Patent Grant
7235751
This application claims priority to Swedish patent application 0100074-4 filed 11 Jan. 2001 and is the national phase under 35 U.S.C. § 371 of PCT/SE02/00034.
The present invention relates firstly to an electric device comprising an electric switch having a plurality of contact members arranged in series to form a plurality of breaking points arranged in series, at least one of the contact members at each breaking point being movable, and drive means arranged to actuate each movable contact member.
The invention relates secondly to a current limiter.
The invention relates thirdly to a dynamic voltage restorer.
The invention relates fourthly to an electric power network.
Finally, the invention relates fifthly to use of the current limiter in accordance with the invention.
Certain types of electrical apparatus in electrical systems are such that they are seldom activated but must be able to be activated quickly when required. The losses of the apparatus contribute to the losses of the system. Admittedly this contribution is rather slight but the losses of the apparatus affect its cost since, in many cases, it must be water-cooled, which is expensive. An apparatus dimensioned for continuous high power also incurs high costs.
With the objective of overcoming these drawbacks it is already known to use a commutation contact to bypass these types of apparatus. The apparatus therefore need not be dimensioned for a continuous current, but only for brief surges. A high power in the apparatus can then be accepted for a short time since it automatically has a thermal buffer in the form of the masses always present. The apparatus can thus operate without water-cooling. This, together with the slimmer dimensioning, enables great savings.
Important examples of apparatus of these type are current limiters and breakers. However, the invention is not limited to these applications. Breakers based on power semiconductors are expensive and cause losses. For most of its lifetime a breaker is passively in the on position and conducts current. It is active during extremely short periods when it opens the circuit and breaks the current. In the same way it then stays in open position and later becomes active during a short period when it closes the circuit. While the breaker is in closed state and conducting current it develops power in the form of losses that must be cooled off. In open state the current is zero and the losses are thus also zero.
If a commutation contact is connected in parallel with the semiconductor breaker, the commutation contact will conduct all current when the breaker is in closed state. When the circuit is to be broken, the commutation contact opens first and commutates all current over to the semiconductor breaker. The current in the commutation contact becomes zero and it is in open position. The semiconductor breaker can now become active and break the current in the circuit.
A breaker and a current limiter have in principle the same function apart from the speed with which they break the current. A breaker breaks at the current's zero crossing whereas the current limiter intervenes earlier and breaks an extremely high current.
Similarly a commutation contact can be used for several applications involving apparatus with high losses but which are only active for brief periods. A current limiter may consist of an electric switch parallel-connected to a commutation circuit to which the current is commutated when the electric switch breaks. During normal operating conditions, thus, the current is thus permitted to flow through the electric switch without losses. In the event of a fault causing the current to increase strongly the electric switch will commutate the current over to the parallel branch. This must take place extremely fast. The stipulation for commutating current from one branch to another is that a voltage must be generated in the branch conducting the current. The amplitude of the voltage required depends on the amplitude of the current at the instant when commutation is to occur, on the impedance in the parallel branch to which the current shall be commutated and on the duration of the commutation process. The commutations process must take place fast in order to minimise power development in the commutation apparatus and thus the damages or the dimensioning of the commutation apparatus. The commutation is facilitated if it can be delayed until the natural zero crossing of the current in alternating current networks. A mechanical contact gives lower loses when it conducts current. However, the voltage it can build up when the contacts open is limited to the voltage over the arc formed between the contacts. High arc voltage is a condition for rapid commutation with a mechanical contact.
Against this background, one object of the present invention is to provide an electric device suitable for use in a current limiter and in other contexts requiring equivalent properties in the electric device, e.g. a breaker that utilises semiconductors as breaking elements, or other electrical equipment that utilises semiconductors. From the first aspect of the invention this object is achieved in that an electric device of the type that includes the drive means being arranged to effect simultaneous movement of the movable contact members so that simultaneous breaking is achieved at all the breaking points; a commutation circuit being connected in parallel with the electric switch and each contact member constituting a part of a contact element, which contact elements are arranged in series, a contact surface of each contact element abutting each immediately adjacent contact element, which contact surfaces are substantially flat and parallel, and each contact element comprising at least one conducting part and at least one insulating part. Furthermore, the contact elements are divided into a first and a second group of contact elements, so arranged that every second contact element belongs to the first group and every second contact element belongs to the second group, the contact elements of the first group and the contact elements of the second group being arranged movable in relation to each other in planes parallel with the contact surfaces, between a first position in which conducting part(s) of each contact element is/are in contact with conducting part(s) of the immediately adjacent contact element, and a second position in which the conducting part(s) of the first group of contact elements is/are exposed only to the insulating part(s) of immediately adjacent contact elements in the second group, the drive means being arranged to effect relative movement of the contact elements between said first and second positions.
A high arc voltage is obtained over the electric switch thanks to breaking taking place simultaneous at all the breaking points, thus enabling the switch to be used in applications where this is required. Thanks also to breaking taking place simultaneously at all the breaking points, rapid and reliable commutation occurs through the commutation circuit. A high arc voltage is a condition for commutating a high current.
An electric switch designed in this manner is able to commutate a high current from the electric switch to the commutation circuit. It is advantageous if the losses in the electric power system are reduced, particularly when using apparatus with large losses that are seldom active. Low losses are then obtained even with high currents. The high voltage is maintained even after commutation has taken place. Simultaneous breaking at several breakers connected in series causes several arcs and the voltage drop over the arcs is added to a high total arc voltage, e.g. 100 V, thus enabling the short commutation time, i.e. in the order of less than 1 ms. The short commutation time means that the energy developed only gives rise to very small damages occurring on the electric switch, which is acceptable from the functioning aspect.
The device is primarily intended for high voltages but is not limited thereto. Typical voltage levels are 12–36 kV.
During normal operation the device will be loss-free, as well as being reliable, robust and substantially maintenance-free. The position between the two groups of discs is not sensitive in either closed or open state. This means that contact bounces or mechanical stress due to high retardation at the end positions are eliminated.
In accordance with a preferred embodiment of the electric device according to the invention a drive means is arranged to impart a simultaneous movement to the contact elements of the first group and retaining means are arranged to keep the contact elements of the second group stationary.
Allowing the contact elements of only one group perform the simultaneous movement, while the other group is retained is an alternative that offers a relatively simple and robust construction.
In accordance with another preferred embodiment the movement is a rotary movement and each contact element is in the form of a flat, circular disc, the discs being coaxial. A rotary movement is advantageous for several reasons. It ensures that the drive mechanism will be simple, the device compact and the mass forces relatively low.
In accordance with yet another preferred embodiment each of the contact elements in the first group is mechanically joined at the periphery to a drive means common to these contact elements, and each of the contact elements in the second group is mechanically joined at the centre to a retaining means common to these contact elements.
The drive and retaining means being in the form of a means common to the first and second group, respectively, ensures in a simple manner that the breaking movement occurs simultaneously at all the breaking points. The positioning of the drive and retaining means at the periphery and centre, respectively, enables a simple and reliable driving connection while, at the same time, retaining can be achieved in the simplest possible way.
In accordance with yet another preferred embodiment the angle of rotation between the first and the second position is within the interval (180°/n)±20%, preferably ±5%, where n=the number of conducting parts in a contact element. A rotary angle within this interval ensures that the device is optimised as regards dimensioning in relation to the required distance of movement.
In accordance with a further preferred embodiment the movement is a linear movement and each contact element is in the form of a flat disc.
This may facilitate achieving high cross-sectional area in the conducting parts, which is particularly advantageous at high nominal current strengths.
In accordance with yet another preferred embodiment the insulating part(s) of each contact element in the first and/or second group comprise an opening extending from one side of the disc to the other side.
This embodiment enables an arc distance between the conductor parts in the contact elements of one group to be easily obtained when the electric switch is turned to the breaking position, in which these conducting parts are exposed to the relevant opening.
In accordance with yet another preferred embodiment the number of contact elements is at least five.
As described above, a higher total arc voltage is obtained the larger the number of breaking points in the electric switch. From this point of view, therefore, the larger the number of breaking points, the more advantageous. However, other aspects naturally place practical limits on the number.
As mentioned above, a condition for efficient commutation is that the electric switch breaks rapidly, preferably at a speed of <1 ms.
In accordance with a further preferred embodiment the driving means is connected to a driving power source arranged to effect movement from the first to the second position in less than 1 ms.
Suitable driving sources to achieve such rapid actuation are a mechanical spring, e.g. a torsion spring or alternatively a Thomson coil. Both these types of driving power sources thus constitute preferred embodiments. In another preferred embodiment the driving power source is a conventional electric motor, which may be suitable in applications where a rapid movement is not necessary.
In accordance with another preferred embodiment the number of conducting parts in each contact element is two or more in order to form a plurality of parallel current paths.
A large contact area can then be achieved, with relatively short stroke length for the movement of the movable contact elements.
A second object of the present invention is to provide a current limiter that enables elimination of losses in the form of heat.
This object is achieved in the second aspect of the invention that includes a current limiter of the type that includes an electric device in accordance with the first aspect of the invention.
As stated in the introduction, the electric device is intended for and designed to be incorporated in a current limiter, but is not restricted to this application. The current limiter as claimed thus exhibits advantages equivalent to those described above regarding the claimed electric device and the various preferred embodiments thereof.
In accordance with a preferred embodiment of the claimed current limiter the commutating circuit includes a fuse.
This provides a simple, reliable and robust alternative that fulfils the requirements of the commutation circuit in the current limiter. The drawback is, of course, that it is a disposable component. However, this drawback can be reduced by arranging several fuses in a revolver arrangement. Since the electric switch normally conducts the current no losses will occur in the fuse during operation. The current with only be commutated over to the fuse in the event of a short circuit.
According to an alternative preferred embodiment of the claimed current limiter the commutating circuit includes power semiconductor components. This alternative is suitable in power systems that are subjected to a large number of short-circuits, such as in distribution systems with overhead lines. It is naturally more complicated than the fuse alternative but instead permits repeated operations.
A third object of the invention is to exploit the advantages of the electric device in a dynamic voltage restorer (DVR). This object has been achieved in the third aspect of the invention in that a dynamic voltage restorer that includes an electric device in accordance with the first aspect of the invention.
A fourth object of the invention is to provide an electric power network in which the losses are small.
This object has been achieved according to the fourth aspect of the invention in that the electric power network comprises a current limiter in accordance with the second aspect of the invention and/or a dynamic voltage restorer in accordance with the third aspect of the invention. The fifth aspect of the invention is achieved by the use of such a current limiter and/or dynamic voltage restorer in an electric power network.
The advantages described above in connection with the first and second aspects of the invention are exploited in a power network so designed or in such use.
These advantages may be of particular interest in applications such as distributed generation in electric networks such as industrial networks or in wind power plants as well as electric networks in which distributed energy is generated by solar arrays, gas turbines, fuel cells or other energy sources. Such applications therefore constitute preferred embodiments of the use.
The invention will be explained in more detail in the following detailed description of embodiments by way of example, with reference to the accompanying drawings.
In
The central rod 9 is connected to a driving power source (not shown) arranged able to rotate the rod 9. Upon rotation of the rod 9 this drive means performs a rotary movement, marked by the arrow A in
Upon activation of the drive means 9 the driving power source is arranged to turn this so that the electric switch assumes the breaking position shown in
The current path B is thus broken. Each contact plane between discs from different groups will therefore constitute a breaking point where the conducting part 12, 14 of respective discs constitutes a contact member. Each disc thus constitutes a contact element having two contact members, one for the breaking point on each surface. The two outermost discs naturally have only one contact member each.
As can be seen most clearly in
Another difference that the drive means consists of the cylinder 108 cooperating with the discs of the first group, whereas the retaining member consists of the central, quadratic rod 109.
A third difference is that each conducting part 112a, 112b, 114a, 114b has considerably less angular extension than each conducting part in the embodiment shown in
A fourth difference is that neither of the groups has any aperture through the insulating part of each disc. In breaking position, as illustrated in
It will be understood that the drive means shown in
The positions of the current limiter illustrated in
Comparing fuses with semiconductors, such as thyristors, the prospective short-circuiting current, i.e. the short-circuiting current obtained if no current limitation takes place, is not dimensioned in the same way for a fuse as for a power semiconductor. This is because it always limits the current, as opposed to thyristors which may fail to break, which destroys the thyristors. The result will be a full non-limited short-circuiting current.
Yet another application is connection of large motors to a high voltage network where the short-circuiting effect is already at the limit. Installation of a new motor will increase the short-circuiting effect on the high-voltage network above what it is was dimensioned for since the motor will supply current to the high-voltage network at a short circuit in the high-voltage network. In principle this is the same problem as in distributed generation where generators are installed in a power network previously dimensioned for a certain short-circuiting effect. The new generators increase the short-circuiting effect above the permitted level. In many cases distributed generation requires the installation of current limiters, or for the switchgear to be rebuilt for the new short-circuiting effect-which may be an extremely costly process. In such cases it is often advisable to connect a number of generators to one current limiter, since the effect on each generator is slight.
In conjunction with respective figures the situation is also illustrated symbolically.
In
In the open position illustrated in
| Number | Date | Country | Kind |
|---|---|---|---|
| 0100074 | Jan 2001 | SE | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/SE02/00034 | 1/10/2002 | WO | 00 | 10/16/2003 |
| Publishing Document | Publishing Date | Country | Kind |
|---|---|---|---|
| WO02/056326 | 7/18/2002 | WO | A |
| Number | Name | Date | Kind |
|---|---|---|---|
| 3430062 | Roth | Feb 1969 | A |
| 3448287 | Giammona | Jun 1969 | A |
| 4027123 | Ihara | May 1977 | A |
| Number | Date | Country | |
|---|---|---|---|
| 20040245857 A1 | Dec 2004 | US |
