WO2005045853A1

WO2005045853A1 - System for transmission of electric power

Info

Publication number: WO2005045853A1
Application number: PCT/SE2004/001615
Authority: WO
Inventors: Rongsheng Liu; Stefan Johansson; Björn HOLMGREN; Stefan Valdemarsson; David Larsson; Kenneth Johansson; Tord Bengtsson; Gerhard Brosig; Lin Jiang; Dierk Bormann
Original assignee: Abb Research Ltd.
Priority date: 2003-11-07
Filing date: 2004-11-05
Publication date: 2005-05-19
Also published as: WO2005045853B1; SE527008C2; SE0302966L; SE0302966D0

Abstract

The invention concerns a high voltage transmission wire or cable for transmitting AC or DC power between two points. Preferably at least one point is connected to one or more power networks. The conductor of the wire or cable comprises a semiconducting outer layer (10) including distributed magnetic particles. Other embodiments include an insulation layer (12) including distributed magnetic material, micro-particles, ribbons tapes etc, and an insulation screen. The invention increases the distance over which power may be transmitted and/or eliminates the necessity for power compensation reactors partially or totally. This is achieved by increasing inductance and reducing AC capacitance leakage currents in AC lines and, when operated as a DC line, by reducing high frequency harmonics.

Description

System for transmission of electric power

TECHNICAL FIELD.

The invention relates to transmission cable systems for power transmission. In particular, the wire or cable and system including it are particularly suitable for minimising power losses with long distance power transmission due to effects of reactive losses, capacitive charging losses and/or harmonic currents that are associated with power transmission, especially those within and between power networks.

BACKGROUND ART

Energy transmission by means of power cables is of particular importance applied in densely populated areas and when passing over stretches of open water. In densely populated areas land values, reliability and aesthetic factors have great importance whereas for passage over open water, the costs of building large number of pylon foundations is what steers the choice towards cable solutions. The problem with extending existing transmission cable installations is principally with generation and transport of reactive power. The risks of resonance problems for very long cable connections as a result of harmonics in the power network also needs to be reduced. Losses due to currents induced in the cable insulation will also affect the maximum transmission length for cable circuits . US 4,843,356 entitled; Electrical cable having improved signal transmission characteristics, assigned to Stanford University, describes a signal transmission line with improved signal characteristics. In this disclosure it is described that a low- loss inductance is may be achieved by inclusion of particulate magnetic material in an insulation layer surrounding the conductor. Although the disclosure is primarily directed to signal transmission, it is also stated that this may be applied to high voltage power transmission lines. However, it does not provide any information about reducing capacitive losses in AC power lines .

5 With even shorter transmission cable circuits for AC transmission of less than, say 50km, shunt compensation is used in order to compensate for the cable losses due to capacitive generation effects. Sometimes an additional dynamic compensation in the form of SVC, Static Var Compensation, is

.0 required. The shunt compensation devices are usually installed at both ends of the cable. There are also examples of installations where shunt compensation devices are installed at several places along the cable. AC transmission cables circuits longer than approximately 50 km or so only exist for low

L5 voltage levels (typically <100 kV) and low power (<100 MVA) . Because of these effects, high voltage direct current (HVDC) installations are today used almost exclusively for long power cable transmission circuits. However, for HVDC installations equipment such as reactors and capacitors are required to

.0 reduce power losses that would otherwise occur due to ripple voltages and harmonic currents.

SUMMARY OF THE INVENTION The present invention solves one or more of the above problems .

25 In a first aspect of the invention a power conductor is provided for a transmission cable system with a conductor wire or cable having a first outer layer with semiconducting properties and comprising distributed magnetic material with a

30 magnetic permeability greater than 1. The magnetic material comprises magnetic nano-particles . The combination of the conductor and the magnetic first outer layer provides for increased impedance in a line operating with AC power and reduce high frequency harmonics in a line operating with DC power, and increased tolerance of temporary power overloads under both AC and DC operation. In a second aspect of the invention a power conductor is provided for a transmission cable system with a conductor wire

5 or cable having a second outer layer comprising insulation material with magnetic material distributed in it such that the electrical permittivity of the second outer layer is greater than 1. The insulating material comprises magnetic nano- particles. The combination of the conductor with both the first0 and second outer layers provide for increased impedance and reduced capacitance losses in a line operating with AC power, and reduced high frequency harmonics for a line operating with DC power, and increased tolerance of temporary power overloads with both AC and DC operation.

.5 In another aspect of the invention, a power transmission system is provided that comprises one or more wires or cables in which a conductor has a first outer layer comprising magnetic nano- particles, providing a conductor material with increase

!0 magnetic permeability, and which may also have a second outer layer wherein an insulation material comprises magnetic nano- particles providing an insulation material with reduced electrical permittivity.

15 The invention increases cable inductance by adding magnetic particles or nano particles into a semiconducting conductor screen or conductor screen compound arranged as a first outer layer on one or more conductors . Thus a conductor is surrounded by a high permeability semiconducting layer, which increases 0 cable inductance and reduce reactive power losses due to wave impedance in the case of AC power lines, and reduce high frequency harmonics, especially for DC and air-cooled DC lines.

The invention applied to the insulation decreases cable 5 insulation permittivity of the second outer layer by adding magnetic nano-particles of controlled distribution, shape, size in insulators such as polyethylene (PE) , cross-linked polyethylene (XLPE) , ethylene-propylene rubber, (EPR) , ethylene-propylene-diene-monomer rubber (EPDM) , silicone rubber (SR) , Polyvinyl chloride (PVC) , Polypropylene laminated paper (PPLP) and celluloses. This reduces the capacitive leakage current, for AC cables. In the case of celluloses, permittivity reduction may also be obtained by adjusting the fibres in series with the gas (or oil) phase inside a composite.

The term cable system is used to designate one or several reaches of power cable, and any shunt reactors that may be connected at the joints between cable reaches. Shunt reactors at the cable terminal may or may not be included in the cable system. The transmission cable system described comprises also associated joints, terminals, breakers and protection devices.

However, as well as the resistive losses in power transmission, there are also considerable losses due to dielectric effects in the cable and to resistive losses in the compensation equipment, typically reactors. These losses can also be minimised according to the present invention.

A principal advantage of the invention is that minimal power losses due to increased inductance and reduced dielectric, capacitive and resistive losses mean that the length of an AC transmission cable reach with a conductor according to the invention is not limited to around 50 km or so but may in fact be several hundred kilometres in length.

Another advantage of the invention is that reactive power compensation by shunt reactors is not required to the same extent at the ends of an AC transmission circuit or, even more disadvantageously, at intervals along the length of a prior art circuit. Reduced requirement for reactive power shunt reactors is advantageous because these reactors also have power losses associated with them.

Yet another and perhaps unexpected advantage of the invention is that it may be used in a line under DC operation to reduce high frequency harmonics. This is accomplished by the magnetic characteristics of the first outer layer on the surface or the wire or cable conductor which layer acts as a filter and removes, or at least greatly reduces, harmonic currents in DC operation. As described briefly above, the magnetic first outer layer comprises magnetic nano-particles . The magnetic first outer layer may also comprise semiconducting material.

Another important result is that the reduced no-load losses will result in a slightly cooler cable or wire. This can either be used to reduce the specification and thereby material costs and manufacturing costs for the cable, or allow for temporary overload of the cable i.e. introduce a temperature dependent dynamic rating. Thus the thermal overload capacity of the cable or wire in a transmission circuit described is greater than for Prior Art cable systems. The conductor and system, and the method for operation of such a system permits greater freedom in running under temporary overloads for AC or DC lines to ease problems in a power network.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the method and system of the present invention may be had by reference to the following detailed description when taken in conjunction with the accompanying drawings wherein:

Figure 1 shows schematically a simplified diagram an example of an HV AC cable transmission circuit including with reactors for compensating capacitive power losses according to the Prior Art; Figure 2 shows a simplified diagram for an example of a conductor having a magnetic outer layer with semiconducting properties according to an embodiment of the invention;

Figure 3 shows a simplified diagram for an example of a conductor insulated with insulation containing magnetic material according to another embodiment of the invention;

Figure 4 shows a simplified schematic of a power transmission line;

Figure 5 shows a simplified diagram of current flow direction for the insulated conductor shown in Figure 3 ;

Figure 6 shows a simplified diagram for an example of a cable including the magnetically screened conductor insulated with insulation containing magnetic material, which insulation is in turn enclosed by a semiconducting insulation screen according to a yet further embodiment of the invention.

Figure 7 shows schematically a simplified diagram for an example of a conductor arranged with an insulation containing magnetic rings or areas according to another embodiment of the invention;

Figure 8 shows a diagram for an example of a cable including the magnetically screened conductor insulated with insulation containing magnetic material, which insulation is in turn enclosed by a current carrying and mechanical shield according to another further embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Figure 1 (Prior Art) a HVAC transmission cable system 1 is shown according to the Prior Art. A transmission circuit is ₍ arranged between two points A, B, including an AC cable 1 and two shunt reactors 2_A, 2_B for compensation of reactive power.

In the Prior Art arrangement of Figure 1 the problem of reactive power losses is overcome by using the shunt reactors to prevent losses due to transport of reactive power. For longer transmission lines, for example lines longer than 50 km, the problem with reactive power losses in the Prior Art is usually only overcome by rectifying from AC to DC current. Thus a HVDC system is most often used for such lines because of the high costs in terms of equipment and power losses associated with reactive power compensation methods of the Prior Art.

AC power transmission equipment is simple compared to DC transmission equipment. However, with increase of the transmission length, cost of AC transmission may increase compared to a DC one. Furthermore, capacitive leakage current increases with transmission length for an AC case, and inductive compensation may be needed since LG < RC, where L is inductance, G is insulation conductance, C is capacitance and R conductor resistance per unit length; and the equation shows the most important parameters of a transmission (cable) line. It follows from the above equation that it is beneficial that L inductance should be increased, and/or C capacitance should be decreased. It is possible to minimize reactive power losses in AC transmission by using an effect or a phenomenon known as the Surge Impedance loading or Natural Load for a transmission conductor, which is defined and may be expressed as:

P natural - where V is voltage and Z_v is (the real part of) the surge impedance. This load level is especially beneficial where the transmission cable consumes the same amount of reactive power per unit length as it generates . Reactive power therefore does not need to be transmitted in any direction. However, a problem with existing transmission cables is that wave impedance is relatively low which gives a high natural load in relation to the cable diameters that are practical or preferred for manufacturing reasons. Therefore it is possible and would be preferable to influence the natural load level of cables by adjusting the wave impedance of the cables to a favourable value relative to the design operating voltage.

The present invention provides a new cable or cable system to offer favorable conditions for a long distance power transmission by: 1) increasing inductance of at least one cable or wire in a system for the purpose of inductive compensation, and 2) decreasing permittivity of the cable or wire insulation and so reducing capacitive leakage current (Fig 2, 3) .

A cable or wire to achieve the above results may be provided in the following ways, which, as the man skilled in the art knows are exemplary and not in any way limiting: 1) A high inductance value can best be obtained by adding magnetic nanoparticles (coated or non-coated magnetic nano- particles) containing elements or compounds such as Fe₃0₄, γFe₂0₃, FeO, Cr0₂, Fe₃S₄, EuO, NiZn-ferrite, MnZn-ferrite, Yttrium-iron garnet, and Indium into a cable conductor screen or screen compound. The conductor, usually an alloy of Al or Cu is by this means surrounded by a high permeability (>1) and, to some extent semiconducting first outer layer. The conductor screen containing distributed magnetic nano-particles also provides an increased thermal conductivity in the conductor screen, and thus a longer life and/or higher power transmission capacity is to be expected.

2) A low permittivity insulation is be obtained by adding nano- 5 particles in a controlled way to insulation material such as PE, XLPE, EPR, EPDM, SR, PVC, PPLP, and impregnated celluloses. For PPLP and cellulose insulation, low permittivity can also be obtained by adjusting the fibres in series (mainly) with gas (or oil) phase inside the composite. The nano-particles are LO distributed in a controlled way so that factors such as concentration and distribution are known and optimised. The above inventive features can even offer benefits for DC cable systems.

L5 Figure 2 shows a conductor 11 in cross section with a first outer layer 10 containing magnetic material arranged abutting the surface of the conductor. The conductor 11 may be any conductor, or superconductor, but will most commonly be an electrical grade alloy of copper or aluminium. The first outer

20 layer 10 comprises magnetic nanoparticles (which may be coated or non-coated particles) containing any of elements or compounds such as Fe₃0₄, 7Fe₂0₃, FeO, Cr0₂, Fe₃S , EuO, NiZn- ferrite, MnZn-ferrite, Yttrium-iron garnet, and Indium. These nano-particles, preferably mixed with larger magnetic

25 particles, may be distributed in a medium arranged in the form of the first outer layer as a conductor screen 11. The conductor screen may be provided as a semiconducting material, with an electrical resistivity of between 1 and 10⁵ ohm-cm, or a composite comprising the nano particles, and may also comprise

30 magnetic ribbons, wires or tapes. The conductor screen described provides a magnetic permeability greater than 1, or that of air. Preferably the magnetic permeability is between 3 and 10,000 and, for AC operation, preferably between 3 and 500. As the man skilled in the art knows, the skin effect observed in conductors means that the AC electrical current conducted does not flow in such a way that it is evenly distributed across the cross section of the conductor, as with DC currents,

5 but instead a disproportionately greater part of the current flows through the conductor cross section close to the skin. This also applies to the high frequency harmonic currents that arise under DC power transmission; so that the close proximity, i.e. contact between the conductor screen and the conductor

.0 surface makes this aspect of the invention very effective.

The thickness of the conductor screen may be chosen dependent on a number of technical and manufacturing considerations for each type of aerial or buried cable to carry a given AC or DC

L5 power transmission load. A magnetic layer may be wrapped around a conductor or a conductor comprising a solid conductor or a quantity of strands of conductor wires or a combination of large diameter conductors and stranded conductors . The magnetic layer may consist of wires, tapes or ribbons comprising

20 distributed magnetic nano-particles. The magnetic wires or ribbons may further be wrapped in such a way that the direction of the wrap is perpendicular to the axial direction of the conductor, (see Figure 5, where the axial direction is in the same direction as arrow 19) . The wrapped magnetic wires or

25 ribbons may or may not be coated with an enamel to act as an insulator at the outer surface.

Power losses due to high frequency harmonics is a particular problem for long distance DC lines. This problem may also be 30 overcome by use of a conductor with a conductor screen according to the present invention because these harmonic currents are largely conducted along the magnetic material enclosing the conductor. The design of the magnetic conductor screen may be varied to include wires, tapes and/or ribbons (eg Fig 5, 10a) wrapped around the conductor in their wrap direction (14, Fig 5) . The length of the magnetic wires or ribbons conducting the harmonic currents may then be increased, compared to the length of the conductor, by up to around two 5 orders of magnitude. Thus due in part to a lower conductivity of the magnetic material and wires or ribbons, and in part to a greater conduction length for the wires, tapes or ribbons, this layer has a greater electrical resistance than the conductor. The comparative increase in length also gives a strong benefit

LO in damping ripple voltages, which may be demonstrated according to the equation

where U(x) is the voltage at a location distance x from the reference point; L5 U(0) is the voltage at the reference point 0 is an amplitude attenuation coefficient β is a phase shift coefficient e is the exponent and j represents an imaginary part. 20 Another benefit of the conductor screen containing magnetic nano-particles is that as it is a very efficient conductor screen in relation to higher frequencies, EMI (electromagnetic interference) and RFI (radio frequency interference) so that 25 such disturbances affecting electronic equipment are greatly suppressed.

Figure 3 shows an arrangement in which the conductor 11 in cross section, with the first outer layer 10 containing

30 magnetic material arranged on the surface of the conductor, is arranged with a second outer layer 12 around the first layer 10. The second outer layer comprises an insulation material which contains a distributed magnetic material such that the insulation has a reduced value of electrical permittivity. The

35 low permittivity insulation is be obtained by adding magnetic nano-particles to PE, XLPE, EPR, EPDM, SR, PVC, PPLP, and impregnated celluloses. For PPLP and cellulose insulation, low permittivity can also be obtained by adjusting the fibres in series (mainly) with gas (or oil) phase inside the composite. The nano-particles are distributed in a controlled way so that factors such as: concentration, distribution, size, size distribution and shape (e.g. an ellipsoidal, hollow or solid cylindrical, rectangular, flat shape and so on) . Carbon nano- tubes are for example produced in magnetic forms and semiconducting forms which may be cylindrical in shape.

The distribution of nano-particles may be random, homogenous, or homogenous in certain parts of the cross section or material of the second outer layer. The insulation layer formed by the second outer layer 12 has a decreased electrical permittivity of typically being a value between 1 and 8 which results in reduced power losses due to capacitive leakage currents, particularly under AC operation.

Figure 4 shows a schematic for a system according to the invention. The figure shows an in-current Iχ and voltage Ui and an out-current I₂, voltage U₂. Voltages Ui and U₂ are the voltages between the line and ground at those points . The line resistance 15 is indicated by resistance R, and line inductance 16 by inductance symbol L. Line capacitance 17 between the line and ground 20 is indicated by the capacitor symbol C, and the insulation conductance 18 is indicated by the resistance symbol G. It can be seen from Figure 4 that a leakage current will go through the capacitor C, which will limit the length of the power transmission; but with increased inductance 16, compensation of the reactor power is obtained, and thus transmission over a longer distance is possible. Figure 5 shows in a simplified diagram the embodiment of Figure 3 arranged as a cross section across the axial direction of the conductor and as along the axial direction. The magnetic semiconducting layer 10 may be in the form of magnetic wires or 5 magnetic ribbons 10a. The direction of wrapping of a magnetic ribbon or wires or tapes are shown by the arrow 14 : this is in relation to the direction of current flow along the axial direction of the conductor shown by arrow 19. The formula:

L0 S = ExH where S is the Poynting vector E is electric field and H is magnetic field

L5 shows how and in which direction energy is transmitted along a transmission line.

Figure 6 shows schematically a further embodiment of the invention. A conductor, essentially the same as conductor 11 in

20 Figure 3, but in this example a stranded conductor 11a is shown enclosed in a first outer semiconducting layer 10 with magnetic material distributed in it. The conductor and conductor screen are enclosed by a second outer layer, an insulator 12 containing distributed magnetic material . In the embodiment

25 shown, an third outer layer in the form of a semiconductor layer 13 with magnetic material distributed in it is placed around the insulator layer 12. A wire screen 15 is also included and enclosed by an inner cable sheath 17a and an outer cable sheath 17b in the cable construction shown.

30 As described above the semiconducting conductor screen 10, 10a provides increased inductance for an AC operation and reduced high frequency harmonics under DC operation and the low permittivity insulation 12 provides for reduced capacitance

35 losses under AC operation. The provision of the insulator screen, the semiconductor layer 13 with magnetic properties has the effect of increasing the inductance, in a similar way as layer 10, and it also provides a means to effect a permanent ground on the exterior of the cable.

Wave impedance (sometimes referred to as surge impedance) of a cable ( Z ) may also be defined as : _z= \ R + jaL G + jωC where L is inductance (should be increased) , C is capacitance (should be decreased) ,

G and R are the insulation conductance and conductor resistance per unit length.

The embodiment of Figure 6 provides a wave impedance value Z that is greater than that of transmission conductors and cables of the prior art, and is preferably between 10 and 1000 ohm.

Another important result is that the reduced power losses and an increased thermal conductivity (due to the inclusion of magnetic particles in 10, 12) will result in a slightly cooler cable or wire. This can be either be used to allow for temporary overload of the cable i.e. introduce a temperature dependent dynamic rating, or instead to reduce the specification and thereby material costs and manufacturing costs for the cable system.

As the man skilled in the art knows, other considerations and parameters may also be taken into account in the design and construction or conductor wires and transmission cables. The cable system may comprise standard equipment for AC over- voltage protection and shielding. This may include for example transposings and sheath sectionalizing insulators fitted to the cables to reduce shield induced currents. Similarly, to guard against known disturbances in long AC circuits such as overtones the system may be equipped with a high frequency filter such as for frequencies of around 100 Hz or higher.

Other configurations for the magnetic first outer layer comprising distributed magnetic nano-particles are possible. A conductor screen comprising iron or steel wires may be extruded together with, or wrapped in, a mixture of semi-conducting material and magnetic nano-particles, see Figure 5.

For example, a magnetic shield may be arranged around an outer semiconducting layer of a cable to increase the series inductance of the cable under AC operation. Figure 8 shows a cable comprising a conductor 11, a semiconducting first layer with magnetic nano-particles 10 and an insulator 12', an outer semiconducting layer 13' (insulation screen), a magnetic shield 83, and an outer mechanical and current carrying shield 85. Conductor 11 may be stranded as shown or one single conductor. Soft magnetic material in outer magnetic shield 83 may include iron powder. However closed loops of that material must be avoided otherwise heating will occur. Insulated laminations or strips may be wound around the cable to form the outer magnetic shield 83 and create a large number of isolated airgaps around the cable. In this embodiment, the magnetic energy is stored mostly in those airgaps. Figure 8 shows a twisted ribbon structure. Dependant on cable specification and design choices, the magnetic shield layer may also be used to carry currents and mechanical loads, and thus completely replace the traditional shield. Conductor insulation layer 12' and insulation outer semiconducting layer 13' may or may not comprise magnetic nano-particles, dependant on various design choices and combinations to achieve a high series reactance and low power losses due reactive power transport, line to ground capacitance and/or high frequency harmonics. Figure 7 shows an additional embodiment in which the wave impedance of the wire or cable is changed in discrete amounts in one or more places along the length of the line. The figure shows a cable with a conductor 11, a first outer layer 10, and 5 an insulation layer 12 that includes distributed magnetic particles. An incoming transient 71 is shown, and the spike after smoothing 72. Magnetic rings 73, 75, are shown. Incoming high transients such as 72 on a cable for example connecting a switchyard and an overhead transmission line can be damped by

.0 means of a series of sudden, discrete changes of the wave impedance. The local, discrete wave impedance changes are introduced in one or more of several ways : magnetic rings with airgaps, ferrite rings or and/or by introducing magnetic (nano-)particles into an outer semiconductor (for example into

L5 or around layer 13 of figure 6) .

By adjusting the distances and the relative wave impedance changes of the 'rings' we can tune the properties of what is in effect a filter. The rings are tuned with magnetic material 20 properties and the induced airgap. The distance between the magnetic areas or rings, ie between 73 and 75, depends on the wavelength of the expected transients, and maximum damping is obtained when the distance is around one-quarter or the wavelength, ^■ . The cable must be at least 1 km long before the 5 damping effects are significant due to the low frequencies involved. This is particularly advantageous for reducing transient stress for example, on transformers. It is not limited to use with overhead lines subject to lightening strikes but may also be used for buried lines when the risk of 0 transients and/or transients transmitted into a buried section of the line exists.

In another embodiment a wire or cable according to the invention may be used as DC high voltage transmission line, 5 HVDC. In the embodiment of an air-cooled transmission line, no significant second outer insulation layer is present. In relation to Figure 2, a conductor wire or bus with a conductive metallic alloy 10 in cross section may be arranged with the first outer layer 11. Arranging the first outer layer, which has a higher electrical resistivity than the conductor, on the outside surface of the conductor, reduces those harmonics, acting in a way as a filter or high frequency filter, as described above. Such a conductor for DC operation may also be provided with an insulation layer, which may be a plain insulator, or an insulator with magnetic properties, or semiconducting properties, or both.

It should be noted that while the above describes exemplifying embodiments of the invention, there are several variations and modifications which may be made to the disclosed solution without departing from the scope of the present invention as defined in the appended claims .

Claims

1. A power conductor for transmitting HV electric power between two points (A, B) at least one of which is connected to one or 5 more power networks, characterised in that said conductor comprises a first outer layer (10) that has magnetic properties and that comprises a magnetic permeability of greater than 1.

2. A power conductor according to claim 1, characterised in LO that said conductor comprises a second outer layer (12) with insulating properties .

3. A power conductor according to claim 2 , characterised in that the second outer layer (12) has magnetic properties and

L5 comprises an electrical permittivity of between 1 and 8.

4. A power conductor according to claim 3 , characterised in that the first outer layer (10) abuts conductor 11 and comprises nano-particles including any element from the list

20 of; iron, chromium, nickel, zinc, manganese, yttrium, indium.

5. A power conductor according to claim 1 , characterised in that the first outer layer has a magnetic permeability of between 3 and 10,000 and most preferably for AC operation

25 between 3 and 500.

6. A power conductor according to claim 5, characterised in that the first outer layer has properties of a semiconductor including an electrical resistivity of between 1 and 10⁵ ohm-cm.

30 7. A power conductor according to claim 2 , characterised in that the first outer layer and the second outer layer each comprise micro-particles with a particle size of between 1 nm and 50μm any of which may be coated or uncoated.

35

8. A power conductor according to any previous claim, characterised in that said conductor comprises a first outer layer with magnetic permeability greater than 1 and a second outer layer with electric permittivity greater than 1.

9. A power conductor according to claim 8, characterised in that the first outer layer and the second outer layer comprise micro-particles formed with a shape of any from the list of: spherical, ellipsoidal, elongated, plate-like, cylindrical.

LO 10. A power conductor according to claim 9, characterised in that the micro-particles comprise nano-particles with a size of between lnm and 500nm.

L5 11. A power conductor according to claim 10, characterised in that the micro-particles comprise particles with a size of between 0.1 and 50μm.

12. A power conductor according to any of claims 7-11,

20 characterised in that the micro-particles comprise particles with a plurality of sizes of between lnm and 50μm.

13. A power conductor according to claims 7, 10-12, characterised in that the micro-particles and nano-particles

25 are arranged distributed in the first outer layer and/or the second outer layer in a substantially homogenous distribution.

14. A power conductor according to claims 7, 10-12, characterised in that the micro-particles and nano-particles

30 are arranged homogenously distributed at each level of depth of the first and/or second outer layers and that the concentration of micro-particles and nano-particles may vary at different depths in said layers .

15. A power conductor according to any of claims 2-14, characterised by a third outer layer (13, 13') enclosing the second and insulating layer (12) which comprises a semiconducting material with distributed magnetic particles, which layer forms an insulation screen.

16. A power conductor according to any of claims 10-15, characterised by magnetic means (73, 75) for smoothing voltage transients (71) .

17. A power conductor according to claim 16, characterised by magnetic areas or rings (73, 75) in or on an insulating layer (12, 12') or semiconducting layer (13, 13') arranged to form areas of discrete changes in wave impedance for smoothing voltage transients.

18. A power conductor according to claim 16, characterised in that discrete magnetic areas or rings (73, 75) are arranged separated by a distance dependent on a wavelength of a transient.

19. A power conductor according to any of claims 8 to 15, characterised in that the first outer layer (10, 10a) comprises magnetic material in a linear form and is arranged wrapped around a conductor (11, 11a) of said power conductor.

20. A power conductor according to claim 19, characterised in that the first outer layer (10, 10a) is arranged to have a greater length than the length of the conductor (11, 11a) over a given length of said power conductor.

21. A power conductor according to claim 19, characterised in that the first outer layer (10, 10a) wrapped radially around a conductor (11, 11a) of said power conductor in a direction (14) dependent on the axial direction (19) of power transmission.

22. A power conductor according to claim 19, characterised by an outer magnetic shield (83) comprising one or more airgaps.

5 23. A system for high voltage transmission (1) transmitting electric power between two points (A, B) at least one of which is connected to one or more power networks, characterised in that at least one cable or wire of said system is arranged with a first outer layer (10) that has magnetic properties and that LO comprises a magnetic permeability of greater than 1.

24. A system according to claim 23, characterised in that the at least one cable or wire comprises magnetic nano-particles distributed in a layer (10) surrounding a conductor (11, 11a) ,

L5 whereby the transmitted power includes a reduced proportion of reactive power and capacitive losses under AC operation and reduced high frequency harmonics under DC operation.

25. A system according to claim 23, characterised in that at 20 least one cable or wire for transmission of said system is arranged with a second and insulating outer layer (12, 12') comprising distributed magnetic nano-particles, thereby having an insulator with decreased electrical permittivity, whereby the transmitted AC power comprises reduced reactive power and 25 minimized losses due to capacitive leakage currents.

26. A system according to claim 23, characterised in that impedance of said conductor is increased close to that of a wave impedance value in order to give reduced transport of

30 reactive power in the cable lines under AC operation.

27. A system according to claim 23, characterised in that at least one cable or wire for transmission of said system is arranged with a first outer layer (10, 10a) comprising magnetic material and wrapped around a conductor (11, 11a) and having a length greater than the conductor (11, 11a) .

28. A system according to claim 23, characterised in that at 5 least one cable or wire for transmission of said system is arranged with one or more series of pluralities of magnetic areas (73, 75) in which the magnetic areas or rings are separated by a distance dependent on a transient wavelength.

0 29. A system according to claim 23, characterised in that said power conductor has a first outer layer (10, 10a) arranged to reduce high frequency harmonics under DC operation.

30. A system according to claim 23, characterised in that one .5 or more AC transmission cables may be an oil and paper insulated cable.

31. A system according to claim 23, characterised in that said one or more AC or DC transmission cables may be insulated with

!0 a polymer, elastomer or other plastic-based material.

32. Use of a system for high voltage transmission (1) for transmitting power between two points (A, B) according to claims 23-31 as a power feeder for large, densely populated

25 urban or suburban areas .

33. Use of a system for high voltage transmission (1) for transmitting power over a distance between two points (A, B) according to claims 23-31 in which a part of the distance is 0 across water.

34. A power conductor for transmitting HV electric power between two points (A, B) at least one of which is connected to one or more power networks, characterised in that said 5 conductor comprises a first outer layer (10) that has magnetic properties and that comprises a magnetic permeability of greater than 1, and a second outer layer (12) with insulating properties .

35. A system for high voltage transmission (1) transmitting electric power between two points (A, B) at least one of which is connected to one or more power networks, characterised in that at least one cable or wire of said system is arranged with a first outer layer (10) that has magnetic properties and that comprises a magnetic permeability of greater than 1, and a second outer layer (12) with insulating properties.