GB2318913A - Performing a switching operation on a switching device - Google Patents

Abstract

A method of actuating an HV circuit breaker at, eg, an AC zero-crossing point involves forming an estimate of time at which an actuating means for opening and closing a set of contacts will reach a contacts-open or contacts-closed position, eg by measuring a velocity of the actuating means, comparing this time with an assumed time at which the voltage/current waveform will pass through zero and, where the difference between these two times is greater than a predetermined amount, adjusting the velocity of the actuating means so that the contacts will open/close at an appropriate zero-crossing.

Description

SWITCHING APPARATUS AND METHOD OF PERFORMING A SWITCHING OPERATION ON A SWITCHING DEVICE The invention concerns a switching apparatus for interfacing an alternating-current electricity supply to a load and a method of performing a switching operation on a switching device, both the apparatus and method involving in particular, but not exclusively, the use of a high-voltage alternating-current supply in conjunction with a switching device employing SF6 as an insulant and arc-extinguishing medium.

A typical hydraulically operated three-phase 145kV switching arrangement is illustrated in Figure 1. In Figure 1, three SF6 interrupters 10, 20, 30 each having a set of contacts 12, 22, 32 - one fixed, one moving - and an actuator mechanism 14, 24, 34 are connected to a hydraulic actuating arrangement 40. Hydraulic actuating arrangement 40 comprises a high-pressure storage accumulator 42 with nitrogen back-up cylinder 44, a main valve 46, a pilot/servo valve arrangement 48, and an oil reservoir 50 (which is at atmospheric pressure). The actuators 14, 24, 34 are connected to the main valve 46 via oil lines 16, 26, 36 and to the accumulator 42 via oil lines 18, 28, 38.

The hydraulic actuating arrangement is shown in an "open" position corresponding to the circuit-breaker contacts being open. To close the contacts, a close coil solenoid 52 is energised which operates the pilot/servo valve and thereby the main valve 46, this in turn cutting off the lines 16, 26, 36 from the reservoir 50 and connecting them to the accumulator 42, placing both the top and bottom of each actuator at high pressure.

The effect of this is seen more clearly by reference to Figure 2, which shows an actuator 10 in both a closing (Fig. 2a) and an opening (Fig. 2b) State. The actuator has an actuating means in the form of an operating rod 11 at the bottom end of which is attached a piston 13. Due to action of the main valve 46, as just described, oil is forced from the accumulator 42 through the line 16 into the lower part of the actuator 10 below the piston, oil already being present at the same pressure in the upper part of the actuator above the piston. A difference in area between one end of the piston 13 and the other causes the piston and the operating rod 11 to be forced upwards closing the contacts 12.

Opening of the contacts is the reverse of this sequence, being initiated by the operation of an opening solenoid 56, which again operates the main valve via the pilot/servo valve to cut off the high-pressure supply to the lines 16, 26, 36 while connecting them to the reservoir. A pressure difference between the two sides of the actuator pistons 13 then causes the piston to move downwards, opening the circuit breaker contacts 12 and forcing the oil in the lower part of the actuators 14, 24, 34 to transfer into the reservoir.

An alternative to using hydraulic actuation is to employ a spring system or pneumatic system. In the first case a large spring, either wound or helical, is employed as an energy storage medium and used to close the circuit breaker contacts, charging a smaller trip spring in the process.

Circuit breakers such as these are required to supply electrical current to, and conversely break current from, a variety of loads. A typical example would be the need to be able to switch a high voltage onto a capacitive load or interrupt current to an inductive load. In the former case it is desirable that the AC supply be switched to the load when the voltage waveform has a low instantaneous value, and preferably a zero value, so that any inrush current is kept to a minimum. In the latter case it is usually required that the supply be removed when the current waveform has reached a low instantaneous value, again preferably a zero value, so as to minimise transient switching overvoltages.

Considerable difficulty has been experienced in striving for, for example, reliable zero-voltage switching. This is due to the presence of uncertainties in the operating parameters of a number of elements in the switching process. These elements include stored energy (the accumulator is capable of handling several consecutive actions, e.g. close - open - close, and the energy stored in the accumulator will decrease each time the circuit breakers are operated), ambient temperature, control voltage (the close and trip solenoids are normally energised from a battery source and this may be at various stages of recharge), and short-term repeatability under norninal conditions of ambient temperature, auxiliary voltage and stored energy. In addition there will be factors such as ageing over a long period, uncertainties associated with the actuating mechanism - for example, where a hydraulic system such as described above is employed, variations in hydraulic oil pressure, nitrogen density, valve timing, presence of air and moisture in the hydraulic oil - and also electrical system variables, e.g. frequency, harmonics, power factor and phase imbalance.

A desired switching operation for the driving of a capacitive load is shown in Figure 3, where the abscissa represents time (in ms) from the start of travel of the actuator rod to the "make" point of the actuator contacts (in this case, 80ms) and beyond, and the ordinate represents the actuator-rod travel itself (in this case 250mm to "contacts make", 340mm to end of travel). It should be noted, incidentally, that the travel of the actuator rod beyond the "contacts make" point allows the puffer cylinder 15, 25, 35 (see Figure 1) containing the SF6 gas to enclose the fixed contact so that in a subsequent "open" operation the arc that is produced during breaking of the contacts can be extinguished by the flow of the gas which is compressed when the puffer cylinder moves downwards and the volume occupied by the gas is decreased. Also shown in this diagram is the waveform of a reference voltage derived from one of the AC system phases.

As shown by the solid-line characteristics, the actuator rod moves at a substantially uniform velocity over its travel and arrives at the "contacts make" point exactly as the phase voltage reaches a zero-crossing point, which is the preferred situation. It is required to provide such coincidence in each phase of the AC system, not just in one phase.

The dotted-line characteristics illustrate the situation where the system variables are such that there is a time difference between the "contacts make" point of the actuator rod and the zero-crossing point of the AC voltage. This time difference may be due to uncertainties in the actuating system itself, e.g. oil pressure, control voltage, etc, and/or due to uncertainties in the AC system, e.g. frequency variations, phase imbalance, etc. Figure 3 shows both the case where the actuator-system variables are responsible (dotted interrupter-travel line) and where the AC system variables are responsible (dotted phase voltage waveform).

In an attempt to secure coincidence between the "contacts make" point of the actuator and a suitable point of the reference waveform, a so-called "control switching relay" is commonly employed between the "close" (or "open") initiation signal and the impulse to the circuit breaker. This relay continuously monitors the supply voltage of a reference phase (or phases) and inititates circuit breaker operation after a preset time with respect to the reference voltage zero crossing. All three phases are initiated independently to ensure that their respective contacts make or break at the point on the wave most beneficial to the application (in the above case, the voltage zero crossing). The limitation with this system, however, is that the variation in circuit breaker closing or opening time must be kept within a very close tolerance to obtain any benefit over random operation.

This is generally more readily obtained with circuit breakers having three independent spring mechanisms than those equipped with hydraulic mechanisms in view of the additional variables outlined earlier (e.g. oil pressure, nitrogen density, etc).

A known way of reducing the effect of such variables is to employ what is called "adaptive contror'. This is a technique in which the control switching relay is provided with a means for compensating for known variables (e.g. short-term repeatability, control voltage, stored energy, etc.) and modifying the "close" command signal timing delay accordingly.

Adjustment of the time delay may be based on a measurement of the various parameters for the most recent operation, or alternatively may be based on a moving average in case parameters for the most recent event are not typical, i.e. do not follow what may be a certain trend. While this may compensate to some extent for random variables, it will not compensate for those which oppose the general trend; also, it does not usually take into account AC system variables such as frequency variations, harmonics, power factor, etc.

Thus, if a circuit breaker has a closing time of 100 ms and the 50 Hz system frequency varies by i1Hz, the tolerance on the "contacts make" timing would be +9 ms relative to the desired point on the voltage wave.

In summary, the present state of the art readily achieves a tolerance of, say, is ms for a hydraulic system and, say, i3 ms for a spring system, though this can be improved with difficulty to +9 ms, whereas it is desirable to be able to achieve timing accuracies of +lems or even better. Furthermore, the performance that can be presently achieved is at the expense of design simplicity and repeatability.

In accordance with a first aspect of the present invention, there is provided a method of performing a switching operation on a switching device having a set of contacts and an actuating means movable between a contacts-open position and a contacts-closed position, the switching device being used to interface an alternating-current electricity supply to a load, the method comprising the steps of: (1) initiating movement of said actuating means from one of said contacts-open position and contacts-closed position to the other of said contacts-open position and contacts closed position; (2) during movement of said actuating means to the other of said contacts-open position and contacts-closed position, forming an estimate of time at which said other position will be reached; (3) comparing said estimated time with an anticipated time at which a parameter of said alternating-current electricity supply will have a desired instantaneous value; (4) in dependence on said comparison, adjusting a rate of movement of said actuating means such as to reduce a difference between said estimated and anticipated times.

Step (2) may comprise the steps of measuring a velocity of said actuating means at a position of said actuating means intermediate said contacts-open and contacts-closed positions and subsequently forming said time estimate on the basis of a known distance between said intermediate position and said other of said contacts-open and contacts-closed positions. Instead of measuring velocity once only, it may be measured twice or more at a plurality of positions of said actuating means intermediate said contacts-open and contactsclosed positions and after each measurement said time estimate may be derived on the basis of a known distance between respective said intermediate position and said other of said contacts-open and contacts-closed positions, said actuating means velocity being adjusted after each successive derivation of time estimate by respectively successively finer amounts.

Step (2) may comprise at a position of said actuating means intermediate said contacts-open and contact-closed positions deriving said estimated time on the basis of an assumed value of actuating means velocity and a known distance between said intermediate position and said other of said contacts-open and contacts-closed positions.

According to a second aspect of the invention, there is provided a switching apparatus for interfacing an alternating-current electricity supply to a load, comprising: a switching device having a set of contacts and an actuating means movable between a contacts-open position and a contacts-closed position; a position-sensing means connected to said actuating means and adapted to provide a signal representative of a position of said actuating means relative to said other of said contacts-open and contacts-closed positions; an alternating-current signal inputting means for providing a signal representative of said electricity supply; and a processing means connected to said position-sensing means and to said signal inputting means and adapted to form from said position-representative signal and from said supply-representative signal an estimate of a time at which said actuating means will reach said other of said contacts-open and contacts-closed positions, to compare said estimated time with an anticipated time at which a parameter of said alternating-current electricity supply will have a desired instantaneous value and, in dependence on said comparison, to adjust a rate of movement of said actuating means such as to reduce a difference between said estimated and anticipated times.

The position-sensing means may comprise at least one auxiliary switch disposed in actuable relationship with said actuating means at a position intermediate said contacts-open and contacts-closed positions, said intermediate position being at a known distance from said other of said contacts-open and contacts-closed positions.

The processing means may be adapted to form said time estimate following actuation of said auxiliary switch by said actuating means, said estimate being derived from said known distance between said intermediate position and said other of said contacts-open and contacts-closed positions and from an assumed value of actuating-means velocity.

The position-sensing means may comprise a pair of said auxiliary switches disposed in spaced-apart relationship along a direction of travel of said actuating-means, said processing means being adapted to derive a value of actuating-means velocity on the basis of a known distance between said auxiliary switches and a measured time interval between actuation of one of said auxiliary switches and subsequent actuation of the other of said auxiliary switches by said actuating means, said time estimate being derived from a known distance between one of said auxiliary switches and said other of said contacts-open and contacts-closed positions and from said derived value of actuating-means velocity.

The position-sensing means may comprise a potentiometer means having a wiper coupled to said actuating means such that movement of said actuating means produces corresponding movement of said wiper, said potentiometer providing in use an electrical quantity the value of which is proportional to a distance of said actuating means from said other of said contacts-open and contacts-closed positions.

An embodiment of the present invention will now be described, by way of example only, with reference to the drawings, of which: Figure 1 is a simplified schematic diagram of a typical hydraulically actuated circuit breaker arrangement; Figure 2 is a sectional diagram of a typical circuit breaker hydraulic actuator in both a closing and an opening state; Figure 3 is a graphical representation of an exemplary behaviour of a circuit breaker arrangement showing both coincidence and non-coincidence with a desired point on a reference AC waveform; Figure 4 is a diagram illustrating the principle of velocity adjustment during circuitbreaker actuation in accordance with the present invention; Figure 5 is a schematic diagram of a circuit-breaker actuation system including a switching apparatus in accordance with the invention; Figure 6 is a schematic diagram of a switching apparatus in accordance with the invention; Figure 7 is a schematic diagram of part of a switching apparatus in accordance with the invention employing a potentiometer position-sensing means, and Figure 8 is a diagram illustrating an alternative switching method in accordance with the invention.

An embodiment of the invention will now be described with the aid of Figure 4.

Figure 4 shows, as Figure 3, a characteristic of actuator-rod travel between an open position and a fully closed position, and also a voltage waveform characteristic at a zero crossing of which it is desired that the circuit-breaker contacts should be at the instant of being closed (i.e. contacts "made"). A number of measurements and adjustments are carried out while the actuator rod is en route to its "contacts make" position, namely: (1) A measurement of velocity of the actuator rod is initiated at a particular point along the actuator travel.

(2) From this velocity measurement and a known distance from the measurement initiation point to the "contacts make" point an estimate is formed as to the time at which the actuator will have reached the "contacts make" point.

(3) This time estimate is compared with an anticipated time at which the voltage waveform will be at its next zero-crossing point (this is calculated on the basis of a previous zero-crossing point and a nominal value of AC system frequency).

(4) The actuator-rod is slowed down by an amount dependent on a time difference determined from the above comparison.

(5) Normal actuator drive is restored, i.e. the actuator rod is allowed to resume its "normal" velocity, such that the "contacts make" point in time is as near as possible coincident with the AC zero-crossing point in time.

Formation of the velocity estimate is accomplished in this embodiment by means of an auxiliary-switch arrangement 19, 29, 39 (see Figure 5) contained in the lower part 17, 27, 37 of the circuit-breaker housing. Figure 6 shows in more detail a representative auxiliaryswitch arrangement for this scheme in which a pair of microswitches 58, 59 are mounted within the housing of the circuit breaker by suitable mounting means and an actuating collar 62 is attached to the operating rod 64 of the actuator, such that when the rod is moved by the hydraulic system the collar engages momentarily with each switch actuator button in turn (times t1, t2 in Figure 4), the switches thereupon transmitting respective signals to a processing unit 70 via lines 72, 73.

Processing unit 70 takes the two signals output along lines 72, 73 and forms a measurement of the time delay At between them. Unit 70 is also preprogrammed with the known distance a between the switches and the known distance b between one switch (e.g.

the top switch) and the "contacts make" position of the collar 62. The velocity of the actuator rod is then calculated as the ratio of the known distance a and the time delay At.

This occurs at, or very shortly after, time t2. The next step is that the processing unit 70 calculates an estimate of the time at which the contacts will reach their "make" position (time t4 in Figure 4), doing this by dividing the known distance b by the calculated velocity.

Also applied to the processing unit 70 is an AC signal 74 derived from the system AC voltage. The processing unit registers on a continuous basis each zero crossing of this AC signal and uses this zero-crossing to trigger a 10 ms count-down routine, this figure representing the time interval between successive zero crossings of a 50 Hz waveform This therefore gives an indication of time to the next anticipated zero-crossing, and is based on an assumption that system frequency will be at its nominal (50 Hz) value.

Directly after calculation of velocity, the processing unit 70 forms a comparison between the estimated time to "contacts make" and the anticipated time to next zero crossing and, if the estimated "contacts make" time is greater than the assumed zerocrossing time by more than a given amount (e.g. 1 ms), then the normal hydraulic oil supply to the bottom of the actuator section of the particular phase in question (the phases are treated separately in this regard) is momentarily reduced. This reduction takes place by way of an electro-hydraulic valve 80 in the respective line 16, 26, 36 (see Figures 5 and 6), the valve being actuated through a solenoid 82 driven from the processing unit 70. In the preferred embodiment of the invention this valve 80 has a bleed path allowing some residual flow when the valve is closed, alternatively there is a bypass in each of lines 16, 26, 36 shunting the valve 80.

Referring again to Figure 4, in the illustrated case the estimated time at which the contacts will close (time t4) is due to occur approximately 2 ms after the previous zero crossing, hence the processing unit 70 makes the decision at, or very shortly after, time t2 to delay the actuator travel by throttling back the hydraulic drive to the actuator. In a typical system the hydraulics may take in the region of 7 ms to respond, so that even if a command is issued to the solenoid 82 to throttle back the valve 80, this will only be done at around time t3 Thus, time interval t3-t2 is around 7 ms. This Figure of 7 ms is representative only and the actual (nominal) response time will be known for the application in question. Thus at time t3 drive to the actuator is greatly reduced, being restricted to the bleed flow previously mentioned.

This restricted flow is continued for as long as is necessary to ensure that, when full flow is resumed at time t5, the contacts will close at the next zero-crossing point, t6. The processor calculates the required restricted-flow time on the basis of the anticipated time to next zero-crossing t6, the known restricted-flow rate and hence the known lower actuatorrod velocity (this reduced velocity can be measured initially for the particular application in the same way as the normal velocity using the auxiliary switches 58, 59), and the known (measured) normal actuator-rod velocity a/#t.

Referring to Figure 4 once again, the time t5 can be found as follows, letting times t3 - t2 = t, ts - t3 = t, t6- t5 = t, t6 - t4 = T, normal actuator- rod velocity = v, reduced actuator-rod velocity = v ', actuator travel over time t '= x t, actuator travel over time t = and known distance between auxiliary switch 59 and "contacts make" position = dx: ####+##=##+#=##-#### (1) Let v'= Mv, where 0 < M < 1 .. Mt/+t= -t/ > (2) v Also, t+t=x x +T= Ax - t# + T (3) v v Now (2) becomes: t = -t"-Mt' (4) v and (3) becomes: Ar t = - (t11 + t') + T (5) # (5) into (4): t -t"-Mt' = ax - t" - t/ + T (6) V V Hence, T / (7) 1 -M Time t5 at which normal hydraulic drive is to be resumed is now simply (t "+ t') ms after time t2, interval t "being already known and interval t 'being given by equation (7).

Figure 4 assumes that, when the estimate of "contacts make" time is made, this point in time corresponds to a point on the AC waveform which is later than a zero-crossing point by greater than a certain amount (e.g. 1 ms). It may be found, however, that the "contacts make" point occurs before such a zero by greater than the threshold amount, in which case reduction of actuator-rod velocity still takes place, but with greatly reduced time period ts t3. Thus, if the "contacts make" point is estimated to occur at a time P on the voltage waveform, a delay T of only approximately 2 rns will be introduced in order to bring coincidence this time not at the t6 voltage zero, but at the previous zero following point P.

Where "contacts make" is estimated to occur very close to a voltage zero (e.g. < Ims before or after), no delay is introduced and normal hydraulic drive is continued.

This procedure is followed for all three phases independently, using either a separate phase-voltage reference per phase or a single reference from one particular phase with prediction of zero-crossing occurrences being suitable shifted for control of the other two phases. Where only a single reference is used, and assuming a 50 Hz supply, this will mean programming the processor unit 70 to adjust anticipated zeroes by 3.33 ms earlier and later, respectively, for the other two phases.

The advantage of the present invention over the known adaptive-control system is that no compensation is required for determinate variables, nor indeed for random variables.

For each operation the closing velocity of the actuator rod is known, say 15-25 ms before the contacts close. This means that variables such as control-voltage variations, stored energy variations, short-term repeatability and ageing are all taken care of by the "up-todate" velocity measurement (these parameters all affect velocity, which is freshly measured each time). To give an example of the effectiveness of the invention, assuming that the time to "contacts make" is 100 ms + 5% and velocity measurement occurred 20 ms prior to "contacts make", then the final tolerance in closing time relative to voltage zero-crossing would be not 100 ms + 5% = 5 ms, but 20 ms + 5% = 1 ms.

The invention may also be realised in other ways. A second embodiment (not shown) of the invention, for example, utilises only one auxiliary switch in the circuit breaker and bases its estimate of "contacts-make" time on a known "rated" actuator-rod velocity. This technique is suitable where the closing time is short (e.g. 50 ms) and the actuator-rod velocity shows little variation.

A third embodiment of the invention employs instead of two auxiliary switches three or four in order to measure the actuator-rod velocity several times during a switching operation. Velocity measurement occurs by measuring the time taken for a point on the actuator rod to pass successive pairs of switches and dividing this by the distance between the respective pair of switches, the result being a new velocity calculation after each successive pair. This can be used to provide successively finer estimates of "contacts-make" time and give rise to successively finer control of actuator-rod velocity until closing time is reached. A particular realisation of this embodiment is to have three auxiliary switches giving rise to a first velocity measurement after the second switch has been actuated and then a second such measurement after the third switch has been actuated, the distances between the first and second and the second and third switches being used to determine the two velocities, in conjunction with the measured times taken for the actuator rod to cover those distances.

A fourth embodiment is shown in Figure 7, where the actuator rod 64 is extended at its lower end and mechanically linked to the wiper 90 of a potentiometer 92. The potentiometer, which is preferably of the sliding type, has a voltage applied across it and a connection is taken from the wiper to the processing unit 70 (see Figure 6) to provide an electrical signal proportional to the position of the operating rod inside the circuit breaker.

Preferably the potentiometer is a linear-law device such that the picked-off voltage is an accurate indication of actuator position; alternatively, where for instance the potentiometer was a rotary-type device with a suitable linkage to the operating rod 64, other resistance laws may be required to be employed in order to ensure linearity of output signal relative to actuator position.

Clearly, the extended-operating-rod arrangement shown in Figure 7 could also be used in the first and second embodiments, the auxiliary switches then being mounted not inside the circuit breaker but outside it, like the potentiometer 92. A further variant would be the use of magnetic switches (e.g. Hall-Effect devices) instead of mechanical switches.

Whereas the above description has assumed that closer coincidence between "contacts-make" time and zero-crossing time will be achieved by slowing the actuator down at some point in order to match up with a later zero crossing, an alternative arrangement is to arrange for the normal actuator travel velocity to be throttled back somewhat and then to increase this velocity so as to bring about contacts-make at an earlier zero-crossing. This can be achieved by throtting back the valve 80 (see Figure 6) to begin with and instructing the valve, via the processing unit 70, to open fully at a point equivalent to time t3 in Figure 4. Such an arrangement is shown in Figure 8. In this example the velocity of the actuator rod is maintained at its higher level through the "contacts-make" point and beyond to the final closing stage. Alternatively, the higher velocity may be maintained for a limited period only, returning then to the normal (lower) velocity either before the "contacts-make" point or some time thereafter.

In practice, the Figure 4 arrangement is to be preferred, since it is usually considered desirable, for reasons of dielectric strength and the prevention of pre-arcing, for the actuator rod to reach its maximum velocity at least 5 ms prior to "contacts make", and the Figure 8 arrangement could not guarantee that.

As mentioned earlier in the specification, it is not necessary that this invention be used in conjunction with a hydraulic actuation system, but it could instead be employed with a spring-operated system, for example. In this case, and assuming the use of a velocityreduction delay arrangement as shown in Figure 4, it would be necessary to provide some form of braking effect in order to achieve delay in contacts closing. In the case of a wound spring, for example, this might take the form of a disc brake. At all events, it would be necessary to employ a spring with excess energy capacity in order that energy could be taken out of the system during velocity reduction and still complete the required actuator travel.

A suitable spring energy-storage figure might be of the order of 5,000 Joules, which would include the energy needed to charge the trip spring.

Finally, although the invention has been described in connection with achieving closure of the circuit-breaker contacts at a zero-crossing point (in order to switch on a capacitive load, for example), it could equally well be employed to achieve zero-crossing performance when opening the contacts, it being assumed in this case that contact opening would be required to occur at a zero level of current rather than voltage. In this arrangement, then, the actuator rod would start at its uppermost position (puffer cylinder at the top of its travel) and would descend at a substantially uniform velocity, operating the two auxiliary switches on its way down (assuming the dual-switch embodiment of the invention were employed) and triggering off calculation of a "contacts open" estimate. This estimate would then, as before, be compared with an anticipated future zero-crossing time and the velocity of the actuator rod reduced for as long as necessary before normal velocity was resumed in order to achieve as close a coincidence as possible between the "contactsopen" point in time and the next suitable current zero-crossing. Operation of this embodiment, as just described, is therefore directly analogous to that of the first embodiment and is therefore not shown in a separate drawing. Velocity increase could be employed as an alternative measure, similar to the scheme shown in Figure 8.

Claims (11)

1. A method of performing a switching operation on a switching device having a set of contacts and an actuating means movable between a contacts-open position and a contactsclosed position, the switching device being used to interlace an alternatingcurrent electricity supply to a load, the method comprising the steps of: (1) initiating movement of said actuating means from one of said contacts-open position and contacts-closed position to the other of said contacts-open position and contacts closed position; (2) during movement of said actuating means to the other of said contacts-open position and contacts-closed position, forming an estimate of time at which said other position will be reached; (3) comparing said estimated time with an anticipated time at which a parameter of said alternating-current electricity supply will have a desired instantaneous value; (4) in dependence on said comparison, adjusting a rate of movement of said actuating means such as to reduce a difference between said estimated and anticipated times.

2. Method according to Claim 1, in which step (2) comprises the steps of measuring a velocity of said actuating means at a position of said actuating means intermediate said contacts-open and contacts-closed positions and subsequently forming said time estimate on the basis of a known distance between said intermediate position and said other of said contacts-open and contacts-closed positions.

3. Method according to Claim 1, in which step (2) comprises at a position of said actuating means interrnediate said contacts-open and contact-closed positions deriving said estimated time on the basis of an assumed value of actuating means velocity and a known distance between said intermediate position and said other of said contacts-open and contacts-closed positions.

4. Method according to Claim 2, in which step (2) comprises the steps of measuring a velocity of said actuating means at a plurality of positions of said actuating means intermediate said contacts-open and contacts-closed positions and after each measurement deriving said time estimate on the basis of a known distance between respective said intermediate position and said other of said contacts-open and contacts-closed positions, said actuating means velocity being adjusted after each successive derivation of time estimate by respectively successively finer amounts.

5. Switching apparatus for interfacing an alternating-current electricity supply to a load, comprising: a switching device having a set of contacts and an actuating means movable between a contacts-open position and a contacts-closed position; a position-sensing means connected to said actuating means and adapted to provide a signal representative of a position of said actuating means relative to said other of said contacts-open and contactsclosed positions; an alternating-current signal inputting means for providing a signal representative of said electricity supply; and a processing means connected to said positionsensing means and to said signal inputting means and adapted to form from said positionrepresentative signal and from said supply-representative signal an estimate of a time at which said actuating means will reach said other of said contacts-open and contacts-closed positions, to compare said estimated time with an anticipated time at which a parameter of said alternatingcurrent electricity supply will have a desired instantaneous value and, in dependence on said comparison, to adjust a rate of movement of said actuating means such as to reduce a difference between said estimated and anticipated times.

6. Switching apparatus according to Claim 5, in which said position-sensing means comprises at least one auxiliary switch disposed in actuable relationship with said actuating means at a position intermediate said contacts-open and contacts-closed positions, said intermediate position being at a known distance from said other of said contacts-open and contacts-closed positions.

7. Switching apparatus according to Claim 6, in which said processing means is adapted to form said time estimate following actuation of said auxiliary switch by said actuating means, said estimate being derived from said known distance between said intermediate position and said other of said contacts-open and contacts-closed positions and from an assumed value of actuating-means velocity.

8. Switching apparatus according to Claim 6, in which said position-sensing means comprises a pair of said auxiliary switches disposed in spaced-apart relationship along a direction of travel of said actuating-means, said processing means being adapted to derive a value of actuating-means velocity on the basis of a known distance between said auxiliary switches and a measured time interval between actuation of one of said auxiliary switches and subsequent actuation of the other of said auxiliary switches by said actuating means, said time estimate being derived from a known distance between one of said auxiliary switches and said other of said contacts-open and contacts-closed positions and from said derived value of actuating-means velocity.

9. Switching apparatus according to Claim 5, in which said position-sensing means comprises a potentiometer means having a wiper coupled to said actuating means such that movement of said actuating means produces corresponding movement of said wiper, said potentiometer providing in use an electrical quantity the value of which is proportional to a distance of said actuating means from said other of said contacts-open and contacts-closed positions.

10. Switching apparatus substantially as shown in, or as hereinbefore described with reference to, Figures 5 and 6, or 5 and 7, with Figure 4, or Figures 5 and 6, or 5 and 7, with Figure 8 of the drawings.

11. Method of performing a switching operation on a switching device substantially as hereinbefore described.