GB2338837A

GB2338837A - Current control element switched by a magnetic field

Info

Publication number: GB2338837A
Application number: GB9813526A
Authority: GB
Inventors: Hans Bernhoff; Jan Isberg; Joergen Karlsson; Pan Min
Original assignee: Asea Brown Boveri AB
Current assignee: ABB AB
Priority date: 1998-06-23
Filing date: 1998-06-23
Publication date: 1999-12-29
Also published as: AU4386899A; GB9813526D0; WO1999067866A1

Abstract

A current control or switching device comprises a contact switch 1, a varistor 2 and a variable resistance element 3. This element is made from an electrically conductive soft magnetic powder, such as grains of steel coated with a boride, carbide or nitride. These grains can be immersed in a liquid such as oil or perfluorinatedpolyether, or can be surrounded by a solid or gel. An electromagnet 4 applies a magnetic field to the element, and the resistance of the element is varied by varying the strength of that field, the element only being conductive in the presence of the field. Alternatively, a current-carrying coil housed around the element can be used. In an alternative embodiment, the element is responsive to both the magnetic field and to mechanical stress, and is rendered conductive only in the presence of both the field and the stress. The element can be used as a part of the switching device shown, this arrangement preventing the occurrence of arcing across the contact switch 1.

Description

1 ELECTRICAL CONTROL DEVICES 2338837 The present invention' relates to

electrical control devices and, in particular, to switching devices for use in ac or dc circuits and/or current-limiting devices comprising an 5 element having a variable electrical resistance which can be controlled.

In high-voltage electric circuits which are arranged to carry large currents, problems arise whenever the current is to be switched on or off using a conventional contact switch. If no protective device is provided in the circuit, arcing can occur at the switching contacts, resulting in wear of the contact surfaces of the switch, possible failure to interrupt or event destruction of the device. The potential power released during switching is proportional to the square of the system voltage. Thus, in highvoltage systems, the power release is considerable.

In traditional circuit breakers the arc is allowed to burn, between two special arcing contacts, for several half periods (10 ms for 50 Hz) and is extinguished with the aid of a gas blast at zero current. To facilitate arc extinguishing (and thus interruption) at zero current a damping circuit can be arranged in parallel with the arcing contacts. Such an arranaement is illustrated in Figure 1 where it can be seen that the damping circuit is in the form of a series combination of a resistor R and a capacitor Q the series combination being connected in parallel with a contact switch 1. When the contact switch 1 is closed there is no voltage difference across it, and so no current flows in the damping circuit. At zero current, when the arc is extinguished, part of the current that otherwise would be forced into the electrode gap by the recovery voltage is absorbed by the damping circuit.

Another possibility, as shown in Figure 2a, is to use a hybrid breaker wherein a solidstate device SS is arranged in parallel with the contact switch 1 to avoid arcing during interruption. The solid-state device SS may consist of a gate turn-off thryistor (GTO) or an insulated-gate bipolar transistor (IGBT). The overvoltages associated with circuit interruption (turn off) are absorbed by the RC filter.

1 2 Thus, with both a traditional circuit breaker (such as an SF6-breaker) and more modem solid-state devices (such as thyristors, GTOs and IGBTs), a filter is used to lower the electrical stress on the electrical currentcontrol device which is associated with current interruption. An alternative arrangement is shown in Figure 2b, wherein the damping circuit is replaced by a varistor 2, typically made from zinc oxide (ZnO), which is an electrical component which exhibits a resistivity which decreases with increasing voltage across its terminals. Thus, when the switch is closed the varistor 2 presents a high resistance, but as the switch is opened so as to interrupt the current and the voltage rises to an overvoltage, the varistor 2 will become conductive and thus limit the voltage 10 across the switch to a certain limit value V,,.. Thus the varistor serves to absorb the mag J2 gnetic energy of the system (=L /2) that otherwise could damage or impair the function of the current control device (e.g. the GTO). Finally, it is stressed that a damping device can consist of a combination of the two schemes presented above.

As semiconductors are relatively expensive for power switching applications and traditional switchgear (SF6 breakers, oil breakers, air blast breakers and vacuum 0 breakers) can only interrupt the current at zero current, it would be advantageous to provide a means of interrupting current without arcing or using solid-state technology.

One way of achieving this would be to provide an electrical element in circuit with the contact switch which can exhibit a large change in resistivity over a very short period. For example, at a relatively low system voltage, e.g. 300 V, the required change in 103 resistivity would be around t, at a medium voltage, e.g. 10 kV, the required change would be around 10 6 and at a high voltage, e.g. 400 kV, the required change would be 25 around 109.

It would be possible to provide a semiconductor device with such a characteristic, but this would be exceedingly expensive to manufacture for high voltage levels.

0 0 Soft magnetic powders are known which are typically in the form of small spherical 0 particles. Although the material of the powder is electrically conductive, each particle 0 has an insulating layer such that a mass of the particles constitutes an electrical insulator.

3 The present inventors have found that if the insulating layer is removed, then such a mass of the particles can be rendered conductive upon the application of a magnetic field. Such powders are thus magnetoresistive and can be considered as a large number of small electrical relays. The magnetic field gives rise to a contact force between individual grains which reduces the bulk resistivity of the powder.

In accordance with a first aspect of the present invention there is provided an electrical current-control element comprising a mass of a pulverised material, such as a ferromagnetic material, which is rendered electrically conductive by the application of a magnetic field.

Furthermore, the effect has been found to be reversible in that removal of the magnetic field causes the element to revert to an insulating state.

is However, the surface of such particles can become oxidised which renders the particles non-conducting. This effect is particularly pronounced at the temperatures typically encountered in high-power switching applications. Therefore, each grain of the pulverised material preferably comprises a chemically inert electrically conductive surface in the form of a coating, such as a boride, e.g. titanium boride (TiB2) and/or a carbide, e.g. titanium carbide (TiC) and/or nitride, e.g. titanium nitride (TiN). Such coatings have been found to be stable at temperatures up to 3000 K in an inert atmosphere. The coatings may alternatively or additionally be an oxide, a silicide, a phosphide and/or a refractory metal such as tungsten..

1 ' 25 In addition, it is thought by the present inventors that the presence of a hard coating on a W relatively elastic core improves the current-switching action of the powder. Elements in accordance with the present invention have been found to exhibit a switching performance of about 50 A cm2 and 30 V mm-' The grains of the pulverised material are preferably substantially spherical but may alternatively be elongate and rounded. Such particles exhibit low resistivity, compared with particles having sharp or pointed edges, since the current constriction between the 4 particles is reduced in the conductive state. Furthermore, such particles can readily be manufactured by means of gas or vacuum atomisation. The resistivity of such material has been found to vary over five orders of magnitude or more between the conducting and the non-conducting states.

The element advantageously comprises a mass of such grains of pulverised material immersed in a fluid such as an insulating liquid such as oil or a perfluorinatedpolyether (PFPE) within a suitable container. The advantage of immersion in such a liquid is an increased electrical resistance in the insulating state and thereby an increased field strength in which the switching element can operate. This is particularly important, since the performance of the switching device is proportional to the square of this field strength, but only inversely proportional to the resistivity in the conducting state.

The presence of liquid is believed by the present inventors to improve the performance of the switching device for two reasons, namely: (1) the small areas of contact between adjacent powder grains are cooled by the presence of the liquid and (2) capillary forces cause a liquid film to form, thereby separating the grains in the absence of a magnetic field. Furthermore, by suitable choice of liquid, the grain surfaces can be protected against oxidation or other degradation of electrical performance due to adverse chemical reactions, and the long-term stability of the power is thereby improved.

Alternatively, the fluid may be a gas, such as one or more of nitrogen, carbon dioxide and sulphurhexafluoride, or the fluid may be replaced by a partial vacuum or by a solid, such as a gel.

--- 25 The invention extends to the combination of such an electrical current- control element and means for applying a magnetic field to the element, since this enables the conductivity to be controlled easily.

Thus, in accordance with a second aspect of the present invention there is provided a method of controlling the current in an electrical circuit including an element of the above-described type, the method comprising varying the strength of a magnetic field applied to the element.

1-1 1.

The means for applying a magnetic field preferably comprises a currentcarrying coil wound around the element or may alternatively comprise an electromagnet. In the latter case, the electromagnet is preferably arranged to be energised by a short current pulse in a conductor which is insulated from the element. The degree of insulation is preferably sufficient to allow the conductor to be operated at substantially earth potential. This could be achieved, for example, by supplying the electromagnet winding with a polymer insulation.

The core of the electromagnet and the element are preferably arranged to form a magnetic circuit. The means for applying a magnetic field may comprise, in addition to the electromagnet, a component of soft magnetic material for completing the magnetic circuit. Alternatively, a permanent mag ,net is included within the magnetic circuit. In the latter case, the direction within the element of the magnetic field generated by the electromagnet is opposite to that of the magnetic field within the element generated by the permanent magnet. This arrangement is particularly suitable when the element is used as a current-limiting device, since current can still flow in the main circuit, even in the absence of a control current in the electromagnet. In this case, the current is limited by the (low) resistance of the element.

The element, the electromagnet core and the permanent magnet are preferably arranged to form a magnetic circuit. A further element is advantageously provided, the two elements being substantially identical, and the magnetic circuit comprises, in sequence, the electromacnet core, the first element, the permanent magnet and the second element.

0 The electrical circuit thus comprises a parallel combination of two elements.

The permanent magnet and the electromagnet core may both be square Cshaped and the elements elongate, each end of each element abutting a respective end of either the permanent magnet or the electromagnet core, such that, again, the magnetic circuit is in 30 the form of a rectangle.

The permanent magnet may advantageously have a cross-sectional area which is substantially greater than that of the elements. The magnetic flux in the elements which 6 results from the electromagnet, and which opposes the flux in the permanent, is therefore less concentrated in the permanent magnet and is therefore less likely to cause demagnetisation of the permanent magnet. In this case, two connecting pieces of soft magnetic material are used to complete the magnetic circuit between the two elements and the permanent magnet. The connecting pieces are each shaped so as to provide a narrow face which abuts a respective element and a wide face which abuts the permanent magnet.

In accordance with a third aspect of the present invention there is provided an electric 10 circuit comprising a parallel combination of a contact switch and an element of the above type. Such an arrangement is preferred when the element is used as an electrical switching device.

In accordance with a fourth aspect of the present invention there is provided a method 15 of switching off an electric current flowing in such an electric circuit, the method comprising varying the magnetic field applied to the current-control element, thereby to cause electric current to flow through the current-control element and so divert current from the contact switch, and subsequently breaking the contact switch.

1 ' 25 In accordance with a fifth aspect of the present invention there is provided an electric circuit comprising a series combination of a contact switch and an element of the above type. As with the parallel configuration, the arrangement may perform the function of an electrical switching device but can also act as a current-limiting device, in which case the contact switch remains closed. The required static magnetic field can, in this case, be provided by one or more permanent magnets.

Thus, in accordance with a sixth aspect of the present invention there is provided a method of switching on an electrical current in such an electrical circuit, the method comprising, in sequence, varying the strength of the magnetic field applied to the current-control element so as to render the element substantially non- conducting, closing the contact switch and varying the strength of the magnetic field so as to render the element conducting thereby to establish a current path through the contact switch and the element.

7 In either type of electric circuit, there is preferably connected in parallel with the contact switch a variabl e-resi stance device, such as a varistor, or alternatively a traditional damping circuit, such as the parallel combination of a resistor and a capacitor. When the variable-resistance device or the damping circuit is connected in parallel with the contact switch which is connected in series with the element, this is preferably connected in parallel with both the contact switch and the element.

In preferred embodiments, a varistor is included, which serves to absorb magnetic energy. However, this could be replaced by a resistor, and the device then functions as a current limiter when the contact switch is open.

The invention extends to the parallel combination of an element and either a variableresistance device or a damping circuit.

In accordance with a seventh aspect of the present invention there is provided a method of controlling the current flowing in an electric circuit of the above-described type, the method comprising varying the strength of the magnetic field applied to the element.

In the case of breaking the current, the strength of a magnetic field applied to the element may be reduced or substantially eliminated.

The magnetic field is advantageously applied by an electromagnet, and the strength of the magnetic field may be reduced by reducing the level of the electric current flowing in said electromagnet.

Alternatively, the magnetic field may be applied by the combination of an electromagnet and a permanent magnet, and the step of reducing the strength of the magnetic field is then preferably effected by increasing the level of the electric current flowing in the electromagnet.

In accordance with an eighth aspect of the present invention there is provided the use of an element of the above-described type as an electrical switching device.

11 - 1 8 In accordance with a ninth aspect of the present invention there is provided the use of an element of the above-described type as an electrical current limiter.

Whilst the present invention has been discussed in relation to controlling the element by a magnetic field alone, the inventors have found that the resistivity of electricallyconductive powders can be controlled by the combination of a magnetic field and mechanical compression and decompression.

Thus, in accordance with a tenth aspect of the present invention there is provided an electrical current-control element comprising a mass of pulverised material which is rendered electrically conductive when subjected to mechanical stress in combination with the application of a magnetic field but which becomes insulating when the stress is reduced and/or the magnetic field removed.

Preferred embodiments of the present invention will now be described with reference to the accompanying drawings, wherein:

Figure 1 illustrates a prior-art arrangement of a damping circuit connected in 20 parallel with a contact switch; Figure 2 illustrates a prior-art arrangement of a varistor connected in parallel with a contact switch; Figure 3 illustrates a first type of current-control device in accordance with the present invention; Figure 4 illustrates a second type of current-control device in accordance with the present invention; Figure 5 illustrates a switching device of the type shown in Figure 3 incorporating a control system; Figure 6 illustrates a switching device in accordance with a first embodiment of 30 the present invention; Figure 7 illustrates a switching device in accordance with a second embodiment of the present invention; 9 Figure 8 illustrates a switching device in accordance with a third embodiment of the present invention; Figure 9 illustrates a switching device in accordance with a fourth embodiment of the present invention; Figure 10 illustrates an apparatus for testing magnetoresistive powders; Figures 11 (a) - (c) are graphs showing data from a first experiment; and Figures 12 (a) - (c) are graphs showing data from a second experiment.

With reference to Figure 3, a first type of current-control device comprises a parallel combination of a contact switch 1 and varistor 2, as described above with reference to Figure 2, and, in addition, a variab 1 e-resi stance element 3 the resistance of which is controlled by the magnetic field produced by an electromagnet 4. The field strength is of the order of 1 tesla.

The element 3 comprises a mass of steel powder or other soft magnetic material, coated with an electrical ly-conducti ve refractory ceramic, such as titanium diboride, preferably immersed in a perfluorinatedpolyether within a container made from an electrically conducting material. The resistivity of the powder is controlled by a magnetic field applied to the element. The steel powder is contained in a housing (not shown) made from a polymer or a ceramic, such as porcelain.

When it is desired that the current flowing in the main circuit be switched on, the electromagnet 4 is energised by a current pulse, which causes the element 3 to become conductive. At the time of the current pulse, the contact switch 1 is closed.

Equally, when it is desired that the current be switched off, the electromagnet 4 is again energised by a current pulse, which again causes the element 3 to become conductive. During the time of the current pulse, the contact switch 1 is opened.

Since the element 3 is conductive at the time when the contact switch 1 is closed or opened, the voltage across the contact switch 1, immediately before closing and immediately after opening the contact switch 1, is low enough for the problem of arcing effectively to be eliminated.

/I- - The varistor serves to absorb magnetic energy, e.g. any back emrs generated by the switching action, and the varistor characteristic is selected such that current can flow in the varistor when the voltage across it is a given percentage of the line-to-ground 5 voltage, e.g. 150% for a line-to-ground voltage of 1OkV.

With reference to Figure 4, a second type of current-control device comprises a parallel combination of a contact switch 1 and varistor 2, as described above with reference to Figure 2. As in the first type of device shown in Figure 3, a variabl e-resi stance element 3, controlled by a magnet 4, is also included in the circuit, but this is connected in series with the contact switch 1. When it is desired that the current flowing in the main circuit be switched on, the electromagnet 4 is controlled so as to cause the element 3 to become non-conductive. During this time, the contact switch 1 is closed.

Equally, when it is desired that the current be switched off, the electromagnet is again controlled so as to cause the element 3 to become non-conductive. At this time, the contact switch 1 is opened.

Since the element 3 is non-conductive at the time when the contact switch 1 is closed or opened, the voltage across the contact switch 1, immediately before closing and immediately after opening the contact switch 1, is low enough for the problem of arcing effectively to be eliminated.

In the arranaements shown in Figures 3 and 4, the magnet 4 may alternatively be a combination of an electromagnet and a permanent magnet arranged to provide substantially equal and opposite magnetic fields in the element 3. Thus, when the electromaanet is energised, the magnetic field of the electromagnet effiectively counteracts that of the permanent magnet, so that there is no over-all magnetic field in the element 3 which is therefore non-conductive. When the electromagnet is de- energised, the magnetic field of the permanent magnet causes the element to become conductive.

Figure 5 illustrates a control system incorporating the current-switching device of Figure 3. The current in the main circuit is measure by an ammeter 5 which supplies a signal indicating the current to a control module 6. The control module 6 supplies signals to both the contact switch 1 and the electromagnet 4 at appropriate times such 5 that arcing across the contact switch 1 is avoided.

A first embodiment of the first type of current-control device is shown in Figure 6. The current-control element 7 is controlled by a control circuit supplied to a coil surrounding the element 7 from a control circuit 8. The macnetic field associated with the control current causes the powder within the element to become electrically conductive, thereby permitting current to flare in the main circuit 9. This arrangement thus involves an air magnetic circuit, i.e. the magnetic field lines pass through the air surrounding the element 7. This embodiment is suitable for use with either of the arrangements shown in Figures 3 or 4. An outer bowing (not shown) is provided which constitutes a shield to eliminate leakaee fields which could give rise to an electromagnetic disturbance to other equipment.

A second embodiment is shown in Figure 7, which is particularly suited for use in the arrangement shown in Figure 3. The magnetic circuit includes two identical elements 10 which are connected at one end thereof to a bar-shaped core 11 of an electromagnet and at the other end to a bar 12 of soft magnetic material. The main circuit is connected at two connection points 13, 14 on the electromagnet core 11 and the magnetic bar 12 respectively. When energised, the electromagnet generates a magnetic field in the direction indicated by the dotted arrows.

1 - 25 A third embodiment is shown in Figure 8, which is similar to that shown in Figure 7, except that the electromagnet core 16 and the soft magnetic material 17 are both square C-shaped and the main circuit is connected directly to the elements 15 by means of connecting conductors 18, 19. Again, this arrangement is particularly suitable for use in 0 0 the circuit configuration of Figure 3.

A fourth embodiment is shown in Figure 9. As with the embodiments shown in Figures 7 and 8, two identical elements 20 are connected to an electromagnet core 21.

12 However, the bar of soft magnetic material is replaced in this embodiment with a permanent magnet 22 which opposes the field from the energised electromagnet. Thus, when the electromagnet is energised, the resulting magnetic field across the element 20 is substantially zero and the elements 20 are therefore non-conductive. In the normal state, with no control current supplied to the electromagnet, the element is therefore conductive, and such an arrangement is therefore particularly suited to the circuit configuration of Figure 4. The directions of the opposed fields are indicated by the dotted arrows. The permanent -magnet is a relatively short bar member 22, and two connecting pieces 23 serve to complete the magnetic circuit between the elements 20 10 and the permanent magnet 22. As can be seen from the drawing, the connecting pieces 23 are tapered, with a narrow end 24 abutting a respective element 20 and a wide side face 25 abutting a respective end of the permanent magnet 22. In this way, the flux fl-om the permanent magnet 22 is concentrated within the elements 20, but, conversely, when the electromagnet is energised, the resulting flux in the permanent magnet is 15 reduced and serves to prevent demagnetisation of the permanent magnet.

The embodiments described above all concern current-control elements and devices which are controlled solely by a magnetic field. However, the invention extends to current-control elements and devices which are additionally controlled by application of 20 pressure. The electrical] yconductive powders used in such elements are both magnetoresistive and piezoresistive, exhibiting a high resistivity at low pressure and a low resistivity at -high pressure.

The arrangements illustrated in Figures 3), 4 and 5 can alternatively comprise an element whose resistivity is additionally controlled by varying the applied pressure.

Numerous methods are available for producing metal powders. However, atomisation processes, e.g. water and gas atomisation of a metal melt, are most widespread, as described in Metals Handbook, Ninth Edition, Volume 7, Powder Metallurgy, American 30 Society for Metals.

Atomisation may be defined as the break-up of a metal melt into fine droplets by pouring the melt into water or a gas jet stream. For high volume and low cost I- 1 - 25 13 production, water atomisation is preferred over gas atomisation. However, when reactive elements are involved, vacuum melting in combination with inert gas atomisation is often necessary. The production of powders of reftactory ceramics, e.g. TIC, TiB2, ZrC and ZrB, is somewhat different. Many techniques are based on oxide reduction of milled powders of the natural form of the ceramics (Ti02 and ZrO). The production of such powders is often directed towards producers of hard metal cutting tools.

Traditionally, the powder type and the production method are chosen with respect to the 10 desired final properties of a detail, i.e. after hot pressing to a Ifinal shape. However, there will be other requirements when powder is to be used as an electrical switching medium, e.g. processing powder of non-commercial alloys and refining of commercial powder qualities.

In principle, a powder with good performance in a magnetoresistive switch can be used in a mechanically controlled switch, but not vice versa, since a conducting, temperaturestable and non-oxidising surface must be combined with a soft magnetic core of the particles. In practice it is therefore necessary to modify the surface of a ferritic or a Cobased powder for a magnetoresistive switch. Such powders are not commercial available.

The first powder tested by the present inventors in a magnetoresistive switch was a ferritic stainless steel powder, surface-modified by boronizing. Boronizing is a diffusion process, and Fe-B layers, 20-100 [im thick, can be processed from many commercial iron-based alloys. When boronizing iron powder, preferably the outer 1-2 gm of the particles surface is boronized. Therefore much shorter process times are needed for an iron-based powder in comparison to conventional boronizing.

Below, some properties of a boronized powder are presented. Inert gas atomisation of a 30 13% CrFe melt was used in the production of this powder. In contrast to wateratomised powders, most gas-atomised powders have spherical shape, since the gravitational force acting on small metal droplets is negligible compared to the surface tension during solidification. The main fraction of this powder has a diameter of around 1 14 microns. When boronizing, the powder was mixed with EKabor (RTM), another powder based on activated boron carbide which releases boron when heated between 800-1000'C. (see A. Kumar Sinha: "Boronizing", ASM Handbook, Heat treating, Vol. 4 (199 1), ASM International Materials, Park, Ohio, pp. 43 7 - 447). The surface structure of the particles changes by boronizing. The

boronized particles were separated from the EKabor powder using a permanent magnet, but some remnants of the EKabor, the particles with an irreclular shape, are however left.

0 Scanning-electron micrograph (SEM) images of etched metallographical cross-sections through different fractions of the powder show that the smallest fraction of the powder is completely boronized, while a welldefined layer is present in the surface of the largest fraction. The surface of this boronized powder is not well characterised chemically. However, in the above-mentioned reference by A. Kumar Sinh, it is reported that a double-phase layer may form consisting of an outer FeB phase and an inner Fe2B phase which grows with a saw-tooth morphology due to a preferred diffusion direction. Possibly this double-phase layer is observed in the prepared crosssections.

In this case, the main reason for boronizing the powder was to provide the powder with an inert surface, since tests of ferritic steel powders indicated that insulating surface oxides decrease performance.

Limited magnetoresistive effects were observed when testing an unmodified ferritic 25 powder as a switching medium. Switching results with the boronized powder are described below.

In the case of nitroaenisation, it is advantageous firstly to alloy the soft magnetic powder with titanium. During the subsequent nitrogenisation at 700 - 8000C, the titanium migrates to the surface of the particles so as to form the TiN coating. This 0 process has the particular advantage of being economical and applicable to large-scale production. Furthermore, the resulting TiN coatings have a melting point of 3 200T.

0 0 The use of chemical vapour deposition (CVD) for coating ferritic powders is highly advantageous, since many materials from the reftactory ceramic group may be deposited with this technique. In general, these coatings are hard, chemically stable and have a high melting point. The CVI) process can be very precisely controlled, and the thickness of the coating can be varied, e.g. from 0. 1 to 10 gm, in order to process powder with optimum performance.

The CV1) process may be regarded as an exclusive method for large-scale applications, and a diffusion process for surface modification is therefore preferred. Therefore traditional diffusion processes such as boronizing, carburizing and nitrogenisation is of 0 interest when developing a powder commercially. In contract to the CV1) technique, such processes, as demonstrated above, affect the different fractions of the powder differently. Thus, a narrow particle size distribution is desirable.

--- 25 From the point of view of surface modification, a powder with a narrow size distribution, around 20-30 gm, should be used. Likewise, from an electrical point of view, such a particle distribution is also appropriate. Particles which are too large impair the ability of the powder to withstand high voltages, due to the presence of fewer contact points per unit length, while particles which are too small tend to form ag lomerates which decrease interruption performance. Furthermore, small particles 119 decrease the over-all conductivity (in the conductive state). The size of such particles is also well above the size (approx. 0.1 gm) for which ferromagnetic particles can be magnetised as a single domain [see C. Kittel, Introduction to Solid State Physics, 6th Edition, John Wiley and Sons Inc. (1986j], which would result in a hard magnetic material due to domain pinning at the grain surface.

To look for maaneto resistivity effects in powder, an experiment was set up, using the apparatus shown schematically in Figure 10, in which a current-control element 28, an electromagnet 29 and two bars 30,31 of soft magnetic material together constitute a magnetic circuit. Electrically insulating material 32 is provided between the electromagnet core 33 and surrounding the element 28. The coil 34 of the electromagnet 29 is within an electric circuit containing a capacitor C, and a thyristor 35. By triggering the thyristor 35, the capacitor C, (3.3 mF) is discharged through the 16 electromagnet coil 34 having 20 turns which surrounds a massive iron core 36. In an improved design, the iron core 36 would be laminated, as e.g. in a transformer, to eliminate eddy currents and to guarantee magnetic saturation of the powder.

As the magnetic field renders the powder conductive, capacitor C2 (680 pF) discharges through the powder. The powder is contained in a cylinder with diameter 22 mm and height 46 mm.

The pre-stress on the powder is not measured in this preliminary experiment.

The powder used in this experiment was steel powder (containing 13% Cr) with an average grain size of approximately 20 gm and boronized with to form FeB on the surface of the grains, as described above. The steel particles were separated from the EKabor grains in a magnetic process.

In the first experiment, it was shown that a short magnetic pulse can render the powder conductive in less than one microsecond with a resistivity substantially below 1 ú2cm. However, complete interruption of the current could not be achieved in this first experiment.

The results are illustrated in Figures 11 (a)-(c). The curves indicate respectively: (a) coil current., (b) powder current; and (c) powder voltage; all plotted vs. time. The voltage across capacitor C2 is 250V.

By carefully adjusting the set-up it was possible to achieve current interruption, as 0 shown in Figures 12 (a)-(c) below, in which the curves again indicate respectively: (a) 0 coil current; (b) powder current; and (c) powder voltage; all plotted vs. time.

0 The results serve merely to prove the potential, as the experiment was done with poor powder and a solid iron magnetic circuit. The latter implies that a large portion of the control current was lost to eddy currents in the iron core instead of contributing to the magnetization. The experiment has been repeated with a lamellar magnetic core, and 0 improved results were achieved.

17 It is believed that the current "tail" in Figures 12 (a)-(c) is due to large-scale motion of the powder or of the entire set-up and could be avoided by simple means such as utilizing a more rigid set-up, immersing the powder in an insulating liquid or using a 0 softer magnetic material, e.g. pure iron powder.

However, it is stressed that the first part of the curve in Figures 12 (a)-(c) (first ms) demonstrates a fast (sub ms) on- and off-switching of 150 A against a voltage of 150 V.

A new magnetoresistive arc-free medium for circuit breaking with no moving parts has been identified. It is emphasised that this is a new medium distinct from that of oil, gas (air and SF6), semiconductors or mechanically-controlled powder. This medium should allow development of a new generation of switchgear (including breakers, current limiters and selector switches). The lack of moving parts is highly desirable, since this allows for a greater degree of control (most likely far larger than that of the mechanically-controlled powder), and it will also facilitate scaling to higher voltages, as no macroscopic movement of individual particles is involved.

The above-described initial experiment demonstrates that the magnetoresistive effect is large enough to allow switching of "realistic" currents and voltages for future 0 switchgear.

Although the present invention has been described above with reference to preferred embodiments thereof, it will be appreciated that many modifications may be made without departing from the scope of the invention, which is defined by the appended claims. For example, although the magnetic switching as described above is achieved by applying a current to the electromagnet for a short time, and then removing the current to achieve current breaking, an additional current, such as a current spike or an ac pulse, could be applied to assist in the current breaking, since that has been found to achieve a reduction in the contact force between the powder grains.

18

Claims

CLAIMS:

An electrical current-control element comprising a mass of a pulverised material which is rendered electrically conductive by the application of a magnetic field.

2. An element as claimed in Claim 1, wherein said pulverised material comprises ferromagnetic material.

3. An element as claimed in Claim 2, wherein each grain of said pulverised material comprises an electrically conductive surface.

4. An element as claimed in Claim 3, wherein said surface comprises an electrical ly-conductive coating.

5. An element device as claimed in Claim 4, wherein said coating comprises a conductive surface of one or more of the following: a boride; a nitride; a carbide; an oxide; a silicide, a phosphide; and a refractory metal.

6. An element as claimed in any preceding claim, wherein said pulverised material is in the form of substantially spherical grains.

An element as claimed in any one of Claims 1 to 5, wherein said pulverised material is in the form of elongate grains.

0 8. An element as claimed in any one of Claims 1 to 7 comprising a mass of pulverised material immersed in a fluid.

9. An element as claimed in Claim 8, wherein said fluid is an oil.

10. An element as claimed in Claim 8, wherein said fluid is a liquid perfluorinatedpolyether.

19 11.

12.

13.

An element as claimed in Claim 8, wherein said fluid is a gas comprising one or more of nitrogen, carbon dioxide and sulpurhexafluoride.

An element as claimed in any one of Claims 1 to 7, comprising a mass of pulverised material surrounded by a partial vacuum.

An element as claimed in any one of Claims 1 to 7, comprising a mass of pulverised material surrounded by a solid.

14. An element as claimed in Claim 131, wherein said solid is a gel. 15. An electrical current-control device comprising an element as claimed in any preceding claim and means for applying a magnetic field to said element.

16.A device as claimed in Claim 15, wherein said means for applying a magnetic field comprises a current-carrying coil housed around said element.

17. A device as claimed in Claim 15, wherein said means for applying a magnetic field comprises an electromagnet.

18.

A device as claimed in Claim 17, wherein the electromagnet is arranged to be energised by current flowing an a conductor which is insulated from the element.

A device as claimed in Claim 18, wherein the conductor is arranged to be at substantially earth potential.

20. A device as claimed in any one of Claims 17 to 19, wherein the core of the electromaanet and the element are arranged to form a magnetic circuit.

0 A device as claimed in any one of Claims 17 to 19, wherein said means for applying a magnetic field further comprises a permanent magnet.

22. A device as claimed in Claim 2 1, wherein the direction within the element of the magnetic field generated by the electromagnet is opposite to that of the magnetic field within the element generated by the permanent magnet.

23. A device as claimed in Claim 21 or Claim 22, wherein the element, the electromagnet core and the permanent magnet are arranged to form a magnetic circuit.

24. A device as claimed in anyone of Claims 21 to 23, further comprising a second element substantially identical to the first-mentioned element, the magnetic circuit comprising, in sequence, said electromagnet core, said first-mentioned element, said permanent magnet and said second element.

25. A device as claimed in Claim 24, wherein each of said elements, said permanent magnet and said electromagnet core are elongate, the resulting magnetic circuit being substantially rectangular.

26. A device as claimed in Claim 24, wherein said permanent magnet and said electromagnet core are both square C-shaped and said elements are elongate, each end of each element abutting a respective end of either said permanent magnet or the electromagnet core, the resulting magnetic circuit being substantially rectangular.

27. A device as claimed in any one of Claims 24 to 26, wherein said permanent 25 magnet has a cross-sectional area which is substantially greater than that of the elements and wherein the combination further comprises a respective connecting piece between each of said elements and said permanent magnets, each connecting piece being formed from soft magnetic material and being tapered, the narrow end abutting its associated element and its wider end abutting the 30 permanent magnet.

28. An electric circuit comprising a parallel combination of a contact switch and an element as claimed in any one of Claims 1 to 14.

21 29. An electric circuit comprising a series combination of a contact switch and an 0 element as claimed in any one of Claims 1 to 14.

30. An electric circuit as claimed in Claim 28 or Claim 29, further comprising a damping circuit connected in parallel with said contact switch.

31. An electric circuit as claimed in Claim 30, when dependent on Claim 29, wherein said damping circuit is connected in parallel with said series connection of said contact switch and said element.

32. An electric circuit comprising a parallel comb. ination of an element as claimed in any one of Claims 1 to 14 and a damping circuit.

3 3. An electric circuit as claimed in any one of Claims 30 to 32, wherein said damping circuit comprises a parallel combination of a resistor and a capacitor.

34. An electric circuit as claimed in Claim 28 or Claim 29, further comprising a vari abl e-resi stance device connected in parallel with said contact switch.

35. An electric circuit as claimed in Claim 33, when dependent on Claim 29, wherein said variab 1 e-resi stance device is connected in parallel with said series connection of said contact switch and said element.

3) 6. An electric circuit comprising a parallel combination of an element as claimed in any one of Claims 1 to 14 and a variab 1 e-resi stance device.

37. An electric circuit as claimed in any one of Claims 34 to 36, wherein said variable-resistance device comprises a varistor.

38. A method of switching off an electric current flowing in an electric circuit as claimed in Claim 28, or any claim dependent thereon, the method comprising varying the magnetic field applied to said current- control element, thereby to 22 cause electrical current to flow through said current-control element and so 0 divert current from said contact switch, and subsequently breaking the contact switch.

39. A method of switching on an electric current in an electric circuit as claimed in Claim 29, or any claim dependent thereon, the method comprising, in sequence, varying the strength of the magnetic field applied to said current-control element so as to render said element substantially non-conducting, closing the contact switch and varying the strength of said magnetic field so as to render said element conducting, thereby to establish a current path through said contact switch and said element.

40. A method of controlling the current in an electrical circuit including an element as claimed in any one of Claims 1 to 14, the method comprising varying the strength of a magneticfield applied to said element.

A method of controlling the current flowing in an electrical circuit as claimed in 0 any one of Claims 28 to 37, the method comprising varying the strength of the magnetic field applied to said element.

42. A method of breaking the current flowing in an electrical circuit including an element as claimed in any one of Claims 1 to 14, the method comprising reducing the strength of a magnetic field applied to said element.

43. A method of breaking the current flowing in an electrical circuit as claimed in Claim 28, or any claim dependent thereon, the method comprising reducing the strength of a magnetic field applied to said element.

44. A method as claimed in Claim 42 or Claim 43, wherein the step of reducing the strength of said magnetic field comprises substantially eliminating said field.

45. A method as claimed in any one of Claims 42 to 44, wherein the magnetic field is applied by an electromagnet and the step of reducing the strength of said 23 magnetic field is effected by reducing the level of the electric current flowing in said electromagnet.

46. A method as claimed in any one of Claims 42 to 44, wherein the magnetic field is applied by the combination of an electromagnet and a permanent magnet and the step of reducing the strength of said magnetic field is effected by increasing 0 the level of the electric current flowing in said electromagnet.

47. The use of an element as claimed in any one of Claims 1 to 14 as an electrical switching device.

48. The use of an element as claimed in any one of Claim 1 to 14 as an electrical current limiter.

49. An electrical current-control element comprising a mass of a pulverised material which is rendered electrically conductive when subjected to mechanical stress in combination with the application of a magnetic field but which becomes insulating when the stress is reduced and/or the magnetic field removed.

50. An electrical current-control device comprising an element as claimed in Claim 49 and means for applying mechanical stress to said element.

51. An electrical current-control element substantially as hereinbefore described with reference to the accompanying drawings.

0 52. A current-control device comprising an electrical current-control element and 0 means for applying a magnetic field to said element substantially as hereinbefore 0 0 described with reference to the accompanying drawings.

0 53.An electric circuit substantially as hereinbefore described with reference to the accompanying drawings.

0 24 54, A method of controlling the current in an electric circuit including an electrital current-control element, the method being substantially as hereitibefore described with reference to the accompanying drawings.

55. A method of switching off an electric current flowing in an electrical circuit substantially as hereinbefore described with reference to the accompanying drawings.

56. A method of switching on an electric current flowing in an electrical circuit substantially as hereinbefore described with reference to the accompanying drawinas.

0 57. The use of an electrically conductive powdered material substantially as hereinbefore described with reference to the accompanying drawings.