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Title:
CONTROLLABLE INDUCTOR
Document Type and Number:
WIPO Patent Application WO/1994/011891
Kind Code:
A1
Abstract:
The invention relates to an inductor with variable inductance based on the principle of orthogonal magnetization, that is, control of the permeability of the magnetic material with the aid of a cross-directional control field. The inductor has a main winding (2) which is intended for alternating current and which surrounds a tubular core (3) wound from tape-formed magnetic material. The core is composed of a plurality of ring cores (31) which are stacked coaxially one above the other and which are axially divided and angularly displaced relative to each other. Axially through the core extends a control winding (4) intended for direct current. By changing the current in the control winding, the permeability of the core is changed in its axial direction and hence also the inductance of the inductor. The eddy-current losses in the core can be considerably reduced by making the core conically tapering towards its ends or by arranging so-called flux funnels (8) with radially bladed laminations near the ends of the core. The inductor is especially suitable for use in power plants, for example in tunable harmonic filters for high-voltage direct current.

Inventors:
VALDEMARSSON STEFAN (SE)
LILJESTRAND LARS (SE)
Application Number:
PCT/SE1993/000946
Publication Date:
May 26, 1994
Filing Date:
November 09, 1993
Export Citation:
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Assignee:
ASEA BROWN BOVERI (SE)
VALDEMARSSON STEFAN (SE)
LILJESTRAND LARS (SE)
International Classes:
H01F29/14; (IPC1-7): H01F29/14
Foreign References:
DE911293C1954-05-13
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Claims:
CLAIMS
1. A controllable inductor comprising at least one tubular core (3, 6) wound from tapeformed magnetic material and composed of a plurality of ring cores (31) stacked coaxially on top of each other, a main winding (2) which surrounds the core (3, 6), and a control winding (4) which passes axially through the core, wherein the two end portions of the core (3, 6) or any magnetic fieldcontrolling members (8), arranged near the end portions, are designed such that the magnetic flux in the core, generated by the main winding (2), is at least approximately axially directed in the length of the entire core, characterized in that the ring cores (31) are axially divided and arranged such that the parting planes (33) of adjoining ring cores are angularly displaced relative to each other.
2. An inductor according to claim 1, characterized in that the ring cores (31) are funnelshaped.
3. An inductor according to claim 1 or 2, characterized in that the angular displacement between the parting planes (33) of adjoining ring cores (31) is greater than 5 degrees.
4. An inductor according to claim 1, 2 or 3, characterized in that the core (3) is tapering towards its two ends.
5. An inductor according to claim 4, characterized in that the tapering of the iron core (3) towards the ends thereof is brought about by stacking ring cores of different diame¬ ters on top of each other.
6. An inductor according to claim 4, characterized in that the iron core (3) is conically tapering towards its two ends, preferably with increasing conicity in a direction towards the ends. SUBSTITUTESHEET .
7. An inductor according to claim 6, characterized in that the tapering of the iron core (3) towards the ends thereof is brought about by turning.
8. An inductor according to claim 1, 2 or 3, characterized in that said magnetic fieldcontrolling members (8) consist of stacks of sheet which are arranged axially outside and close to each one of the two end portions of the core (6), the laminations of the sheet stack being perpendicular to the end surfaces of the core and being directed substan¬ tially radially in relation to the centre line of the core.
9. An inductor according to claim 1, 2 or 3, characterized in that said magnetic fieldcontrolling members consist of shield members which surround at least the end portions of the core and which consist of several turns of a tape of electrically conducting material, wound one above the other, with an insulating film arranged at least on one side of the tape.
10. An inductor according to any of the preceding claims, characterized in that it comprises two cores of said kind, arranged adjacent to each other, through which the control winding passes.
11. The use of a controllable inductor according to any of the preceding claims in a harmonic filter for highvoltage direct current or highvoltage alternating current.
Description:
Controllable inductor

TECHNICAL FIELD

The present invention relates to a variable inductor, that is, a reactor with variable inductance, of the kind desc¬ ribed in the preamble to claim 1. The inductor has a main winding intended for alternating current and surrounding at least one elongated tubular core of magnetic material. Through the axial channel of the tubular core extends a control winding intended for direct current. Inductors of this type, which have an open (i.e. not closed) core of magnetic material, the permeability of which is controlled with the aid of a transversely directed control field, so- called cross-magnetization, are particularly suitable for use in tunable harmonic filter's for both high-voltage direct current and high-voltage alternating current, but also other applications are feasible, for example in static reactive power compensators.

BACKGROUND ART

Controllable inductors, in which the permeability of the magnetic material of the inductor is controlled with the aid of cross-magnetization, are previously known from, for example, EP-C- 0 010 502 and SU-A- 678 542. These inductors are designed with a closed iron core, of which, however, only a small part is influenced by the control field gene- • rated by the d.c. winding. This results in poor utilization of the number of ampere turns of the a.c. winding, since a reduction of the hysteresis losses and the permeability of the core, which is of importance for the energy storage capacity, is achieved only in that part of the core which is being influenced by both the alternating and the direct field. This means that the known inductors are heavy and bulky in relation to their energy storage capacity. Furthermore, in prior art designs the magnetic cores are sometimes designed such that "sly paths" for the alternating

flux are created, which may, inter alia, result in distor¬ tion in the a.c. curve because of superposition of the direct field.

From DE-C-911 293 an inductor which is controllable by cross-magnetization is previously known, the inductor being provided with a hollow cylindrical open core, through which the control winding passes. For the purpose of reducing the eddy-current losses, the core is designed with conically tapering end portions. To be able to achieve a sufficiently strong cross-magnetization using a moderate control current, the control winding must be designed with a large number of turns. This entails difficulties in the manufacture since, for each turn, the winding wire must be drawn through the axial hole of the core, which hole is relatively elongated and has a limited cross section.

SUMMARY OF THE INVENTION

The object of the invention is to provide a controllable inductor intended for the above purpose, which makes possible a high energy density, has low iron losses, and exhibits a linear ratio of the current to the voltage. As far as the above-mentioned application to tunable harmonic filters for HVDC is concerned, on the other hand, the demands for the maximum inductance variation of the inductor are relatively moderate, for example ±10%. In addition, it shall be possible to manufacture the inductor in a relati- • vely simple way. An inductor with these properties is achieved according to the invention with a design which has the characteristic features described in claim 1.

By composing the iron core of the inductor from several ring-cores stacked one above the other and being axially divided, the core can be mounted around a control winding which has been wound in advance. This greatly facilitates the manufacture of the inductor. In spite of the fact that the ring-cores are axially divided, a core with a high

mechanical stability can be obtained by angularly displacing the parting planes of adjoining ring-cores relative to each other.

According to a further development of the invention, the stability of the core can be further improved by designing the ring-cores funnel-shaped. This results in the ring-cores being automatically centred during mounting, and because the axial parting planes of adjoining ring-cores are angularly displaced relative to each other, the ring-cores are preven¬ ted from moving in relation to each other. By displacing the winding turns axially, during the winding of the cores, for achieving the funnel-like shape, the additional advantage is obtained that the risk of turns being subjected to a short circuit owing to burrs on the sheet tapes is reduced, which entails reduced eddy-current losses.

In an inductor according to the invention, the entire core will be under the influence of the cross-directional control field generated by the d.c. winding. This means that all the material in the core can be efficiently utilized for storage of magnetic energy. The reason is that the reactive power density (VA/kg) is inversely proportional to the relative permeability of the core material. By maintaining a high field strength (A/m) in the control field, a low permeabi¬ lity in the longitudinal direction of the core and hence a very high power density can be obtained; for example, in a control field of 10 5 A/m a power density of about 1000 VA/kg can be obtained at 50 Hz. Higher frequencies give proportio- nally higher power density. An additional advantage of a high control field strength is that the hysteresis losses in the core are practically eliminated.

With regard to the eddy-current losses, these can be consi- derably reduced according to the invention by making the core conically tapering towards its two ends, and preferably such that the conicity increases in a direction towards the ends. In this way, the alternating flux generated by the

main winding of the inductor may become axially directed, such that the field lines follow the sheet direction in the whole length of the core.

According to an alternative embodiment of the invention, the eddy-current losses can be reduced by arranging stacks of sheets, so-called flux funnels, near the ends of the core, the laminations of which are directed perpendicular to the end surfaces of the core and substantially radially in relation to the centre line of the core.

In an inductor according to the invention, the inductance variation, that is, the dynamics, which is achieved by changing the direct current in the control winding, will greatly depend on the length of the core in relation to its cross-section area and on the 'ratio of the area of the main winding to the area of the core. However, the dynamics will generally be low, but fully sufficient for the above- mentioned application in tunable harmonic filters for HVDC.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be explained in greater detail by desc¬ ription of embodiments with reference to the accompanying drawing, wherein

Figure 1 shows an axial section through an inductor designed according to the invention,

Figure 2 shows a cross section through the inductor along the line II-II in Figure 1,

Figure 3 shows a picture of the magnetic field at one end portion of the magnetic core of the inductor, pro- duced by computer calculation,

Figure 4 shows curves of the relative permeability of an inductor core of mu-metal as a function of a longi-

tudinal alternating field at different values of a cross-magnetizing direct field,

Figure 5 shows an axial section through a second embodiment of an inductor according to the invention,

Figure 6 shows an axial section through a third embodiment of an inductor according to the invention,

Figure 7 shows in side view an iron core provided with a control winding and included in the inductor accor¬ ding to Figure 6, and

Figure 8 shows in axial section two of the ring-cores of the inductor core in- an alternative embodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The inductor shown in Figures 1 and 2 has a main winding 2 which is wound around a tube 1 of insulating material and which coaxially surrounds a tubular iron core 3. The core is built up of a plurality of ring cores 31, stacked on top of each other, which are wound from sheet tapes the thickness of which may be, for example, 0.3 mm or smaller, depending on the frequencies for which the inductor is intended and the eddy-current losses associated therewith. The core mate¬ rial may, for example, be oriented transformer sheet, amorphous sheet, mu-metal, etc. A number of the ring cores located nearest the ends of the core are conically turned such that the core is continuously tapering towards the ends. Through the axial hole 32 of the core extends a con¬ trol winding 4 divided into several coils. The mounting of this control winding is facilitated by dividing the ring cores 31 axially along a parting plane 33 (Fig. 2) . The halves of the ring cores are retained by surrounding straps of insulating material, and the whole core is compressed in the axial direction by a number of insulated clamp bolts (not shown) which extend between two end rings 34, 35.

When a direct current I flows through the control winding 4, a circulating direct flux is generated in the ring cores, the permeability of the core in the axial direction thus being reduced. The main winding 2, which is connected into an a.c. circuit and generates an axially directed alterna¬ ting flux in the iron core 3, will thus change its induc¬ tance when the control current I is changed.

The tapering of the iron core towards its ends is adapted such that the alternating flux inside each ring core becomes axially directed, that is, along the direction of the sheet. Radial flux inside the core material has thus become elimi¬ nated, whereby the eddy-current losses are reduced conside¬ rably and a high factor of merit (Q-value) for the inductor can be achieved. Figure 3 shows the magnetic field lines 5 at one end portion of the inductor core 3 in an embodiment where the relative permeability of the core is μ r = 30. As is clear from the field picture, it is only in the very outermost part of the end portion that the field lines exhibit a radial directional component, which, however, because of the limited area, does not give rise to eddy- current losses of any practical importance.

Figure 4 shows how the relative permeability μ r of a core of the shown kind, made of mu-metal, varies as a function of a longitudinal alternating field H ac at different values of a transverse direct field Ha c varying between 0 and 42600 A/m.

The inductor shown in Figure 5 differs from that shown in Figure 1 in that it has two cores 3 of the shown kind arranged adjacent to each other, the control winding 4 passing through both of these cores. Since a greater part of the control winding is thereby surrounded by core material, the winding material is utilized in a better way, and it is sufficient with a lower control power.

Also the inductor shown in Figure 6 has two tubular cores 6 arranged adjacent to each other, through both of which the

control winding 4 passes, as is clear from Figure 7. Each such core is built up of a number of ring cores, which are stacked on top of each other into a suitable length. For the control winding 4 to be manufactured and mounted in a simple way in the axial channels of the cores, the ring cores are divided into two halves along an axial parting plane. The ring cores are retained by surrounding straps and axial clamp bolts in the same way as described with reference to Figure 1. When a direct current flows in the control winding 4, a circulating magnetic direct field is generated in the tubular cores. By varying this field from 1-10 6 A/m, the relative permeability in the longitudinal direction of the core may be changed from about 20000 to 2.

Figure 8 shows an alternative embodiment of the ring cores from which the iron core of the inductor is composed. The funnel-like shape of these cores can suitably be achieved by winding the sheet tape, of which the cores consist, on a cylindrical coil form which has a flange with a conical guide surface for the sheet tape. With ring cores of this design, the mounting of the inductor core is facilitated by the ring cores becoming self-centring and, in addition, fixed relative to each other, if the joint between the core halves is rotated through an angle of, for example, 10-90 degrees in relation to the preceding core. This makes the design more stable and less sensitive to mechanical stresses.

The core type shown in Figure 7, provided with its own d.c. winding, constitutes a core with a variable permeability, as described above. This core may constitute a construction part of its own, which may, for example, be built into an ordinary air coil to achieve a controllable inductor, for example of the design shown in Figure 6. The inductor according to Figure 6 has a main winding 2 which is wound on a plastic tube 1 and which surrounds the variable core 6, 4, which is provided with so-called flux funnels 8 at each end. When the permeability of the core is changed with the aid of

direct current in the control winding 4, the inductance of the coil 2 with the core 6 will vary.

The task of the flux funnels 8 is to control the magnetic alternating flux in the longitudinal direction of the core. By designing the flux funnels with a relatively large air area, all the flux will leave the core via the funnels. The flux funnels consist of stacks .of sheets, the laminations of which are perpendicular to the end surfaces of the core and are directed substantially radially in relation to the centre line of the core. The manufacture of these funnels may, for example, be performed in the manner described in SE-B- 324 404.

Another possibility of controlling the magnetic flux in the longitudinal direction of the 'core is to arrange an eddy current-conducting shield around the core, or, alternati¬ vely, two shields around each of the end portions of the core. Such a shield can suitably be built up of a plurality of turns of a tape of electrically conducting material, for example copper or aluminium, wound one above the other, with an insulating film arranged at least on one side of the tape. The shield may, for example, have a thickness of 5 mm and be wound from 0.2 mm thick tape.

The invention is not limited to the embodiments shown but several modifications are possible within the scope of the claims. In, for example, an embodiment where the ratio of the length of the iron core to the diameter thereof is con- siderably smaller than in the examples shown in Figures 1 and 5, it may be suitable to form the iron core with both ah outside diameter decreasing towards the ends and an inside diameter increasing towards its ends.

Further, it is not absolutely necessary that the current in the control winding be direct current, but in certain cases the control current may be alternating current as well.

As mentioned above, an inductor according to the invention is particularly suitable for use in harmonic filters in converter stations for high-voltage direct current (HVDC) . In this way, these filters can be made controllable and the filter tuning continuously adjustable according to the operating conditions prevailing. This makes possible a reduction of the cost of construction of the whole HVDC station.