WO2001037300A1

WO2001037300A1 - An electric switching device

Info

Publication number: WO2001037300A1
Application number: PCT/SE2000/002275
Authority: WO
Inventors: Lars Jonsson; Per Larsson; Falah Al-Hosini; Magnus Backman
Original assignee: Abb Ab
Priority date: 1999-11-18
Filing date: 2000-11-20
Publication date: 2001-05-25
Also published as: SE9904164L; EP1232505A1; SE9904164D0; SE517613C2; AU1908001A

Abstract

An electric switching device for alternating current comprises two contact members and a component (21, 22) having ability to block current in at least one direction and conduct current in at least one direction therethrough. It also comprises a unit (14) adapted to control the first contact member to open for transferring the current to the component when this is in or going into conducting state and then the second contact member to open when the component is in a state of blocking current therethrough for breaking the current through the switching device. It also comprises a movable part (4) and a unit (14) adapted to control this part to carry out one single mechanical movement for opening or closing the two contact members.

Description

An electric switching device

FIELD OF THE INVENTION AND PRIOR ART

The present invention relates to an electric switching device for alternating current comprising in a current path two branches connected in parallel to each other, a first of them comprising a first contact member having two contacts movable with respect to each other for opening and closing and the second comprising a component able to block current therethrough in at least a first 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 the component, and in which the switching device also comprises a unit adapted to control opening of said current path by controlling the first contact member to open for transferring the current to the component when this is in or is going into the conducting state and then the second contact member to open when the component is in a state of blocking current therethrough for breaking the current through the switching device.

Such electric switching devices are usually called hybrid break- ers, and it is characterizing for them that they are able to achieve an arc-free breaking of the current path through the switching device, since this takes place when the component is in blocking state and no current flows through the switching device. In switching devices having contact members breaking the current therethrough, and in which accordingly an arc is generated, the gas pressure inside the breaker used has to be high for achieving a sufficient insulation and breaking performance or a vacuum has to be provided inside the breaker for the same reason. Quite an amount of energy is needed in the first case for blowing out the arc, while in the second case a comparatively high contact pressure for a good contact is needed, which consumes a not neglectible amount of energy. The corresponding amount of energy may in a switching device according to the introduction having an arc-free breaking in the way mentioned instead be used for making the breaking more rapid so as to bet- ter protect different types of electrical equipment connected to said current path upon occurrence of faults and reduce the material wear of contacts included in the second contact member.

The component is usually a semiconductor device, but it could be of any conceivable type having said ability, such as for example a variable electric resistor according WO 98/49694.

The invention is of course not restricted to any particular range of the operation current through such an electric switching de- vice in the closed state, and neither to any particular voltage levels existing in said current path, but it may nevertheless be mentioned that it is particularly useful for intermediate voltage, i.e. corresponding to 1 -52 kV system voltage, in which the operation current in question typically may be 1 kA, but both lower and higher voltages and currents than these are conceivable.

Such an electric switching device is generally used for obtaining breaking of a current path upon occurrence of any fault, such as a shortcircuit, along the current path. The fault may for example be caused by cutting off a cable of an alternating voltage distribution network by a digging machine. It is then important to break the current rapidly for minimizing damage on persons or material. It is not necessary, but well possible that the second contact member of such a switching device accomplishes a breaking visible to the eye, i.e. functions as a disconnector, which is necessary when the breaking of the current is made for carrying out any type of maintenance work.

A switching device of this type is particularly well suited to be arranged within a switch gear for supply of electricity within industries or in distribution or transmission networks. It may also be mentioned that it may advantageously be used for being able to rapidly disconnect a generator from an alternating voltage network for protecting the generator against different types of disturbances or faults on the alternating voltage network.

It is pointed out that "conducting state" above is to be given a broad sense, and it is not necessary that a component going into or being in the conducting state really conducts, but this is also intended to cover that it may be brought to conduct in that moment should that be desired, which could be the case for a semiconductor device of turn-on type, such as a thyristor, while a passive semiconductor device in the form of a diode instead always will conduct in the conducting state as defined here.

Furthermore, it is pointed out that "first contact member" and "second contact member" comprise all types of physical appearances of a contact member in which a physical separation of two parts while forming a gap therebetween takes place when opening the contact member, and this may for example take place by moving a movable contact interconnecting two contacts mutually spaced so that these are no longer in connection with each other or by the fact that a movable contact bears against a fixed contact and is moved away therefrom. Two movable con- tacts are also conceivable. "Gap" is also to be given a broad sense. It comprises also that an insulator is introduced between the contacts and this may be a solid body bearing against both contacts. Thus, a contact surface may also be followed by an insulating surface in the same body (which may comprise a movable or fixed contact) and in the same plane and "the gap" will then be formed by the fact that two bodies forming a contact in a bearing position are displaced with respect to each other so that such an insulating surface forms the bearing surface of one or both bodies.

Electric switching devices of the type according to the introduction already known, such as for example the one known from US 4 459 629, have a comparatively costly control electronic so as to accomplish opening of the two contact members when there is a desire to break said current path or closing the contact members when re-establishing the current path in a well defined way through an exact co-ordination required of the control of the two contact members.

SUMMARY OF THE INVENTION

The object of the present invention is to provide an electric switching device of the type defined in the introduction, which is reliable and which may be controlled with a high accuracy by simple means being by that favourable from the cost point of view.

This object is according to the invention obtained by providing such a switching device with a movable part and by designing the unit to control this part to carry out one single mechanical movement for opening and closing the two contact members.

By controlling one single movable part to carry out one single mechanical movement for opening the two contact members this may take place in a perfectly synchronized way, without any re- quirement of any complicated control electronic for co-ordinating these openings with each other. Another advantage is that one single driving arrangement may be used for accomplishing both openings by driving the movable part to carry out one movement. By the fact that the opening or closing of the electric switching device takes place through one single mechanical movement improved possibilities to make the control faster are obtained, since only one acceleration of one movable part is necessary.

According to a preferred embodiment of the invention the switching device comprises at least two contacts consecutively arranged along the path of the movable part for said mechanical movement, the movable part is adapted to form a galvanic connection between said two contacts in the closed state of the switching device, the switching device is adapted to be switched in a current path going through the switching device through one of the contacts and the movable part, the semiconductor component connects the two contacts with each other, and the control unit is adapted to control opening of said current path by operating the movable part to move along such a path and in such a direction that the galvanic connection between one of said two contacts and the movable part is firstly broken for opening the first contact member and the galvanic connection between the second of said two contacts and the movable part is then broken for opening the second contact member. This constitutes a very simple way to design the switching device according to the invention to a low cost, since it only has to contain two fixed contacts, one movable contact part and one single semiconductor device so as to ensure arc-free breaking of the current path.

According to another preferred embodiment of the invention the switching device comprises at least three contacts consecutively arranged along the path of said movable part for said mechanical movement, and the component connects a first and second adjacent of these three contacts with each other, the movable part is adapted to form a galvanic connection between said three contacts in the closed state of the switching device, the control unit is adapted to control the movable part to move along such a path and in such a direction for breaking the current through the switching device that firstly the galvanic connection between the first contact most far away from the third of the contacts and the two other contacts is broken for opening the first contact member and then the galvanic connection between the second and the third contact is broken for opening the second contact member. This embodiment requires likewise only one component, which makes it worth its price. This embodiment is suited for the case that the switching device is adapted to be connected in a current path through the switching device through the two outer contacts along this path.

According to another preferred embodiment of the invention the switching device comprises at least two said semiconductor devices connected between an outer contact each with respect to the movement path of the movable member and a contact located next to the latter, the movable part is adapted to form a galvanic connection between all contacts between and including said outer contacts in the closed state of the switching device, the switching device comprises members for detecting the direction of the current through the switching device in the closed state, and the control unit is adapted to control the movable part on the basis of information from said detecting members when breaking the current through the switching device to move in one or the other direction along the movement path thereon so as to open the first contact member by breaking the galvanic connection between a first outer contact and a first contact ad- jacent thereto and then open the second contact member through continued movement. By enabling a breaking of said current path in this way through movement of the movable part in two different directions the movable part may in principle instantaneously upon arising of a desire of breaking be acceler- ated so as to open the first contact member, since the movement direction may be selected so that the semiconductor device then intended to take over the current is in the conducting state. This means that the harmful influence which may be caused by a fault along the current path may be reduced to a minimum. This is a great advantage with respect to the preferred embodiment firstly mentioned above, in which the movement di- rection of the movable part when breaking the current path is determined and it has to be awaited for the correct current direction before the operation may take place, which may cause a delay of as much as half a period of the alternating voltage.

According to another preferred embodiment of the invention the control unit is adapted to control the movable part along a substantially arc-shaped movement path. The switching device may in an opened state thereof be brought to the closed state through a movement in the direction desired by rotating the movable part, so that the closed state may be obtained in the most rapid possible way, so that in the case of opening and closing through movement in two different directions as above the rotation direction giving possibility to the quickest closing with respect to the position of the alternating voltage across the switching device may be chosen. It will also be easier to operate the switching device through for example an electric motor.

According to a very preferred embodiment of the invention the switching device comprises an electrically controlled driving member adapted to carry out said single mechanical movement, and it is in particular advantageous if this driving member is an electromagnetic machine in the form of an electric motor. It is possible by using such a driving member to control the move- ment of the movable part for breaking and closing very accurately and for example ensure that the separation of the two contacts takes place at a particular phase position of the alternating current. By arranging a control unit in the form of an electronic unit adapted to control the driving member it is then also possible to influence the movement of the movable part also when this has already been started so as to make adaptions to new measured values of parameters, such as current and voltage, and possibly interrupt the entire procedure, if it is discovered that there is no longer any need thereof or that the movement should for example rather take place in the opposite direction. Furthermore, this embodiment is suited for co-ordina- tion with a prediction of the future development of the current through the switching device, such as a future zero-crossing of the current so as to co-ordinate a breaking of the current through the switching device with such a prediction, for example for ensuring that said component with ability to block current will only conduct current during a so called short half wave.

According to another preferred embodiment of the invention the movable part is rigidly connected to an axle, and the control unit is adapted to control a driving arrangement to rotate the axle for moving the movable part along said path. This way of accomplishing the movement of the movable part enables a simple design of the driving arrangement and a possibility to a movement with a high acceleration.

According to another preferred embodiment of the invention the switching device comprises a first piece rotatably received in a second piece, the movable part is arranged on one of the pieces and the contacts along said movement path are arranged on an- other of the pieces, and the two pieces through the contacts form entirely or partially a cylinder each having the height and diameter being adapted to the operation current through the switching device in the closed position and the voltage it has to hold in the opened state, respectively. A switching device adapted to the electrical conditions prevailing where it is to be used may in this way be realized in a simple way.

According to another preferred embodiment of the invention the switching device comprises members adapted to substantially continuously detect the direction and the magnitude of the current through the switching device and send information thereabout to the control unit, which makes it possible for the control unit to instantaneously react upon irregularities of the current, which could motivate a breaking of the current path in question. According to another preferred embodiment of the invention the switching device comprises a current limiting device connected in parallel with the semiconductor device, and said current limiting device is adapted to start conducting at a voltage the- reacross close to the maximum voltage withstood by the component. By the fact that in the closed and opened state of the switching device no voltage will be applied across the component and thereby neither across the current limiting device this is possible, so that this will not be heated by any leak currents therethrough. Through the voltage limiting device, which may be a varistor, the first voltage peak occurring across the component through the returning voltage after opening the first contact member may be limited, which in the case of one single component makes it possible to dimension it for being able to hold a lower returning voltage in the blocking direction thereof and thereby be less expensive than otherwise. In the case of a plurality of components connected in series the number of such components connected in series having a determined voltage withstanding capability may be reduced through an arrangement of such a voltage limiting device in parallel with each component. It is hereby avoided that any individual component gets a higher voltage thereacross than it may withstand, while other components get a lower voltage thereacross.

According to another preferred embodiment of the invention the switching device comprises means adapted to influence the voltage to increase when separating two contacts in connection with opening of the first contact member. The voltage at the contact separation is normally in the order of 12-15 V, and it drives the transfer of the current to the component connected in parallel therewith. The higher this voltage the quicker the current may be fed into the component. Less material wear is obtained by the arrangement of this means and the contact position will also be more stable with respect to the insulation. According to another preferred embodiment of the invention said means comprises a plurality of first contact members connected in series and adapted to be opened substantially simultaneously for transferring the current to the component. The voltage for driving the conduction of the component may be increased through such a series connection of a plurality of contact members, since this voltage will be formed by an addition of the voltages of the contact members connected in series with exactly said advantageous result as a consequence.

According to another preferred embodiment of the invention said means are formed by the fact that the contacts included in the first contact member have at least a part of ablating material adapted to be heated and evaporated to gases for gas blowing on an arc when separating two contacts when opening the first contact member, which also causes a higher arc-voltage and a faster commutation of the current to the component.

According to a preferred embodiment of the invention the semi- conductor device is a diode, which often will be preferred, since such a solution is inexpensive with respect to other controllable semiconductor devices and also very reliable. However, it is also conceivable that the semiconductor device is controllable, such as a thyristor, and it may also be of turn-off type, such as a GTO or an IGBT, for enabling a quicker breaking process. It could also in some situations be advantageous to arrange a bidirectional semiconductor device, i.e. a semiconductor device which may block and conduct in both directions, such as a BCT (bidirectionally controlled thyristor).

If a semiconductor device of a material having a wide energy gap between the valence band and the conduction band is used, i.e. an energy gap exceeding 2.5 eV, such as SiC and diamond, is used, comparatively high voltages may be handled by the switching device while utilizing a low number of semiconductor devices. The invention also relates to advantageous uses of a switching device as above in accordance with the appended claims, and advantages thereof appear without any doubt from the discus- sion above.

The invention also relates to a switch gear for supply of electricity within industry or in distribution and transmission networks provided with an electric switching device according to the invention. The method according to the invention is also excellently suited for being carried out through a computer program provided with suitable program steps, and the invention also relates to such a program as well as computer readable medium onto which such a program is recorded.

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:

Figs 1 -3 are simplified views illustrating an electric switching device according to a first preferred embodiment of the invention in a closed, temporary closed and opened position, respectively,

Fig 4 is a simplified view of an electric switching device according to a second preferred embodiment of the invention in closed position, Fig 5 is a simplified circuit diagram illustrating the principle of the function of the switching device according to Fig 4,

Figs 6-8 illustrate the switching device according to Fig 4 in an- other closed, a temporary closed and an opened position, respectively,

Fig 9 is a simplified view of an electric switching device according to a third preferred embodiment of the invention in a closed position,

Fig 1 0 is a simplified circuit diagram illustrating the principle of the function of the switching device according to Fig 9,

Figs 1 1 and 12 are views illustrating the switching device according to Fig 9 in temporary closed and opened position, respectively,

Fig 13 illustrates schematically additional embodiments of the invention,

Figs 14-16 illustrate schematically further additional preferred embodiments of the invention,

Fig 17 illustrates how the current I through and the voltage U across the semiconductor devices of the embodiments according to Fig 16 is developed versus time in comparison with the embodiment according to Fig 14,

Figs 18 and 19 are schematic circuit diagrams illustrating two possible ways of arranging electric switching devices according to the invention for start of an electric motor,

Figs 20 and 21 illustrate a part of a switching device in two dif- ferent positions when breaking the current therethrough, Figs 22 and 23 illustrate very schematically switching devices according to two further preferred embodiments of the invention, and

Fig 24 illustrates how a switching device according to the invention may be arranged together with two other such switching devices in a current path in the form of a three-phase alternating voltage line.

DETAI LED DESCRI PTION OF PREFERRED EMBODIMENTS OF TH E INVENTION

An electric switching device for alternating current according to a first preferred embodiment of the invention is schematically illustrated in Fig 1 and this is connected in a current path 1 for being able to rapidly open and close this path. 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 2 being ar- ranged to rotate around an axie 3 and has a movable contact part 4. A second cylinder 5 is arranged externally of the cylinder 2 and has four contacts 6-9 arranged along the movement path of the movable part 4 and adapted to form good electric contacts while bearing against the movable part 4. The switching device is connected in said current path through the two outer contacts 6 and 9, respectively.

A semiconductor device in the form of a diode 1 0, 1 1 is connected between the two outer contacts and the inner contact lo- cated next thereto with the conducting direction from the outer to the adjacent contact. The diodes could just as well both be directed with the conducting direction towards the outer contact. A member 48 increasing the resistance is arranged between the connection of the respective semiconductor device 10, 1 1 to the current path 1 and the closest contact 6 and 9, respectively. This resistance increasing member is intended to be controlled either to have a neglectible resistance in the closed state of the switching device according to Fig 1 or receive a comparatively high resistance for taking voltage thereacross in the way described further below. The resistance increasing member could be a resistor having a controllable resistance, such as a powder having a very low resistance when applying an outer pressure thereonto, but which gets a high resistance when the pressure is removed, or a controllable semiconductor device having a low on-state voltage, but which may be turned off for considerably increasing the resistance thereby.

The switching device has also a driving arrangement adapted to drive the inner cylinder 2 to rotate for movement of the movable contact part 4 with respect to the other contacts 6-9. If the driv- ing arrangement is in this case constituted by an integrated electric motor 12, schematically indicated, which may be of many different types, such as for example a PM-motor, reluctance motor, step motor, DC-motor. The motor does not have to be located in the same plane as the contacts and will thereby not influence the voltage resistance. However, it is pointed out that the driving may very well take place in another way, such as through a motor arranged to act directly upon the axle 3.

Furthermore, the switching device is provided with a detecting member 13 schematically indicated and adapted to detect the direction and the magnitude of the current in the current path and send information thereabout to a unit 14 adapted to control the motor 12 and thereby the movement of the movable contact part 4 on the basis of this information. The control unit will in this way all the time be aware of what the current instantaneously looks like so as to be able to instantaneously control the movable contact part in a desired way.

The height of the two cylinders 2, 5 and by that the length of the contacts perpendicularly to the drawing plane is proportional to the operation current for which the switching device is dimen- sioned , i.e. the current allowed to continuously flow in the current path 1 during normal conditions. The diameters of the cylinders are in their turn proportional to the magnitude of the alternating voltage of the current path.

The function of this electric switching device is as follows: When there is a desire to break the current path 1 , for example since the detecting member 13 detects a very high current in the current path 1 , which may be caused by a shortcircuit therealong , the control unit 14 then firstly decides in which direction the movable contact part 4 shall be moved for enabling breaking as quick as possible. This decision depends upon in which position the current in the current path is in exactly that moment. In the closed position according to Fig 1 the entire current flows in fact through the switching device between the two outer contacts 6, 9 through the movable part 4 interconnecting the former gal- vanically. When now a breaking is to take place the current has to be transferred to flow through one of the diodes instead as soon as possible. The current may be switched in to one diode during the part of an alternating current period being between the time shortly before the diode gets forward biased until the diode gets reverse biased next time. This means when a whole period is 20 ms in the practice that an opening of the conducting path in parallel with the diode in question may take place for ex- ample about 2 ms before zero-crossing towards the forward biased direction until the next zero-crossing. By using an electrically controlled driving member, an electronic unit for controlling thereof and a prediction of a future zero-crossing of the current the opening of this first contact member may be controlled to take place substantially at such a zero-crossing , which means within about 0.5 ms before and about 0.5 ms after such a zero- crossing. This means that the current to be commutated over to flow through the diode is small and the commutation may therefore take place quickly without any high demand on means for increasing the voltage across this contact member. We now assume that the current in the current path 1 upon a desire of breaking has the direction indicated through the arrows 15 in Fig 1 and is within the interval allowing transfer of the current to the diode 10. The control unit 14 controls then through the motor 12 the movable contact 4 to rotate clockwise so as to open a first contact member connected in parallel with the diode 10, which here is constituted by the contacts 6, 7 and the movable part 4. The temporary closed position shown in Fig 2 is then obtained. When this takes place a small spark is formed in the gap 16 between the movable part 4 and the contact 6, which results in a voltage of usually 12-15 V, which will drive the transfer of the current through the diode 10, so that the current will now instead flow through the diode 10, the contact 7, the movable part 4 and the contact 9. By co-ordinating the separa- tion of the contact 6 and the movable part 4 with a control through the control unit 14 of the resistance increasing member 48 to increase its resistance a voltage may very rapidly be built up across the diode 10 and thereby the transfer of the current to flow through the diode may be fast. With reference to Figs 20 and 21 a further possible way to make the transfer of the current quicker will be described further below.

When the voltage across the switching device then changes direction no current will flow therethrough, but a voltage will be built up across the diode 10 then reverse biased and the rotation movement of the movable contact part 4 is now continued in the same direction as before, so that the galvanic connection between the contact 7 and the contact 9 is broken, in which this breaking may take place arc-free, since no current flows through the contact site at the time for the breaking. The completely opened position according to Fig 3 is thereby obtained. It is in this breaking important that it takes place so fast that the voltage across the diode 10 will not change direction again and this starts to conduct. Should the voltage across the switching de- vice be during the second half period when detecting the need of breaking the current path 1 the control unit would instead control the movable contact part 4 to rotate counter-clockwise as seen in the Figures for utilizing a diode 1 1 and breaking instead . A possibility to a very rapid breaking of the current path is obtained in this way, and the switching device may even in the most unfavourable position of the alternating voltage when detecting a need of a breaking be brought between the closed position and the completely opened position according to Fig 3 within a substantially shorter time than a period, usually always within 15 ms for a frequency of 50 Hz of the alternating voltage.

By the fact that the current in the closed position of the switching device never flows through the diodes the contacts 4, 6 and 9 have only to be dimensioned for the operation current, which for example may be 1000 A, while the diodes 10, 1 1 are dimen- sioned for a possible shortcircuit current, which in such a case could be 25 kA. However, they only have to withstand that voltage during a very short time, and the dimensioning of the diode may be made without any considerations taken to any continuous operation current through the switching device. Further- more, the diodes have to be dimensioned for a returning voltage that during a short period of time is applied thereacross after opening said first contact member. This may in the case of a network voltage of 12 kV for example be about 20 kV. However, the very contact members of the switching device have in the opened position according to Fig 3 to be able to withstand a considerably higher so called impulse voltage, which in this case could be 75 kV.

The switching device may advantageously be arranged in such a way that the breaking location 17 in the position according to Fig 3 is visible, i.e. as disconnector, so that works may be carried out along the current path in this position. The opening and the closing of the switching device through one single mechanical movement of one single movable part results in the advan- tages mentioned further above. En electric switching device for alternating current according to a second preferred embodiment of the invention is illustrated in Figs 4-8, and this will now be described somewhat more briefly than the one according to Figs 1 -3, while concentrating upon the differences of these embodiments. The main difference therebetween consists in the fact that the movable part 4 is here directly connected to said current path on one side of the switching device. The movable part and the three contacts 18-20 arranged along the movement path thereof could for the rest be arranged on cylinders in the same way as for the embodiment describe above and the movable part may be driven to rotate in the same way as there. The movable part is in this embodiment driven to move through an electrically controlled driving member 49 in the form of an electric motor, which is arranged to be con- trolled by a control unit 14 in the form of an electronic unit, so that the movement of the movable part 4 may be controlled very accurately and be adjusted or interrupted as long as it goes on. Voltage increasing means 50 corresponding to the resistance increasing members 48 of the first embodiment are also present in this embodiment, which here comprise a charged capacitor adapted to be switched in between the two contacts 18, 19 and 19, 20, respectively, of the first contact member when this is to be opened so as to quickly transfer the current to the diode 21 and 22, respectively.

A semiconductor device in the form of a diode 21 , 22 here connects each outer contact 18, 20 as seen in the movement path with the intermediate contact 1 9, in which one of the diodes has its conducting direction towards the mid contact and the other its conducting direction away from the latter. The equivalent circuit diagram to the switching device according to Fig 4 is shown in Fig 5, in which it is shown how a load 23 may be connected to the movable contact 4.

The function of this switching device is as follows. When a desire of an opening of the current path 1 occurs the choice of ro- tation direction of the movable part 4 is made according to the same conditions depending upon information from the detecting member 13 as for the embodiment according to Figs 1 -3. We now imagine that the current through the switching device has the direction shown through the arrow 15 in Fig 4 and this then flows through the switching device through the movable part 4 and the mid contact 19. Breaking now takes place by bringing the movable part 4 to rotate clockwise in the Figures firstly past the position according to Fig 6 and then further to the one ac- cording to Fig 7, in which the current is transferred over to the diode 22 in the same way as described above when breaking the galvanic connection between the contacts 19 and 20. The rotation is then continued until the entirely opened position according to Fig 8.

An electric switching device for alternating current according to a third preferred embodiment of the invention is illustrated in Fig 9, and this functions according to the same principles as the two embodiments mentioned above, but the movable contact part 4 is here instead movable along a rectilinear movement path. It functions for the rest in the same way as if one diode and one of the mid contacts, for example the diode 1 1 and the contact 8, should have been removed from the embodiment according to Fig 1 . Accordingly, this means that the opening of the current path could take longer time should the current not have such a direction that the breaking process may be started immediately when a need of breaking occurs. Costs are instead saved by dividing the number of required semiconductor devices by 2. The equivalent circuit is here shown in Fig 10, where it is indicated that a load 23 is connected to the one outer 26 of the contacts 24-26. It is shown in Figs 1 1 and 12 how the movable part 4 is firstly moved along a rectilinear movement path for obtaining the temporarily opened position, in which the current is transferred to the semiconductor device 27, to the entirely opened position according to Fig 12. The reliability of this switching device will be very high thanks to a minimum number of components, and it also gets inexpensive to produce. This embodiment comprises also a possibility to ground the electric circuit after carrying out the breaking , which is obtained by the fact that a component 51 connected to ground is arranged along the movement path of the movable part 4 so as to come into contact with the movable part after the current breaking is carried out.

A further preferred embodiment of the invention is schematically illustrated in Fig 13 and this is based upon the idea to try to in- crease the voltage of at least a spark created when separating two contacts in connection with opening of the first contact member of a switching device according to the invention, i.e. a voltage that will then be created across the semiconductor device and drives the transfer of the current therethrough. This is in the example shown achieved by a series connection of a plurality of such first contact members 28, 29 in parallel with the semiconductor device 30. For each such contact member connected in series the voltage will be for example about 12-15 V, so that the voltage driving the current through the diode 30 in the example shown will be twice as high as when arranging only one contact member in parallel therewith. Advantages of this have been discussed further above. When three contact members are connected in series the voltage gets three times higher and so on. The series connection of the contact members may for example be achieved by arranging a plurality of contact discs, one for each contact member, having different diameters, on an axle with the contacts of the largest disc connected to the current path on both sides of the series connection. It would also be possible to design the contacts included in such a first contact member with at least one part of ablating material adapted to be heated and vaporised to gases for gas blowing of the arc when separating two contacts when opening the first member, which would mean a higher arc voltage. Such an ablating material is a material that may be evaporated to gases. It is schematically illustrated in Fig 14 how a plurality of semiconductor devices 31 -33 may be connected in series for jointly being able to hold a determined returning voltage after breaking the current path. Thus, each diode symbol may in all the em- bodiments shown above be replaced by a number of diodes connected in series in this way. It is also possible to achieve that the number of semiconductor devices required for a certain voltage gets low by choosing a material having a wide bandgap between the valence band and the conduction band, such as SiC or diamond.

Furthermore, it is shown in Fig 15 how it is possible to connect semiconductor devices 31 , 31 ' in parallel with each other so as to take a certain shortcircuit current or simply for redundancy reason, so that the switching device may function in a desired way even if any diode in one so called package of diodes connected in series gets broken.

Furthermore, it is illustrated in Fig 16 how a varistor 34, pref- erably of ZnO, may be connected in parallel with the respective semiconductor device 35, in which the varistor is adapted to start conducting at a voltage thereacross close to the maximum voltage that may be withstood by the semiconductor device. This may be accomplished by the fact that the varistors do not nor- mally conduct any current at all , since no voltage will be applied thereacross, but they will only receive a voltage thereacross in connection with the transition between the temporarily closed and the completely opened position. It is illustrated in Fig 17 how the voltage U over the semiconductor devices 35 in the re- verse direction thereof is developed over time t when the voltage increases thereacross in the temporarily closed position at the time zero. The dashed line shows how the voltage across the diodes is developed in the absence of varistors and the solid line with varistors. Thus, it appears that the varistors cut the first voltage peak off. This would in a system having a network voltage of 12 kV and a normal returning voltage of 22 kV mean that the number of five 5 kV diodes normally required may through the arrangement of the varistors be reduced to four, since the first voltage peak then could be reduced to 18 kV. The change of the current I is illustrated to the left of (before) the time 0. By connecting a separate varistor in parallel with each semiconductor device in this way it is avoided that any individual semiconductor device gets a higher voltage thereacross than it may withstand, while other semiconductor devices get a lower voltage thereacross. It is also possible to arrange resistances or capacitances connected in parallel with the semiconductor devices for distributing the voltage substantially equally over the semiconductor devices.

A possible application of a breaker according to the invention for motor starts is illustrated in Fig 18 and 19. It is shown how two switching devices according to Fig 4 have been arranged, in which these may have a movable part 4 in common, which could be achieved by arranging three contacts and two semiconductor devices in Fig 4 in the same way at the bottom and at the top. One switching device is then connected to the motor 36 through a reactor 37 while the other is directly connected to the motor. Most power networks are not sufficiently stiff for allowing a start of big motors directly connected thereto, since these absorb so much power that the voltage on the network would fall far too much. This problem may be solved by starting the motor according to different start methods, such as reactor start, capacitor start or transformer start, in which the reactor start is illustrated here. When the motor is to be started it is the left switching device shown in Fig 18 which is brought into a closed position, which corresponds to the position according to Fig 4, so that the motor 36 gets its feeding through the reactor 37. Should there now for some reason be a desire to interrupt this start the contacts 39-41 of this switching device are opened according to the sequence 41 , 40, 39. When the motor has then obtained a synchronous number of revolutions the reactor 37 may be disconnected by bringing the switching device 38 in opened position, while the switching device 42 is brought into closed position.

Should a shortcircuit occur in any equipment connected to the current path 1 , the motor 36 will then start to operate as a generator and contribute with power to the fault location before the fault has been disconnected. It is here a possibility to restrict the effect of this by closing the switching device 38 and opening the switching device 42 in such a case, so that the shortcircuit contribution from the motor to the fault location is limited and the breaking of the motor is at the same time reduced. Should a short circuit occur in the motor or a planned stop being made, then the switching device 42 is opened.

Both switching devices 38 and 42 are in Fig 19 summarized through a box 43 and it is here shown that the switching devices just as well may be arranged in direct connection to the motor with a reactor arranged between the switching devices and the alternating voltage network.

It is illustrated in Fig 20 what means adapted to influence the voltage to increase upon separation of the two contacts in connection with opening of the first contact member may look like. We now assume that the first contact member has two fixed contacts 44, 45, which are adapted to be galvanically connected through a movable part 46 in the closed state. The movable part 46 is at one end thereof provided with a portion 47 of a material having a comparatively high resistivity, so that the resistance between the movable part 46 and the contact 44 and thereby between the two contacts 44 and 45 is increased in the beginning of said separation (the position according to Fig 21 ) while allowing current between these contacts therethrough, so that a voltage that will drive the transfer of the current through the semiconductor device will be increased. The portion 46 may for example be made of graphite. An electric switching device according to a further preferred embodiment of the invention of the type where the movable part carries out a rectilinear movement is illustrated in Fig 22. The electric switching device is shown in different positions corre- sponding to the positions according to Figs 1 -3 of the embodiment shown there. The main contacts 52, 53 are in this embodiment provided by tubes 57, 59 displaceable in each other and are adapted to give the switching device a low-ohmic contact. When the current through the switching device is to be bro- ken the movement of the two contacts 52, 53 are synchronized out of engagement with each other in the way described above, so that the current is directed in the conducting direction of the diode 54 and this may take over the current. The continued movement of the contacts 52, 53 away from each other is syn- chronized so that the current through the diode 54 manages to reach zero and a voltage is built up thereover in the reverse direction before a contact 55 of rod-like type leaves the physical contact with contacts 56 of an inner tube 58 connected to the tube 57. Breaking will hereby take place at zero current.

Fig 23 illustrates a further development of the embodiment according to Fig 22, which enables a quick breaking of the current, since it is not necessary to wait until the current through the switching device has a certain direction. Opposite parts of the switching device have here fixedly arranged an inner contact tube 60 connected in series with the diode 54 and an outer contact tube 61 . A contact tube 62 having contacts 63 for making contact with the respective tube 61 and contacts 64 for making contact with the inner tube 60 is arranged between these. The function of this switching device is apparent, and we assume that the current flows through the switching device in the direction shown in Fig 23 when a desire of breaking occurs. Thus, the current flows through the two tubes 61 and the tube 62 in the main current path. The tube 62 is then controlled to move downwardly as seen in Fig 23, so that the tube 62 is separated from the upper tube 61 and the current thereby is com- mutated into the upper diode 54. When then the current has passed zero breaking is completed by continuing the movement of the tube 62 downwardly, so that the contact 64 thereof leaves the contact with the upper contact tube 61 . It is evident that the movement of the movable contact tube 62 takes place in the opposite direction, i.e. upwardly as seen in Fig 23, should the current through the switching device be of the opposite direction when a desire of breaking occurs.

It is illustrated in Fig 24 how a plurality of electric switching devices 65 according to the invention are arranged to be each connected to a phase 66-68 each of a three-phase alternating current line. The movable parts 4 of the switching devices are connected to each other and arranged to be controlled through a movement of one single axle 69, so that the different phases are broken or closed in time displacement with respect to each other. Connecting members 70, 71 are arranged with each movable part for connection thereof to the movement of the axle so that they begin to be influenced by the axle to move in positions being mutually displaced along the rotation path of the axle, so that it gets possible to open one phase at a time, and preferably chose which phase is to be opened first depending upon detection of for example the current in the different phases.

Another advantage of an electric switching device according to the invention is obtained thanks to the fact that in the case of a three-phase voltage, which is most usual, the three electric switching devices for each phase are arranged controllable entirely independently of each other, which is not the case for such switching devices already known , which are mechanically interconnected with each other, so that they have to be all opened or closed simultaneously. When a fault occurs close to a generator connected to an alternating current network it is possible that an asymmetry of voltage may exist in any of the phases and it takes several periods before it gets zero, which means that it has for electric switching devices already known been a neces- sity to wait with the breaking until it is certain that a zero crossing has been obtained for all phases, which may mean a delay in the order of 100 ms. A breaking of the phases where symmetry exists may thanks to the arrangement according to the in- vention of electric switching devices being independently controllable take place earlier than for a phase with said asymmetry, so that the harmful consequences of the currents created through a fault may be reduced considerably.

Preferred uses of an electric switching device according to the invention is as current limiter or connected in series with a current limiter or as a breaker, as protection for obtaining current breaking and/or disconnecting of parts in an electric circuit located on both sides thereof upon occurrence of faults, such as shortcircuits, for switching in and/or out normal operation currents of an electric circuit, as disconnector, as grounder for grounding an electric circuit, for switching in and out a generator with respect to an alternating voltage network, for switching in and switching out a resistive load with respect to an alternating voltage network, for switching in and switching out a resistive, capacitive or inductive load with respect to an alternating voltage network, for breaking current paths in switch gears for supply of electricity in industry or in distribution or transmission networks and for reactor start of an electric motor connected to an alternating voltage network.

Preferred is also a switching device according to claim 1 comprising current measuring members, an electronic unit adapted to carry out a current prediction algorithm and an electrically controlled driving member, such as a motor, for obtaining opening of the first contact member substantially at a zero crossing of the current through the switching device.

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 skilled in the art without departing from the basic idea of the invention as defined in the claims.

It would for example be possible to remove one contact and the semiconductor device connected thereto from the embodiment according to Fig 4, but the opening may then only take place by movement in one single direction and the breaking time will be longer. One of the inner contacts and a semiconductor device connected thereto may also be removed from the embodiment according to Fig 1 with the same result as a consequence.

The embodiments above having a rotation movement could be modified to have a rectilinear movement of the movable part instead, while a change in the opposite direction may take place for the embodiment according to Fig 9.

It would also be possible to exchange the diodes shown above against other semiconductor devices having ability to block in at least one direction in accordance with the discussion above.

It is furthermore pointed out that "comprising two branches connected in parallel with each other in the current path" as used in the claims also comprises the existence of more than two branches connected in parallel with each other in the current path.

Claims

1 . An electric switching device for alternating current comprising in a current path two branches connected in parallel with each other, a first of them comprising a first contact member having two contacts (4, 6, 7) movable with respect to each other for opening and closing and the second comprising a component (10) able to block current therethrough in at least a first blocking direction and conduct current therethrough in at least one direc- tion, in which a second contact member having two contacts (4, 7, 8) movable with respect to each other for opening and closing is connected in series with the component, and in which the switching device also comprises a unit (14) adapted to control opening of said current path by controlling the first contact member to open for transferring the current to the component when this is in or is going into the conducting state and then the second contact member to open when the component is in a state of blocking current therethrough for breaking the current through the switching device, characterized in that the switch- ing device comprises a movable part (4) and that the unit is adapted to control this part to carry out one single mechanical movement for opening and closing the two contact members.

2. A switching device according to claim 1 , characterized in that it comprises at least two (19, 20) contacts consecutively arranged along the path of the movable part (4) for said mechanical movement, that the movable part is adapted to form a galvanic connection between said two contacts in the closed state of the switching device, that the switching device is adapted to be switched in in a current path going through the switching device through one (19) of the contacts and the movable part, that the semiconductor component (22) connects the two contacts with each other, and that the control unit (14) is adapted to control opening of said current path by controlling the movable part to move along such a path and in such a direction that the galvanic connection between one (19) of said two contacts and the movable part is firstly broken for opening the first contact member and the galvanic connection between the second (20) of said contacts and the movable part is then broken for opening the second contact member.

3. A switching device according to claim 1 or 2, characterized in that it comprises at least three (6-9, 18-20, 24-26) contacts consecutively arranged along the path of said movable part (4) for said mechanical movement, and that the component (1 0, 1 1 , 21 , 22, 27) connects a first and second adjacent of these three contacts with each other.

4. A switching device according to claim 3 not dependent upon claim 2, characterized in that the movable part (4) is adapted to form a galvanic connection between said three contacts (6-9, 24-26) in the closed state of the switching device, that the control unit is adapted to control the movable part to move along such a path and in such a direction for breaking the current through the switching device that first the galvanic connection between the first contact (6; 9, 24) most far away from the third of the contacts and the two other contacts is broken for opening the first contact member and then the galvanic connection between the second (7; 8, 25) and the third contact (8; 7, 26) is broken for opening the second contact member.

5. A switching device according to any of claims 1 -4, characterized in that the component is a semiconductor device (10) and that the unit (14) is adapted to control the first contact member to open when the semiconductor device is in or going into a conducting state.

6. A switching device according to claims 3 and 5 or 4 and 5, characterized in that it comprises at least two said semiconductor devices (10, 1 1 , 21 , 22) connected between an outer contact each with respect to the movement path of the movable member and a contact located next to the latter, that the mov- able part (4) is adapted to form a galvanic connection between all contacts between and including said outer contacts in the closed state of the switching device, that the switching device comprises members ( 13) for detecting the direction of the cur- rent through the switching device in the closed state, and that the control unit (14) is adapted to control the movable part on the basis of information from said detecting members when breaking the current through the switching device to move in one or the other direction along the movement path thereof so as to open the first contact member by breaking the galvanic connection between a first outer contact and a first contact adjacent thereto and then open the second contact member through continued movement.

7. A switching device according to claim 6 not dependent upon claim 2, characterized in that it is arranged to be connected into a current path through the switching device through said two outer contacts (6, 9), that the contacts (6-9) along the path of the movable member for the mechanical movement are four to the number, that the semiconductor devices (1 1 , 12) are adapted to block current therethrough either both of them to or both of them from the respective outer contact, and that the control unit (14) is adapted to control the movable part (4) so that the galvanic connection between said first adjacent contact and the contact adjacent to the other outer contact is broken after opening the first contact member for opening the second contact member.

8. A switching device according to claim 6 not dependent upon claim 4, characterized in that the contacts (18-20) arranged along the movement path of the movable part are three to the number, that the switching device is adapted to be connected into the current path through the switching device through the mid contact (19) and the movable part (4), that one (21 ) of the semiconductor devices is adapted to block current therethrough to the mid contact and the other (22) to block current there- through from the mid contact, and that the control unit (14) is adapted to control the movable part so that the galvanic connection between the first outer contact (18, 20) and the movable part (4) is broken after opening the first contact member for opening the second contact member.

9. A switching device according to any of claims 2 to 8, characterized in that the control unit is adapted to control the movable part (4) along a substantially rectilinear movement path.

10. A switching device according to any of claims 2-8, characterized in that the control unit is adapted to control the movable part (4) along a substantially arc-shaped movement path.

1 1 . A switching device according to claim 10, characterized in that the movable part is rigidly connected to an axle (3), and that the control unit is adapted to control a driving arrangement to rotate the axle for moving the movable part along said path.

12. A switching device according to claim 10 or 1 1 , characterized in that it comprises a first piece (2) rotatably received in a second piece (5), and that the movable part (4) is arranged on one of the pieces and the contacts (6-9) along said movement path are arranged on another of the pieces.

13. A switching device according to claim 12, characterized in that the two pieces through the contacts form entirely or partially a cylinder (2, 5) each having the height and diameter being adapted to the operation current through the switching device in the closed position and the voltage it has to hold in the opened state, respectively.

14. A switching device according to claim 12 or 1 3, characterized in that the movable part (4) is arranged on the first piece, and that a motor (12) is integrated into the pieces so as to drive them to rotate with respect to each other.

15. A switching device according to any of claims 1 -13, characterized in that is comprises a driving member (49) being electrically controlled and adapted to carry out said single mechanical movement.

16. A switching device according to claim 15, characterized in that the driving member (49) is an electromagnetic machine.

17. A switching device according to claim 16, characterized in that the driving member (49) is an electric motor.

18. A switching device according to any of claims 15-17, characterized in that it comprises a control unit in the form of an electronic unit adapted to control said driving member.

19. A switching device according to any of the preceding claims, characterized in that it comprises members (13) adapted to substantially continuously detect the direction and the magni- tude of the current through the switching device and send information thereabout to the control unit (14).

20. A switching device according to any of the preceding claims, characterized in that it comprises a plurality of said compo- nents (31 -33) connected in series and adapted to together hold a voltage over the switching device in a blocking state.

21 . A switching device according to any of the preceding claims, characterized in that it comprises a plurality of said compo- nents (31 , 31 ') connected in parallel with each other and adapted to together take care of the current through the switching device after opening the first contact member.

22. A switching device according to any of the preceding claims, characterized in that a voltage limiting device (34) is connected in parallel with the component (35), and that said device is adapted to start conducting at a voltage thereacross close to the maximum voltage withstood by the component.

23. A switching device according to claims 20 and 22, charac- terized in that each component has a said voltage limiting device (34) each connected in parallel therewith for a substantial even distribution of the voltage across the series connection of the components onto the individual components.

24. A switching device according to claim 22 or 23, characterized in that the voltage limiting device is a varistor.

25. A switching device according to any of the preceding claims, characterized in that it comprises means adapted to act in- creasingly upon the voltage when separating two contacts in connection with opening the first contact member.

26. A switching device according to claim 25, characterized in that said means comprise a plurality of first contact members connected in series adapted to be opened substantially simultaneously for transferring the current to the component.

27. A switching device according to claim 25 or 26, characterized in that said means comprise a component (47, 48) adapted to increase the resistance between said two contacts (44, 46; 6, 7; 8, 9) at the beginning of said separation thereof while allowing a current between these contacts therethrough.

28. A switching device according to claim 27, characterized in that said component (48) increasing the resistance is formed by a semiconductor device being controllable to be turned off so as to increase the voltage between the two contacts of the first contact member.

29. A switching device according to claim 27, characterized in that said component increasing the resistance is formed by a re- sistor (48) having a controllable resistance and adapted to have an unimportant resistance in the closed state of the switching device and be controlled to get a substantial resistance for increasing the voltage between the two contacts of the first con- tact member.

30. A switching device according to claim 25, characterized in that said means increasing the voltage comprise a charged capacitor (50) adapted to be connected between the two contacts of the first contact member when this is to be opened.

31 . A switching device according to claim 25 or 26, characterized in that said means are formed by the fact that the contacts included in the first contact member have at least a part of ab- lating material adapted to be heated and evaporated to gases for gas blowing on an arc when separating two contacts when opening the first contact member.

32. A switching device according to any of the preceding claims, characterized in that it is adapted to be connected together with two other such switching devices to a phase (66-68) of its own of a three phase alternating current line, and that the movable parts (4) of the switching devices are mechanically connected to each other and adapted to be controlled through a movement of one single part (69), so that the different phases are broken or closed timely displaced with respect to each other.

33. A switching device according to claims 10 and 32, characterized in that said single part is an axle (69) and that the mov- able parts of the respective switching device (65) are adapted to start to be influenced by the axle to move in positions mutually displaced along the rotation path of the axle.

34. A switching device according to claim 5 or claim 5 and any of the other preceding claims, characterized in that the semiconductor device is a diode.

35. A switching device according to claim 5 or claim 5 and any of claims 1 -33, characterized in that the semiconductor device is controllable.

36. A switching device according to claim 5 or claim 5 and any of claims 1 -33, characterized in that the semiconductor device is of turn-off type.

37. A switching device according to claim 35, characterized in that the semiconductor device is a thyristor.

38. A switching device according to claim 36, characterized in that the semiconductor device is bi-directional, i.e. it is able to block and conduct in both directions.

39. A switching device according to claim 5 or any of claim 5 and the other preceding claims, characterized in that the semiconductor device is of a material having an energy gap between the valence band and the conduction band exceeding 2.5 eV, such as SiC and diamond.

40. A switching device according to any of the preceding claims, characterized in that it is designed to operate at a system volt- age between 1 -52 kV.

41 . A switching device according to any of the preceding claims, characterized in that it is adapted to withstand at least an operation current of 1 kV, preferably at least 2 kA, in the closed state.

42. A use of a switching device according to any of the preceding claims as current limiter or connected in series with a current limiter or as a breaker.

43. A use of a switching device according to any of claims 1 -41 as a protection for obtaining current breaking and/or disconnection of parts in an electric circuit located on both sides thereof when a fault occur, such as a shortcircuit.

44. A use of a switching device according to any of claims 1 -41 for switching in and/or switching out normal operation currents of an electric circuit.

45. A use of a switching device according to any of claims 1 -41 as a disconnector.

46. A use of a switching device according to any of claims 1 -41 as a grounder for grounding an electric circuit.

47. A use of a switching device according to any of claims 1 -32 for switching in and switching out a generator with respect to an alternating voltage network.

48. A use of a switching device according to any of claims 1 -41 for switching in and switching out a resistive load with respect to an alternating voltage network.

49. A use of a switching device according to any of claims 1 -41 for switching in and switching out a capacitive load with respect to an alternating voltage network.

50. A use of a switching device according to any of claims 1 -41 for switching in and switching out an inductive load with respect to an alternating voltage network.

51 . A use of a switching device according to any of claims 1 -41 for breaking current paths in switch gears for supply of electricity within the industry or in distribution and transmission net- works.

52. A use of a switching device according to any of claims 1 -41 for reactor start of an electric motor connected to an alternating voltage network.

53. A plant for a multi-phase network having a plurality of switching devices according to any of claims 1 -41 , one for each phase, characterized in that the switching devices are controllable independently of each other.

54. A switch gear for supply of electricity within the industry or in distribution or transmission networks, characterized in that it comprises an electric switching device according to any of claims 1 -41 .

55. A method for breaking a current path through an electric switching device for alternating current, in which a main current path through the switching device is opened and the current is transferred into a temporary current path through a component able to block current therethrough in at least a first blocking di- rection and the temporary current path is then broken and the current through the switching device is by that broken, characterized in that a movable part (4) opens both the main and the temporary current path by carrying out one single mechanical movement.

56. A method according to claim 55, characterized in that the current through the switching device is detected and a future zero-crossing of the current is predicted on the basis of this detection, and that the opening of the main current path is con- trolled to take place substantially at a zero-crossing of the current.

57. A computer program product adapted to be loaded directly into the internal memory of a computer and comprising software code portions for instructing a processor to carry out the steps according to claim 55 or 56 when the product is run on a computer.

58. A computer program product according to claim 57, provided at least partially over a network as the Internet.

59. A computer readable medium having a program adapted to make a computer control the steps according to claim 55 or 56 recorded thereon.