GB2361107A

GB2361107A - Magnetic bias of a magnetic core portion used to adjust a core's reluctance

Info

Publication number: GB2361107A
Application number: GB0008150A
Authority: GB
Inventors: Mikael Dahlgren; Tomas Jonsson
Original assignee: ABB AB
Current assignee: ABB AB
Priority date: 2000-04-03
Filing date: 2000-04-03
Publication date: 2001-10-10
Also published as: GB0008150D0

Abstract

A method or means of increasing the reluctance of a closed magnetic circuit 20 comprises providing a portion 21 of the magnetic circuit with a magnetic bias towards magnetic fields transverse <I>X</I> to that of the magnetic flux flow direction in the magnetic circuit for the said portion 21. The magnetic bias may be created by the application of a transverse magnetic field using electromagnetic means with a coil 23 on either side of the portion 21 and a magnetic core 25 arranged to boost the magnetic field produced. Alternatively the portion 21 of the magnetic circuit may be formed from a hard or semi-hard magnetic material with a preferred magnetic direction (anisotropy) transverse to the said magnetic flux flow direction. The hard or semi-hard magnetic material portion may be demagnetised or subjected to a transverse magnetic field generated by electromagnetic means. The arrangement may be used in a high voltage induction devices such as transformers or reactors used in power supply systems.

Description

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2361107 - 1 Core of a High Voltage induction Device Technical Field

This invention relates to a high voltage (HV) induction device of the kind having a solid closed magnetic core and at least one winding wound on the closed magnetic core. The invention also relates to a method of influencing the reluctance of the magnetic circuit of an induction device. 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 HV 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 form the air gap. The core may support the windings which, when energized by a current, produce a magnetic flux 45 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 4, 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 somewhat alleviated by the use of one or more inserts in the gap designed to stabilize the structure and thereby reduce vibrations. in addition, the structure, or insert, is formed of materials which are designed to reduce the fringing effects in the gap. However, these devices are difficult to manufacture and are expensive.

A typical 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.

- 3 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 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.

is An aim of the present invention is to provide an induction device in which the reluctance of the magnetic circuit is increased by an alternative method to the use of air gaps in the core of the induction device.

According to one aspect of the present invention there is provided an induction device of the kind referred to characterised in that a portion of the closed magnetic core has its magnetic moments aligned in a direction transverse to the direction of the said portion of the magnetic core.

The principle of the invention is based on aligning the magnetic moments in the portion of the endless magnetic core in a direction transverse, preferably substantially perpendicular to, the general magnetic path direction on either side of the core portion. By aligning the magnetic moments in this way, the permeability, g, is made considerably lower in the core portion than in the rest of the core. The effective permeability, g, of the whole magnetic circuit of the closed magnetic core is thus decreased, i.e. the reluctance is increased, without the use - 4 of conventional air gaps in the core and thus core loses are reduced.

Suitably, the magnetic moments are aligned by passing a d.c. current through electromagnet means positioned on either side of the core portion, which is suitably made of soft magnetic material. Conveniently the electromagnet means is formed by coils on either side of the core portion and a further closed electromagnetic core including the said core portion. The magnetic field can be easily adjusted by varying the strength of the current through the coils. In this way, the field strength can be adjusted.

Alternatively, the core portion may comprise a hard, or semi-hard, magnetic material with its anisotropy axis transverse to, preferably substantially perpendicular to, the direction of said core portion. In this case, the hard or semi-hard magnetic material should preferably be macroscopically demagnetised in order to minimise stray fields in the air around said core portion. Furthermore, for practical reasons, a demagnetised hard magnetic material is easier to handle.

According to another aspect of the present invention there is provided a method of increasing the reluctance of the magnetic circuit of an induction device having core means with a closed magnetic core, characterised in that the method comprises aligning the magnetic moments of a portion of the closed magnetic core in a direction transverse to the direction of the said portion of the core.

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; Fig. 2 is a perspective fragmentary view of a cable which may be used in the winding of 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 one embodiment of a power induction device according to the invention having a closed magnetic core; and Fig. 5 is a schematic view of another embodiment of a power induction device according to the invention having a closed magnetic core.

DESCRIPTION OF THE INVENTION

Fig. 1 shows a known induction device 1, such as a power transformer or reactor, having at least one winding 2 and a core 3. Fig. 1 also shows a simplified 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 flux 4 is shown by the arrow in Figs. 1 and 2 and, in 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 Pig. 1, the upper part of the winding is subjected to the highest dielectric - 6 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 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 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 - 7 97/45931 and such cable is incorporated herein by way of reference.

Devices for use in high power applications may have a power rating ranging from 10 kVA up to over 1000 MVA with a 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 10-12, results in a structure which may be integrated so that adjoining semiconducting 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.

Referring to Fig. 4, there is shown a simplified view of an induction device 20 according to the invention. The induction device 20 is similar to the induction device 1 described with reference to Figs. 1-3 and where possible, similar reference numerals have been used to identify similar part of the two induction devices. In particular, the induction device 20 has at least one winding 2 arranged on a endless or closed solid core 3 having no air gap therein. The core 3 has a core window 5 and may be made from materials as described with reference to the core 3 shown in Fig. 1.

A pair of d.c. coils 23 are arranged on either side of a portion 21 of the core 3 and, in use, produce a d.c.

field 24 across the core portion 21 transverse to, preferably substantially perpendicular to, the magnetic path direction of the core through the core portion 21. The core portion is of a "soft" magnetic material and the d.c. coils act as electromagnets to at least substantially align the magnetic moments in the core portion 21 transversely to the

8 general direction of the magnetic core in the core portion. Depending on the d.c.-field strength, the magnetic domains in the core portion 21 can be aligned more or less in the xdirection. As long as the d.c.-field is considerably larger than the peak value of the a.c.-field (produced by the primary coil 2), the magnetic moments are mainly aligned in the direction of the d.c.- field. The closed magnetic core 25 shown in dashed lines is used to guide the d.c.-field in a well defined loop which crosses the main core 3 only once at the core portion 21. By aligning the magnetic moments in the core portion in the manner described, the permeability g of the core portion 21 is reduced and is considerably lower than that of the rest of the core. Thus the effective permeability g of the whole magnetic circuit (i.e. of the main core 3) is reduced. The reluctance is increased using a solid closed core without the need to use conventional air gaps in the core and core losses are reduced. By the use of d.c. coils, the field strength can be easily adjusted to provide a variable, adjustable reluctance (and inductance).

An alternative arrangement is shown in Fig. 5 which shows an alternative embodiment of an induction device 30 according to the invention. In the induction device 30, where similar reference numerals have been employed to identify similar parts, the electromagnetic coils 23 have dispensed with and the core portion 21 comprises a "hard" or "semi-hard" magnetic material with its anisotropy axis (see the double-headed arrow) transverse, preferably perpendicular, to the core direction of the core portion 21. The (semi) hard magnetic material is macroscopically demagnetised in order to minimise stray fields in the air around the core portion 21. For practical reasons, a demagnetised (semi) hard magnetic material is generally easier to handle.

preferably The induction devices of Figs. 4 and 5 may be modified to operate as transformers if a second winding is added. If such a second winding (not shown) is provided, it - 9 may, for example, be wound concentrically with the first winding.

Claims

- 10 CLAIMS

1. A high voltage induction device having a solid closed magnetic core and at least one winding wound on the closed magnetic core, characterised in that a portion of the closed magnetic core has its magnetic moments aligned in a direction transverse to the general direction of the said portion of the magnetic core.

2. A high voltage induction device according to claim 1, characterised in that the magnetic moments in the said portion of the closed magnetic core are aligned substantially perpendicular to the general magnetic path direction on either side of the said core portion.

3. A high voltage induction device according to claim 1 or 2, characterised in that electromagnetic means are arranged on either side of the said core portion, whereby the magnetic moments are aligned in said core portion by passing a d.c. current through said electromagnet means.

4. A high voltage induction device according to claim 3, characterised in that the said core portion is made of soft magnetic material.

5. A high voltage induction device according to claim 3 or 4, characterised in that the electromagnet means is formed by coils on either side of the core portion and a further closed electromagnetic core including the said core portion.

6. A high voltage induction device according to claim 1 or 2, characterised in that the said core portion comprises a hard, or semihard, magnetic material with its anisotropy axis transverse to, preferably substantially perpendicular to, the direction of said core portion.

7. A high voltage induction device according to claim 6, characterised in that the said hard or semi-hard magnetic material of said core portion is macroscopically demagnetised in order to minimise stray fields in the air 5 around said core portion.

8. A method of increasing the reluctance of the magnetic circuit of a high voltage induction device having core means with a closed magnetic core, characterised in that the method comprises aligning the magnetic moments of a portion of the closed magnetic core in a direction transverse to the direction of the said portion of the core.

9. A method according to claim 8, characterised in that the said portion of the core comprises a soft magnetic material the magnetic moments of which are aligned by passing a d.c. current through electromagnet means arranged on either side of the said core portion.

10. A method according to claim 8, characterised in that the said core portion comprises a hard, or semi-hard, magnetic material with its anisotropy axis transverse to, preferably substantially perpendicular to, the direction of said core portion.

11. A method according to claim 8, characterised in that the said core portion comprises a hard or semi-hard magnetic material with its anisotropy axis transverse to, preferably substantially perpendicular to, the direction of said core portion, and in that an additional magnetic field is induced in the direction of said magnetic anisotropy by passing a d.c. current through electromagnetic means arranged on either side of said core portion.

12. A method according to claim 10 or 11, characterised in that the said hard or semi-hard magnetic material of said core portion is macroscopically demagnetised in order to minimise stray fields in the air around said core portion.