Field of the Invention The present invention relates to the use of permanent magnets in magnetic circuits, such as may be particularly useful in. fault current limiters for alternating currents, but also in other electric power related circuits that include magnetic circuits, for example, transformers, generators, motors and actuators.

Background of the Invention An increasing number of power systems require devices to be included in their circuits to limit AC fault currents in semiconductors and switchgear. These fault current limiters must be reliable, compact and inexpensive.

One known solution to the problem of fault current limiting is the superconducting fault current limiter. For an example of such a device, see B.P.Raju. KLCParton and T.CBartram, "A Current Limiting Device using Superconducting D.C. Bias Applications and Prospects", IEEE Transc. on PAS, Vol. PAS-101. No.9, Sept. 1982, pp3173-3177. However, such devices can take only a limited maximum operating current. They also require a sophistictted refrigeration system and need a reset time subsequent to fault! clearing. The latter two limitations deleteriously affect reliabilily.

It has also previously been proposed to construct a passive current limiter utilising a permanent magnet as part of a magnetic circuit, the rest of which comprises a steel or iron core to complete the flux circuit As an. example of such a proposal, see "INVESTIGATION OF THE PERFORMANCES OF A PERMANENT MAGNET BIASED FAULT CURRENT LIMITING REACTOR WITH A STEEL CORE", by S.CMukhopadhyay, Mlwaharø, S.Yamada and F .P.Dawson. This paper is available on the Internet at h://magmacl.ec.t.kanazawa-u.ac.jp/magcap/research/fcl.html and a hard copy is filed with the present patent application to provide a more permanent record. Figures 1 and 2 illustrate diagrammaticaϊly why a permanent magnet makes an effective fault current limiter. Ia Figure 1, a magnetic circuit 1 in a fault current limiter for AC comprises a "C"-shaped magnetically soft iron core 10 and a demagnetised permanent magnet 12, around which a coil 14 is wound. Magnetic flux can flow around the magnetic circuit, e.g., as indicated by the arrows. During normal operation (no fault current flowing in the coil 14), the permanent magnet 12 is in the demagnetised state. Its permeability is close to one (similar to air) and it thus has a low inductance. During a fault, the current flowing in the coil 14 increases, thus magnetising the permanent magnet, ie., the energy of the fault is used to magnetise the permanent magnet. As the polarity of the AC fault current reverses, the permanent magnet 12 is first demagnetised and then magnetised with opposite polarity. Again, the energy of the fault current is used to magnetise the permanent magnet, thus the energy is effectively dissipated into the permanent magnet

During faults, the permanent magnet will follow a characteristic hystereses loop as shown in Figure 2, which plots magnetic flux against current. The area inside the curve represents the energy dissipated into the magnet The large area inside the loop illustrates why a permanent magnet is, from one point of view, ideal for limiting fault currents.

A problem with using permanent magnets in fault current lirniters is that permanent magnets by definition comprise magnetically hard material, and therefore require use of high power electric currents to magnetise them and equally high powers to demagnetise them. A so-called soft magnet is made of a material with a slender hystereses curve, and thus has small values of remanence MR (the remaining magnetisation, in the material for zero external magnetic field) and coercivity (or coercive field strength Hc, the magnitude of the external field needed to bring the magnetisation of the material down to zero again). Conversely, as already explained above, a hard or permanent magnefs hystereses curve encloses a large area, and its material has large values of remaαence and coercivity.

Consequently, after a fault is cleared from the electrical system being protected, e.g., by the operation of autoreclosing circuit breakers, and normal operation of the system has resumed, the permanent magnet 12 in the fault current Hmiter circuit of Figure 1 remains magnetised. This is not desirable because it offsets (biases) the flux in the magnetic circuit 1 under normal operation. In general, it is essential to demagnetise the magnet again. The problem is that this requires a very large current: impulse and thus a large power supply, similar in size to the fault current, to demagnetise the permanent magnet

Summary of the Invention According to the present invention, a magnetic circuit device comprises: permanent magnet means for conducting magnetic flux around part of a magnetic circuit, electrical coil means wound around the permanent magnet means, and means for connecting the electrical coil means to an electrical power supply, thereby to selectively magnetise and demagnetise the permanent magnet means, wherein the permanent magnet means comprises a plurality of smaller permanent magnets connected into the magnetic circuit in parallel with each other, at least some of the plurality of permanent magnets each having an electrical coil wound around it, each coil being individually connectable to the electrical power supply.

Preferably, each one of the plurality of permanent magnets has an electrical coil wound around it

The magnetic circuit devices of the present invention can, for example, be used in fault current limiting devices that in normal (non-fault) operation have low inductances and thus are not seen as a significant impedance in a circuit, but under fault current conditions show a higher inductance that limits the fault current The use of a plurality of relatively small permanent magnets hi parallel with each other as part of a magnetic circuit in a fault current limiter mitigates the problems associated with demagnetising permanent magnets.

The individual permanent magnets may have cross-sectional shapes selected from the group comprising round, rectangular, and polygonal cross-sections. Advantageously, the power supply is selectively connectable to each one of the coils through switches controlled by a control unit that also controls the power supply; the power supply and the switches are preferably of the solid state type.

To obtain the maximum number of steps ra the value of the inductance possessed by the permanent magnet means, the control unit is operative to control the value and sense of the current applied to the coils through the switches such that the polarity and strength of each permanent magnet is individually selectable.

Brief Description of the Drawings Figure 1 is a diagrammatic side elevation of a known type of magnetic circuit forming part of e.g., a fault current limiter for alternating currents; Figure 2 is a graph of a hystereses curve for magnetisation and demagnetisation, of a permanent magnet; Figure 3 is a view similar to Figure I5 but modified in accordance with the invention; Figure 4 js a diagram illustrating how a magnetic circuit constructed with the invention may be controlled; and Figures 5A and 5B illustrate how the principle of the invention may be employed to accomplish magnetic flux commutation or switching.

Detailed Description of Some Exemplary Embodiments As illustrated in. Figure 3, the invention divides the large cross-section permanent magnet part 121 of the magnetic circuit into a number (typically ten, but may be more or less) of smaller cross-section permanent magnets 20 connected in parallel with each other such that each of the smaller permanent magnets can be relatively easily magnetised or demagnetised by a coil 22. The individual permanent magnets 20 may be of any convenient cross-sectional shape, such as round, rectangular, or polygonal. For illustrative convenience only one magnetising/demagnetising coil 22 is shown in Figure 3, but as shown in the more detailed equivalent circuit of Figure 4, each permanent magnet 20 (of which only three are shown for illustrative convenience) is in fact provided with its own coil 22, a demagnetising power supply 24 being connectable to each of the coils through switches 26 controlled by a control unit 28 which also controls the demagnetising power supply 24 by activating it upon receipt of a command signal 30.

La normal operation of the invention as part of a magnetic fault current limiter for AC, the reactance of each magnet-cored winding 22 may be compensated by a polarised electrolytic capacitor (not shown), of appropriate capacitance, the arrangement being such that the compensating capacitors are temporarily switched out of the limiter circuits while the demagnetisation process proceeds. The demagnetising power supply 24 is a reversible polarity DC supply so that the individual permanent magnets 20 can be demagnetised at will. Depending on the value and sense of the current applied to the coils 22 through the switches 26, the polarities of the small permanent magnets 20 can be individually selected, e.g., neutralised or reversed as desired.

For example, to achieve demagnetisation of all the permanent magnets 20, the switches 26 are closed and opened in sequence so that individual permanent magnets 20 are demagnetised sequentially. Because the power supply has to demagnetise only one relatively small permanent magnet at a time, the invention achieves the advantage of considerably reducing the size of the power supply needed to perform the demagnetisation in comparison with the power supply required to demagnetise the much larger permanent magnet of Figure 1. Stated another way, if a large permanent magnet is sub-divided into N smaller permanent magnets, then the power supply will only require 1/N of the power of the original source.

The power supply 24 and switches 26 are of course preferably of the solid state type, the switches being GTO's or the like, as known per se. The controller 28 is preferably a programmable controller (e.g., a PID controller) that can be programmed so that the power supply operates -with desired voltage and current characteristics and the switches operate with desired switching frequency characteristics. The only limit on switching frequency is imposed by the time taken to magnetise, demagnetise, or reverse the polarity of the permanent magnets, tut this is not a severe limitation because this time is in the range microseconds to milliseconds, according to the size of the permanent magnets and the power applied to their coils.

The principle of operation of the. invention can also be utilised to commutate flmq i.e., the invention can be used to provide a sort of magnetic switch or variable resistor. For example, the permanent magnets 20 will have a high inductance - i.e., they will be magnetically conductive to the flux in the magnetic circuit - when in their magnetised state and when aligned with the same polarity as the magnetic field in the iron core 10, but will tend to block the flux when either demagnetised, or magnetised in. reverse polarity to the flux field.

This principle is illustrated by Figures 4 aad 5. Referring again to Figure 4, the polarity and degree of magnetisation of the permanent magnets 20 can be controlled by means of one power supply 24 linked to the magnets through switches and controlled by programmable controller 28. For example, Figure 5A diagramrnaticaϊly illustrates a flux path A around a magnetic circuit containing a group of small permanent magnets 40 (only three are shown) that are magnetised with the same polarity as the flux. The permanent magnets 40 would therefore be conductive of the magnetic flux. On the otter hand, flux path B in Figure 5B would exhibit much higher reluctance, because the permanent magnets 40 are magnetised with opposite polarity to the flux. Alternatively, if the permanent magnets 40 were put into a demagnetised state by the circuit of Figure 4, they would exhibit an intermediate value of reluctance.

Although tihe above description has focussed on the use of the invention in connection with fault current limiters, it is envisaged that the invention could be used in other power electrical devices that incorporate magnetic circuits, such as transformers, generators, motors and actuators.

Furthermore, inductance/reluctance could be increased or decreased in a number of steps according to the number of parallel permanent magnets used, and according to how many of the parallel permanent magnets are magnetised -with the same polarity as the field, demagnetised, or magnetised with opposite polarity.