EP1131829A1

EP1131829A1 - A switchable flux control for high power static electromagnetic devices

Info

Publication number: EP1131829A1
Application number: EP98950250A
Authority: EP
Inventors: Christian Sasse; Gunnar Russberg; Mats Leijon; Udo Fromm; Par Holmberg
Original assignee: ABB AB
Current assignee: ABB AB
Priority date: 1998-09-29
Filing date: 1998-09-29
Publication date: 2001-09-12
Also published as: AU9640198A; CN1327601A; JP2002526912A; WO2000019459A1

Abstract

A high power static electromagnetic device with a flux path, a main winding and one or more regulation windings surrounding portions of the flux path. A control device is coupled to the flux path for selectively admitting the flux therein. In an exemplary embodiment, multiple flux paths are selectively turned on and off for including and excluding the regulation windings from the circuit. The windings may be formed of a magnetically permeable, field-confining insulating cable.

Description

A SWITCHABLE FLUX CONTROL FOR HIGH POWER STATIC

ELECTROMAGNETIC DEVICES

BACKGROUND OF THE INVENTION

The present invention relates to a selectively controllable high power static

electromagnetic device, and in particular to a controllable high power transformer, reactor,

inductance, or regulator with switchable step function selectively. As used herein the high

power devices include those having a rated power ranging from a few hundred kVA up to

more than 1000 MVA with a rated voltage ranging from 3-4 kV and up to very high

transmission voltages, 400 kV to 800 kV or higher.

In the transmission and distribution of electric energy, various known static inductive

devices such as transformers, reactors, regulators and the like are used. The purpose of such

devices is to allow exchange or control of electric energy in and between two or more electric

systems. Such devices belong to an electrical product group known as static inductive

devices. Energy transfer is achieved by electromagnetic induction. There are a great number

of textbooks, patents and articles which describe the theory, operation and manufacture of

such devices and associated systems, and a detailed discussion is not necessary.

Conventional electric high voltage control is generally achieved by transformers

having one or more windings wound on one or more legs of the transformer core. The

windings often include taps making it possible to supply different voltage levels from the

transformer. Known power transformers and distribution transformers used in high voltage trunk lines involve tap-changers for the voltage regulation. These are mechanically

complicated and are subject to mechanical wear and electrophysical erosion due to discharges

between contacts.

SUMMARY OF THE INVENTION

The invention provides a high power static electromagnetic or induction device with a

rated power ranging from a few hundred kVA up to over 1000 MVA with a rated voltage

ranging from 3-4 kV and up to very high transmission voltages, such as 400 kV to 800 kV or

higher, and which does not entail the disadvantages, problems and limitations which are

associated with the prior art power devices.

The invention is based on the discovery that selective switchable control of the flux

paths in the device enables broad control functions not hereinbefore available.

In a particular embodiment the invention comprises a high power static induction

device having a flux bearing path, a main winding and a at least one regulation winding in

operative relation therewith. A control in operative relationship with the flux bearing region

selectively admits or blocks flux therein. The control may be in the form of a switchable

conductive ring having one or more turns. At least one of the windings is formed of one or

more current-carrying conductors surrounded by a magnetically permeable, electric field

confining insulating cover.

In a particular exemplary embodiment, the cover comprises a solid insulation

surrounded by an outer and an inner potential-equalizing layer being partially conductive or

having semiconducting properties. The electric conductor is located within the inner layer. As a result the electric field is confined within the winding. The electric conductor, according

to the invention, is arranged so that it has conducting contact with the inner semiconducting

layer. As a result no harmful potential differences arise in the boundary layer between the

innermost part of the solid insulation and the surrounding inner semiconductor along the

length of the conductor.

According to an exemplary embodiment of the invention, the device has a flux

bearing region and a control in operative relationship therewith for selectively admitting or

blocking the flux there through for regulating the device. In a transformer having a plurality

of legs or flux paths in the flux bearing region, the flux may be selectively admitted or

blocked in each of said plurality of the legs so that various voltage outputs may be achieved.

In a reactor, selective control of the flux in the core results in a switchable flux bearing region

in the reactor. In a regulator, switchable voltage control is achieved. Depending on the type

of control used, regulation may be in discrete steps corresponding to discrete or selective

opening or closing of flux paths.

The invention employs windings having semiconducting layers which exhibit similar

thermal properties to the solid insulation as regards the coefficient of thermal expansion. The

semiconducting layers according to the invention may be integrated with the solid insulation

so that these layers and the adjoining insulation exhibit similar thermal properties to ensure

good contact independently of the variations in temperature which arise in the line at different

loads. At temperature gradients the insulating layer and semiconducting layers form a

monolithic core for the conduction and defects caused by different temperature expansion in the insulation and the surrounding layers do not arise.

The electric load on the material is reduced because the semiconducting layers form

equipotential surfaces and the electric field in the insulating part is distributed nearly

uniformly over the thickness of the insulation.

In particular, the outer semiconducting layer exhibits such electrical properties that

potential equalization along the conductor is achieved. The semiconducting layer does not,

however, exhibit such conductivity properties that the induced current causes an unwanted

thermal load. Further, the conductive properties of the layer are sufficient result in that an

equipotential surface. Exemplary thereof, the resistivity, p, of the semiconducting layer

generally exhibits a minimum value, pmin = 1 Ωcm, and a maximum value, pmax = 100

kΩcm, and, in addition, the resistance of the semiconducting layer per unit of length in the

axial extent, R, of the cable generally exhibits a minimum value R_min = 50 Ω/m and a

maximum value R_max = 50 MΩ/m.

The inner semiconducting layer exhibits sufficient electrical conductivity in order for

it to function in a potential-equalizing manner and hence equalizing with respect to the

electric field outside the inner layer. In this connection the inner layer has such properties

that any irregularities in the surface of the conductor are equalized, and the inner layer forms

an equipotential surface with a high surface finish at the boundary layer with the solid

insulation. The layer may, as such, be formed with a varying thickness but to ensure an even

surface with respect to the conductor and the solid insulation, its thickness is generally

between 0.5 and 1 mm. However, the inner layer does not exhibit such a great conductivity that it contributes to induce voltages. Exemplary thereof, for the inner semiconducting layer,

thus, Pmin = 10"⁶ Ωcm, R_min = 50 μΩ/m and, in a corresponding way, Pmax = 100 kΩcm,

R_max = 5 MΩ/m.

In an exemplary embodiment, a transformer according to the invention operates as a

series element with selectable leakage inductance and thus reactance. Such a transformer is

capable of controlling power flow by redistribution of active or reactive effects between

networks connected to the primary and secondary. Such a transformer is capable of limiting

short circuit currents, and provides for good transient stability. The transformer is also

capable of damping power oscillations and providing good voltage stability.

The present invention, allows for a flexible AC transmission system with control of

the components wherein the power flow can be controlled. In the particular embodiment, the

ability to control or regulate power flow is implemented in a component which is normally

needed for other purposes. Thus, the invention allows for dual use without significant

increase in cost.

In accordance with another embodiment of the invention, a reactor may be switchably

operable either as a series or shunt element with selectable inductance and thus reactance.

There is no need for power electronics in the main power circuit. Accordingly, losses are

lower. Further, the control equipment is generally low voltage equipment and thus, simpler

and more economical. The arrangement also avoids the problem of harmonics generation.

As a shunt element, the reactor can perform fast variable reactive power compensation. As a

series element, the reactor is capable of performing power flow control by redistribution of active or reactive effect between lines. The reactor can limit short circuit currents, provide

transient stability, damp power oscillations and provide voltage stability. These features are

likewise important for flexible AC transmission systems.

The drawbacks of prior art voltage regulation are avoided by a switchable voltage

regulator according to the invention, wherein the magnetic circuit of the regulator includes at

least one regulation leg having a flux bearing region switchable between open and closed

states, and by at least one regulation winding wound around said regulation leg, said

regulation winding being connected to the main winding. It is also possible to place at least

one winding loaded with a variable capacity on at least one magnetic flux path or leg having a

zone with reduced permeability across the magnetic flux, to vary the reluctance of the leg by

varying the impedance.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described with reference to the accompanying drawings,

wherein

Fig. 1 shows the electric field distribution around a winding of a conventional

inductive device such as a power transformer or reactor;

Fig. 2 shows an embodiment of a winding in the form of a cable in a high power

inductive device according to the invention;

Fig. 3 shows an embodiment of a power transformer according to the invention;

Fig. 3A illustrates a magnetic switch in accordance with the invention;

Fig. 3B shows an open and closed flux path corresponding to open and closed magnetic switches;

Fig. 4 is a schematic illustration of a regulation leg portion of the transformer of Fig.

3;

Fig. 5 is a schematic illustration of a reactor in accordance with the present invention;

Figs. 6A and 6B are respective, perspective and sectional schematic illustrations of a

device in accordance with an embodiment of the present invention;

Figs. 7A and 7B are respective, perspective and sectional schematic illustrations of a

device in accordance with another embodiment of the invention;

Figs. 8 A and 8B are respective, perspective and sectional schematic illustrations of a device in accordance with yet another embodiment of the invention; and

Fig. 9 is a schematic illustration of a three phase transformer according to the

invention.

DESCRIPTION OF THE INVENTION

The inventive concept which forms the basis of the present invention is applicable to

various static inductive devices including, power transformers, reactors and regulators. As is

known, the devices herein categorized may be designed as single-phase and three-phase

systems. Also, air-insulated and oil-insulated, self-cooled, oil cooled, etc., devices are

available. Although devices have one or more windings (per phase) and may be designed

both with and without an iron core, the description generally shows devices with an iron core

having a selectable region of variable high reluctance.

The invention further relates more specifically to a controllable inductance wherein the magnetic flux is selectively redistributed among and between different flux paths by

affecting the reluctance of at least one of such paths. In a reactor the invention operates as a

series or shunt element with a selectable variable inductance.

Fig. 1 shows a simplified and fundamental view of the electric field distribution

around a winding of a conventional static induction device such as a power

transformer/reactor 1, including a winding 2 and a core 3. Equipotential lines 4 show 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 a window 4.

The potential distribution determines the composition of the insulation system since it

is necessary to have sufficient insulation both between adjacent turns of the winding and

between each turn and earth. 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 are in this

way determined substantially by the electric field distribution in the core window 4.

Fig. 2 shows an example of an exemplary cable 5 which may be used in windings

which are included in high power inductive devices according to the invention. Such a cable

5 comprises at least one conductor 6 including a number of strands 6A with a covering 7

surrounding the conductor. The covering includes an inner semiconducting layer 8 disposed

around the strands. Outside of this inner semiconducting layer is the main insulation layer 9

of the cable in the form of a solid insulation, and surrounding this solid insulation is an outer

semiconducting layer 10. The cable 5 may be provided with other additional layers for

special purposes, for example for preventing too high electric stresses on other regions of the device. From the point of view of geometrical dimension, the cables 5 in question will

generally have a conductor area which is between about 30 and 3000 mm² and an outer cable

diameter which is between about 20 and 250 mm.

Fig. 3 shows a high power inductive device in the form of a single phase core type

transformer 11 in accordance with the present invention. The transformer 11 comprises a

core 12 which is formed with main or outer legs 14,16 and short or inner legs 18 and 20, and

respective lower, middle and upper arms 22, 24 and 26. The core 12 may be made of

laminated iron sheets having a main or large aperture or window 28 and a plurality of small or

regulation windows 30-1, 30-2 and 30-m, in a regulation region 32 located generally between

the middle and upper arms 24 and 26 as shown. In the exemplary embodiment, m=3.

In order to form a core type transformer, a primary winding 34 is wrapped around the

leg 14. In a similar manner, a secondary winding 36 may be wrapped concentrically with the

primary winding 34 around the leg 14 or on another leg. A regulation winding 37 formed of

one or more regulation sub-windings or coils 38-1..., 38-n in series of the primary winding 34

may be wrapped around the respective inner legs 18 and 20 as shown.

Control means in the form of one or more conductive short circuit rings 40-1..., 40-n

may be located as shown. For example, rings 40-1, 40-2 and 40-3 surround the middle arm

24 and extend through the windows 28 and 30-1, 30-2 and 30-m respectively. In the similar

manner rings 40-4, 40-5 and 40-n surround the upper arm 26 in the windows 30-1, 30-2 and

30-m respectively. It should be understood that the suffix 1, 2, 3, m and n are used to

designate the position of the corresponding element, and are otherwise not used when the position is not relevant to the discussion.

In the exemplary embodiment, and as shown in Fig. 3 A, ring 40 comprises one or

more turns of a conductor 42, e.g. copper terminated such as switch 44. When the switch 44

is closed the corresponding ring forms a short circuit. In other embodiments, the control may

be an active or passive filter, a reactance in voltage or current supply.

The windings 34, 36 and 38 produce the flux φ, which is carried by the core 12 along

one or more possible alternative paths as shown by the dotted lines in each of the legs 14, 16,

18, 20 and the arms 22, 24 and 26. In a device 46 shown in Fig. 3B, when any switch 44 of a

corresponding ring 40 is open, the corresponding flux path through the leg or arm of the core,

as the case may be, surrounded by ring is open. Likewise, when a switch 44 is closed, the

flux path through the core, at that point, is blocked.

Fig. 4 is a fragmentary portion of the regulation region 32 of the transformer 11

shown in Fig. 3, illustrating in greater detail stepwise magnetic flux regulation according to

the invention. In the exemplary embodiment of Fig. 3, the magnetically regulated

transformer 11 has the low voltage (LV) winding 34 (N_LV turns), the high voltage (HV)

winding 36 ^ turns) and the at least one additional regulation (R) winding 37 (N_R0 turns)

in series with the LV winding 34. Voltage regulation is then obtained by changing the

transformer ratio N_HV/^_LV+N_R), where N_R is an effective number of regulation turns. N_R can

be varied over some subinterval of [-N_R + N_R] by actively linking the main magnetic flux

through different parts of the regulation windings. The linking is performed with an

arrangement of switchable magnetic rings 40 in the core 12, each of which should as completely as possible exclude the flux from a selected region of the core, or admit the flux

through with a minimum of reluctance. In the regulation winding 37 the separate subcoils 38-

1..., 38-n (n=2) are wound in series through the windows 30-1..., 30-m (m=3) in the

regulation or upper portion 32 of the core 12.

The principle of the invention illustrated in Fig. 4 shows that magnetic switching is

achieved with the short circuit rings 40, which, when switched closed, block the passage of

flux through the corresponding sub-coil 38. Likewise, when opening, the rings 40 admit the

flux φ into the core segment and direct it through or past the subcoils. Depending on the

arrangement, flux control occurs in a number of ways, each representing a single

noncirculating path through the regulation region 32 and a unique value of N_R. In the

example of Fig. 4, N_R = 1-3 = -2. The regulation region 32 is dimensioned for maximum flux

along any allowed path. Accordingly, the regulation region 32 is at least twice the size of a

conventional core without regulation.

In accordance with another embodiment of the invention, a reactor 60 is shown in Fig.

5. The reactor 60 has a main flux path 62 and a regulating flux path 64. A main winding 66

in the main path 62 is in series with a regulating winding 68 in the regulating path 64. A

magnetic contact switch 70 is in the regulating path 64 as shown. When closed, the magnetic

switch 70 blocks the regulating path 64, and when open the magnetic switch 70 opens the

magnetic path. An additional winding 72 and a magnetic switch 74 may be added to the main

path, as shown, so that more complex regulation of the reactor 60 may be provided.

Figs. 6A - 6B; 7A - 7B; and 8A - 8B illustrate the regulation portion 70 of a transformer, reactor or regulator, as the case may be, depending on the application. The

regulation winding 72 having N_R=4 turns is divided into spatially well separated subcoils 74-

1..., 74-n having Nl ...n terms where Nl=3 and n=l . Regulation is achieved by linking the

magnetic flux past or through each such sub-coil 74 to omit, add, or subtract its

corresponding number of turns, n_i5 to the total number of regulation turns, N_R.

Three regulation winding arrangements of interest can be identified and are named

after the first three elements in the sequence of subcoil turn rations: 1 :2:4, 1 :3:7, and 1 :3:9,

respectively. The arrangements are restricted to a construction with 2x4 magnetic switches.

Each of these arrangements is illustrated in Figs. 6 A - 6B; 7 A - 7B; and 8 A - 8B respectively

as follows.

Figs. 6 A - 6B illustrate a 1 :2:4 arrangement. The winding 72 in the form of a cable

discussed above in Fig. 2 is wound around a common axis A_ppl parallel to the direction of the

main magnetic flux φ and with one magnetic switch 40 -1 A in 40 NA inside each sub-coil

74-1 in 74-n and one switch 40- IB in 40 NB outside each coil. The number of turns is

doubled for each coil in the sequence, i.e., = 2-^_1n,, i = 1,2,3,..., n] = 1,2,3,... The magnetic

flux can pass through a coil in just one direction. Accordingly, turns can be omitted or added,

but not subtracted. The number of switches 40 required is 2m, where m is the number of

subcoils, and the number of possible regulation levels in 2^m. Figs. 2 A, 6 A - 6B show sixteen

possible values of N_r:

0,1,2,3 (=2+1), 4,5 (=4+1),..., 15 (=8+4+2+1).

Figs. 7A - 7B illustrate a 1 :3:9 arrangement. The cable is wound around alternate legs 90-1..., 90-n with axes AP, perpendicular to the main magnetic flux direction. Every second

leg 50-2..., 50-(N-l) is left unwound as a bridge between the upper and the lower horizontal

part of the core. The number of turns is tripled for each sub-coil 74-1..., 74-n in the sequence;

n_; = 3'"'n,. Switches 40-1A, 40-1B..., 40-NA, 40-NB are positioned on the sides of each leg

so that the flux ma be linked past or in both directions through a sub-coil 38-1... 38-n. The

number of switches required is 4m and the number of possible regulation levels is 3^m. Figs.

7 A - 7B show an example with nine possible values of N_R:

.4 (=-3-1), -3, -2 (=-3+1), -1, 0, 1, 2 (=3-1), 3, 4(=3+l).

Figs. 8A - 8B illustrate a 1 :3:7 arrangement. The cable is wound around legs 94-1...,

94-n with axes AP perpendicular to the main magnetic flux direction. In contrast to the 1 :3:9

case above all legs 94-1... 94-n are wound. The number of turns is approximately doubled for

each sub-coil 38 in the sequence; n_; = (2^*-l)n,. Switches 40-1 A, 40-1B..., 40-NA, 40-NB are

positioned on the sides of each leg so that the flux may be linked past or in both directions

through sub-coil 5 74-1..., 74-n, with the restriction than in a sequence of incorporated coils,

turns are added with alternating sign. The number of switches required is 2m+2 and the

number of possible regulation levels is 2^ra+1-l. Figs. 8A - 8B show an example with fifteen

possible values of N_R:

_7,_6 (=-7+1), -5 (=-7+3-1), -4 (=-7+3), -3 -2 (=-3+1), -1,0,1,2 (=3-l)J,4 (=7-3),

5(=7-3+l), 6 (=7-l),7.

Thus, in accordance with the invention, a selectable static induction device has been

provided in which one or more magnetic switches selectively open and close flux paths in the device. It should be understood that in addition to the short circuit rings described, providing

a step function like flux response, variable impedances of various kinds may be used. For

example, if a variable inductor is used to load a ring 40, the reluctance varies inversely with

the inductance. Thus, high inductive loading will result in a corresponding high flux

distribution in the leg. If a variable capacitance is used, reluctance varies directly. If a

variable or high resistance is used as a load for the ring 40, a variable or high flux distribution

results in the leg. If the ring is shorted, the effect is as described in that the flux will be

blocked. Various combinations of fixed and variable, real and reactive loading may also be

provided. In addition, loading or activation may be provided by an active element, for

example, an active filter. Such a filter could be programmable.

It is also possible to provide a variable power source, e.g., a voltage or current source

to produce an input on the ring which is adapted to modulate the flux in the leg. Modulation

may be in terms of amplitude, phase and frequency. It is also possible to provide an active

filter to load the ring to thereby vary the performance of the ring and thus modulate the device

output.

Fig. 9 illustrates another embodiment of the invention wherein a three phase

transformer 100 of a shell or core type having a main winding 102 and a regulation winding

104 for each phase wrapped on a core 106 is illustrated. The various flux paths are shown in

dotted line in the legs 108, 110 and 112 and the yokes 114, 116 and 118. According to the

invention, a one or more magnetic switches 120 may be employed as hereinabove described.

In the exemplary embodiment shown, switches 120 are located in yokes 114 and 116 to control the flux through the regulation windings 104. The windings may be in series or shunt

as may be the flux bearing paths.

While there have been provided what are considered to be exemplary embodiments of

the invention, it will be apparent to those skilled in the art that various changes and

modifications therein may be made without departing from the invention, and it is intended in

the appended claims to cover such changes and modifications as fall within the true spirit and

scope of the invention.

Claims

I claim:

1. A static high power electromagnetic device comprising:

at least one main winding for producing a flux when energized comprising at

least one current-carrying conductor and a magnetically permeable, electric field confining,

insulating covering surrounding the conductor;

at least one regulation winding in operative relationship with the main

winding;

a flux bearing region for the main winding and the at least one regulation

winding; and

control means in operative relationship with the flux bearing region for

selectively admitting the flux in the flux bearing region.

2. The electromagnetic device according to claim 1, wherein the control means is

operable in first and second states, said first state is operative for admitting flux in the flux

bearing region and the second state is operative for blocking flux in the flux bearing region.

3. The electromagnetic device according to claim 1 , wherein the control means

includes switching means for operating the control means in the first and second states.

4. The electromagnetic device according to claim 1, wherein the control means

comprises a winding having terminals and at least one turn surrounding the flux bearing

region, and a switch coupled to the terminals for opening and closing the winding.

5. The electromagnetic device according to claim 1, wherein the control means

comprises at least one conductive mean surrounding the flux bearing region and means for switching the ring into and out of operative relationship therewith for selectively blocking

and admitting the flux therein.

6. The electromagnetic device according to claim 1 , wherein the flux bearing region

comprises at least two selectable flux paths.

7. The electromagnetic device according to claim 1 , wherein the flux bearing region

comprises a main flux path for the main winding and at least one selectable flux path in

operative relation with said at least one regulation winding.

8. The electromagnetic device according to claim 1, wherein the flux bearing region

comprises a main flux path for the main winding and a selectable flux path for each

regulation winding.

9. The electromagnetic device according to claim 1, wherein the at least one

regulation winding includes a plurality of subwindings, and the flux bearing region comprises

a main flux path for the main winding and a selectable flux path for each subwinding.

10. The electromagnetic device according to claim 9, wherein the subwinding

comprises windings having turns in at least one of a ration of 1 :2:4; 1 :3 :7; and 1 :3 :9.

11. The electromagnetic device according to claim 1 , wherein the flux bearing region

includes a main flux path for the main winding having a main flux direction at least one

selectable flux path having an orientation in at least one of a direction perpendicular and

parallel to the main flux path.

12. The electromagnetic device according to claim 11, wherein the at least one flux

path has a selectable orientation.

13. A device according to claim 1, wherein the covering comprises at least one solid

insulating layer surrounding the conductor and at least one partially conductive layer

surrounding the conductor.

14. The device according to claim 1, further wherein the flux bearing region is

magnetizable and is in operative relationship with the main winding and the regulation

winding.

15. A device according to claim 1, wherein the magnetizable flux bearing region in

operative relationship with the main winding and the regulation winding includes at least one

of a shell and core.

16. A device according to claim 1, further including a selectable region of relatively

high reluctance in the flux bearing region in operative relationship with at least one of the

main winding and the regulation winding.

17. A device according to claim 1, wherein the main winding and the at least one

regulation winding are in at least one of a shunt and series relationship.

18. A device according to claim 1, including a magnetic circuit having at least one of

serial and parallel paths and wherein the at least one regulation winding is located in at least

one of said serial and parallel paths.

19. The device according to claim 1, wherein the control means comprises at least

one of active and passive impedances.

20. The device of claim 19, wherein the impedances comprise a reactive impedance.

21. The device according to claim 19, wherein the impedance comprises a real impedance including at least one of an open circuit, a short circuit, and a resistance in

operative relationship with the at least one regulation.

22. The device according to claim 1, wherein the main winding comprises a flexible

cable.

23. A device according to claim 1, wherein the covering comprises an inner layer

surrounding the conductor having semiconducting properties; a solid insulating layer

surrounding the inner layer; and an outer layer having semiconducting properties surrounding

the insulating layer.

24. A device according to claim 23, wherein the inner layer is in electrical contact

with the conductor and is operative at the same potential thereof.

25. A device according to claim 23, wherein the outer layer comprises an

equipotential surface surrounding the insulating layer.

26. A device according to claim 23, wherein the outer layer is connectable to at least

one selectable potential.

27. A device according to claim 26, wherein the selected potential is ground.

28. The device according to claim 26, wherein at least one of said semiconducting

layers has substantially the same coefficient of thermal expansion as the insulating layer.

29. A device according to claim 26, wherein the cover is substantially void free.

30. A device according to claim 26, wherein each semiconducting layer has a contact

surface in confronting relationship with the corresponding surfaces of the insulating layer and

wherein said contacting surfaces are joined therealong.

31. A device according to claim 26, wherein the covering is formed of at least one

polymeric material.

32. A device according to claim 1, wherein the main winding comprises a

transmission line call.

33. A device according to claim 32, wherein the cable is manufactured with a

conductor area which is between about 30 and 300 mm² and with an outer cable diameter

which is between about 20 and 250 mm.

34. A device according to claim 1, wherein the covering comprises an extruded solid

insulation.

35. A device according to claim 1, wherein the at least one current-carrying conductor

comprises at least one insulated strand and at least one uninsulated strand.

36. A device according to claim 35, wherein the at least one uninsulated strand is

arranged in electrical contact with the covering.

37. A device according to claim 1, wherein the flux bearing region includes a zone of

reduced permeability comprising at least one of an air gap and a conductive element and solid

inserts of a material with low permeability.

38. A device according to claim 37, wherein said zone of reduced permeability

comprises cavities formed in said conductive element.

39. A device according to claim 1, including a core comprising a main leg and at least

two sub-legs, at least one of the sub-legs forming a leg for the regulation winding.

40. A device according to claim 1, including a core comprising a main leg and at least two sub-legs.

41. A device according to claim 1, wherein said device comprises a multiphase

transformer having a regulation leg in each phase, wherein the at least one regulation winding

includes at least one winding for each regulation leg and being connected for having joint

regulation.

42. A device according to claim 1, wherein said device comprises at least one of an

autotransformer and a booster transformer.

43. A high power variable inductance device comprising:

a magnetic circuit including a flux path;

a main winding surrounding the flux path;

at least one regulation winding surrounding the flux path; and

magnetic switch means in operative relationship with the flux path operable

when energized, for selectively varying the flux in the flux path between open and closed

states.

44. The device of claim 43, wherein the switch means comprises at least one

conductive turn surrounding the flux path and a switch for opening and closing the turn.

45. The device of claim 44, wherein the control means includes an impedance

comprising at least one of a reactive and real impedance.

46. The device of claim 45, wherein the reactive impedance includes at least one of a

capacitive and inductive load.

47. The device of claim 45, wherein the impedance is variable.

48. The device of claim 45, wherein the impedance is a short circuit.

49. The device of claim 43, wherein the switch means includes at least one of an

active and passive filter.

50. The device of claim 43, wherein the switch means includes a power source

including means for varying at least one of the amplitude, frequency and phase of the flux in

the flux path.

51. A high power variable inductance device comprising:

a magnetic circuit including a flux path having selectively variable flux

bearing properties; at least one main winding in operative relation with the flux path;

at least one regulation winding surrounding the flux path; and

control means coupled to the flux path operable when activated, for selectively

varying the flux in the flux path region.

52. The device according to claim 51 , wherein at least one of the windings comprises

a current-carrying conductor and a magnetically permeable field-confining insulating cover.

53. The device of claim 51, wherein the flux path includes spacer means in the flux

path.

54. The device according to claim 51, wherein the control means comprises a power

source for producing at least one of amplitude, phase and frequency modulation for the

regulation winding.

55. The device according to claim 51, wherein the flux path comprises a plurality of selectable flux bearing regions.

56. The device according to claim 55, wherein the control means includes switch

means for selectively varying the flux between for respective on and off states.

57. The device according to claim 55, wherein the switch means includes a switch for

controlling the flux in each regulation winding.