GB2361108A - A magnetic core with a conductive ring or a core portion with a modified shape - Google Patents

Abstract

A magnetic core 20, 40 comprises at least one closed electrically conductive ring 22, 41 in or around a portion of the core 20, 40 for influencing the magnetic field D B in the said portion of the core. The core may be formed from magnetic wire, ribbon, strip or compacted magnetic particulate material. In a wire wound core conductive rings may be placed around different wires or wire portions. The conductive rings 22 may be embedded in a synthetic material 23 forming a portion 21 of the core 20. Also disclosed is a magnetic core comprising a portion of the core with a modified shape such that it has an influence on the magnetic field D B in the region of the said portion. The shaped portions may be arranged to prevent the magnetic field D B from leaving and re-entering the core perpendicular to the laminated sheets forming the core 40. This shaping technique may be combined with a conductive ring arrangement 41.

Description

2361108 Magnetic Core of a High Voltage Induction Device

Technical Field

This invention relates to a magnetic core of a high voltage (HV) induction device and to a high voltage induction device provided with such a magnetic core. In this specification the term "high voltage,, is intended to mean in excess of 2 kV and preferably in excess of 10 kV.

Background of the invention

HV induction devices such as reactors are used in power systems, for example, in order to compensate for the Ferranti effect from long overhead lines or extended cable systems causing high voltages in the open circuit or lightly loaded lines. 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 light load conditions. Similarly, transformers are used in power systems to step up. and step down voltages to useful levels.

Such RV induction devices are manufactured from similar components. Typically, one or more coils are wound around a laminated core to form windings, which may be coupled to the line or load and switched in and out of the circuit in a desirable manner.. The equivalent magnetic circuit of a static induction device comprises a source of magnetomotive force, which is a function of the number turns of the winding, in series with the reluctance of the core, which may be of iron and which may include 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.

A known core may be visualized as a body having a closed magnetic circuit, for example, a pair of legs and interconnecting yokes. one of the legs may be cut through to f orm the air gap. The core may support the windings which, when energized by a current, produce a magnetic field 4, in the core which extends across the air gap. At high current densities the magnetic field is intense.

Although useful and desirable, the air gap represents a weak link in the structure of the core. The core tends to vibrate at a frequency twice that of the alternating input current. This is the source of vibrational noise and stress in such induction devices.

Another problem associated with an air gap is that the flux cD 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.

These problems can be alleviated by the use of one or more inserts in the air gap designed to stabilize the structure and thereby reduce vibrations. Such cores are known as distributed air gap cores. These known structures or inserts are formed of materials which are designed to reduce the fringing effects in the air gaps. However, heretofore, these devices have generally been difficult and expensive to manufacture.

A typical known insert comprises a cylindrical segment of radially laminated core steel plates arranged in a wedge shaped pattern. The laminated segments are moulded 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 f ield 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 kilo-newtons (M). 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. As a result, such devices are relatively expensive.

An aim of the present invention is to provide a modified closed magnetic core for a high voltage induction device.

According to one aspect of the present invention there is provided a solid closed magnetic core for a high voltage induction device, characterised in that a portion of the core includes at least one closed ring of electrically conducting material for influencing the magnetic flux path in the region of said portion of the core.

The or each closed ring of electrically conducting material does not, according to LenzIs law, allow the magnetic field to pass through it. Thus the magnetic flux is forced to follow a different path in the region of said core portion. In this way the effective permeability of the core is reduced.

The core may comprise high permeability magnetic material with a gap therein closed by an insert of polymeric material within which the at least one closed ring of electrically conducing material is embedded. In this case the insert stabilises the gap in a simple and relatively - 4 cheap manner and effectively reduces or eliminates the problems of noise and mechanical weakness. associated with cores with air gaps. The effective permeability of the core can be adjusted by, for example, by varying the proportion of closed rings of electrically conducting material embedded in the polymeric material. Typically the permeability of the insert will be less than 1. The insert being essentially made of polymeric material may be moulded in the gap.

As an alternative to the core having a filled gap, the magnetic core may comprise high permeability material arranged in a closed path and including, in the core portion, said at least one closed ring of electrically conducting material.

The core may be formed of laminated sheet material, e.g. electrical steel, wire, powdered magnetic material or the like. In the case where the core is made of wire, the wire may be wound so as to pass through the closed rings of electrically conducting material.

According to another aspect of the present invention, there is provided a solid closed magnetic core for a high voltage induction device, characterised in that a portion of the core has its shape modified for influencing the magnetic f lux path in the region of said portion of the core.

According to a still further aspect of the present invention, there is provided a high voltage induction device having a closed magnetic core according to said on or said other aspect of the present invention.

Embodiments of the invention will now be described, by way of example only, with particular reference to the accompanying drawings, in which:

Fig. 1 illustrates schematically a known induction device, e.g. a power reactor or power transformer; - 5 Fig. 2 is a perspective fragmentary view of a cable which may be used in the winding 9f a high power static induction device for a power system; Fig. 3 is a cross-section through the cable shown in Fig. 1; Fig. 4 is a schematic view of a f irst embodiment of part of a closed magnetic core according to the invention f or a high voltage induction device; Fig. 5 is a schematic view of a second embodiment of part of a closed magnetic core according to the invention for a high voltage induction device; Fig. 6 is a schematic view of a third embodiment of part of a closed magnetic core according to the invention for a high voltage induction device; and Fig. 7 is a schematic view of a fourth embodiment of part of a closed magne,tic core according to the invention for a high voltage induction device.

DESCRIPTION OF THE INVENTION

Fig. 1 shows a known induction device 1, such as a 20 power transformer or reactor, having at least one winding 2 and a core 3. Fig. 1 also shows a simplif ied view of the electric field distribution around the turns of the winding 2, with lines of equipotential designated E and indicating where the electric field has the same magnitude. The lower part of the winding is assumed to be at earth potential. The core 3 has an optional distributed air gap 4 and a window 5. The core is typically formed of laminated sheets of magnetically permeable material, e.g. silicon steel, but may, alternatively, be formed of magnetic wire, ribbon or powder metallurgy material. The direction of the magnetic f lux 4 is shown by the arrow in Figs. 1 and 2 and, in 6 general, is confined, or is at least nearly confined, within the core 3.

The potential distribution determines the composition of the insulation system, especially in high power systems, because it is necessary to have sufficient insulation between adjacent turns of the winding. In Fig. 1, the upper part of the winding is subjected to the highest dielectric stress. The design and location of a winding relative to the core 3 are in this way determined substantially by the electric field distribution in the core window 5. The windings 2 may be formed of a conventional multiturn insulated wire, as shown, or the windings 2 may be in the form of a high power transmission line cable discussed below. In the former case, the device may be operated at power levels typical for such devices in known power generating systems. In the latter case, the device may be operated at much high power levels not typical for such devices.

Figs. 2 and 3 illustrate an exemplary cable 6 for 20 manufacturing windings 2 useful in high voltage, high current and high power induction devices. Such cable 6 comprises at least one conductor 7 which may include a number of strands 8 with a cover 9 surrounding the conductor 7. In the exemplary embodiment shown, the cover 9 includes a semiconducting inner layer 10 disposed around the strands 8, a solid main electrically insulating layer 11 surrounding the semiconducting inner layer 10, and a semiconducting outer layer 12 surrounding the main electrically insulating layer 11 as shown. The inner and outer layers 10 and 12 have a similar coefficient of thermal expansion as the main electrically insulating layer 11. The cable 6 may be provided with additional layers (not shown) for special purposes. In a high power static conductor device, for example, the cable 6 may have a conductor area which is between about 30 and 3000 mm and the outer cable diameter may be between about 20 and 250 mm. Depending upon the application, the individual strands 8 may be individually - 7 insulated. A small number of the strands near the interface between the conductor 7 and the semiconducting inner layer 10 may be uninsulated for establishing good electrical contact therewith. As a result, no harmful potential differences arise in the boundary layer between the innermost part of the solid insulation and the surrounding inner semiconducting layer along the length of the conductor. The cable 7 may typically be as described in WO 97/45931 and such cable is incorporated herein by way of reference.

Devices for use in high power applications may have power rating ranging from 10 kVA up to over 1000 MVA with greater voltage ranging from about 3-4 kV and up to very high transmission voltages, such as 400 kV to 800 kV or higher.

The similar thermal properties of the various layers 1012, results in a structure which may be integrated so that adjoining.emiconducting and insulation layers exhibit good contact independently of variations and temperatures which arise in different parts of the cable. The insulating layer and the semiconducting layers form a monolithic structure and defects caused by different temperature expansion of the insulation and the surrounding layers do not arise.

Fig. 4 shows one embodiment of part of a closed or endless magnetic core 20 of a high voltage induction device.

The core 20 is formed of high permeability material with a gap therein which is filled with an insert 21 comprising closed rings 22 of electrically conducting material embedded in a matrix 23 of synthetic polymeric material. Typically the polymeric material may be an epoxy resin, polyester, polyamide, polyimide, polyethylene, cross-linked polyethylene, polytetrafluoroethylene (PTFE) and polyformaldehyde (PFA) sold under the trademark Teflon by Dupont, rubber, ethylene propylene rubber (EPR), acrylonitrile-butadiene-styrene (ABS), polyacetal, polycarbonate, polymethyl methacrylate (PMMA), polyphenylene sulphone (PPS), PSU, polyetherimide (PEI), PEEK, silicone rubber, polymers containing silicone and the like.

The closed rings 22 of electrically conducting material do not allow the magnetic field lines to pass therethrough. Fig. 4 shows the magnetic f lux 4>, in the core 20 and it can be seen that the magnetic flux path diverges around, and is modified by the presence of, the rings 22.

The conducting rings 22 may be of molecular size. Alternatively, the rings 22 may comprise larger loops of electrically conducting material. in this respect, it will be appreciated that Fig. 4 is only schematic.

Figs 5, 6 and 7 show three different embodiments of magnetic cores 30, 40 and 50, respectively, each having high permeability material arranged in a closed path and including closed rings of electrically conducting material in a portion of the core for influencing the magnetic flux path in that portion of the core.

The magnetic core 30 shown partially in Fig. 5 has an electrically conducting ring 31 arranged around a portion of the core. The presence of the ring 31 causes the magnetic flux path to diverge around the ring 31 as shown. In this embodiment the core may be made, for example, of laminated sheets of magnetically permeable material, e.g. electrical steel, but may, alternatively, be formed of magnetic wire, ribbon or powder metallurgy material.

One disadvantage of the embodiment shown in Fig. 51 especially when the core is made of laminated sheets of magnetically permeable material, is that the magnetic flux exits and enters the core on either side of the ring 31 in directions generally perpendicular to the laminated sheets. This tends to cause heating of the core with associated core losses and to create leakage fluxes. The embodiment of the - 9 magnetic core 40 shown partly in Fig. 6 addresses these disadvantages by shaping the core on eiher side of the electrically conducting ring 41. In the case of the core 40 being formed of laminated sheets of magnetically pe=eable material, the laminated sheets are shaped on either side of the ring 41 so that the magnetic flux path leaves and enters the laminated sheets generally parallel to the directions of the sheets. Although not shown, it may also be possible to influence the magnetic flux path in a region of the magnetic core by merely adapting the shape of the core, e. g. in the manner described above with respect to the laminated sheets of the Fig. 6 core embodiment, without the presence of the electrically conducting ring 41.

The magnetic core 50 shown partly in Fig. 7 is formed from wire which is arranged to pass through a plurality of closed electrically conducting rings 51. The use of wire ensures that the leakage flux is reduced. It is also possible for a single wire to be wound into any desired shape of endless core, e.g. a toroidal core.

Although not shown, the magnetic cores described herein are intended to be incorporated in highvoltage induction devices, e.g. reactors or transformers.

The invention provides alternative methods of adjusting the effective permeability of a magnetic core by incorporating rings or loops of electrically conducting material in the core. Additionally the shape of the core may be adjusted.

Claims (16)

1. A solid closed magnetic core for a high voltage induction device, characterised in that a portion of the core includes at least one closed ring of electrically conducting material for influencing the magnetic flux path in the region of said portion of the core.

2. A magnetic core according to claim 11 characterised in that the core comprises high permeability magnetic material with a gap therein closed by an insert of polymeric material within which the said at least one closed ring of electrically conducing material is embedded.

3. A magnetic core according to claim 2, characterised in that the Insert is moulded in the said gap.

4. A magnetic core according to claim 2 or 3, characterised in that said polymeric material comprises epoxy resin, polyester, polyamide, polyimide, polyethylene, cross-linked poly-ethylene, polytetrafluoroethylene (PTFE) and poly-formaldehyde (PFA) sold under the trademark Teflon by Dupont, rubber, ethylene propylene rubber (EPR), ac ryl oni tr i 1 e - butadi ene - s tyrene (ABS), polyacetal, polycarbonate, polymethyl methacrylate (PMMA), polyphenylene sulphone (PPS), PSU, polyetherimide (PEI), PEEK, silicone rubber, polymers containing silicone and the like.

5. A magnetic core according to claim 1, characterised in that the core comprises high permeability material arranged in a closed path and including, in the said core portion, said at least one closed ring of electrically conducting material.

6. A magnetic core according to any one of the preceding claims, characterised in that the said core portion has a varying cross-section.

7. A magnetic core according to any one of the preceding claims, characterised in that the. said portion has a relatively low permeability, preferably less than 1.

8. A magnetic core according to any one of the preceding claims, characterised in that the core is formed of laminated sheet material, e.g. electrical steel.

9. A magnetic core according to any one of claims 1 to 7, characterised in that the core comprises wire.

10. A magnetic core according to any one of claims 1 to 7, characterised in that the core comprises magnetic ribbon material.

11. Amagnetic core according to claim 9 or 10, each when dependent directly or indirectly on claim 5, characterised in that the said wire or ribbon material passes through said at least one closed ring.

12 A magnetic core according to any one of claims 1 to 7, characterised in that the core comprises compacted magnetic particulate material, e.g. powder.

13. A magnetic core for a high voltage induction device according to claim 1, characterised in that a portion of the core has its shape modified for influencing the magnetic flux path in the region of said portion of the core.

14. A magnetic core according to claim 13, characterised in that the core comprises magnetic ribbon material or wire.

15. A magnetic core according to claim 13, characterised in that the core comprises compacted magnetic particulate material, e.g. powder.

12 -

16. A high voltage induction device having a magnetic core as claimed in any one of the preceding claims.