A TRANSFER SWITCH

Abstract: In a transfer switch for alternating current two electric switching devices of hybrid type are adapted to establish and break, respectively, a current path between a load and an alternating current supply each for transferring the supply of the load with electric power from one alternating current supply (5) to the other (6). The switching devices have components (15-18) blocking in at least one direction for establishing a temporary current path therethrough when breaking the current path through the switching device and enabling an arc-free completion of the breaking. A control unit (20) is adapted to control a movable part (4) to carry out one single mechanical movement for opening one of the switching devices and closing the other for changing the alternating current supply to the load.

Description: A TRANSFER SWITCH FIELD OF THE INVENTION AND PRIOR ART The present invention relates to a transfer switch for alternating current comprising two electric switching devices adapted to establish and breaking, respectively, a current path between a load and an alternating current supply for transferring the supply of electric power to the load from one alternating current supply to the other, each of said switching devices comprising in a cur- rent path therethrough two branches connected in parallel with each other, the first of the branches 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 di- rection 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 transfer switch also comprises a unit adapted to control opening of said current path through the switching device being closed by controlling the first contact member of this switching device 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 and the other electric switching device to close. Such transfer switches are usually used for rapidly switching in a spare supply to a load, such as an equipment within any industrial process, should the regular supply fail. Such failure may depend on for example a power failure of the regular supply or any fault thereof, such as a shortcircuit through for example an insulation fault with respect to ground. It is then usually a question of such loads that are sensitive to power failure, and for which power failures could cause high costs or other severe in- conveniences, such as danger for persons. One example of a technical field in which a failure of the supply of electric power to an industrial process would have consequences being devastating from the cost point of view are high temperature processes within the steel industry. However, it is within the scope of the invention to use such a transfer switch for switching between two different supplies depending upon completely other parameters, such as for example the price of electric power delivered from different places with respect to each other during different periods of time. "Load" is here to be given a broad sense and an alternating voltage network is for example also comprised, which in this way through a transfer switch may be provided with supply of electric power from different places. 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 to WO 98/149694. The invention is of course not restricted to any particular range of the operation current through such an electric switching device in the closed state of such a transfer switch, 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. 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 in conducting state really conducts, but this is also intended to cover that it may be brought to conduct in that moment if 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 such as defined here. It is also 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 spaced contacts so that these are no longer in connection with each other or by having a movable contact bearing against a fixed contact and this is moved away therefrom. Two movable contacts are also conceivable. "Gap" is also to be given a broad sense. It comprises also that an insulator is brought in 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" is then formed by the fact that two bodies forming a contact in a bearing position are displaced with respect to each other so that such in insulating surface forms the bearing surface of one or both bodies. The electric switching devices arranged in the transfer switch defined in the introduction are normally called hybrid breakers, and characterizing for them is that they are able to achieve an arc-free breaking of the current path through the switching device, since this takes place when the semiconductor device is in the 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 com- paratively high contact pressure for a good contact is needed , which consumes a not negligible amount of energy. Said first contact member is in transfer switches of the type defined in the introduction already known formed by a vacuum breaker with the inconvenience mentioned above as consequence, while the second contact member is formed by a disconnector. Furthermore, the component is in some cases switched in during normal operation, which results in a great need of cooling thereof. This together with requirements of a comparatively complicated control electronic means that such transfer switches already known get very voluminous and costly. SUMMARY OF THE INVENTION The object of the present invention is to provide a transfer switch of the type defined in the introduction, which may be designed in a considerably simpler way, with substantially lower demand on space and by that to a lower cost than such transfer switches already known discussed above. This object is according to the invention obtained by providing such a transfer switch, which comprises at least one movable part and by designing the unit to control this part to carry out one single mechanical movement for opening the two contact members of the first switching device and close the two contact members of the second switching device. By controlling one single movable part to carry out one single mechanical movement for opening the two contact members of one switching device and closing the two contact members of the other switching device this may take place in a perfectly synchronized way, without any requirement of any complicated control apparatus for co-ordinating these openings and closings, respectively, with each other. Another advantage is that one single driving arrangement may be used for accomplishing the entire transfer of supply to the load from one alternating current supply to the other by driving the movable part to carry out one movement. By the fact that the opening and the closing of each electric switching device of the transfer switch according to the invention takes place by one single mechanical movement im- proved possibilities to make the control faster are obtained, since only one acceleration of one movable part is necessary. This is amplified by the fact that the switching devices of the transfer switch according to the invention in this way may ensure arc-free breaking of the current through the first contact member without arranging any vacuum breaker, so that instead the corresponding amount of energy may be used for making the breaking faster. Different types of electrical equipment connected to said current path may by that be better protected upon occurrence of a fault and material wear of the contacts included in the switching devices may be reduced. All this means that the transfer switch according to the invention may be made considerably more compact than such transfer switches already known and it may also be provided to a lower cost. According to a preferred embodiment of the invention the transfer switch 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 re- spective switching device, the respective switching device is adapted to be switched in in a current path going through the switching device through one of the contacts and the movable part, the component of the respective switching device connects the two contacts with each other, and the control unit is adapted to control opening of said current path through the respective switching device by controlling 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 broken for opening the first contact member and then the galvanic connection between the second of said two contacts and the movable part is broken for opening the second contact member This is a very simple and price worth way to design the transfer switch according to the invention , since each switching device only has to include two fixed contacts, one movable contact part in common with the other switching device and one single component for ensuring arc-free breaking of the current path to one alternating current supply and establishing of the current path to the other alternating current supply According to another preferred embodiment of the invention at least one of said switching devices comprises at least three contacts consecutively arranged along the path of the movable part for said mechanical movement, and the component of this switching device 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 said at least one switching device, and 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 this switching device that first 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 also only one semiconductor device per switching device, which makes it worth its price According to another preferred embodiment of the invention said at least one switching device of the transfer switch comprises at least two said components in the form of 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 this switching device, this 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 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. By enabling a breaking and also an establishing of a current path through the movement of the movable part in two different directions in this way, the movable part may in principal instantaneously upon arising of a desire to interrupt one alternating current supply be accelerated for opening the first contact member of the switching device in question, since the movement direction may be chosen so that the semiconductor device intended to have the current transferred thereto is in the conducting state. This means that the harmful influence that a fault along the current path in question may procure may be reduced to a minimum at the same time as the time period during which the load is without supply may be made very short. This is a great advantage with respect to the preferred embodiment first mentioned above, in which the movement direction of the movable part when breaking the current path is determined and the correct current direction has to be waited for before the actuation 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 movable part is connected to the load, which enables a transfer of the supply of the load from one alternating current supply to the other by very simple means and while using a minimum number of contacts According to another preferred embodiment of the invention the two switching devices have a contact in common along the movement path of the movable part, so that the contacts along this movement path on one side of the contact in common belong to a first of the switching devices and those on the other side belong to the other of the switching devices, and the con- tact in common is adapted to be connected to the load By designing the transfer switch for connecting to the load in this way it may be made very simple and compact 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 along which the contact members of both switching devices are arranged In an open state of one switching device this may by this be brought to the closed state through a movement in the direction desired by ro- tatmg the movable part, so that the closed state may be obtained in the fastest possible way, so that in the case of opening and closing in two different directions according to above the rotation direction providing possibility to the fastest opening with respect to the position of the alternating voltage across the switching device in question may always be chosen It is then particularly advantageous if both switching devices are arranged substantially 180° displaced with respect to each other along said arc, since this makes a transfer of the supply to the load through one switching device to the other possible in the short- est possible maximum time irrespectively of the position of the alternating current of the switching device closed in the moment for occurring of a need of transfer It also gets simpler to operate the switching device by for example an electric motor 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 drive 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 move- ment with a high acceleration . According to a very preferred embodiment of the invention the transfer switch comprises a driving member being electrically controlled and adapted to carry out said single mechanical movement, and it is particularly advantageous if this driving member is an electromagnetic machine in the form of an electric motor. By using such a driving member it gets possible to control the movement of the movable part for breaking or closing very accurately and for example ensure that a separation of two contacts takes place at a particular phase position of the alternating current. By the fact that the control unit of the device in the form of an electronic unit is adapted to control the driving member it is also possible to influence the movement of the movable part also when this has already started for making ad- aptations to values of parameters newly measured, such as current and voltage, and possibly interrupt the entire procedure, should it be discovered that there is no longer any need thereof or the movement should for example better take place in the opposite direction. Furthermore, this embodiment is suitable for co-ordination with a prediction of the future development of the current through the respective switching device of the transfer switch, such as a coming zero crossing of the current, so as to co-ordinate a breaking or closing of the current path through the respective switching device with such a prediction, so as to for example ensure that said component with ability to block current only will conduct current during a so-called short half wave. By the fact that it is possible to in this way ensure that said component, which usually is a semiconductor device, such as a diode, only has to conduct current during a very short period of time, in the order of half a current period, this component has not to be dimensioned for being able to over a longer time withstand op- eration currents, but it may instead be allowed to be considerably overloaded once it is conducting, since this only takes place during a very short period of time. This results in the possibility to use a lower number of such components than would other- wise be the case would they have to withstand the currents in question over a longer period of time. According to another preferred embodiment of the invention the transfer switch comprises members adapted to substantially continuously detect the direction and magnitude of the current through the switching device being closed for the moment and send information thereabout to the control unit. This means that the control unit all the time "knows" how transfer from one supply to the other shall take place, so that this may be carried out in the quickest and most flexible way as soon as a need thereof arises. According to another preferred embodiment of the invention the switching device comprises a voltage limiting device connected in parallel with the semiconductor device, and the device is adapted to start conducting at a voltage thereacross close to the maximum voltage withstood by the semiconductor device. This is possible thanks to the fact that in the closed and open state of the switching device no voltage will be applied across the semiconductor device and by that neither across the device, so that this will not be heated up by any leakage currents therethrough. By means of the device, which may be a varistor, but also could be of any other type, for example a resistor, snubber or the like, the first voltage peak occurring across the semicon- ductor device through the returning voltage after opening the first contact member may be limited, which in the case of one single semiconductor device makes it possible to dimension it for being able to hold a lower returning voltage in the blocking direction thereof and by that gets less costly than otherwise. In the case of a plurality of semiconductor devices or components connected in series, through an arrangement of such a device in parallel with each component, the number of such components having a determined ability to withstand voltage connected in series may be reduced. It is by that avoided that any individual component gets a higher voltage thereacross than it may with- stand, while other components get a lower voltage thereacross. According to another preferred embodiment of the invention at least one of the switching devices comprises means adapted act increasingly upon the voltage when separating two contacts in connection with opening 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. The higher this voltage is the faster may the current be fed into the component. By arranging said means less mate- rial wear is obtained and the contact place will also take more from the insulation point of view. According to another preferred embodiment of the invention said means comprises a plurality of first contact members connected in series adapted to be opened substantially simultaneously for transferring the current to the component. By such a series connection of a plurality of contact members the voltage driving the conduction of the component will be increased, since this will be formed by an addition of the voltages of the contact members connected in series having 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 achieves a higher arc-voltage and a faster commutation of the current to the component. According to another preferred embodiment of the invention the semiconductor device of at least one of the switching devices is a diode, which often would be preferred, since such a solution is cheap compared 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 could also be of turn-off type, such as a GTO or an IGBT, so as to enable a quicker breaking procedure of the closed switching device. It could also in some situations be advantageous to ar- range a bi-directional semiconductor device, i.e. a semiconductor device being able to block and conduct in both directions, such as a BCT (bi-directionally controlled thyristor). Comparatively high voltages may be handled by the two switch- ing devices of the transfer switch and by that by the transfer switch while using a low number of semiconductor devices, when 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. According to another preferred embodiment of the invention the transfer switch comprises means adapted to continuously or intermittently determine which of the two supplies is for the moment the most suitable for the load, and the control unit is adapted to control said movable part on the basis of information from said means for transferring the supply of the load from one alternating current supply to the other if the alternating current supply switched in to the load in that moment does not correspond to said most suitable supply. The transfer switch may by this be used to always ensure that the one of the two alternating current supplies according to certain criterions most suitable is connected to the load. The invention also relates to advantageous uses of a transfer switch according to above in accordance with the appended claims, and the advantages of most of them appear without any doubt from the discussion above, but it may here be mentioned that a particularly advantageous use of the transfer switch is for connecting an electrical equipment to an alternating voltage network through two different current paths, firstly the first one for starting the electrical equipment through closing the first of the switching devices and then the second one for operation of this equipment through opening the first switching device and closing the second one, in which a more specific such use is constituted by reactor start of an electric motor connected to an alternating voltage network for firstly through the first switching device connect the motor to the alternating voltage network through a reactor, i.e. an inductor, and then through the second switching device connect the motor directly to the alternating voltage network. It is by this avoided that a large such motor re- duces the voltage level of the alternating voltage network to un- acceptably low levels at said start, and a soft start may instead take place through the reactor. The invention also relates to a switch gear for supply of electricity within industry or in distribution and transmission networks provided with a transfer switch according to the invention. The method according to the invention is also well 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 a computer readable medium onto which such a program is recorded . Further advantages as well as advantageous features of the invention will appear from the following description and the other dependent claims. BRIEF DESCRIPTION OF THE DRAWINGS With reference to the appended drawings, below follows a de- scription of preferred embodiments of the invention cited as examples. In the drawings: Fig 1 and 3-8 are simplified views illustrating a transfer switch according to a first preferred embodiment of the invention in different operation positions between a load connected to a first alternating voltage supply and the load connected to a second alternating voltage supply, Fig 2 is a simplified circuit diagram illustrating the principle of the operation of the transfer switch according to Fig 1 , Fig 9 is a simplified view illustrating a transfer switch according to a second preferred embodiment of the invention in a parking position with the load separated from both supplies, Fig 10 and 12-14 are simplified views illustrating a transfer switch according to a third preferred embodiment of the invention in different operation positions between a load connected to a first alternating voltage supply and the load connected to a second alternating voltage supply, Fig 1 1 is a simplified circuit diagram illustrating the principle for the function of the transfer switch according to Fig 10, Fig 15-17 illustrate schematically an electric switching device applicable in a transfer switch according to a fourth preferred embodiment of the invention in closed, temporary closed and open position, respectively, Fig 1 8 and 1 9 illustrates electric switching devices usable in transfer switches according to further preferred embodiments of the invention, Fig 20 illustrates how the current I through and the voltage U across the semiconductor devices of a switching device accord- mg to Fig 19 develop over time in comparison with a switching device substantially according to Fig 18, Fig 21 and 22 illustrates a part of a switching device included in a transfer switch according to the invention in two different positions when breaking the current therethrough, Fig 23 and 24 are schematical circuit diagrams illustrating two possible ways to arrange a transfer switch according to the m- vention for start of an electric motor, Fig 25 is a simplified circuit diagram illustrating a possible use of a transfer switch according to the invention for switching in and switching out capacitors to an alternating voltage network for reactive power compensation, Fig 26 is a simplified sketch illustrating an advantageous use of a transfer switch according to the invention, Fig 27-29 illustrate schematically a use of a transfer switch according to a preferred embodiment of the invention in a ring circuit for feeding loads connected to the ring, Fig 30 is a very simplified view illustrating how the two switching devices of the transfer switch according to the embodiment shown in Fig 27-29 may be controlled, and Figs 31 -34 illustrate a transfer switch according to a further preferred embodiment of the invention used in a said ring circuit in different states DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION A transfer switch for alternating current according to a first preferred embodiment of the invention is schematically illustrated in Fig 1 and comprises two electric switching devices 1 , 2 adapted to establish and break, respectively, a current path between a load 3 (see Fig 2) here connected to a movable contact part 4 and an alternating current supply 5. 6 each indicated through an electric line for transferring the supply for feeding of the load with electric power from one alternating current supply to the other. One such transfer switch is arranged per phase, so that a three-phase network has three such transfer switches on one and the same location. The movable contact part 4 is arranged on a carrier 7, which is arranged to rotate around an axle 8. Three contacts 9-1 1 and 12-14, respectively, are arranged externally of the carrier 7 with the movable part 4 for the respective switching device along the movement path of the movable part 4. and these contacts are arranged to form good electric contacts when bearing against the movable part 4. The diameter of the circle along which the movable part is adapted to establish electric contacts with the contacts 9-14 is proportional to the alternating voltage of the two supplies. A semiconductor device 1 5-1 8 in the form of a diode connects each outer contact 9, 1 1 , 12, 14 as seen in the movement path with the mid-contact 1 0, 1 3, in which one diode of each switching device has the conducting direction towards the mid-contact and the other the conducting direction away from the latter. The transfer switch has also a driving arrangement in the form of an electric motor 65 adapted to drive the carrier 7 and by that the movable contact 4 to rotate around the axle 8. By using such an electrically controlled driving member in the form of an electric motor the movement of the movable part 4 may be controlled very accurately and be adjusted or interrupted as long as it is going on. Each switching device is also provided with a detecting member 19 schematically indicated for the switching device 1 and adapted to detect the direction and the magnitude of the current in the current path through the switching device being closed for the moment and send information thereabout to a unit 20 adapted to control said driving arrangement and by that 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. Means 56 of are also arranged to on the basis of other pa- rameters, such as prices of power fed from the two supplies 5, 6, determine which one is the most suitable for the supply of the load at a given time and send information thereabout to the control unit 20. A component 66 increasing the resistance is arranged between the connection of the respective semiconductor device 15-18 to the respective supply 5, 6 and the closest contact 1 0, 1 3. This component increasing the resistance is intended to be controlled to either have a negligible resistance when the respective switching device is closed or get a comparatively high resistance for take voltage thereacross in a way described further below. The component increasing the resistance could be a resistor having a controllable resistance, such as a powder having a very low resistance when applying an outer pressure thereon, but which gets a high resistance when the pressure is removed, or a controllable semiconductor de- vice, which has a low on-state voltage, but which may be brought to turn-off for then considerably increasing the resistance therethrough. The function of this transfer switch is as follows: We assume that the transfer switch is in the position according to Fig 1 , in which the first switching device 1 is closed, so that the load is connected to the alternating voltage supply 5, while the alternating voltage supply 6 is disconnected, since the elec- trie switching device 2 is open . For the sake of example it is here assumed that the alternating current supply 5 is the regular or normal supply, while the alternating current supply 6 is a spare supply. When a desire to switch in the spare supply 6 arises, for example by the fact that the detecting member detects that no current at all flows in the alternating current supply as a consequence of a power failure there, the control unit 20 firstly decides in which direction the movable contact part 4 shall be moved for being able to break the current path through the first switching device 1 as quick as possible. This decision is depending upon in which position the current in the current path through this switching device is located exactly then. In the closed position according to Fig 1 the entire current goes through the switching device between the first contact 10 and the movable part 4. When now breaking shall take place the current shall as quick as possible be transferred to go through one of the diodes 15, 16 instead. The current may be switched in to a diode during that part of an alternating current period being between the time the diode became forward biased in that direction until the diode gets reverse biased next time. This means for a full period of 20 ms in the practice that an opening of the conducting path in parallel with the diode in question may take place for example 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. That means that the current to be corn- mutated over to flow through the diode is small and the com- mutation 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 arrow 21 in Fig 1 and is within the interval allowing transfer of the cur- rent to the diode 15. The control unit 20 controls then the movable contact 4 to rotate clockwise so as to open a first contact member connected in parallel with the diode 15, which here is constituted by the contacts 1 0, 1 1 and the movable part 4. The temporary closed position shown in Fig 4 is then obtained. When this takes place a small spark is formed in the gap 22 between the movable part 4 and the contact 10, which results in a voltage of usually 12-15 V, which will drive the transfer of the current through the diode 15, so that the current now instead will flow through the movable part 4, the contact 1 1 and the diode 1 5. By co-ordinating the separation of the contact 1 0 and the movable part 4 with the control through the control unit 20 of the resistance increasing component 66 to increase its resistance a voltage may very rapidly be built up across the diode 15 and thereby the transfer of the current to flow through the diode may be fast. The transfer of the current to the semiconductor device may also be accelerated by switching in a resistance in the regular current path and by that increasing the voltage also across the semiconductor device. When then the current through the switching device 1 changes direction the diode blocks the current therethrough, since a voltage will be built up across the diode 15 now 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 1 1 and the movable part 4 is broken, in which this breaking may take place arc-free, since no current goes through the contact place at the moment of breaking . The completely open position according to Fig 8 is thereby obtained. It is in this breaking important that it takes place so fast that the voltage across the diode 15 will not change direction again and this starts to conduct. Should the voltage across the switching device 1 be during the other half period when detecting the need of breaking the current path through the switching device the control unit would instead control the movable contact part 4 to rotate counter-clockwise as seen in the figures for utilizing the diode 16 at the 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 8 within a substantially shorter time than one period, usually always within 15 ms for a frequency of 50 Hz of the alternating voltage. The position shown in Fig 8 constitutes also a possible parking position, which may be assumed during a longer period of time should there be a requirement to separate the load from the supplies, for example for carrying out maintenance works between the load and the transfer switch . Voltage increasing means 67 corresponding to the resistance increasing component 66 according to above have also been shown in this figure, which here comprises a charged capacitor adapted to be switched in between the respective outer contact and the mid- contact 1 0, 13 when the connection therebetween through the movable part is to be broken for rapidly transfer the current to flow through the respective diode. By the fact that the current in the closed position of the switching device never flows through the diodes the contacts 10, 1 3 and 4 have only to be dimensioned for the operation current, which for example may be 1000 A, while the diodes 15, 16 are dimensioned for a possible shortcircuit current, which in such a case could be 25 kA. However, they only have to withstand that current during a very short period of time and the dimensioning of the diode may be made without any considerations taken to any continuous operation current through the switching device. Furthermore, 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. How- ever, the contact gap 23 in Fig 8 has to be able to withstand a considerably higher so-called i mpulse voltage, which in th is case could be 75 kV. When the position according to Fig 5 is obtained the spare sup- ply 2 is immediately switched in should the current direction thereof be so that a temporary current path may be established through the diode 1 7 , and otherwise the switching in of the spare supply is delayed until the position according to Fig 6 is obtained, in which the movable part 4 establish a galvanic con- tact connection with the mid-contact 1 3 of the second electric switching device 2. The movement of the movable part is then continued to the position according to Fig 7 , so that the transfer switch has assumed a ready position for the quickest possible return to supply of the load through the regular alternating cur- rent supply 5 when a desire thereof arises . It is pointed out that it may for sure be very advantageous, when the movable part 4 reaches the contact 14 , to have the diodes 1 7 and 1 8 directed as shown with respect to the diodes 1 5 and 1 6, but they could also be oppositely directed . The opening of the first switchi ng device 1 and the closing of the second switching device 2 takes place through one single mechanical movement of one sing le movable part, which results in the advantages already mentioned further above. Further advantages of this embodiment of the transfer switch worth to be mentioned are very low manufacturing costs , since it is easy to produce and the material consumption is low and the number of components included therein is low. Furthermore, a small amount of control energy is needed thanks to a light movable mass. The diodes may be used without cooling . The principle for the transfer switch according to Fig 1 is very schematically illustrated in Fig 2. A transfer switch according to a second preferred embodiment of the invention is schematically illustrated in Fig 9, in which the two switching devices 1 , 2 are arranged displaced with respect to each other along and arc of an angle of substantially 120° , which means that the insulation distances 23 between the movable part 4 and the contacts 1 1 , 14 may be increased in the parking position of the transfer switch shown in Fig 9. The transfer switch may by this in the broken parking position withstand higher impulse voltages than in the embodiment according to Fig 1 . However, this embodiment means that the movable part has a longer distance to move when rotating the movable part 4 counter-clockwise in Fig 9 for transferring the load from one supply to the other. Another possibility to increase said insulation distance is to arrange the two switching devices displaced with respect to each other in the direction of the axle 8. A transfer switch for alternating current according to another preferred embodiment of the invention is illustrated in Fig 1 0 and 12-14, which functions according to the same principle as the one just described, but the movable contact part 4 is here instead movable along a rectilinear movement path. The two switching devices 1 and 2 have here a fixed contact 24 in common along the rectilinear movement path of the movable part 4. and to this contact 24 in common the load 3 to which the trans- fer switch is intended to accomplish feeding is intended to be connected. The two other contacts 25, 26 and 27, 28, respectively, of the respective switching device are connected with a semiconductor device 29, 30 each in the form of a diode, which is directed with the conducting direction thereof towards the in- ner one of the contacts 26, 27. However, all diodes could be directed in the opposite direction instead. When now the alternating current supply 5 is connected to the load 3, but for any reason a desire of switching in the alternating current supply 6 to the load arises instead, the movement of the movable part 4 is then started in Fig 1 0 to the right. However, the opening of the current path through the contact 25, the movable part 4 and the contact 24 has possibly to be delayed somewhat, should the current when a need of breaking occurs not have such a direction that the breaking procedure may be started immediately. Costs are instead saved by dividing the number of semiconduc- tor device required by two with respect to the embodiment according to Fig 1 . The equivalent circuit for this embodiment is shown in Fig 1 1 , where also the load 3 is shown. It is shown in Fig 12, 1 3 and 14 how firstly the movable part 4 is moved along a rectilinear movement path for obtaining the temporary closed position according to Fig 12, in which the current is transferred to the semiconductor device 29, to the completely open position according to Fig 1 3 of the first switching device 1 and then further to the other completely closed position according to Fig 14 of the switching device 2, so that the spare supply 6 is con- nected to the load 3 through the contact 28, the movable part 4 and the contact 24. A modified electric switching device is schematically illustrated in Fig 1 5, which in combination with an identical such one could form a transfer switch according to a further preferred embodiment of the invention. However, only this switching device will here be described, and it is apparent for a man with skill in the art how two such switching devices could be combined for obtaining a transfer switch while using one and the same movable part 4. The switching device has an inner cylinder 31 , which is rotatably arranged at an axle 32 and has a movable part 4. A second cylinder 33 is arranged externally of the cylinder 31 and has four contacts 34-37 arranged along the movement path of the movable part 4 and adapted to form good electric contacts when bearing against the movable part 4. The switching device is switched in in a current path 38 therethrough through the two outer contacts 34 and 37, respectively. A semiconductor device in the form of a diode 39, 40 is switched in between the two outer contacts and the closest adjacent inner contacts and it has a conducting direction from the outer to the adjacent contact, but it could instead be switched in with the opposite conducting direction. The switching device has also a driving arrangement adapted to drive the inner cylinder 31 to rotate for movement of the movable contact part 4 with respect to the other contacts 34-37. The driving arrangement is in this case an integrated electric motor 41 schematically indicated, which may be of an amount of different types, such as for example a PM-motor, a reluctant motor, a step motor, a DC-motor. The motor is here not located in the same plane as the contacts, but it could be, and it will by that not influence the voltage withstandability. However, it is pointed out that the driving very well may take place in another way, such as through a motor being adapted to act directly upon the axle 32. With respect to the dimensioning of the cylinders with respect to diameter and height this is carried out according to the same criterions as for the embodiment according to Fig 1 . The function of the electric switching device will now be shortly described, and the decisions made with respect to rotation direction of the inner cylinder when there is a desire of breaking are based on the same considerations with respect to the position of the alternating current as for the embodiment according to Fig 1 . We now assume that the current in the current path 38 when there is a desire of breaking has a direction indicated through the arrows 42 in Fig 15 and is within the interval allow- ing transfer of the current to the diode 39. The control unit 20 then controls through the motor 41 the movable part 4 to rotate clockwise so as to open a first contact member being arranged in parallel with the diode 39, which here is constituted by the contacts 34, 35 and the movable part 4. The temporarily closed position shown in Fig 14 is then obtained. When this takes place a small spark is formed in the gap 43 between the movable part 4 and the contact 34, which results in a voltage of usually 12-1 5 V, which will drive the transfer of the current through the diode 39, so that the current now instead will flow through the diode 39, the contact 35, the movable part 4 and the contact 37. When the current through the switching device then changes direction no current will flow therethrough, but a voltage will be built up across the diode then reverse biased and a rotation movement of the movable part 4 is now continued in the same direction as before, so that the galvanic connection between the contact 35 and the contact 37 is broken, in which this breaking may take place arc-free, since no current flows through the contact place at the breaking moment. The completely open position shown in Fig 17 is by that obtained. It would of course be well possible to arrange a further switching device of this type having four fixed contacts arranged along the movement path of the movable part 4 by a suitable modification of the outer cylinder in the same way as for the switching device shown for obtaining a transfer switch. However, in the case that we assume that the contact 35 would be connected to the load, this contact could for example be in common for both switching devices. Three additional fixed contacts may then for the rest be arranged following upon each other in the counter-clockwise di- rection from the contact 34 with semiconductor devices switched in the same way between them as for the contacts shown and the last contact as seen in this direction could be connected to an additional alternating current supply, such as a spare supply. It is schematically illustrated in Fig 1 8 how a plurality of semiconductor devices 44-46 can be connected in series so as to together be able to take a determined returning voltage after breaking the current path in a switching device included in a transfer switch according to the invention. Thus, in all embodi- ments shown above each diode symbol may be replaced by a number diodes connected in series in this way. It is here also possible to obtain that the number of semiconductor devices required for a given voltage will be low by choosing a material having a wide bandgap between the valence band and the conduction band, such as SiC or diamond It is also shown in this figure how it is possible to connect semiconductor devices in parallel by here arranging two packages of diodes connected in series in parallel with each other, and this is made for being able to take a certain shortcircuit current or just for redundancy reasons, so that the switching device will function in the desired way even if any diode in any package of the diodes connected in series fails Furthermore, it is illustrated in Fig 19 how a vaπstor 47, preferably of ZnO, may be connected in parallel with the respective semiconductor device 48 , in which the vaπstor 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 vaπstors do not normally 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 open position It is illustrated in Fig 20 how the voltage U over the semiconductor devices 48 in the reverse direction thereof is developed over time t when the voltage m- creases 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 vaπstors and the solid line with vaπstors Thus, it appears that the vaπstors cut the first voltage peak off Are for example four 5 kV diodes arranged in series in a system having a network voltage of 12 kV and a normal returning voltage of 22 kV the vaπstors may in this way start conduct a small current during this short period of time (about 10 μs) as the peak of the returning voltage last, so that this voltage peak could be brought down to 1 8 kV To the left of (before) the time 0 the change of the current I is illustrated This means that there is no requirement of five diodes connected in series but only four, for being able to take the returning voltage By switching a separate vaπstor 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 and capacitances connected in parallel with the semiconductor devices so as to distribute the voltage substantially equal over the semiconductor devices It is illustrated in Fig 21 what means adapted to influence the voltage to increase upon separation of the two contacts in connection with opening of the first contact member in any of the switching devices may look like We now assume that the first conduct member has two fixed contacts 56, 57, which are adapted to be galvanically connected through a movable part 58 in the closed state The movable part 58 is at one end thereof provided with a portion 59 of a material having a comparatively high resistivity, so that the resistance between the movable part 58 and the contact 56 and by that between the two contacts 56 and 57 is increased in the beginning of said separation (the position according to Fig 22) 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 58 may for example be made of graphite A possible application of a transfer switch according to the invention for motor starts is also illustrated in Fig 23 and 24 It is illustrated in Fig 23 how a transfer switch of the type according to the invention (see the principle sketch according to Fig 2) is switched in between a motor 49 and an alternating voltage network 50 We may here consider the alternating voltage supply through one switching device direct to the motor 49 as a main supply, while the alternating current supply through the other switching device 2 through a reactor 51 as a spare supply This way of consideration is even more understandable when study- ing the embodiment according to Fig 24, which differs from that according to Fig 23 by the fact that the transfer switch 52 has there been arranged in direct connection to the motor. Most power networks are not sufficiently stiff for allowing a start of large motors directly connected thereto, since they take so much power that the voltage on the network will sink much 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 reactor start is illustrated here. When the motor is to be started the left switching device 2 shown in Fig 23 is brought in a closed position , so that the motor gets its supply through the reactor 51 . Would there now for any reason be a desire to interrupt the start the contacts 53-55 of this switching device may be opened according to the sequence 55, 54, 53. When the motor then has obtained a synchronous number of revolutions the reactor 51 may be by-passed by bringing the switching device 2 in open position while the switching device 1 is brought into closed position . Would a shortcircuit occur in any equipment connected to the alternating current network 50, the motor 49 will then start to act as a generator and contribute with power to the fault place be- fore the fault gets disconnected. It is here a possibility to limit the effect of this by closing the switching device 2 and opening the switching device 1 in such a case, so that the shortcircuit contribution from the motor to the fault place is restricted and breaking of the motor is at the same time reduced. Would a shortcircuit occur in the motor or a planned stop be made, the switching device 1 will then be opened. A possible application of a transfer switch according to the invention for switching in capacitors 60 to a three-phase alternat- ing voltage network 61 for reactive power compensation is illustrated in Fig 25. A transfer switch according to the invention may then be replaced by two breakers 62, 63, such as illustrated in Fig 25 When switching in the capacitor 60 to the phase in question of the alternating voltage network the breaker 62 may firstly be closed Thyπstors 64 connecting the breaker 62 to the phase in question are then turned on so that the capacitor 60 is switched in at a desired time Also the breaker 63 is then closed The breaker 62 is then opened, so that the thyπstors not have to conduct any longer, while the breaker 63 is closed and the switching in of the capacitor is completed This advanta- geous sequence for switching in reactive power compensation may be comfortably realised by suitably designing a transfer switch according to the invention Another advantage of a transfer switch according to the mven- tion is obtained thanks to the fact that in the case of a three- phase voltage, which is the most common, the three transfer switches for each phase are arranged controllable completely independently of each other, which is not the case for such switches already known, which are mechanically interconnected so that they all have to be 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 transfer switches already known been a necessity 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 invention of transfer switches being mde- pendently 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 It is schematically illustrated in Fig 26 how a transfer switch ac- cording to the invention may be arranged in a ring circuit 70 for feeding loads connected to the ring circuit, which are indicated through a load line 71 . The ring has two parts 72, 73 arriving to the location for feeding the loads for opposite directions. The two electric switching devices 1 , 2 of the transfer switch are arranged to function as circuit breakers, while also a load breaker 80 is arranged in the line to the load. Two preferred embodiments according to the invention of a transfer switch used in this way will hereinafter be described. A transfer switch according to the invention formed by two elec- trie switching devices 1 , 2 of the type being described in detail in the Swedish patent application 9904166-7 of the applicant is illustrated in Fig 27, and reference is made to that application for more details with respect to the function of such a switching device. Characterizing for this switching device is that only one single rectifying semiconductor device 68 is required for enabling an immediate start of a breaking operation independently of the current direction through the switching device. The movable parts 4 have here an arc-like or banana-like shape and are connected to each other, for example like a wheel with a hub. Thus, they may be rotated about a centre point 69 for breaking or closing the current through the respective switching device. This transfer switch is arranged in a ring circuit 70 emanating from an electric power central and is intended for feeding loads connected to the ring with alternating current. 71 indicates the line to a load, while 72 and 73 indicated the ring parts meeting at the location for the output to the load. By controlling the two electric switching devices of the transfer switch the feeding of alternating current to the load may be controlled. The two electric switching devices are preferably controllable completely in- dependently of each other, and the following alternative may then be obtained: Both electric switching devices are closed and the load gets feeding from the two ring parts 72 and 73 of the ring circuit. Only one of the switching devices are closed, and the load then gets feeding only from the ring part connected to the switching device in question. None of the switching devices is closed, which means that the load does not get any feeding at all A load breaker is normally also arranged in the line 71 for making it possible to break away the load there and simultaneously enable feeding in the ring beyond the transfer switch It is illustrated by the sequence according to Fig 27-29 how the feeding to the load may be shifted from one ring part 72 to the other ring part 73 In the position according to Fig 27 current flows through the first switching device 1 to the load line 71 , while the second switching device 2 is open When then electric switching devices are to be changed the movable parts 4 of the electric switching device 1 are rotated as shown in Fig 28, so that the current through that switching device is transferred to go through the diode 68, while the second switching device 2 is transferred to a position so that the current thereof also flows through the diode thereof The rotation of the movable parts 4 of the first switching device 1 is then continued so that the contact with the diode branch is left when the current has become zero and a voltage has started to be built up across the diode, so that an arc-free breaking at current zero may take place through the continuation to the broken position according to Fig 29 The ro- tation of the movable part 24 of the switching device 2 is simultaneously continued , so that the regular current path through the switching device is established It is schematically illustrated in Fig 30 how it would be possible to be able to control both switching devices separately or together An electric motor 65 is intended to control a rotation of a hollow axle 74 connected to the movable part of one switching device and an axle 75 inserted therein and connected to the movable parts of the second switching device The function of the motor and the function of a magnet clutch 76 schematically indicated for optionally interconnecting the two axles 74, 75 is intended to be controlled by an electronic unit 20 The construction and the function of a transfer switch according to another preferred embodiment of the invention is very schematically illustrated in Fig 31 -34, and this transfer switch is also built in in a ring circuit 70 having ring parts 72, 73 and a load connection 71 . The two electric switching device 1 , 2 here share the diode branch 77 by arranging an elongated contact member 78 controllable through a driving member not shown and which may optionally connect one of the electric switching devices or none of them to the diode branch 77. The two ring parts 72, 73 may in the position according to Fig 31 feed alternating current to the load line 71 through the movable contact parts thereof. We now assume that the ring part 73 shall be switched out and the feeding after that only takes place through the ring part 72. The movable part 4 of the electric switching device 2 is then operated to turn while opening the contact member 79 preceded by a displacement of the contact component 78 into contact with the movable part, so that the current through the switching de- vice 2 may be commutated over to flow through the diode branch 77. When the current then has passed zero and a voltage starts to be built up in the reverse direction of the diode 68 the rotation of the movable part 4 of the switching device 2 is continued to the position according to Fig 33 for breaking the current through the switching device 2. The rotation of the movable part may then be continued to the position according to Fig 34 for grounding the ring part 73 through a ground or earth contact 81 , should this be desired . Preferred uses of a transfer switch according to the invention is as current limiter or connected in series with a current limiter or as breaker, as protection for obtaining current breaking and/or disconnection 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 eiectric 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, eapacitive 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 transfer switch 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 being closed. The invention is of course not in any way restricted to the preferred 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 an outer contact of each switching device and the semiconductor device connected thereto in the embodiment according to Fig 1 , but the opening of the switching devices may then only take place by movement in one single direction and the breaking time will be longer. One of the inner contacts and the semiconductor device connected thereto may also be removed from each switching device in the embodiment according to Fig 13 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 in- stead, while a change in the opposite direction may take place for the embodiment according to Fig 8. 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 also pointed out that it is for sure conceivable to design the two switching devices of the transfer switch in different ways, such as one with three fixed contacts and two semiconductor devices according to Fig 1 and the other with two fixed contacts and one semiconductor device according to the lower part of Fig 1 without for example the contact 12 and the semiconductor device 18, would for example the requirement of rapidness be differently high depending upon in which direction the switch is operated. However, the two switching devices would most often be equally designed. The electric quantities, such as voltage and current, may possibly have slightly different levels in for example a spare supply and the switching devices may then be somewhat differently designed. It has above also been shown how the "second supply" has been switched in, i.e. the second switching device has been closed, not before the complete switching out of the first supply, but it would be well possible to design the transfer switch so that the second supply is switched in already before the first supply has been completely switched out, i.e. the movable part then bridges the gap between the switching devices. The claims are to be interpreted as comprising also this case.

Claims: Claims 1 . A transfer switch for alternating current comprising two electric switching devices (1 , 2) adapted to establish and break- ing , respectively, a current path between a load (3) and an alternating current supply (5, 6) for transferring the supply of electric power to the load from one alternating current supply to the other, each of said switching devices comprising in a current path therethrough two branches connected in parallel with each other, the first of the branches comprising a first contact member (4, 10) having two contacts (4, 10) movable with respect to each other for opening and closing and the second comprising a component (1 5) 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 (4, 1 1 ) having two contacts (4, 1 1 ) movable with respect to each other for opening and closing is connected in series with the component, and in which the transfer switch also comprises a unit (20) adapted to control opening of said current path through the switching device being closed by controlling the first contact member of this switching device 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 and the other electric switching device to close, characterized in that it comprises at least one movable part (4) and that the unit (20) is adapted to control this part to carry out one single mechanical movement for opening the two contact members of the first switching device and closing the two contact members of the second switching device. 2. A transfer switch according to claim 1 , characterized in that each switching device comprises at least two contacts (10, 1 1 ) consecutively arranged along the path of the movable part for said mechanical movement, that the movable part (4) is adapted to form a galvanic connection between said two contacts in the closed state of the respective switching device, that the respective switching device is adapted to be switched in in a current path going through the switching device through one (10, 13) of the contacts and the movable part, that the component ( 15) of the respective switching device connects the two contacts with each other, and that the control unit is adapted to control opening of said current path through the respective switching device by controlling the movable part (4) to move along such a path and in such a direction that the galvanic connection between one (10) of said two contacts and the movable part is broken for opening the first contact member and then the galvanic connection be- tween the second (1 1 ) of said two contacts and the movable part is broken for opening the second contact member. 3. A transfer switch according to claim 1 or 2, characterized in that at least one of said switching devices comprises at least three contacts (24-26) consecutively arranged along the path of the movable part for said mechanical movement, and that the component (29) of this switching device connects a first and second adjacent of these three contacts with each other. 4. A switching device according to claim 3 not dependant upon claim 2, characterized in that the movable part is adapted to form a galvanic connection between said three contacts (24- 26) in the closed state of said at least one switching device, and that the control unit (20) is adapted to control the mov- able part to move along such a path and in such a direction for breaking the current through this switching device that first the galvanic connection between the first contact (25) 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 (26) and the third (24) contact is broken for opening the second contact member. 5. A transfer switch according to any of claims 1 -4, character- ized in that the component is a semiconductor device (15), and that the unit (20) is adapted to control the first contact member to open when the semiconductor device is in or going into a conducing state. 6. A transfer switch according to claims 3 and 5 or 4 and 5, characterized in that said at least one switching device comprises at least two said semiconductor devices (15-18, 39, 40) connected between an outer contact each with respect to the movement path of the movable member and a contact located next to the iatter, that the movable part is adapted to form a galvanic connection between all contacts between and including said outer contacts in the closed state of this switching device, that this switching device comprises members (19) for detecting the direction of the current through the switching device in the closed state, and that the control unit (20) 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 transfer switch according to claim 6 not dependent upon claim 2, characterized in that the at least one switching device is arranged to be connected into a current path through the switching device through said two outer contacts (34, 37), that the contacts along the path of the movable part (4) for the mechanical movement of this switching device are four to the number, that in each switching device the semiconductor devices (39, 40) are adapted to block current therethrough either both of them to or both of them from the respective outer contact (34, 37), and that the control unit is adapted to control the movable part so that the galvanic connection be- tween said first adjacent contact (35) and the contact (36) adjacent to the other outer contact is broken after opening the first contact member for opening the second contact member. 8. A transfer switch according to claim 6 not dependent upon claim 4, characterized in that in said at least one switching device the contacts (9-1 1 ) arranged along the movement path of the movable part are three to the number, that this switching device is adapted to be connected into a current path through the switching device through the mid contact (10), that one (16) of the semiconductor devices is adapted to block current therethrough to the mid contact and the other (15) to block current therethrough from the mid contact, and that the control unit is adapted to control the movable part so that the galvanic connection between the first outer contact (1 1 ) and the movable part (4) is broken after opening the first contact member of this switching device for opening the second contact member. 9. A transfer switch according to claim 2 or 8, characterized in that the movable part (4) is connected to the load (3). 10. A transfer switch according to any of claims 3-7, characterized in that the two switching devices have a contact (24) in common along the movement path of the movable part, so that the contacts along this movement path on one side of the contact in common belong to a first of the switching devices and those on the other side belong to a second of the switching devices, and that the contact in common is adapted to be connected to the load (3). 1 1 . A transfer switch according to any of claims 1 to 10, characterized in that the control unit is adapted to control the movable part (4) along a substantially rectilinear movement path along which the contact members of the two switching devices are arranged. 12. A transfer switch according to any of claims 1 -10, characterized in that the control unit is adapted to control the movable part (4) along a substantially arc-shaped movement path along which the contact members of the two switching devices are arranged. 13. A transfer switch according to claim 12, characterized in that the two switching devices (1 , 2) are arranged substaπ- tially 180° displaced with respect to each other along said arc. 14. A transfer switch according to claim 12, characterized in that the two switching devices (12) are arranged displaced with respect to each other along said arc by an angle between 100° and 140°, preferably substantially 120°, and that a rest position for the movable part (4) with the two supplies separated with respect to the load is arranged along the portion of the arc where the angle distance between the two switching devices are the greatest. 15. A transfer switch according to any of claims 12-14, characterized in that the movable part is rigidly connected to an axle (8, 32), and that the control unit is adapted to control a driving arrangement to rotate the axle for moving the movable part along said path. 16. A transfer switch according to any of claims 1 1 -1 5, characterized in that it comprises a first piece (31 ) rotatably re- ceived in a second piece (33), and that the movable part (4) is arranged on one of the pieces and the contacts of the two switching devices along said movement path are arranged on another of the pieces. 17. A transfer switch according to claim 16, characterized in that the two pieces (31 , 33) through the contacts form entirely or partially a cylinder each having the height and diameter thereof adapted to the operation current through the respective switching device in the closed state and the voltage it has to hold in the opened state, respectively. 1 8. A transfer switch according to claim 16 or 17, characterized in that the movable part (4) is arranged on the first piece (31 ), and that a motor (41 ) is integrated into the pieces so as to drive them to rotate with respect to each other. 19. A transfer switch according to any of claims 1 -17, characterized in that it comprises a driving member (65) being electrically controlled and adapted to carry out said single mechanical movement. 20. A transfer switch according to claim 19, characterized in that the driving member (65) is an electromagnetic machine. 21 . A transfer switch according to claim 20, characterized in that the driving member (65) is an electric motor. 22. A transfer switch according to any of claims 1 9-21 , characterized in that it comprises a control unit (20) in the form of an electronic unit adapted to control said driving member (65). 23. A transfer switch according to any of the preceding claims, characterized in that it comprises members (1 9) adapted to substantially continuously detect the direction and the mag- nitude of the current through the switching device being closed for the moment and send information thereabout to the control unit (20). 24. A transfer switch according to any of the preceding claims, characterized in that at least one of said switching devices comprises a plurality of said components (44-46) connected in series and adapted to together hold a voltage over this switching device in a blocking state. 25. A transfer switch according to any of the preceding claims, characterized in that at least one of the switching devices comprises a plurality of said components (44-46) connected in parallel with each other and adapted to together take care of the current through this switching device after opening the first contact member. 26. A transfer switch according to any of the preceding claims, characterized in that a voltage limiting device (47) is connected in parallel with the component (48), and that said de- vice is adapted to start conducting at a voltage thereacross close to the maximum voltage withstood by the component. 27. A transfer switch according to claims 24 and 26, characterized in that each component has a said voltage limiting device (47) connected in parallel therewith for a substantial even distribution of the voltage across the series connection of the components onto the individual components. 28. A switching device according to claim 26 or 27, character- ized in that the voltage limiting device is a varistor. 29. A transfer switch according to any of the preceding claims, characterized in that at least one of the switching devices comprises means adapted to act increasingly upon the volt- age when separating two contacts in connection with opening the first contact member. 30. A transfer switch according to claim 29, 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. 31 . A transfer switch according to claim 29 or 30, characterized in that said means comprise a component (59) adapted to increase the resistance between said two contacts (56, 58) at the beginning of said separation thereof while allowing a current between these contacts therethrough. 32. A transfer switch according to claim 31 , characterized in that said component (66) 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. 33. A transfer switch according to claim 31 , characterized in that said component (66) increasing the resistance is formed by a resistor having a controllable resistance and adapted to have an unimportant resistance in the closed state of the respective switching device and be controlled to get a substantial resistance for increasing the voltage between the two contacts of the first contact member. 34. A transfer switch according to claim 29, characterized in that said means (67) increasing the voltage comprise a charged capacitor adapted to be connected between the two contacts of the first contact member when this is to be opened. 35. A transfer switch according to claim 29 or 30, characterized in that said means are formed by the fact that the con- tacts 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. 36. A transfer switch according to any of the preceding claims, characterized in that the control unit (20) is adapted to optionally control the two switching devices (1 , 2) to close the current path between their alternating current supply and a load simultaneously or both simultaneously break the current path between their alternating current supply and a load . 37. A transfer switch according to claim 5 or claim 5 and any of the other preceding claims, characterized in that the semiconductor device of at least one of the switching devices is a diode. 38. A transfer switch according to claim 5 or claim 5 and any of claims 1 -37, characterized in that the semiconductor device of at least one of the switching devices is controllable. 39. A transfer switch according to claim 5 or claim 5 and any of claims 1 -37, characterized in that the semiconductor device of at least one of the switching devices is of turn-off type. 40. A transfer switch according to claim 38, characterized in that the semiconductor device of at least one of the switching devices is a thyristor. 41 . A transfer switch according to claim 39, characterized in that the semiconductor device of at least one of the switching devices is bi-directional, i.e. it is able to block and conduct in both directions. 42. A transfer switch according to claim 5 or claim 5 and any of the other preceding claims, characterized in that the semi- conductor device of at least one of the switching devices 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. 43. A transfer switch according to any of the preceding claims, characterized in that the two switching devices thereof are designed to operate at a system voltage between 1 -52 kV. 44. A transfer switch according to any of the preceding claims, characterized in that the two switching devices thereof are adapted to withstand at least an operation current of 1 kA, preferably at least 2 kA, in the closed state. 45. A transfer switch according to any of the preceding claims, characterized in that a first of the switching devices is adapted to be connected to a regular alternating current supply and to be closed in the normal state of the transfer switch, and that the second switching device is adapted to be connected to a spare supply and be open in said normal state. 46. A transfer switch according to any of the preceding claims, characterized in that it has a rest position for the movable part (4) with the two switching devices (1 , 2) open and the load separated from both supplies. 47. A transfer switch according to any of claims 1 -44, characterized in that it comprises means (56) adapted to continuously or intermittently determine which of the two supplies is for the moment the most suitable for the load, and that the control unit (20) is adapted to control said movable part (4) on the basis of information from said means for transferring the supply of the load from one alternating current supply to the other if the alternating current supply switched in to the load in that moment does not correspond to said most suit- able supply. 48. A use of a transfer switch according to any of the preceding claims for connecting electrical equipment to an alternating current network through two different current paths, firstly the first one for starting the electrical equipment through closing the first (2) of the switching devices and then the second for operation of this equipment through opening the first switching device and closing the second one ( 1 ). 49. A use according to claim 48, characterized in that the first current path has a current limiting function. 50. A use according to claim 48 or 49, characterized in that the transfer switch is arranged for start of an electric motor (49) connected to an alternating current network (50). 51 . A use according to claim 50, characterized in that the transfer switch is used to start the motor (49) by firstly through the first switching device connect the motor to the alternating voltage network through a reactor (51 ) and then through the second switching device connect the motor directly to the alternating voltage network. 52. A use of a transfer switch according to any of claims 1 -47 for switching in and switching out normal operation currents of an electric circuit. 53. A use of a transfer switch according to any of claims 1 -47 with the respective electric switching device thereof as disconnector in the breaking condition. 54. A use of a transfer switch according to any of claims 1 -47 as grounder for grounding the electric circuit. 55. A use of a transfer switch according to any of claims 1 -47 for switching in and switching out a resistive load with respect to an alternating voltage network. 56. A use of a transfer switch according to any of claims 1 -47 for switching in and switching out a eapacitive load with respect to an alternating voltage network. 57. A use of a transfer switch according to any of claims 1 -47 for switching in and switching out an inductive load with respect to an alternating voltage network. 58. A use of a transfer switch according to any of claims 1 -47 in a ring circuit for feeding loads connected to the ring with alternating current so as to connect the ring part on each side of the point for the connection of the load to the ring with the load or break this connection through the switching devices. 59. A use of a transfer switch according to any of claims 1 -47 for connection of a spare alternating current supply to said load when there is a power failure or a fault of the regular alternating current supply. 60. A use of a transfer switch according to any of claims 1 -47 in switch gears for power supply within industry or in distribution or transmission networks. 61 . A use of a transfer switch according to any of claims 1 -47 for switching in and switching out a device for reactive power compensation with respect to an alternating voltage line. 62. A use according to claim 61 , characterized in that one contact member of each of the two branches connected in parallel is intended to be connected to the alternating voltage line and the two other contact members of these branches are intended to be connected to components (60) included in the device for reactive power compensation. 63. A use according to claim 61 or 62, characterized in that the transfer switch is used for switching in and switching out capacitors (60) to the alternating voltage line. 64. A plant for a multi-phase network having a plurality of transfer switches according to any of claims 1 -47, one for each phase, characterized in that the transfer switches are controllable independently of each other. 65. A switch gear for supply of electricity within the industry or in distribution or transmission networks, characterized in that it comprises a transfer switch according to any of claims 1 -47. 66. A method for transferring a supply of a load with alternating current from one alternating current supply to another by opening a current path between a first alternating current supply and the load through a first electric switching device and after that closing a current path between another alter- nating current supply and the load through a second electric switching device, in which a regular current path through the first switching device is opened and the current is transferred to a temporary current path through a component able to block current therethrough in at least a first blocking direction and then the temporary current path is broken and the current through the first switching device is broken, and the second electric switching device is closed, characterized in that one and the same movable part (4) opens the regular and the temporary current path as well as closes the current path through the second switching device by carrying out one single mechanical movement. 67. A method according to claim 66, characterized in that both switching devices (1 , 2) close the current path between the alternating current supply thereof and the load simultaneously upon desire or they do simultaneously both break the current path between the alternating current supply thereof and a load. 68. A method according to claim 66, 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 controlled to take place substantially at a zero-crossing of the current. 69. 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 any of claims 66-68 when the product is run on a computer. 70. A computer program product according to claim 69 provided at least partially over a network as the Internet. 71 . A computer readable medium having a program adapted to make a computer control the steps according to any of claims 66-68 recorded thereon.

Inventors: AL-HOSINI FALAH, BACKMAN MAGNUS, FRANK HARRY, JONSSON LARS, LARSSON PER, LILJESTRAND LARS, AL-HOSINI, FALAH, BACKMAN, MAGNUS, FRANK, HARRY, JONSSON, LARS, LARSSON, PER, LILJESTRAND, LARS
Applicants: ABB AB, AL HOSINI FALAH, BACKMAN MAGNUS, FRANK HARRY, JONSSON LARS, LARSSON PER, LILJESTRAND LARS, AL-HOSINI, FALAH, BACKMAN, MAGNUS, FRANK, HARRY, JONSSON, LARS, LARSSON, PER, LILJESTRAND, LARS
Application year: 2000
Main classification: H01H9
IPC codes: H01H9