Normally, such arrangements for diverting over-voltages are con- structed such that they comprise varistors. When the voltage over the varistor increases above a certain level, the resistance will de- crease such that the varistor becomes highly electrically conducting, resulting in the possibility to keep the voltage low through a substan- tial current diversion. Over-voltages might, for instance, be caused by atmospheric phenomena, such as strokes of lightning, connec- tions in the net, or other distorsions that occur.

Varistors have certain disadvantages in themselves. Such a disad- vantage is that the voltage level at which the varistor starts to oper- ate is as high as between 150 and 200 per cent of the voltage value "normally"accepted due to their inherent characteristics. Often, this implies a troublesome load on the included elements.

It is pointed out that, even though the present invention also could be applied in connection with low voltages, intermediate and high- voltage applications are preferred. According to the IEC Standards, intermediate voltage is referred to as 1-72.5 kV, while high voltage is referred to as >72.5 kV. Thus, the transmission, sub-transmission, and distribution levels are included.

OBJECT OF THE INVENTION The object of the present invention is to provide an over-voltage di- verting arrangement capable of very rapidly being made operable for the diversion of over-voltages, whereby the arrangement should have the capability of diverting large currents in order to fulfil the over- voltage reducing operation in high-voltage connections.

Another object of the invention is to provide favourable conditions for a significant reduction of the"over-voltage/maximum normal voltage" accepted, such that advantageous conditions for dimensioning are created as to a specific electric power plant and its contributive parts.

SUMMARY OF THE INVENTION According to the invention, the over-voltage diverting arrangement is designed as is more precisely defined in the characterising part of the following claim 1. Thereby, the electrode gap brought to a con- ductive state by means of the switch means will be able to rapidly divert current such that the over-voltage is delimited to an acceptable level. Because the conductivity of the electrode gap is present as a consequence of a plasma in the electrode gap, a very high conduc- tivity is obtained. The plasma is, more precisely, intended to have the character of an electrically conducting channel between the electrodes when a closure has taken place.

The inventive solution based upon a switch means triggered by ra- diation accordingly implies a very advantageous fulfilling of demands which may be set up in order to achieve a satisfactory protection function. Thus, a very rapid triggering may be achieved by the switch means so that currents will be diverted over the switch means with a very small time delay as soon as the electrode gap has assumed the electrically conducting state. It is pointed out that the term "triggering"in this connection means bringing the switch means into

an electrically conducting state. Because of the constitution of the switch means, said switch means may easily be dimensioned to be able to conduct very large currents. In order to obtain a satisfactory protection function it is, namely, desirable that the current conduct- ing channel, which is established through the switch means, has a very low resistance. This means the largest possible strain-relieving of the object, which is to be protected from fault-currents. Besides, a switch means according to claim 1 may with a small effort be caused to function with a particularly high triggering safety. The triggering must not, in order to divert occurring fault-currents as soon as possi- ble, therefore fail in a critical situation. The switch means according to the invention gives on the other hand rise to the possibility to di- mensioning in order to achieve a very high electric strength in a non- triggered condition. The probability for a spontaneous breakdown is thus to be at a minimum. It is especially preferred to thereby use at least one laser for triggering.

Preferable developments with respect to a. o. the means for supplying radiation energy to the electrode gap are defined in the enclose claims. According to one embodiment, the radiation energy is sup- plied to the electrode gap in two or more spots or areas for the pur- pose of achieving the highest possible certainty with regard to bringing the electrode gap to assume an electrically conducting state. According to one alternative the energy supply means may be designed to supply the radiation energy along an elongated area in the conduction path which is aimed at between the electrodes. Ac- cording to an optimal embodiment this elongated area may, entirely or substantially entirely, bridge the gap between the electrodes. Al- though it is possible, in a case with two or more spots or areas for radiation supply, that these spots or areas are applied successively corresponding to the propagation with respect to the electrical con- duction path between the electrodes in such a way that the spots or areas are successively applied with a time delay, it is, according to the invention, normally preferred to apply these spots or areas sub- stantially simultaneously for making the electrode gap conducting momentarily.

Furthermore, the means for supply of triggering energy may accord- ing to the invention be adapted to apply the radiation energy in a volume having a tubular shape. This is particularly preferable when one of the electrodes comprises an opening, through which the ra- diation energy is supplied, and when the radiation energy supplied in a tubular volume is applied relatively close to the electrode provided with an opening.

According to an alternative embodiment, the energy supply means may be designed to supply the radiation energy in a plurality of sub- stantially parallel, elongated areas extending between the elec- trodes.

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

Further advantages and features of the invention, also with respect to the inventive method, appear from the following description and claims.

BRIEF DESCRIPTION OF THE DRAWINGS With reference to the enclose drawings, a more specific description of an embodiment example of the invention follows hereinafter.

In the drawings: figs. 1-3 are schematic views illustrating how arrangements for diverting over-voltages may be arranged, figs. 4 and 5 are diagrams illustrating the operation of the over- voltage diverting device according to the invention,

fig. 6 and 7 are diagrams illustrating how an over-voltage pulse behaves without (fig. 6) and with (fig. 7) application of the inventive device. fig. 8 is a schematic, detailed view illustrating a possible embodiment of the arrangement for diverting over- voltages, figs. 9-14 are schematic views of different variants, fig. 15 is a principe view illustrating a switch means accord- ing to the invention and applied in connection with a varistor arrangement, fig. 16 is a diagram which, for comparison, illustrates the op- eration of a varistor arrangement with and without a switch means according to the invention, fig. 17 is a block scheme of a further embodiment, and fig. 18 is a partly cut view of an embodiment which is princi- pally similar to the one of fig. 15, but in another form.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Fig. 1 illustrates a device for over-voltage protection comprising an arrangement for diverting over-voltages which comprises a switch means 1, the design of which will be discussed hereinafter. The switch means 1 is connected to a line 2 which may by of any arbi- trary type as long as it is intended to conduct electricity. The switch means 1 is connected between this line 2 and earth, indicated by 3.

The device also comprises a control unit 4 which is connected with a control member 5 for the switch means 1 and an arrangement 6 for detecting over-voltages. The arrangement 6 may present an arbitrary number of individual detectors and provides the control unit 4 with information about the appearance of an over-voltage.

Fig. 2 illustrates how, by a net with a plurality of phases R, S, T, switch means 1 can be connected between each respective phase and earth 3.

Fig. 3 also illustrates how the switch means denoted 1a may be con- nected between phases in order to obtain protection against induc- tively transmitted over-voltages and against a certain connection over-voltages which may appear between phases.

It should be noted that, in the following description, the inventive de- vice thus is usable by one as well as by a plurality of phases, and both by AC and DC.

Fig. 4 illustrates the operation of the inventive switch means in its role as an over-voltage diverter. When an over-voltage exceeds the normal threshold voltage Uth, the electrode gap in the inventive switch means is immediately activated to close, whereby the current through the switch means increases rapidly while, at the same time, the voltage is delimited.

Fig. 5 corresponds to what is shown in fig. 4, but the axes in the co- ordinate system have been given inverted magnitudes.

Fig. 6 illustrates how an over-voltage pulse arrives. Fig. 7 illustrates how the over-voltage may be reduced to a very significant extent by means of the inventive switch means, given that it is brought into a conducting state with an extreme velocity when the over-voltage pulse arrives. Because the inventive switch means has a significantly low voltage loss when being in its conducting state, there are possi- bilities of having a better over-voltage elimination than with conven- tional varistor arrangements.

The voltage loss over the inventive switch means in its conducting state will possibly be as low as 100-400 volts.

In systems (electric power systems) for very high voltage levels, for example above 300 kV, it is of the utmost importance that a fault is taken care of with great rapidity in order to avoid costly breakdowns in the system. The voltage diversion by means of the switch means of the present invention may be effected within a few microseconds, as its operation is intended to be based upon the propagation veloc- ity of the light.

Fig. 8 illustrates a first embodiment of the arrangement for reducing over-currents provided with a switch means denoted 1. The switch means 1 presents electrodes 7 and a gap 8 located between said electrodes. As has been described earlier, the switch means pres- ents members 9 for triggering the electrode gap 8 to form an electri- cally conducting path between the electrodes. A control member 5 is adapted to control the operation of the members 9 through activation by the control unit 4. In the example, the members 9 are adapted to cause or at least initiate the electrode gap to assume electric con- ductivity by making the gap or at least a part thereof forming a plasma. Thereby, it is essential that the members 9 are capable of supplying triggering energy to the electrode gap in a very rapid way.

The triggering energy is supplied in the form of radiation energy ca- pable of effecting ionisation/piasma initiation in the electrode gap.

According to a particularly preferred embodiment of the invention, the members 9 comprise at least one laser which, by means of en- ergy supply to the electrode gap, accomplishes ionisation/plasma formation in at least a part of the electrode gap.

According to the invention, it is preferred to supply energy to the electrode gap 8 by means of one or more lasers or other members 9, such that the whole electrode gap is ionised and brought to the form of a plasma almost momentarily, and such that the whole gap 8 is brought to electric conductivity almost immediately. In order to economise on and optimise the use of the (normally) delimited, ac- cessible laser energy/effect, the members 9 may, during application of the invention, be arranged such that they are able to accomplish

ionisation/plasma formation in one or more parts of the gap 8. In the embodiment according to fig. 8, there is illustrated how the member 9 supplies the radiation energy to one single spot or area 10. As will be described later, the invention also includes that the radiation energy may be applied in a plurality of spots or areas in the electrode gap, including also on one of or both the electrodes or in one or more rod- like areas extending continuously or generally continuously between the electrodes.

Upon connection of the switch means 1 between the line 2 and earth 3 (or any other unit with a lower potential), such as schematically in- dicated in fig. 8, i. e. with one of the electrodes 7 connected to the line 2 and the other electrode connected to earth 3, there will be a voltage difference between the electrodes which gives rise to an electrical field. The electrical field in the gap 8 may be used in order to promote or generate an electric breakdown between the elec- trodes as soon as the members 9 have been controlled to triggering, i. e. given rise to ionisation/plasma formation in one or more parts of the electrode gap. The established ionisation/plasma formation will be forced by the electrical field to bridge the gap between the elec- trodes in order to obtain an electrically conducting channel with low resistivity, i. e. an electric arc between the electrodes 7. However, it is pointed out that the invention is not intended to be delimited only to application upon presence of such an electric field. Accordingly, the intention is that, also without such a field, the members 9 should be capable of establishing an electric conduction between the elec- trodes.

Due to the demand on the switch means 1 to close very rapidly for current diversion, it is thus desirable when only a restricted part, e. g. a spot like part of the gap is ionised, that the switch means is di- mensioned in such a way that the strength of the electric field in the gap 8 will be sufficiently high for safe closing. It is however on the other hand a desire that the switch means 1 should have a very high electric strength against breakdowns between electrodes in its iso- lating rest position. The strength of the electric field in the gap 8

should therefore be proportionally low. This will on the other hand reduce the speed, with which the switch means may be caused to establish the current diverting arc between the electrodes. In order to achieve an advantageous relation between the desire for a safe trig- gering of the switch means and on the other hand high electric strength against undesired triggering, it is according to the invention preferred that the switch means is formed in such a way that regard- ing its complete operational environment the electric field in the gap 8 has a field strength which is not more than 30% of the field strength at which a spontaneous breakdown normally takes place, when the gap forms electric isolation. This causes a proportionally low probability of a spontaneous breakdown.

The strength of the electric field in the electrode gap 8 in its isolating state is suitably not more than 20% and preferably not more than 10% of the field strength at which a spontaneous breakdown nor- mally takes place. In order to on the other hand achieve an electric field in the electrode gap 8, which promotes forming of an arc at ini- tiation of ionising/forming of plasma in a part of the electrode gap in a relatively rapid way, it is preferred that the strength in the electric field is at least 0,1% and suitably at least 1%, and preferably at least 5% of the field strength at which a spontaneous breakdown normally takes place.

The electrode gap 8 is, as may be seen in fig. 8, enclosed in a suit- able casing 11. A vacuum as well as a suitable medium in the form of gas or even fluid, also pressurised, may for this purpose be present in the gap 8. In the case of a gas/fluid the medium in the gap is in- tended to be formed in such a way that it might be ionised and brought to plasma by triggering. It would in such a case be suitable to initiate ionisation/forming of plasma in the gap 8 at a point some- where between the electrodes 7. It is however in fig. 8 illustrated the conceived case where there either is a vacuum or a suitable medium in the gap 25. It is then preferred that initiation of closing takes place by way of making the laser 9, which is illustrated in fig. 8, to focus the emitted radiation energy in at least one area 10 on or in the vi-

cinity of one of the electrodes via a suitable optical system 12. This implies that the electrode will operate as an electron and ion emitter for establishing an ionised environment/a plasma in the electrode gap 8 in such a way that thus an arc will be formed between the electrodes. One of the electrodes 7 may according to fig. 8 have an opening 13, through which the laser 25a is arranged to emit the ra- diation energy to the area 10 with support of the optical system 12.

Fig 9 illustrates a variant 1 of the switch means, where instead the system laser 9/optics 12 focus the radiation energy in a triggering area 10, which is situated between the electrodes and in a medium between these electrodes. Plasma is accordingly, on triggering, in- tended to be developed from this area to bridging of the electrodes.

In order to achieve the above discussed conditions regarding the op- eration of the switch means the characteristics of the switch means must of course be adequately adapted to the intended situation of use, i. e. the voltage conditions which will arise over the electrodes 7.

The constructive steps available regard of course forming of the electrodes, distance between the electrodes, the medium between the electrodes and the presence of possible further field affecting components between the electrodes.

Diffractive, refractive and reflexive optical elements may be used by the invention.

Fig. 10 illustrates an embodiment based upon an optical system 12 comprising a lens system 14, via which arriving laser pulses are con- veyed to a diffractive optical phase element 15, a kinoform. This element is designed to have a plurality of focal points or spots 10 generated starting from a single incoming laser pulse. These focal spots 10 are distributed along the axis of symmetry between the electrodes 7. As a consequence of the focal spots 10 being distrib- uted along a line between the electrodes 7, a more safe establish- ment of an electrical conduction path between the electrodes is achieved, meaning as high a probability for triggering as possible at

a voltage/electrical field strength as low as possible and with a time delay as short as possible.

The kinoform 15 is low absorbing and may, accordingly, resist ex- tremely high optical energy densities. The kinoform is, accordingly, produced from a dielectrical material so that it will not disturb the electrical field between the electrodes in any serious degree.

In the embodiment according to fig. 10, the radiation energy is supplied through an opening 13 in one of the electrodes as before.

Fig. 11 illustrates a variant where, generally speaking, the only dif- ference as compared to the embodiment according to fig. 10 is that the diffractive optical element (kinoform 15) is placed radially exter- nally of one of the electrodes 7. The optical element 15 is as before designed to deflect the laser light and focus the same in a number of spots or points distributed along the intended electrical conduction path between the electrodes. The bunches of beams forming the spots 10 have each their own deflection angle. Thus, the bunches of beams have to travel different distances to the respective spots 10.

The advantage in locating the kinoform 15 according to fig. 11 at the side of one of the electrodes is that the kinoform will be located sidewardly of the strongest electrical field so that the field distur- bance will be at a minimum. The electrode design is also simplified since no opening for the laser light is required.

Fig. 12 illustrates an embodiment where a laser 9 supplies the laser radiation via an optical system 12 symmetrically in a number of focal spots or points 10 distributed along the length of the electrode gap without requiring any opening in the electrodes 7. The optical system 12 comprises a prism or beam divider 16 arranged to deflect the la- ser beam around the adjacent electrode 7. Around this electrode 7 there is provided one or preferably more kinoforms 15 (diffractive optical elements) designed to focus, possibly by means of further lenses, the laser beam in the desired focal spot 10 so that plasma formations are generated in these spots.

Fig. 13 illustrates a variant where a laser beam is conveyed by means of an optical system 12 comprising optical fibres 17 for for- mation of focal spots 10 located at various places between the elec- trodes 7. The optical fibres 17 may be arranged to emit the light via lenses 18.

It appears from fig. 14 how the laser light may be focused in an elon- gated focal area 10 located between the electrodes 7 by means of an axicone 15. This elongated focal area may, according to one em- bodiment of the invention, extend continuously all the way between the electrodes, but could also occupy only a part of the gap there- between. If the radiation energy is applied in the electrode gap such that ionisation takes place along a rod-like, almost continuous ioni- sation/plasma channel, excellent preconditions for a secure closure of the switch means are created. In this connection, it should be pointed out that it would also be possible to shape the elongated fo- cal area in the electrode gap by means of axicones or other optical elements, such that it would become tubular. Furthermore, a plurality of rod-like or tubular focal areas could be arranged such that they extend generally in parallel between the electrodes, such that an ini- tiation of a closure between the electrodes takes place at a plurality of locations along their surface. This further increases the triggering secu rity.

The embodiment of the over-voltage diverting arrangement according to the invention and illustrated in fig. 15 comprises a varistor ar- rangement denoted 19 which presents a plurality of varistor elements 20 arranged in a stack. This stack of varistors is intended to be con- nected to a line, earth, etc., by means of the connections denoted 21. According to the invention, a switch means 1 of the type dis- cussed above is connected in parallel over at ieast a part of the varistor element 19. In the example, the switch means 1 is connected in parallel over approximately 27% of the stack of varistors.

The design illustrated in fig. 15 is supposed to reduce the problem associated with the fact that varistors demand that the permitted

over-voltages are dimensioned to between 150 and 200% of the normal voltage level. This is due to deficiences as to varistor char- acteristics.

By combining a varistor stack 19 according to fig. 15 with a switch means 1 connected in parallel over a part of the varistor stack, a protection function which delimits the over-voltages to maybe 10% above the voltage level normally admitted can be obtained. This ap- pears from the following example: as an example an AC-system with a varistor stack, the characteristic of which requires 150% over- voltage (1,5 Vtop) for substantial currents (kiloampere) to flow. At an over-voltage of 11% (i. e. 1,11 Vtop), the switch means 1 is activated and will disconnect 27% of the stack very rapidly. This leads to a voltage over the remaining varistor block corresponding to 1.11/0.73=152%, meaning that a large current flows through the varistor stack and the switch means 1, and that the voltage is delim- ited. When the over-voltage subsequently fades out and goes below Vtop, the over-voltage over the"remaining"varistors decreases to 137% at maximum, which means a limitation of the current, and then the current decreases rapidly to 0 through varistor and switch means (when one gets away from the top value and then gets closer to the zero cross-over). Because the switch means 1 is supposed to be a plasma switch, the next voltage top, which arrives 10 milliseconds later, is not enough to generate any current through the plasma, as the latter then has had time to de-ionise.

In the diagram in fig. 16, there is illustrated how the voltage level U, at which over-voltages are admitted, can be reduced from the full- line curve to the dotted line curve by adding the inventive switch means 1 in parallel connection over at least a part of the varistor stack.

Fig. 17 illustrates an embodiment reminding of the one in fig. 15 in the sense that also here there is a switch means 1 designed in ac- cordance with the invention which is located in parallel over a varis- tor element. Here, there are more precisely two or more varistors 22

arranged in series. As appears from fig. 17, switch means 1 accord- ing to the invention may be connected in parallel over one, two, or more of the varistors 22. These switch means 1 are connected to a control unit 4. As previously, the control unit is connected to an ar- rangement 6 for detecting over-voltages, said arrangement being connected to a line or the like 2 which is to be supervised. The de- vice illustrated in fig. 17 could, for instance, be intended for protect- ing long HVDC (High Voltage Direct Current) cables against coupling over-voltages. By such cables, especially if they are long, there is often a need of a very low protection limit as to faults or over- voltages. The protection device must be capable of diverting low DC- currents in order to delimit a driving DC over-voltage.

The special point about the embodiment according to fig. 17 is, thus, that one or more of the existing plasma switch means 1 controlled by the control unit 4 may be selectively activated to short-circuit one or more of the varistors 22 present in accordance with the present con- ditions and protection requirements.

Fig. 18 illustrates how the device schematically depicted in fig. 15 could be realised in practice. As in fig. 15, there are also varistor elements 20 arranged in a stack. In the varistor stack 19, there is provided a space 23 in connection to which the electrodes 7 of the switch means 1 are arranged, such that the space 23 will form the electrode gap. As appears from fig. 18, more precisely, one of the electrodes 7 is arranged at one of the ends of the varistor stack 19, while the other electrode 7 is located at the inner end of the space 23. According to a preferred embodiment, the space 23 is formed be- cause of the annular shape of the varistor elements 20a located at the space 23. The space/electrode gap 23 may, as earlier, have an atmosphere which is anything from vacuum to overpressure, and with a gas content set in accordance with the operational requirements. A tube 24 forms a sealed casing together with the electrodes.

A radiation source in the shape of a laser is symbolise by the arrow denoted 9. The optical system 12 is symbolise by an axicone 12.

One advantage of having an integrated switch means 1 in the varis- tor stack is that only one outer insulation 24 is required. There is also the advantage of having inductances, which counteract current from commuting from the varistor branch to the switch gap, mini- mised in order to obtain a commuting time as short as possible and, thereby, a dangerous over-voltage which is as brief as possible. It is a further advantage that the varistor stack gives an even field distri- bution between the electrodes.

The operation by the embodiment according to fig. 18 is the same as the one by the embodiment according to fig. 15, i. e. when the gap 23 between the electrodes 7 is brought to a conducting state, the con- ducting path generated between the electrodes will short-circuit a part of the varistor stack, such that a very low over-voltage threshold (e. g. 10%) can be obtained.

It should be noted that the description presented above only is to be regarded as an example of the inventive idea upon which the inven- tion is based. It is thus obvious to a man skilled in the art that de- tailed modifications can be made without thereby leaving the scope of the invention. As an example, it could be mentioned that, accord- ing to the invention, it is not necessary to use a laser for supplying ionising/plasma forming energy to the gap 24. Also other radiation sources, e. g. electron guns or other energy supply solutions may be applied as long as the rapidity and reliability demands according to the invention are fulfilled.