The present invention relates to the operation of electrical switches, especially circuit breakers.
Circuit breakers, including reclosers, typically comprise an electromagnetic actuator for moving an electrical contact between open and closed states. Closing the actuator usually involves energising one or more electromagnetic coils to move the contact against a mechanical bias such as a spring. In order to preserve the mechanical life of the circuit breaker, the speed at which the contact moves should be restricted. This adversely affects the efficiency of the actuator, resulting in increased weight size and power consumption for the circuit breaker.
It would be desirable to provide an improved method for controlling the operation of circuit breakers that mitigates the problem outlined above.
A first aspect of the invention provides a method of controlling an electrical switch, the electrical switch comprising a movable contact and an electromagnetic actuator for causing said movable contact to move between an open position and a closed position, said method comprising:
with said movable contact in said open position, applying a voltage to said actuator to cause a motive force to be applied to said movable contact to cause said movable contact to move towards said closed position, wherein said voltage is applied for a first time period ending before said movable contact reaches said closed position, and
at the end of said first time period, adjusting said voltage to reduce said motive force.
In typical embodiments, said method further includes, after said voltage is adjusted to reduce said motive force, further adjusting said voltage to increase said motive force. Said further adjusting of said voltage is preferably performed before said movable contact reaches said closed position, especially immediately before said movable contact reaches said closed position. In particular, it is preferred that said further adjusting of said voltage is performed sufficiently close to the moment when said movable contact reaches said closed position that said further voltage adjusting does not appreciably affect the speed of said movable contact. For example, said further adjusting of said voltage may be performed up to 2 ms, preferably up to 1 ms, and more preferably up to 0.5 ms, before said movable contact reaches said closed position. Said further adjusting of said voltage may be performed substantially at the same time as said movable contact reaches said closed position.
Optionally, said adjusting said voltage to reduce said motive force involves reducing said voltage to a non-zero level. Said adjusting said voltage to reduce said motive force may involve reducing said voltage by at least approximately 50% to a non-zero level.
Alternatively, said adjusting said voltage to reduce said motive force involves reducing said voltage to zero.
Alternatively still, said adjusting said voltage to reduce said motive force involves reversing the polarity of said voltage.
Alternatively, said adjusting said voltage to reduce said motive force involves modulating said voltage. Said adjusting said voltage to reduce said motive force may involve pulse width modulating said voltage. Said pulse width modulation may be arranged to cause zero volts to be applied to said actuator between pulses.
In typical embodiments, said switch includes a control circuit, said control circuit including at least one capacitor for storing said voltage, and wherein said applying a voltage to said actuator to cause a motive force to be applied to said movable contact involves applying said voltage from said at least one capacitor to said actuator. Adjusting said voltage to reduce said motive force may therefore involve adjusting said voltage applied from said at least one capacitor to said actuator.
In preferred embodiments, said actuator comprises at least one electromagnetic coil, and wherein said applying a voltage to said actuator to cause a motive force to be applied to said movable contact involves applying said voltage to said at least one coil. Typically adjusting said voltage to reduce said motive force involves adjusting said voltage applied to said at least one coil.
From a second aspect the invention provides an electrical switch comprising a movable contact and an electromagnetic actuator for causing said movable contact to move between an open position and a closed position, said switch further comprising
a voltage source,
a controller for selectably applying voltage from said voltage source to said actuator,
wherein said controller is arranged to, with said movable contact in said open position, cause a voltage to be applied to said actuator from said voltage source to cause a motive force to be applied to said movable contact to cause said movable contact to move towards said closed position,
and wherein said controller is arranged to apply said voltage for a first time period ending before said movable contact reaches said closed position,
and wherein said controller is further arranged to, at the end of said first time period, adjust said voltage to reduce said motive force.
Preferably, said voltage source comprises at least one capacitor.
Typically, said actuator comprises at least one electromagnetic coil, said controller being arranged to selectably apply voltage to said at least one electromagnetic coil.
Said actuator may include a movable part movable into and out of a closed position in response to changes in the energization of said at least one electromagnetic coil. Preferably, said actuator includes a non-movable part, and wherein said movable and non-movable parts are configured to latch magnetically with one another in a closed position as a result of residual magnetism of said movable and non-movable parts (said residual magnetism resulting from the prior effect of said at least one coil when energised (i.e. by the flow of current) on said movable and non-movable parts).
Said electrical switch may comprise a circuit breaker or a vacuum interrupter.
Further advantageous aspects of the invention will become apparent to those ordinarily skilled in the art upon review of the following description of a specific embodiment and with reference to the accompanying drawings.
An embodiment of the invention is now described by way of example and with reference to the accompanying drawings in which:
Referring now in particular to
The switch 10, which is hereinafter referred to as a circuit breaker, comprises first and second electrical contacts 12, 14. The first contact 12 is movable between an open position (as shown in
In the illustrated embodiment, the contacts 12, 14 are located in a vacuum chamber 16 and the circuit breaker 10 may be referred to as a vacuum circuit breaker.
Movement of the contact 12 between its open and closed positions is effected by an electromagnetic actuator 18, which is described in further detail hereinafter with reference to
Referring now to
The actuator 18 comprises a stem 28 which conveniently carries the spring 26. In the illustrated embodiment, the free end 30 of the stem 28 is coupled to the coupling member 22. In use, as part 24A moves towards part 24B, it causes rod 30 to move upwardly (as viewed in the drawings). Corresponding movement is imparted to a second stem 29 via the coupling member 22, the second stem 29 being coupled between the coupling member 22 and the movable contact 12. This movement of the second stem 29 causes the contact 12 to move towards and ultimately into the closed position. Resilient biasing means, for example comprising one or more compression springs 27, may be coupled between the movable part 24A and the stem 28. The preferred arrangement is such that, when the part 24A is in its closed position, spring 27 is compressed and so imparts force to the stem 28 to help maintain contact 12 in its closed position.
Hence, movement of the part 24A towards its closed position causes movement of the contact 12 towards its closed position. It is noted that the part 24A and contact 12 may not reach their respective closed positions at the same time. For example, in the illustrated embodiment, contact 12 reaches its closed position before part 24A does. The preferred arrangement is such that the movement of the part 24A that occurs after contact 12 is closed serves to compress spring 27.
The actuator 18 includes an electromagnetic operating device 32 comprising one or more electromagnetic coil 36 (which may comprising one or more windings), and typically a coil holder. The coil 36 is typically annular and is shown in
In the preferred embodiment, a solid core is not present within the coil 36. However, movable part 24A may be regarded as an electromagnetic core for the coil 36, while non-movable part 24B may be regarded as a yoke. Typically, parts 24A, 24B are formed at least partly from magnetisable, or ferromagnetic, material that is non-permanently magnetised but is susceptible of being magnetised by the electromagnetic field generated in use by the coil 36. Alternatively, one or both of parts 24A, 24B may be formed at least partly from permanently magnetised material.
The coil 36 is carried by, typically fixed to, one of the parts 24A, 24B, in this example the second part 24B. The preferred arrangement is that the coil 36 projects from the second part 24B and the first part 24B is shaped to receive the projecting portion of the coil 36 when the parts 24A, 24B are together. The first part 24A may be held in the closed position by one or more of a variety of ways depending on the embodiment. For example, where one or both of the first or second parts 24A, 24B comprises a permanent magnet, or is otherwise formed at least partly from magnetisable material, the first part 24A may be held closed by residual magnetism (indicated by magnetic flux lines RM in
The coil 36 may be operated to release the first part 24A by controlling the voltage applied to the coil 36, and in particular by controlling the current flowing in the coil. For example, in embodiments where the coil 36 is energised to maintain the latching state by electromagnetism, the coil 36 may be released by de-energising the coil 36 (i.e. reducing the current flowing in the coil). In preferred embodiments, a suitable voltage may be applied to the coil 36 resulting in an electromagnetic field that has the effect of overcoming or cancelling any residual magnetism (including permanent magnetism) that is maintaining the latched state. Conveniently, this is achieved by applying a voltage to the coil with opposite polarity to the voltage used to close the actuator 18.
When the coil 36 is operated as described above (i.e. when the first and second parts 24A, 24B are de-magnetised), the spring 26 actuates the first part 24A of the body into its open position (
Referring now to
In a simple embodiment (not illustrated), the control circuit may be arranged to apply an energising voltage to the coil 36 when it is desired to close the actuator 18 or keep it closed (i.e. keep the parts 24A, 24B magnetised), and to de-energise the coil 36, e.g. cut or reduce the voltage, when it is desired to open the actuator 18 (wherein the parts 24A, 24B are such that residual magnetism does not continue to hold them together).
In preferred embodiments, however, where the coil 36 is held in its latching state by residual magnetism, the control circuit 40 is configured to respectively apply a voltage to the coil 36 to open the actuator 18 and to close the actuator 18. When opening the actuator 18, the applied voltage is selected such that it has the effect of de-magnetising the first and second parts 24A, 24B of the actuator as described above. When closing the actuator, the applied voltage is selected such that the coil 36 creates an electromagnetic field causing the first part 24A to be drawn to the closed position (overcoming the bias of the spring 26), i.e. the energised coil 36 creates a motive force acting on the movable part 24A of the actuator, causing the movable part 24A to move towards the closed position, which in turn creates a motive force on the movable contact 12, causing the contact 12 to move towards the closed position.
Typically, the circuit 40 includes one or more storage capacitors 44, 46 for energising the coil 36. In particular, the coil 36 is energised by discharging the capacitor voltage across the coil, thereby causing current to flow through the coil to energise the coil. To this end, the circuit 40 includes one or more switches for selectably applying the or each capacitor voltage to the coil 36. In preferred embodiments, a respective one or more capacitors are provided for opening the actuator 18 and for closing the actuator 18. In
Closing the actuator 18 consumes much more energy than opening the actuator 18 especially where the bias of the spring 26 must be overcome. One way of controlling the closing process involves direct connection of the respective capacitor 44, 46 to the actuator coil 36 for a limited duration (i.e. application of a transient voltage). A disadvantage of this method is the substantial energy required for actuator closing. This energy could be reduced if there were no limitation on the speed at which the actuator closes, since with increasing closing speed actuator efficiency increases. However, closing velocity should be limited in order to preserve the mechanical life of the circuit breaker 10. For example, the closing velocity of the movable contact 12 should typically not exceed 1-1.5 m/s. Therefore, the parameters of the actuator are selected in such a way that the closing velocity does not exceed the acceptable limit. However, in this case the actuator operates with relatively low efficiency, resulting in increased weight, size and power consumption.
For example,
In preferred embodiments, the controller 42 is configured to control the application of voltage to the coil 36 during the closing process as is now described with reference to
At the end of time period P1, the controller 42 is configured to adjust the voltage applied to the coil 36, preferably for a second time period P2 ending at time T4, where T4 is before or substantially at the same time as the contact 12 reaches its closed position. The adjustment of the voltage is such that it reduces the motive force exerted on, and therefore the acceleration of, the movable part 24A (by de-energisation of the coil 36) and correspondingly on the movable contact 12.
In one embodiment, as exemplified by
In another embodiment, as exemplified by
In a further embodiment, as exemplified by
In a still further embodiment, as exemplified by
Advantageously, at the end of time period P2, the controller 42 is configured to increase the voltage (including the option of increasing the effective voltage, e.g. by adjusting the modulation) applied to the coil 36, preferably to the maximum level attainable by the control circuit 40 (which in the present embodiment is determined by the voltage across capacitor 44 and is typically less than the voltage V1), for a time period P3 ending at time T5, where T5 typically ends after contact 12 has reached the closed position. This has the effect of re-energising the coil 36 to create sufficient residual magnetism in parts 24A, 24B to hold the actuator 18 in its closed state after the capacitor voltage has gone. In the illustrated embodiment, the voltage is increased during P3 to increase the current in coil 36 in order to increase the magnetic flux in parts 24A, 24B to such a level that the parts 24A, 24B are held closed by residual magnetism (magnetic latching). In embodiments where residual magnetism is not required to hold the latch in its closing state, increasing the voltage during P3 is not necessary.
Period P3 may begin before (preferably just before, e.g. up to 2 ms, preferably up to 1 ms, and more preferably up to 0.5 ms before), at substantially the same moment as, or after the movable contact 12 reaches its closed position. As a result, increasing the voltage at this time does not appreciably increase the speed of the contact 12.
In preferred embodiments, the desired initial speed of the contact 12 at time T3 is determined by the desired maximum speed of the contact 12 when it engages with the fixed contact 14. The desired maximum speed depends on the physical characteristics of the circuit breaker 10 but in general is selected so as not to cause undue damage to the contacts 12, 14. Once the initial speed is known, the duration of period P1 can be determined. This will depend not only on the physical characteristics of the circuit breaker 10 (e.g. respective masses of the movable parts 24A, 12, strength of the spring 26 etc.) but also on the voltage available from the capacitor 44. It is preferred to accelerate the contact 12 to the initial speed as quickly as possible since this reduces the energy required to do so. Therefore, it is preferred to use a capacitor 44 that allows the highest practicable voltage to be provided to the coil 36. In practice, the control circuit 40 has current limitations and so the capacitor 44 is chosen to provide the highest voltage possible without exceeding the current limitations. For example, in the circuit 40 of
It will be seen therefore that in the preferred embodiment, the entire available capacitor voltage is applied to the coil 36 during the initial stage P1 to begin to close the actuator 18 and to accelerate the movable contact 12 to the desired initial velocity. Then, the voltage (or effective voltage) is decreased deliberately (as opposed to decreasing as a result of capacitor voltage decay) by the controller 42 to suppress acceleration of the contact 12. When the movable contact 12 approaches the closed position (and there is no time left to accelerate the respective movable parts beyond the desired maximum speed), or afterwards, the voltage is increased again, providing growth of coil current to a level sufficient for effective magnetization of the actuator's components to allow magnetic latching in the closed position.
In the example of
In practice, the speed of moving contact 12 is important as it affects the mechanical life of the vacuum interrupter or other device. Typically, the respective speeds of movable contact 12 and part 24A of the actuator 18 are substantially equal until movable contact 12 hits the fixed contact 14 (due to the fact that part 24AB during upward movement pushes stem 28 of the insulator 22 with the aid of additional contact pressure spring 27). At the moment when contacts 12, 14 close together, there is a gap, e.g. of approximately 2 mm, between the parts 24A, 24B of the actuator 18. After this moment movable contact 12 does not move but part 24A keeps moving until the gap is closed.
The invention is not limited to the embodiment described herein, which may be modified or varied without departing from the scope of the invention.
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