The present invention relates to a high-voltage line conductor for overhead lines for voltages of approx¬ imately 60 kV and higher.

The high-voltage overhead lines hitherto employed in which the voltage over the phase conductors exceeds about 20 kV comprise bare, i.e. substantially unsheathed, conductors. Therefore the conductors must be installed on pole constructions such as to allow sufficient spacing between the conductors to prevent hitting together of the conductors.

On the other hand, PAS lines - which are overhead conductors provided with a simple plastic sheathing - are employed in the voltage range of 20 kV to replace bare conductors. The insulator is often cross-linked poly¬ ethylene XLPE (PEX) . The insulation is dimensioned to withstand voltage stresses resulting from hitting together of the conductors, but not to insulate the conductor com¬ pletely as in cables. Often a PAS conductor provides an alternative to the much costlier underground cable solu¬ tion.

Insulations or sheathings are not used in con- ductors for voltages in excess of about 60 kV and particu¬ larly not in high-voltage conductors for 110 kV and higher, since the known insulation layers employed in overhead lines should have considerable thickness to pro¬ vide sufficient insulation. Therefore, the pole construc- tions, insulators and insulating and conductor fittings are mainly designed for bare conductors.

Increasingly during the past decade, attention has been paid to the electric and magnetic fields gener¬ ated by power lines and their possible effects on the health of those living nearby. Limits for the magnetic and

electric fields of power lines have already been specified in some states of the U.S.A. and in Italy.

Electric and magnetic fields produced by overhead lines can be affected by the relative location of the con- ductors in the cross-directional plane. Disposing the con¬ ductors as close as possible to one another, for example at the apices of an equilateral triangle, affords minimum fields to be obtained. Also, conductors brought closer to one another require narrower conductor streets, and this would afford savings at least in land acquisition. How¬ ever, the minimum spacings required for high-voltage bare conductors to prevent short circuit, flash-over and corona effects have in practice prevented any substantial reduc¬ tion of the electric and magnetic fields in conductor streets and in their immediate vicinity.

It is an object of the present invention to pro¬ vide a high-voltage conductor wherewith overhead lines that are suitable for narrower conductor streets and gen¬ erate smaller electric and magnetic fields can be real- ized. For achieving this object, the high-voltage conduc¬ tor of the invention is characterized in that the conduc¬ tor is covered with an insulation and shielded against spark-over generated by contact with another conductor, the insulator coating on the conductor comprising a semi- conductive layer enveloping the conductor, an outermost surface insulation layer, and an actual insulation layer therebetween.

With suitable selection of the insulation layer in the conductor of the invention and with considerable work invested in different tests for determining the safety and strength of conductors, a thin high-voltage conductor that is economical to manufacture has unexpec¬ tedly been achieved. This conductor solves the majority of the problems associated with power lines of conventional construction. By using the conductor of the invention, the

conductor streets can be diminished from the present width of about 15 m to about half (vertical setout) with volt¬ ages of e.g. 110 kV, and for instance the strength of the magnetic fields is considerably diminished from the field strength of the current lines.

Other preferred embodiments of the invention are characterized in that which is set forth in the ensuing claims.

In the following the invention will be described in greater detail by means of examples with reference to the accompanying figure illustrating a conductor of the invention.

Figure 1 shows an example of a 110 kV conductor comprising a circular conductor of aluminium alloy, stranded of wires 3 and having a diameter of about 20 mm. Ingress of water between the layers of the conductor has been prevented for example by fat or a hygroexpansible powder. Conductor sheathing 4 is of a semi-conductive plastic or rubber material. The semi-conductivity has normally been achieved by doping a cross-linkable insu¬ lator material with about 30-40% carbon black (when insu¬ lator carbon black is used, semi-conductivity can be achieved with as little as 10% doping). In a 110 kV insu¬ lation-coated conductor, the conductor sheathing layer is about 1-2 mm in thickness. The purpose of the conductor sheathing is to neutralize voltage peaks on the uneven surface of the conductor stranded of metal wires and to prevent creation of discharge sites.

The actual insulating layer 1 surrounding the semi-conductive layer 4 is of high-purity XLPE plastic of a thickness of for instance about 5 mm for 110 kV voltage. High purity is required to minimize the risk of spark-over through the insulating material at high voltages. Cross- linked polyethylene is mainly used on account of its high degree of purity, heat resistivity, strength and insulat-

ing properties. The outer layer is an insulating layer of about 1.5 mm doped for instance with carbon black to achieve weatherproo ness. The content of carbon black is preferably 2-3%, which ensures sufficient shielding prop- erties for example against UV radiation, yet not imparting too high conductivity to the surface layer of the conduc¬ tor.

Also ethylene propylene gum, i.e. EP gum (EDPM or EPR), may be used as raw material instead of a cross- linkable polyethylene (XLPE, PEX).

Further data for the exemplary 110 kV conductor of the invention: outer diameter about 39 mm, mass 1730 kg/km, breaking load 108 kN and load rating 660 A. In addition to alternating current lines, the conductor of the invention can equally well be employed for direct current power transmission, in which case the three phase conductors are replaced for example by two conductors (current + earth) .

The conductor of the invention is thus sheathed with an insulating layer that is far thinner than in a normal cable construction. The insulation layer is spe¬ cifically dimensioned to withstand hitting together of the phase conductors within the span. Hence no attempt is made to insulate the conductor completely with an insulating layer, but leakage currents of the order of milliamperes exist at the outer layer, and therefore it is highly dan¬ gerous to touch for example a 110 kV conductor with one's bare hands.

In tests in which a voltage of 120 kV was applied between two conductors, the conductors were hit together 540 000 times without spark-over. Furthermore, a leaning test of 17 days was performed on the same conductors; in this test the conductors leaned against one another with¬ out spark-over. Thus the conductors of the invention are permitted to hit one another for example by the action of

short-circuit forces or the wind. The necessary minimum spacings of the conductors must thus be calculated on other grounds applicable to the situation than on the criteria for a case of short circuit. It has proved poss- ible to reduce the spacings between 110 kV phase conduc¬ tors from the 2 m spacings used at present to about half of said distance.

At any rate, it has proved that shielded conduc¬ tors of the invention enable considerable reduction of phase spacings and cutback in conductor streets. A conse¬ quence of this is the smallness of the electric and mag¬ netic fields generated by the shielded-conductor line in comparison with conventional lines.

CONDUCTOR §„ 0.1 uT 0.2 uT

Bare horiz. 1 33 23

Bare vert. 0.68 33 21

Bare triang. 0.45 25 16 Shielded triang. 0.39 21 13

Shielded horiz. 0.33 16 10

Shielded vert. 0.32 18 11

Shielded delta 0.21 13 6 B _max = relative maximum value of flux density of magnetic field

0.1 μT and 0.2 μT = flux density decreases to this level at the distance indicated from the centre of the line.

TABLE 1

Table 1 shows curves illustrating the magnetic flux densities of a high-voltage overhead line on the ground for different conductor types. The comparison included a conventional non-insulated line with horizon¬ tal, triangular and vertical configurations with standard 2 m phase spacings, and a shielded PAS line according to the invention with horizontal, vertical, triangular and delta configurations with phase spacings of 1.15 m. The

basic data for the measuring results shown in Table 1 are as follows:

- U = 123 kV

- Load current 100 A, P = 18 MW - shielded conductor: SAX 355, σ ₀ = 40 N/mm ²

- Bare conductor: Duck, σ ₀ = 40 N/mm ²

- Lightning conductor: Sustrong, σ ₀ = 60 N/mm ²

- Temperature of current conductor +15°C

- Temperature of lightning conductor +5°C - Clearance of lowermost current conductor from earth 5.9 m at +70°C (permitted minimum height)

- Span a _e = a = 200 m

In terms of the magnetic field, the following observations can be made on the basis of Table 1:

- when the setout of the conductors is horizon¬ tal, the maximum value of flux density in a PAS line de¬ creases to about one third compared with a corresponding non-insulated line. The flux density decreases to the level of background radiation (=0.1 μT) with clearings of 33 m and 16 m respectively from the centre of the line.

- with vertical setout of the conductors, the maximum value of the flux density of a PAS line decreases to about one half compared with a regular line. The flux density decreases to the level of background radiation with clearings of 33 m and 18 m respectively from the centre of the line.

- with triangular setout of the conductors, the maximum value of the flux density of the line does not differ to any appreciable extent from the value for a regular line. However, the flux density decreases with PAS conductors to the level of background radiation with a clearing of 21 m from the centre of the line, while in the case of a corresponding non-insulated line a clearing of 25 m is needed. This rather minor difference is due to the

fact that other factors than the spacing of the conduc¬ tors, for example the free clearance, determine the loca¬ tion of the conductors. Thus the construction is roughly the same with both conductors. However, with a PAS line reduction to one half of the peak value of the electric field strength was found in the measurements.

- delta setout (equilateral triangle) of a PAS line is clearly the best solution in view of the fields generated. Compared with a regular non-insulated horizon- tal tower line, the maximum value for the flux density is only about one fifth, and the flux density decreases to the level of background radiation already at a distance of 13 metres from the line.

A conductor according to the invention can be manufactured by known methods without any significant changes to insulating lines manufacturing underground cables, for instance. By the triple extrusion technique, all coating layers on the conductor can be produced in one step. It is obvious to one skilled in the art that the different embodiments of the invention are not limited to the examples presented above, but can vary freely within the scope of the ensuing claims.