WO2014055010A1

WO2014055010A1 - An overhead electric power cable

Info

Publication number: WO2014055010A1
Application number: PCT/SE2013/051091
Authority: WO
Inventors: Anders SÖDERMAN
Original assignee: Sandvik Intellectual Property AB
Current assignee: Sandvik Intellectual Property AB
Priority date: 2012-10-05
Filing date: 2013-09-18
Publication date: 2014-04-10
Anticipated expiration: 2015-04-05
Also published as: IN2014KN00981A; SE1251127A1; SE536835C2

Description

An overhead electric power cable

TECHNICAL FIELD

The present invention relates to an overhead electric power cable according to the preamble of claim 1 .

BACKGROUND ART

Overhead electric power cables, also called overhead electric conductors are used for conducting electric power from power plants over land or sea. The power cables typically consist of several conducting wires, such as aluminum wires, which surround a central steel wire core. The purpose of the steel wire core is to support the weak aluminum wires when the electric power cable is suspended in the air on between towers. A general problem with overhead electrical power cables is that the length of the power cable increases due to thermal expansion since heat is generated in the power cable when electric current is conducted there through. Thermal expansion may lead to sagging of the cable and in extreme cases cause the cable to touch the ground or nearby constructions.

Traditionally, carbon steel is used in the supporting wires of overhead electric power cables. Carbon steel is a material which is available at low cost and has sufficiently high strength and sufficiently low thermal expansion to be suitable as a supporting wire in overhead electric power cables. However, the thermal expansion coefficient of carbon steel, which lies in the range of 1 1 .5 - 12.5 x10^"6/°C, limits its use in power grids to maximum cable temperatures below 200 °C.

In recent development of power distribution the voltage of the electric power which is conducted in the cables is increased. This results in the generation of more heat in the power cable, typically up to 250 °C. The voltage and hence the temperature is believed to increase even further in the future. To meet these demands, the alloy Invar is used in the supporting wires. Invar has low thermal expansion coefficient, however due to its high nickel content (35-40%) Invar is a very expensive material. A further drawback with Invar™ is its relatively low strength, 30-35% of the strength of carbons steel. One problem related to both carbon steel and Invar is the low corrosion resistance of these materials. The low resistance to corrosion can to some extent be compensated with e.g. galvanization. However in certain environments such as coastal areas the corrosion is so high that the life length of Invar and carbon steel becomes unacceptable short. JP61266558 shows a further material which has been proposed in the past for use as reinforcement in electrical cables. However, this material has not sufficiently low thermal expansion to be used in modern power grids.

Hence, it is an object of the present invention to achieve an inexpensive overhead electric power cable which is corrosion resistant, has high strength and may be used at temperatures exceeding 200 °C.

SUMMARY OF THE INVENTION

This object is achieved by an overhead electric power cable (1 ) comprising conducting wires (2) and a supporting wire (3) characterized in that the supporting wire comprises a ferritic-austenitic steel alloy which essentially consists of 30 - 70 vol% ferrite and 30 - 70 vol% austenite, whereby the steel alloy has the following composition in percent by weight (wt%):

C: < 0.1

N: 0.1 - 0.5

Ni: 0.1 - 3 Cr: 18 - 30

Mn: 1 - 10 Si: < 2.0

Cu: < 3

Co: < 3

Mo: < 2

the balance Fe and normally occurring impurities.

The material that is used in the supporting wire of the inventive overhead cable is variant of so called duplex stainless steel alloy, i.e a stainless steel having a structure of both austenite and ferrite. The steel alloy in the supporting wire of the inventive power cable is known to have high strength, good ductility and very good resistance to corrosion. Due to these properties the steel alloy have found use as reinforcement in constructions, for example as supporting wires in bridges or load bearing elements in marine

constructions. However, until yet it has not been reported that this steel alloy have been used in applications where a low thermal expansion is important.

When the inventor performed measurements on the steel alloy above it was surprisingly discovered that the steels showed an unexpected low thermal expansion coefficient at high temperatures. The Coefficient of Thermal Expansion (CTE) was found to lie in the range of 9.2 - 9.6 x10^"6/°C in the temperature range of 200 - 300 °C. Expected values, based on other duplex steels, are approximately 1 1 .5 x10^"6/°C in same temperature range. Due to its low thermal expansion coefficient at high temperatures in combination with high strength, high torsion ductility and good corrosion resistance the steel alloy is very suitable as supporting wire in overhead electric power cables.

The reason for the low thermal expansion of the steel is not entirely understood.

However it is believed to at least in part depend on the balanced additions of

manganese and nitrogen in the steel. The low thermal expansion effect is extended by increasing the density of crystal defects, which is dependent on the degree of

deformation, the structure condition during deformation and the alloy composition.

Manganese and nitrogen increase the deformation hardening and thereby causing an increased density of crystal defects in the structure. The duplex matrix itself may also have an impact on thermal expansion, due to the extended amount of phase

boundaries.

The inventive overhead cable further provides the following advantages:

The good corrosion resistance ensures long lifetime of the cable in coastal areas. The steel alloy has a Critical Pitting Temperature (CPT) of 45 -60 °C (0.1 %Nacl) +300mV) which ensures sufficient corrosion resistance in all environments.

The high strength of the supporting wire makes it possible to use the inventive overhead cable in cold climates where ice formation could weigh down the cable to a point where it breaks. The wire has the following mechanical properties: Tensile strength, Rm = 1877 MPa, Yield strenght, Rp = 1416 MPa.

The wire further has a torsion ductility of 24 - 28 turns over a length of wire which is equal to the wire diameter x 100. This torsion ductility is very high in comparison with other materials. Torsion ductility is an important property of the wire since electric power cables are manufactured by twinning of several conductors in to a cable which may have a length of several hundred meters. According to international standards a material which is designated for use in cables must have a torsion ductility of at least 20 turns.

Due to the high strength, the diameter of the supporting wire can be reduced, this makes it possible to increase the number of alumina conductors in the inventive power cable with maintained total diameter of the cable. The advantage thereof is that the capacity of the cable can be increased without increasing the total diameter of the cable.

In particular the supporting steel wire may have a composition (in weigth%) of: C: 0.01 - 0.07; N: 0.1 - 0.3; Ni 1 - 3; Cr: 18 - 25; Mn: 2 -8; Si: 0.1 - 1 ; Mo: <1 .0. According to alternatives the amount of carbon may be 0.01 - 0.03 weight%.

According to alternatives the amount of nitrogen may be 0.20 - 0.25 weight% .

According to alternatives the amount of manganese may be 2 -6 weight%. According to alternatives Silica may be 0.1 - 1 .5 weight%.

In particular the supporting steel wire may have a composition (in weight%) of: C: 0.03; N: 0.22; Ni 1 .49; Cr 21 .6; Si: 0,71 ; Mo 0.15; Mn: 4.8, Mo 0.15.

BRIEF DESCRIPTION OF DRAWINGS

Figure 1 : a schematic drawing of a cross-section of an overhead electric power cable according to the invention.

Figure 2: a schematic drawing of a side view of a portion of an overhead electric power cable according to the invention. DETAILED DESCRIPTION OF THE INVENTION

Figure 1 shows the inventive overhead power cable 1 in cross-section. Figure 2 shows a side view of a portion of the inventive power cable. The general design of the cable is known in the art and its construction will therefore on be briefly described here. The power cable, which could be of any length and diameter, comprises several alumina conductors 2, i.e. alumina wires. The cable could comprise any number of alumina conductors for example 3-10, at least 20 at least 50, in figure 1 the cable comprises 30 alumina conductors. The alumina conductors are surrounds a supporting wire 3 which is arranged in the center of the power cable. The supporting wire 3 may be one single wire or consist of several wires, such as 2 or 3 or more wires that have been twined together, in figure 1 the supporting wire comprises 7 wires that are twined together. The wire may have any suitable diameter for example 1 - 5 mm, typically 1 .78 - 4.75mm . The entire power cable is manufacture by winding and twining alumina wires around the central supporting wire.

The supporting wire consists of, i.e. is entierly manufactured from, a ferritic-austenitic steel alloy which consists of the following alloying elements:

Carbon (C) strongly promotes the formation of austenite. Carbon further increases the mechanical strength by forming carbides with other alloying elements in the steel, such as chromium. However, a high amount of carbon drastically reduces the ductility and the corrosion resistance of the steel alloy. The amount of carbon should therefore be limited to < 0.1 wt%, alternatively 0.01 - 0.07 wt%, alternatively 0.01 - 0.03 wt%.

Nitrogen (N) strongly promotes the formation of austenite and increases the resistance of the steel alloy against pitting corrosion. Nitrogen further increases the mechanical strength of the steel alloy by precipitation of nitrides. However, too high amounts of nitrogen may lead to the precipitation of a brittle phase of chromium nitrides. Higher amounts of nitrogen also increase the risk of exceeding the solubility limit for nitrogen in the solid phase, giving rise to gas phase (bubbles) in the steel. The content of nitrogen in the steel should therefore be 0.1 - 0.6 wt%, alternatively 0.1 - 0.3 wt%, alternatively 0.20 - 0.25 wt%.

Nickel (Ni) is an expensive alloying element giving a large contribution to the alloy cost of a standard austenitic stainless steel alloy. Nickel promotes the formation of austenite consequently inhibits the formation of ferrite. Nickel is therefore an important constituent in the steel of the supporting wire which should be duplex, i.e. have both an austenitic and a ferritic phase. Nickel further improves ductility and to some extent also the corrosion resistance. To ensure ductility and sufficient amount of austenitic phase in the steel the amount of nickel should be 0.1 - 3 wt%.

Chromium (Cr) is an important element of the stainless steel alloy since it provides corrosion resistance by the formation of a chromium-oxide layer on the surface of the steel alloy.Chromium is further a ferrite stabilizing alloy element and is therefore important for ensuring a duplex phase of both austenite and ferrite in the steel of the supporting wire. The content of chromium should be 18 - 30 weight%, alternatively 18- 25 weight%. Manganese (Mn) stabilizes the austenite phase and is therefore an important element as a replacement for nickel, in order to control the amount of ferrite phase formed in the steel alloy. The cost of manganese is lower than the cost of nickel and therefore the total cost of the alloy may be reduced by replacing nickel with manganese. Another positive effect of manganese is that it promotes the solubility of nitrogen in the solid phase, and by that also indirectly increases the stability of the austenitic microstructure. Manganese will however increase the deformation hardening of the steel alloy, which increases the the mechanical strength and lowers the ductility, causing an enlarged risk of formation of cracks in the steel alloy during cold working. High amounts of manganese also reduce the corrosion resistance of the steel alloy, especially the resistance against pitting corrosion. The amount of manganese in the steel alloy should therefore be limited to a range from 1 - 10 wt%, alternatively 2 - 8 wt%, alternatively 2 -6 wt%.

Silicon (Si) is necessary for removing oxygen from the steel melt during manufacturing of the steel alloy and therefore the steel alloy will contain some amounts of silicon.

However, silicon increases the tendency for precipitation of intermetallic phases which makes the material brittle and therefore has a negative impact on the torsion ductility of the material. High torsion ductility is an important property of the supporting wire since the wire is twisted several turns during manufacture of the overhead power cable and it is therefore important to keep the content of silicon as low as possible in the supporting steel wire. Consequently, the amount of silicon in the supporting steel wire hould therefore be limited to a maximum of 2.0 wt%, alternatively 0.1 - 1 .5 wt%.

Copper (Cu) increases the ductility of the steel and stabilizes the austenite phase. At high temperatures, a too high amount of copper sharply reduces the hot workability of the steel. The amount of copper in the steel alloy should therefore be limited to a maximum of 3.0 wt%.

Cobalt (Co) can be used to replace some Ni as an austenite-stabilizing element however, cobalt is an expensive element so it should be limited to a maximum of 3.0 wt%.

Molybdenum (Mo) greatly improves the corrosion resistance in most environments. It also has a strong stabilizing effect on the ferrite phase. However, molybdenum is an expensive alloying element and therefore, the amount of molybdenum in the steel alloy should be limited to a maximum of 2.0 wt%

The supporting steel wire of the inventive overhead power cable should have a so called duplex structure. By duplex structure is meant that the structure of the supportive steel wire consists of both ferrite and austenite. Ferrite has low thermal expansion and it is therefore important that sufficient ferritic phase is present in the alloy. However, in comparison with austenite, ferrite is soft and has low ductility. The austenitic phase has high thermal expansion and is tough and has high strength. In a duplex steel, the presence of a mixture of ferrite and austenite yields good ductility. In the supporting steel wire of the inventive overhead power cable the amounts of the alloy elements described above are balanced so that the amount of measurable ferritic phase is 30 to 70 vol% and the amount of austenitic phase is 30 - 70 vol%. The amount of ferrite may for example be measured by microscope, i.e. ocular. The amount of ferrite may also be measured by magnetic induction, for example by using a Fischer Feritscope® M10B from the company Helmut Fischer GmbH.

EXAMPLE

Following is a concrete example showing the unexpected high thermal expansion coefficient that was measured in the supporting wire of inventive overhead electric power cable.

A heat of a steel alloy having the following chemical composition was prepared by conventional steel making processes. The steel alloy had the following composition:

Table 1 : Chemical composition of heat

The steel alloy was cast, rolled and finally drawn to a wire having a diameter of 3.1 mm. The wire was cut into 50 mm long samples.

The corrosion resistance of the samples were determined as Critical Pitting

Temperature (CPT) 45 -60°C (0.1 % NaCI +300mV).

The mechanical properties of the samples were determined. It was found that the samples had the following mechanical properties: Tensile strength, Rm = 1877 MPa. Yield strenght, Rp = 1416 MPa, torsion ductility of 24 - 28 turns over a length of wire which is equal to the wire diameter x 100.

The CTE data (Coefficient of Thermal Expansion) was thereafter determined on two wire samples (AL 167 and AL 168) in the temperature interval from a relation temperature RT of 30°C to a final temperature of 400^<€ by a dilatometer (Bahr DIL 802). A measuring system of fused silica was used which was calibrated against a reference sample of platinum.

The relative thermal expansion was determined to be below 0.4% for both samples over the temperature range. Table 2 shows the results from the measurements of the Coefficient of Thermal

Expansion [CTE].

From table 2 it can be concluded that for both samples the CTE was in the interval 9.1 - 1 1 .5 x10^"6/°C ^m/(m°C)]. However, at temperatures from 150°C and above, the thermal expansion was below 9.8 x10^"6/°C, where the lowest values, with an average around 9.2 x1 0^"6/°C, were seen at 200°C, followed by a slight increase with increasing temperatures. At 250°C the average was around 9.4 x10^"6/°C and at 300 °C the average was around 9.6 x10^"6/°C The highest values were found initially, but decreased rapidly with temperature.

Temperature Thermal Expansion Coefficient x10^"6/°C

AL 167 AL168 Average

50 °C 1 1 .32 1 1 .54 1 1 .43

100°C 10.73 1 1 .02 10.88

150°C 9.28 9.54 9.41

160°C 9.18 9.43 9.30

170°C 9.10 9.35 9.23

180°C 9.06 9.29 9.18

190°C 9.04 9.26 9.15

200 °C 9.05 9.25 9.15

210°C 9.08 9.26 9.17

250 °C 9.36 9.50 9.43

300 °C 9.59 9.70 9.64 350 °C 9.68 9.79 9.73

400 °C 9.75 9.85 9.80

Table 2: CTE-Values for inventive samples

In order to demonstrate which CTE-values that could be expected from other duplex steels, the CTE-measurement described above was also performed on a traditional duplex steel which is used in constructions. The comparative samples Com 1 and 2 Com 2 had the following chemical composition:

Table 3: Chemical composition of test samples

The following results were recorded on the comparative samples Com 1 and Com 2

Table 4: CTE-values on comparative samples

The above measurements show that the comparative samples Com 1 and Com 2 has substantially higher CTE-values, 1 1 .52 and 1 1 .45 x10^"6/°C than the CTE values that were measured on the samples of the steel of the inventive supporting wire.

Claims

1. An overhead electric power cable (1 ) comprising conducting wires (2) and a supporting wire (3) characterized in that the supporting wire comprises a ferritic- austenitic steel alloy which essentially consists of 30 - 70 vol% ferrite and 30 - 70 vol% austenite whereby the steel alloy has the following composition in percent by weight (wt%):

C: < 0.1

N: 0.1 - 0.5

Ni: 0.1 - 3 Cr: 18 - 30

Mn: 1 - 10

Si: < 2

Cu: < 3

Co: < 3 Mo: < 2 the balance Fe and normally occurring impurities.

2. The overhead electric power cable according to claim 1 , wherein the steel alloy has a carbon content of 0.01 - 0.07 weight%.

3 The overhead electric power cable according to claim 1 or 2, wherein the steel alloy has a carbon content of 0.01 - 0.03 weight%.

4. The overhead electric power cable according to any of claims 1 - 3, wherein the steel alloy has a nitrogen content of 0.1 - 0.3 weight%.

5. The overhead electric power cable according to any of claims 1 - 4, wherein the steel alloy has a nitrogen content of 0.20 - 0.24 weight%.

6. The overhead electric power cable according to any of claims 1 - 5, wherein the steel alloy has a manganese content of 2 - 8 weight%.

7. The overhead electric power cable according to any of claims 1 - 6, wherein the steel alloy has a manganese content of 2 - 6 weight%.

8. The overhead electric power cable according to any of claims 1 - 7, wherein the steel alloy has a silica content of 0.1 - 1.5 weight%.

9. The overhead electric power cable according to any of claims 1 - 8, wherein the steel alloy has a silica content of 0.1 - 1 weight%.

10. The overhead electric power cable according to claim 1 , wherein the steel alloy has the following composition in percent by weight (wt%):

C: 0.01 -0.07; N: 0.1- 0.3; Ni 1 - 3; Cr; 18 - 25; Mn: 2 -8; Si: 0.1 - 1.5; Mo: <1.0.

11. The overhead electric power cable according to claim 1 , wherein the steel alloy has the following composition in percent by weight (wt%): C: 0.03; N: 0.22; Ni 1.49; Cr 21.6;

Si: 0.71 ; Mo 0.15; Mn: 4.8.