WO1999067864A1

WO1999067864A1 - A protection device

Info

Publication number: WO1999067864A1
Application number: PCT/SE1999/001099
Authority: WO
Inventors: Mikael Bergkvist; Hans Bernhoff; Mats Ekberg; Jan Isberg; Anders Sunesson
Original assignee: Abb Ab
Priority date: 1998-06-17
Filing date: 1999-06-17
Publication date: 1999-12-29
Also published as: AU4815999A

Abstract

A device for protection of an electrical object (2) comprises an electric connection member (10) coupled in parallel to the object, said connection member comprising an electrode gap which normally is electrically insulating. The connection member is adapted to establish an electric connection over the object (2) on occurrence of an abnormal condition. The connection member (10) forms an electric closing device comprising members to cause the electrode gap to assume electric conductivity by triggering energy supply to the electrode gap to form a plasma, by means of said energy, in the gap.

Description

A protection device

FIELD OF THE INVENTION AND PRIOR ART

This invention relates to device for protection of an electrical object comprising at least one electric connection member coupled in parallel to the object, said connection member comprising at least one electrode gap normally electrically substantially isolating and which is adapted to establish an electric connection of the object on occurrence of an abnormal condition.

Said abnormal condition may regard both current and voltage. Over voltages may arise as a consequence of atmospheric dis- turbances, such as lightning strokes, coupling over voltages etc. Over currents may occur as a consequence of various faults, such as ground faults.

The device according to the invention may be used for protec- tion of a variety of different objects, for instance objects connected in series. A concrete example of application relates to protection of capacitors, in particularly capacitor banks for series compensation in electric power networks.

Plants of the series compensator type may be very large. As an example may be mentioned system voltages up to 800 kV, nominal currents up to 4 kA (effect level 2 GW at 500 kV) and fault currents up to 100 kA. The surface of a large plant is considerable, in the order of a football ground or more. Protection of series compensation capacitor banks occurs traditionally by means of spark gaps and breakers and possibly surge diverters (varistors). These devices allow protection against over voltages with varying speed.

The capacitors must be protected against over currents and over voltages by diversions/restriction measures. With regard to over voltages, the varistor enters into operation at a certain level and allows passage of such a current required for the voltage reduction. The varistor may also up to a certain limit adopt fault currents and allows the capacitor to remain in the network so that compensation still occurs. This may be important from the point of view of stability. On more serious faults (short circuits etc) the capacitor is generally removed from the network by means of the spark gap. Until the spark gap is fired, the varistor is subjected to the full fault current. In this connection it should be observed that the so called thermal limit of the varistor is not exceeded. The consideration in this connection is directed towards stability. When there is a smaller fault on the own line, the capacitor may as a rule be short circuited, but when there are faults on an adjacent line, it may be a requirement that the capacitor remains irrespective of whether over currents and over voltages exist.

A problem with the prior protection devices is that the spark gaps have a comparatively prolonged closing time. The time is not shorter than 2-5 milliseconds (ms). During the time required before the spark gap has been placed in a fully developed conducting state the plant may be subjected to substantial strains.

A negative consequence of the spark gaps having a comparatively slow closing time according to the prior art is that the varistors must be dimensioned excessively large. FIELD OF THE I NVENTION

The object of the present invention is to devise ways to develop the protection device so that it is provided with better possibilities to efficiently protect the electrical object, in particular the capacitor plant. The invention especially aims at creating possibilities to reduce the disadvantage being associated to the fact that the spark gaps according to the prior art react comparatively slowly.

SUMMARY OF THE I NVENTION

According to the invention the object presented is fulfilled by the closing device comprising members to cause or at least initiate the electrode gap or at least a part thereof to assume electric conductivity by supply of triggering energy to the electrode gap to bring the gap or at least a part thereof to the form of a plasma by means of this energy. Thus, the invention is based upon the idea to establish a very rapid closing of a conduction path with low resistivity through the electrode gap by supplying to the same triggering energy and in this way rapidly establish a plasma in the electrode gap.

The rapid operation of the closing means of the invention as compared to conventionally occurring spark gaps means that in case of a varistor occurs, it may be dimensioned in an optimum manner with focus on the over voltage protection aspect as a consequence of the fact that the closing means according to the invention react very rapidly to over currents and divert these over currents past the varistor/capacitors.

The rapid operation of the closing means according to the invention of course also means a reduced load on the capacitor banks. The closing means according to the invention will by its character be imparted conductivity by supply of triggering energy obtain a very high triggering safety. On the other hand a closing means according to the invention create possibilities of dimen- sioning for achieving a very high strength against spontaneous electric brake through in untrigged state.

According to the invention it is preferred that the triggering energy is supplied as radiation energy. Although the radiation may be of different character, for example electrons, electromagnetic wave motions are preferred and particularly such wave motion which emanates from lasers. To take advantage of the velocity of propagation of light in such a way promotes, of course, rapid functioning.

In the enlclosed claims preferable developments are defined regarding a.o. the members for supply of radiation energy to the electrode gap. According to one embodiment, the radiation energy is supplied to the electrode gap in two or more spots or ar- eas for the purpose of achieving the highest possible safety as concerns bringing the electrode gap to assume an electrically conducting state. According to an alternative, the energy supply members may be designed to supply the radiation energy along an elongated area in the desired conduction path between the electrodes. According to an optimum design this elongated area may entirely or substantially entirely bridge the gap between the electrodes. Although it is possible, in the case with to or more spots or areas for radiation supply, that these spots or areas are successively applied in correspondence to the propagation with regard to the desired electric conduction path between the electrodes in such a way that the spots or areas are successively applied with a time delay, it is normally preferred according to the invention that these spots or areas are applied substantially simultaneously for the purpose of momentarily making the electrode gap conducting . Furthermore, it is according to the invention proposed that the members for supply of triggering energy may be arranged to apply the radiation energy in a volume assuming tubular shape. This is particularly preferable when one of the electrodes present an opening, through which the radiation energy is supplied and the radiation energy supplied in a tubular volume is applied relatively closely to the electrode provided with an opening .

According to an alternative embodiment, the energy supply members may be designed to supply the radiation energy in several substantially parallel elongated areas extending between the electrodes.

The radiation energy may also be supplied to the electron gap transversely relative to an axis of the electrodes in one or more spots located between the electrodes.

Further advantages and features of the invention, in particular with regard to the method according to the invention, appear from the following description and the claims.

SHORT DESCRIPTION OF THE DRAWINGS

With reference to the enclosed drawings, a more specific description of an embodiment example of the invention will follow hereafter.

In the drawings:

Fig 1 is a diagram illustrating a first embodiment of the protection device;

Fig 2 is a view illustration an embodiment completed with a varistor; Fig 3 is a view of an embodiment comprising two closing means according to the invention;

Fig 4 is a view illustrating the embodiment according to Fig 2 with a more developed electric switch in parallel over the closing means;

Fig 5 is a view similar to the embodiment according to Fig 4 but illustrating an electric switch in series with the closing means;

Fig 6 is a diagramatical detail view illustrating a possible design of the over current reducing arrangement;

Fig 7 is a view similar to Fig 6 of a variant;

Fig 8 is a diagramatical view illustrating an optical system for energy supply to the electrode gap;

Fig 9 is a view illustrating an alternative optical system placed at the side of one of the electrodes;

Fig 1 0 is a further alternative to an optical system arranged to supply the radiation energy, without need for an opening in one of the electrodes, around one of the electrodes and coaxially relative to the same;

Fig 1 1 is a view of an optical system based on the use of optical fibres;

Fig 12 is a view illustrating the function of detractive axicone for producing an elongated focal area between the electrodes;

Fig 13 is a view illustrating the use of a diffractive axicone (a kinoform) capable of generating focal areas with different geo- metrical shapes; Fig 14 is a view illustrating how the radiation energy may be supplied so that several substantially parallel electrically conducting channels are formed between the electrodes; and

Fig 15 is a view illustrating a preferred axicone embodiment.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The closing means according to the invention and transferable to an electrically conducting state by supply of triggering energy is denoted 1 0 and will be described more closely hereafter. In a line 1 there is provided a capacitor assembly 2 diagrammatically indicated. The closing means 10 is coupled in parallel over the capacitor. A current limiter 3 is arranged in series with the closing means 10. The current limiter may have the character of a damping reactor. An electric switch is coupled in parallel over the capacitor 2 as well as the closing means 10.

A control unit is generally denoted 5. An arrangement for de- tecting abnormal conditions, such as over currents, over voltages etc, is connected to the control unit 5 and denoted 6. It is stressed that the illustrations of this detector arrangement are diagramatical. In reality the arrangement may have an arbitrary number of sensors of differing character for establishing whether abnormal conditions exist or not.

The control unit 5 actuates the closing means 10 and the electric switch 4 respectively via control members 7 and 8 respectively.

The device illustrated in Fig operates in the following manner: the closing means 1 0 normally extinguished and the electric switch 4 open.

If the detector arrangement 6 now detects a fault condition, for example an over current or an over voltage, a corresponding o

signal is delivered to the control unit 5, which via the control member 7 activates the closing means 10 by supply of triggering energy.

This energy transfers the closing means 10 to an electrically conducting state within a time period which is within 1 millisecond (ms) after detection, and preferably within a time period which is a slow as near or below 1 microsecond. The electric current path through the closing means 10 relieves the capacitor 2 via the current limiter 3. The latter functions normally as a current maximiser, for instance at the level 100 kA or what is otherwise suitable for the individual case.

When the detector arrangement 6 has established that the fault condition has ceased to exist, a corresponding signal is delivered to the control unit 5, which via the control member 8 closes the electric switch 4. This causes a conduction path to be established in parallel over the closing means 10 so that accordingly the voltage drop over the same disappears. This causes deionisation of the electrode gap in the closing means 10 so that the closing means is transferred to an electrically insulating state. The control unit 5 thereafter controls the electric switch 4 to opening, whereupon the device according to Fig 1 is present in a normal position of operation.

The described function of the electric switch 4 motivates it to be denominated as a shunt breaker.

Fig 2 illustrates a variant differing from what has been described with assistance of Fig 1 only with regard to a surge diverter in the form of a varistor 1 1 having been added in Fig 2. This varistor acts essentially as an over voltage protection but may to a certain limit adopt over currents. In Fig 2 the embodiment is normally intended to be such that the varistor 1 1 functions as an over voltage protection whereas the closing means 10 primarily functions as an over current protection. As appears by a comparison with Fig 1 , the closing means 10 according to the invention could in practice function independently to fulfil over current as well as over voltage reducing tasks.

The embodiment illustrated in Fig 3 differs from the one in Fig 1 by a further closing means 10' according to the invention having been added. Furthermore, a second electric switch 4' has been provided. This electric switch 4' has been provided. This electric switch 4' and the further closing means 10' are arranged in series with each other in a branch which is parallel with the branches, in which the closing means 10 and the electric switch 4 are present. The closing means 10 is intended to define a higher protection level whereas the closing means 10' is conceived to function at a lover level. As has been pointed out earlier, the electric switch 4 is open in normal operation. On the contrary, the electric switch 4' is normally closed. If a fault case now would occur, this is registered via the detector arrangement (not illustrated in Fig 3) and the control unit 5 (neither illustrated in Fig 3) controls at a first lower level the closing means 10' to closing for dirversion of the actual over voltage or over current. In case the over current or over voltage would reach above a certain predetermined higher limit, the control unit also controls the closing means 10 to open for achieving a good protection of the capacitor 2. When the over current or the over voltage has ceased to exist, the closing means 10 and 10' respectively are extinguished by closing the electric switch 4 and opening the electric switch 4'. This provides for removal of the voltage drop over the closing means and thus extinguishing thereof. Thereafter, the electric switches return to the positions according to Fig 3.

The embodiment according to Fig 3 could be completed with a varistor in a manner appearing from Fig 2 if deemed appropriate. The embodiment illustrated in Fig 4 differs from the one shown in Fig 2 by the electric switch 4a in Fig 4 having been illustrated in a specific preferred embodiment. More specifically, a shunt line denoted 12 is coupled parallel over the electric switch 4a, said shunt line comprising one or more components 13 for avoiding arcs on separation of contacts of the electric switch 4a by causing the shunt line 12 to take over current conduction from the contacts. The shunt line 12 is closeable to conduction via the control unit 5a and a control member denoted 14. The components 13 may for instance be formed by controllable semiconductor components. A surge diverter 15 (over voltage protection) may be coupled in parallel over the semiconductor components 13. In the shunt line 12 there is provided a switch 16, which is controlled by the control unit 5a via control member 1 7.

The electric switch 4a described is intended to be of relatively rapid design and operates in the following manner: the electric switch 4a is normally open. When the closing means 10a in a fault situation is brought to conduction and then again is to be extinguished, this occurs by the electric switch 4a being closed. The voltage drop over the closing means 10 ceases to exist and the arc is extinguished. Thereafter the control unit 5a provides for closing of the switch 16 in the shunt line. The control unit 5a insures furthermore, via the control member 14, that the semiconductor components 13 are conductive. This means that the current conduction over the electric switch 4a ceases or is low. Opening of the electric switch 4a is therefore possible without arc or at least such that the arc is soon extinguished. In the next step the control unit 5a controls the components 13 to breake the shunt line 12. Thereafter, if so desired, the switch 16 may be opened so that the shunt line 12 obtains a galvanic interruption.

The embodiment in Fig 5 reminds about the embodiment according to Fig 3. In the embodiment according to Fig 5 an elec- trie switch 4b' provided with a shunt line 12b has been placed in series with the closing means 10b' whereas the further breaker 10b is in a branch line of its own in parallel with the closing means 10b' and furthermore, the electric switch 4b is placed in parallel over the closing means 10b. In the normal position the electric switch 4b is open whereas the electric switch 4b' is closed. The components 13b' maintain the shunt line 12b broken. In case of a fault, the closing means 10b' is first closed, which means by-pass of the consequences of the fault past the capacitor 2b. When the fault ceases to exist, the shunt breaker 4b is closed via the control unit 5b so that the voltage drop over the closing means 10b' is eliminated. It is then extinguished. The closing means 1 0b is normally adjusted to be brought into an electrically conducting state at a higher level of occurring faults then the closing means 10b'. Thus, the closing means 10b may be said to function as backup.

Fig 6 illustrates a first embodiment of the over-current reducing arrangement having a closing means denoted 10. The closing means 1 0 comprises electrodes 23 and a gap 24 present therebetween. As previously described, the closing means comprises members 25 to trigger the electrode gap 24 to form an electrically conducting path between the electrodes. A control member 7 is adapted to control, via the control unit 5, operation of the members 25. The members 25 are in the example arranged to cause or at least initiate the electrode gap to assume electric conductivity by ensuring that the gap or a part thereof is caused to form a plasma. It is then essential that the members 25 are capable of supplying triggering energy to the electrode gap with a great speed. It is then preferred that the triggering energy is supplied in the form of radiation energy capable of effecting ionisation/plasma initiation in the electrode gap.

According to a particularly preferred embodiment of the inven- tion the members 25 comprise at least one laser, which by en- ergy supply to the electrode gap provides for ionisation/plasma formation in at least a part of the electrode gap.

According to the invention it is preferred to supply, by means of one of more lasers or other members 25, energy to the electrode gap 24 so that almost momentarily the entire electrode gap is ionised and brought to the form of a plasma respectively so that also the entire gap 24 is immediately brought to electric conductivity. In order to spare, and optimise the use of, the (normally) limitedly available laser energy/effect the members 25 may, however, in use of the invention be adapted so that they are capable of achieving ionisation/plasma formation in only one or more parts of the gap 24. In the embodiment according to Fig 6 it is illustrated that the members 25 supply radiation energy in one single point or area 28. As will be described later the invention includes also application of radiation energy in several spots or areas in the electrode gap, including also on one of or on both of the electrodes, or in one or more rod like areas extending continuously or substantially continuously between the electrodes.

When the closing means 10 is coupled between two lines or units 2, 8 as is diagrammatically indicated in Fig 6, i.e. with one of the electrodes 23 connected to 2 and the other electrode connected to 8, there will between the electrodes normally occur a voltage difference giving rise to an electric field. The electric field in the gap 24 may be used to promote or cause an electric breakthrough between the electrodes as soon as the members 25 have been controlled to triggering, i.e. given rise to ionisa- tion/plasma formation in one or more parts of the electrode gap. This established ionisation/plasma formation will be driven by the electric field to bridge the gap between the electrodes so as to provide in this way an electrically conducting channel with low resistivity, i.e. an arc, between the electrodes 23. However, it is pointed out that the invention is not intended to be restricted only to use on occurrence of such an electric field. Thus, the intention is that the members 25 should be capable of establishing electric conduction between the electrodes also without such a field, or with a weak field.

As a consequence of the requirement on the closing means 10 to close very rapidly for current diversion, it is, accordingly, desirable, when only a restricted part, e.g. a spot like part, of the gap is ionised, that the closing means is dimensioned so that the strength of the electric field in the gap 24 becomes sufficient for safe closing. On the other hand, it is a desire that the closing means 10 in its insulating state of rest should have a very high electric strength to break-through between the electrodes. Seen against this background, the strength of the electric field in the gap 24 should be comparatively low. However, this reduces in one spot ionisation the speed, with which the closing means may be caused to establish the current diverting arc between the electrodes. In order to achieve an advantageous balancing between the desire of a safe triggering of the closing means and a high electric strength against non-desired triggering , it is ac- cording to the invention in such a case preferred that the closing means is designed in such a way with regard taken to its operational environment that the electric field in the gap 24, when the gap forms an electrical insulation , has a field strength which is not more than 30% of the field strength at which spontantaneous breakthrough normally occurs. This provides for an extremely low probability for a spontantaneous breakthrough.

The strength of the electric field in the electrode gap 24 in the insulating state thereof is suitably not more than 20% and pref- erably not more than 10% of the field strength, at which spontantaneous breakthrough normally occurs. In order to obtain an electric field in the electrode gap 24 which, on the other hand, promotes arc formation on initiation of ionisation/plasma formation in a part of the electrode gap in a relatively rapid manner 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 spontantaneous breakthrough normally occurs.

The electrode gap 24 is as appears from Fig 4 enclosed in a suitable housing 32. In the gap 24 there may exist a vacuum as well as a medium in the form of gas or even a liquid suitable for the purpose. In the case of gas/liquid, the medium in the gap is intended to be adapted so that it on triggering may be ionised and brought to a plasma form. In such a case it would be suit- able to initiate ionisation/plasma formation in the gap 24 in at least one spot somewhere between the electrodes 23. However, in Fig 6 is illustrated a case where there exists in the gap 24 either vacuum or a suitable medium. It is then preferred that initiation of closing occurs by the laser 25 illustrated in Fig 6 being caused to focus, via a suitable optical system 27, the emitted radiation energy in at least one area 28 on or adjacent to one of the electrodes. This causes the electrode to function as an electrone and ion emitter for establishing an ionised environment/a plasma in the electrode gap 24 so that, accordingly, an arc will be formed between the electrodes. As appears from Fig 6, one of the electrodes 23 may comprise an opening 29, through which the laser 25a is adapted to deliver radiation energy to the area 28 with the assistance of the optic system 27. Fig 7 illustrates a closing means variant 10b where instead the system laser 25b/optics 27b focuses the radiation energy in at least one triggering area 28b located between the electrodes and in a medium therebetween. On triggering a development of plasma to bridging of the electrodes is, accordingly, intended to occur from this area.

In order to achieve the conditions discussed hereinabove as far as the field strength relations between the electrodes 23 in the insulating state of the closing means are concerned, the characteristics of the closing means must of course be adequately adapted to the intended situation of use, i.e. the voltage conditions which will occur over the electrodes 23. The constructive WO 99/67864 -. ς PCT/SE99/01099

measures available concern of course electrode design, distance between the electrodes, the medium between the electrodes and the occurrence of possible further field influencing components between the electrodes.

Diffractive, refractive and reflective optical elements may be used in the invention. Diffractive optical elements are elements, in which the wave fronts of the light, which wave fronts determine the propagation of the light, are formed by diffraction rather than refraction. Diffractive optical elements may be produced by holographic technique, which does not admit arbitrary functions to be realised. A more flexible mode of production is computer generation, in which the optical function may be calculated in a computer. In principle entirely arbitrary optical functions may then be realised, which functions often are impossible to provide by conventional refractive and reflective optics. Such computer generated, phase controlling surface relief components are often termed kinoforms. A well known example is the Fresnel-lens. This may, as all difractive optics, be designed as a binary structure consisting of only two relief levels or as a multi-level relief, which provides for a considerably better diffraction efficiency (functional efficiency of the optical element).

Fig 8 illustrates an embodiment based upon an optical system 27e comprising a lens system 35, via which arriving laser pulses are delivered to a diffractive optical phase element 36, a kino- form. This element is designed to have a plurality of focal points 28e generated starting from a single arriving laser pulse. These focal points 28e are distributed along the axis of symmetry between the electrodes 23e. Since the focal points 28e are distributed along a line between the electrodes 23e, a more safe establishment of an electric conduction path between the electrodes is achieved, which means as high a probability for trig- gering as possible at as low a voltage/electric field strength as possible and with as small a time delay as possible. The kinoform 36 is low-absorbing and may, accordingly, withstand extremely high optical energy densities. The kinoform is produced from a dielectrical material so that it will not in a seri- ous degree disturb the electric field between the electrodes.

In the embodiment according to Fig 8 the radiation energy is supplied through an opening 29e in one of the electrodes as before. Fig 9 illustrates a variant where generally the only dif- ference as compared to the embodiment according to Fig 8 is that here the diffractive optical element (kinoform) 36f is placed radially outwardly of one of the electrodes 23f. The optical element 36f is as before designed to divert the laser light and focus the same in a number of spots distributed along the desired electric conduction path between the electrodes. The radiation bundles forming the spots 28f have each their own deflection angle. Thus, the radiation bundles have to move different distances to the respective spots 28f. The advantage of locating , according to Fig 9, the kinoform 36f at the side of one of the electrodes is that the kinoform will be located beside the largest electric field so that the field disturbance becomes a minimum. The electrode design is also simplified since there is not needed any opening for the laser light.

Fig 10 illustrates an embodiment where a laser 25g via an optical system 27g supplies the laser radiation symmetrically in a plurality of focal points 28g distributed along the length of the electrode gap without any opening being required in the electrodes 23g . The optical system 27g comprises a prisma or a ra- diation divider 37 arranged to break up the laser beam around the adjacent electrode 23g. Around this electrode 23g there is provided one or preferably more kinoforms 36g (diffractive optical elements) designed to, possibly with the assistance of further lenses, focus the laser beam in the desired focal points 28g so that plasma formations are generated therein. WO 99/67864 A -J PCT/SE99/01099

Fig 1 1 illustrates a variant where a laser beam by means of an optical system 27h comprising optical fibres 38 is directed for formation of focal points 28h located at different places between the electrodes 23h. The optical fibres 38 may be arranged to emit the light via lenses 39.

It appears from Fig 12 that the light may be focused, by means of an axicone 36i, in an elongated focal area 28i located between the electrodes 23i. This elongated focal area may ac- cording to one embodiment of the invention extend continuously the whole distance between the electrodes but could also assume only a part of the gap therebetween. For the rest, it is pointed out that the invention is not only limited to such axi- cones which are purely linearly conical. Thus, within the frame of the invention also axicones are included, the mantle surface of which deviates from the linear cone, which will get a direct influence on the focal intensity distribution.

Fig 13 illustrates an embodiment where a specially shaped dif- tractive axicone 36n, a kinoform, has been designed to provide focal areas 28n and 28n' respectively with different shapes. In the example it is illustrated that the focal area 28n is elongated and provided on the axis of symmetry of the axicone 36n and the electrodes. The focal area 28n' on the contrary has as is in- dicated to the left in Fig 18 obtained a cross-sectionally tubular shape. This tubular shape is advantageous most closely to an electrode 23n provided with an opening 29n since the periphery of the tubular focal area 28n' will be located relatively close to the electrode 29n provided with the opening. Both focal areas 28n and 28n' have in Fig 1 8 a substantially constant intensity along the axis of symmetry but perpendicularly thereto there occurs, as concerns the focal area 28n, a substantially Gauss- shaped or Bessel-function shaped intensity distribution.

An advantage of an entirely or substantially conical or diffractive, coaxially focusing component as for example in Fig 8, 9, 10, 13 is that along the efficient direction of propagation of the radiation energy, which direction may be said to be a straight line, that plasma volume which is formed firstly, which occurs most closely to that electrode, at which the supply of the radia- tion energy occurs, will not screen, reflect or influence to a serious degree the radiation energy focused in points/areas located further away from the supply electrode. This "shadowing effect" of the plasma volumes first formed could otherwise have prevented the radiation energy from efficiently reaching later foci. This is a consequence of the fact that the plasma has the property to be able to reflect or absorb radiation energy.

It is illustrated in Fig 14 that several substantially parallel electrically conducting channels may be formed between the elec- trodes 23p. The electrically conducting channels may, viewed from the side, be located in one single row. However, it is possible to arrange a plurality of electrically conducting plasma channels in not only rows by also columns between the electrodes. The occurrence of a plurality of simultaneously electri- cally conducting channels increases the conduction capacity of the closing means.

The definition of an axicone may be said to be each rotationally symmetrical wave movement directing element, which by refrac- tion, reflection, diffraction or combinations thereof deflects light from a point source on the axis of symmetry of the element in such a way that the wave movement intersects this axis of symmetry not in one single point, as would be the case with a conventional spherical lens, but along a continuous line of points along a considerable extent of this axis of symmetry.

Fig 1 5 illustrates an embodiment of the invention where such an axicone 36r is used. This axicone forms more specifically a radiation energy line 28r between the electrodes 23r. The axicone 36r is so designed and the substantially collimated radiation 40 incident to the axicone so directed that the radiation energy line 28r is at least partly displaced laterally a distance d in relation to a centre axis 41 of the incident, substantially colli- mated radiation.

In the case of an axicone 36r as in Fig 15, this means that the axicone 36r has its optical axis/axis of symmetry 42 laterally displaced from the axis 41 of the incident collimated radiation. It appears from Fig 15 that the incident radiation 40 passes through the axicone 36r and is deflected thereby in a peripheri- cal area thereof. The consequence thereof is that the axicone 36r will direct the radiation energy obliquely as indicated by means of the arrow 43 but nevertheless the radiation energy line 28r, along which the radiation energy is focused, will be substantially parallel to the incident collimated radiation 40.

It is preferred that the axicone 36r is adapted to apply the radiation energy along the radiation energy line so that a substan- tially rod-shaped area results along said line, said area being ionised/formed to a plasma and bridging, entirely or substantially entirely, the distance between the electrodes 23r for creating favourable, to a maximum degree, conditions for arc formation between the electrodes.

At least one of the electrodes has an opening 29r, through which the axicone 36r is adapted to direct the radiation energy. The opening 29r extends obliquely relative to an axis 44 of symmetry of the electrodes 23r. The opening 29r is eccentric relative to the axis 44 of symmetry of the electrodes. The axis 42 of the axicone coincides with the radiation energy line 28r.

The axicone 36r is adapted to apply the radiation energy so that it arrives upon a lateral surface, denoted 45, of the opening 29r in one of the electrodes 23r. It is pointed out that the axicone

36r is adapted to apply the radiation energy line 28r so that it is substantially parallel to the axis 44 of symmetry of the electrodes 23r. The lateral surface 45 forms an angle α to the axis 44 of symmetry. The radiation directed to the adjacent electrode 23r by the axicone 36r forms an angle γ with the axis 44 of sym- metry. The angle α is smaller than the angle γ, preferably about half the angle γ. This means that the radiation 43 will hit upon the lateral surface 45, a fact which promotes formation of a plasma extending between the electrodes 23r.

Even if in Fig 15 an "entire" axicone has been drawn, it is realised that only one part of an axicone is required according to that described hereinabove, namely that part which actually is penetrated by incident radiation 40.

According to a preferred embodiment, the axicone 36r is designed to be rotatable about its axis 42 of symmetry. This means the advantage that if a portion of the axicone 36r present opposite to the opening 29r would be influenced negatively by the arc between the electrodes, it is possible to move forward, by rota- tion of the axicone 36r, such a portion of the axicone, which is in good condition, to such an area that the collimated radiation energy from the radiation source in an adequate manner may be deflected and applied along the previously discussed radiation energy line 28r.

It should be noted that the description given hereinabove only should be considered as exemplifying for the inventive concept, on which the invention is based. Thus, it is obvious for the men skilled in the art that detail modifications may be made without therefore leaving the scope of the invention. As an example it may be mentioned that it is not necessary according to the invention to use a laser for supply of ionisation/plasma formation energy to the gap 24. Also other radiation sources, for instance electron cannons, or other energy supply solutions may be re- sorted to provided that they fulfil requirements with respect to speed and reliability defined according to the invention. Fur- thermore, it is pointed out that the invention is well suited for both alternating and direct voltage.

Claims

CLAIMS:

1 . A device for protection of an electrical object (2) comprising at least one electric connection member coupled in parallel with the object, said connection member comprising at least one electrode gap which normally is electrically substantially insulating and which is adapted to establish an electric connection across the object on occurrence of an abnormal condition, said connection member forming an electric closing device comprising at least one closing means, characterized in that the closing device (10) comprises members (25, 27) for causing or at lest initiating the electrode gap or at least a part thereof to assume electric conductivity by supply of triggering energy to the electrode gap to bring , by means of this energy, the gap or at least a part thereof to the form of a plasma.

2. A device according to claim 1 , characterized in that the triggering energy is radiation energy.

3. A device according to any preceding claim, characterized in that the members (25, 27) for causing or at least initiating the electrode gap or a part thereof to assume electric conductivity comprise at least one laser.

4. A device according to any preceding claim, characterized in that the members (25, 27) for supply of triggering energy to the electrode gap are arranged to apply the radiation energy on or in the vicinity of at least one of the electrodes (23).

5. A device according to any preceding claim, characterized in that the members (25, 27) for supply of triggering energy to the electrode gap are adapted to locate the radiation energy in a spot or an area (28) in the gap (24) between the electrodes (24).

6. A device according to any preceding claim, characterized in that the members (25, 27) for supply of triggering energy to the electrode gap are adapted to locate the radiation energy in two or more spots or areas (28) between the electrodes.

7. A device according to claim 6, characterized in that the members for supply of triggering energy to the electrode gap are arranged to locate said two or more spots or areas of radiation energy along a line extending between the electrodes and corresponding to the course of an electric conduction path desired between the electrodes.

8. A device according to any preceding claim, characterized in that the members (25, 27) for supply of triggering energy to the electrode gap are adapted to apply the radiation energy in one or more elongated areas (28), the longitudinal axis of which resides substantially along the direction of the desired electric conduction path between the electrodes.

9. A device according to claim 8, characterized in that the members (25, 27) for supply of triggering energy to the electrode gap are arranged to shape the elongated focal area to a tubular shape.

10. A device according to claim 8 or 9, characterized in that the members for supply of triggering energy to the electrode gap are adapted to shape said at least one elongated area so that it entirely or substantially entirely bridges the distance between the electrodes.

1 1 . A device according to any of claims 8 or 9, characterized in that the members (25, 27) for supply of triggering energy to the electrode gap are adapted to form, in said gap, two or more elongated areas (28) located in the longitudinal direction after each other along the desired electric conduction path between the electrodes.

12. A device according to any preceding claim, characterized in that the members to supply triggering energy to the electrode gap are adapted to apply the radiation energy in several substantially parallel elongated areas, the longitudinal axes of which extend substantially along the direction of the desired electric conduction path between the electrodes.

13. A device according to any preceding claim, characterized in that the members for supply of triggering energy to the electrode gap are adapted to apply the energy both on at least one of the electrodes and between them.

14. A device according to any preceding claim, characterized in that at least one of the electrodes at the electrode gap has an opening (29) through which the members (25) for triggering energy supply are adapted to direct the radiation energy.

15. A device according to claim 9 and 14, characterized in that the members (25, 27) for supply of triggering energy to the electrode gap are adapted to locate the tubular radiation energy area (28) adjacent to the electrode provided with the opening (29) and such that the axis of the tubular radiation energy area is substantially concentric to the axis of the opening in the electrode.

16. A device according to any preceding claim, characterized in that the members for supply of triggering energy to the elec- trode gap comprises a system for directing electro-magnetic wave energy.

17. A device according to claim 16, characterized in that the direction system comprises at least one refractive, reflective and/or diffractive element.

18. A device according to claim 17, characterized in that the element is formed by an axicone.

19. A device according to claim 17, characterized in that the element is formed by a kinoform.

20. A device according to claim 17, characterized in that the elements comprise optical fibres (38).

21 .A device according to any of the claims 17-20, characterized in that the direction system (27f, h) is placed radially outwardly of the electrodes and arranged to direct bundles of radiation inwardly towards the gap between the electrodes.

22. A device according to any of claims 16-21 , characterized in that the direction system (27g) is adapted to brake up the supplied energy to an annular configuration around the electrodes.

23. A device according to any of claims 17-18, characterized in that the direction element (36r) of the radiation direction system is adapted to apply the radiation energy along a line (28r) extending between the electrodes and that the direction element (36r) is so designed and the substantially collimated radiation inciding to the direction system so directed that the radiation energy line (28r) is at least partly displaced laterally (d) in relation to the axis (41 ) of the incident, substantially collimated radiation.

24. A device according to claim 23, characterized in that the direction element (36r) is adapted to orientate the radiation energy line (28r) substantially parallel to the substantially collimated radiation (40) incident to the direction element.

25. A device according to any of claims 23-24, characterized in that the direction element (36r) is adapted to direct the ra- diation energy obliquely relative to an axis (44) of symmetry of the electrodes.

26. A device according to claim 14 and any of claims 23-25, characterized in that the opening (29r) extends obliquely relative to an axis (44) of symmetry of the electrodes.

27. A device according to claim 26, characterized in that the opening (29r) is eccentric relative to an axis (44) of symme- try of the electrodes.

28. A device according to any of claims 26-27, characterized in that the direction element (36r) is adapted to direct the radiation energy so as to be incident on a side surface (45) of the opening in the electrode.

29. A device according to any of claims 23-28, characterized in that the direction element (36r) is rotatable about its axis (42) of symmetry.

30. A device according to any preceding claim, characterized in that it comprises a control unit (5) connected to said at least one closing means (10) and to an arrangement (6) detecting abnormal conditions, said control unit being adapted to bring the closing means to an electrically conducting state when required for reasonsof protection.

31 . A device according to any preceding claim, characterized in that an electric switch (4), which is open during normal operation, is arranged in parallel with the closing means (10).

32. A device according to any preceding claim, characterized in that an electric switch (4', 4b') is arranged in series with the closing means (10', 10b').

33. A device according to any of the claims 30-32, characterized in that the control unit (5) besides is connected to the electric switch for controlling the same to closing alternatively opening for the purpose of deionising the closing means.

34. A device according to any of the claims 30-33, characterized in that a shunt line (12) is coupled in parallel over the electric switch, said shunt line having one or more components (13) for avoiding arcs on separation of contacts of the electric switch by causing the shunt line to take over current conduction from the contacts.

35. A device according to claim 34, characterized in that said one or more components (13) in the shunt line are closeable to conduction by control via the control unit (5).

36. A device according to any of the claims 34 and 35, characterized in that said one or more components (13) are formed by controllable semiconductor components.

37. A device according to any preceding claim, characterized in that at least one search diverter (1 1 ) is coupled parallel to the closing means (10).

38. A device according to any preceding claim, characterized in that a current limiting reactor (3) is arranged in series with the closing means (10).