in or more elongated areas extending continuously or generally continuously between the electrodes. Thereby, it is preferred that the radiation energy is applied along one or more lines 13 extending between the electrodes 3, 4.

When the switch means 1 is connected between a high-voltage point and earth (or any other unit which has a lower poten- tial) as schematically indicated in fig. 1, i.e. with the electrode 3 connected by means of the connecting member 8 and the electrode 4 connected to earth 9 by means of a connecting member 10, there will be a voltage difference between the electrodes, said voltage difference giving rise to an electrical field. The electrical field in the electrical gap 5 may be used in order to promote or generate an electric breakdown between the electrodes as soon as the members 6, 7 have been controlled to triggering, i.e. given rise to ionisation/plasma formation in one or more parts of the electrode gap. This established ionisation/plasma formation will be induced by the electrical field to bridge the gap between the electrodes in order to obtain an electrically conducting channel, i.e. an electric arc, with low resistivity between the electrodes 3, 4. However, it is pointed out that the invention is not intended to be delimited to application only upon presence of such an electric field. Accordingly, the intention is to make the members 6, 7 capable of establishing an electric conduction between the electrodes also when there is a relatively weak field. Because of the demand on the switch means 2 to be able to close very rapidly for current diversion, it is thus desired that at least a part of the gap is ionised and that the switch means is dimensioned such that the strength of the electric field in the gap 5 becomes sufficiently high for a secure closure. On the other hand, however, it is desired that the switch means 2 shall have a very high electric resistance against breakdowns between the electrodes when being in its insulating state of rest. With reference to this background, the strength of the electric field in the gap 5 should thus be relatively low. However, upon one spot ioni- sation, this reduces the rapidity with which the switch means can establish the current-diverting electric arc between the electrodes. According to the invention, in order to obtain an advantageous balance between the desire of having a safe triggering of the switch means and, on the other hand, a high electric resistance against undesired triggering, it is preferred that the switch means is designed with reference to its operational environment, such that the electrical field in the gap 5, when the gap forms an electric insulation, has a field strength which is not more than 30% of the field strength at which there is normally a spontaneous breakdown. This results in a very low probability of a spontaneous breakdown.

Suitably, the strength of the electric field in the electrode gap 5 in the insulating state thereof is not more than 20%, and preferably not more than 10% of the field strength at which a spontaneous breakdown normally takes place. In order to obtain an electric field in the electrode gap 5 which promotes generation of an electric arc upon initiation of ionisation/plasma formation in a part of the electrode gap in a relatively rapid way, it is preferred that the strength in the electric field is at least 0.1%, suitably at least 1%, and preferably at least 5% of the field strength at which there is normally a spontaneous breakdown. As seen in fig. 1, the electrode gap 5 is enclosed in a suitable casing 11. In the gap 5 there might be a vacuum as well as a medium suitable for the purpose, in the form of a gas or even a liquid, possibly over-pressurised. As can be seen in fig. 1, one 4 of the electrodes may present an opening 12 through which the laser 6 is adapted to emit the radiation energy to the area 13 with support of the radiation- directing system 7.

Here, the radiation-directing system comprises a mirror 14 for changing the direction of the collimated radiation emanating from the laser 6 and reaching a directing element 15. This directing element applies the radiation energy into the electrode gap 5 via the opening 12 in the electrode 4, and the radiation energy is applied along a line, either such that the radiation energy is substantially constant along this line, or such that the energy is applied to a plurality of spots or sub-areas along this line. In other words, the intention is to apply the radiation energy in an elongated, rod-like shape between the electrodes 3, 4 in order to, as far as possible, create favourable conditions for inducing an electric arc between the electrodes. The radiation energy is intended to give rise to ionisation/plasma formation in said elongated, rod-like area, such that an electric arc is formed between the electrodes, either with the support of a voltage difference between the electrodes or without such a voltage difference.

In the preferred embodiment, the casing 11, and thus the electrode gap 5, contains a gas or a gas mixture which is over-pressurised. As indicated in fig. 1, there are suitable sealing members 16 at all channels through the casing 11.

The principle view in fig. 1 illustrates a deficiency of the switch means; this deficiency is due to the fact that the radiation energy line 13 extends generally co-axially in relation to the electrodes, and straight through the opening 12 arranged in the electrode 4. This means that when an electric arc has been established between the electrodes, the directing element 15 will be situated opposite the electric arc, which might result in the directing element 15 being subjected to unacceptably high temperatures or obtaining undesired coatings. Furthermore, there is the problem of making the electric arc in the region of the opening 12 "jump-over" to the adjacent electrode 4 without having an undesired high resistance against electric arc generation. As will be described later on, the embodiment, for instance the one shown in fig. 1, remedies this deficiency.

However, to begin with, and with the aid of fig. 2a, the basic principle of a conical lens, a so-called axicone, will be explained. Per definition, an axicone may be seen as any rotational, symmetric element for directing waves, and thereby, through a refraction, reflection, diffraction, or combinations thereof, bending light from a spot source on the symmetry axis 17 of the element in such a way that the wave movement intersects this symmetry axis not in one point, such as would be the case with a conventional spherical lens, but along a continuos line of spots along a sub- stantial length of this symmetry axis 17.

In fig. 2b, there is illustrated how collimated (parallel) light beams are bent with the same angle by the axicone. Due to the rotational symmetry of the axicone, each beam will intersect the symmetry axis 17 in one spot. The result, according to fig. 2b, being that the collimated beams 18 will be concentrated along a radiation energy line coinciding with the symmetry axis 17, i.e. the radiation energy will be focused along this line. In fig. 3, there is illustrated an embodiment of the invention in which such an axicone 15 is used. This axicone more precisely forms a radiation energy line 13 between the electrodes 3, 4.

The axicone 15 is designed such that, and the substantially collimated radiation 19 inciding onto the axicone 15 is directed such that the radiation energy line 13 is at least partly laterally displaced a distance d in relation to a centre axis of the substantially collimated, inciding radiation.

In the case of an axicone 15 as in fig. 3, this results in the axicone 15 having its optical axis/symmetry axis 21 lat- erally displaced from the axis 20 of the inciding, collimated radiation. From fig. 3 appears that the inciding radiation 20 goes through and is deflected by the axicone 15 along a peripheral area thereof. The consequece thereof is that the axicone 15 will direct the radiation energy obliquely, as is indicated by means of the arrow 22, but nonetheless will the radiation energy line 13 along which the radiation energy is focused be generally parallel with the inciding, collimated radiation 20.

It is preferred that the axicone 15 is adapted to apply the radiation energy along the radiation energy line such that there is a resulting ionised/plasma-formed, generally rodlike area which totally or substantially totally bridges the distance between the electrodes 3, 4 in order to create con- ditions as favourable as possible for the generation of an electric arc between the electrodes.

At least one of the electrodes, viz. the one denoted 4, has an opening 23 through which the axicone 15 is adapted to di- rect the radiation energy. The opening 23 extends obliquely in relation to a symmetry axis 24 of the electrodes 3, 4. The opening 23 is eccentric in relation to the symmetry axis 24 of the electrodes.

The axicone 15 is adapted to apply the radiation energy line 13 such that an ionised plasma channel formed along this radiation energy line through the supply of radiation energy hits or is located close to an edge, denoted 25, of the opening 23 in the electrode 23. Thereby, it is pointed out that the axicone 15 is adapted to apply the radiation energy line 13 such that it is generally parallel to the symmetry axis 24 of the electrodes 3, 4.

As seen in fig. 3, the radiation energy line 13 is generally parallel to the symmetry axis 24 of the electrodes but lat- erally displaced in relation thereto. This has the favourable effect of making the initial ionisation/plasma formation take place eccentrically on the electrodes, but as soon as an electric arc has been developed between the electrodes, the latter will take the centre position in relation to the electrode and thus becoming located in the area of the symmetry axis 24. This also means that the electric arc developed between the electrodes will be located by the side of the opening 23, which means that the axicone 15 located on the other side of the electrode obtains a well-protected position.

It is preferred that the opening 23 in the electrode 4 is relatively limited. More precisely, the opening 23 should not be unnecessarily large in relation to the energy beam which is supposed to pass through the opening.

According to a preferred embodiment, the axicone 15 is designed to be rotatable around its symmetry axis. This has the advantage that, if a portion of the axicone 15 located opposite the opening 23 would be negatively affected by the electric arc between the electrodes, it will be possible to, by rotating the axicone 15, forward a portion of the axicone which is in good condition to such an area, such that the collimated radiation energy from the radiation source can be deflected in a faultless way and applied along the radiation energy line 13 discussed above.

The opening 23 in the electrode 4 may have a completely circular cross-section. However, it would also be possible to design the cross-section of the opening 23 in the way shown in fig. 4, i.e. kidney-shaped. In other words, the opening should present a bulge 26 located on that part of the limitation surface of the opening 23 which is closest to the symmetry axis 24 of the electrode. The object is thus to make the axicone 15 direct the radiation energy along or possibly directly onto this bulge 26 such that, through this intimacy, the establishment of a totally developed electric arc is favoured. The term "kidney-shaped" referred to above with reference to the opening should not be interpreted as more delimiting than defining that said bulge 26 should be arranged in the opening 23.

The variant of fig. 5 shows that the electrode could be provided with a projection 27, the object of which is to act in a locating way on the plasma formed between the electrodes. More precisely, this projection 27 is arranged at that part of the opening 23 which is closest to the electrode 3, and in such a position that the projection is close to the symmetry axis 24 of the electrodes.

The present invention can also be applied by means of diffractive elements for directing wave movements, said elements being produced by means of the holographic technique or computer generation. Such computer-generated, phase- directing relief components are often called kinoforms . Fig. 6 illustrates the use of a diffractive element in the form of a so-called kinoform 15a. Radiation energy is provided to this kinoform in a collimated way as is indicated by the arrow 20, and more precisely with a centre axis lo- cated at a radial distance from the symmetry axis 24a of the electrodes. The kinoform 15a deflects the inciding radiation and locates it along a radiation energy line 13a which is laterally displaced from the centre axis of the inciding, collimated radiation 20a, and, in the example, also later- ally displaced in relation to the symmetry axis 24a of the electrodes. As in the preceding embodiment, the electrode 4a presents an opening 23a, and the kinoform 15a is adapted to locate the radiation energy line 13a such that it hits or is located close to an edge portion 25a of the electrode 4a in order to promote the development of a conducting plasma channel and also an electric arc between the electrodes.

Diffractive, radiation-directing elements may be defined as elements in which the wave fronts of the radiation are shaped through diffraction rather than refraction, said wave fronts deciding the propagation of the radiation.

Fig. 7 illustrates a variant in which a kinoform 15b is designed such that it directs the radiation energy line 13b obliquely in relation to the symmetry axis 24b of the electrodes 4b and 3b, and also obliquely in relation to the inciding, substantially collimated radiation 20b. Thereby, it is favourable if the kinoform 15b directs the radiation energy line 13b such that it will intersect the symmetry axis 24b of the electrodes 3b and 4b between the electrodes, preferably approximately halfway between the electrodes.

Fig. 8 illustrates how a radiation directing element 15c designed as a kinoform is located radially externally of the electrode 4c, and arranged to direct radiation bundles towards the gap between the electrodes 3c and 4c, spots or ar- eas with increased radiation energy densities being denoted 28 and positioned on a radiation energy line 13c which, in this example, generally coincides with a symmetry axis of the electrodes. Such an embodiment eliminates the need of an opening through one of the electrodes.

Figs. 9 and 10 illustrate a variant of the embodiment according to fig. 5. In figs. 9 and 10 the projection 27 already mentioned in fig. 5 has obtained another position in order to better protect the radiation directing system, and then primarily the directing element 15 from the burning electric arc. In fig. 5, the projection 27 is positioned relatively close to the opening 23. This means that the electric arc will tend to remain at the projection 27, and, accordingly, the high-temperature conditions will be communicated to the radiation directing system via the opening 23. In figs. 9 and 10, on the other hand, the projection 37 has now been moved away from the opening 23. The task of the projection 37 is more precisely to define a minimum distance between the electrodes 3, 4 such that the fully established electric arc, denoted 13' , will tend to be located and remain in the area of the projection 37. Fig. 9 illustrates the initiation of the electric arc. The plasma channel 13 will then be located relatively close to the opening 23, but as the electric arc grows, it will spontaneously move to the projection 37 where the shortest distance between the electrodes is found. The area denoted 38 in fig. 10 is an area in which a direct view-line to the directing element 15 can be obtained. As can be seen in fig. 10, the projection 37 should be located such that the developed electric arc 13' will be outside the area 38. This means that the radiation directing system obtains a well-protected position while, a the same time, the distance between the electrodes, which is decreased due to the projection 37, only decreases the limit for an electrically spontaneous breakdown marginally. In all the illustrated embodiments, the electrode denoted 3 is a high-voltage electrode, while the electrode 4 is connected to earth or a potential lower than the one of the high-voltage electrode. From a practical point of view, it is most suitable to position the radiation directing system in connection to the electrode having the lower voltage.

When the inventive switch device is positioned in a concrete operational environment, its control unit 8 should be con- nected to other equipment located in the plant in question, such as arrangements for delivering control signals, for example arrangements for detecting over-currents and/or over- voltages. When there is a need of closing by means of the switch means 2, the radiation source 6 is activated, said source suitably being a laser and delivering an energy-rich pulse which is delivered to the directing element 15 which applies the energy in the most suitable way to the electrode gap in order to accomplish a completed electrical breakdown between the electrodes.

It should be noted that the description given above only is to be regarded as an example of the inventive idea, upon which the invention is based. Accordingly, it is obvious to men skilled in the art that detailed modifications can be done without thereby leaving the scope of the invention. As an example, it is not necessary, according to the invention, to use a laser for supplying ionisation/plasma formation energy to the gap 5. Also other radiation sources, for example electron guns or other radiation energy sources may be ap- plied as long as the rapidity and reliability demands according to the invention are fulfilled. Finally, it should be pointed out that the invention is perfectly suited for both alternating voltage and direct voltage.

Claims

1. Electric switching device, characterised in that it comprises at least one switch means (2) which presents an elec- trode gap (5) which is convertible between an electrically generally insulating state and an electrically conducting state, and members (6, 7) for causing or at least initiating the electrode gap or at least a part thereof to assume electrical conductivity and comprising a source (6) for gener- ally collimated radiation and a system (7) for directing the radiation towards the electrode gap for supplying said electrode gap with energy in order to bring the gap or at least a part thereof to the form of a plasma, that the radiation directing system comprises at least one directing element (15) for applying the radiation energy along a line (13) extending between the electrodes, and that the directing element (15) is designed such that, and the generally collimated radiation inciding onto the directing system is directed such that the radiation energy line (13) is at least partly laterally displaced (d) in relation to the axis (20) of the inciding, generally collimated radiation.

2. A device according to claim 1, characterised in that the radiation source comprises at least one laser (6) .

3. A device according to claim 1 or 2, characterised in that the directing element (15) is adapted to apply the radiation energy in an area extending along the radiation energy line, and wherein said area totally or generally to- tally bridges the distance between the electrodes.

4. A device according to any preceding claim, characterised in that at least one (4) of the electrodes by the electrode gap has an opening (23) through which the directing element (15) is adapted to direct the radiation energy.

5. A device according to any preceding claim, characterised in that the directing element (15) is adapted to orientate the radiation energy line (13) generally in parallel with the generally collimated radiation inciding on the directing element.

6. A device according to any one of claims 1-4, characterised in that the directing element (15) is adapted to orientate the radiation energy line (13) such that it forms an angle with the generally collimated radiation (20) inciding onto the directing element.

7. A device according to any preceding claim, characterised in that the directing element (15) is adapted to direct the radiation energy obliquely in relation to a symmetry axis (24) of the electrodes.

8. A device according to any one of claims 4-7, characterised in that the opening (23) extends obliquely in relation to a symmetry axis (24) of the electrodes.

9. A device according to any one of claims 4-8, characterised in that the opening (23) is eccentric in relation to a symmetry axis (24) of the electrodes.

10. A device according to any one of claims 4-9, characterised in that the directing element (15) is adapted to locate the radiation energy line such that it hits or is located close to an edge (25, 27) of the opening in the electrode.

11. A device according to any one of claims 1-6 and 8-10, characterised in that the directing element (15) is adapted to apply the radiation energy line (13) such that it is generally parallel with a symmetry axis (24) of the electrodes.

12. A device according to any preceding claim, characterised in that, on one of the electrodes (4), on the side directed towards the second electrode, there is provided a projection (27, 37) intended to act locatingly on the formed plasma.

13. A device according to any one of claims 4-12, characterised in that the projection (37) is located at a distance from the opening (23) in the electrode in order to locate the electric arc at a substantial distance from the opening.

14. A device according to any preceding claim, characterised in that the directing element (15) is a refractive, reflective and/or diffractive element.

15. A device according to claim 14, characterised in that the directing element (15) is constituted by an axicone.

16. A device according to claim 14, characterised in that the directing element (14) is constituted by a kinoform.

17. A device according to any one of claims 1-3 or 5-16, characterised in that the at least one directing element (15) is positioned radially outwardly of one of the electrodes and adapted to direct bundles of radiation towards the gap between the electrodes.

18. A device according to claim 15, characterised in that the axicone is turnable around its symmetry axis (21) .

19. A device according to claim 4, characterised in that the opening (23) has a generally kidney-shaped cross-section.