WO1999067867A1

WO1999067867A1 - A protection device and method

Info

Publication number: WO1999067867A1
Application number: PCT/SE1999/001100
Authority: WO
Inventors: Marie Sundin; Per Skytt; Bertil Berggren
Original assignee: Abb Ab
Priority date: 1998-06-17
Filing date: 1999-06-17
Publication date: 1999-12-29
Also published as: AU4816099A

Abstract

A device for protecting, in a multiphase electric power plant, one or more objects (1) from over-currents, said objects being connected to an electric power network (5) or another equipment contained in the electric power plant, said device comprising an over-current reducing arrangement (7), which is activatable for over-current reduction with the assistance of an arrangement (8) detecting over-current conditions. The over-current reducing arrangement comprises a plurality of electric closing means (9), which each are coupled between phase conductors (R, S, T) in the plant and which are closable to bring each individual phase conductor into electric connection to a further phase conductor.

Description

A protection device and method

FIELD OF THE INVENTION AND PRIOR ART

This invention is related to a device according to the prechar- acterising part of enclosed claim 1 . Furthermore, the invention comprises a method for protecting objects.

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

In prior electric power plants of this nature one has relied, to protect the object in question, on a conventional circuit breaker (electric switch) of such a design that it provides for galvanic separation on breaking. Since this circuit breaker must be designed to be able to break very high currents and voltages, it will obtain a comparatively bulky design having a large inertia, which reflects itself in a comparatively long breaking time. It is pointed out that the over-current primarily intended is the short circuit current arising in connection with the protected object, e.g. as a consequence of faults in the electrical insulation system of the protected object. Such faults cause the fault current (short circuit current) of the external network/equipment to tend to flow through the light arc. The result thereof may be a very large failure. It may be mentioned that the dimensioning short circuit current/fault current for the Swedish power network is 63 kA. In reality, the short circuit current may be 40-50 kA.

A problem with the circuit breaker mentioned is the extended breaking time thereof. The dimensioning breaking time (IEC- standard) for completely performed breaking is 150 milliseconds (ms). It is difficult to reduce this breaking time to less than 50- 90 ms depending on the operational case. The consequence is that when a fault in the protected object occurs, a very high cur- rent will flow through the same during the entire time required for causing the circuit breaker to break. During this time the full fault current of the external power network involves a substantial strain on the protected object. In order to avoid damages and total failures with regard to the protected object, one has con- structed, in accordance with the prior art used until now, the object so that it should be capable of withstanding the short circuit current/fault current during the breaking time of the circuit breaker without appreciable damages. In this connection it is pointed out that a short circuit current (fault current) in the pro- tected object may be composed of the own contribution to the fault current of the object itself and the current addition emanating from the network/equipment. The own contribution of the object to the fault current is not influenced by the operation of the circuit breaker but the contribution to the fault current from the network/equipment depends on the operation of the circuit breaker. The need for constructing the protected object so that it endures a high short circuit current/fault current during a considerable time involves substantial disadvantages in the form of a more expensive design and reduced performance. OBJECT OF THE INVENTION

The object of the present invention is to devise ways to design the device and method so as to achieve a better protection for the object and, accordingly, a decreased load thereon, which means that the object itself no longer has to be constructed to withstand a maximum of short circuit currents/fault currents during relatively prolonged time periods.

SUMMARY OF THE INVENTION

The object mentioned hereinabove is achieved according to the invention by the device according to the invention comprising an over-current reducing arrangement, which is activateable for over-current reduction with the assistance of an arrangement detecting over-current conditions and by the over-current reducing arrangement comprising a plurality of electric closing means, which each are coupled between phase conductors in the plant and which are closable to bring each individual phase conductor into electric connection to a further phase conductor. The solution according to the invention means that upon ocur- rence of a fault, the phase conductors are short circuited to each other by means of the closing means.

The solution according to the invention involves the advantage that the fault current which will flow to ground becomes rather limited if it is assumed that the protected object Is either ungrounded or has a relatively high-ohm system grounding. A relatively small current will then flow from the location of the fault to the system grounding . This causes the own contribution of the object to the fault to be delimited. Instead this current will circulate in the object and heat the occurring windings until magnetisation has been reduced.

According to a preferred embodiment of the invention, the closing means are designed as defined in enclosed claim 8, i.e. such that an electrode gap thereof may be imparted electric conductivity by supply of radiation energy to the electrode gap so as to create therein such ionisation/plasma that an electrically conducting channel between the electrodes is formed.

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

SHORT DESCRIPTION OF THE DRAWINGS

With reference to the enclosed drawings, a more specific description of embodiment examples of the invention follows hereunder.

In the drawings;

Fig 1 is a diagrammatical view illustrating the protection device according to the invention applied in an electric power plant for protection of a rotating electric machine, here in the form of a generator;

Fig 2 is a diagram illustrating the electric consequence of closing of closing means comprised in the diagram ac- cording to Fig 1 ;

Fig 3 is a diagrammatical view illustrating how a mechanical closing means is coupled in parallel with a closing means, which is intended to be of a non-mechanical type, to provide galvanic closing in parallel over the non-mechanical closing means;

Fig 4 is a diagrammatical detail view illustrating a possible embodiment of the closing means according to the in- vention; Fig 5-13 are views similar to Fig 4 of different variants.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Fig 1 illustrates an electric power plant, in which an object 1 to be protected is comprised. This object is in the example conceived to consist of a generator, the stator windings of which are denoted 2. The zero point 3 of the generator is either without system grounding or otherwise there exist as "system ground- ing" a relatively high-ohm grounding, for instance by means of the resistor 4. In the example the plant has three phases with the phase conductors R, S and T. The invention is applicable to multiphase embodiments where the number of phases are lower as well as higher than three.

The generator 1 is in connection with an external delivery network 5 via the phase conductors R, S, T. Instead of such a network, the unit denoted 5 could be formed by another equipment contained in the electric power plant.

It is pointed out that it primarily is the generator 1 proper which is intended to be protected against fault currents from the network/equipment 5 when there occurs in the generator 1 proper a fault giving rise to a fault current from the network/equipment 5 towards the generator so that the fault current will flow therethrough. Said fault may e.g . consist in a ground fault having been formed in the generator 1 .

It is pointed out that detrimental short circuit currents/fault cur- rents in some types of protected electrical objects 1 , as for instance generators, may flow from the object 1 towards the network/equipment 5. It is within the frame of the invention that it may be used for protection purposes not only for protecting the object 1 from externally emanating fault currents flowing to the object but also internal fault currents in the object flowing in the opposite direction. In the following the reference character 5 will to simplify the description always be denominated as consisting of an external electric power network. However, it should be kept in mind that instead of such a network it may be the question of another equipment which when a fault occurs causes violent current flow through the object 1 .

It appears from Fig 1 that conventional circuit breakers 6 are provided in the phase conductors R, S, T. These circuit breakers comprise at least one own sensor for sensing circumstances indicative of an over-current flowing in one or some of the phase conductors R, S, T. Such circumstances may be currents/voltages but also other indicating that a fault is at hand. For instance, the sensor may be an arc sensor or a sensor detecting short circuit sound etc. When the sensor indicates that the over-current exceeds a certain level, the circuit breakers 6 are activated for breaking the connections between the generator 1 and the network 5. However, the circuit breakers 6 have to break the total short circuit current/fault current. Thus, the circuit breakers must be designed to fulfil severe requirements, which in practice means that they will operate relatively slowly. The circuit breakers 6 are of such a design that they establish galvanic separation by moving metallic contacts apart. Accord- ingly, the circuit breakers 6 comprise normally required auxiliary equipment for arc extinguishing.

According to the invention there is an arrangement generally denoted 7 for over-current reduction, said arrangement being activateable for over-current reduction with the assistance of an arrangement 8 detecting over-current conditions. The over-current reducing arrangement 7 comprises a plurality of electric closing means 9, which each are coupled between phase conductors in the plant and which are closable to bring each indi- vidual phase conductor into electric connection to a further phase conductor. In the embodiment illustrated in Fig 1 , there are three phase conductors. In such a case the enclosing means 9 are two in number and they are coupled between two respective phase conductors as indicated in Fig I .The over-current reducing arrangement 7 comprises further a control unit 10 adapted to control means denoted 1 1 and being in their turn adapted to cause the closing means 9 to assume an electrically conducting state. The over-current reducing arrangement 7 is activateable for over-current reduction within a time period substantially shorter than the breaking time of the circuit breakers 6.

When a fault occurs, e.g. the one indicated at 12 in Fig 2, this is registered by the arrangement 8 detecting over-current conditions and information is supplied to the control unit 10. This controls in its turn the closing means 9 to short circuit the three phase conductors R, S, T to each other. This short circuiting is intended to be effected within some or few ms after unacceptable fault conditions having been detected. It is aimed at to carry out current reduction in a shorter time period than 1 ms, and preferably more rapidly than 1 micro second. Simultaneously, the circuit breakers 6 are controlled to open but this requires a considerable time. Immediately after closing of the closing means 9, the situation in the plant can be said to be the one according to Fig 2.

From the point of view of the network 5, two two-phase short circuits are provided in a three phase network.

Two short circuits between two phase conductors in the way in- dicated in Fig 1 are equivalent in the relevant regards with short circuiting each of the occurring phases individually directly to ground.

To apply the closing means 9, in the way prescribed by the in- vention, between phase conductors to establish connection therebetween involves a solution equivalent to short circuiting all occurring phases to ground or otherwise a relatively low potential.

The embodiment according to Fig 1 and 2 has several advan- tages:

If it is assumed that the generator 1 (the rotation machine) has a high-ohm system grounding, the fault current flowing to ground will be rather limited. One gets a small current closing from the fault location in the system grounding (e.g. a step-up transformer) but that current should be rather low. The own contribution of the machine to the fault is restricted; instead this current will circulate in the machine and heat the occurring windings until the magnetisation has been reduced . If instead a rotating electric machine with low-ohm grounding is involved, this alternative is not equally attractive. The arrows 13 occurring in Fig 2 illustrate how a current will circulate in the generator and heat the windings thereof until the magnetisation has been reduced. On the netside, the closing means 9 will connect the phases R, S, T so that also there connections between the phases will be established until the circuit breakers 6 break. Typically, the circuit breakers 6 break after the order of 50-90 ms where as the closing means 9 are intended to be brought into electrically conducting state in about 1 ms or less. In absence of the closing means 9 according to the invention, a considerably larger fault current will flow in the fault 12 in any case if the fault occurs on the high voltage side of the stator winding in question.

Fig 4 illustrates a first embodiment of the closing means 9 of the over-current reducing arrangement 7. The closing means 9 comprises electrodes 14 and a gap 15 present there between. As previously described, the closing means comprises members 16 to trigger the electrode gap 15 to form an electrically conducting path between the electrodes. A control means 1 1 is adapted to control, via the control unit 10, operation of the members 16. The members 16 are in the example arranged to cause or at least initiate the electrode gap to assume electric conductivity by ensuring that the gap or a part thereof is caused to form a plasma. It is then essential that the members 16 are capable of supplying triggering energy to the electrode gap with a great speed. It is then preferred that the triggering energy is supplied in the form of radiation energy capable of effecting ionisation/plasma initiation in the electrode gap.

According to a particularly preferred embodiment of the inven- tion the members 16 comprise at least one laser, which by energy supply to the electrode gap provides for ionisation/plasma formation in at least a part of the electrode gap.

According to the invention it is preferred to supply, by means of one of more lasers or other members 16, energy to the electrode gap 15 so that almost momentarily the entire electrode gap is ionised and brought to the form of a plasma respectively so that also the entire gap 15 is immediately brought to electric conductivity. In order to spare, and optimise the use of, the (normally) limitedly available laser energy/effect the members 16 may, however, in use of the invention be adapted so that they are capable of achieving ionisation/plasma formation in only one or more parts of the gap 15. In the embodiment according to Fig 4 it is illustrated that the members 16 supply radiation energy in one single point or area 19. As will be described later the invention includes also application of radiation energy in several spots or areas in the electrode gap, including also on one of or on both of the electrodes, or in one or more rod like areas extending continuously or substantially continuously between the electrodes.

When the closing means 9 is coupled between two phase conductors, e.g. R and S as diagrammatically indicated in Fig 4, i. e. with one of the electrodes 14 connected to the phase con- ductor and the other connected to the conductors S, there will between the electrodes mostly occur a voltage difference giving rise to an electric field. The electric field in the gap 15 may be used to promote or cause an electric breakthrough between the electrodes as soon as the members 16 have been controlled to triggering , i.e. given rise to ionisation/plasma formation in one or more parts of the electrode gap. This established ionisation/plasma formation will be driven by the electric field to bridge the gap between the electrodes so as to provide in this way an electrically conducting channel with low resistivity, i.e. an arc, between the electrodes 14. However, it is pointed out that the operation of the closing means 9 is not intended to be restricted only to the occurrence of such an electric field. Thus, the intention is that the members 16 should be capable of establishing electric conduction between the electrodes also without such a field.

It is pointed out that the term "triggering" in the present context means to bring the closing means to an electrically conducting state.

As a consequence of the requirement on the closing means 9 to close very rapidly for current diversion, it is, accordingly, desirable, when only a restricted part, e.g. a spot like part, of the gap is ionised, that the closing means is dimensioned so that the strength of the electric field in the gap 15 becomes sufficient for safe closing. On the other hand, it is a desire that the closing means 9 in its insulating state of rest should have a very high electric strength to break-through between the electrodes. Seen against this background, the strength of the electric field in the gap 15 should be comparatively low. However, this reduces in one spot ionisation the speed, with which the closing means may be caused to establish the current diverting arc between the electrodes. In order to achieve an advantageous balancing between the desire of a safe triggering of the closing means and a high electric strength against non-desired triggering , it is ac- cording to the invention in such a case preferred that the closing means is designed in such a way with regard taken to its opera- tional environment that the electric field in the gap 15, when the gap forms an electrical insulation, has a field strength which is not more than 30% of the field strength at which spontaneous breakthrough normally occurs. This provides for an extremely low probability for a spontaneous breakthrough.

The strength of the electric field in the electrode gap 15 in the insulating state thereof is suitably not more than 20% and preferably not more than 10% of the field strength, at which spon- taneous breakthrough normally occurs. In order to obtain an electric field in the electrode gap 1 5 which, on the other hand, promotes arc formation on initiation of ionisation/plasma formation in a part of the electrode gap in a relatively rapid manner it is preferred that the strength in the electric field is at least 0, 1 %, suitably at least 1 % and preferably at least 5% of the field strength, at which spontaneous breakthrough normally occurs.

The electrode gap 15 is as appears from Fig 4 enclosed in a suitable housing 17. In the gap 15 there may exist a vacuum as well as a medium in the form of gas or even a liquid suitable for the purpose. The pressure in the housing 17 may be anything from vacuum to over-pressure. In the case of gas/liquid, the medium in the gap is intended to be adapted so that it on triggering may be ionised and brought to a plasma form. In such a case it would be suitable to initiate ionisation/plasma formation in the gap 15 in at least one spot somewhere between the electrodes 14. However, in Fig 4 is illustrated a case where there exists in the gap 15 either vacuum or a suitable medium. It is then preferred that initiation of closing occurs by the laser 16 illustrated in Fig 4 being caused to focus, via a suitable optical system 18, the emitted radiation energy in at least one area 19 on or adjacent to one of the electrodes. This causes the electrode to function as an electron and ion emitter for establishing an ionised environment/a plasma in the electrode gap 15 so that, accord- ingly, an arc will be formed between the electrodes. As appears from Fig 4, one of the electrodes 14 may comprise an opening 27, through which the laser 16 is adapted to deliver radiation energy to the area 19 with the assistance of the optic system 18. Fig 5 illustrates a closing means variant 9a where instead the system laser 16a/optics 18a focuses the radiation energy in at least one triggering area 19a located between the electrodes and in a medium there between. On triggering a development of plasma to bridging of the electrodes is, accordingly, intended to occur from this area.

In order to achieve the conditions discussed hereinabove as far as the field strength relations between the electrodes 14 in the insulating state of the closing means are concerned, the characteristics of the closing means must of course be adequately adapted to the intended situation of use, i.e. the voltage condi- tions which will occur over the electrodes 14. The constructive measures available concern of course electrode design, distance between the electrodes, the medium between the electrodes and the occurrence of possible further field influencing components between the electrodes.

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

Fig 6 illustrates an embodiment based upon an optical system 18b comprising a lens system 20, via which arriving laser pulses are delivered to a diffractive optical phase element 21 , a kino- form. This element is designed to have a plurality of focal points 19b generated starting from a single arriving laser pulse. These focal points 19b are distributed along the axis of symmetry be- tween the electrodes 14b. Since the focal points 19b are distributed along a line between the electrodes 14b, a more safe establishment of an electric conduction path between the electrodes is achieved, which means as high a probability for triggering as possible at as low a voltage/electric field strength as possible and with as small a time delay as possible. The kinoform 21 is low-absorbing and may, accordingly, withstand extremely high optical energy densities. The kinoform is produced from a dielectrical material so that it will not in a serious degree disturb the electric field between the electrodes.

In the embodiment according to Fig 6 the radiation energy is supplied through an opening 27b in one of the electrodes as before. Fig 7 illustrates a variant where generally the only difference as compared to the embodiment according to Fig 6 is that here the diffractive optical element (kinoform) 21 c is placed radially outwardly of one of the electrodes 14c. The optical element 21 c is as before designed to divert the laser light and focus the same in a number of spots distributed along the desired electric conduction path between the electrodes. The radiation bundles forming the spots 19c have each their own deflection angle. Thus, the radiation bundles have to move different distances to the respective spots 19c. The advantage of locating, according to Fig 7, the kinoform 21 c at the side of one of the electrodes is that the kinoform will be located beside the largest electric field so that the field disturbance becomes a minimum. The electrode design is also simplified since there is not needed any opening for the laser light.

Fig 8 illustrates an embodiment where a laser 16d via an optical system 18d supplies the laser radiation symmetrically in a plurality of focal points 19d distributed along the length of the electrode gap without any opening being required in the electrodes 14d. The optical system 18d comprises a prisma or a radiation divider 22 arranged to break up the laser beam around the adjacent electrode 14d. Around this electrode 14d there is provided one or preferably more kinoforms 21 d (diffractive optical elements) designed to, possibly with the assistance of further lenses, focus the laser beam in the desired focal points 19d so that plasma formations are generated therein. Fig 9 illustrates a variant where a laser beam by means of an optical system 18e comprising optical fibres 23 is directed for formation of focal points 19e located at different places between the electrodes 14e. The optical fibres 23 may be arranged to emit the light via lenses 24.

In Fig 10 a conical lens, a so called axicone, is included. The definition of such an axicone may be said to be every rotation- ally symmetrical optical element, which by refraction, reflection, diffraction or combinations thereof deflects light from a point source on the axis of symmetry of the element in such a way that the light intersects this axis of symmetry not in one single point, as would be the case with a conventional spherical lens, but along a continuous line of points along a substantial extent of this axis.

It appears from Fig 10 that the light may be focused, by means of an axicone 21f, in an elongated focal area 19f located between the electrodes 14f. This elongated focal area may ac- cording to one embodiment of the invention extend continuously the whole distance between the electrodes but could also assume only a part of the gap therebetween. For the rest, it is pointed out that the invention is not only limited to such axi- cones which are purely linearly conical. Thus, within the frame of the invention also axicones are included, the mantle surface of which deviates from the linear cone, which will get a direct influence on the focal intensity distribution.

Fig 1 1 illustrates an embodiment where a specially shaped dif- fractive axicone 21 g, a kinoform, has been designed to provide focal areas 19g and 19g' respectively with different shapes. In the example it is illustrated that the focal area 19g is elongated and provided on the axis of symmetry of the axicone 21 g and the electrodes. The focal area 19g' on the contrary has as is in- dicated to the left in Fig 1 1 obtained a cross-sectionally tubular shape. This tubular shape is advantageous most closely to an electrode 14g provided with an opening 20g since the periphery of the tubular focal area 19g' will be located relatively close to the electrode 14g provided with the opening. Both focal areas 19g and 19g' have in Fig 1 1 a substantially constant intensity along the axis of symmetry but perpendicularly thereto there occurs, as concerns the focal area 19g, a substantially Gauss- shaped or Bessel-function shaped intensity distribution.

An advantage of an entirely or substantially conical or diffrac- tive, coaxially focusing component as for example in Fig 6, 7, 8, 9, 10, 1 1 is that along the efficient direction of propagation of the radiation energy, which direction may be said to be a straight line, that plasma volume which is formed firstly, which occurs most closely to that electrode, at which the supply of the radiation energy occurs, will not screen, reflect or influence to a serious degree the radiation energy focused in points/areas located further away from the supply electrode. This "shadowing effect" of the plasma volumes first formed could otherwise have prevented the radiation energy from efficiently reaching later foci. This is a consequence of the fact that a plasma has the property to be able to reflect or absorb radiation energy.

It is illustrated in Fig 12 that several substantially parallel electrically conducting channels may be formed between the elec- trodes 14h. The occurrence of a plurality of simultaneously electrically conducting channels increases the conduction capacity of the closing means.

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

Fig 13 illustrates an embodiment of the invention where such an axicone 21 i is used. This axicone forms more specifically a radiation energy line 19i between the electrodes 14i. The axicone 21 i is so designed and the substantially collimated radiation 26 incident to the axicone so directed that the radiation energy line 19i is at least partly displaced laterally a distance d in relation to a centre axis 30 of the incident, substantially collimated radiation.

In the case of an axicone 21 i as in Fig 13, this means that the axicone 21 i has its optical axis/axis of symmetry 28 laterally displaced from the axis 30 of the incident collimated radiation. It appears from Fig 13 that the incident radiation 26 passes through the axicone 21 i and is deflected thereby in a peripheri- cal area thereof. The consequence thereof is that the axicone 21 i will direct the radiation energy obliquely as indicated by means of the arrow 29 but nevertheless the radiation energy line 19i, along which the radiation energy is focused, will be sub- stantially parallel to the incident collimated radiation 26.

It is preferred that the axicone 21 i is adapted to apply the radiation energy along the radiation energy line so that a substantially rod-shaped area results along said line, said area being ionised/formed to a plasma and bridging, entirely or substantially entirely, the distance between the electrodes 14i for creating favourable, to a maximum degree, conditions for arc formation between the electrodes.

At least one of the electrodes has an opening 27i, through which the axicone 21 i is adapted to direct the radiation energy. The opening 27i extends obliquely relative to an axis 31 of symmetry of the electrodes 14i. The opening 27i is eccentric relative to the axis 31 of symmetry of the electrodes. The axis 28 of the axi- cone coincides with the radiation energy line 19i. The axicone 21 i is adapted to apply the radiation energy so that it arrives upon a lateral surface, denoted 32, of the opening 27i in one of the electrodes 14i. It is pointed out that the axicone 21 i is adapted to apply the radiation energy line 19i so that it is substantially parallel to the axis 31 of symmetry of the electrodes 14i. The lateral surface 32 forms an angle α to the axis 31 of symmetry. The radiation directed to the adjacent electrode 14i by the axicone 21 i forms an angle γ with the axis 31 of symmetry. The angle α is smaller than the angle γ, preferably about half the angle γ. This means that the radiation 29 will hit upon the lateral surface 32, a fact which promotes formation of a plasma extending between the electrodes 14i.

Even if in Fig 13 an "entire" axicone has been drawn, it is real- ised that only one part of an axicone is required according to that described hereinabove, namely that part which actually is penetrated by incident radiation 26.

According to a preferred embodiment, the axicone 21 i is de- signed to be routable about its axis 28 of symmetry. This means the advantage that if a portion of the axicone 21 i present opposite to the opening 27i would be influenced negatively by the arc between the electrodes, it is possible to move forward, by rotation of the axicone 21 i, such a portion of the axicone, which is in good condition, to such an area that the collimated radiation energy from the radiation source in an adequate manner may be deflected and applied along the previously discussed radiation energy line 19i.

It is illustrated in Fig 3 that a mechanical closing means 25 adapted to establish a galvanic connection parallel to radiation triggered closing means 9 may be coupled parallel over the closing means 9 triggerable by radiation energy. The embodiment according to Fig 3 has the advantage that such a mechani- cal closing means 25, in case a great contact pressure may be maintained between the contacts thereof, results in reduction of o

the voltage drop over the radiation triggered closing means 9 from a value, which per se already is comparatively low, to zero or almost zero. Most specifically, the function is such that the control unit 10 (see for instance Fig 1 ) activates, when the closing means 9 is to be brought to an electrically conducting state, the closing function, which will involve the radiation triggered closing means 9 to first form, as a consequence of the extremely rapid reaction time thereof, an electrically conducting current path whereas the mechanical closing means 25 will close, as a rule not until after a certain delay, so that the relatively low voltage drop over the closing means 9 in the conducting state thereof will be further reduced to zero or almost zero. Thus, it is within the frame of the present invention that whenever radiation triggered closing means 9 occur, they may ac- cording to a preferred embodiment of the invention have mechanical closing means coupled in parallel with said radiation triggered closing means.

It should be noted that the description given hereinabove only should be considered as exemplifying for the inventive concept, on which the invention is based. Thus, it is obvious for the men skilled in the art that detail modifications may be made without therefore leaving the scope of the invention. As an example it may be mentioned that it is not necessary according to the in- vention to use a laser for supply of ionisation/plasma formation energy to the gap 15. Also other radiation sources, for instance electron cannons, or other energy supply solutions may be resorted to provided that they fulfil requirements with respect to speed and reliability defined according to the invention. Fur- thermore, it is pointed out that the closing means 9 according to the invention may be used for protection of electrical objects to fault related over-currents also in other operational cases than those illustrated in Figs 1 and 2, where the device according to the invention is arranged to reduce the negative influences of the relatively prolonged breaking time of the circuit breakers 6. The closing means according to the invention must not neces- sarily be functionally related to such circuit breakers 6. Furthermore, the closing means 9 may be mobile.

Finally, it is pointed out that although radiation triggered closing means 9 are extremely preferable, the basic protection aspects discussed with assistance of Figs 1 and 2 could also be realised by means of other suitable closing means, including purely mechanical closing means, provided that such closing means fulfil the requirements associated to a good protection functionality, i.e. that the closing means in any case should be capable of closing appreciably more rapidly than the breaking time of the circuit breakers 6.

Claims

CLAIMS:

1 . A device for protecting, in a multiphase electric power plant for alternating voltage, one or more objects (1 ) from over- currents, said objects being connected to an electric power network (5) or another equipment comprised in the electric power plant, characterized in that the device comprises an over-current reducing arrangement (7), which is activateable for over-current reduction with the assistance of an arrange- ment (8) detecting over-current conditions and that the over- current reducing arrangement (7) comprises a plurality of electric closing means (9), which each are connected between phase conductors (R, S, T) in the plant and which are closable for bringing each individual phase conductor into electric connection with a further phase conductor.

2. A device according to claim 1 , comprising switches (6) in the phase conductors between the object (1 ) and the network/equipment (3), characterized in that the closing means (9) are connected to the phase conductors between the object (1 ) and the switches (6) and that the closing means (9) are activateable for over-current diversion within a time period substantially shorter than the breaking time of the switches (6).

3. A device according to claim 2, characterized in that the switches (6) are constituted by circuit breakers.

4. A device according to any preceding claim, characterized in that the protected object (1 ) is formed by an electrical apparatus having a magnetic circuit.

5. A device according to claim 4, characterized in that the object is formed by a generator, transformer or motor.

6. A device according to any preceding claim, characterized in that the object is in absence of system grounding or is high- ohm grounded.

7. A device according to any preceding claim, characterized in that it comprises a control unit (10) connected to the closing means (9) and to the arrangement (8) detecting over-current conditions, said control unit being adapted to control, with assistance of information from the arrangement, the closing means to close when required for reasons of protection.

8. A device according to any preceding claim, characterized in that the closing means comprise an electrode gap (15), which is convertible between an electrically substantially in- sulating state and an electrically conducting state, and members (16, 18) for causing or at least initiating the electrode gap or at least a part thereof to assume electric conductivity and that the members (16, 18) to cause or at least initiate the electrode gap to assume conductivity are adapted to supply energy to the electrode gap in the form of radiation energy to bring , by means of this radiation energy, the gap or at least a part thereof to the form of a plasma.

9. A device according to claim 8, characterized in that the members (16, 18) for causing or at least initiating the electrode gap or a part thereof to assume electric conductivity comprise at least laser (16).

10. A device according to any of claims 8 and 9, characterized in that the members (16, 18) for supply of triggering energy to the electrode gap are arranged to apply the radiation energy on or in the vicinity of at least one of the electrodes (14).

1 1 . A device according to any of claims 8-10, characterized in that the members (16, 1 8) for supply of triggering energy to the electrode gap are adapted to locate the radiation energy in one or more spots or one or more areas in the gap (15) between the electrodes (14).

12. A device according to claim 1 1 , characterized in that the members for supply of triggering energy to the electrode gap are arranged to locate said two or more spots or areas of radiation energy along a line extending between the electrodes and corresponding to the course of an electric conduction path desired between the electrodes.

13. A device according to any of claims 8-12, characterized in that the members (16, 18) for supply of triggering energy to the electrode gap are adapted to apply the radiation energy in one or more elongated areas (19), the longitudinal axis of which resides substantially along the direction of the desired electric conduction path between the electrodes.

14. A device according to claim 13, characterized in that the members (1 8) for supply of triggering energy to the electrode gap are arranged to shape the elongated focal area to a tubular shape.

15. A device according to claim 13 or 14, characterized in that the members for supply of triggering energy to the electrode gap are adapted to shape said at least one elongated area so that it entirely or substantially entirely bridges the distance between the electrodes.

16. A device according to any of claims 13 or 14, characterized in that the members (16, 1 8) for supply of triggering energy to the electrode gap are adapted to form, in said gap, two or more elongated focal areas (19) located in the longitudinal direction after each other along the desired electric conduction path between the electrodes.

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

18. A device according to any of claims 8 and 17, characterized in that the members for supply of triggering energy to the electrode gap are adapted to apply the radiation energy both on at least one of the electrodes and between them.

19. A device according to any of claims 8-18, characterized in that at least one of the electrodes at the electrode gap has an opening (27) through which the members (16, 1 8) for triggering energy supply are adapted to direct the radiation energy.

20. A device according to claim 14 and 19, characterized in that the members (16, 18) for supply of triggering energy to the electrode gap are adapted to locate the tubular radiation energy area (10) adjacent to the electrode provided with the opening (27) and such that the axis of the tubular radiation energy area is substantially concentric to the axis of the opening in the electrode.

21 . A device according to any of claims 8-20, characterized in that the members for supply of triggering energy to the electrode gap comprises a system for directing electro-magnetic wave energy.

22. A device according to claim 21 , characterized in that the direction system comprises at least one refractive, reflective and/or diffractive element.

23. A device according to claim 22, characterized in that the element is formed by an axicone, a kinoform or optical fibres (38).

24. A device according to any of claims 22-23, characterized in that the direction element (21 i) of the radiation direction system is adapted to apply the radiation energy along a line (19i) extending between the electrodes and that the direction element (21 i) is so designed and the substantially collimated radiation inciding to the direction system so directed that the radiation energy line (19i) is at least partly displaced laterally (d) in relation to the axis (30) of the incident, substantially collimated radiation.

25. A device according to claim 24, characterized in that the direction element (21 i) is adapted to orientate the radiation energy line (19i) substantially parallel to the substantially collimated radiation (26) incident to the direction element.

26. A device according to any of claims 24-25, characterized in that the direction element (21 i) is adapted to direct the radiation energy obliquely relative to an axis (31 ) of symmetry of the electrodes.

27. A device according to claim 19 and any of claims 24-26, characterized in that the opening (27i) extends obliquely relative to an axis (31 ) of symmetry of the electrodes.

28. A device according to claim 27, characterized in that the opening (27i) is eccentric relative to an axis (31 ) of symmetry of the electrodes.

29. A device according to any of claims 27-28, characterized in that the direction element (21 i) is adapted to direct the ra- diation energy so as to be incident on a side surface (32) of the opening in the electrode.

30. A device according to any of claims 24-29, characterized in that the direction element (21 i) is rotatable about its axis (28) of symmetry.

31 . A device according to any of claims 22-23, characterized in that the direction system (18) is placed radially outwardly of the electrodes and adapted to direct bundles of radiation inwardly towards the gap between the electrodes.

32. A device according to any of claims 22-23 and 31 , characterized in that the direction system (18) is adapted to break up laser pulses to an annular conFiguration about one of the electrodes.

33. A device according to any of claims 8-32, characterized in that each of the closing means is coupled in parallel to a mechanical closing means (25) capable of establishing galvanic closing.

34. Electric power plant, characterized in that it comprises one or more devices according to any preceding claim.

35. Use of a device according to any preceding claim for protec- tion of objects against fault-related over-currents.

36. A method to protect, in a multiphase electric power plant, one or more objects from over-currents, said objects being connected to an electric power network or another equipment comprised in the electric power plant, characterized in that over-current reduction is carried out, when over-current conditions have been detected by means of an arrangement intended therefor, by causing a plurality of electric closing means to close so as to establish an electric connection be- tween each individual phase conductor and a further phase conductor.