WO2002023573A1

WO2002023573A1 - A method and a device for switching an electric switching device for alternating current

Info

Publication number: WO2002023573A1
Application number: PCT/SE2001/001936
Authority: WO
Inventors: Magnus Backman; Lars Jonsson; Per Larsson; Lennart BALGÅRD; Jan-Anders Nolemo
Original assignee: Abb T&D Technology Ltd.
Priority date: 2000-09-18
Filing date: 2001-09-11
Publication date: 2002-03-21
Also published as: AU2001284617A1; SE0003305D0

Abstract

In a method for switching an electric switching device (1) for alternating current arranged in a current path (2) between positions for breaking and closing the current path at a time considered to be an optimum for the conditions prevailing by achieving a relative movement of two contacts (30, 31-36) of an electric switching device arranged in the current path, the contacts are adapted to move a deterimed distance with respect to each other beforethe switching of the switching device is completed. Parameters useful for determining an optimum switching time are measured after the relative movement of the contacts for switching the switching device has been initiated and anre compared with values desired for these parameters at the time of the measurement with respect to obtaining a switching at an optimun time, and the relative movement of the contacts if controlled in dependence of the result of this comparison.

Description

Applicant: ABB T&D Technology Ltd.

A METHOD AND A DEVICE FOR SWITCHING AN ELECTRIC SWITCHING DEVICE FOR ALTERNATING CURRENT

FIELD OF THE INVENTION AND PRIOR ART

The present invention relates to a method for switching an electric switching device for alternating current arranged in a current path between positions of breaking and closing the current path at a time considered to be an optimum for the conditions prevailing by achieving a relative movement of two contacts of the electric switching device, in which parameters useful for determining said optimum time are measured and this time is calculated on the basis of this measurement so as to determine when said relative movement is to be initiated, and in which the contacts are arranged to move a determined distance with respect to each other before the switching is completed, as well as an apparatus for carrying out such a method.

The invention is directed to all types of electric switching devices of mechanical type, i.e. which achieve a physical separa- tion of two contacts when breaking a current or move two contacts together when switching in a current path therethrough, and which are arranged in a current path, such as for example in switch gears for electricity supply within the industry or in distribution or transmission networks. "Electric switching devices" comprises also so-called transfer switches through which a current in a current path may be broken, for example upon occurrence of a fault current in the current path, for switching in another current path to a load or the like instead.

The word "switching" comprises breaking of a current in the current path as well as closing of a current path through switching the electric switching device on. A breaking is normally desired to take place when normal load currents occur, but the invention also relates to breaking of fault currents. "Completing the switching of the switching device" means in this disclosure that when breaking the two contacts have been separated from each other and when closing the two contacts have arrived into electric contact with each other, but it is not at all necessary that the two contacts movable with respect to each other have arrived to the final resting position they will assume in the respective operation position of the electric switching device.

The case of switching in the form of breaking of a current through an electric switching device upon occurrence of a fault current in said current path will hereinafter be discussed for il- luminating the invention and the problems it tries to solve without in any way restrict the invention to exactly this specific application.

When such a fault current occurs it is on one hand important that the electric switching device opens the current path, i.e. breaks the current, as soon as possible for not damaging different types of equipment connected to the current path, but it is on the other absolutely necessary that the alternating current is in a certain position, normally has a zero-crossing, before it is broken. For a conventional electric switching device it is desired to obtain a separation of the contacts a short time, such as a few milliseconds, before the current therethrough would be zero for being able to obtain a sufficiently great contact distance at zero current at the same time as this time shall be reduced so much as possible for reducing burning of the contacts. In electric switching devices in which in connection with breaking the current is firstly transferred to flow through a diode or another corresponding member, it is desired to obtain a first contact separation close to zero current so as to facilitate commutation of the current over to the diode and then the final contact separation, which is to be put equal to said switching when the diode blocks. Thus, in the case of switching in the form of breaking "at a time considered to be an optimum for the conditions prevailing" may mean that we speak about a time, at which it is judged to be most appropriate to break the current, and for a conven- tional breaker this may be put equal to achieving an optimum arcing time and for a switching device with diode the time for achieving an optimum contact separation. Accordingly, a balancing between being quick and having a current in a certain position with respect to phase and amplitude is normally made. The direct current part that the alternating current may have as a consequence of the fault occurring may then also be of interest. It may also be a question of a multiple-phase system, for example three-phase, in which one such electric switching device is arranged per phase, and it may then be suitable to consider what is taking place with the other phases when determining said optimum breaking time.

"Useful parameters" could in practise be one single, but it is nevertheless co-ordinated with other data so that it is in such a case neither wrong to speak about "parameters". With respect to the word "relative movement" mentioned in the introduction this also comprises the case that one of the two contacts is stationary and the other is moved with respect thereto. It is also possible that the electric switching device has more than two, for ex- ample four contacts, of which one moves with respect to the three others and when this leaves the contact engagement with the third of the stationary contacts the breaking of the current may have been completed.

In methods of the type defined in the introduction already known said contacts have always a certain period of time after an order has been given to the electric switching device to break the current moved a certain distance with respect to each other. This time is comparatively constant and depends upon the mechani- cal delay time interval in the switching device, i.e. that it takes a certain time before the contacts have moved said determined distance with respect to each other for breaking the current. If this mechanical delay time interval is for example 15 ms, said relative movement is initiated in such a method already known of this type always 15 ms before the calculated optimum break- ing time. This is satisfying if the conditions 15 ms later are the same as predicted when calculating the optimum breaking time, but this is not always the case. Something may for instance have happened during this period of time, which has moved the optimum breaking time forwards or backwards in time. This may for example be that any electric switching device of another phase has broken away that phase or the fault current has had another development than predicted. It is also possible that something making the breaking of the current superfluous or in any way to be delayed has occurred. Possible deficiencies of the calculation of the optimum breaking time may also have existed, since any assumption may have been erroneously or certain approximations have been done for avoiding calculations requiring too much resources.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a method and an apparatus of the type defined in the introduction, which makes it possible to improve such methods and apparatuses al- ready known in at least certain respects.

This object is according to the invention obtained in such a method by measuring parameters useful for determining said optimum time after said relative movement of the contacts for the switching of the switching device has been initiated and comparing them with values desired for these parameters at the time of the measurement with respect to achieving the switching at said optimum time, and controlling the relative movement of the contacts in dependence of the result of this comparison. Events taking place after the relative movement of the contacts has been initiated may in this way be considered and it may accordingly be ensured that the switching really takes place at said optimum time, which could be the same as has been cal- culated before the relative movement was started, but it could also be another time calculated gradually during the movement. This "optimum time" may also have been converted to a time being indefinitely far in the future, if calculations have resulted in a decision that no switching is desired any longer, at least not for the moment. Such a method is particularly advantageous for synchronized breakers, in which it is important to achieve contact separation at a particular current value, such as when the current is zero. As already mentioned, said "switching" may just as well mean a closing of the current path through the electric switching device, and this then preferably takes place so that a mechanical contact between the two movable contacts is obtained at a particular voltage value, such as when the voltage is zero or a maximum. Thus, it gets through the invention possible to influence the switching operation and possibly interrupt it completely during the "mechanical delay time interval of the electric switching device".

According to a preferred embodiment of the invention said measuring of the parameters useful for determining an optimum time and the comparison thereof with said desired values are continuously carried out after said relative movement has been initiated and this movement is continuously adapted to the result of the comparison. An excellent adaptation of the course of the switching procedure to the conditions prevailing exactly for the moment may in this way be obtained.

According to another preferred embodiment of the invention the relative movement of the contacts is initiated a longer time before the calculated optimum switching time than is necessary for the contacts to travel said distance until said time. Good possibilities to gradually adapt the relative movement to the time judged to be an optimum exactly when the switching will take place are by this achieved, since it is easy to control the movement by influencing retardingly thereon. It is also possible to reduce the energy consumption for the very relative movement by using a slower movement than otherwise. This means also that the torque need of the actuating member causing said movement, such as an electric motor, gets lower and this may be made less costly. It is also possible, which is the case for another preferred embodiment of the invention, to make a rough estimation of the optimum switching time and then initiate said relative movements of the contacts and after that establish a first calculated value of said optimum time after initiating the relative movement for adaptation thereof to the calculated optimum time. Thus, the advantages with starting "a little too early" may be utilized and the switching may despite of that take place as soon as possible, since the relative movement is started before it was possible to carry out any completely correct prediction of the optimum switching time, such as a zero current.

According to a preferred embodiment of the invention the relative movement of the contacts is controlled in accelerating direction if said comparison with the desired values indicates that the movement on-going will result in a too late switching and in retarding direction if said comparison with the desired values indicates that the on-going movement will result in a too early switching.

According to another preferred embodiment of the invention said relative movement is stopped before the switching of the switching device has been completed and the contact is moved back to the starting position in which the electric switching device is in the original operation position, if said comparison with said desired value results in a decision that the switching should be substantially delayed or not take place at all. This may in certain cases be extremely advantageous, since it may by that be avoided that different parts of the electric switching device or other electrical equipment is entirely or partly destroyed. A fault current could namely have been that high that in one type of breakers, in which this in connection with the breaking for a short time is transferred to semiconductor members, such as di- odes, these semiconductor members would be completely destroyed. It may in such a case be better to delay the breaking, maybe by a few current periods, or carry out the breaking after the current in another phase has been broken. It may also have been the case that the relative movement could not take place sufficiently fast, for example through the occurrence of problems with the actuating member achieving the relative movement, and the breaking has therefor to be delayed, possibly a current period. In the case of an electric switching device designed symmetrically around at least a first of said positions, so that a said switching to the other position may take place through a relative movement of the contacts in an optional of two opposite directions starting from the first position, it is in such a case according to another preferred embodiment of the invention possible to stop the relative movement and turn back for moving the con- tacts back to their original position and continue past this position for achieving switching through relative movement of the contacts in the opposite direction if the new optimum time calculated is within a time interval for which this is suitable.

Other preferred embodiments of the invention are related to the parameters measured and compared with values instantaneously desired therefor for determining the optimum switching time, and such parameters are especially the current and the voltage in the current path. Different information about the cur- rent may be collected as said parameters, such as the dc-Ievel thereof, the decay with the time of dc-Ievel, the phase position of the current, the ac-decay and the time for a zero-crossing of the current.

According to another preferred embodiment of the invention the switching of the switching device takes place by controlling an electrical actuating member, such as en electric motor, adapted to achieve a relative movement of the two contacts movable with respect to each other between positions of the electric switching device closing and breaking the current path. Especially when using an electrical actuating member, and especially when such a member is present together with an electric switching device co-ordinated with an electronic unit for control of the electrical actuating member, and the relative movement of the contacts is by means of the electronic unit adapted to the result of this comparison, there are excellent possibilities to influence the switching procedure after said relative movement of the contacts has started. The measurements of said parameters, said comparisons and the calculations as well as the control of the relative movement are advantageously carried out through control of the electric actuating member through the electronic unit. Means are by this provided for making it possible for the electric switching device to react upon circumstances occurring after the switching order has been given.

According to a particularly advantageous embodiment of the invention the method is intended for switching an electric switching device arranged in a current path and having at least two contact members arranged in a current path through the switching and a semiconductor member with ability to block current therethrough in at least a first blocking direction and a unit adapted, when switching the switching device from a closed to a breaking position, to control the opening of a 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 member when this is in or going into a conducting state and then the second contact member to open when the semiconductor member is in a state of blocking current therethrough for making the breaking of the current through the switching device permanent, in which the current path has two branches connected in parallel between a first and a second end of the switching device and cross- connected to each other through the semiconductor member, in which for said switching firstly 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 member, in which which of the branches that is opened before and which is opened after said connection is made dependent upon a detection of the current through the switching device, so that the current is transferred into a temporary current path between said two ends through a part of one of the branches, the semiconductor member and a part of the other branch when the semiconductor member is in or going into a conducting state and said breaking of the current is through the switching devices made permanent when the semiconductor member is in a state of blocking current therethrough through opening said temporary current path, in which the switching of the electric switching device between said two positions is achieved by an electric actuating member adapted to achieve a relative movement of the contacts.

The invention also relates to an appliance, a computer program and 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 instruc- tions from a processor that may be influenced by a computer program provided with the program steps in question. This is especially the case for the embodiments with an electronic unit co-ordinated with the electric switching device, since it is very suitable to program this electronic unit to control the entire switching procedure as well as the calculation of an optimum switching time before and after the start of the relative movement of the contacts. Although not explicitly expressed in claims, the invention comprises such appliances, computer programs and computer program products combined with a method according to any of the appended method claims. The invention also relates to an apparatus for switching an electric switching device for alternating current arranged in a current path between positions for breaking and closing the current at a time considered to be an optimum for the conditions prevailing according to the appended apparatus claims. The function and the advantages of such an apparatus appear without any doubt from the discussion above of the corresponding embodiments of the method according to the invention.

Further advantages as well as advantageous features of the invention 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 preferred embodiments of the invention cited as examples.

In the drawings:

Fig 1 is a schematic view of an electric switching device for which the method according to the invention is particularly well suited,

Fig 2-4 are simplified views illustrating the function of an apparatus for breaking an alternating current through an electric switching device according to Fig 1 according to a preferred embodiment of the invention,

Fig 5 is a graph illustrating the movement in geometrical grades of the movable part of the electric switching device according to Fig 1-3 versus time when breaking a fault current through the switching device, and Fig 6-9 are very simplified views of an apparatus according to another preferred embodiment of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

An electric switching device in the form of a so-called hybrid breaker is schematically illustrated in Fig 1 , and the method according to the invention for breaking an alternating current in a current path is especially well suited for this breaker. This electric switching device 1 is connected in a current path 2 for being able to rapidly break a current therein by opening the current path. The electric switching device is intended for alternating current and 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. This electric switching device is already described in the Swedish patent application 9904166-7 of the applicant. The construction of the switching device is more schematically shown in Fig 2-4. It comprises two branches 3, 4 connected in parallel in the current path and having each at least two mechanical contact members 5-8 connected in series. A semiconductor member 9 in the form of a diode is arranged to connect the midpoints 10, 1 1 between the two contact members of each branch with each other.

The switching device has also an electrical actuating member 13 in the form of an electric motor adapted to drive a part (see Fig 1 ) having the movable contact 30 of each contact member to rotate for movement of these movable contacts with respect to the respective fixed contact 31 -36.

Members 14 are adapted to measure the current in the current path 2 and send information thereabout to an electronic unit 15 adapted to control the electric motor 13 and by that the movable contacts 30. The electronic unit here actually controls the electric motor through a converter and a source for energy supply to the motor. The electronic unit 15 comprises also members adapted to calculate a time considered to be an optimum for the conditions prevailing for breaking an alternating current in the current path 2 after occurrence of a fault current in the current path on the basis of the measurement results from the members 14 so as to determine when the relative movement of the movable contacts 30 and the other contacts 31-36 shall be initiated. The electronic unit 15 also comprises members adapted to carry out the current values delivered by the members 14 after begin- ning the relative movement of the contacts with values desired at the measuring time therefor with respect to obtaining breaking at said optimum breaking time, and the electronic unit is adapted to control the electric motor and by that the relative movement of the contacts in dependence of the result of said comparison. These operations are preferably carried out continuously, i.e. the measuring members 14 deliver continuously information about the current to the electronic unit, and said comparison and calculations also takes place continuously. Information about the current may for example be the phase posi- tion, the amplitude, the dc-Ievel, the dc-decay, the ac-decay. Members 16 schematically indicated are also adapted to detect the actual position of the movable contact 30 and the electronic unit 15 is adapted to compare this information with the relative movement instantaneously desired with respect to obtaining breaking at the optimum breaking time so as to control the electric motor 13 on the basis of this comparison.

The function of this electric switching device is as follows: When there is a desire to break the current in the current path 2, for example by the fact that the detecting member 14 detects a very high current in the current path, which may be caused by a short-circuit therealong, it would be possible for obtaining a breaking being as fast as possible to detect the direction of the alternating current and make the rotation direction of the part carrying the movable contacts and by that the movable contacts 30 dependent thereupon. Once it has been determined that the electric switching device shall be opened, the electronic unit 15 firstly decides which two contact members, here the contact members 6 and 7 (see Fig 3), are to be opened for establishing a temporary current path through the semiconductor member 9. Thus, this decision is dependent upon in which position the current in the current path is in exactly that moment. In the position according to Fig 2 the entire current flows through the switching device through the two branches 3 and 4 and nothing through the diode. When now the breaking shall take place the current shall 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 time shortly before the diode gets forward biased in that direction and the time when the diode gets reverse biased next time. This means for a whole period of 20 ms in practise that an opening of the contact members according to Fig 2 may take place for example about 2 ms before zero-crossing towards the forward biasing direction and until the next zero-crossing. When the wrong half period of the alternat- ing voltage for an opening of the contact members 6 and 7 according to these requirements exists, the contact members 5 and 8 may instead be opened for establishing the temporary current path instead. Accordingly, this temporary current path may be established comparatively fast after detecting the need of and the possibility to open the switching device for breaking the current therethrough.

When the temporary closed position illustrated in Fig 2 is obtained through opening the contact members 6, 7 a small spark is formed in the gap between the contacts of the respective contact member, which results in a voltage of usually 12-15 V, which will drive the transfer of the current through the diode 9. When then the current across the switching device changes direction no current will flow therethrough, but a voltage will be built up across the diode 9 then reverse biased, and at least one of the two other contact members 5, 8 is now opened, so that the temporary current path is opened, in which this opening may take place arc-freely, since no current flows through the contact place at the moment of the opening. The completely open position of the switching device shown in Fig 4 is by that obtained, in which the current therethrough is permanently broken. It is at the final opening important that it takes place so fast that the voltage across the diode 9 has no time to change direction again and it starts to conduct. The utilization of the same semiconductor member in the temporary current path independ- ently of in which direction the current flows through the switching enables great savings of costs by a substantial reduction of the number of semiconductor members with respect to in other switching devices of this type.

It is illustrated in Fig 5 how the movement of a movable contact 30 of the electric switching device according to Fig 1 may take place when breaking an alternating current through the switching device. During a first period of time a of for example about 5 ms an acceleration of the movable contact 30 takes place, whereupon during a period of time b following thereon of about 12,5 ms a movement with a substantially constant speed takes place. This time period ends with an angle movement of about 52° of the movable contact 30 with respect to the starting position according to Fig 1 , which corresponds to the position in which it leaves the contact engagement with the stationary contact 32 of the contact member 5 and the breaking of the current is completed. The movement is then continued during a third period of time c with a substantially constant speed until a breaking of the movement to an open position with the movable contact displaced 90° with respect to the position according to Fig 1 (the position according to Fig 4) takes place during a time period d.

The electronic unit 15 may according to the present invention first carry out a rough estimation of when the contact separation when reaching the position A should take place and then when the diode is in the blocking state the position B should be reached, and the movement of the movable contact 30 may after that be started earlier than necessary and this be controlled so that the positions A and B are reached at times turning out to be an optimum according to calculations thereof carried out along the path of the movable contact towards these positions. The current in the current path 2 may then all the time be measured and compared with current values instantaneously desired for determining the optimum breaking time. It is then possible to calculate the dc-Ievel of the current and the decay of the dc- Ievel with the time on the basis of values of the alternating current detected by the members 14 for being used for the calculation of the optimum breaking time. The ac-decay of the alternating current may also be calculated on the basis of said de- tected values of the alternating current and considered when calculating the optimum breaking time. It may then be advantageous to store the value of the alternating current during at least an entire current period in a memory, calculate the dc-Ievel of the current and the dc-decay with the time continuously through integration of the memory, and predict the optimum breaking time by calculating the value of the current for each time a period in advance through the value prevailing at said each time minus the present dc-decay. Another possibility is to measure the alternating current by sampling the value of the alternating current with a sampling frequency during at least an entire current period and store the values sampled in a memory member, and to then calculate the dc-Ievel of the current at a given time by forming an average of the stored current values for the time period a current period backwardly from said time. This dc-Ievel is then used when calculating the optimum breaking time.

It is through the method according to the invention possible to for example start the movement of the contact 5 ms earlier than would be the case if the electric motor 13 would deliver maxi- mum torque for the acceleration of the movable contact 30, so that energy may be saved or alternatively a weaker and less costly electric motor may be used. Things may also occur during the very movement, which makes it possible to reconsider the optimum breaking time. Another electric switching device may for example already have broken the connection to a fault place, so that no breaking of the alternating current is desired any longer. It is also possible that the fault current may have been developed to be higher or lower than when the movement of the movable contact 30 was started, and if it has become higher it may be that high that the diode 9 may not withstand it and the movement therefor has to be interrupted and the breaking possibly be delayed. If the value is sufficiently low perhaps no breaking will be necessary. Problems with the electric switching device may also have occurred, so that the correct speed of the movable contact 30 may not be achieved and no acceptable breaking procedure may therefor be carried out. The electronic unit 15 controls in such a case also the electric motor to stop the movement of the movable contact 30 and bring this back to the starting position according to Fig 1 . A mass not shown in the figures may in practise be connected to the movable contact and be put into movement by the motor 13 for achieving the relative movement of the contacts. The control of the relative movement then takes place by adjusting the speed of the mass.

An embodiment in which the movable contact 30 is designed to carry out a substantially rectilinear relative movement when breaking the alternating current therethrough (Fig 6, 7) and closing the current path therethrough (Fig 8, 9), respectively, is illustrated in Fig 6-9. Parts having correspondence in the apparatus according to Fig 1 -4 have here been provided with the same reference numerals. The case in which the electric switching device is adapted to under normal operation (Fig 6) of an electric plant to which the switching device is connected be closed and conduct a current therethrough is illustrated in Fig 6 and 7. The electric motor 2 is adapted to move the movable contact 30 away from the stationary contact 31 , such as shown in Fig 7, for breaking the alternating current through the switch- ing device during measuring of parameters useful for calculating the optimum breaking time, so that the movement may be adapted to the result of this calculation. It is illustrated in Fig 8 and 9 how a corresponding method is carried out for an electric switching device being arranged to keep the current path therethrough broken during normal operation (Fig 8) of the electric plant. Thus, a switching of the electric switching device from a broken to a closed position here takes place while carrying out the method according to the invention described further above.

The invention is of course not in any way restricted to the preferred embodiments described above, but many possibilities to modifications thereof would 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 electric switching devices intended for all conceivable levels of voltages and currents, even if it is especially well suited for breaking alternating currents through electric switching devices in a current path in switch gears for electricity supply within the industry, distribution or transmission networks with a voltage of the current path on intermediate voltage and high voltage level, i.e. between 1 and 800 kV. The invention is also particularly applicable to electric switching devices adapted to be able to take an operation current of 1 kA, preferably 4 kA.

The notion "comparison" used in the claims is to be given a broad sense and comprises also that a certain measured value quite simply results in a certain reaction, which is dependent upon the magnitude of the parameter in question, so that no real comparison takes place. A certain value may for example always create a certain result.

Claims

1 . A method for switching an electric switching device (1 ) for alternating current arranged in a current path (2) between positions of breaking and closing the current path at a time considered to be an optimum for the conditions prevailing by achieving a relative movement of two contacts (30, 31 -36) of the electric switching device (1 ), in which parameters useful for determining said optimum time are measured and this time is calculated on the basis of this measurement so as to determine when said relative movement is to be initiated, and in which the contacts are arranged to move a determined distance with respect to each other before the switching is completed, characterized in that parameters useful for de- termining said optimum time are measured after said relative movement of the contacts for the switching of the switching device has been initiated and compared with values desired for these parameters at the time of the measurement with respect to achieving the switching at said optimum time, and that the relative movement of the contacts is controlled in dependence of the result of this comparison.

2. A method according to claim 1 , characterized in that said measurement of the parameters useful for determining the optimum time and the comparison thereof with said desired values are carried out continuously after the initiation of said relative movement and this movement is continuously adapted to the results of the comparison.

3. A method according to claim 1 or 2, characterized in that the relative movement of the contacts (30, 31 -36) is initiated a longer time before the calculated optimum time than necessary for the contacts to travel said distance until said time.

4. A method according to claim 3, characterized in that a rough estimation of the optimum time is made and said rela- tive movement of the contacts (30, 31 -36) is then initiated and a first calculated value of said optimum time is established after initiation of the relative movement for adaptation thereof to the calculated optimum time.

5. A method according to any of the preceding claims, characterized in that the mutual position of said contacts (5, 7-10) is dependent upon the position of a mass and the latter is put into movement for initiating said relative movement and the control of the movement then takes place by adjustment of the speed of said mass.

6. A method according to any of the preceding claims, characterized in that if said comparison with the desired values in- dicates that the on-going movement will result in a too late switching the relative movement of the contacts (30, 31 -36) is controlled in accelerating sense.

7. A method according to any of claims 1 -5, characterized in that if said comparison with the desired values indicates that the on-going movement will result in a too early switching the relative movement of the contacts (30, 31 -36) is controlled in a retarding sense.

8. A method according to any of claims 1 -5, characterized in that if said comparison with said desired values results in a recommendation to delay the switching substantially or not carry out the switching at all said relative movement is stopped before the switching of the switching device has been completed and the contacts are moved back to the starting position in which the electric switching device is in the original operation position.

9. A method according to claim 8, characterized in that said measurements are carried out for determining the possibilities to control the relative movement and this possibility is considered as a said parameter and if this possibility is restricted in an unacceptable degree or disappears said relative movement is stopped before the switching of the switching device has been completed and the contacts (30, 31-36) are moved back to the starting position in which the electric switching device is in the original operation position.

10. A method according to any of claims 1 -5, in which the electric switching device (1 ) is designed symmetrically around at least a first of said positions, so that a said switching to the second position may take place through a relative movement of contacts (30, 31 -36) in an optional of two opposite directions starting from the first position, characterized in that if said comparison with said desired values results in a recom- mendation of a substantial delay of said switching with respect to the case if said relative movement is continued the relative movement is stopped and it is returned for moving the contacts (30, 31 -36) back to their starting position and continue past this position so as to achieve a switching through a relative movement of the contacts in the opposite direction if the calculated optimum time is within a time interval for which this is suitable.

1 1 . A method according to any of the preceding claims, in which the switching is carried out from a position of the switching device (1 ) in which the current is closed to a position in which it is broken, characterized in that it is the current in the current path that is measured as a said useful parameter and compared with current values instantaneously desired for determining the optimum breaking time.

12. A method according to claim 1 1 , characterized in that the dc-Ievel of the current, i.e. the displacement of the current with respect to the zero level thereof, and the decay of the dc-Ievel with the time are calculated on the basis of detected values of the alternating current for forming a base of the calculation of said optimum breaking time.

13. A method according to claim 1 1 or 12, characterized in that the ac-decay of the alternating current, i.e. the reduction of the amplitude of the alternating current with the time, is calculated on the basis of detected values of the alternating current and considered when calculating said optimum breaking time.

14. A method according to any of claims 1 1 -13, characterized in that the value of the alternating current during at least an entire current period is stored in a memory, that the dc-Ievel of the current, i.e. the displacement of the symmetry line of the alternating current with respect to the zero line thereof, and the dc-decay with the time are calculated continuously through integration of said memory, and that said optimum breaking time is predicted by, for each time, calculating the value of the current one period in advance through the value prevailing at said each time minus the present dc-decay.

15. A method according to any of claims 1 1 -13, characterized in that the alternating current is measured by sampling the value of the alternating current with a sampling frequency during at least an entire current period and the values sampled are stored in a memory member, and that the dc- Ievel of the current, i.e. the displacement of the symmetry line of the alternating current with respect to the zero level thereof, at a given time is calculated by forming an average of the current values stored for the time period one current period back from said time and this dc-Ievel is then used when calculating said optimum breaking time.

16. A method according to claim 12, characterized in that the optimum breaking time is calculated as a time for a future zero-crossing of the alternating current, which is predicted from at least current values obtained through said current detection, the calculated dc-Ievel, the calculated dc-decay and the period time of the alternating current.

17. A method according to any of claims 1 -10, in which the switching is carried out from a position of the switching device (1 ) in which the current path is broken to a position in which it is closed, characterized in that it is the voltage in the current path that is measured as said useful parameter and compared with voltage values instantaneously desired for determining the optimum closing time.

18. A method according to any of the preceding claims, characterized in that the switching device takes place by control- ling an electrical actuating member (13), such as an electric motor, arranged to achieve a relative movement of the two contacts (30, 31 -36) movable with respect to each other between positions of the electric switching device closing and breaking the current path.

19. A method according to claim 18, characterized in that the actual relative position of the contacts (5, 7-10) is detected during the relative movement and compared with a relative position instantaneously desired for obtaining a switching at the optimum time and the electrical actuating member (13) is controlled on the basis of this comparison.

20. A method according to claim 19, characterized in that the energy supply to the electrical actuating member (13) is con- trolled in dependence of the result of the comparison of the actual relative position between the contacts (5, 7-10) and said desired relative position.

21 . A method according to any of claims 18-20, characterized in that it is an electric switching device (1 ) co-ordinated with an electronic unit (15) for control of the electrical actuating member (13) that is switched, and that the relative movement of the contacts (30, 31 -36) is by means of the electronic unit adapted to the result of said comparison.

22. A method according to claim 21 , characterized in that the measurements of said parameters, said comparisons and said calculations as well as the control of the relative movement through the control of the electric actuating member (13) are carried out through the electronic unit (15).

23. A method according to any of the preceding claims for switching an electric switching device (1 ) arranged in a current path and having at least two contact members arranged in a current path through the switching and a semiconductor member (9) with ability to block current therethrough in at least a first blocking direction and a unit (15) adapted when switching the switching device from a closed to a breaking position, to control the opening of a 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 member when this is in or going into a conducting state and then the second contact member to open when the semiconductor member is in a state of blocking current therethrough for making the breaking of the current through the switching device permanent, that the current path has two branches (3, 4) connected in parallel between a first and a second end of the switching device and cross-connected to each other through the semiconductor member, that for said switching firstly 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 member, in which which of the branches that is opened before and which is opened after said connection is made dependent upon a detection of the current through the switching device, so that the current is transferred into a temporary current path between said two ends through a part of one of the branches, the semiconductor member and a part of the other branch when the semiconductor member 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 member is in a state of blocking current therethrough through opening said temporary current path, characterized in that the switching of the electric switching device between said two positions is achieved by an electric actuating member (13) adapted to achieve a relative movement of the contacts (30, 31 -36).

24. A method according to claim 15, characterized in that if a current being higher than said member (9) withstands is measured as a said parameter after the relative movement of said contacts (30, 31 -36) has been initiated the relative movement is interrupted and the breaking is delayed until the current intensity has decreased below a level being acceptable with respect to the withstandability of the member.

25. An apparatus for switching an electric switching device (1 ) for alternating current arranged in a current path (2) between positions of breaking and closing the current path at a time considered to be an optimum for the conditions prevailing comprising an arrangement (13) adapted to carry out a relative movement of two contacts (30, 31 -36) of an electric switching device arranged in the current path, said apparatus comprising members (14) adapted to measure parameters useful for determining said optimum time and members (15) adapted to calculate said time on the basis of this measurement so as to determine when said relative movement shall be initiated by the arrangement, and in which the contacts are adapted to move a determined distance with respect to each other before the switching is completed, characterized in that said measuring members are adapted to measure said parameters after said relative movement of the contacts for the switching of the switching device has been initiated, that the apparatus also comprises members adapted to compare the parameters so measured with values desired for these parameters at the measuring time with respect to obtaining a switching at said optimum time, and that it comprises means (15) adapted to control the relative movement of the contacts in dependence of the result of this comparison.

26. An apparatus according to claim 25, characterized in that the measuring members (14) and the comparison members (15) are adapted to continuously measure the parameters useful for determining the optimum breaking time and compare these with said desired values, respectively, after a said relative movement has been initiated, and that the control means (15) are adapted to continuously adapt the relative movement to the result of this comparison.

27. An apparatus according to claim 25 or 26, characterized in that said means (15) are adapted to control the arrangement

(13) to start the relative movement of the contacts a longer time before the calculated optimum time than is necessary for the contacts for being able to travel said distance until said time.

28. An apparatus according to claim 27, characterized in that said calculating members (15) are arranged to carry out a rough estimation of said optimum time and said means are adapted to control the arrangement to initiate the relative movement of the contacts before a first calculated value of said optimum time has been established, and that the calculating members are adapted to carry out said calculation of the optimum time during the relative movement of the contacts so that said means may control the arrangement to adapt the relative movement to the optimum time so calculated.

29. An apparatus according to any of claims 25-28, characterized in that said means (15) are adapted to control the relative movement of the contacts (5, 7-10) in an accelerating di- rection when receiving an information from said comparison members (15) that the movement on-going will result in a switching being too late.

30. An apparatus according to any of claims 25-28, character- ized in that said means (15) are adapted to control the relative movement of the contacts in a retarding direction when receiving an information from said comparison members (15) that the on-going movement will result in a too early switching.

31. An apparatus according to any of claims 25-28, characterized in that said means (15) are adapted to control the arrangement (13) to stop the relative movement before the switching of the electric switching device has been completed and move the contacts (30, 31 -36) back to the starting position in which the electric switching device is in the original operation position when the comparison members inform about a desire to delay the switching substantially or stop it from taking place at all.

32. An apparatus according to any of claims 25-31 , characterized in that said measuring members (14) are adapted to measure the current in the current path in the closed position of the switching device as a said useful parameter and the comparison members (15) are adapted to compare the current with current values instantaneously desired for determining the optimum time for switching to the breaking position.

33. An apparatus according to any of claims 25-32, characterized in that the arrangement comprises an electrical actuat- ing member (13) such as an electric motor, arranged to achieve a relative movement of the two contacts (30, 31 -36) movable with respect to each other between positions of the electric switching device for closing and breaking the current path.

34. An apparatus according to claim 33, characterized in that it comprises members (16) adapted to detect the actual relative movement of the contacts during the relative movement thereof for switching between said two positions, and that said comparison members (15) are adapted to compare the actual relative position of the contacts with a relative position instantaneously desired with respect to obtaining a switching of the switching device at said optimum time, and that said means are adapted to control the electrical actuating member

(13) on the basis of this comparison.

35. An apparatus according to claim 34, characterized in that it comprises a source (17) for energy supply to the electrical actuating member and said means are adapted to control the energy supply to the electrical actuating member (13) in dependence of the result of the comparison of the actual relative position of the contacts (30, 31 -36) and said desired relative position.

36. An apparatus according to any of claims 33-35, characterized in that it comprises an electronic unit (15) adapted to control the electrical actuating member (13), and that the electronic unit is adapted to control the electrical actuating member to adapt the relative movement to the result of said comparison.

37. An apparatus according to claim 36, characterized in that the electronic unit (15) is adapted to achieve the measure- ments of said parameters, said comparisons and said calculations as well as the control of the relative movement through control of the electrical actuating member (13).

38. An apparatus according to any of claims 25-37 for switching an electric switching device (1 ) arranged in a current path and having at least two contact members arranged in the current path through the switching device and a semiconductor member (9) with ability to block current therethrough in at least a first blocking direction and a unit (15) adapted, when switching the switching device from a blocking to a breaking position, to control the opening of a 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 member when this is in or going into a conducting state and then the second contact member to open when the semiconductor member is in a state of blocking current therethrough for making the breaking of the current through the switching device permanent, that the current path has two branches (3, 4) connected in parallel be- tween a first and a second end of the switching device and cross-connected to each other through the semiconductor member (9), that the unit (15) is for said switching adapted to first achieve opening of the two branches, 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 member, in which which of the branches is to be opened before and which is to be opened after said connection is made dependent upon the detection of the current through the switching device by a detecting member (14) be- ing a part of the apparatus, so that the current is transferred into a temporary current path between said two ends through a part of one of the branches, the semiconductor member and a part of the other branch when the semiconductor member 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 member is in a state of blocking cur- rent therethrough by opening said temporary current path, characterized in that an electrical actuating member (13) is adapted to achieve the switching of the electric switching device between said two positions by achieving the relative movement of the contacts.

39. An appliance for switching an electric switching device (1 ) for alternating current arranged in a current path (2) between positions for breaking and closing the current path at a time considered to be an optimum for the conditions prevailing by achieving a relative movement of two contacts (30, 31 -36) of an electric switching device (1 ) arranged in the current path, which apparatus comprises a program module containing at least a processor adapted to carry out program instructions to measure parameters useful for determining said optimum time, to calculate this time on the basis of this measurement so as to determine when said relative movement shall be initiated, in which the contacts (30, 31 -36) are adapted to move a determined distance with respect to each other before the switching of the switching device is completed, to measure parameters useful for determining an optimum switching time after said relative movement of the contacts for switching the switching device has been initiated and to compare these with values desired at the measuring time for these parameters with respect to obtaining a switching at said optimum time, and to control the relative movement of the contacts in dependence of the result of this comparison.

40. A computer program for switching an electric switching de- vice (1 ) for alternating current arranged in a current path (2) between positions for breaking and closing the current path at a time considered to be an optimum for the conditions prevailing by achieving a relative movement of two contacts (30, 31 -36) of an electric switching device (1 ) arranged in the cur- rent path, in which the computer program comprises instructions for influencing a processor to cause a measurement of parameters useful for determining said optimum time, a calculation of this time on the basis of this measurement so as to determine when said relative movement shall be initiated, in which the contacts are adapted to move a determined dis- tance with respect to each other before the switching of the switching device is completed, a measurement of parameters useful for determining an optimum switching time after said relative movement of the contacts (30, 31 -36) for the switching of the switching device has been initiated and a compari- son of these with values desired for these parameters at the measuring time with respect to obtaining a switching at said optimum time, and a control of the relative movement of the contacts in dependence of the result of this comparison.

41 . A computer program according to claim 40 provided at least partially through a network as the Internet.

42. A computer program product directly loadable into the internal memory of a digital computer and comprising software code portions for carrying out the steps according to any of claims 1 -24 when the product is run on a computer.