GB2415833A

GB2415833A - Inductive device with parallel permanent magnets in a magnetic circuit

Info

Publication number: GB2415833A
Application number: GB0414603A
Authority: GB
Inventors: Christian Sasse
Original assignee: Areva T&D UK Ltd
Current assignee: UK Grid Solutions Ltd
Priority date: 2004-06-30
Filing date: 2004-06-30
Publication date: 2006-01-04
Also published as: GB0414603D0; WO2006003111A1

Abstract

An inductive device comprises a magnetic circuit 10 with a permanent magnet arrangement 12'. The permanent magnet arrangement 12' comprises a plurality of permanent magnets 20 connected in the magnetic circuit 10 in parallel with each other. At least some of the said permanent magnets 20 have a respective electrical winding 22 around it. The individual permanent magnets 20 have a small cross sectional area which allows them to be magnetised to a desired polarity or demagnetised using less current in the winding 22 surrounding the respective magnets. A programmable switch control system may be used to connect the individual coils 22 associated with a respective magnet to a power supply such that the magnetisation and demagnetisation of the magnets can be controlled. The device may be employed in arrangements such as fault current limiters, transformers, generators, motors or actuators.

Description

MAGNETIC CIRCUIT f)F,V1CE

Field of the Invention

I he 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

lo 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 l 5 current limiter. For an example of such a device, see B.P.Raju, K.C.Parton and T.C.Bartram, "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 sophisticated refrigeration system and need a reset time subsequent to fault clearing. The latter two limitations deleteriously affect reliability.

It has also previously been proposed to construct a passive current limiter utilising a permanent magnet as part oi 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 BIASFI) FAULT CUMEN I LIMITING REACTOR WITH A STEEL CORE", by S.C.Mukhopadhyay, M. lwahara, S.Yamada and F.P.Dawson. This paper is available on the Internet at http://magmacl.ec.t.kanazawa-u.acjp/magcap/research/fcl.html and a hard copy is filed with the present patent application to provide a more permanent record.

Figures l and 2 illustrate diagrammatically why a permanent magnet makes an effective fault current limiter. In Figure l, a magnetic circuit I in a fault current limiter for AC comprises a "C"-shaped magnetically soft iron core 10 and a demagnetized permanent magnet 12, around which a coil 14 is wound. Magnetic flux can flow around the magnetic s 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 demagnetized 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, i.e., 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 demagnetized 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 limiters 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 magnetization 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 magnetization of the material down to zero again).

Conversely, as already explained above, a hard or permanent magnet's hystereses curve encloses a large area, and it's material has large values of remanence 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 limiter circuit of Figure I 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 0 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 in parallel with each other as part of a magnetic circuit in a fault current limiter mitigates the problems associated with demagnetizing permanent magnets.

The individual permanent magnets may have cross-sectional shapes selected from the group comprising round, rectangular, and polygonal crosssections.

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 in 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 I 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 magnetization and demagnetization of a permanent magnet, Figure 3 is a view similar to Figure 1, but modified in accordance with the invention; Figure 4 is 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 2s part 12' 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 demagnetized by a coil 22. The individual permanent magnets 20 may be of any convenient cross- sectional shape, such as round, rectangular, or polygonal. s

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 demagnetizing power supply 24 being connectable to each of the s coils through switches 26 controlled by a control unit 2X which also controls the demagnetising power supply 24 by activating it upon receipt of a command signal 30.

In 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 lo 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 demagnetization process proceeds. The demagnetizing power supply 24 is a reversible polarity DC supply so that the individual permanent magnets 20 can be demagnetized at will. Depending on the value and sense of the current applied to the coils 22 through the l s 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 demagnetization 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 demagnetize 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 demagnetization 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 2s smaller permanent magnets, then the power supply will only require 1/N of the power of the original source.

I he 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, but 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.

I he principle of operation of the invention can also be utilised to commutate flux, i.e., the invention can be used to provide a sort of magnetic switch or variable resistor. For lo 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 demagnetized, or magnetised in reverse polarity to the

flux field.

This principle is illustrated by Figures 4 and S. Referring again to Figure 4, the polarity and degree of magnetization 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 SA diagrammatically 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 other hand, flux path B in Figure SB would exhibit much higher reluctance, because the permanent magnets are magnetised with opposite polarity to the flux. Alternatively, if the permanent magnets 40 were put into a demagnetized state by the circuit of Figure 4, they would exhibit an intermediate value of reluctance.

Although the 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.

Claims

1. A magnetic circuit device comprising: 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 lo 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.

2. A magnetic circuit device according to claim 1, in which each one of the plurality of permanent magnets has an electrical coil wound around it.

3. A magnetic circuit device according to claim 1 or claim 2, in which the individual permanent magnets have cross-sectional shapes selected from the group comprising round, rectangular, and polygonal cross- sections.

4. A magnetic circuit device according to any preceding claim, wherein 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.

5. A magnetic circuit device according to claim 4, in which the power supply and the switches are of the solid state type.

6. A magnetic circuit device according to claim 4 or claim 5, wherein 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 of each permanent magnet is individually selectable.

7. A magnetic circuit device according to any one of claims 4 to 6, in which the controller is programmable to control voltage and current characteristics of the power supply, and switching frequency characteristics of the switches.

8. An electrical fault current limiter incorporating a magnetic circuit device according to any preceding claim.

9. A magnetic circuit device substantially as described herein with reference to and as lo illustrated in any of Figures 3, 4, SA and 5B of the accompanying drawings.

I (). An electrical fault current limiter substantially as described herein with reference to and as illustrated in any of Figures 3, 4, 5A and 5B of the accompanying drawings.