OA11126A - Switching device including spark gap for switchingelectrical power - Google Patents

Abstract

A device for switching electric power comprises at least one electric switching arrangement (5). This switching arrangement comprises at least one switching element (10a) comprising an electrode gap (24). This gap is convertible between an electrically substantially insulating state and an electrically conducting state. Furthermore, the switching element comprises means (25) for causing or at least initiating the electrode gap or at least a part thereof to assume electrical conductivity. The means (25) for causing or at least initiating the electrode gap to assume conductivity are adapted to supply energy to the electrode gap in the form of radiation energy to bring the gap or at least a part thereof to the form of a plasma by means of this radiation energy.

Description

1 01I126 SWITCHING DEVICE INCLUDING SPARK GAP FOR SWITCHING ELECTRI- CAL POWER.

FIELD OF THE INVENTION AND PRIOR ART

This invention is relateâ to a device according to the pre-characterising part of enclosed claim 1. The device accord-ing to the invention may be used in any connection forswitching purposes. Particularly preferred are applicationswhere high power is to be switched. In realitv, high voltageconnections and electric power transmission applications areinvolved. A preferred, but not restricting, application ofthe device according to the invention is to protect, in anelectrical power plant, an electrical object from the consé-quences of faults, primarily as far as current is concernéebut also voltage. Besides, the invention comprises a methodfor protection of the object.

The electric object in question may be of arbitrary natureas long as it is contained in an electric power networkand requires protection against fault-related over-cur-rents, i.e. in practice short-circuit currents. As an ex-ample, it may be mentioned that the object may be formedby an electric apparatus having a magnetic circuit, e.g. agenerator, transformer or motor. Also other objects may bein question, e.g. power lines and cables, switch gearecuipment etc. The présent invention is intended to be ap-piied in connection with medium and high voltage. Ac-cording to IEC norm, medium voltage refers to 1-72,5 kVwhereas high voltage is >72,5 kV. Thus, transmission, sub-transmissiôn and distribution levels are included. d 1 1 26

In prior power plants of this nature one has resorteâ to,for protection of the object in question, a conventionalcircuit-breaker (switching device) of such a design thatit provides galvanic séparation on breaking. Since thiscircuit breaker must be designed to be able to break veryhigh currents and voltages, it will obtain a comparâtivelybulky design with large inertie, which reflects itself ina comparatively long break-time. It is pointed out thatthe over-current primarily intended is the short-circuitcurrent occurring in connection with the protected object,for instance as a conséquence of faults in the electricinsulation system of the protected object. Such faultsmeans that the fault current (short-circuit current) ofthe external network/equipment will tend to flow throughthe arc. The resuit may be a very large breakdown. It maybe mentioned that for the Swedish power network, the di-mensioning short-circuit current/fault-current is 63 kA.In reality, the short-circuit current may amount to 40-50kA. A problem with said circuit-breaker is the long-break timethereof. The dimensioning break-time (IEC-norm) for com-pletely accomplished breaking is 150 milliseconds (ms). Itis associated to difficulties to reduce this break-time toless than 50-130 ms depending upon the actual case. Theconséquence thereof is that when there is a fault in theprotected object, a very high current will flow throughthe saine during the entire time required for actuating thecircuit-breaker to break. During this time the full faultcurrent of the external power network involves a considér-able load on the protected object. In order to avoid dam-age and complété breakdown with respect to the protectedobject, one has, according to the prior art, constructedthe object so that it manages, without appréciable damage,to be subjected to the short-circuit current/fault currentduring the break-time of the circuit breaker. It is 3 0111:6 pointed out that a short-circuit current (fault current)in the protected object may be composed of the own contri-bution of the object to the fault current and the currentaddition emanating from the network/equipment. The owncontribution of the object to the fault current is not in-fluenced by the functioning of the circuit-breaker but thecontribution to the fault current from the net-work/equipment dépends upon the operation of the circuitbreaker. The requirement for constructing the protectedobject so that it may withstand a high short-circuit cur-rent/fault current during a considérable time period meanssubstantiel disadvantages in the form of more expensivedesign and reduced performance.

As pointed out hereinabove, the invention is, however, notonly restricted to protection applications. In otherswitching situations it is a disadvantage to hâve to re-sort to rather costly and bulky switching devices whenhigh power is involved, for instance banks of semi-conductor components, in order to manage the switchingfunction aimed at. Today's semi-conductor comportent, whichpreferably is produced in Silicon even if other materialsmay be in question, has for practical reasons a restric-tion as to the maximum electric field strength which thecomponent may withstand before an electrical breakthroughoccurs in the semi-conductor material. This implicates im-mediately corresponding restrictions of the maximum elec-tric voltage that the component may be subjected to. Inparticular in high voltage connections, one is thereforeforced to couple in sériés (stack) a large number of semi-conductor components in such a way that none of the compo-nents contained in the stack is subjected to a voltagewhich is above a safe level for the component.

Furthermore, complications may occur in the design of thesemi-conductor component in that the semi-conductor mate- 4 0111 0 rial in itself endures to be subjected to, for instancecompared with atmospheric air, very high electric fieldstrength. The sarae is, however, not valid for the insulat-ing material which necessarily must be présent betweenthose électrodes externally of the semi-conductor materialbetween which the high voltage is placed. This also in-volves a restriction: In design of a semi-conductor compo-nent for high voltage use a careful balancing must be madebetween the electrical field strength in the semi-conductor material and the electric résistance in the sur-rounding insulating medium.

In several applications in electric power plants the com-ponents included therein are subjected to not only highelectrical voltages but also to large electrical currents.When a current passes through a component having a certainrésistance, considérable amounts of thermal energy ( so-called Joule heat energy) which is proportional to the ré-sistance in question and to the square of the current.Since each semi-conductor component has a small but negli-gible résistance, the maximum current that the componentstack may endure is restricted. If very large currents areto be conveyed by the semi-conductor component s one isforced to convey the current through several identicalparallel current paths. The number of semi-conductor com-ponents increases, accordingly, multiplicatingly.

At high voltages and at large currents, a large number ofsemi-conductor components must be used. This results imme-diately in a lower reliabilitv since ail components mustfunction in order to make the electric power plant, suchas for instance a. HVDC valve, to be in operation.

The fact that a large number of semi-conductor componentsare stacked means that they must be controlled with veryhigh précision in time. Erroneous "timing” could for in- stance resuit in a far too high a voltage being applied over an individual component causing a certain failure and appendant removal from operation of the entire plant. The "timing" problem increases, of course, if a plurality of parallel current paths must be provided and synchronized.

OBJECT OF THE INVENTION

η 1 Ί 4 ο S υ ι ! ι ζ: ο

The primary object of the présent invention is to providea switching device better suited for switching high elec-tric power in a rapid manner and to a comparatively lowcost than switching devices used today. A secondary object of the présent invention is to deviseways to design the device and the method so as to achievebetter protection for arbitrary objects and, accordinaly,a reduced load on the same, a fact which means that theobjects themselves do not hâve to be designed to withstanda maximum of short-circuit currents/fault currents duringrelatively long time periods.

SUMMARY OF THE INVENTION

According to the invention, the switching arrangement is de-signed in accordance with the characterizing part of claim1. Since the electrode gap of the switching means is broughtto an electrically conducting State by supplying energy di-rectly to the electrode gap proper in the form of radiationin order to establish ionisation/plasma in the electrodegap, conditions are created for a very rapid operation ofthe switching arrangement according to the invention. Theionisation/plasma "ïn the electrode gap causes/initiates anelectrically conducting plasma channel having a very highconductivity so that very large currents may be conveyed andthis more specifically during relatively prolonged time pe- 011126 riods without négative effects, which is in direct contrast to conventional semi-conductor art.

According to the invention, the secondary object indicatedabove is achieved in that the switching arrangement in theform of an over-current reducing arrangement, which is ac-tuatable for over-current réduction with assistance of anover-current conditions detecting arrangement, is connectedto the electric power plant for protection of the object.The switching arrangement may, according to a preferred em-bodiment, form an over-current diverter for diverting over-currents to earth or otherwise another unit having a rela-tively low potential.

Thus, the invention is based upon the principle, as far asthe protection aspect is concerned, to utilise a rapidly op-erating switch arrangement, hereinafter called switch means,which without effecting any real breaking of the over-current, nevertheless reduces the same to such an extentthat the object under protection will be subjected to sub-stantially reduced strains and accordingly a smaller amountof damages. The reduced over-current/fault-current means,accordingly, that the total energy injection into the pro-tected object will be substantially smaller than in absenceof the switch means according to the invention.

The solution according to the invention based upon a switchmeans implies a particularly advantageous fulfilling of de-mands which may be set up in order to achieve a satisfactoryprotection function. Thus, a very rapid triggering may beachieved by the switch means so that occurring fault-relatedover-currents with -a very small delay in time will be di-verted via the switch means as soon as the electrode gap hasadopted an electrically conductive condition. It is pointedout that the term "triggering" in this connection meansbringing the switch means into an electrically conducting 7 01 26

State. By means of the arrangement of the switch means, saidswitch means may easily be dimensioned to be able to conductvery large currents. In order to obtain a satisfactory pro-tection function it is, namely, désirable that the currentconducting channel, which is established through the switchmeans, has a very low résistance. This means the largestpossible strain-relieving of the object, which is to be pro-tected from fault-currents. Besides, a switch means accord-ing to claim 1 may with a small effort be caused to functionwith a particularly high triggering safety. The triggeringmust not, in order to divert occuring fault-currents as soonas possible, therefore fail in a critical situation. Theswitch means according to the invention gives on the otherhand nss to the possrbrlrty to dimensroning in order toachieve a very high electric strength in a non-triggeredcondition. The probability for a spontaneous breakthrough isthus, to be at a minimum. It is especially preferred tothereby use at least one laser for triggering.

Préférable developments with respect to a.o. the means forsupplying radiant energy to the electrode gap are defined inthe enclosed daims. According- to one embodiment, the radi-ant energy is supplied to the electrode gap in two or morespots or areas for the purpose of achieving the highest pos-sible certainty with regard to bringing the electrode gap toassume an electrically conducting State. According to onealternative the energy supply means may be designed to sup-ply the radiant energy along an elongated area in the con-duction pat'n which is aimed at between the électrodes. Ac-cording to an optimal embodiment this elongated area may, entirely or substantially entirely, bridge the gap between.*1 · the électrodes. Although it is possible, in a case with twoor more spots or areas for radiation supply, that thesespots or areas are applied successively corresponding to thepropagation with respect to the electrical conduction pathbetween the électrodes in such a way that the spots or areas 011126 are successively applied with a time delay, it is, according to the invention, normally preferred to apply these spots or areas substantially simultaneously for making the electrode gap conducting momentarily.

Furthermore, the means for supply of triggering energy mayaccording to the invention be adapted to apply the. radiantenergy in a volume having a tubular shape. This is particu-larly préférable when one of the électrodes comprises anopening, through which the radiant energy is supplied, andwhen the radiant energy supplied in a tubular volume is ap-plied relatively close to the electrode provided with anopening.

According to an alternative embodiment, the energy supplymeans may be designed to supply the radiant energy in a plu-rality of substantially parallel, elongated areas extendingbetween the électrodes.

The radiant energy may also be supplied to the electrode gaptransversely relative to an axis of the électrodes in one ormore spots located between the électrodes.

The switching arrangement according to the invention may beused with advantage for realizing various switching func-tionalities obtainable conventionally by means of semi-conductor art. Expressed in other words, electrical compo-nents may be built by means of the switching arrangement ac-cording to the invention in suitable number, such electricalcomponents having properties similar to those known per sewithin for instance semi-conductor art.

Further advantages and features of the invention, particu-larly with respect to the method according to the inven-tion, appear from the following description and daims. 011126

BRIEF DESCRIPTION OF THE DRAWINGS

With reference to the enclosed drawings, a more spécifiedescription of an embodiment example of the invention fol-lows hereinafter.

In the drawings:

Fig 1 is a purely diagrammatical view illustrating the basic aspect behind the solution according tothe invention,

Figs 2a- 2d a^e diagrams illustrating in a diagrammaticalform and in a comparative way fault current de-velopments and the energy development with andwithout the device according to the invention; Fig 3 is a diagrammatical view illustrating a conceiv-able design of a device according to the inven-tion; Fig 4 is a diagrammatical, detailed view illustrating a possible design of the over-current reducing ar-rangement , Figs 5-7 are views similar to Fig 4 of different variants, Fig 8 is a diagrammatical view illustrating an optical System for energy supply to the electrode gap; Fig 9 is a view”illustrating an alternative optical Sys-tem placed at the side of one of the électrodes;

Fig 10 is a further alternative for an optical System ar-ranged to supply the radiant energy around one of 10

Cl ; 6-

Fig 11

Fig 12

Fig 13

Fig 14

Fig 15

Fig 16

Fig 17

Fig 18

Fig 19 the électrodes and co-axially relative theretowithout need for an opening in one of the élec-trodes ; is a view of an optical System based upon use ofoptical fibres; is a principle view illustrating refraction oflight emanating from a point source by means of arefractive axicone; is a view similar to fig 16 but sbowing the actionof the axicone on a collimated laser beam; is a view illustrating the function of a refrac-tive axicone for génération of an elongated focalarea between the électrodes; is a diagram illustrating the power density alongthe focal area in fig 18; is a view similar to fig 18 but illustrating theuse of a diffraction optical component; is a view illustrating focusing in an elongatedarea by means of a reflective axicone; is a view illustrating use of a diffractive axi-cone (a kinoform) capable of generating focal ar-eas having different geometrical shapes; is a diagrammatical view illustrating the deviceaccording to the invention applied in an electricpower plant comprising a generator, a transformerand an electric power network coupled thereto; 11 0111

Fig 20 is a view illustrating how energy may be suppliedto the electrode gap transversely relative to an axis common to the électrodes, fig 20a illustrat-ing radiant energy being supplied in a single spotor area whereas three such spots or areas occur in fig 20b;

Figs 21a and b are views illustrating how the radiant energy maybe supplied such that several substantially parai-lel and electrically conducting channels are formed between the électrodes; Fig 22 is a sideview illustrating an embodinent somewhatsimilar to the one in fig 10, it being apparent f rom Fig 23 that a plurality of individual kinoforms (diffrac- tive optical éléments) are arranged around one of the électrodes. Fig 24 illustrâtes in diagrammatical form that the switching arrangement according to the invention may fulfil a bidirectional triac function, Fig 25 is a view illustrating a unidirectional triac function,

Figs 25- 28 are three différent examples on how bidirectionaltriac fonction may be achieved by msans of switch-ing arrangements according to the invention eachcomprising two switching means, 01 Π 26 12

Figs 29a- d are views illustrating that the switching arrange- ment according to the invention by sériés couplingwith one or more diode functionalities may be pro-vided to function like a thyristor,

Figs 30 and 31 are examples on how switching arrangements accord-ing to the invention may be used with triac func-tion or thyristor function, and

Fig 32 is a diagrammatical view of the switching arrange-ment according to the invention in a sériésswitching function.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

An electric power plant comprising a protected object 1 isshown in Fig 1. This object could for instance consist ofa generator. This object is connected, via a line 2, to anexternal distribution network 3. Instead of such a net-work, the unit denoted 3 could be formed by some otherequipment contained in the electric power plant. The elec-tric power plant involved is conceived to be of such a na-ture that it is the object 1 itself which primarily is in-tended to be protected against fault currents from thenetwork/equipment 3 when there occurs a fault in the ob-ject 1 giving rise to a fault current from the net-work/eguipment 3 towards the object 1 so that the faultcurrent will flow through the object. Said fault may con-sist in a short-circuit having been formed in the object1. A short-circuit is a conduction path, which is not in-tended, between two or more points. The short-circuit mayfor instance consist of an arc. This short-circuit and theresulting violent current flow may involve considérabledamages and even a total break-down of the object 1. 13 011126

It is already pointed out that with at least some types ofprotected electrical objects 1, short-circuit cur-rents/fault currents harmful to the object in question mayflow from the protected object towards the net-work/equipment 3. Within the scope of the invention, it isintended to be used for protection purposes not only forprotection of the object from externally emanating faultcurrents flowing towards the object but also from internaifault currents in the object flowing in the opposite di-rection. This will be discussed in more detail in the fol-lowing.

In the following, the désignation 3 will, to simplify thedescription, always be mentioned as consisting of an ex-tern al electric power network. However, it should be keptin mind that some other equipment may be involved insteadof such a network, as long as said equipment causes vio-lent current flows through the object 1 when there is afault. A conventional circuit breaker 4 is arranged in the line 2between the object 1 and the network 3. This circuitbreaker comprises at least one own sensor for sensing cir-cumstances indicative of the fact that there is an over-current flowing in the line 2. Such circumstances may becurrents/voltages but also other indicating that a faultis at hand. For instance, the sensor may be an arc sensoror a sensor recording short circuit sound etc. When thesensor indicates that the overcurrent is over a certainlevai, the circuit breaker 4 is activated for breaking ofthe connection between the object 1 and the network 3. Thecircuit breaker 4' must, however, break the total shortcircuit current/fault current. Thus, the circuit breakermust be designed to fulfil highly placed requirements,which in practice means that it will operate relativelyslowly. In Fig1 2a it is illustrated in a current/time-dia- 14 011126 gram that when a fault, for instance a short circuit inthe object 1, occurs at the time tfauy^, the fault currentin the line denoted 2 in Fig 1 rapidly assumes the magni-tude ii· This fault current i]_ is broken by means of thecircuit breaker 4 at tj, which is at least within 150 msafter tfauy-fc· Fig 2d illustrâtes the diagram i^-t and, ac-cordingly, the energy developed in the protected object 1as a conséquence of the short circuit therein. The energyinjection into the object occurring as a conséquence ofthe short-circuit current is, accordingly, represented bythe total area of the outer rectangle in Fig 2d.

It is in this connection pointed out that the fault cur-rent in Figs 2a-c and the currents in Fig 2d represent theenvelope of the extreme value. Only one polarity has beendrawn out in the diagram for the sake of simplicity.

The circuit breaker 4 is of such a design that it estab-lishes galvanic séparation by séparation of metallic con-tacts. Accordingly, the circuit breaker 4 comprises, as arule, required auxiliary equipment for arc extinguishing.

According to the invention the line 2 between the object 1and the switching device 4 is connected to an arrangementgenerally denoted 5. This arrangement may in general re-gard be designated as a switching arrangement. In the ap-plication shown, the switching arrangement has the func-tion of an arrangement reducing overcurrents towards theapparatus. The arrangement is actuatable for overcurrentréduction with the assistance of an overcurrent conditionsdetecting arrangement within a time period substantiallyless than the break time of the circuit breaker 4. Thisarrangement 5 is, accordingly, designed such that it doesnot hâve to establish any galvanic séparation. Therefore,conditions are. created to very rapidly establish a currentréduction without having to accomplish any total élimina- 15 011126 tion of the current flowing from the network 3 towards theprotected object 1. Fig 2b illustrâtes in contrast to thecase according to Fig 2a that the overcurrent reducing ar-rangement 5 according to the invention is activated uponoccurrence of a short circuit current at the time tfauitfor overcurrent réduction to the level ±2 at the time t2-The time interval tfauj_^-t2 represents, accordingly, thereaction time of the overcurrent reducing arrangement 5.Since the task of the arrangement 5 is not to break butonly reduce the fault current, the arrangement may becaused to react extremely rapidly, which will be discussedmore closely thereunder. As an example, it may be men-tioned that current réduction from the level ij_ to thelevel ±2 intended to bs scccmpiished within one or afew ms after unacceptable overcurrent conditions havingbeen detected. It is then aimed at to accomplish the cur-rent réduction in a shorter time than 1 ms, and preferablymore rapidly than 1 microsecond.

As appears from Fig 1, the device comprises a furtherbreaker generally denoted 6 and arranged in the line 2 be-tween the circuit breaker 4 and the object 1. This furtherbreaker is designed to break a lower voltage and currentthan the circuit breaker 4 and may, as a conséquencethereof, be designed to operate with shorter break timesthan the circuit breaker. The further breaker 6 is ar-ranged to break not until after the overcurrent from thenetwork 3 towards the object 1 has been reduced by meansof the overcurrent reducing arrangement 5 but substan-earlier than the circuit breaker 4. From thatit appears that the further breaker 6 should be coupled to the linSv2 in such a way that it is the currentreduced by means of the overcurrent reducing arrangement 5which will flow through the further breaker and which, ac-cordingly, is to be broken by means thereof. tiallystated, 011126 16

Fig 2b illustrâtes the action of the further breaker 6.This breaker is, more specifically, designed to break atthe time t3, which means that the duration of the currentÎ2 reduced by means of the overcurrent reducing arrange-ment 5 is substantially delimited, namely to the time pe-riod t2_t3- The conséquence is that the energy injectioninto the protected object 1 caused by a fault current fromthe network 3 is represented by the surfaces marked withoblique lines in Fig 2d. It appears that a drastic réduc-tion of the energy injection is achieved. In this connec-tion it is pointed out that since, according to a spécifiemodel, the energy increases with the square of the cur-rent, a réduction to one half of the current reduces theenergy injection to a fourth. It is illustrated in Fig 2chow the fault current will flow through the arrangement 5.

The dimensioning of the arrangement 5 and the furtherbreaker 6 is conceived to be carried out such that the ar-rangement 5 reduces the fault current and the voltage tobe broken by means of the further breaker 6 to substan-tially lower levels. A realistic break time as to the fur-ther breaker 6 is 1 ms. However, the dimensioning shouldbe made such that the breaker 6 is caused to break not un-til after the arrangement 5 having reduced the currentflowing through the breaker 6 to at least a substantieldegree.

It is illustrated in more detail in Fig 3 how the devicemay be realised. It is then pointed out that the inventionis applicable in direct current (also HVDC = High VoltageDirect Current) and alternating-current connections. Inthe latter case, the line denoted 2 may be considered toform one of the phases in a multiphase alternating-currentSystem. However, it should be kept in mind that the deviceaccording to the invention may be realised so that eitherail phases are >subjected to the protection function ac- cm 26 17 cording to the invention in case of a detected fault or that only that phase or those phases where a fault current occurs are subjected to current réduction.

It appears from Fig 3 that the overcurrent reducing ar-rangement generally denoted 5 comprises an overcurrent di-verter 7 for diverting overcurrents to earth 8 or other-wise another unit having a lower potential than the net-work 3. Thus, the overcurrent diverter may be consideredas forming a current divider which rapidly establishes ashort circuit to earth or otherwise a low potential 8 forthe purpose of diverting at least a substantial part ofthe current flowing in the line 2 so that said currentdoes not reach the object 1 to be protected. If there is aserious fault in the object 1, for instance a short cir-cuit, which is of the same magnitude as the short circuitthat the overcurrent diverter 7 is capable of establish-ing, it may be said that generally speaking a réductionto one half of the current flowing to the object 1 fromthe network 3 is achieved as a conséquence of the overcur-rent diverter 7 in case the fault is close to the latter.

In comparison with Fig 2b, it appears, accordingly, thatthe current level 12 illustrated therein and being indi-cated to amount to approximately half of iq may be said torepresent the worst occurring case. Under normal condi-tions, the purpose is that the overcurrent diverter 7should be able to establish a short circuit having a bet-ter conductivity than the one corresponding to the shortcircuit fault in the object 1 to be protected so that ac-cordingly a main part of the fault current is diverted toearth or otherwise,a lower potential via the overcurrentdiverter 7. It appears from this that, accordingly, in anormal fault case, the energy injection into the object 1in case of a fault becomes substantially smaller than thatwhich is indicated in Fig 2d as a conséquence of lowercurrent level ±2 as w^ll as shorter time span tg-tj· It 18 011126 should be obvious that a certain protection is obtainedalso when a short-circuit, which has been established, hasa somewhat lower conductivity than the one correspondingto the short-circuit fault in the object 1 to be pro-tected.

It has been pointed out that the notation 8 not only in-cludes earth but another unit with a lower potential thanthe network/equipment 3. it is thereby to be noted that theunit 8 possibly could be formed by another power network oranother equipment included in the electric power plant, saidequipment having a lower level of voltage than the one whichis effective for the network/equipment 3, to which the ob-ject 1, which is to be protected, is connected.

The over-current diverter 7 comprises switch means coupledbetween earth 8 or said lower potential and the line 2 be-tween the object 1 and the network 3. This switch means com-prises a control member 9 and a switch member 10. Thisswitch member is arranged to be open in a normal State, i.e.insulating in relation to earth. The switch member 10 mayhowever be brought into a conductive state via the controlmember 9 in a very short time in order to establish currentréduction by diversion to earth.

Fig 3 illustrâtes that an overcurrent conditions detectingarrangement may comprise at least one and preferably sev-eral sensors 11-13 suitable for detecting such overcurrentsituations reauiring activation of the protection func-tion. As also appears from Fig 3, these sensors may in-clude the sensor denoted 13 located in the object 1 or inits vicinity. Fur-thermore, the detector arrangement com-prises a sensor 11 adapted to sense overcurrent conditionsin the line 2 upstreams of the connection of the overcur-rent reducing arrangement 5 and the line 2. As is also ex-plained in the following, it is suitable that a further 19 011126 sensor 12 is provided to sense the current flowing in theline 2 towards the object 1 to be protected, i.e. the cur-rent which has been reduced by means of the overcurrentreducing arrangement 5. In addition, it is pointed outthat the sensor 12, as well as possibly the sensor 13, iscapable of sensing the current flowing in the line 2 in adirection away from the object 1, for instance in caseswhere energy magnetically stored in the object 1 givesrise to a current directed away from the object 1.

It is pointed out that the sensors 11-13 do not necessar-ily hâve to be constituted by only current and/or voltagesensing sensors. Within the scope of the invention, thesensors may be of such nature that they generally speakingmay sense any conditions indicative of the occurrence of afault of the nature requiring initiation of a protectionfunction.

In cases where such a fault occurs that the fault currentwill flow in a direction away from the object 1, the de-vice is designed such that the control unit 14 thereofwill control the further breaker 6 to closing, in case itwould hâve been open, and, in addition, the overcurrentreducing arrangement 5 is activated such that the shortcircuit current may be diverted by means of the same.When, for example, the object 1 is conceived to consist ofa transformer, the function on occurrence of a short cir-cuit therein could be such that the short circuit firstgives rise to a violent flow of current into the trans-former, which is detected and gives rise to activation ofthe arrangement 5 for the purpose of current diversion.When the current'-flowing towards the transformer 1 hasbeen reduced in a required degree, the breaker 5 is causedto break, but, controlled by means of the control unit 14,not earlier than leaving time for the energy, in occurringcases, magnetically stored in the transformer 1 to flow 20 011126 away from the transformer 1 and be diverted via the ar- rangement 5.

Furthermore, the device comprises a control unit generally 5 denoted 14. This is connected to the sensors 11-13, to theovercurrent reducing arrangement 5 and to the furtherbreaker 6. The operation is such that when the controlunit 14 via one or more of the sensors 11-13 receives sig-nais indicating occurrence of unacceptable fault currents 10 towards the object 1, the overcurrent reducing arrangement5 is immediately controlled to rapidly provide the re-quired current réduction. The control unit 14 may be ar-ranged such that when the sensor 12 has sensed that thecurrent or voltage has been reduced to a sufficient de- 15 gree, it Controls the breaker 6 to obtain operationthereof for breaking when the overcurrent is below a pre-determined level. Such a design ensures that the breaker 6is not caused to break until the current really has beenreduced to such a degree that the breaker 6 is not given 20 the task to break such a high current that it is not ade-quately dimensioned for that purpose. However, the embodi-ment may alternatively also be such that the breaker 6 iscontrolled to break a certain predetermined time after theovercurrent reducing arrangement having been controlled to 25 carry out current réduction.

The circuit breaker 4 may comprise a detector arrangementof its own for détection of overcurrent situations or oth-erwise the circuit breaker may be controlled via the con- 30 trol unit 14 based upon information from the same sensors11-13 also controlling the operation of the overcurrentreducing arrangement.

It is illustrated in Fig 3 that the further breaker 6 com- 35 prises a switch 15 having metallic contacts. This switch15 is opérable between breaking and closing positions by ΟΠ 1 26 21 means of an operating member 16, which in turn is con-trolled by the control unit 14. A shunt line 17 is con-nected in parallel over this switch 15, said shunt linecomprising one or more components 18 intended to avoidarcs on séparation of the contacts of the switch 15 bycausing the shunt line 17 to take over the current conduc-tion from the contacts. These components are designed sothat they may break or restrict the current. Thus, thepurpose is that the components 18 normally should keep theconduction path in the shunt line 17 interrupted but closethe shunt line when the switch 15 is to be opened so thataccordingly the current is shunted past the switch 15 andin that way arcs do not occur or possibly occurring arcsare efficiently extinguished. The components 18 compriseone or more associated control members 19 connected to thecontrol unit 14 for control purposes. According to one em-bodiment of the invention, said components 18 are control-lable semiconductor components, for instance GTO thyris-tors, having necessary over-voltage arresters 30. A disconnector 20 for galvanic séparation in the currentconduction path created by means of the shunt line 17 tothe object 1 to be protected is arranged in sériés withsaid one or more components 18. This disconnector 20 isvia an operating member 21 controlled by the control unit14. The disconnector 20 is illustrated in Fig 3 as beingplaced in the shunt line 17 itself. This is of course notnecessary. The disconnector 20 could also be placed in theline 2 as long as it ensures real galvanic séparation, bysériés coupling with said one or more components 18, inthe conduction path established by means of said sériéscoupling so that accordingly there is not any possibilityfor current to flow through the components 18.

The device as it has been described so far opérâtes in thefollowing manner: In absence of a fault, the circuit 22 011126 breaker 4 is closed just like the switch 15 of the furtherbreaker 6. The components 18 in the shunt line 17 are in anon-conducting State. The disconnector 20 is closed. Fi-nally, the switch means 10 of the overcurrent reducing ar-rangement 5 is open, i.e. it is in a non-conducting State.In this situation the switch means 10 must, of course,hâve an adéquate electrical strength so that it is not in-advertently brought into a conducting State. Overvoltageconditions occurring in the line 2 as a conséquence of at-mospheric (lightning stroke) circumstances or couplingmeasures may, accordingly, not involve the voltagestrength of the switch means 10 in its non-conductingState to be exceeded. For this purpose it is suitable tocouple at least one over-voltage arrester 22 in parallelwith the switch means 10. In the example such over-voltagearresters are illustrated on both sides of the switchmeans 10. Accordingly, the over-voltage arresters hâve thepurpose to divert such overvoltages which otherwise couldinvolve a risk for inadvertent -breakthrough in the switchmeans 10.

The over-voltage diverters 22 are illustrated in Fig 3 to beconnected to the line 2 itself on either sides of the con-nection of the switch means 10 to the line. It is in princi-ple désirable that at least one over-voltage diverter hasits connection as close as possible upstreams in relation tothe switch means 10. The over-current diverters 22 could in-stead, which is indicated in Fig. 3 with the dotted lines 26be connected to the branch line forming electric connectionbetween the switch means 10 and the line 2. Such a construc-tion enables intégration of the switch means 10 and at leastone over-voltage diverter 22 to one single electric appara-tus, which apparatus may be brought in electric conductingconnection with the line 2 via one single connection. 23 C11126

When an over-current State has been registered by means ofsonie of the sensors 11-13 or the own sensor (it is ofcourse realized that information from the own sensor ofthe circuit breaker 4 may be used as a basis for controlof the over-current reducing arrangement 5 according tothe invention) of the circuit breaker 4 and this over-cur-rent State is of such magnitude that a serious fault ofthe object 1 is expected to be at hand, a breaking opera-tion is initiated as far as the circuit breaker 4 is con-cerner In addition, the control unit 14 Controls theover-current reducing arrangement 5 to effect such réduc-tion, and this more specifically by bringing, via the con-trol member 9, the switch means 10 into an electricallyconducting State. As described before, this may occur veryrapidly, i.e. in a fraction of the time required forbreaking by means of the circuit breaker 4, for what rea-son the object 1 to be protected immediately is liberatedfrom the full short-circuit current from the network 3 asa conséquence of the switch means 10 diverting at least anessential part, and in practice the main part, of the cur-rent to earth or otherwise a lower potential. As soon asthe current, which flows towards the object 1 via the fur-ther breaker 6, has been reduced in a required degree,which can be established on a pure time basis by a time différence between activation of the switch means 10 andoperation of the breaker 6, or by sensing of the currentflowing in the line 2 by means of, for instance, the sen-sor 12, the operating member 16 of the switch 15 is, viatne control unit 14, controlled to open the contacts ofthe switch 15. For extinguishing or avoiding arcs, thecomponents 18, e..g. GTQ thyristors or gas switches, arevia the control members 19 controlled to establish conduc-tivity of the shunt line 17. When the switch 15 has beenopened and, thus, provided galvanic séparation, the compo-nent 18 is again controlled to bring the shunt line 17into a non-conducting State. In that way the current from 24 011126 the network 3 towards the object 1 has been efficientlyeut off. After having brought the shunt line 17 into anon-conducting state, galvanic séparation may, in addi-tion, be effected by means of the disconnector 20 by con-trolling the operating member 21 thereof from the controlunit 14. When ail these incidents hâve occurred, breakingby means of the circuit breaker 4 occurs as a last inci-dent. It is important to note that the over-current reduc-ing arrangement 5 as well as the further breaker 6 accord-ing to a first embodiment can be operated repeatedly.Thus, when it has been established by means of the sensors11-13 that the circuit breaker 4 has been brought to eutoff, the switch means 10 is reset to a non-conductingstate and the switch 15 and the disconnector 20 are againclosed so that when the circuit breaker 4 next timecloses, the protection device is completelv opérable. Ac-cording to another embodiment, it is, however, contem-plated that the over-current reducing arrangement 5 mayrequire exchange of one or more parts in order to operateagain.

It is pointed out that according to an alternative embodi-ment of the invention, the component or components 18could be brought into a conducting state as soon as theover-current reducing arrangement 5 has been brought intoa closing state and this independently of whether theswitch 15 possibly is not opened thereafter. The controlof the components 18 could then, as described before, oc-cur via the control unit 14 or, alternatively, by means ofa control function involving a slavish following of theclosing of the arrangement 5. '*4.

Fig 4 illustrâtes a first embodiment of the over-current re-ducing arrangement 5 with switch means denoted 10a. Theswitch means 10a has électrodes 23 and a gap 24 prevailingbetween these électrodes. The switch means as it has been 25 011126 described so far has means 25a in order to trigger the élec-trode gap 24 to form an electrically conducting path betweenthe électrodes. A control member 9a is arranged to controlthe operation of the members 25a via the control unit 14a.The means 25a are in the example arranged for causing or atleast initiating the electrode gap to assume electrical con-ductivity by means of causing the gap or part thereof toform a plasma. It is thereby essential that the means 25aare capable of realising a very rapid supply of triggeringenergy to the electrode gap. It is thereby preferred thatthe triggering energy is supplied in the form of radiativeenergy, which in turn is capable of effecting ionis-ing/initiating of plasma in the electrode gap.

The means 25a comprises according to a particularly pre-ferred embodiment of the invention at least one laser, whichby means of energy supply to the electrode gap causes ionis-ing/forming of plasma in at least a part of the electrodegap.

It is preferred in accordance with the invention to supply,with the aid of one or several lasers or other means 25a,energy to the electrode gap 24 in such a way that the com-plété electrode gap will be ionised and brought to the formof a plasma respectively, approximately momentarily in sucha way that also the complété gap 24 immediately is broughtto electrical conductivity. In order to spare with and opti-mise the use of the (normallv) restricted available laserenergy/-effect, the means 25a may, in application of the in-vention, be arranged so that thev can provide ioniza-tion/plasma formation in only one or more parts of the gap24. In the embodiment according to fig 4, it is illustratedthat the means 25a supply the radiant energy in one singlespot or area 28. As will be described later, the inventionalso comprises application of the radiant energy in a plu-rality of spots or areas in the electrode gap, including 26 011126 also on one of or both of the électrodes, or in one or more rodlike areas extending continuously or substantially con- tinuously between the électrodes. 5 By connecting the switch means 10a between the line 2 andearth 8 ( or another unit with lower potential ) as is dia-grammatically indicated in Fig 4, i.e. with one of the élec-trodes 23 connected to the line 2 and the other electrodeconnected to earth 8, there will be a voltage différence be- 10 tween the électrodes causing an electric field. The electricfield in the gap 24 is intended to be utilised in order toconvey or cause an electric breakdown between the électrodesas soon as the means 25a hâve been controlled to triggering,i.e. hâve given rise to ionisina/forming of plasma in one or 15 more parts of the electrode gap. The established ionis-ing/forming of plasma will be driven by the electric fieldto shunt the gap between the électrodes in order to in thisway give rise to a low-resistant electrical conductive chan-nel, i.e. an arc between the électrodes 23. It is pointed 20 out that the invention is not intended to be restricted touse in connection with occurence of such an electric field.Thus, the intention is that the means 25a should be capableof establishing electrical conduction between the électrodesalso without such a field. 25

Due to the demand on the switch means 10a to close very rap-idly for current diversion, it is thus désirable when only arestricted part, e. g. a spot like part of the gap is ion-isée that the switch means is dimensioned in such a way that 30 the strengt’n of the electric field in the gap 24 will besufficiently high "for safe closing. It is however on theother hand a desire that the switch means 10a should hâve avery high electric strength against breakdowns between élec-trodes in its isolating rest position. The strength of the 35 electric field in the gap 24 should therefore be proportion- 27 011126 ally low. This will on the other hand reduce the speed, withwhich the switch means may be caused to establish the cur-rent diverting arc between the électrodes. In order toachieve an advantageous relation between the desire for asafe trigging of the switch means and on the other hand highelectric strength against undesired trigging, it is accord-ing to the invention preferred that the switch means isformed in such a way that regarding its complété operationalenvironment the electric field in the gap 24 has a fieldstrength which is not more than 30% of the field strength atwhich a spontaneous breakdown normally takes place, when thegap forms electric isolation. This causes a proportionallylow probability of a spontaneous breakdown.

The strength of the electric field in the electrode gap 24in its isolating state is suitably not more than 20% andpreferably not more than 10% of the field strength at whicha spontaneous breakdown normally takes place. In order to onthe other hand achieve an electric field in the electrodegap 24, which promotes forming of an arc at initiation ofionising/forming of plasma in a part of the electrode gap ina relatively rapid way, it is preferred that the strength inthe electric field is at least 0,1% and suitably at least 1%(E4), and preferably at least 5% of the field strength, atwhich a spontaneous breakdown normally takes place.

The electrode gap 24 is, as may be seen in Fig. 4, enlcosedin a suitable casing 32. A vacuum as well as a suitable me-dium in the form of gas or even fluid may for this purposebe présent in the gap 24. In the case of a gas/fluid the me-dium in the gap is intended to be formed in such a way thatit might be ionisea. and brought to plasma by trigging. Itwould in such a case be suitable to initiate ionisa-tion/forming of plasma in the gap 24 at a point somewherebetween the électrodes 23. It is however in Fig 4 illus-trated the conceived case where there either is a vacuum or 28 011126 a suitable medium in the gap 24. It is then preferred thatinitiation of closing takes place by way of making the laser25a, which is illustrated in Fig 4, to focus the emitted ra-diative energy in at least one area 28 on or in the vicinityof one of the électrodes via a suitable optical System 27.This implies that the electrode will operate as an électronand ion emitter for establishing an ionised environment/aplasma in the electrode gap 24 in such a way that thus anarc will be formed between the électrodes. One of the élec-trodes 23 may according to Fig 4 hâve an opening 29, throughwhich the laser 25a is arranged to émit the radiative energyto the area 28 with support of the optical System 27.

Fig 5 illustrâtes a variant 10b of the switch means, whereinstead the System laser 25b/optics 27b focus the radiativeenergy in a triggering area 28b, which is situated betweenthe électrodes and in a medium between these électrodes.Plasma is accordingly, on triggering, intended to be devel-oped from this area to bridging of the électrodes.

The variant 10c of the switch means in Fig 6 differs fromthe one in Fig 4 in the way that auxiliary électrodes 31hâve been arranged between the électrodes 23c in this case,said auxiliary électrodes suitably being annuler in a waythat the beam emitted by the laser 25c may pass through theauxiliary électrodes 31. T'nesa électrodes are intended tooperate for smoothing the electric field between the élec-trodes 23c and may be isolated from each other, i.e. theymay be on a floatina potentiel, The auxiliary électrodes re-suit in improved safetv against a spontaneous breakthrough,reduced dimensions of the switch means and a reduced sensi-

·» I tivity to the effect- of externsl fields. The auxiliary élec-trodes may also be exposed to the laser beam/laser puise andbe made to émit free charges, which further promote thetriggering capability. 29 011126

Fig 7 illustrâtes a variant lOd of the switch means with the change that the électrodes 31d are added also here, in simi- larity to what has been described with the reference to Fig 6.

In order to achieve the above discussed relations regardingthe field strength conditions between the électrodes 23 inthe isolating State of the switch means, the characteristicsof the switch means must of course be adeguately adapted tothe intended use, i.e. the voltage conditions which willarise over the électrodes 23. The constructive steps avail-able regard of course forming of the électrodes, distancebetween the électrodes, the medium between the électrodesand the presence of possible further field affecting compo-nents between the électrodes.

Diffractive optical éléments may be used with the invention.Diffractive optical éléments are éléments, in which the wavefronts of the light, which wave fronts détermine the propa-gation of the light, are formed by means of diffractionrather than refraction. A particular type of diffractive op-tics modulâtes only the phase of the light and not the am-plitude, for what reason components of this type has a veryhigh transmittance. Pure phase modulation may be achieved byproviding the surface of the optical component with a reliefstructure, where the relief height should be of the same or-der as the wavelength in order to achieve an optimum func-tion of the component. An alternative way of achieving phasemodulation is to modulate the refractive index of the opti-cal element, which modulation is rather difficult. Diffrac-tive optical éléments may be manufactured by means of holo-graphie technique/’-which does not admit that arbitrary func-tions may be realized. A more flexible manufacturing mode iscomputer génération, in which mode the optical function maybe calculated in a computer. Entirely arbitrary opticalfunctions may 'then, in principle, be realized, said func- 30 011126 tions often being impossible to obtain by means of conven-tional refractive and reflective optics. The resulting facesurface is thereafter transferred to a relief, e.g. by meansof électron beam lithography or optical lithography, both ofwhich are well known within the semi-conductor art. Such da-tor generated, phase controlling surface relief componentsare often called kinoforms. A well-known example is theFresnell lens. This lens may, as ail diffractive optics, bedesigned as a binary structure consisting of only two relieflevels, or as a multilevel relief providing a substantiallyimproved diffraction efficiency (functional efficiency ofthe optical element).

Fig 8 illustrâtes an embodiment based upon an optical System27e comprising a lens System 35, via which arriving laserpuises are conveyed to a diffractive optical phase element36, a kinoform. This element is designed to hâve a pluralityof focal points or spots 28e generated starting from a sin-gle incoming laser puise. These focal spots 28e are distrib-uted along the axis of symmetry between the électrodes 23e.As a conséquence of the focal spot 28e being distributedalong a line between the électrodes 23e, a more safe estab-lishment of an electrical conduction path between the élec-trodes is achieved, meaning as high a probability for trig-gering as possible at a voltage/electrical field strength aslow as possible and with a time delay as short as possible.

The kinoform 36 is low absorbing and may, accordingly,resist extremely high optical energy densities. The kinoformis, accordingly, produced from a dielectrical material sotnat it will not disturb the electrical field between the •M · électrodes in any serious degree.

In the embodiment according to fig 8, the radiant energy issupplied through an opening 29e in one of the électrodes asbefore. Fig 9 illustrâtes a variant where, generally speak- 31 011126 ing, the only différence as compared to the embodiment ac-cording to fig 8 is that the diffractive optical element(kinoform 36f) is placed radially externally of one of theélectrodes 23f. The optical element 35f is as before de-signed to deflect the laser light and focus the same in anumber of spots or points distributed along the intendedelectrical conduction path between the électrodes. Thebunches of beams forming the spots 28f hâve each their owndeflection angle. Thus, the bunches of beams hâve to traveldifferent distances to the respective spots 28f. The advan-tage in locating the kinoform 36f according to fig 9 at theside of one of the électrodes is that the kinoform will belocated sidewardly of the strongest electrical field so that ^ield disturbance will be

Tim: design is also simplified since no opening for the laserlight is required.

Fig 10 illustrâtes an embodiment where a laser 25g suppliesthe laser radiation via an optical System 27g symmetricallyin a number of focal spots or points 28g distributed alongthe length of the electrode gap without requiring any open-ing in the électrodes 23g. The optical System 27g comprisesa prisma or beam divider 37 arranged to deflect the laserbeam around the adjacent electrode 23g. Around this elec-trode 23g there is provided one or preferably more kinoforms36g (diffractive optical éléments) designed to focus, possi-bly by means of further lenses, the laser beam in the de-sired focal spot 28g so that plasma formations are generatecin these spots.

Fig 11 illustrâtes a variant where a laser beam is conveyedby means of an optimal System 27h comprising optical fibres38 for formation of focal spots 28h located at variousplaces between the électrodes 23h; The optical fibres 38 maybe arranged to émit the light via lenses 39. 32 ου 126

Fig 12 illustrâtes the basic principle of a conical lens, aso-called axicone. The définition of such an axicone maysaid to be every optical element having symmetry with regardto rotation and being capable of deflection of light bymeans of a refraction, reflection, diffraction or combina-tions thereof from a point source on the axis of symmetry ofthe element in such a way that the light intersects thisaxis of symmetry not in a single point as would be the casewith a conventional spherical lens, but along a continuousline of points or spots along a substantial extent of thisaxis.

It is illustrated in fig 13 that collimated (non-divergent)light beams are deflected the same angle by the axicone. Asa conséquence of the symmetry as far as rotation is con-cerned, each beam will cross the axis of symmetry in sortiepoint.

It appears from fig 14 that the light may be focused in anelongated focal area 28i located between the électrodes 23iby means of an axicone 36i. This elongated focal area mayaccording to one embodiment of the invention extend continu-ously ail the way between the électrodes but could alsoadopt only a part of the gap therebetween. Fig 15 illus-trâtes how the intensity is related to the distance betweenthe électrodes. The full line curve illustrâtes the inten-sity distribution on illumination with the light beam whichoriginally had a Gaussic intensity distribution whereas thedashed line curve illustrâtes the intensity distribution onillumination with a constant intensity distribution. For therest, it is pointed out that the invention is not only re-stricted to such axieones which are purely linearly conical.Thus, axieones, the mantle surface of which deviate from thelinear cône, which will hâve a direct influence on the focalintensity distribution, are included within the scope of the invention. 33 011126

Fig 16 illustrâtes that a similar resuit as the one in fig14 as far as the focal area 28k is concerned may be achievedby means of a diffraction optical element 36k, a kinoform.

Fig 17 illustrâtes that an elongated focal area 28m in thegap between électrodes 23m may be achieved by means of anaxicone, more specifically a reflective axicone.

Fig 18 illustrâtes an embodiment where a particularlydesigned diffractive axicone 36n, a kinoform, has been de-signed to provide focal areas 28n and 28n' respectively hav-ing different shapes. In the example it is illustrated thatthe focal area 28n is elongated and arranged on the axis ofsymmetry of the axicone 36n and the électrodes. In contrast,the focal area 28n' has, as is indicated to the left in fig18, obtained a tubular shape in cross-section. This tubularshape is advantageous most closely to an electrode 23n pro-vided with an opening 29n since the periphery of the tubularfocal area 28n' will be located relatively close to theelectrode 29n provided with an opening. Both focal areas 28nand 28n' hâve, in fig 18, a substantially constant intensityalong the axis of symmetry but perpendicular to this axisthe intensity distribution, as far as the focal area 28n isconcerned, is substantially Gauss-shaped or shaped in accor-dance with the Bessel function.

An advantage with an entirely or substantially conical ordiffractive, co-axiallv focusing component es for instancein figs 8, 9, 10, 14, 16, 18 is that along the efficient di-rection of propagation of the radiant energy, said directionof propagation beirt'g a straight line, the plasma volumefirst formed, which occurs most closely to the electrode, atwhich the radiant energy is supplied, will not shield, re-flect or to a serious degree affect the radiant energy fo-cused in spots/àreas being located further away from the 34 011126 supply electrode. This "shadow effect” from the first formed plasma volumes could otherwise hâve hindered the radiant en- ergy to efficiently reach later foci. This is a conséquence of a plasma having the ability to be able to reflect or ab- sorb radiant energy.

Fig 19 illustrâtes an embodiment where a generator. 1b isconnected to a power network 3a via a transformer la. Theobjects which are to be protected are therefore representedby the transformer la and the generator lb. The over-currerttreducing arrangement 5a and the further breaker 6a as wellas the ordinary circuit breaker 4a are apparently arrangedin resemblance with what is évident from Fig 1 in the casethat the object 1 in Fig 1 is conceived to form the objectla according to Fig 19 - It is therefore in this regard re-ferred to the descriptions in connection to Fig 1. The sameis true for the protection operation of the over-current re-ducing arrangement 5c and the further breaker 6c in relationto the generator lb. The generator lb should therefore inthis case be équivalent to the object 1 in Fig 1 while thetransformer la should be équivalent to the equipment 3 inFig 1. The over-current reducing arrangement 5c and the fur-ther breaker 6c will therefore in combination with the con-ventional circuit breaker 4b be able to protect the genera-tor lb against a violent current flow in the direction fromthe transformer la.

Fig 19 also illustrâtes the further over-current reducingarrangement 5b with the associated further breaker 6b. Ap-parently, over-current-reducing arrangements 5a and 5b willtherefore be arranged on either sides of the transformer la.It is to be noted that the further breakers 6a and 6b, re-spectively are positioned in the connections between saidover-current reducing arrangements 5a and 5b and the trans-former la. The further over-current reducing arrangement 5bis intended to protect the transformer la from violent cur- 35 0111 ? 6 rent flows towards the -transformer from the generator lb.The circuit breaker 4b will apparently be capable of break-ing independently of in which direction between the objectsla and lb a safety function is desired.

Fig 20 illustrâtes diagrammatically how radiant energy maybe supplied to the gap between électrodes 23o by means ofone or more lasers 25o in one or more spots or areas 28otransversely relative to an axis X of symmetry between theélectrodes 23o, By using a plurality of different lasers25o, a very high power may be supplied to the gap betweenthe électrodes for plasma formation.

Fig 21b illustrâtes that a plurality of substantrally parai—lel, electrically conducting channels may be formed betweenthe électrodes 23p. The view in fig 21a could be formed by avertical view of fig 21b, in which case the electricallyconducting channels, viewed from the side, is in one singlerow. However, it is possible to arrange a plurality of elec-trically conducting plasma channels in not only rows butalso columns between the électrodes. The occurrence of aplurality of simultaneously electrically conducting channelsincreases the conducting capacity of the switch means.

Fig 22 illustrâtes a variant where the optical System 27qcomprises an axicone (refractive or diffractive) dividingthe radiation arriving from a laser or similar into partsand directing these radiation parts to diffractive éléments(kinoforms 36g). These kinoforms are distributed about oneof the électrodes, namely the one denoted 23g in fig 22. Thesame structure as in fig 22 is shown in perspective in fig23. It appears in fig 23 that in the example 4 kinoforms 36gare arranged around the electroae 23q to cause the radiantenergy to be focused by diffraction in a number of spots orareas 28q présent along the axis of symmetry of the élec-trodes. The use of several discrète kinoforms 36q would ap- 36 011126 pear to be more simple and non-expensive to realize than a continuously annular kinoform even if the latter wouldn't be impossible.

Semi-conductor components, such as thyristor, triac, GTO,IGBT and several others, are common in electric power Sys-tems of today, where they are used primarily as electronicvalves to control, i.e. convey or block, the flow of elec-tric current.

Even if semi-conductor components hâve a high efficiency,présent good performance and hâve become relatively non-expensive by development of modem manuf acturing methods,there are - primarily at very high electrical voltage levels- problems requiring complicated, bulky and cost requiringtechnical solutions.

By means of the technical embodiments/solutions presented inthe présent spécification, alternatives to semi-conductorcomponents are presented/explained, said alternatives pro-viding more simple designs with considerably fewer compo-nents and to lower costs. Besides, the technique presentedallows design of valve components which may endure consid-erably higher voltages than corresponding semi-conductorcomponents. Besides, it is of fundamental weight that compo-nents based upon the technique presented herein may endurealmost unrestricted electrical currents and current densi-ties.

Within the electric power art semi-conductor éléments areused in a large number of applications. Tnis part of theelectric power art usually is called power electronics.These applications are commonly denominated converters. Aconverter is an operative unit consisting of semi-conductorunits (electronical valves) and necessary peripheral equip-ment used to change one or more of the characteristic vari- 37 0111?6 ables and parameters of an electric power System. Thus, the converter may change voltage and current level, frequency and number of phases. Also electronic switches may be con- sidered as converters.

As a converter (current redirector) also an apparatus inter-connecting a DC-system with an AC-system is considered. Incase the power flow is in a direction from the AC- to theDC-side, the converter opérâtes as a rectifier. In case thepower flow instead would occur in a direction from the DC-side to the AC-side, the converter opérâtes as an alterna-tor. An AC-AC-converter is dominated a frequency converterand converts an AC-signal to another AC-signal with an arhi-trary relation between frequency, amplitude, phase and phaseposition as well as the number of phases of the voltage. ADC-DC-converter converts DC-voltage to another DC-voltage.

An electronical switch may be designed for AC or DC. It maybe used for connection or disconnection of an apparatus orfor controlling or checking active or reactive power.

An electronic valve is controllable if it can switch/changefrom a blocking State with a high voltage and a low current(off-state) to conducting State with a low voltage and ahigh current (on-state). The great efficiency of electronicconverters dépends upon this bistable function of thevalves. A valve may either be stable in itself, such as athyristor, or be controllable so as to operate bistably,such as a transistor.

The terminologv is unfortunately not entirely consistent. AnIEC-compilation carr be found in "International Electrotech-nical Dictionary" and in Publ. 60050-551 IEV, "Power Elec-tronics". There is a very large number of different semi-conductor components which entirely or partly may be re-placed by the technique which is the subject matter of the 38 présent patent application. Two examples of a présentationof the State of the art are "Modem Power Electronics" byBose et al, IEEE Industrial Electronics Society, ISBN: 0-87942-282-3, and "Power Electronics - in Theory and Prac-tice" o£ K. Thorborg, Chartwell-Bratt, ISBN: 0-86238-341-2.Among available semi-conductor components dealt with inthese literature references the following should be men-tioned: -thyristor, diode, triac, GTO (gâte turn-off thyristor), bi-polar transistor (BJT), PWM-transistor, MOSFET, IGBT (insu-lated gâte bipolar transistor), SIT (static induction tran-sistor), SITH (static induction thyristor), MCT (M0S-controlled thyristor),. etc. I. A thyristor is turned-off (transferred to a blockingstate ) when its current is brought to zéro by externalmeans. In self-commutating alternators, the valves areturned-off by turn-off circuits consisting of condensators,inductors and resistors. The thyristor is the dominatingsemi-conductor component for high voltages and power levels.

The thyristor is definea as a semi-conductor component hav-ing a bistable function. It consists of three pn-transitions. It can be switched from off-State to on-stateand vice versa in one or two directions. The most commonlyused thyristor type is the εο-called "reverse blocking tri-ode thyristor". The thyristor has three connections: anode,cathode and gâte. In absence of control puise on the gâte,the thyristor blocks the current flow in both directions.With an imposed voltage which is positive on the anode andnégative on the cathode the thyristor is in its off-Stateand blocks the voltage. If the voltage imposed has an oppo-site polarity, the thyristor is in its reverse directionblocking State and reverse-blocks the voltage. 39 011126

The leak currents in the reverse blocking and blocking

States increase with the size of the thyristor and the tem- pérature and may for very large thyristors be up to a few hundred of mA.

If the thyristor is triggered by imposing a current or volt-age puise on the gâte, with adéquate amplitude and duration,the thyristor switches from the off-state to the on-stateand a current may flow in the forward direction from anodeto cathode.

The voltage drop (the voltage over the thyristor) the so-called on-state voltage, is typically 1-2 Volt for normalvalues on the on-state current.

If the blocking (in forward direction anode to cathode)voltage exceeds the break over voltage specified for thethyristor, it changes spontaneously from the off-state tothe on-state. This self-triggering voltage may seriouslydamage the thyristor and should, accordingly, never be ex-ceeded.

In high voltage applications, where the System voltage sub-stantially exceeds the maximum voltage that an individualthyristor element may endure, several thyristors must becoupled in sériés or cascade. In order to achieve an adé-quate voltage division between the thyristors coupled in sé-riés, each of them must be provided with an individual RC-circuit and with a résistive voltage divider: The RC-circuitacts as a transient voltage divider and the résister dividesthe blocking and rearwardly blocking respectively voltagesto approximately the same voltage différence per thyristornot withstanding the fact that different thyristors hâvedifferent leak currents. Besides, the resistors make thevoltages over the capacitors in the RC-circuits to beequally large. For thyristors connected in sériés it is of 40 011126 the utmost importance that the triggering puises of ail thy-ristors are simultaneous and hâve identical amplitudes. Dé-viations from simultaneous triggering resuit in an over-voltage over the thyristor which is triggered (becomes con-ductive) as the last as a conséquence of the current whichwill flow through the RC-circuit thereof and through theother thyristors. It is often required that only thyristorswhich hâve been matched to each other are used, i.e. hâvebeen selected to présent performances adapted to the otherthyristors, in particular in high freguency applications, afact which complicates the structure and makes the same moreexpensive. A triac ii bi-directional thyristor, which means has two blocking directions or forward directions. A triacis équivalent to two thyristors connected antiparallel andhaving a common gâte. A triac is at the outset in its block-ing State. Transfer to the on-state may, however, be con-trolled by a négative as well as a positive puise on thegâte and this may be achieved for both polarities over thetriac. Technical performances for a triac corresponds inmost cases with those of a thyristor having a correspondingsise and performance. Restricting exceptions are caused bythe triac not having equally short rise and fall times as athyristor and not the same résistance to voltage transients(dU/dt) either. Therefore, they are mostly used in voltageregulators having a résistive load and for net frequencies,where rapid fluctuations in currents and voltages do not oc-cur. A property of a triac to be bi-directional and only re-auiring one single cooling elsment as well as one singletriggering puise unit implies that simple and relativelynon-expensive structures may be designed, in particular for low electrical power levels. Light triggered thyristorshâve, for natural reasons, a great interest for high volt-ages, as in HVDC-systems and in Systems for thyristorswitched phase compensating Systems. The primary reason is 41 011126 the high demande regarding electrical insulation. Besides,the risks are reduced that the thyristor fires/opens sponta-neously as a conséquence of noise coupled to its gâte. Lightpuises to the thyristor are transmitted via a light conduc-tor to the thyristor from a control unit at earth potential.Since the light conductor is formed by di-electrical mate-riel, a high voltage insulation may be obtained.

However, the light energy which may be transferred by meansof light conductors is restricted and there is a risk thatthe thyristor System obtains a long delay time for the con-trol signal and, accordingly, an associated low increaserate of the on-state current of the thyristor unless thethyristor is equipped with a much more complex controlstructure comprising an amplification function for the sig-nal finally applied on the gâte of the thyristor. However,such a structure means that the thyristor again becomes moresensitive to noise which may be coupled via the gâte andlead to inadvertent switchings. A laser triggered plasmaswitch may fulfil the same functions as a plurality of powersemi-conductors and in some cases with very great technicaland economical advantages as a resuit. In the présent patentapplication the function of a laser triggered plasma switchas a triac is specifically dealt with. A laser triggeredplasma switch, which is based on a momentary short- circuit-ing of a gasfilled electrode gap by means of an elongated, ionised and electrically conducting channel generated by thelaser light has, primarily, the following great advantages:

The distance between the électrodes contained in the elec-trode System may be made large such that bigger problems donot occur in dimensioning and design of the external elec-trical insulation System. The structure is in that way con-siderably simplified and may be manufactured with lowercosts. 42 C11126

Another advantage which is substantially greater is thatthere are in principle not any restrictions as to the maxi-mum current which may be conducted by the plasma switch whenit has been brought into its conducting State by the lasertriggering. The conduction of the electrical current occursthrough a laser light generated ionised channel which rap-idly is developed to an arc. An arc is not subjected to anyfundamental restrictions as far as the maximum current,which flows through the sanie, is concerned and there is, ac-cordingly, not any maximum limit for the current density asis the case with semi-conductor components. When the currentincreases, the arc maintains an energetically advantageousc-urrent density by expanding radialiy. This self-adjustingfunction does not hâve any correspondance in a semi-conductor component. A third fundamental advantage is formed by the fact that aplasma switch may be designed for very high voltage levels,and may then be constituted by only one single component. Incomparison with a stacked semi-conductor structure havingthe same function and for the same high voltage level, thisresults in a considerably reduced complexity not only in thestructure itself, which does not hâve to consist of a largenumber of accurately connected semi-conductor éléments, butalso in the drastically reduced demands on timing of thesecomponents relative to each other. A substantially reduced number of active components in anapplication, compared to the corresponding applicationachieved with semi-conductor components, results in an in-creased reliability. Besides, reduced electrical losses, re-duced device costs and less complicated control Systems areachieved. A further great advantage is that triggering may be carried 43 011126 out very rapidly, in the order of some hundreds of microsec- onds, a fact which increases the possibility of précision modulation.

Triac function

Laser triggering in an elongated focal area allows forswitching from the off-state to the on-state at an arbitrarypoint in time. By its construction, with an adéquat© combi-nation of electrode distance, gas pressure, gas compositionand the partial pressures of the part gases and the overallgeometry of the encapsulating vessel, the plasma switch ac-cording to the invention présents properties identical tothose of a triac. Without triggering, the plasma switch isprésent in its blocking, non-conducting off-state. Thisblocking property is bi-directional, i.e. the component iselectrically insulating for voltages of both polarities overthe plasma switch. On triggering, the plasma switch istransferred almost momentarily to its conducting on-state,in which it remains as long as the current in the arc ismaintained above a certain design spécifie value, and aslong as the voltage of the électrodes of the plasma switchare maintained above a certain, as well, design spécifievalue. Also this conducting on-state is bi-directional: Aplasma switch element may be made conducting for both po-larities by the laser triggering. A switching arrangementwith radiant energy triggering is diagrammatically illus-trated in fig 24. A laser, for instance, may be used for the triggering. A switching arrangement 5 is triggerable on bothpolarities, which gives a bi-directional function.

Triac function witîT'turn-off circuit

In an application the function of the switching arrangementmust be completed with a possibility to shut off the switch,i.e. transfer it to the off-state. This is achieved by the 44 011126 switch either (1) becoming automatically self extinguishingthrough its construction with an adéquate combination ofelectrode distance, gas pressure, gas composition and par-tial pressures of the part gases or (2) the switch beinggiven properties enabling external turn-off. In case (1) thetime period after which self turn-off occurs may be checkedand determined by the design of the structure. This selfturning-off function may be realized for DC as well as ACSystems. The AC case is a more simple case in that selfturn-off automatically is assisted by the current throughthe conducting arc changing polarity, and, accordingly,passing through the current zéro. In accordance with thatstated hereinabove, conditions are created to efficientlyturn-off the plasma switch in a manner which is entirelyéquivalent to those used for turning-off of a thyristor.Thus, the AC case places less stringent demands on thestructure of the plasma switch. In case (2) the plasmaswitch may therefore be provided with external impédanceéléments, which assures that the on-state current is reducedto zéro whereby the plasma switch is opened and assumes itsoff-state. The saine effect may be achieved by using the op-eration circumstances in an AC System: Just before the cur- rent changes polarity in its passage through zéro, theplasma switch is turned-off by itself as a conséquence ofinsufficient ionisation of gas along the discharge channel.During the time it takes for the current to change polarityand for the new polarity to reach a voltage, for which theentirely ionised plasma channel aaain coula be electricallyconducting, a sufficiently large proportion of the plasmaconstituants has time to recombine to sucn a high degreethat the conductivity of the channel becomes too low to sup-port repeated turning-on of the arc. Thus, refiring is pre-vented and the plasma switch has assumed its off-state. 45 011126

In contrast, the DC case involves more stringent demands onthe structure of the plasma switch. By accurately balancedchoice of design parameters, such as electrode distance and,primarily, total gas pressure, included gas components andthe partial pressures thereof, these demands may, however,be fulfilled such that self turning-off may be achieved. Anatural alternative is to allow the current to commutate toanother line or component, the current in the plasma switchdecreasing to zéro and the plasma switch being turned-off.However, the most simple technical solution is to complétéalso the DC component with an external current restrictingcircuit element. A more coarse, but nevertheless efficient,solution is to couple in a suitable inannex- in connectionwith the plasma switch mechanical breakers, by means ofwhich the plasma switch may be entirely separated from thenetwork under voltage.

Unidirectional Triac function A triac may conduct in two directions in its on-state. Theplasma switch as it has been described here is in its natureprimarily bi-directional since it does not hâve any type ofdiode function. If, however, the laser triggering is onlyeffected during one of the two polarities of an AC System,the function becomes practically unidirectional, providedthat the rests of plasma possibly remaining since the pre-ceding triggering after recombination has a sufficiently lowconductivity to resist a spontaneous triggering under thepolarity (the half-period of the AC voltage) which is notlaser triggered.

Fig 25 illustrâtes a plasma switch according to the inven-tion with unidirectional triac function. 46 011126

Bi-directional triac function with two plasma switch élé- ments

An alternative to the embodiment described above is formed 5 by two plasma switch éléments having a turn-off function(self turning-off or external turning-off function) saidplasma switch éléments being connected anti-parallel betweenthe higher and the lower potential. The two plasma switches,which now hâve been constructed and connected to the AC sys- 10 tem to form two unidirectional units according to thatstated hereinabove, are connected to the network and in re-lation to each other in such a manner that the current con-ducting directions of the two switches are opposite- Sincethe plasma switch in itself is bi-directional, this means 15 that the two éléments are designed to be laser triggeredduring their respective polarities, i.e. during a half pe-riod of its own, of the AC voltage. The two plasma switchesmay be triggered by one and the sanie laser, which requiresthe optical System to be provided with a light flux- 20 controlling shutter. The laser is triggered at least twotimes per period and one time per half-period and the shut-ter may in one embodiment be designed so that the entireemitted light amount of the laser is alternatingly directeoto one or the other of the two plasma switch éléments. Such 25 a light flux directing shutter may for instance be consti-tuted by a rotatable highly reflecting mirror, which by re-flection from its two end positions directs the laser lightto each of the two light channels leading to the respectiveplasma switches. Another embodiment is to divide the laser 30 light into two channels with an equal amount of laser effectin both channels. Each channel leads to one of the rwoplasma switches. A-..light flux-controlling shutter has beeninstalled in each channel, said shutter ensuring, by con-trolled action at each triggering operation, that only one 35 of the two plasma switch éléments is subjected to the trig-gering laser light. The two plasma switch éléments may also 47 011126 very well be triggered by a laser for each plasma switchelement, the operation of the lasers being controlled,checked and synchronized by an external electronic unit.Figs 26,27 and 28 illustrate the possibilities just de-scribed for forming bi-directional triac functions.

The same bidirectionality is of course also achievéd withcorresponding coupling of two plasma switch éléments, whichdo not présent self-turning-off function or which do nothâve been provided with external means for turning-off.

Thyristor functions

As already has been described in the prior art, the thyris-tor has a blocking State called off-state for voltages andcurrents in both directions. When the thyristor is triggeredon its gâte, it assumes its conducting State, denominatedon-state, in which current may flow in the forward directionof the thyristor, but not in the opposite direction. A pre-ferred manner of achieving the same function by means of alaser triggered plasma switch means that the laser triggeredplasma switch is coupled in sériés with one or more diodefunctions, which may be of semi-conductor type. The numberof diodes is entirely determined by the maximum voltage,which in an application in view may hâve to be placed overthem. There are two different possibilities to orientate thediodes relative to the plasma switch: With the forward di-rection towards the plasma switch and with the forward di-rection directed away from the plasma switch. Accordingly,two different thyristor functions may be realized, where theresulting thyristor-'-unit consisting of plasma switch in sé-riés with a number of diodes obtains different directional-ity or polarity. It is preferred that the diodes containedin such a constellation are equipped with individual RC net-works or other more general impédance networks and a résis- tive voltage divider in order to efficiently achieve an 48 011126 equalizing voltage division. The diodes are, accordingly,not subjected to different voltage levels, which could ex-ceed their specified voltage résistance. Figs 29a-d illus-trate that a plasma switch according to the invention mayachieve different directionality or polarity by means of oneor more diodes.

As an example on how the plasma switch described hereinabovemay be used with a triac function or rather with a thyristorfunction reference is made to figs 30 and 31. The circuitspresented in the figures act as change over switches. Thefunction in fig 30 is as follows: The upper conduotor Lx isconnected to an alternating voltage whereas the lower con-ductor L2 is connected to earth or a lower potential. Aslong as the voltage in the upper conductor Lx is positivewithout any of the plasma switches PS2 and PS2 having beentriggered, no current may flow through the circuit. If, how-ever, the plasma switch PS2 is triggered to close, a currentwill flow through diode Dx from the upper conductor Lj toearth through PS2. This current flows as long as the voltagehas a positive polarity. After polarity change to négativepolarity on the upper conductor Lx, a current would flow inthe opposite direction. However, such a current may onlyflow after PSX having been triggered and it flows then fromthe upper conductor through the diode D2 and PSX and thelower conductor through the diode D2 and PS2 to the upperconductor. Since the voltage drop over the diode Dz may bemade lower than the voltage drop over the arc in PS2 in di-rection from L2 towards Llz the current will preferablv flowthrough D2 after the polarity change described instead ofthrough PS2. PS2 may, then not be refired spontaneously in wrong direction, and·· is accordingly left to rest during theprésent half-period with a négative polarity.

In the circuit just described the plasma switch is used inthe function of a unipolar triac, i.e. generally a thyris- 49 011126 tor. The circuit in fig 31 has been provided with two fur-ther diodes in sériés with the respective plasma switch,which entirely guarantees that the current after polaritychange is not erroneously seeking to pass the plasma switchto be turned-off. Thus, the further diode prevents, as anexample of an external means, this switch from refiring("back firing").

It is considered to be obvious from the présentation givenhereinabove that more technical functions than those herepresented may be realized while using a laser triggeredplasma switch as a starting point.

Fig 32 illustrâtes diagrammatically that a switching ar-rangement 5r is coupled in sériés in the line 2r previouslydiscussed between the network 3r and the object lr. Theswitching arrangement 5r comprises, suitably, a switch meanslOr with the previously described character, i.e. a switchmeans having an electrode gap adapted to be brought intoelectrically conducting closing by means of radiant energy.However, this is not shown more closely in fig 32. As ap-pears from fig 32, the switching arrangement 5r is intendedto hâve a purely switching function, i.e. the feeding of theobject lr or possibly feeding in the opposite direction mayoccur via the switch means lOr when this is in a conductingstate. When there is a need, the switch means lOr may bemade to inhibit current passage relatively rapidly, e.g. forprotection of the object lr or possibly even the network 3rfrom current flow from the object ld. In order to achieveswitching-off by means of the switch means lOr in alternat-ing current connections, it is sufficient that the means forenergy supply to t*he electrode gap are caused to cease with such energy supply. On the following passage through zéro,extinguishing of the arc in the switch means lOr is intendedto take place so that the current feeding ceases. In directcurrent application, it is probablv necessary to support the 50 011126 turning-off function by taking measures to reduce or elimi-nate the voltage différence over the switch means lOr. Suchmeans may consist in a switch 31 coupled parallel to theswitch means lOr. Closing of the switch 31 means that thecurrent is shunted passed the switch means lOr, a fact whichcauses the arc in the switch means lOr to be extinguished.In case such a measure wouldn't be sufficient, furtherswitches could as a complément thereto be arranged on eithersides of the switch means lOd in line 2r to totally discon-nect the switch means lOr from the line 2r.

The purpose with fig 32 is to illustrate that the switch ar-rangement 5r according to the invention may find generalswitching applications, in which it may be the question ofprotecting various apparatus but also of switching power invarious load situations in a more general sense.

It should be noted that the description presented here-inabove only should be considered as exemplifying for theinventive idea, on which the invention is built. Thus, it isobvious for the men skilled in the art that detailed modifi-cations may be made without leaving the scope of the inven-tion. As an example, it may be mentioned that according tothe invention, it is not necessary to use a laser for supplyof ionising/plasma forming energy to the gap 24. Also otherradiative sources, for example électron guns, or other en-ergy supply solutions may be applied as long as the rapid-ness and reliability demands according to the invention arefulfilled. It should be observed that the switch means 10may according to the invention be applied for protection ofelectric objects against fault-related over-currents also inother operative cases than the ones illustrated in Figs 1, 3and 19, where the device according to the invention is ar-ranged in order to reduce the négative effects of the rela-tively slow breaking time of the circuit breaker 4. Thus,the switch means' according to the invention does not neces- 51 011126 sarily need to be operation-related to such a circuitbreaker 4. It should finally be observed that the inventionis well suited for alternating current as well as directcarrent.

Claims (48)

1. 011126 52 Claims

   1. A device for switching electric power comprising at leastone electric switching arrangement ( 5 ), characterized inthat the switching arrangement (5 ) comprises at least oneswitch means (10), which comprises an electrode gap (24),which is convertible between an electrically substantiallyisolating State and an electrically conducting State, andmeans (25) for causing or at least initiating the electrodegap or at least a part thereof to assume electrical conduc-tivity and that said means (25) for causing or at least ini-tiating the electrode gap to assume conductivity are adaptedto sunply energv to the electrode gap in the form cf radiantenergy to bring the gap or at least a part thereof to theshape of a plasma.
2. 2. A device according to any preceding claim, characterizedby said means (25) for causing or at least initiating theelectrode gap or a part thereof to assume electrical conduc-tivity comprising at least one laser (25).
3. 3. A device according to any preceding claim, characterizedin that the switch means (10) is formed in such a way thatan electric field is présent in its isolating condition be-tween its électrodes (23), which field promotes or generatesan electric flash-over between the électrodes on causing orinitiating the electrode gap to assume electrical conductiv-ity.
4. 4. A device according to claim 3, characterized in that theelectric field in the isolating condition of the electrodegap (24) has substantially less field strength than thefield strength, at which a spontaneous breakthrough takesplace. 53 011126
5. 5. A device according to claim 3 or 4, characterized in thatthe electric field in the insulating condition of the élec-trode gap (24) has a field strength which is not more than30%, suitably not more than 20% and preferably not morethan 10% of the field strength, at which a spontaneousbreakthrough takes place.

   5. A device according to any of the daims 3-5, character-ized in that the electric field in the insulating conditionof the electrode gap ( 24 ) has a field strength which is atleast 0,1%, suitably at least 1%, and preferably at least5%, of the field strength, at which a spontaneous break-through takes place.
6. 7. A device according to any preceding claim, characterizedin that the means (25) for causing or at least initiatingthe electrode gap (24) to assume electrical conductivity arearranged to supply the radiant energy in such a manner thatthe lowest electrical field strength, at which the electrodegap may be triggered to assume electrical conductivity, isminimized.
7. 8. A device according to any preceding claim, characterizedin that the means (25) for causing or at least initiatingthe electrode gap ( 24 ) to assume electrical conductivity arearranged to supply the radiant energy to the electrode gapin a such a manner that a time delay between the arrivingradiant energy and a developed conductive ability of theelectrode gap is minimized.
8. 9. A device according to any preceding claim, characterizedin that the switch means (10) and the means (25) for causingor at least initiating the electrode gap to assume electricconductivity are arranged such that the establishment of theelectric conductivity in the electrode gap is substantially 54 011126 independent of the electric field strength présent between the électrodes of the switch means in its insulating State.
9. 10. A device according to any preceding claim, characterized 5 in that the means (25) for supplying triggering energy tothe electrode gap are arranged to apply the radiative energyon or at least in the vicinity of at least one of the élec-trodes ( 23 ). 10 11. A device according to any preceding claim, characterized in that the means (25) for supplying triggering energy tothe electrode gap are arranged to locate the radiative en-ergy in one spot or area in the gap ( 24 ) between the élec-trodes ( 23 ). 15
10. 12. A device according to any preceding claim, characterized in that the members (25, 27) for supplying the triggering energy to the electrode gap are arranged to apply the radi-ant energy in two or more spots or areas (28) between the 20 électrodes.
11. 13. A device according to claim 12, characterized in thatthe means for supplying triggering energy to the electrodegap are arranged to locate said two or more spots or areas 25 of radiant energy along a line extending between the élec-trodes, said line corresponding to the extent of an electricconduction path desired between the électrodes.
12. 14. A device according to any preceding claim, characterized 30 in that the means (25) for supplying triggering energy to the electrode gap „are arranged to apply the radiant energy in one or more elongated areas (28i,k, m, n), the longitudi- nal axes of which extend substantially along the direction of the electric conduction path which is intended between 35 the électrodes.. 55 011126
13. 15. A device according to claim 14, characterized in thatthe means ( 27 ) for supplying triggering energy to the élec-trode gap are adapted to shape the elongated focal area intoa tubular shape.
14. 16. A device according to claim 14 or 15, characterized inthat the means for supplying triggering energy to the élec-trode gap are adapted to shape the elongated area so that itbridges, entirely or substantially entirely, the space be-tween the électrodes.
15. 17. A device according to any of daims 14 or 15, character-f zed in that the means ( 27 ) for supplying triggering energyto the electrode gap are adapted to form two or more elon-gated focal areas (28) in the electrode gap, said focal ar-eas being located longitudinally after each other along theelectric conduction path intended between the électrodes.
16. 18. A device according to any of daims 1 and 10, character-ized in that the means for supplying triggering energy tothe electrode gap are adapted to apply .the radiant energy onat least one of the électrodes as well as between them.
17. 19. A device according to any of the daims 10-18, charac-terized in that at least one of the électrodes at the elec-trode gap has an opening (29), through which the means (25)for supplying triggering energy are arranged to direct theradiative energy.
18. 20. A device according to daims 15 and 19, characterized inthat the means ( 27·) for supplying triggering energy to theelectrode gap are adapted to locate the tubular radiant en-ergy area (28) in the vicinity of that electrode which hasan opening (29) and such that the axis of the tubular radi-ant energy area is substantially concentric to the axis ofthe opening in the electrode. 56 011126
19. 21. A device according to any preœding daim, characterizedin that auxiliary électrodes (31, 31d) for equalizing theelectric field and/or for active participation in the trig-gering process by said auxiliar électrodes being exposed tothe radiant energy and, as a resuit, being capable of emit-ting free charges, are arranged at the gap (24) between theélectrodes.
20. 22. A device according to any of daims 10-21, characterizedin that the means for supplying triggering energy to theelectrode gap comprise a System for controlling electromag-netic wave energy.
21. 23. A device according to claim 22, characterized in thatthe control System comprises at least one refractive, re-flective and/or diffractive element.
22. 24. A device according to claim 23, characterized in thatthe element is formed by an axicone.
23. 25. A device according to claim 24, characterized in thatthe element is formed by a kinoform.
24. 26. A device according to claim 23, characterized in thatthe éléments comprise optical fibres (38).
25. 27. A device according to any of daims 23-26, characterizedin that the control System (27f, h) is located radially out-wardly of the électrodes and adapted to direct bunches ofrays towards the gap between the électrodes.
26. 28. A device according to any of daims 23-27, characterizedin that the control System (27g) is adapted to divide laserpuises into an annuler configuration about one of the élec-trodes . 57 011126
27. 29. A device according to any preceding claim, characterized in that at least one over-voltage diverter (22) is connected in parallel to the switch means (10). 5 30. A device according to any preceding claim, wherein the electric object (1) is connected to an electric power net-work ( 3 ) or another equipment included in the electric powerplant, the device comprising a . switching device (4) in aline (2) between the object and the network/equipment, char- 10 acterized in that the switch means (10) is connected to theline (2) between the object (1) and the switching device(4), and that the switch means (10) is actuatable for over-current diversion within a time period substantially shorterthan the break-time of the switching device (4). 15
28. 31. A device according to claim 30, characterized in thatthe switching device (4) is formed by a circuit breaker.
29. 32. A device according to claim 21 or 31, characterized in 20 that it comprises a further breaker ( 6 ) arranged in the line between the switching device (4) and the object, said fur-ther breaker being arranged between the switching means (10)and the obj ect (1 ) and being adapted to break lower voltagesand currents than the switching device (4) and therefore ca- 25 pable of performing a shorter break-time than the switchingdevice and that the further breaker is adapted to break whenthe over-current towards or away from the object (1) hasbeen reduced by means of the switch means (10) but sub-stantially earlier than the switching device. 30
30. 33. A device according to claim 32, characterized in that itcomprises a control unit (14) connected to the detecting ar-rangement (11-13) and to the further breaker (6) in order toachieve actuation of the further breaker for breaking pur- 35 poses when the over-current towards or away from the object 58 011126 (1) is indicated, by means of the detecting arrangement, tobe below a predetermined level.
31. 34. A device according to any of the daims 32-33, charac-terized in that the further breaker (6) comprises a switch (15) over which there is coupled a shunt line (17) havingone or more components ( 18 ) for avoiding arcs on séparationof contacts of the switch (15) by causing the shuntline (17)to take over current conduction from the contacts.
32. 35. A device according to claim 34, characterized in thatsaid one or more components (18) in the shunt line (17) areclosable into conduction by means of control via the controlunit (14).
33. 36. A device according to claim 34 or 35, characterized inthat said one or more components (18) are formed by control-lable semiconductor components.
34. 37. A device according to any of the claims 34-36, charac-terized in that said one or more components (18) are pro-vided with at least one over-voltage arrester (30).
35. 38. A device according to any of the claims 34-37, charac-terized in that a disconnector (20) for galvanic séparationis arranged in sériés with said one or more components (18).
36. 39. A device according to claim 38, characterized in thatthe disconnector (20) is coupled to the control unit (14) tobe controlled thereof for opening after the switch (15) hav-ing been controlled .ίο hâve closed and said one or more com-ponents ( 18) having "been placed in a condition for breakingthe shunt line (17). 59 011126
37. 40. A device according to any preceding claim, characterizedin that the protected object (1) is formed by an electricapparatus with a magnetic circuit.
38. 41. A device according to claim 40, characterized in thatthe object is formed by a generator, transformer or motor.
39. 42. A device according to any of the daims 1-41, character-ized in that the object is formed by a power line, e.g. acable.
40. 43. A device according to any preceding claim, characterizedin that two switch means (10) are arranged on either sidesof the object to protect the same from two sides.
41. 44. A device according to claim 1, characterized in that itcomprises a control unit (14) connected to the switch means(10) and to the over-current conditions detecting arrange-ments (11-13), said control unit (14) being arranged to con-trol the switch means to closing based upon information fromthe over-current conditions detecting arrangement when re-quired for reasons of protection.
42. 45. A device according to claim 44 and one or more of theclaims 34, 36 and 40, characterized in that one and the samecontrol unit (14) is arranged to control, based upon infor-mation from the over-current conditions detecting arrange- ment (11-13), the switch means(6).
43. 46. Use of a deviûe accordingprotection of an object against (10) and the further breaker to any preceding claim forfault-related over-currents.
44. 47. A device according to any preceding claim, characterizedin that the means for supplying triggering energy to theelectrode gap are adapted to focus the radiant energy in a 60 011126 plurality of substantially parallel, elongated focal areas,the longitudinal axes of which are located substantiallyalong the direction of the electrical conduction path aimedat between the électrodes (fig 21).
45. 48. A device according to any preceding claim, characterizedin that one or more switch means (10), possibly in additionto complementary diodes or other components, are arrangedfor forming switching or converter functionalities.
46. 49. À device according to claim 48, characterized in thatthe functionalities are triac and thyristor functionalities.
47. 50. A method in an electric power plant for protection of anelectric object (1) from fault-related over-currents, char-acterized in that over-current diversion is accomplished bymeans of a switch means (10) when over-current conditionshâve been detected by means of an arrangement (11-13) forsuch détection, said switch means (10), which is arrangedfor diversion of over-currents to earth ( 8 ) or some otherunit with a relatively low potential, being closed for over-current diversion by imparting an electrode gap (24), whichis présent in the switch means, electrical conductivity bysupply of radiant energy to the electrode gap with the aidof triggering means (25 ) .
48. 51. A method according to claim 50, characterized in that afurther breaker (6), which is arranged in a line (2) betweena switching device (4) and the object (1) and between theswitch means (10) and the object (1), is actuated for break-ing after the over-ourrent towards or away from the object(1) having been reduced by means of the switch means (10).