WO1999067857A1

WO1999067857A1 - A switching device

Info

Publication number: WO1999067857A1
Application number: PCT/SE1999/001097
Authority: WO
Inventors: Mikael Bergkvist; Mats Ekberg; Anders Sunesson; Li Ming; Arne Gustafsson
Original assignee: Asea Brown Boveri AB; ABB AB
Current assignee: ABB AB
Priority date: 1998-06-17
Filing date: 1999-06-17
Publication date: 1999-12-29
Anticipated expiration: 2000-12-17
Also published as: AU4815799A

Abstract

A device for switching electrical effects comprises at least one electric switching arrangement (5). This arrangement comprises at least one switching element (6) presenting at least one electrode gap (15). This gap is convertible between an electrically substantially insulating state and an electrically conducting state. The switching element comprises, furthermore, members (16) for causing or at least initiating the electrode gap or at least a part thereof to assume electric conductivity. The members (16) for causing or at least initiating the electrode gap to assume conductivity are arranged to supply energy to the electrode gap in the form of radiation energy to cause the gap or at least a part thereof to the form of a plasma by means of this radiation energy. The gap (15) is arranged in an enclosure (20) containing a gas mixture combining a high strength to spontaneous breakthrough with a low resistance to radiation induced breakthrough.

Description

A SWITCHING DEVICE

FIELD OF THE INVENTION AND PRIOR ART

This invention is related to a device according to the prechar- acterizing part of the enclosed claim 1 . The device according to the invention may be used in any connections for switching purposes. Applications where high effects are to be switched are particularly preferred. In reality high voltage connections and electric power transmission applications are involved. A preferred but non-restricting application of the device according to the invention is to protect, in an electric power plant, an electrical object from the consequences of faults, primarily with regard to current but also voltage.

The electrical object in question may be of arbitrary nature as long as it is contained in an electric power plant 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 electrical 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, switchgear etc. The present invention is intended to be applied with medium and high voltage. According to lEC-standard, me- dium voltage concerns 1 -72.5 kV whereas high voltage is > 72.5 kV. Accordingly, transmission, sub-transmission and distribution levels are included.

In prior electric power plants of this nature one has relied, for protection of objects against over-currents, on relatively slowly operating protection equipment, such as conventional circuit breakers of such a design that they on breaking provide for galvanic separation. However, the break-time becomes, with such circuit breakers, necessarily relatively extended, for example in the order of 50-90 milliseconds (ms), since they must be de- signed to be able to break very high currents and voltages and thereby obtain the character of comparatively complicated apparatus. Thus, it would be desirable to provide protection devices having a substantially more rapid reaction time.

Surge diverters are conventionally used for protection against over-voltages. Such surge diverters suffer from the disadvantage that they due to inherent characteristics must be dimensioned to allow relatively great over-voltages, which in turn involves cost consequences for the electric power plant.

As pointed out hereinabove, the invention is, however, not only restricted to protection applications.

OBJECT OF THE INVENTION

The primary object of the present invention is to provide a switch device better suited to be able to connect high electrical effects with a high rapidity and to a comparatively low cost as compared to the switch devices used today.

A secondary object of the present invention is to devise ways to design the device so that a better protection for arbitrary objects is achieved and, accordingly, a decreased strain on the same, which means that the objects themselves no longer must be constructed to be able to resist maximum short circuit currents/fault currents during relatively extended time periods.

SUMMARY OF THE INVENTION

The switch arrangement is according to the invention designed in accordance with the characterizing part of claims 1 and 2 re- spectively. By bringing the electrode gap of the switch element into an electrically conducting state by supplying, directly to the electrode gap itself, triggering energy to establish ioniza- tion/plasma in the electrode gap, conditions are created for a very rapid function of the switch arrangement according to the invention. The ionization/plasma in the electrode gap provides/initiates an electrically conducting plasma channel having a very high conductivity so that very large currents may be conducted and this more specifically during relatively extended time periods without negative effects, a fact which stands in sharp contrast to conventional semiconductor art.

As concerns the invention defined in claim 1 it also comprises the feature that the electrode gap is arranged in an enclosure containing a mixture of gases, at least one first gas thereof imparting the gas mixture a relatively high strength against spon- tantaneous electric breakthrough between the electrodes whereas at least one second gas has the property to produce a relatively low resistance to breakthrough between the electrodes with that type of triggering energy used for triggering, e.g. laser radiation. The second gas could even promote triggering induced breakthrough. Thus, the conditions in the enclosure are optimized so that the risk for undesired spontantaneous dielectric breakthroughs is minimized at the same time as the elec- trode gap comparatively easily and rapidly, as a consequence of the second gas, may be imparted electrically conducting ability. Thus, the electrode gap can be made electrically conducting in a very short time, with very low voltage differences over the electrodes and besides with a very great certainty and low energy. The gas mixture proposed according to the invention fulfils, accordingly, as a paradox diametrically opposite desires with respect to the electrode gap.

The second gas could for instance present a threshold value for triggering energy induced, e.g. laser radiation induced, breakthrough between the electrodes lower than the threshold value of the first gas against spontantaneous electric breakthrough with at least a factor 3, suitably a factor 6, preferably a factor 10.

The aspect of the invention defined in claim 2 is based upon the realization that a radiation induced breakthrough and a spontantaneous breakthrough in the electrode gap will be located at different locations along the so called Paschen graph, which as appears from Fig 5 clarifies the connection between p x d (where p is pressure and d distance between the electrodes) (abscissa) and the voltage U (in reality the voltage U required for breakthrough) (ordinate). More specifically, the graph A in Fig 5 is representative for the dielectric strength against breakthrough whereas the graph B is representative for optical breakthrough conditions. Thus, the graph B would be relevant for clarifying the voltage, at which a laser-induced breakthrough occurs. The two graphs A and B in Fig 5 are valid for the first and second gases contained in the gas mixture at one and the same pressure P^ As can be seen, the two graphs are dis- placed so that a pressure increase would tend to reduce the voltage required for a breakthrough by means of laser radiation or other optical conditions (graph B) whereas this pressure increase would cause an increase of the voltage required for spontantaneous electric breakthrough. This circumstance speaks, accordingly, in favour of the gas mixture selected according to the invention. This is of course a consequence of where on the Paschen graph the different gases will end up at one and the same pressure.

One can draw the conclusion from Fig 5 that at given conditions it would be recommendable to increase the pressure so that the value with regard to the resistance of the second gas against radiation induced breakthrough would end up in the area of the minimum of the Paschen graph. An optimization in functional regard is of course obtainable in case the aspects defined in claim 1 as concerns the gas mixture are combined with the aspects defined in claim 2 with regard to the pressure conditions.

According to the invention the above mentioned secondary object is obtained by the at least one switch element of the switch arrangement being activatable for over-current and/or over- voltage reduction with the assistance of an arrangement detecting over-current and/or over-voltage conditions for protecting the object. The switch element may according to a preferred embodiment form a diverter for diverting currents to ground or otherwise another unit.

Thus, the invention is based on the principle, as concerns the over-current protection aspect, to use a rapidly operating switch arrangement which without effecting any real breaking of the over-current nevertheless reduces the same to such an extent that the protected object will be subjected to substantially re- duced strains and accordingly a reduced amount of damages. The reduced over-current/fault current means, thus, that the total energy injection into the protected object becomes substantially smaller than in absence of the switch arrangement according to the invention.

The solution according to the invention based upon at least one switch element in the form of a closing means involves a particularly preferable fulfilling of requirements which may be erected in order to obtain a good protection function. Thus, a very rapid triggering may be achieved by the closing means so that the occurring over-currents and over-voltages will be reduced, with a very small time delay, by current diversion over the closing means as soon as the electrode gap has assumed electrically conducting state. In this regard it must be pointed out that the term "triggering" refers to bringing of the closing means to an electrically conducting state. By the design of the closing means it can easily be dimensioned to conduct very large currents. In order to achieve a good protection function it is, namely, desirable that the current conducting channel established through the closing means should have a very low resis- tance. This means the highest possible relief of the object to be protected from fault currents. A closing means in accordance with claim 1 may besides with a small effort be made to operate with an extremely high triggering safety. In order to divert occurring fault currents as soon as possible, the triggering may not fail in a critical situation. On the other hand the closing means according to the invention creates possibility to dimension for achieving a very high electric strength in an untriggered state. The probability of a spontantaneous breakthrough should, accordingly, be minimal. It is particularly preferred to use at least one laser for triggering.

In the enclosed claims preferable developments as concerns the members for supply of radiation energy to the electrode gap are a.o. defined.

Further advantages and features of the invention appear from the following description and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

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

In the drawings:

Fig 1 is a purely diagrammatical view illustrating the basic concept of the solution according to the invention; is a diagrammatical view illustrating a different coupling of switch elements according to the invention as over- current protection;

is a diagrammatical view illustrating switch elements according to the invention as over-voltage protection;

is a view similar to Fig 3 and besides illustrating switch elements for over-voltage protection between phases;

are Paschen graphs illustrating, at the same pressure in an enclosure, the position of the radiation induced breakthrough on the Pashen graph and the position respectively for a spontantaneous electric break- through;

is a diagrammatical detail view illustrating a possible embodiment of the switch arrangement according to the invention;

is a view similar to Fig 6 of a variant;

is a diagrammatical view illustrating an optical system for energy supply to the electrode gap;

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

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

is a view of an optical system based on the use of optical fibres; Fig 12 is a view illustrating the function of a refractive 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 producing focal areas with different geometrical 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;

Fig 15 is a diagrammatical view of the switch arrangement ac- cording to the invention in a switch function in series; and

Fig 16 is a view illustrating a preferred axicone embodiment.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Fig 1 illustrates an electric power plant, in which a protected object 1 is included. This may for instance consist of a generator. This object is via a line 2 in connection with an external de- livery network 3. Instead of such a network, the unit denoted 3 could be formed by another equipment contained in the electric power plant. The electric power plant here in view is conceived to be of such a nature that it is the object 1 itself which primarily is intended to be protected against over-currents and/or over- voltages from the network/equipment 3. Over-currents may arise when there occurs a fault in the object 1 proper which gives rise to a fault current from the network/equipment 3 towards the object 1 so that the fault current will flow through the object. Said fault may consist in a short circuit having been formed in the object 1 . A short circuit refers to an unintended conducting current path between two or more points. The short circuit may for instance consist of a light arc. This short circuit and the resulting violent flow or current may cause substantial damages or even a total failure of the object 1 .

It is already here pointed out that at least in some types of protected electrical objects 1 short circuit currents/fault currents harmful to the object in question may flow from the protected object towards the network/equipment 3. Thus, within the frame of the invention it is intended to be used for protection purposes not only for protection of the object from externally emanating fault currents flowing to the object but also from internal fault currents in the object flowing in the opposite direction.

Hereafter the reference 3 will always, to simplify the description, be denominated as consisting of an external electric power network. However, it should be kept in mind that it, instead of such a network, may be the question of another equipment which on occurrence of faults cause violent current flow through the object 1 .

In the line 2 between the object 1 and the network 3 there is provided a conventional circuit breaker 4. This circuit breaker comprises at least one own sensor for sensing circumstances indicative for an over-current flowing in the line 2. Such circum- stances may be currents/voltages but also other indicating that a fault is at hand. For instance, the sensor may be a light arc sensor or a sensor registering short circuit sound etc. When the sensor indicates that the over-current exceeds a certain level, the circuit breaker 4 is activated for breaking the connection between the object and the network 3. However, the circuit breaker 4 has to break to total short circuit current/fault current. Thus, the circuit breaker must be designed to fulfil highly placed requirements, which in practice means that it will operate relatively slowly. The switch arrangement 5 according to the inven- tion comprises a switch element 6, the structure of which will be discussed more closely hereafter. This switch element 6 is ca- pable of operating as a closing means. In Fig 1 the closing means 6 is coupled between the line 2 and ground 7 to form a diverting low resistance channel when the closing means 6 is caused to transfer from an electrical insulating state to a con- ducting state. As will be discussed more closely later, this closing is intended to occur very rapidly so that the object 1 and other constituents of the plant are subjected to a minimum of fault current strain. The arrangement 5 comprises a control unit 8. An arrangement 9 having the ability to detect occurrence of over-currents or over-voltages is coupled thereto. This arrangement 9 is for the sake of simplicity indicated in Fig 1 as comprising one single sensor but it could of course comprise a plurality of sensors for various sensing tasks. The control unit 8 is in connection with a control member 10 capable of bringing the closing means 6 into an electrically conducting state.

It is not necessary that the closing means 6 is coupled to ground 7 as in Fig 1 but the closing means 6 could also be connected to some other unit for fault diversion purposes. For efficient fault diversion, said second unit should at least at times have a potential lower than the line 2.

The view in Fig 1 is diagrammatical. The line 2 may comprise one as well as several phase conductors. In the case of several phase conductors, it may be preferable to provide one closing means 6 for each phase conductor.

Fig 2 illustrates another variant according to the invention. More specifically, Fig 2 illustrates an embodiment where a three- phase generator 1 having stator windings denoted 1 1 feeds the network 3. The phase conductors are denoted R, S and T. Circuit breakers 4 are placed in these phase conductors in conventional manner. The generator 1 has its zero point ungrounded or, as in the example, grounded in a high-ohm manner by means of a resistor 12. In order to achieve an over- current protection function, two closing means 6 according to the invention are coupled so as to connect two of the phases. As can be seen, one of the closing means 6 connects more specifically the phase conductors R and S whereas the other connects the phase conductors S and T. The closing means 6 are normally electrically insulating but when a fault has been detected, they are brought to conduction and will thereby reduce the fault current flowing to ground in an insulation fault indicated as an example at 13.

Fig 3 illustrates diagrammatically an over-voltage application of closing means 6 coupled between phase conductors in a three- phase system and ground. When an over-voltage has been detected by means the previously described arrangement 9, the control unit 8 not illustrated in Fig 3 controls the closing means 6 in question for diversion of current to such an extent that the over-voltage pulse is reduced to an acceptable level.

Fig 4 illustrates a more developed embodiment for over-voltage protection. More specifically, this embodiment is also capable of efficiently protecting against inductively transferred over-voltages and against some coupling over-voltages which may occur between phases. For this purpose closing means 6 are in addition coupled directly between the phases according to Fig 4.

Figs 1 -4 illustrate that the closing means according to the invention may be coupled in many different ways in order to provide for protection functions both with regard to over-voltages and over-currents and independently of whether these result from direct faults or not.

Fig 5 has already been discussed above. As mentioned, the gas pressure in the enclosure containing the electrode gap should be selected so that the position of the radiation induced breakthrough will arrive at the bottom of the Paschen graph and so that the position of a spontantaneous electric breakthrough will arrive at such a location the Paschen graph that it there is rising strongly, i.e. that the pressure relation in the enclosure of the closing means 6 is selected so that the strength against a spontantaneous electric breakthrough is substantially higher than the strength against a radiation induced breakthrough.

Fig 6 illustrates a first embodiment of the over-current reducing arrangement 5 having a closing means denoted 6. The closing means 6 comprises electrodes 14 and a gap 15 present therebetween. As previously described, the closing means comprises members 16 to trigger the electrode gap 15 to form an electrically conducting path between the electrodes. A control member 10 is adapted to control, via the control unit 8, operation of the members 16. The members 16 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 16 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 invention the members 16 comprise at least one laser, which by energy 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 16, energy to the electrode gap 15 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 15 is immediately brought to electric conductivity. In order to spare, and optimise the use of, the (normally) limitedly available laser energy/effect the members 16 may, however, in use of the invention be adapted so that they are ca- pable of achieving ionisation/plasma formation in only one or more parts of the gap 15. In the embodiment according to Fig 6 it is illustrated that the member 16 supplies radiation energy in one single point or area 17. 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 6 is coupled between a line and ground or otherwise between two lines or units as diagrammatically indicated in Fig 6, there will between the electrodes mostly occur a voltage difference giving rise to an electric field. The electric field in the gap 15 may be used to promote or cause an electric breakthrough between the electrodes as soon as the members 16 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 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 14. However, it is pointed out that the invention is not intended to be restricted only to use on occurrence of a strong electric field. Thus, the intention is that the members 16 should be capable of establishing electric conduction between the electrodes also with a relatively weak field, and this preferably with a low energy need.

As a consequence of the requirement on the closing means 6 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 15 becomes sufficient for safe closing. On the other hand, it is a desire that the closing means 6 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 15 should be comparatively low. However, this reduces in one spot ionisation the speed and reliability, 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 according 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 15, 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 15 in the insulating state thereof is suitably not more than 20% and preferably 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 15 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 oc- curs.

The electrode gap 15 is as appears from Fig 6 enclosed in a suitable housing 20. The medium in the electrode gap is intended to be such that it is ionized, on triggering, to a plasma. In such a case it would be suitable to initiate ionisation/plasma formation in the gap 15 in at least one spot somewhere between the electrodes 14. However, in Fig 6 is illustrated a case where there exists in the gap 15 either vacuum or a suitable medium. It is then preferred that initiation of closing occurs by the laser 16 illustrated in Fig 6 being caused to focus, via a suitable optical system 18, the emitted radiation energy in at least one area 17 on or adjacent to one of the electrodes. This causes the electrode to function as an electron and ion emitter for establishing an ionised environment/a plasma in the electrode gap 15 so that, accordingly, an arc will be formed between the electrodes. As appears from Fig 6, one of the electrodes 14 may comprise an opening 19, through which the laser 16 is adapted to deliver radiation energy to the area 17 with the assistance of the optic system 18.

Fig 7 illustrates a closing means variant 6a where instead the system laser 16a/optics 18a focuses the radiation energy in at least one triggering area 17a 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 14 in the insulating state of the closing means are concerned, the char- acteristics 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 14. The constructive measures available concern of course electrode design, distance between the electrodes, the medium between the elec- trodes and the occurrence of possible further field influencing components between the electrodes.

Diffractive, refractive and reflective optical elements may be used in the invention.

Fig 8 illustrates an embodiment based upon an optical system 18b comprising a lens system 21 , via which arriving laser pulses are delivered to a diffractive optical phase element 22, a kino- form. This element is designed to have a plurality of focal points 17b generated starting from a single arriving laser pulse. These focal points 17b are distributed along the axis of symmetry be- tween the electrodes 14b. Since the focal points 17b are distributed along a line between the electrodes 14b, 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 21 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 serious degree disturb the electric field between the electrodes.

In the embodiment according to Fig 8 the radiation energy is supplied through an opening 19b in one of the electrodes as before. Fig 9 illustrates a variant where generally the only difference as compared to the embodiment according to Fig 8 is that here the diffractive optical element (kinoform) 22c is placed radially outwardly of one of the electrodes 14c. The optical element 22c is as before designed to divert the laser light and fo- cus the same in a number of spots distributed along the desired electric conduction path between the electrodes. The radiation bundles forming the spots 17c have each their own deflection angle. Thus, the radiation bundles have to move different distances to the respective spots 17c. The advantage of locating, according to Fig 9, the kinoform 22c 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 16d via an optical system 18d supplies the laser radiation symmetrically in a plurality of focal points 17d distributed along the length of the electrode gap without any opening being required in the elec- trodes 14d. The optical system 18d comprises a prisma or a radiation divider 23 arranged to break up the laser beam around the adjacent electrode 14d. Around this electrode 14d there is provided one or preferably more kinoforms 22d (diffractive optical elements) designed to, possibly with the assistance of further lenses, focus the laser beam in the desired focal points 17d so that plasma formations are generated therein.

Fig 1 1 illustrates a variant where a laser beam by means of an optical system 18e comprising optical fibres 24 is directed for formation of focal points 17e located at different places between the electrodes 14e. The optical fibres 24 may be arranged to emit the light via lenses 25.

The definition of a conical lens, a so called axicone, may be said to be every rotationally symmetrical optical element, which by refraction, 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 light 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 substantial extent of this axis.

It appears from Fig 12 that the light may be focused, by means of an axicone 22f, in an elongated focal area 17f located between the electrodes 14f. 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.

Fig 13 illustrates an embodiment where a specially shaped dif- fractive axicone 22g, a kinoform, has been designed to provide focal areas 17g and 17g' respectively with different shapes. In the example it is illustrated that the focal area 17g is elongated and provided on the axis of symmetry of the axicone 22g and the electrodes. The focal area 17g' on the contrary has as is in- dicated to the left in Fig 13 obtained a cross-sectionally tubular shape. This tubular shape is advantageous most closely to an electrode 19g provided with an opening 19g since the periphery of the tubular focal area 17g' will be located relatively close to the electrode 19g provided with the opening. Both focal areas 17g and 17g' have in Fig 14 a substantially constant intensity along the axis of symmetry but perpendicularly thereto there occurs, as concerns the focal area 17g, a substantially Gauss- shaped or Bessel-function shaped intensity distribution.

An advantage of an entirely or substantially conical or diffrac- tive, coaxially focusing component as for example in Fig 8, 9, 10, 12 and 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 radiation 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 14h. The occurrence of a plurality of simultaneously electrically conducting channels increases the conduction capacity of the closing means.

Fig 15 illustrates diagrammatically that a switch arrangement 5i is coupled in series in the previously discussed line 2i between the network 3i and the object 1 i. The switch arrangement 5i comprises suitably a switch element 6i with the previously described character, i.e. a switch element having an electrode gap intended to be brought to an electrically conducting state by means of radiation energy. However, this is not shown more closely in Fig 15. As appears from Fig 15 the switch arrange- ment 5i is intended to have a purely switching function, i.e. the supply of the object 1 i or possibly supply in the opposite direction, may occur via the switch element 6i when it is in a conducting state. If necessary, the switch element 6i can be made to inhibit the current passage relatively rapidly, for instance for protecting the object 1 i or possibly even the network 3i from current flow from the object 1 i. In order to provide coupling-off in alternating current connections with the assistance of the switch element 6i, it is sufficient that the members for energy supply to the electrode gap are made to cease with such energy supply. On the following passage through zero it is intended that extinguishing of the light arc in the switch element 6i occurs so that current supply ceases. On direct current applications it would appear to be necessary to support the breaking function by tak- ing measures to reduce or eliminate the voltage difference over the switch element 6i. Such a means may consist in an electric switch 26 coupled in parallel over the switch element 6i. Closing of the electric switch 26 means that the current is shunted past the switch element 6i, a fact which causes the light arc in the switch element 6i to be extinguished. If such a measure wouldn't be sufficient, further electric switches could, as complement thereto, be arranged on either sides of the switch element 6i in the line 2i to totally disconnect the switch element 6i from the line 2i.

The purpose of Fig 15 is to illustrate that the switch arrangement 5i according to the invention may find general switch applications, in which it may be the question of protection of various apparatus but also of more generally switching effects in various load situations.

The definition of an axicone may be said to be each rotationally symmetrical wave movement directing element, which by refraction, 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 sym- metry 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 16 illustrates an embodiment of the invention where such an axicone 22k is used. This axicone forms more specifically a radiation energy line 17k between the electrodes 14k.

The axicone 22k is so designed and the substantially collimated radiation 27 incident to the axicone so directed that the radiation energy line 17k is at least partly displaced laterally a distance d in relation to a centre axis 28 of the incident, substantially collimated radiation.

In the case of an axicone 22k as in Fig 16, this means that the axicone 22k has its optical axis/axis of symmetry 29 laterally displaced from the axis 28 of the incident collimated radiation. It appears from Fig 16 that the incident radiation 27 passes through the axicone 22k and is deflected thereby in a peripheri- cal area thereof. The consequence thereof is that the axicone 22k will direct the radiation energy obliquely as indicated by means of the arrow 30 but nevertheless the radiation energy line 17k, along which the radiation energy is focused, will be substantially parallel to the incident collimated radiation 27.

It is preferred that the axicone 22k is adapted to apply the radiation energy along the radiation energy line so that a substantially rod-shaped area results along said line, said area being ionised/formed to a plasma and bridging, entirely or substan- tially entirely, the distance between the electrodes 14k for creating favourable, to a maximum degree, conditions for arc formation between the electrodes.

At least one of the electrodes has an opening 19k, through which the axicone 22k is adapted to direct the radiation energy.

The opening 19k extends obliquely relative to an axis 31 of symmetry of the electrodes 14k. The opening 19k is eccentric relative to the axis 31 of symmetry of the electrodes. The axis 29 of the axicone coincides with the radiation energy line 17k.

The axicone 22k is adapted to apply the radiation energy so that it arrives upon a lateral surface, denoted 32, of the opening 19k in one of the electrodes 14k. It is pointed out that the axicone 22k is adapted to apply the radiation energy line 17k so that it is substantially parallel to the axis 31 of symmetry of the elec- trodes 14k. The lateral surface 32 forms an angle α to the axis 31 of symmetry. The radiation directed to the adjacent electrode 14k by the axicone 22k forms an angle γ with the axis 31 of symmetry. The angle α smaller than the angle γ, preferably about half the angle γ. This means that the radiation 30 will hit upon the lateral surface 32, a fact which promotes formation of a plasma extending between the electrodes 14k.

Even if in Fig 16 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 27.

According to a preferred embodiment, the axicone 22k is designed to be rotatable about its axis 29 of symmetry. This means the advantage that if a portion of the axicone 22k present opposite to the opening 19k would be influenced negatively by the arc between the electrodes, it is possible to move forward, by rotation of the axicone 22k, such a portion of the axicone, which is in good condition, to such an area that the collimated radia- tion energy from the radiation source in an adequate manner may be deflected and applied along the previously discussed radiation energy line 17k.

By adequate selection of constituents in the gas mixture in the electrode gap/enclosure 15, an optimum function can be achieved as already indicated above. In the following three main types of gases will be discussed, namely strength gases, ignition gases and extinguishing gases.

As a strength gas any gas having a high strength against spontantaneous electric breakthrough may be in question. Examples of strength gases are N₂, Co₂, 0₂, air, SF₆ and mixtures containing one or more of them. These gases have a strength against spontantaneous electric breakthrough which is at least 25 kV/cm x bar.

As ignition gases may be in question

1 . Any gas having a low resistance to radiation induced breakthrough, e.g. rare gases (argon, xenon, krypton). These gases have a strength less than or substantially less than 5 kV/cm x bar.

2. Gases having a high absorption at the radiation wave length used for ionisation/plasma formation in the electrode gap.

3. Aerosols, which considerably reduce the strength against radiation induced plasmas and in addition increases the spontantaneous strength.

Each gas which is more or less markedly electro-negative may be used as an extinguishing gas. Such electro-negativity means that free electrons in the gas volume in the enclosure will be caught, a fact which rapidly deionizes the electrode gap. SF₆ is an extreme example of an electro-negative gas. Other examples are C0₂ and 0₂. As further extinguishing gases, gases may be in question which also are useful as strength gases.

In order to provide a rapid and current conduction efficient closing of a conducting path between the electrodes it is favour- able to create between them extended plasmas by means of special radiation directing optics. The elongated plasmas short circuit momentarily the entire electrode gap and result in extremely low ignition voltages and rapid switch times. The problem is to create sufficiently elongated plasmas to achieve a high strength. It turns out to be practically difficult to achieve elon- gated plasmas in the strength gases just due to their strength - the effects of the Paschen graph is not sufficiently strong. On the contrary it is possible in ignition gases, in particular rare gases, to create very long plasmas having a relatively restricted radiation energy. On the other hand these gases have a too low electric strength against spontantaneous breakthrough to provide plasma switches to the highest voltages.

It has turned out that it is possible to increase the strength against spontantaneous breakthrough very considerably by mixing small amounts of typical strength gases into ignition gases. An example is that only some per cent of nitrogen gas in argon doubles the strength as compared to pure argon.

In a laboratory the following has in addition been proven:

The plasma length of extended plasmas in argon-nitrogen mixtures depend substantially on the partial pressure for argon as long as the pressure is so low that nitrogen gas plasmas are hardly formed.

Mixtures with typically 25% nitrogen gas and 75% argon have strengths which are close to 50-50-mixtures, up to 60kV/cm at a pressure of 6 bar. Pure nitrogen gas is on 96 kV/cm and pure argon at about 18 kV/cm.

This example illustrates that by using to the gas mixture in the enclosure a combination of nitrogen gas (high strength) and rare gas, in particular argon, (high trigger ability) a very good functionality is obtained. In this connection it is pointed out that the proportion of nitrogen gas in the gas mixture in the enclosure is between 0.1 and 70%, suitably between 0.5 and 50% and preferably not over 30%. An advantage in using rare gases, in particular argon and xenon, is that they do not react easily with other atoms or molecules. Thus, these gases will not or only to a small degree be effected by the established light arc in the closing means.

Nitrogen gas is a simple biatomary molecule which will not form any stabile compounds with the rare gases. Formation of some nitride compounds with metal ions eroded away from the electrodes may be expected. However, this does not have to form any problem but can in fact be used for plasma formation and maintaining thereof.

Rare gases, in particular argon and xenon, and nitrogen gas are non-toxic and do not have any known negative effects on the environment. Neither the metal nitrides formed are expected to be a health risk or dangerous to the environment.

It is to be noted that the description presented above only should be considered as exemplifying for the inventive concept, on which the invention is based. Thus, it is obvious to men skilled within the art that detail modifications may be made without leaving the frame 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 ionization/piasma formation energy to the gap 15. Also other radiation sources, for example electron cannons, or other energy supply solutions may be used as long as they fulfil the requirements with respect to rapidity and reli- ability according to the invention. The switch device 6 and the switch element 5 may be designed to be mobile. Finally, it is pointed out that the invention is well suited for alternating voltage as well as direct voltage.

Claims

CLAIMS:

1 . A device for switching electrical effects, comprising at least one electric switch arrangement (5), characterized in that the switch arrangement (5) comprises at least one switch element (6) comprising an electrode gap (15), which is convertible between an electrically substantially insulating state and an electrically conducting state, and members (16) to cause or at least initiate the electrode gap or at least a part thereof to assume electric conductivity, the members (16) for causing or at least initiating the electrode gap to assume conductivity being arranged to apply 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, and that the electrode gap (15) is arranged in an enclosure containing a mixture of gases, at least one first gas thereof imparting the gas mixture a relatively high strength against spontantaneous electric breakthrough between the electrodes whereas at least one second gas has the property to promote break- through between the electrodes with that type of radiation energy which is used or present a relatively moderate resistance to such breakthrough.

2. A device for switching electrical effects comprising at least one electric switch arrangement (5), characterized in that the switch arrangement (5) comprises at least one switch element (6) comprising an electrode gap (15), which is convertible between electrically substantially insulating state and electrically conducting state, and members (16) for causing or at least initiating the electrode gap or at least a part thereof to assume electric conductivity, the members (16) for causing or at least initiating the electrode gap to assume conductivity being arranged to supply 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, and that the electrode gap is arranged in a pressure tight enclosure (20), in which there is a pressurized gas atmosphere, the pressure in the enclosure being adjusted, while considering Pashen's law, so that the electric strength against spontantaneous breakthrough is greater than the strength against triggering energy induced electric breakthrough between the electrodes.

3. A device according to claim 1 , characterized in that the first gas is selected among N₂, C0₂, 0₂, air, SF₆ and mixtures containing one or more of them.

4. A device according to claims 1 and 3, characterized in that the second gas is selected among rare gases, gases with a high absorption of electrons or other charged particles at that or those radiation wave lengths used and aerosols as well as mixtures containing one or more of these gases.

5. A device according to any preceding claim, characterized in that the first gas has a strength to spontantaneous electric breakthrough which is at least 25 kV/cm x bar.

6. A device according to any preceding claim, characterized in that the second gas has a strength against breakthrough induced by the radiation in question which is less than 5 kV/cm x bar.

7. A device according to any preceding claim, characterized in that the gas mixture in the enclosure contains at least one rare gas and nitrogen gas.

8. A device according to claim 4 or 7, characterized in that the rare gas is argon, xenon or krypton.

9. A device according to any of claims 3-8, characterized in that the proportion of nitrogen gas in the gas mixture in the en- closure is between 0.1 and 70%, suitably between 0.5 and 50% and preferably not over 30%.

10. A device according to any preceding claim, characterized in that the members (16) for causing or at least initiating the electrode gap or a part thereof to assume electric conductivity comprise at least one laser.

1 1 . A device according to any preceding claim, characterized in that the switch element (6) is designed so that there is, in its insulating state, an electric field between the electrodes (14) thereof, said electric field promoting or causing an electric breakthrough between the electrodes on bringing or initiating the electrode gap to assume electric conductivity.

12. A device according to claim 1 1 , characterized in that the electric field in the insulating state of the electrode gap (15) has a substantially smaller field strength than the field strength, at which spontantaneous breakthrough occurs.

13. A device according to claim 1 1 or 12, characterized in that the electric field in the insulating state of the electrode gap (24) has a field strength which is not more than 30%, suitable not more than 20%, and preferably not more than 10% of the field strength, at which spontantaneous breakthrough occurs.

14. A device according to any of claims 1 1 -13, characterized in that the electric field in the insulating state of the electrode gap (15) has a field strength which is at least 0.1 %, suitably at least 1 % and preferably at least 5% of the field strength, at which spontantaneous breakthrough occurs.

15. A device according to any preceding claim, characterized in that the members (16) for causing or at least initiating the electrode gap (15) to assume electric conductivity are ar- ranged to supply the radiation energy in such a way that the lowest electric field strength, at which the electrode gap may be triggered to assume electric conductivity, is minimized.

16. A device according to any preceding claim, characterized in that the members (15) for causing or at least initiating the electrode gap (15) to assume electric conductivity are arranged to supply the radiation energy to the electrode gap in such a way that the time delay between the arriving radiation energy and the developed conductivity of the electrode gap is minimized.

17. A device according to any preceding claim, characterized in that the members (16 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 (14).

18. A device according to any preceding claim, characterized in that the members (16) for supply of triggering energy to the electrode gap are adapted to locate the radiation energy in one or more spots or one or more areas in the gap (15) between the electrodes (14).

19. A device according to claim 18, 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.

20. A device according to any preceding claim, characterized in that the members (16) for supply of triggering energy to the electrode gap are adapted to apply the radiation energy in one or more elongated areas (17), the longitudinal axis of which resides substantially along the direction of the desired electric conduction path between the electrodes.

21. A device according to claim 20, characterized in that the members (18) for supply of triggering energy to the electrode gap are arranged to shape the elongated focal area to a tubular shape.

22. A device according to claim 20 or 21 , characterized in that the members for supply of triggering energy to the electrode gap are adapted to shape said elongated area so that it entirely or substantially entirely bridges the distance between the electrodes.

23. A device according to any of claims 20 or 21 , characterized in that the members (18) for supply of triggering energy to the electrode gap are adapted to form, in said gap, two or more elongated focal areas (17) located in the longitudinal direction after each other along the desired electric conduction path between the electrodes.

24. 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 focal areas, the longitudinal axes of which extend substantially along the direction of the desired electric conduction path between the electrodes.

25. 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 radiation energy both on at least one of the electrodes and between them.

26. A device according to any preceding claim, characterized in that at least one of the electrodes at the electrode gap has an opening (19) through which the members (16) for triggering energy supply are adapted to direct the radiation energy.

27. A device according to claim 21 and 26, characterized in that the members (18) for supply of triggering energy to the electrode gap are adapted to locate the tubular radiation energy area (17) adjacent to the electrode provided with the opening (19) and such that the axis of the tubular radiation energy area is substantially concentric to the axis of the opening in the electrode.

28. A device according to any preceding claim, characterized in that the members for supply of triggering energy to the electrode gap comprises a system (18) for directing electro-magnetic wave energy.

29. A device according to claim 28, characterized in that the di- rection system comprises at least one refractive, reflective and/or diffractive element.

30. A device according to claim 29, characterized in that the element is formed by an axicone, a kinoform or optical fibres (38).

31 . A device according to any of claims 29-30, characterized in that the direction system (18) is placed radially outwardly of the electrodes and adapted to direct bundles of radiation in- wardly towards the gap between the electrodes.

32. A device according to any of claims 29-31 , characterized in that the direction system (18) is adapted to break up laser pulses to an annular configuration about one of the elec- trodes.

33. A device according to any of claims 29-30, characterized in that the direction element (22k) of the radiation direction system is adapted to apply the radiation energy along a line (17k) extending between the electrodes and that the direction element (22k) is so designed and the substantially collimated radiation inciding to the direction system so directed that the radiation energy line (17k) is at least partly displaced laterally (d) in relation to the axis (28) of the incident, substantially collimated radiation.

34. A device according to claim 33, characterized in that the direction element (22k) is adapted to orientate the radiation energy line (17k) substantially parallel to the substantially collimated radiation (27) incident to the direction element.

35. A device according to any of claims 33-34, characterized in that the direction element (22k) is adapted to direct the radiation energy obliquely relative to an axis (31 ) of symmetry of the electrodes.

36. A device according to claim 26 and any of claims 33-35, characterized in that the opening (19k) extends obliquely relative to an axis (31 ) of symmetry of the electrodes.

37. A device according to claim 36, characterized in that the opening (19ki) is eccentric relative to an axis (31 ) of symmetry of the electrodes.

38. A device according to any of claims 36-37, characterized in that the direction element (22k) is adapted to direct the radiation energy so as to be incident on a side surface (32) of the opening in the electrode.

39. A device according to any of claims 33-38, characterized in that the direction element (22k) is a rotatable about its axis

(29) of symmetry.

40. A device according to any preceding claim, characterized in that the switch element (6) is adapted to protect, in an elec- trie power plant, the object (1 ) against over-currents and/or over-voltages by being imparted conductivity by radiation en- ergy supply when over-currents and over-voltages are detected by means of an arrangement (9) therefor.

41 . A device according to any preceding claim, characterized in that it comprises a control unit (8) connected to the switch element (6) and to the arrangement (9) detecting over-current and/over-voltage conditions, said control unit being adapted to control the switch element to closing when required for reasons of protection with assistance of information from the arrangement.

42. A device according to any preceding claim, characterized in that the gas mixture in the enclosure in addition contains at least one gas, which may be formed by said at least one first gas or by a third gas and having the property of being elec- tronegative for the purpose of deionizing the electrode gap.

43. A device according to claim 35, characterized in that the de- ionization gas mentioned in claim 42 is C0₂, 0₂, SF₆ or mixtures of one of more thereof.

44. Use of a device according to any preceding claim for protection of an object against over-currents and/or over-voltages.

45. Electric power plant, characterized in that it contains one or more devices according to any of claims 1 -43.