WO2000001050A1

WO2000001050A1 - A device for protecting an object against fault related over-currents

Info

Publication number: WO2000001050A1
Application number: PCT/SE1999/001043
Authority: WO
Inventors: Per Skytt; Jan Isberg; Mats Ii Ekberg; Hans Bernhoff
Original assignee: Abb Ab
Priority date: 1998-06-18
Filing date: 1999-06-17
Publication date: 2000-01-06
Also published as: AU4812999A; SE9802202D0; SE9802202L

Abstract

This invention is related to a device for protecting, in an electric power plant, an object (1) against over-currents from a network (3) or another equipment comprised in the high voltage plant, the device comprising a first electric switch (4) in a line (2) between the object and the network/equipment. The line (2) between the object and the first electric switch (4) is connected to an arrangement (5) reducing over-currents towards the object (1). This arrangement is activatable for over-current reduction with the assistance of an arrangement (11-13) detecting over-current conditions within a time period substantially shorter than the breaking time of the first electric switch (4). A second electric switch (27) is arranged in the line between the object (1) and the connection of the over-current reducing arrangement (5) to the line (2).

Description

A DEVICE FOR PROTECTING AN OBJECT AGAINST FAULT-RELATED OVER-CURRENTS

FIELD OF THE INVENTION AND PRIOR ART

This invention is related to a device in an electric power plant for protection of an object connected to an electric power network or another equipment in the electric power plant from fault-related over-currents, the device comprising an electric switch in a line between the object and the network/equipment.

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

In prior power plants of this nature one has resorted to, for protection of the object in question, a conventional circuit breaker (electric switch) of such a design that it provides 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 with large inertia, which reflects itself in a comparatively long break-time. It is pointed out that the over-current primarily intended is the short-circuit current occurring in connection with the protected object, for instance as a conse- quence of faults in the electrical insulation system of the protected object. Such faults mean that the fault current (short-circuit current) of the external network/equipment will tend to flow through the arc. The result may be a very large breakdown. It may be mentioned that for the Swed- ish power network, the dimensioning short-circuit current/fault current is 63 kA. In reality, the short-circuit current may amount to 40-50 kA.

A problem with said circuit breaker is the long break-time thereof. The dimensioning break-time (lEC-norm) for completely accomplished breaking is 150 milliseconds (ms). It is associated to difficulties to reduce this break-time to less than 50-90 ms depending upon the operational case. The consequence thereof is that when there is a fault in the protected object, a very high current will flow through the same during the entire time required for actuating the circuit breaker to break. During this time the full fault current of the external power network involves a considerable load on the protected object. In order to avoid damage and complete breakdown with respect to the protected object, one has, according to the prior art, constructed the object so that it manages, without appreciable damage, to be subjected to the short-circuit current/fault current during the break-time of the circuit breaker. It is pointed out that a short-circuit current (fault current) in the protected object may be composed of the own contribution of the object to the fault current and the current addition emanating from the network/equipment. The own contribution of the object to the fault current is not influenced by the function- ing of the circuit breaker but the contribution to the fault current from the network/equipment depends upon the operation of the circuit breaker. The requirements for constructing the protected object so that it may withstand a high short-circuit current/fault current during a considerable time period means substantial disadvantages in the form of a more ex- pensive design and reduced performance. OBJECT OF THE INVENTION

The object of the present invention is to devise ways to design the device and the method so that a better protection for the object is achieved and, accordingly, a reduced load on the same, a fact which means that the object itself does not have to be designed to withstand a maximum of short-circuit currents/fault currents during relatively long time periods.

SUMMARY OF THE INVENTION

According to the invention, the object indicated above is achieved in that there is provided in the line between the object and the first electric switch, in addition to the first mentioned electric switch, a second electric switch and that an over-current reducing arrangement, which is ac- tuatable for over-current reduction with the assistance of an arrangement detecting over-current conditions within a time period substantially shorter than the break-time of the first electric switch, is connected to the line between the two electric switches.

Thus, the invention is based upon the principle not to rely for breaking purposes only upon electric switches, which finally establish galvanic separation, but instead use a rapidly operating over-current reducing arrangement, which, without effecting any real breaking of the over-current, nevertheless reduces the same to such an extent that the protected object will be subjected to substantially reduced strains and, accordingly, a smaller amount of damages. The reduced over-current/fault current means, accordingly, that when the electric switches establish galvanic separation, the total energy injection into the protected object will have been much smaller than in absence of over-current reducing arrange- ments. First of all, it is established that the two electric switches are intended to be efficient circuit breakers, i.e. electric switches capable of independently effecting breaking of the line at the occurring voltages and currents. The first circuit breaker is required to switch off, on occurrence of a fault, that area of the plant, which suffers from the fault, from the plant for the rest. Also the second electric switch is valuable in many operational situations. If the detection arrangement would by mistake trig- ger the over-current reducing arrangement a situation is created, which is difficult in for instance such cases where the object is formed by a generator, which will be loaded with its internal short circuit current until the generator has been allowed to reduce its number of revolutions. By means of the second electric switch, the over-current reducing arrangement and, accordingly, the cause to the violent flow of current obtained in mistake may be switched away from the generator and the latter does not have to stop. This reduces the off-time of the generator and decreases the short circuit load.

If the over-current reducing arrangement would suffer from a dielectric breakthrough, the second electric switch involves a possibility to separate the over-current reducing arrangement from the object. Such a dielectric breakthrough could occur due to an excessively high applied voltage, e.g. from an adjacent lightning.

According to a particularly preferred embodiment of the invention, the over-current reducing arrangement is designed as comprising an over- current diverter for diverting the over-currents. Such diversion may occur to ground, to another phase conductor, or otherwise another unit having at least at times a lower potential than the network/equipment.

According to a particularly preferred embodiment of the invention, measures have been taken to obtain a reduction of the time period, during which the current already reduced by means of the over-current reducing arrangement may flow into the protected object. For this purpose the device comprises a further breaker arranged in the line between the first electric switch and the object, said further breaker being designed to break at a lower voltage and current than the first electric switch and therefore may be designed with a shorter break-time than the first electric switch as a consequence of a smaller need for movement and a smaller weight of the movable contact(s) of the breaker, said further breaker being arranged to break not until a time when the over-current towards or away from the protected object has been reduced by means of the over-current reducing arrangement. More specifically, the movement required of the movable contact(s) of the further breaker is smaller due to lower voltage whereas the weight of the contact(s) may be held lower due to the fact that the lower current does not require such large contact areas. The further breaker is also designed to break at a lower voltage and current than the second electric switch and has, accordingly, the same pre-requisites to break more rapidly than both of the first and second electric switches. The second electric switch is provided in the line between the object and the further breaker. This creates freedom to break away the fault from important plant parts during all occurring circumstances. The position of the fault could for instance make it impossi- ble to open the further breaker. If an internal fault in an object in the form of e.g. a generator occurs at a low point of the stator winding this may mean that the voltage that the further breaker must break away exceeds the performance of the further breaker. In such a case the further breaker cannot be opened. This means that the uncomfortable situation may arise that current may flow until the generator has stopped or been demagnetized. This presupposes of course that the further breaker, in order to achieve a rapid functioning, is constructed such that it is incapable of breaking when a full system voltage is present.

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

BRIEF DESCRIPTION OF THE DRAWINGS

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

In the drawings:

Fig 1 is a purely diagrammatical view illustrating the basic idea behind the solution according to the invention;

Figs 2a-2d are diagrams illustrating in a diagrammatical form and in a comparative way fault current developments and the energy development with and without the device according to the invention; Fig 3 is a diagrammatical view illustrating a conceivable design of the device according to the invention;

Fig 4 is a diagrammatical view illustrating a possible design of the over- current reducing arrangement;

Fig 5 is a diagrammatical view illustrating the device according to the invention applied in an electric power plant comprising a generator, a transformer and an electric power network connected thereto; and

Fig 6 is a view illustrating an alternative design of the device.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

An electric power plant comprising a protected object 1 is shown in Fig 1. The object could for instance consist of a generator. This object is connected, via a line 2, to an external distribution network 3. Instead of such a network, the unit denoted 3 could be formed by some other equipment contained in the electric power plant. The electric power plant here involved is conceived to be of such a nature that it is the object 1 itself which primarily is intended to be protected against fault currents from the network/equipment 3 when there occurs a fault in the object 1 giving rise to a fault current from the network/equipment 3 towards the object 1 so that the fault current will flow through the object. Said fault may consist in a short circuit having been formed in the object 1. A short circuit is a conduction path, which is not intended, between two or more points. The short circuit may for instance consist of an arc. This short circuit and the resulting violent current flow may involve considerable damage and even a total breakdown of the object 1.

It is already now pointed out that with at least some types of protected electrical objects 1 , short circuit currents/fault currents harmful to the object in question may flow from the protected object towards the net- work/equipment 3. Within the scope of the invention, it is intended to be used for protection purposes not only for protection of the object from externally emanating fault currents flowing towards the object but also from internal fault current in the object flowing in the opposite direction. This will be discussed in more detail in the following.

In the following, the designation 3 will, to simplify the description, always be mentioned as consisting of an external electric power network. However, it should be kept in mind that some other equipment might be involved instead of such a network, as long as said equipment causes a violent current flow through the object 1 when there is a fault.

A conventional circuit breaker 4 is arranged in the line 2 between the object 1 and the network 3. This circuit breaker comprises at least one own sensor for sensing circumstances indicative of the fact that there is an over-current flowing in the line 2. Such circumstances may be cur- rents/voltages but also other indicating that a fault is at hand. For instance, the sensor may be an arc sensor or a sensor recording short circuit sound etc. When the sensor indicates that the over-current is above a certain level, the circuit breaker 4 is activated for breaking of the connection between the object 1 and the network 3. The circuit breaker 4 must, however, break the total short circuit current/fault current. Thus, the circuit breaker must be designed to fulfil highly placed requirements, which in practice means that it will operate relatively slowly. In Fig 2a it is illustrated in a current/time-diagram that when a fault, for instance a short circuit in the object 1 , occurs at a time t_fauι_t, the fault current in the line denoted 2 in Fig 1 rapidly assumes the magnitude . This fault current is broken by means of the circuit breaker 4 at ti, which is at least within 150 ms after t_fauι_t. Fig 2d illustrates the diagram i² x t and, accordingly, the energy developed in the protected object 1 as a consequence of the short circuit therein. The energy injection into the object occurring as a consequence of the short circuit current is, accordingly, represented by the total area of the outer rectangle in Fig 2d.

It is in this connection pointed out that the fault current in Figs 2a-c and the currents in Fig 2d represent the envelope of the extreme value. Only one polarity has been drawn out in the diagram for the sake of simplicity. The circuit breaker 4 is of such a design that it establishes galvanic separation by separation of metallic contacts. Accordingly, the circuit breaker 4 comprises, as a rule, required auxiliary equipment for arc extinguishing.

According to the invention, the line 2 between the object 1 and the electric switch 4 is connected to an arrangement reducing over-currents towards the apparatus 1 and generally denoted 5. The arrangement is actuatable for over-current reduction with the assistance of an over-cur- rent conditions detecting arrangement within a time period substantially less than the break-time of the circuit breaker 4. This arrangement 5 is, accordingly, designed such that it does not have to establish any galvanic separation. Therefore, conditions are created to very rapidly establish a current reduction without having to accomplish any total elimina- tion of the current flowing from the network 3 towards the protected object 1. Fig 2b illustrates in contrast to the case according to Fig 2a that the over-current reducing arrangement 5 according to the invention is activated upon occurrence of a short circuit current at a time t_fauι_t for over-current reducing to the level i₂ at the time t₂. The time interval t_fauιt-t₂ represents, accordingly, the reaction time of the over-current arrangement 5. By the task of the arrangement 5 not to break but only reduce the fault current, the arrangement may be caused to react extremely rapidly, which will be discussed more closely hereunder. As an example, it may be mentioned that current reduction from the level to the level i₂ is intended to be accomplished within one or a few ms after unacceptable over-current conditions having been detected. It is then aimed at to accomplish the current reduction in a shorter time than 1 ms, and preferably more rapidly than 1 microsecond.

As appears from Fig 1 , the device comprises a further breaker generally denoted 6 and arranged in the line 2 between the circuit breaker 4 and the object 1. This further breaker is designed to break a lower voltage and current than the circuit breaker 4 and may, as a consequence thereof, be designed to operate with shorter break-times than the circuit breaker. The further breaker 6 is arranged to break not until after the over-current from the network 3 towards the object 1 has been reduced by means of the over-current reducing arrangement 5 but substantially earlier than the circuit breaker 4. From what is stated it appears that the further breaker 6 should be coupled to the line 2 in such a way that it is the current reduced by means of the over-current reducing arrangement 5 that will flow through the further breaker and, accordingly, is the one to be broken by means thereof.

Fig 2b illustrates the action of the further breaker 6. This breaker is, more specifically, designed to break at a time t₃, which means that the duration of the current i₂ reduced by means of the over-current reducing arrangement 5 is substantially delimited, namely to the time period t₂-t₃. The consequence is that the energy injection into the protected object 1 caused by a fault current from the network 3 is represented by the surfaces marked with oblique lines in Fig 2d. It appears that a drastic re- duction of the energy injection is achieved. In this connection it is pointed out that since, according to a specific model, the energy increases with the square of the current, a reduction to one half of the current reduces the energy injection to a fourth. It is illustrated in Fig 2c how the fault current will flow through the arrangement 5.

The dimensioning of the arrangement 5 and the further breaker 6 is conceived to be carried out such that the arrangement 5 reduces the fault current and the voltage to be broken by means of the further breaker 6 to substantially lower levels. A realistic break-time as to the further breaker 6 is 1 ms. However, the dimensioning should be made such that the breaker 6 is caused to break not until after the arrangement 5 having reduced the current flowing through the breaker 6 to at least a substantial degree. In addition to the first mentioned electric switch 4 there is also a second electric switch 27. This is likewise defined as a circuit breaker 27 and is coupled into the line 2 between the object and the further breaker 6. Also the electric switch 27 is conceived to have its own sensors for being brought to breaking when required.

It is illustrated in more detail in Fig 3 how the device may be realized. It is then pointed out that the invention is applicable in direct current (also

HVDC = High Voltage Direct Current) and alternating current connec- tions. In the latter case and in a multiphase arrangement, the line denoted 2 may be considered to constitute one of the phases in a multiphase alternating-current system. However, it should be kept in mind that the device according to the invention may be realized so that either all phases are subjected to the protection function according 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 reduction.

It appears from Fig 3 that the over-current reducing arrangement gener- ally denoted 5 comprises an over-current diverter 7 for diverting over- currents to ground 8 or otherwise another unit having a lower potential than the network 3. Thus, the over-current diverter may be considered as forming a current divider which rapidly establishes a short circuit to ground or otherwise a low potential 8 for the purpose of diverting at least a substantial part of the current flowing in the line 2 so that said current does not reach the object 1 to be protected. If there is a serious fault in the object 1 , for instance a short circuit, which is of the same magnitude as the short circuit that the over-current diverter 7 is capable of establishing, it may be said that generally speaking a reduction to one half of the current flowing to the object 1 from the network 3 is achieved as a consequence of the over-current diverter 7 in case the fault is close to the latter. In comparison with Fig 2b, it appears, accordingly, that the current level i₂ illustrated therein and being indicated to amount to approximately one half of i₁ may be said to represent the worst occurring case. Under normal conditions, the purpose is that the over-current diverter 7 should be able to establish a short circuit having a better conductivity than the one corresponding to the short circuit fault in the object 1 to be protected so that accordingly a main part of the fault current is diverted to earth or otherwise a lower potential via the over-current di- verter 7. It appears from this that, accordingly, in a normal fault case, the energy injection into the object 1 in case of a fault becomes substantially smaller than that which is illustrated in Fig 2d as a consequence of lower current level i₂ as well as shorter time span t₂-t₃.

The over-current diverter 7 comprises a closing means coupled between earth 8 or said lower potential and the line 2 between the object 1 and the network 3. This closing means comprises a control member 9 and a closing member 10. This closing member may for instance be formed by at least one semiconductor component, for instance a thyristor, which is open in a normal state, i.e. isolating in relation to ground, but via the control member 9 may be brought into an active, conducting state in a very short time in order to establish current reduction by diversion to ground.

Fig 3 also illustrates that an over-current conditions detecting arrange- ment may comprise at least one and preferably several sensors 11-13 suitable for detecting such over-current situations requiring activation of the protection function. As also appears from Fig 3, these sensors may include a sensor denoted 13 and located in the object 1 or in its vicinity. Furthermore, the detector arrangement comprises a sensor 11 adapted to sense over-current conditions in the line 2 upstream of the connection of the over-current reducing arrangement 5 and the line 2. As is also explained in the following, it is suitable that a further sensor 12 is provided to sense the current flowing in the line 2 towards the object 1 to be protected, i.e. the current which has been reduced by means of the over- current reducing arrangement 5. In addition, it is pointed out that the sensor 12, as well as possibly the sensor 13, is capable of sensing the current flowing in the line 2 in a direction away from the object 1 , for instance in cases where energy magnetically stored in the object 1 gives rise to a current directed away from the object 1.

It is pointed out that the sensors 11-13 do not necessarily have to be constituted by only current and/or voltage sensing sensors. Within the scope of the invention, the sensors may be of such nature that they generally speaking may sense any conditions indicative of the occurrence of a fault of the nature requiring initiation of a protection function.

In cases where such a fault occurs that a fault current will flow in a direction away from the object 1 , the device is designed such that the control unit 14 thereof controls the further breaker 6 to closing, in case it would have been open, and in addition the over-current reducing arrangement 5 is activated such that the short circuit current may be diverted by means of the same. When, for example, the object 1 is conceived to consist of a transformer, the function on occurrence of a short circuit therein could be such that the short circuit first gives rise to a violent flow of current into the transformer, which is detected and gives rise to acti- vation of the arrangement 5 for the purpose of current diversion. When the current flowing towards the transformer 1 has been reduced in a required degree, the breaker 6 is caused to break, but, controlled by means of the control unit 14, not earlier than leaving time for the energy, in occurring cases, magnetically stored in the transformer 1 to flow away from the transformer 1 and be diverted via the arrangement 5.

Furthermore, the device comprises a control unit generally denoted 14. This is connected to the sensors 11-13, to the over-current reducing arrangement 5 and to the further breaker 6. The operation is such that when the control unit 14 via one or more of the sensors 11-13 receives signals indicating occurrence of unacceptable fault currents towards the object 1 , the over-current reducing arrangement 5 is immediately controlled to rapidly provide the required current reduction. The control unit 14 may be arranged such that when the sensor 12 has senses that the current or voltage has been reduced to a sufficient degree, it controls the breaker 6 to obtain operation thereof for breaking when the over-current is below a predetermined level. Such a design ensures that the breaker 6 is not caused to break until the current has really been reduced to such a degree that the breaker 6 is not given the task to break such a high current that it is not adequately dimensioned for that purpose. However, the embodiment may alternatively also be such that the breaker 6 is controlled to break a certain predetermined time after the over-current reducing arrangement having been controlled to carry out current reduction.

The circuit breaker 4 may comprise a detector arrangement of its own for detection of over-current situations or otherwise the circuit breaker may be controlled via the control unit 14 based upon information from the same sensors 11-13 also controlling the operation of the over-current reducing arrangement. It is illustrated in Fig 3 that the further breaker 6 comprises a switch 15 having metallic contacts. This switch 15 is operable between breaking and closing positions by means of an operating member 16, which in turn is controlled by the control unit 14. A shunt line 17 is connected in parallel over this switch 15, said shunt line comprising one or more components 18 intended to avoid arcs on separation of the contacts of the switch 15 by causing the shunt line 17 to take over the current conduction from the contacts. These components are designed so that they may break or restrict the current. Thus, the purpose is that the compo- nents 18 should normally keep the conduction path in the shunt line 17 interrupted but close the shunt line when the switch 15 is to be opened so that accordingly the current is shunted past the switch 15 and in that way arcs do not occur or possibly occurring arcs are efficiently extinguished. The components 18 comprise one or more associated control members 19 connected to the control unit 14 for control purposes. According to one embodiment of the invention, said components 18 are controllable semiconductor components, for instance GTO thyristors, having necessary surge arresters 30.

A disconnector 20 for galvanic separation in the current conduction path created by means of the shunt line 17 to the object 1 to be protected is arranged in series with said one or more components 18. This disconnector 20 is via an operating member 21 controlled by the control unit 14. The disconnector 20 is illustrated in Fig 3 as being placed in the shunt line 17 itself. This is of course not necessary. The disconnector 20 could also be placed in the line 2 as long as it ensures real galvanic separation, by series coupling with said one or more components 18, in the conduction path established by means of said series coupling so that accordingly there is not any possibility for current to flow through the components 18.

The device as it has been described so far operates in the following manner: in absence of a fault, the circuit breaker 4 is closed just like the switch 15 of the further breaker 6. The components 18 in the shunt line 17 are in a non-conducting state. The disconnector 20 is closed. Finally, the closing means 10 of the over-current reducing arrangement 5 is open, i.e. it is in a non-conducting state. In this situation the closing means 10 must of course have an adequate electrical strength so that it is not inadvertently brought into a conducting state. Over-voltage conditions occurring in the line 2 as a consequence of atmospheric (lightning stroke) circumstances or coupling measures may, accordingly, not involve the voltage strength of the closing means 10 in its non-conducting state to be exceeded. For this purpose it is suitable to couple at least one surge arrester 22 in parallel with the closing means 10. In the example, such surge arresters are illustrated on both sides of the closing means 10. Accordingly, the surge arresters have the purpose to divert such over-voltages, which could otherwise involve risk for inadvertent breakthrough in the closing means 10.

When an over-current state has been registered by means of some of the sensors 1 1-13 or the own sensor (it is of course realized that information from the own sensor of the circuit breaker 4 may be used as a basis for control of the over-current reducing arrangement 5 according to the invention) of the circuit breaker 4 and this over-current state is of such magnitude that a serious fault of the object 1 is expected to be at hand, a breaking operation is initiated as far as the circuit breaker 4 is concerned. In addition, the control unit 14 controls the over-current reducing arrangement 5 to effect such reduction, and this more specifically by bringing, via the control member 9, the closing means 10 into an electrically conducting state. As described before, this may occur very rapidly, i.e. in a fraction of the time required for breaking by means of the circuit breaker 4, for what reason the object 1 to be protected is immediately liberated from the full short circuit current from the network 3 as a consequence of the closing means 10 diverting at least an essential part, and in practice the main part, of the current to ground or otherwise lower potential. As soon as the current, which flows towards the object 1 via the further breaker 6, has been reduced in a required degree, which can be established on a pure time basis by a time difference between activation of the closing means 10 and operation of the breaker 6, or by sensing of the current flowing in the line 2 by means of, for instance, the sen- sor 12, the operating member 16 of the switch 15 is, via the control unit 14, controlled to open the contacts of the switch. For extinguishing or avoiding arcs, the components 18, e.g. GTO thyristors or gas switches, are via the control members 19 controlled to establish conductivity of the shunt line 17. When the switch 15 has been opened and, thus, provided galvanic separation, the component 18 is again controlled to bring the shunt line 17 into a non-conducting state. In that way the current from the network 3 towards the object 1 has been efficiently cut off. After having brought the shunt line 17 into a non-conducting state, galvanic separation may, in addition, be effected by means of the disconnector 20 by controlling the operating member 21 thereof from the control unit 14. When all these incidents have occurred, breaking by means of the circuit breaker 4 occurs as a last incident. It is important to note that the over- current reducing arrangement 5 as well as the further breaker 6 according to a first embodiment can be operated repeatedly. Thus, when it has been established by means of the sensors 11-13 that the circuit breaker 4 has been brought to cut off, the closing means 10 is reset to a nonconducting state and the switch 15 and the disconnector 20 are again closed so that when the circuit breaker 4 closes next time, the protection device is completely operable. According to another embodiment, it is, however, contemplated that the over-current reducing arrangement 5 may require exchange of one or more parts in order to operate again.

It is pointed out that according to an alternative embodiment of the invention, the component or components 18 could be brought into a conducting state as soon as the over-current reducing arrangement 5 has been brought into a closing state, and this independently of whether the switch 15 is not possibly opened thereafter. The control of the components 18 could then, as described before, occur via the control unit 14 or, alternatively, by means of a control function involving a slavish following of the closing of the arrangement 5.

Fig 4 illustrates an alternative embodiment of the over-current reducing arrangement 5. Instead of relying on a semiconductor closing means as in Fig 3, the embodiment according to Fig 4 is intended to involve causing of a medium present in a gap 24 between electrodes 23 to assume electrical conductivity by means of a control member 9a. This control member is arranged to control the operation of members 25 for causing or at least initiating the medium or a part thereof in the gap 24 into a conducting state. Said members 25 are in the example arranged to cause the medium in the gap 24 to assume electric conductivity by causing or at least assisting in causing the medium to ionization/plasma. It is preferred that the members 25 comprise at least one laser, which by energy supply to the medium in the gap 24 provides for the ionization. As appears from Fig 4, a mirror 26 may be used for required deflection of the laser beam bundle. It is in this connection pointed out that the embodiment according to Fig 4 may be such that the members 25 do not alone give rise to ionization/plasma in the entire electrode gap. Thus, the intention may be that an electric field imposed over the gap should contribute in ionization/plasma formation, only a part of the medium in the gap being ionized by means of the members 25 so that thereafter the electric field in the gap gives rise to establishment of plasma in the entire gap. It is in this connection pointed out that there may be in the electrode gap not only a medium consisting of various gases or gas mixtures but also vacuum. In the case of vacuum, initiation by means of laser occurs at at least one of the electrodes, which, accordingly, will function as an electrone and ion transmitter for establishment of an ionized environ- ment a plasma in the electrode gap.

Fig 5 illustrates an embodiment where a generator 1 b is coupled to an electric power network 3a via a transformer 1a. The objects to be protected are, accordingly, represented by the transformer 1a and the gen- erator 1 b. The over-current reducing arrangement 5a and the further breaker 6a and the ordinary first circuit breaker 4a are, as can be seen, arranged similarly to what appears from Fig 1 for the case that the object 1 shown therein is conceived to form the object 1a according to Fig 5. Accordingly, reference is in this regard made to the descriptions delivered with respect to Fig 1. The same is due for the protection function of the over-current reducing arrangement 5c and the further breaker 6c with respect to the generator 1 b. In this case the transformer 1a could, accordingly, be considered equivalent to the object 1 in Fig 1 whereas the transformer 1a could be considered equivalent to the equipment 3 in Fig 1. Thus, the over-current reducing arrangement 5c and the further breaker 6c will, in combination with the conventional first circuit breaker 4b, be able to protect the generator 1b against violent flow of current in a direction away from the transformer 1a.

As an additional aspect in Fig 5, the additional over-current reducing ar- rangement 5b with associated further breaker 6b is present. As can be seen, there will be over-current reducing arrangements 5a and 5b on either side of the transformer 1a. It is then pointed out that the respective further breakers 6a and 6b are arranged in the connections between said over-current reducing arrangements 5a and 5b and the transformer 1a. The further over-current reducing arrangement 5b is intended to protect the transformer 1 a from current flows towards the transformer from the generator 1 b. As can be seen, the first circuit breaker 4b will be able to break independently of in which direction between the objects 1a and 1b the protection function is desired. Furthermore, there occurs in Fig 5 other circuit breakers 27a-c.

Above it has in the first place been described that closing means 10 provide for connection to ground. This is not critical to the invention. In e.g. Fig 6 an embodiment is illustrated, where closing means 10 ac- cording to the invention are connected not between phase and ground as before but between two phase conductors R, S and S, T respectively. Such a short circuit between phases is equivalent to grounding of all three phases. It also appears from Fig 6 that there are, in addition to the first electric switches 4, second electric switches 27 in the three phases. It should be observed that when short-circuiting occurs between phases, the further breakers 6 in the previous embodiment might be omitted.

It should be noted that the description presented hereinabove only should be considered as exemplifying for the inventive idea, on which the invention is based. Thus, it is obvious to a man skilled in the art that detail modifications may be made without leaving the scope of the invention. As an example, it may be mentioned that it would be possible to use as the closing means 10 a mechanical closing means.

Claims

1. A device in an electric power plant for protection of an object (1) connected to an electric power network (3) or another equipment included in the electric power plant from fault-related over-currents, the device comprising an electric switch (4) in a line (2) between the object and the network/equipment, characterized in that in addition to the first mentioned electric switch there is provided, in the line (2) between the object (1) and the network/equipment (3), a second electric switch (27) and that to the line between the two electric switches (4, 27) is connected an over- current reducing arrangement (5), which is actuatable for over-current reduction with assistance of an over-current conditions detecting arrangement (11-13) within a time period substantially shorter than the break-time of the first electric switch (4).

2. A device according to claim 1 , characterized in that the electric switches (4, 27) are formed by circuit-breakers.

3. A device according to claim 1 or 2, characterized in that both electric switches are capable on their own to break the line with occurring voltages and currents.

4. A device according to any preceding claim, characterized in that the over-current reducing arrangement (5) comprises an over-current di- verter (7) for diverting over-currents from the line (2) to another unit.

5. A device according to claim 4, characterized in that said other unit is ground (8), another phase conductor than the line or otherwise another unit having at least at times a lower potential than the net- work/equipment.

6. A device according to claim 3, characterized in that the over-current diverter (7) comprises a closing means (10).

7. A device according to claim 6, characterized in that the closing means (10) comprises at least one semiconductor component.

8. A device according to claim 6, characterized in that the switch (10a) comprises an electrode gap (24) and means (25) for causing or at least initiating the electrode gap or at least a part thereof to assume electric conductivity.

9. A device according to claim 8, characterized by said means (25) for causing or at least initiating the electrode gap to assume electrical conductivity being arranged to cause the gap or a part thereof to assume the form of a plasma.

10. A device according to claim 9, characterized by said members (25) for causing or at least initiating the electrode gap or a part thereof to assume electrical conductivity comprising at least one laser.

11. A device according to any preceding claim, characterized in that it comprises a further breaker (6) arranged in the line between the first electric switch (4) and the object, said further breaker being arranged between the over-current reducing arrangement (5) and the object (1) and being adapted to break lower voltages and currents than the first electric switch (4) and therefore capable of performing a shorter break- time than the first electric switch, and that the further breaker is adapted to break when the over-current towards or away from the object (1 ) has been reduced by means of the over-current reducing arrangement (5) but substantially earlier than the first electric switch.

12. A device according to claim 11 , characterized in that the second electric switch (27) is provided between the object (1 ) and the further breaker (6).

13. A device according to claim 11 , characterized in that it comprises a control unit (14) connected to the detecting arrangement (11-13) and to the further breaker (6) in order to achieve actuation of the further breaker for breaking purposes when the over-current towards or away from the object (1 ) is indicated, by means of the detecting arrangement, to be below a predetermined level.

14. A device according to any of claims 11-13, characterized in that the further breaker (6) comprises a switch (15), over which there is coupled a shunt line (17) having one or more components (18) for avoiding arcs on separation of contacts of the switch (15) by causing the shunt line (17) to take over current conduction from the contacts.

15. A device according to claim 14, characterized in that said one or more components (18) in the shunt line (17) are closeable into conduc- tion by means of control via the control unit (14).

16. A device according to claim 14 or 15, characterized in that said one or more components (18) are formed by controllable semiconductor components.

17. A device according to any of claims 14-16, characterized in that said one or more components (18) are provided with at least one surge arrester (30).

18. A device according to any of claims 14-17, characterized in that a disconnector (20) for galvanic separation is arranged in series with said one or more components (18).

19. A device according to claim 18, characterized in that the discon- nector (20) is coupled to the control unit (14) to be controlled thereof for opening after the switch (15) having been controlled to have closed and said one or more components (18) having been placed in a condition for breaking the shunt line (17).

20. A device according to any preceding claim, characterized in that at least one surge arrester (22) is coupled in parallel with the over-current reducing arrangement (5).

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

22. A device according to claim 21, characterized in that the object is formed by a generator, transformer or motor.

23. A device according to any of claims 1-20, characterized in that the object is formed by a power line, e.g. a cable.

24. A device according to any preceding claim, characterized in that two over-current reducing arrangements are provided on either sides of the object to protect the same double-sidedly.

25. A device according to claim 1 , characterized in that it comprises a control unit (14) connected to the over-current reducing arrangement (5) and to the arrangement (11-13) detecting over-current conditions, said control unit being adapted to control the over-current reducing arrangement to closing when motivated by reasons of protection and with assistance of information from the arrangement (11-13) detecting over-current conditions.

26. A device according to claim 25 and one or more of the claims 13, 15 and 19, characterized in that one and the same control unit (14) is adapted to control, based on information from the arrangement (11-13) detecting over-current conditions, the over-current reducing arrangement (5) and the further breaker (6).

27. Use of a device according to any preceding claim for protection of an object against fault-related over-currents.