GB2305560A - Switching circuit for a bistable magnetic actuator - Google Patents

Abstract

A switching circuit for a two-coil bistable magnetic actuator, e.g for a reclosing circuit breaker, uses two switching devices 11,12, such as MOS or bipolar transistors, and freewheel diodes 23,24 which are switched out of circuit when an activating signal is being applied to the other of the transistors, but switched into circuit when the transistor with which they are coupled is turned off. If the diodes were left in circuit they would reduce the actuator's speed of response by allowing an induced circulating current to flow, opposing armature movement. The diodes 23,24 may be switched into circuit by normally open contacts 32,33 which are closed by the coils 30 and 31. A suitable relay response time response time (e.g. 5 ms) ensures that the diodes are in circuit for a sufficient time to absorb most of the magnetic energy of the magnetic actuator, thereby protecting the transistors from damaging overvoltages. The metal oxide varistors 20,21 also absorb some of the magnetic energy.

Description

SWITCHING CIRCUIT FOR A BISTABLE MAGNETIC ACTUATOR The invention relates to a switching circuit for a bistable magnetic actuator, and especially, but not exclusively, a switching circuit for a bistable magnetic actuator employed in an autorecloser device.

Bistable magnetic actuators are used in many applications where some form of mechanical change of state is required to be carried out in response to an electrical signal. Often, this mechanical change of state will result in the execution of an electrical switching operation.

One example of a switching actuator is found in an outdoor automatic recloser, which is a circuit breaker having fault detection circuitry causing it to open on excessive or unbalanced currents in power lines to which the recloser is connected. To avoid manual attention in cases where the fault is transitory, the relay circuitry is arranged to make several attempts to reclose the contact mechanism at short intervals, failing which, the mechanism is locked out.

An example of an autorecloser which is manufactured by the Applicants under the designation "OXR" is illustrated in Figure 1. This is a 3-phase recloser, each phase having a tubular insulating casting 1 which contains a vacuum interrupter 3 between HV terminals 5 and 7. The vacuum interrupters 3 are operated by insulating rods 9 which are driven from a common drive shaft 11. This drive shaft is in turn driven by an electromagnetic actuator 13.

The electromagnetic actuator, which is controlled from a so-called "protection relay" unit 15 containing switching elements, e.g. MOSFETs, is shown in Figure 2 (very diagrammatically) and comprises an armature 17 fixedly mounted on a drive shaft 19, the armature and shaft being movable axially within a cylindrical permanent magnet 21, closing coil 23 and opening coil 25. The armature is shown in its open, i.e.

downward, position, the switch having been tripped on a fault detection for example.

The permanent magnet 21 has a north pole on its inner cylindrical face and a south pole on its outer face. The resulting flux paths 27 through the armature and upper housing and 29 through the armature and lower housing are shown. In order to change the state of the circuit breaker from the illustrated open position to the closed position, the closing coil 23 is connected to the LiSO2 battery for a short time, e.g. less than 500 milliseconds. The resulting flux 31 is arranged to be in such a direction as to enhance the upper permanent magnet flux 27 and oppose the lower permanent magnet flux 29. The armature therefore moves to reduce the reluctance of the predominant upper flux path. The armature thus moves upwards, taking the drive shaft 11 with it and attempting to close the vacuum interrupter contacts.

A similar operation, but involving the opening coil instead of the closing coil, changes the circuit breaker from the closed to the open position.

It has been noted that the speed at which the armature of the magnetic actuator is moved directly influences the speed at which the autorecloser opens. The operating pulse is provided using a power MOSFET, the latter being chosen because it has simple drive requirements in comparison with many other devices.

The iron circuit of the magnetic actuator causes the coils to be seen as a highly inductive load. Due to the high switch-off speed of the MOSFET, a large turn-off transient voltage is seen between the drain and source of the device (usually greater than 300V). This voltage exceeds the voltage withstand of the MOSFET and, in conjunction with the current that is flowing through the device, exceeds the so-called "safe operating area" for that device. This can lead to device failure.

To avoid this, it is known to limit the turn-off voltage appearing across the switching device by placing a metal oxide varistor (MOV) across the device. This is shown in Figure 3.

In Figure 3, a switching circuit for a bistable magnetic actuator 10 is shown comprising switching elements in the form of power MOSFETs 11 and 12, the sources of which are connected to a voltage reference level 13 (nominally ground) and the drains of which are connected to respective operating coils 14, 15 of the actuator 10.

The other ends of the coils 14, 15 are taken to a voltage supply rail 22, which is constituted by one pole of a 140V lithium battery, the other pole of the battery being connected to the reference level 13.

The gates of the MOSFETs are protected by back-to-back connected zener diodes 16, 17 and are referenced to ground level 13 by resistors 18 and 19. The coils 14, 15 are energised by respective drive signals appearing on the gates of the MOSFETs 11, 12, these drive signals being +15V to energise the coil and 0V to remove the energising current from the coil. Because the actuator is bistable due to the action of the permanent magnet 21 (see Figure 2), the drive signals take the form of pulses, as shown, rather than continous levels.

Also included are 180V varistors 20, 21 between drain and source of respective MOSFETs. The varistors are designed to start conducting below the rated drain-source voltage limit of the switching device, thereby limiting the voltage across the device and dissipating the excess energy. However, a drawback of the varistor is that it takes a finite time to start conducting, and therefore an initial voltage transient will still be seen across the MOSFET. Whilst the MOSFET is turning off it is still breaking the full-load current of the device at the clamping voltage of the varistor, and, while this may not exceed the safe operating area limits for the switching device, it will still mean that the MOSFET switches off a large amount of energy. In a fast operating sequence, the MOSFET may have to be switched on and off on a number of occasions in a short space of time.Each operation cause the junction of the MOSFET to increase in temperature. This increase in temperature will result in a decrease in the switching capability of the device during the sequence of switching operations, and eventually the device may fail. It is necessary, therefore, with this type of circuit to limit the maximum duty cycle of the switching circuit to minimise the probability of failure.

A second known technique for switching inductive loads (e.g. relay coils) safely is the connection of a so-called "freewheel" diode across the load coil (see Figure 4).

In this arrangement, the diodes 23, 24 provide switch-off current circulating paths for current generated by the stored energy in the magnetic circuits of the actuator, the freewheeling current flowing preferentially through the diodes rather than through the switching devices. Now, on switch-off, the increase in voltage across the switching device due to the stored magnetic energy is clamped to approximately 1V above the positive voltage supply rail 22 by the diodes 23, 24, thereby protecting the MOSFETs by keeping these devices within their safe operating area.

It is, incidentally, possible to use a combination of both freewheel diodes across the coils and varistors across the MOSFETs for better protection.

While the use of freewheel diodes is largely beneficial, it has one significant drawback, namely the slowing down of the response time of the actuator. This occurs for the following reason. The armature 19 of the actuator passes through the centre of both the trip (open) and close coils 25, 23 (see Figure 2). Similarly, the casing of the actuator (not shown) surrounds both coils. To move the armature, current is applied to the relevant coil in the actuator, e.g. the trip coil 25. Once the armature has started moving towards the open position, the magnetic flux linked with the close coil will change. This will give rise to an induced EMF (electromotive force) across that coil by the well-known Lenz's law. Now, since a freewheel diode has been added across the close coil, current will flow through that coil creating flux which will tend to oppose the motion of the armature.In practice, the undesired current flowing through the close coil is smaller than the desired flowing through the trip coil, but nevertheless the existence of the smaller current will have the effect of opposing the required armature motion and will slow down that motion.

The lengthening of the response time is shown in Figure 5. In this diagram, which is a typical representation of the lengthening trend only, the solid waveform is that for the opening (trip) operation without freewheel diodes, while the dotted waveform is that for the same operation with freewheel diodes. A similar characteristic applies to the closing operation as well.

Such slowing of the response time of the actuator is a positive disadvantage in an application such as a battery-operated recloser, where it is required in the interest of battery life that the coil-energising pulses supplied by the battery be kept to as short a duration as possible.

It is an aim of the present invention to provide a switching circuit for an actuator which provides protection for the switching devices used while at the same minimising the response time of the actuator.

In accordance with a first aspect of the invention, there is provided a switching circuit for a bistable magnetic actuator having first and second actuator coils for moving an armature of the actuator into respective first and second positions, the switching circuit comprising first and second main switching elements connected to respective said coils for the application of operating currents thereto, first and second freewheel diode means for connection across respective said coils, and first and second auxiliary switching means connected to respective said coils, to respective said diode means and to respective said main switching elements for selective connection or disconnection of said diode means across respective said coils, said first and second auxiliary switching means being arranged such that respective said diode means are out of circuit during a time when a switch-on signal is being applied to the other of said main switching elements, and are in circuit when a switch-on signal is removed from respective said main switching elements.

Said first and second auxiliary switching means may be arranged to keep respective said diode means in circuit for a period of time following removal of a switch-on signal from respective said main switching elements.

Said first and second auxiliary switching means may be arranged to take respective said diode means out of circuit after a period of time sufficient to allow magnetic energy associated with respective said coils to decay to a residual level, whereby a voltage appearing across respective switching elements as a result of said residual magnetic energy is below a rated voltage for the respective switching elements.

Each of said auxiliary switching means may be a relay comprising a relay operating coil and a set of normally-open switching contacts, said relay operating coil being connected in parallel with an input drive circuit of the associated main switching means and said switching contacts being connected in series with the associated diode means across the associated actuator coil.

Said relay may be arranged to have a switch-off response time of at least 5ms.

The main switching element may be a MOSFET device.

In accordance with a second aspect of the invention, there is provide a method of switching a bistable magnetic actuator having first and second actuator coils for moving an armature of the actuator into respective first and second positions, said switching taking place by means of first and second switching elements connected to respective coils, first and second freewheel diode means being also associated with respective coils, the method comprising the steps of:: (a) removing from circuit a diode means associated with a coil which was last energised in order to place the armature in an existing position, (b) applying a switch-on signal to the switching element associated with the other of said coils in order to move the armature to a new position, (c) connecting into circuit the diode means associated with said other of said coils, and (d) when said armature has reached, or almost reached, its new position, removing said switch-on signal.

The method may include the step of (e) after a delay following step (d), removing from circuit the diode means connected into circuit in step (c), said delay being sufficient to allow magnetic energy associated with said other of said coils to decay to a residual level, whereby a voltage appearing across the switching element associated with said other of said coils as a result of said residual magnetic energy is below a rated voltage for the relevant switching element.

Step (e) may correspond to step (a) for a subsequent armature-reversing actuator switching operation.

An embodiment of the invention will now be described, by way of example only, with reference to the drawings, of which: Figure 1 is a schematic diagram of the internal arrangement of a known autorecloser; Figure 2 is a schematic representation of a magnetic actuator used in the autorecloser of Figure 1 and showing the application of a closing pulse; Figure 3 is a circuit diagram of a known bistable magnetic actuator drive circuit employing varistors; Figure 4 is a circuit diagram of a known bistable magnetic actuator drive circuit employing freewheel diodes; Figure 5 is a representational waveform diagram showing actuator response with and without freewheel diodes, and Figure 6 is a circuit diagram of a bistable magnetic actuator drive circuit in accordance with the invention.

Referring now to Figure 6, an autorecloser magnetic actuator drive circuit according to the invention is shown comprising, as with the known arrangements, two MOSFET devices 11, 12 coupled to respective actuator coils 14, 15, bias and input protection components 16, 17, 18, 19 being provided between the gates of the MOSFETs and ground potential. As before, a 140V lithium battery supplies power to the actuator coils 14 and 15. There is also a varistor 20, 21 between drain and ground of the respective MOSFETs.

In this embodiment of the invention, however, there is included in each half of the circuit a relay, whose operating coil 30, 31 is connected in parallel with the MOSFET drive circuit, i.e. between gate and ground, and whose normally-open contacts 32, 33 are connected between a freewheel diode 23, 24 and respective drains of the MOSFETs 11, 12. The other end of the diodes are connected to the positive supply rail 22.

When the operating pulse is applied to, say, MOSFET 11 (i.e. a "trip" pulse), relay coil 30 is energised, closing contacts 32 and bringing the diode 23 into circuit across the coil 14. Note that, since no operating pulse has been applied to the MOSFET 12, relay coil 31 will not be energised and diode 24 will remain out of circuit. At the end of the operating pulse, voltage is removed from the gate of the MOSFET, and the switching device begins to switch off. Power is also removed from the relay coil 30, but since the relay has a finite response time, in a typical case around 5ms, it takes this long for the contacts 32 to open again. This ensures that diode 23 is still in circuit at the moment of switch-off of MOSFET 11, allowing the stored magnetic energy associated with the coil 14 and the actuator iron to be dissipated as current through the diode 23.Diode 32 therefore switches on and clamps the drain voltage to within 1V of the positive supply rail 22.

When the contacts 32 finally open, the recirculating current flowing through the diode 23 will have decayed so that a much smaller over-voltage is seen. By this time, the MOSFET 11 has switched off completely and the voltage appearing across the drain and source of the MOSFET is well within the rating of the device used. Furthermore, it has a slow enough rise time to be clipped by the varistor 20.

The same mode of operation applies equally to a closing operation involving the close coil 15, MOSFET 12, diode 24, etc.

It can thus be seen that the two desired functions of MOSFET protection and minimisation of actuator response time have been accomplished by virtue of the inclusion of diode 32 at the moment of switch-off and by the exclusion of diode 33 during the application of the operating pulse to MOSFET 11, respectively. The freewheeling diode is only allowed in circuit when it is actually needed, and excluded otherwise.

While it has been assumed that MOSFETs are used as the switching devices, in practice any suitable switching device may be employed, e.g. bipolar transistors.

Also, while the use of a relay to switch the freewheel diodes into and out of circuit has been described, other switching means may be possible, e.g. the use of a semiconductor switching device in place of the relay contacts 32, 33. The main proviso in this case is that the device be able to handle the recirculating current levels through the diode and that a delay be provided between the moment the switch-off command appears at the device's control terminal (e.g. gate or base) and the moment the device has switched from its low-impedance state to its high-impedance state.

Claims (11)

1. A switching circuit for a bistable magnetic actuator having first and second actuator coils for moving an armature of the actuator into respective first and second positions, the switching circuit comprising first and second main switching elements connected to respective said coils for the application of operating currents thereto, first and second freewheel diode means for connection across respective said coils, and first and second auxiliary switching means connected to respective said coils, to respective said diode means and to respective said main switching elements for selective connection or disconnection of said diode means across respective said coils, said first and second auxiliary switching means being arranged such that respective said diode means are out of circuit during a time when a switch-on signal is being applied to the other of said main switching elements, and are in circuit when a switch-on signal is removed from respective said main switching elements.

2. A switching circuit as claimed in Claim 1, in which said first and second auxiliary switching means are arranged to keep respective said diode means in circuit for a period of time following removal of a switch-on signal from respective said main switching elements.

3. A switching circuit as claimed in Claim 2, in which said first and second auxiliary switching means are arranged to take respective said diode means out of circuit after a period of time sufficient to allow magnetic energy associated with respective said coils to decay to a residual level, whereby a voltage appearing across respective switching elements as a result of said residual magnetic energy is below a rated voltage for the respective switching elements.

4. A switching circuit as claimed in Claim 3, in which each of said auxiliary switching means is a relay comprising a relay operating coil and a set of normally-open switching contacts, said relay operating coil being connected in parallel with an input drive circuit of the associated main switching means and said switching contacts being connected in series with the associated diode means across the associated actuator coil.

5. A switching circuit as claimed in Claim 4, in which said relay is arranged to have a switch-off response time of at least 5ms.

6. A switching circuit as claimed in any one of the preceding claims, in which said main switching element is a MOSFET device.

7. A method of switching a bistable magnetic actuator having first and second actuator coils for moving an armature of the actuator into respective first and second positions, said switching taking place by means of first and second switching elements connected to respective coils, first and second freewheel diode means being also associated with respective coils, the method comprising the steps of: (a) removing from circuit a diode means associated with a coil which was last energised in order to place the armature in an existing position, (b) applying a switch-on signal to the switching element associated with the other of said coils in order to move the armature to a new position, (c) connecting into circuit the diode means associated with said other of said coils, and (d) when said armature has reached, or almost reached, its new position, removing said switch-on signal.

8. A method of switching a bistable magnetic actuator as claimed in Claim 7, including the step of (e) after a delay following step (d), removing from circuit the diode means connected into circuit in step (c), said delay being sufficient to allow magnetic energy associated with said other of said coils to decay to a residual level, whereby a voltage appearing across the switching element associated with said other of said coils as a result of said residual magnetic energy is below a rated voltage for the relevant switching element.

9. A method of switching a bistable magnetic actuator as claimed in Claim 8, in which step (e) corresponds to step (a) for a subsequent armature-reversing actuator switching operation.

10. A switching circuit substantially as hereinbefore described with reference to Figure 6.

11. A method of switching a bistable magnetic actuator substantially as hereinbefore described.