WO2001095355A1

WO2001095355A1 - A method and an apparatus for opening a current path

Info

Publication number: WO2001095355A1
Application number: PCT/SE2001/001264
Authority: WO
Inventors: Per Larsson; Magnus Backman; Lars Liljestrand
Original assignee: Abb Ab
Priority date: 2000-06-07
Filing date: 2001-06-07
Publication date: 2001-12-13
Also published as: SE0002127D0; AU6447801A

Abstract

A device for obtaining breaking of the current in a current path (2) for an alternating current with a plurality of phases and a dc-component of at least one phase current, i.e. displacement of the symmetry line of the phase current with respect to the zero-level thereof, through opening of an electric switching device (1) arranged in a part current path for each phase and thereby breaking the current in the current path after occurrence of a fault current in the current path, comprises members (15) adapted to detect the current in the part current path of each phase and a unit (19) adapted to control the electric switching devices to break the current in the respective part current path at a point of time being dependent upon the result of the current detection.

Description

A method and an apparatus for opening a current path

FIELD OF THE INVENTION AND PRIOR ART

The present invention relates to an apparatus for obtaining breaking of a current in a current path for an alternating current with a plurality of phases and a dc-component of at least one phase current, i.e. displacement of the symmetry line of the phase current with respect to the zero-level thereof, through opening an electric switching device arranged in a part current path for each phase and thereby breaking the current in the current path after occurrence of a fault current in the current path.

Such apparatuses and methods are applicable to all types of fields of use of an electric switching device for breaking a cur- rent in a current path upon occurrence of fault currents, such as for example in switch gears for electricity supply within the industry or in distribution or transmission networks. When such a fault current emanating from a short circuiting occurs, it is important that the electric switching device on one hand open so that the current is broken as soon as possible for not damaging different types of equipment connected to the current path and making the damage in the fault position too great, but it is on the other absolutely necessary that the alternating current may change direction, i.e. has a zero-crossing, before it is broken. However, the alternating voltage receives upon occurrence of said fault usually a direct current component (dc-component), the magnitude of which being dependent upon the phase position of the alternating current at the point of time for the occurrence of the fault, and this dc-component superposes the alternating current, which in the worst case means that it may take several periods of the alternating current before any zero- crossing occurs in the case that the amplitude of the alternating current decreases, as upon short circuiting of synchronous machines.

"Electric switching device" is to be given a broad sense and does not only cover such ones with a mechanical movement of different parts for obtaining an opening through physical separation of two parts in a current path, but also semiconductor devices, such as IGBT-s or the like, opening by going to blocking state and thereby breaking the current therethrough. "Electric switching device" also comprises so-called transfer switches through which then a current in a current path may be broken upon occurrence of a fault current in the current path for switching in another current path instead to a load or the like.

It is illustrated in Fig 1 how the voltage U and the current I in the current path in the case of a purely inductive load are displaced 90° with respect to each other. We now assume that a short circuiting along the current path occurs at the point of time t-i , when the voltage is a maximum and the current zer-O, This means then that a current symmetric with respect to the zero- line after the fault and having a certain ac-decay, i.e. an amplitude decreasing with the time, is obtained. However, should the short circuiting occur at the point of time t₂, when the voltage U is zero and the current I is a maximum, a maximum asymmetry is obtained, i.e. the dc-component of the current l₂ will be a maximum. This dc-component has also a decay over the time. However, should this decay of the dc-component be slower than the initially existing ac-decay it may take a not negligible time before a zero-crossing is obtained and a breaking of the fault current may be achieved. This problem is accentuated in uses where said ac-decay is important, such as when arranging such an electric switching device for breaking the current in a current path to synchronous machines, both generators and motors, so that the invention is particularly, but not at all exclusively, di- rected to such applications.

Breaking the current in the current path upon occurrence of a fault therealong and thereby a fault current therein has so far been achieved through a predetermined time delay after occur- rence of the fault current. All phases, normally three, have then been simultaneously opened and the delay has been chosen to be sufficiently long for taking the worst case, i.e. a maximum asymmetry of the alternating current. This means that the time delay in most cases will be far too long with unnecessary stresses on electric equipment connected to the current path. Should the time delay in some cases be too short this would have fatal consequences in the form of an arching time being too long for a mechanical switching device for the switching device in connection with the breaking, which results in a break- down of the switching device which has to be replaced.

SUMMARY OF THE INVENTION

The object of the present invention is to provide an apparatus and a method of the type defined in the introduction,-=which makes it possible to to a large extent find a remedy to the inconveniences mentioned above of such already known apparatuses and methods.

This object is according to the invention obtained by providing such an apparatus, which comprises members adapted to detect the current in the part current path of each phase and a unit adapted to control the electric switching devices to break the current in the respective part current path at a point of time be- ing dependent upon the result of the current detection, as well as a method according to the appended independent method claim.

The detection of the current in each part current path and the control of the electric switching device in dependence of the detection makes it possible in most fault cases to obtain a considerably earlier breaking of the current in said current path than should no such detection have taken place. This is also valid in the case one waits until the detection through predicted or measured zero-crossings in the different phases shows that it is possible to open all switching devices and this is made simultaneously.

According to another preferred embodiment of the invention the unit is adapted to control the electric switching devices separately to break the current in the respective part current path at a point of time being dependent upon the result of the current detection.

By detecting the current in each part current path and make the point of time for the breaking of the current in each individual part current path dependent upon the result of the current detection in this way the breaking of the current in each individual part current path may take place at the point of time being the - ost suitable in the respective fault case. Unnecessarily long time delays for breaking the current in the current path are hereby avoided, and instead they get only as long as each individual case requires. This means that the damage at the fault location and in electrical equipment connected to the current path may be reduced.

According to a very preferred embodiment of the invention the unit is adapted to control the electric switching devices separately to break the current in the part current paths in an order determined in dependence of the result of the current detection. It has been found that in some cases the order in which the cur- rent in the part current paths is broken may have an influence upon how fast the breaking procedure may be carried out and thereby how great the stresses on the fault location and said equipment will be. This means that the breaking procedure may be improved by choosing the most suitable order for breaking the current in the different part current paths.

According to another preferred embodiment of the invention, which constitutes a further development of the embodiment last discussed, the unit is adapted to first break the phase current of a part current path for a phase having a lower dc-level, i.e. displacement of the symmetry line of the phase current with respect to the zero-level thereof, than the phase current of at least another part current path. Such an order is particularly advanta- geous in fault cases having a considerable ac-decay and a large asymmetry, i.e. dc-component of any of the phases. By at this point breaking the phase current which does not have the highest dc-level when this is allowed, i.e. when this has zero-crossings, a phase current having a larger asymmetry may be posi- tively influenced in the sense that the dc-level thereof may be reduced. This makes it possible to reduce the time delay for breaking that phase current and the entire breaking procedure may thereby be shortened. This is particularly the case for all synchronous machines, since they have a zero-point not being directly connected to ground and breaking of a phase influences thereby the other phases, so that a forced zero-crossing may be obtained for the phase current of a phase having a larger asymmetry than the phase broken.

According to another preferred embodiment of the invention applicable to an apparatus for obtaining breaking of a current in a current path for an alternating current having three phases, a unit is adapted to firstly break the phase current having the second lowest dc-level of the phase current determined by said detection. By firstly breaking this intermediate phase current a maximum influence upon the two remaining phases in a positive direction is obtained, since this means a lower sum of the dc- components thereof than if the most symmetric phase current would have been broken first. The symmetry will in this case be considerably better, i.e. the dc-level will be considerably re- duced, for the most asymmetric phase current, so that a zero- crossing may be obtained for this and this may be broken considerably earlier, while the asymmetry will increase somewhat for the most symmetric phase current.

According to another preferred embodiment of the invention the control unit is adapted to control the breaking of the current in the part current path for the respective phase after detecting at least one zero-crossing of the alternating current for the phase in question. This means that there is no risk of a too early at- tempt to break the current in any of the phases, at the same time as it is possible by obtaining breaking of a detected or predicted zero-crossing to break the phase current in question as early as possible.

According to another preferred embodiment of the invention said current detection members are adapted to deliver the value of the alternating current of two succeeding current peaks to the unit, and the unit is adapted to form an average of these two current values for use as said dc-level when determining said order. This constitutes a very simple but nevertheless reliable way to determine the relations between the dc-levels of the different phases for breaking the respective phase at a suitable point of time.

According to another preferred embodiment of the invention the apparatus is adapted to obtain an opening of a current path for an alternating current with an electric switching device for each phase, said electric switching device having two branches connected in parallel in the current path, the first one of which com- prising a first contact member with two contacts movable with respect to each other for opening and closing and the second comprising a part able to block current therethrough in at least a blocking direction and conduct current therethrough in at least one direction, in which a second contact member having two contacts movable with respect to each other for opening and closing is connected in series with said part, said switching device also comprising a unit adapted to control breaking of the respective current path at a point of time being dependent upon the result of the current detection by controlling the first contact member to open for transferring the current to said part when this is in or going into a conducting state and then the second contact member to open when the part is in a state of blocking current therethrough for making the breaking of the current through the switching device permanent. An electric switching device of this type is described in the Swedish patent applica- tion 9904164-2, not yet available to the public, of the applicant, and a so called hybrid breaker of this type may particularly well be separately controlled for each phase. This is also the case for an apparatus according to the appended claim 18, which has electric switching devices of the type described in the Swedish patent application 9904166-7, not yet available to the public, of the applicant.

According to a preferred embodiment of the invention the apparatus comprises an electrically controlled driving member adapted to obtain opening of the respective electric switching device, and it is particularly advantageous if this driving member is an electromagnetic machine in the form of an electric motor. By using such a driving member it will be possible to very accurately control the movement of one or more movable parts of the respective switching device for opening thereof and for example ensure that a separation of two contacts takes place in a quite particular phase position of the alternating current. By the fact that the control unit of the apparatus in the form of an electronic unit is adapted to control the driving member it is possible to obtain an accurate co-ordination of the different electric switching devices and breaking of the respective phase current. The invention also relates to a use of an apparatus according to the invention for obtaining a breaking of a current in a current path connected to a synchronous machine. This application of an apparatus according to the invention is particularly advantageous, since the ac-decay may be considerable after occurrence of a fault in a synchronous machine, and it is through the zero- point being not directly connected to ground for such a machine possible to influence the phases not yet switched in by breaking one phase.

The invention also relates to an arrangement, computer program as well as a computer program product according to the corresponding appended claims. It is easily understood that the method according to the invention defined in the appended set of method claims is well suited to be carried out through program instructions from a processor that may be influenced by a computer program provided with the program steps in question. Although not explicitly expressed in the claims, the invention comprises such arrangements, computer programs and computer program products combined with a method according to any of the appended method claims.

Further advantages as well as advantageous features of the in- vention appear from the following description and the other dependent claims.

BRIEF DESCRIPTION OF THE DRAWINGS

With reference to the appended drawings, below follows a description of a preferred embodiment of the invention cited as an example.

In the drawings: Fig 1 is a graph illustrating the development of the voltage U and the current I over time to the point of time for occurrence of a short circuiting along a current path and after such a short circuiting for two different points of time for the short circuiting,

Fig 2 is a graph corresponding to Fig 1 illustrating the development of the current of three different phases of a three-phase alternating current after occurrence of a short circuiting,

Fig 3 illustrates the development of the currents over time in three different phases of a three-phase alternating current to a synchronous machine after occurrence of a short circuiting,

Fig 4 illustrates what is happening with the phase currents ac- cording to Fig 3 when one phase current is broken,

Figs 5-7 are simplified views illustrating an apparatus for controlling an electric switching device for alternating current arranged in a current path according to a preferred embodiment of the invention, and

Figs 8-10 are simplified circuit diagrams illustrating an apparatus for controlling an electric switching device for alternating current arranged in a current path in a closed, a temporarily closed and an open position, respectively. —--

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION

The construction and the function of a hybrid breaker of the type described in the previously mentioned Swedish patent application 9904164-2 will first of all be briefly described for making it easier to understand the invention and since the invention has particular advantages for exactly such an electric switching de- vice. The description is made with reference to Figs 5-7. The electric switching device 1 is connected in a current path 2 for being able to break a current therein by opening or closing thereof. One such switching device is arranged per phase, so that a three-phase network has three such switching devices on one and the same location. The switching device has an inner cylinder 3, which may be rotated around an axle 4 and has a movable contact part 5. Another cylinder 6 is arranged externally of the cylinder 3 and has four contacts 7-10 arranged along the movement path of the movable part 5 and adapted to form good electric contacts when applied against the movable part 5. The switching device is connected in the current path through the two outer contacts 7 and 10, respectively.

A semiconductor device in the form of a diode 1 1 , 12 is con- nected between the two outer contacts and the adjacent inner contact next thereto and has the conducting direction from the outer to the adjacent contact. The diodes could just as well both be directed with a conducting direction towards the outer contact.

The switching device has also a driving arrangement adapted to drive the inner cylinder 3 to rotate for movement of the movable contact part 5 with respect to the other contacts 7-10. The driving arrangement is in this case constituted by an integrated electric motor 13 schematically indicated, which may be of-many different types.

An apparatus 14 according to the invention for controlling the electric switching device is connected thereto. The apparatus has members 15 schematically indicated and adapted to detect the current in the current path by detecting the direction and the magnitude thereof and thereby also detecting the point of time for a zero-crossing of the current. The detecting members are adapted to send signals with information about the current fur- ther to an analogous/digital-converter 16 for converting the analogous signals to digital signals. Filters 17, 18 are arranged in the signal path before and after the converter for filtering out noise signals, especially high frequency noise signals, from the signals from the detecting members 15. The current information is sent further to a unit 19 in common for three such electric switching devices, one for each phase of a three-phase alternating current. The unit is adapted to control opening of the electric switching devices based on the information from the detecting members separately to break the current in the respective part current path at a point of time being dependent upon the result of the current detection, and more exactly to break the current in the current paths in an order determined in dependence of the result of the current detection. How this order is established in the practice and the result thereof will be discussed further below with reference to Figs 2 and 3. By using an electrically controlled driving member 13 in the form of an electric motor the movement of the movable part 5 of the respective switching device may be controlled very accurately. This in combination with utilizing a control unit 19 in the form of an electronic unit makes it possible to well co-ordinate the function of the different electric switching devices so as to obtain breaking of the phase currents in the order desired.

The function of a switching device of the type illustrated appears more in detail from the Swedish patent application 9904164-2 mentioned above, but it will here be briefly summarized: When a desire to break the current in the current path 2 occurs, for example by the fact that the detecting members 15 detect a very high current in the current path 2, which may be caused by a short circuiting therealong, it would then be possible to detect the direction of the alternating current and make the rotation direction of the cylinder 3 and thereby the movable contact part 5 depending thereupon for obtaining the quickest possible breaking, but a very high accuracy at the very breaking is givien priority with respect to being as quick as possible. In the closed position according to Fig 4 the entire current through the switching device flows between the two outer contacts 7, 10 through the movable part 5 interconnecting them galvanically. We assume that a decision has been made to carry out the breaking through rotating the inner cylinder 3 clockwise as seen in Fig 1 . This shall then preferably be made so that an opening of the contact member formed by the contacts 7 and 8 is made through a zero-crossing of the current, so that this may take place without forming any remarkable arc. This shall take place when the diode is going to be forward biased, so that the current is then switched over to the diode 1 1 instead.

Then when the current across the switching device changes direction no current will flow therethrough, but a voltage will be built up across the diode 1 1 then reverse biased and the rotation movement of the movable contact member 5 is now continued in the same direction as before, so that the galvanic connection between the contact 8 and the contact 10 is broken and the breaking of the current is thereby made permanent. This may take place without any arcing, since no current is then flowing through the contact position. The completely open position according to Fig 7 is thereby obtained, in which the current is permanently broken.

The general construction of an electric switching device according to the Swedish patent application 9904166-7 mentioned above is schematically illustrated in Fig 8, and this switching device is connected in a current path 2 for rapidly open and close it. One such switching device is arranged per phase, so that a three-phase network has three such switching devices on one and the same location. The switching device comprises two branches 21 , 22 connected in parallel in the current path and having each at least two mechanical contact members 23-26 connected in series. A semiconductor device 27 in the form of a diode is adapted to connect the midpoints 28, 29 between the two contact members of each branch to each other. An apparatus 14 according to the invention for controlling the electric switching device is connected thereto and the construction thereof is the same as described above for the embodiment according to Figs 5-7.

The function of this electric switching device is as follows: When a desire for breaking the current in the current path 2 occurs, for example by a detection of a very high current in the current path by the detecting member 15, which may be caused by a short circuiting therealong, it is determined through the result of the detection in the way described above when it is suitable to break the current through the respective electric switching device. Once it has been determined that a given electric switching device is to be opened, the control unit 19 then firstly decides which two contact members, here the contact members 24 and 25 (see Fig 9), are to be opened for establishing a temporary current path through the semiconductor device 27. Thus, this decision depends upon the position of the current in the current path at that moment. In the position according to Fig 8 the entire current through the switching device flows through the two branches 21 , 22 and nothing through the diode. When now breaking is to take place the current must as quick as possible be transferred to flow through the diode instead. The current may be switched in to the diode from a certain direction during that part of an alternating current period that is located between the point of time just before the diode gets forward biased in that direction and the point of time when the diode gets reverse biased next time. This means for a full period of 20ms in the practice that an opening of the contact members according to Fig 9 may take place for example about 2ms before a zero- crossing towards the forward biased direction and until the next zero-crossing. When the wrong half-period of the alternating voltage for an opening of the contact members 24 and 25 according to these premises exists, the contact members 23 and 26 may instead be immediately opened for establishing that temporary current path instead. Accordingly, this temporary current path may be established immediately after detecting the need of and possibility of opening the switching device for closing the current therethrough.

When the temporarily closed position illustrated in Fig 9 is obtained through opening of the contact members 24, 25 a small spark is created in the gap between the contacts of the respective contact member, which results in a voltage of usually 12-15 V, which will drive a transfer of the current through the diode 27. When then the current over the switching device changes direction no current will flow therethrough, but a voltage will be built up across the diode 27 then reversed biased, and one of the two other contact members 23, 26 is now opened, so that the temporary current path is opened, in which this opening takes place without any arcing, since no current flows through the contact place at the opening moment. The completely open position of the switching device shown in Fig 10 is thereby obtained, in which the current therethrough is permanently broken. It is in this terminating opening important that it takes place so quick that the voltage across the diode 27 will not have time to change direction again and this starts to conduct. The utilizing of the same semiconductor device in the temporary current path independently of in which direction the current flows through the switching device enables large savings of costs by a substantial reduction of the number of semiconductor devices with respect to switching devices of this type already known.

It is illustrated in Fig 2 what is happening with the phase currents of three different phases of a symmetric three-phase alter- nating current, accordingly with 120 electrical degrees between each phase, when a short circuiting occurs at a point of time t₀. It appears that the different phase currents will receive different size dc-components depending upon the position of the voltage U of the phase at the point of time for the short circuiting. It ap- pears that this direct voltage component has a decay over time, at the same time as the amplitude of the alternating current decreases over time. The phase current of the second phase S is almost symmetric and could be broken comparatively early, while the current of the first phase R has a large asymmetry, which delays the point of time for a suitable breaking. In a great ac-decay, such as for synchronous machines, it is even possible that it takes many periods of the alternating current before any zero-crossing is obtained of the phase current with the largest asymmetry. However, this may to a substantial extent be counteracted by a preferred embodiment of the invention. This is more exactly obtained by the fact that the current detecting members 15 deliver values of the respective phase current and the unit 19 is adapted to control the electric switching devices separately to break the current in the part current paths in an order determined in dependence of the result of the current detection. It is then advantageous to first break the current in the "second best" part current path, i.e. the part current path with the second smallest asymmetry, which in the case of Fig 2 would mean a breaking of the phase current T first. The dc- component of the remaining phase currents is in this way influ- enced mostly in the reducing sense.

Fig 3 illustrates the development of the three phase currents after short circuiting of a synchronous generator at the point of time t₀ as a result of simulations. It is shown in Fig 4 what is happening if the current in the part current path of the phase T with the smallest asymmetry is broken according to the invention at the point of time t-,, in which, however, the asymmetry of this phase is nearly the same as of the phase S. This means that at the point of time ti the two other phase currents R and S will then jump, so that the phase current R with the largest asymmetry gets more symmetric, the dc-component thereof is considerably reduced, and it gets a forced or accelerated current zero-crossing. However, the asymmetry of the phase current S is somewhat increased. However, this is altogether a great ad- vantage, since the phase current R gets a zero-crossing considerably earlier than otherwise would be the case, so that the two phase currents R and S may be broken earlier and thereby the damage on the fault location and the stresses on the arrangements and components upstream of the fault may be reduced.

Although it is advantageous to open the different part current paths in said determined order, it is within the scope of the invention to detect the respective phase current and break it as soon as it fulfils the requirements, primarily current zero-crossing, required for breaking thereof. However, this will in the prac- tice mean that it will be any of the two most symmetric phase currents that will be broken first and the most asymmetric phase current will thereby get a forced zero-crossing and the breaking of the current in the current path may be completed earlier than would all three phase currents have been broken simultane- ously.

An apparatus according to the invention is advantageously used for obtaining breaking of the current in a current path through an electric switching device in switch gears for electricity supply within industry or in distribution or transmission networks, and the voltage of the current path is then preferably on intermediate voltage level, i.e. between 1 and 52 kV. However, the invention is not restricted to alternating voltages having these levels.

Furthermore, the invention is especially applicable to electric switching devices adapted to take an operation current of 1 kA, preferably 2 kA.

The invention is of course not in any way restricted to the pre- ferred embodiments described above, but many possibilities to modifications thereof will be apparent to a person with skill in the art without departing from the basic idea of the invention as defined in the appended claims. The invention is applicable to all types of electric switching devices having a breaking function, and not only to the type of hybrid breakers illustrated in the figures.

The invention is particularly interesting in the case of a multiple phase alternating current with a zero-point being not directly connected to ground, in which other applications with these conditions than that mentioned above are conceivable.

Claims

1. A method for breaking a current in a current path (2) for an alternating current with a plurality of phases and a dc-compo- nent of at least one phase current, i.e. a displacement of the symmetry line of the phase current with respect to the zero-level thereof, through opening an electric switching device (1 ) arranged in a part current path for each phase and thereby breaking the current in the current path after occurrence of a fault current in the current path, characterized in that the current is detected in the part current path of each phase, and that the electric switching devices are controlled to break the current in the respective part current path at a point of time depending upon the result of the current detection.

2. A method according to claim 1 , characterized in that the electric switching devices are separately controlled to break the current in the respective part current path at a point of time depending upon the result of the current detection.

3. A method according to claim 2, characterized in that the electric switching devices are separately controlled to break the current in the part current paths in an order determined in dependence of the result of the current detection.

4. A method according to claim 3, characterized in that the phase current of a part current path for one phase with a lower dc-level, i.e. displacement of the symmetry line of the phase current with respect to the zero-level thereof, than the phase current of at least one other part current path is the first of the phase currents of the part current paths that is broken.

5. A method according to claim 4 for breaking a current in a current path for an alternating current with three phases, charac- terized in that the phase current with the second lowest dc-level determined by the detection is broken first.

6. A method according to any of the preceding claims, characterized in that the breaking of the current in the part current path for the respective phase is carried out after detecting at least one zero-crossing of the alternating current for the phase in question.

7. A method according to claim 4, characterized in that through said current detection of each part current path the values of the alternating current of two succeeding current peaks are determined and an average of these two current values is formed as a value of the dc-level of the phase current in question.

8. A method according to any of the preceding claims, charac- terized in that a breaking of a current in a current path for an alternating current with one electric switching device (1 ) for each phase is achieved, said one electric switching device comprising two branches connected in parallel in the current path, the first one of which comprises a first contact member with two contacts (5, 7, 8) movable with respect to each other for opening and closing and the second comprises a part (10, 1 1 ) able to block current therethrough in at least a blocking direction and conduct current therethrough in at least one direction, in which the second contact member having two contacts movable with respect to each other for opening and closing is connected in series with said part, said switching device also comprising a unit (19) adapted to control breaking of the current in the respective part current path to a point of time depending upon the result of the current detection for transferring the current to the part when this is in or going into a conducting state and then the second contact member to open when the part is in a state of blocking current therethrough for making the breaking of the current through the switching device permanent.

9. A method according to any of claims 1 -7, characterized in that a breaking of a current in a current path (2) for an alternat- ing current with one electric switching device for each phase is achieved, said one electric switching device having at least two contact members arranged in a part current path through the switching device and a semiconductor device able to block cur- rent therethrough in at least a first blocking direction and a unit (19) adapted to control opening of a part current path through the switching device by controlling a first of the contact members to open for transferring the current through the switching device to the semiconductor device when this is in or going into a conducting state and then the second contact member to open when the semiconductor device is in a state of blocking current therethrough for making the breaking of the current through the switching device permanent, that each part current path has two branches (21 , 22) connected in parallel between a first and a second end of the respective switching device and cross-connected to each other through the semiconductor device (27), that the direction and the magnitude of the current through the switching device is detected, that for said breaking of the current in the part current path through the switching device first both branches are opened, one as seen from said first end before and the other as seen from the first end after the connection of the respective branch to the semiconductor device, in which which of the branches being opened before and which of them being opened after said connection is made dependent upon the detection of the current, so that the current is transferred into a temporary current path between said two ends through one part of one branch, the semiconductor device and a part of the other branch when the semiconductor device is in or going into a conducting state and the breaking of the current through the switching device is then made permanent when the semiconductor device is in a state of blocking current therethrough through opening said temporary current path, and that the breaking of the current in the respective part current path is controlled to a point of time being dependent upon the result of the current detection in the respective part current path.

10. An apparatus for breaking a current in a current path (2) for an alternating current with a plurality of phases and a dc-component of at least one phase current, i.e. a displacement of the symmetry line of the phase current with respect to the zero-level thereof, through opening an electric switching device (1 ) arranged in a part current path for each phase and thereby breaking the current in the current path after occurrence of a fault current in the current path, characterized in that it comprises members (15) adapted to detect the current in the part current path of each phase and a unit (19) adapted to control the electric switching devices to break the current in the respective part current path at a point of time being dependent upon the result of the current detection.

11. An apparatus according to claim 10, characterized in that the unit is adapted to control the electric switching devices separately to break the current in the respective part current path at a point of time being dependent upon the result of the current detection.

12. An apparatus according to claim 1 1 , characterized in that the unit is adapted to control the electric switching devices separately to break the current in the part current paths in an order determined in dependence of the result of the current de- tection.

13. An apparatus according to claim 12, characterized in that the unit (19) is adapted to first break the phase current of a part current path for a phase having a lower dc-level, i.e. displace- ment of the symmetry line of the phase current with respect to the zero-level thereof, than the phase current of at least another part current path.

14. An apparatus according to claim 13 for breaking a current in a current path for an alternating current with three phases (R, S,

T), characterized in that the unit is adapted to first break the phase current having the second lowest dc-level determined by said detection.

15. An apparatus according to any of claims 10-14, character- ized in that the control unit (19) is adapted to control the breaking of the current in the part current path for the respective phase after detecting at least a zero-crossing of the alternating current for the phase in question.

16. An apparatus according to any of claims 12-14, characterized in that said current detection members (15) are adapted to deliver values of the alternating current of two succeeding current peaks to the unit, and that the unit is adapted to form an average of these two current values for using them as said dc- level when determining said order.

17. An apparatus according to any of claims 10-16, characterized in that it is adapted to achieve breaking of a current in a current path for an alternating current with an electric switching device (1 ) for each phase, said electric switching device having two branches connected in parallel in the current path, the first one of which comprising a first contact member with two contacts (5, 7, 8) movable with respect to each other for opening and closing and the second comprising a part (10, 1 1 ) able to block current therethrough in at least a blocking direction and conduct current therethrough in at least one direction, in which a second contact member having two contacts movable with respect to each other for opening and closing is connected in series with said part, said switching device also comprising a unit (19) adapted to control breaking of the respective current path at a point of time being dependent upon the result of the current detection by controlling the first contact member to open for transferring the current to said part when this is in or going into a conducting state and then the second contact member to open when the part is in a state of blocking current therethrough for making the breaking of the current through the switching device permanent.

18. An apparatus according to any of claims 10-16, charac- terized in that it is adapted to obtain breaking of a current in a current path for an alternating current with an electric switching device for each phase, said switching device comprising at least two contact members arranged in a part current path through the switching device and a semiconductor device able to block cur- rent therethrough in at least a first blocking direction as well as a unit (19) adapted to control breaking of a current in the part current path through the switching device by controlling a first of the contact members to open for transferring the current through the switching device to the semiconductor device when this is in or going into a conducting state and then the second member to open when the semiconductor device is in a state of blocking current therethrough for making the breaking of the current through the switching device permanent, that the total number of contact members (23-26) of the switching device is at least four with two connected in series in each of two branches (21 , 22) connected in parallel in said part current path, that the semiconductor device (27) is adapted to connect the midpoints (28, 29) between the two contact members of each branch with each other, that the switching device comprises at least one member (15) adapted to detect the direction of the current through the switching device, that the control unit (19) is adapted to control the breaking of the current in the current path by controlling a first contact member of one, first branch located before said midpoint as seen in the current direction prevailing to open and a second contact member of the second branch located after the midpoint as seen in the current direction to open for transferring the current to a temporary current path through the semiconductor device when this is in or going into a conducting state and then making the breaking of the current in the current path through the switching device permanent when the semiconductor device is in a state of blocking current therethrough by opening at least one contact member of the switching device arranged in the temporary current path through the semiconductor device, and that the control unit is adapted to select the branch to be the first one on the basis of information from the current detecting member and control the breaking of the current in the respective part current path to a point of time being dependent upon the result of said current detection.

19. An apparatus according to any of claims 10-18, charac- terized in that it comprises an electrically controlled driving member (13) adapted to cause opening of the respective electric switching device.

20. An apparatus according to claim 19, characterized in that the driving member (13) is an electromagnetic machine.

21. An apparatus according to claim 20, characterized in that the driving member (13) is an electric motor.

22. An apparatus according to any of claims 19-21 , characterized in that it comprises a control unit (19) in the form of an electronic unit adapted to control said driving member (13).

23. A use of an apparatus according to any of claims 10-22 for obtaining breaking of a current in a current path in a switch gear for electricity supply within the industry or in distribution or transmission networks.

24. A use of an apparatus according to any of claims 10-22 for obtaining breaking of a current in a current path connected to a synchronous machine.

25. A use of an apparatus according to any of claims 10-22 for obtaining a breaking of a current in a current path for an alter- nating current having a voltage between 1-52 kV.

26. A use of an apparatus according to any of claims 10-22 for obtaining a breaking of a current in a current path through an electric switching device adapted to be able to handle an operation current of 1 kA, preferably at least 2 kA.

27. An arrangement for obtaining breaking of a current in a current path (2) for an alternating current with a plurality of phases and a dc-component of at least one phase current, i.e. a displacement of the symmetry line of the phase current with respect to the zero-level thereof, through opening of an electric switching device (1 ) arranged in a part current path for each phase and thereby breaking the current in the current path after occurrence of a fault current in the current path, in which the apparatus comprises a program module containing at least one processor adapted to carry out program instructions to detect the current in the part current path in each phase, and to control the electric switching devices to break the current in the respective part current path at a point of time being dependent upon the result of the current detection.

28. An arrangement according to claim 27, characterized in that the processor is adapted to carry out program instructions to control the electric switching devices separately to break the current in the respective part current path at a point of time be- ing dependent upon the result of the current detection.

29. An arrangement according to claim 28, characterized in that the processor is adapted to carry out program instructions to control the electric switching devices separately to break the current in the part current paths in an order determined in dependence of the result of the current detection.

30. An arrangement according to claim 29, characterized in that the processor is adapted to carry out program instructions to first of the phase currents of the part current paths break the phase current of a part current path for a phase having a lower dc-level, i.e. displacement of the symmetry line of the phase current with respect to the zero-level thereof, than the phase current of at least another part current path.

31. A computer program for obtaining breaking of a current in a current path for an alternating current with a plurality of phases and a dc-component of at least one phase current, i.e. displacement of the symmetry line of the phase current with respect to the zero-level thereof, through opening of an electric switching device (1) arranged in a part current path of each phase and thereby breaking the current in the current path after occurrence of a fault current in the current path, in which the computer program comprises instructions for influencing a processor to cause detection of the current in the part current path of each phase and control of the electric switching devices to break the current in the respective part current path at a point of time being dependent upon the result of the current detection.

32. A computer program according to claim 31 , characterized in that it comprises instructions for influencing a processor to cause control of the electric switching devices separately to break the current in the respective part current path at a point of time being dependent upon the result of the current detection.

33. A computer program according to claim 32, characterized in that it comprises instructions for influencing a processor to cause control of the electric switching devices separately to break the current in the part current paths in an order determined in dependence of the result of the current detection.

34. A computer program according to claim 33, characterized in that it comprises instructions for influencing a processor to cause breaking among the phase currents of the part current paths firstly of the phase current of a part current path for a phase having a lower dc-level, i.e. displacement of the symmetry line of the phase current with respect to the zero-level thereof, than the phase current of at least another part current path.

35. A computer program according to any of claims 31 -34 pro- vided at least partially through a network as the Internet.

36. A computer program product being loadable directly into the internal memory of a digital computer and comprising software code portions for carrying out the steps according to any of claims 1 -9 when the product is run on a computer.