The invention is particularly applicable to a large reactor for use in a power system, for example in order to compensate for the Ferranti effect in long overhead lines or extended cable systems causing high voltages under open circuit or lightly loaded conditions. Reactors are sometimes required to provide stability to long line systems.

They may also be used for voltage control and switched into and out of the system during lightly loaded conditions.

Transformers are used in power systems to step up and step down voltages to useful levels.

A typical known induction device comprises one or more coils wrapped around a laminated core to form windings, which may be coupled to the line or load and switched in and out of the circuit. The equivalent magnetic circuit of a static induction device comprises a source of magnetomotive force, which is a function of the number of turns in the winding, in series with the reluctance of the core, which may include iron and optionally an air gap.

While the air gap is not strictly speaking necessary, reactors and transformers without air gaps tend to saturate at high magnetic field densities. Thus, control is less precise and fault currents may produce catastrophic failures. Although useful and desirable, the air gap represents a weak link in the structure of the core, which tends to vibrate at a frequency twice that of the alternating input current. This is a source of vibrational noise and high mechanical stress. Another problem associated with the air gap is that the magnetic field fringes, spreads out and is less confined. Thus, field lines tend to

enter and leave the core with a non-zero component transverse to the core laminations which can cause a concentration in unwanted eddy currents and hot spots in the core.

It is known to alleviate these problems by placing one or more inserts in the air gap. However, such inserts are complicated and difficult to manufacture and are therefore expensive.

A typical known insert comprises a cylindrical segment of radially laminated core steel plates arranged in a wedge shaped pattern. The laminated segments are molded in an epoxy resin as a solid piece or module. Ceramic spacers are placed on the surface of the module to space it from the core or, when multiple modules are used, from an adjacent module. In the latter case, the modules and ceramic spacers are accurately stacked and cemented together to make a solid core limb for the device.

The magnetic field in the core creates pulsating forces across all air gaps which, in the case of devices used in power systems, can amount to hundreds of kilonewtons (kN) The core must be stiff to eliminate these objectionable vibrations. The radial laminations in the modules reduce fringing flux entering flat surfaces of core steel which thereby reduce current overheating and hot spots.

These structures are difficult to build and require precise alignment of a number of specially designed laminated wedge shaped pieces to form the circular module. The machining must be precise and the ceramic spacers are likewise difficult to size and position accurately. Therefore, such devices are relatively expensive. Accordingly, it is desirable to produce an air gap spacer which has magnetic material distributed, preferably evenly, through non-magnetic plastics material which is substantially less expensive to produce than the described prior arrangements.

Another problem with manufacturing inserts for reactors with air gaps is that the volume of the insert will be large. For instance, in a typical"motor/cylinder"type reactor, the air gap will be about 2 m in length and have a diameter of about 1 m resulting in a volume of about 1. 5 m3. Such large volume inserts are conventionally relatively difficult

and expensive to manufacture since the stresses involved in their manufacture exclude the use of cheap, large volume thermoplastics materials. The use of thermosetting, e. g. epoxy, materials is troublesome since it is difficult to cast large volume parts due to shrinkage and heat generation.

Accordingly, in the invention, inserts for the magnetic core are formed from a "distributed air gap material"comprising finely divided magnetic particles in a matrix of a dielectric material. Such a material can have a magnetic permeability low enough to prevent saturation of the core under normal operating conditions and yet high enough to provide a path for magnetic flux.

Due to seasonal variations, it is desirable to change the inductance of reactors used in connection with power transmission lines.

Summary of the Invention The present invention provides an induction device comprising a core of magnetically permeable material, characterized in that the core comprises at least one component comprising finely divided magnetic particles in a matrix of a dielectric material, and the inductance of the device is variable by changing the effective magnetic permeability of the at least one component.

The effective magnetic permeability of the component can be varied in a number of ways. In one embodiment of the invention, the component is removable and interchangeable with at least one further component having a different magnetic permeability. Alternatively, the component can be movable between different positions in which it lies in the path of the magnetic flux to a greater or lesser extent. This can be achieved by forming the component in two or more sections and moving one, both, some or all of the sections into and out of alignment with the magnetic core of the device.

In embodiments of the invention, the component comprises regions of different magnetic permeability, and is movable so as to include a different one or different ones of

the regions in the magnetic circuit of the device or so as to include the regions in different configurations. For example, the component may be elongate, having a longitudinal axis, and the different regions may be arranged at different locations along the axis, the component being axially movable to include a different one of the regions in the magnetic circuit in each of a plurality of different locations of the component. Alternatively, the component may be rotatable about an axis, and the different regions may be arranged at different circumferential or angular locations around the component. Such a component is particularly appropriate for use in a multiphase induction device comprising a plurality of magnetic core limbs, of iron or other highly permeable material, extending radially from the component, in which the magnetic field rotates, or a combination of alternating and rotating fields occurs.

In another embodiment of the invention, also comprising a rotating-field or combined altemating/rotating-field distributed air gap material component, the magnetic permeability of the component is changed by applying a magnetic field thereto at an angle to the rotating field (and the alternating field where present), preferably approaching 90 degrees. The magnitude of the resultant magnetic field vector in the radial direction is reduced by the applied magnetic field, which may be applied using one or more permanent magnets or solenoids.

The magnetic particles of the component may, for example, be of iron, an amorphous iron-based material, an alloy such as Ni-Fe, Co-Fe, Fe-Si or a ferrite based on at least one of manganese, zinc, nickel and magnesium (and preferably on an alloy such as Mn-Zn, Ni-Zn or Mn-Mg), and may have a size of about I nm to about 1 mm, preferably about 0.1 ujn to about 200 pm, and a volume fraction of up to about 60%. The magnetic permeability of the magnetic component is suitably from 0 to 20. The magnetic particles may be coated with a dielectric coating comprising metal oxide or another inorganic material.

Suitable dielectric materials for the matrix include thermosetting polymers, such as epoxy resin, phenolic resin, urea-formaldehyde resin, melamine-formaldehyde resin, polyester resin or polyvinyl ester resin ; thermoplastic polymers, such as polyamide,

polyimide, polyethylene, cross-linked polyethylene, polypropylene, acrylnitride-butadiene- styrene, polycarbonate, polymethyl methacrylate, polyphenylene sulphone, polysulphone, polyetheretherketone, polytetrafluoroethylene or polyformaldehyde ; elastomers, such as natural rubber, vinylidine fluoride-hexafluoropropylene rubber, vinylidine fluoride- hexafluoropropylene-tetrafluoroethylene rubber, ethylene propylene rubber, thermoplastic polyolefin rubbers, or polyether-ester thermoplastic rubbers; or concrete, foundry sand or a fluid such as water or a gas.

To form the component, a plurality of elongate members can be assembled in a pack with their elongate axes parallel to each other, at least some, typically all, of the elongate members comprising finely divided magnetic particles in a matrix of a dielectric material. If the component is to have regions of different magnetic permeability, at least one of the members may have a different magnetic permeability from at least one other.

The proportion of the particulate magnetic material in the different elongate members may be varied as required to produce the desired regions of different magnetic permeability. It is also possible for the different elongate members to display magnetic anisotropy in different directions through the assembled component. The members may be of substantially identical size and shape and may comprise strands of solid material, wires, powder filled hoses or pipes, or rolls of ribbon. Any of the members which do not comprise finely divided magnetic particles in a matrix of a dielectric material comprise magnetic material. The members are either permanently assembled or one or more of the members is removable.

If the dielectric material of the matrix comprises thermoplastics material, the elongate members may be extruded, and if the dielectric material comprises thermosetting, e. g. epoxy material, the elongate members may be molded. Typically the elongate members may have a length of up to several meters with a diameter or width of from 4 to 5 cm although these dimensions are by way of example and are not intended to limit the scope of the invention. Preferably, the members are in the form of regular prisms which interfit together although they may have other cylindrical shapes, e. g. circular cylinders.

An assembled pack of elongate members may be mechanically united together by binding the pack with elongate binding material, e. g. wire, string, tape or the like.

Preferably, however, the elongate members are at least partly bonded or united together under the application of electromagnetic radiation, e. g. heat or ultraviolet radiation, and/or pressure. The elongate members may be bonded to each other, e. g. using a curable resin such as an epoxy resin containing a catalyst or accelerator. Alternatively, or in addition, the assembled pack of members may be wrapped in a curable resin, or curable resin impregnated matting or the like, which is subsequently cured to produce a strong "composite"construction. Such a construction has excellent mechanical properties.

After having been assembled from a plurality of elongate members, the component of the invention may subsequently be shaped, e. g. machined, into a desired form.

In an embodiment of the invention, the members comprise one or more hollow cylinders surrounding a solid cylinder which is slidable out of the component. The solid cylinder preferably has a higher magnetic permeability than the or each hollow cylinder.

The magnetic permeability of a prism can be determined by the concentration and/or the size of the magnetic particles in the dielectric matrix.

A communications unit is preferably included in the induction device. The communications unit typically comprises at least one Input/Output (I/O) interface and a processor. Measured values for one or more sensors in the induction device may be received via the I/O interface and routed to the processor. An output channel of the I/O interface may be used to send a control signal to an actuator of any sort arranged in the induction device. The communications unit may also be used to send data out of the induction device by wire or wireless means, for supervision, data collection and/or control purposes. The communications unit may, for example, be mounted on the core.

Brief Description of the Drawings Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which : Figure 1 schematically shows an induction device having an interchangeable distributed air gap material component; Figure 2 schematically shows an induction device having a separable distributed air gap material component; Figure 3 schematically shows a device having an axially movable distributed air gap material component; Figure 4 schematically shows a rotatable component ; Figure 5 schematically shows a component comprising solid and hollow cylinders; Figures 6,7 and 8 are schematic perspective views of distributed air gap material components formed from members of uniform cross-section; Figures 9,10 and 11 show alternative arrangements of magnetic permeability for the components of Figures 6,7 and 8; and Figure 12 is a schematic perspective view of a rotating-field distributed air gap component with an applied magnetic field.

Detailed Description of the Preferred Embodiments Figure 1 shows a cross section of a reactor having a core 1 of highly magnetically permeable material including a gap 2. Large components of distributed air gap material 3, 4,5, each having a different concentration of magnetic particles, and hence a different

magnetic permeability, can be interchangeably placed in the gap 2 by insertion into an end of the gap. Thus, the inductance of the reactor is varied.

The reactor may be rotationally symmetrical, the core comprising core limbs extending at equiangular intervals from the distributed air gap material component.

Figure 2 shows an alternative embodiment in which forces exerted during regulation are symmetrical. A distributed air gap material component is divided into two halves 6 which can be moved apart or together. This allows fine adjustment of the effective magnetic permeability and even continuous regulation of the reactor during operation.

Figure 3 shows a more simple reactor which can be regulated quickly and continuously. An elongated distributed air gap material component 7 comprises a number of regions (three in this example) 8,9,10, each having a different magnetic permeability, at different locations along the axis of the component. The component 7 is moved axially, for example by raising and lowering, so that a different one of the regions, or a different ratio of two adjacent regions, is present in the gap 2 in the core 1.

Figure 4 shows a three phase reactor having three core limbs spaced at 120° from each other. A cylindrical distributed air gap material component 11 comprises alternating regions 12,13 occupying an annulus around its circumference. The regions 12 have a different magnetic permeability from the regions 13. A central region 14 of the component can have a much higher magnetic permeability than the regions 12,13 and can even be of a soft magnetic material such as iron. To vary the inductance of the reactor, the component 11 is rotated about its center through up to 60°. The inductance varies continuously between a minimum value, when the regions of lower magnetic permeability are adjacent the core limbs, and a maximum value when the regions of higher magnetic permeability are adjacent the core limbs.

Figure 5 shows a cylindrical distributed air gap material component comprising concentric regions of different magnetic permeability 14, 15,16. The outer regions 14, 15

are formed from hollow cylinders whilst the innermost region 16 is formed from a solid cylinder. In a preferred configuration, the magnetic permeability of the intermediate cylinder 15 is greater than that of the outermost cylinder 14 and the permeability of the innermost cylinder 16 is greater still. The effective permeability of the entire component can be reduced by sliding out the innermost cylinder 16 or the inner two cylinders 15,16.

The composite distributed air gap material is particularly suitable for use in a multiphase cylindrical reactor having a magnetic core resembling the stator of a known rotating machine, the stationary distributed air gap material component taking the place of the rotor of the machine.

Figures 6,7 and 8 show components each formed from a number of uniform cross-section members of distributed air gap material. In Figure 6, the members are hexagonal, in Figure 7, triangular and in Figure 8, circular. The members typically each have a cross-sectional area of from 10-40 cm2, preferably from 15-30 Cm, and a length of up to several meters. Each member comprises a strand of solid material, or one or more wires, or a powder filled hose or pipe, or a roll of ribbon. One or more of the members can be removable from the remainder of the component.

The elongate members are preferably made of plastics material such as extruded thermoplastics material or molded curable material, e. g. a thermosetting material such as an epoxy resin, containing magnetic particles. The elongate members may be coated with a suitable cross-linkable polymeric substance, such as curable resin, so that they are subsequently joined together in an assembled pack on curing of the resin. Alternatively, or in addition, the assembled packs of members may be surrounded by reinforcing material such as, for example, curable resin embedded in glass fiber matting or the like.

This produces an assembled united"composite"pack having excellent mechanical properties.

If the elongate components are made from thermoplastics materials, they can be extruded using readily available, low cost thermoplastics materials. Examples of thermoplastics materials and curable resin materials for the elongate components are

epoxy resin, polyester, polyamide, polyimide, polyethylene, cross-linked polyethylene, polytetrafluoroethylene (PTFE) and polyformaldehyde (PFA) sold under the trademark Teflon by Dumont, rubber, ethylene propylene rubber (EPR), acrylonitrile-butadiene- styrene (ABS), polyacetal, polycarbonate, polymethyl methacrylate (PMMA), polyphenylene sulphone (PPS), polysulphone (PSU,) polyetherimide (PEI), PEEK, silicone rubber, polymers containing silicone and the like. The magnetic particles may be formed of iron, amorphous iron based materials, or alloys of Ni-Fe, Co-Fe, Fe-Si, or ferrites based on at least one of magnesium, zinc, nickel and magnesium.

Although generally all the elongate members will comprise distributed particulate magnetic material in a matrix of dielectric material, this is not essential. For example, the assembled pack of members may include air spaces of magnetically significant sizes. It is also possible for the elongate members to be in the form, for instance, of strands of solid magnetic material, wires, magnetic powder filled hoses or tubes, rolls or ribbons of magnetic material or the like.

Although it is preferred to bond or unite the elongate members together or to surround the assembled pack with reinforcing, resin impregnated material or the like, it is also possible to mechanically unite the elongate components together by binding the pack with elongate binding material, e. g. wire, string, tape or the like. Other examples of surrounding uniting materials are nylon or polyester based web, braid or"tire cord" material.

As shown in Figures 9,10 and 11, some of the members in any of Figures 6,7 and 8 have a different magnetic permeability from others. In particular, the dark circles represent members of greater magnetic permeability than the white circles. Figure 9 shows a component formed from an"alloy"of the two kinds of member, Figure 10 shows a coma anent in which the magnetic permeability varies radially in cross section, and Figure 11 a component in which the magnetic permeability varies angularly.

It will be appreciated that different distributed air gap material components can easily be customized, and by suitable variations of the geometry, this method can be used to form the components exemplified in Figures 3,4 and 5.

In an alternative distributed air gap material component of uniform cross-section, transition regions at the ends have a higher magnetic permeability than the center of the component.

Figure 12 shows an alternative method of varying the effective permeability in a given direction of a cylindrical distributed air gap material component 17 in a rotating- field multiphase reactor. A static coaxial magnetic field Bz is applied, in this example by means of permanent magnets 18 at the ends of the reactor. This static field combines with the original rotating field vector Br, which sweeps out a disc, to form a resultant field vector Btot, sweeping out a conical surface whenever Bz>O. As long as the core is of a soft magnetic material and Bz is non-zero, the radial component of Btot is less than Br, and the permeability of the component perpendicular to the z-axis is reduced. Br, the rotating field vector, could also represent a combination of alternating and rotating magnetic fields.