SE536835C2 - An overhead line for electric power - Google Patents

Abstract

13 ABSTRACT An overhead electric power cable (1) comprising conducting wires (2) and a supportingwire (3) wherein the supporting wire comprises ferritic-austenitic steel alloy whichessentially consists of 30 - 70 vol% ferrite and 30 - 70 vol% austenite whereby the steelalloy has the following composition in percent by weight (wt°/°): CI

Description

An overhead electric power cable TECHNICAL FIELD The present invention relates to an overhead electric power cable according to thepreamble of claim 1.

BACKG ROUND ART Overhead electric power cables, also called overhead electric conductors are used forconducting electric power from power plants over land or sea. The power cablestypically consist of several conducting wires, such as aluminum wires, which surround acentral steel wire core. The purpose of the steel wire core is to support the weakaluminum wires when the electric power cable is suspended in the air on betweentowers. A general problem with overhead electrical power cables is that the length ofthe power cable increases due to thermal expansion since heat is generated in thepower cable when electric current is conducted there through. Thermal expansion maylead to sagging of the wire and in extreme cases cause the cable to touch the ground ornearby constructions.

Traditionally, carbon steel is used in the supporting wires of overhead electric powercables. Carbon steel is a material which is available at low cost and has sufficiently highstrength and sufficiently low thermal expansion to be suitable as a supporting wire inoverhead electric power cables. However, the thermal expansion coefficient of carbonsteel, which lies in the range of 11.5 - 12.5 x1O'6/°C, limits its use in power grids tomaximum cable temperatures below 200°C. ln recent development of power distribution the voltage of the electric power which isconducted in the cables is increased. This results in the generation of more heat in thepower cable, typically up to 250 °C. The voltage and hence the temperature is believedto increase even further in the future.

To meet these demands, the alloy lnvar is used in the supporting wires. lnvar has lowthermal expansion coefficient, however due to its high nickel content (35-40%) lnvar is avery expensive material. Further drawbacks with lnvarTM is its relatively low strength, 30-35% of the strength of carbons steel _ One problem related to both carbon steel and lnvar is the low corrosion resistance ofthese materials. The low resistance to corrosion can to some extent be compensatedwith e.g. galvanization. However in certain environments such as coastal areas thecorrosion is so high that the life length of lnvar and carbon steel becomes unacceptableshort.

JP61266558 shows a further material which has been proposed in the past for use asreinforcement in electrical cables. However, this material has not sufficiently low thermalexpansion to be used in modern power grids.

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

SUMMARY OF THE INVENTION This object is achieved by an overhead electric power cable (1) comprising conductingwires (2) and a supporting wire (3) characterized in that the supporting wire comprises aferritic-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 byweight (wt%): CI NI 0.1 -O.5NiïOfl -3Cr: 18-30 lVlni1-1O Si: 5 2.0 Cu: 5 3 C02 5 3 IVIOI 5 2 the balance Fe and normally occurring impurities.

The material that is used in the supporting wire of the inventive overhead cable is variantof so called duplex stainless steel alloy, i.e a stainless steel having a structure of bothaustenite and ferrite. The steel alloy in the supporting wire of the inventive power cableis known to have high strength, good ductility and very good resistance to corrosion.

Due to these properties the steel alloy have found use as reinforcment in constructions,for example as supporting wires in bridges or load bearing elements in marineconstructions. However, until yet it has not been reported that this steel alloy have beenused in applications where a low thermal expansion is important.

When the inventor performed measurements on the steel alloy above it was surprisinglydiscovered that the steels showed an unexpected low thermal expansion coefficient athigh temperatures. The Coefficient of Thermal Expansion (CTE) was found to lie in therange of 9.2 - 9.6 x1O'6/°C in the temperature range of 200 - 300°C. Expected values,based on other duplex steels, are approximately 11.5 x1O'6/°C in same temperature range.

Due to its low thermal expansion coefficient at high temperatures in combination withhigh strength, high torsion ductility and good corrosion resistance the steel alloy is verysuitable 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 ofmanganese and nitrogen in the steel. The low thermal expansion effect is extended byincreasing the density of crystal defects, which is dependent on the degree ofdeformation, the structure condition during deformation and the alloy composition.l\/langanese and nitrogen increase the deformation hardening and thereby causing an increased density of crystal defects in the structure. The duplex matrix itself may alsohave an impact on thermal expansion, due to the extended amount of phaseboundaries.

The inventive overhead cable further provides the following advantages: The good corrosion resistance ensures long lifetime of the cable in coastal areas. Thesteel 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 overheadcable in cold climates where ice formation could weigh down the cable to a point whereit breaks. The wire has the following mechanical properties: Tensile strength, Fšm = 1877l\/lPa, Yield strenght, Rp = 1416 l\/lPa.

The wire further has a torsion ductility of 24 - 28 turns over a length of wire which isequal to the wire diameter x 100. This torsion ductility is very high in comparison withother materials. Torsion ductility is an important property of the wire since electric powercables are manufactured by twinning of several conductors in to a cable which mayhave a length of several hundred meters. According to international standards a materialwhich 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, thismakes it possible to increase the number of alumina conductors in the inventive powercable with maintained total diameter of the cable. The advantage thereof is that thecapacity of the cable can be increased without increasing the total diameter of the cable. ln particular the supporting steel wire may have a composition of: C: 0.01 -0.07; N: 0.1-0.3; Ni 1 -3; Cr: 18 - 25; l\/ln: 2 -8; Si: 0.1 - 1; l\/lo: <1.0.

According to alternatives the amount of carbon may be 0.01 - 0.03.According to alternatives the amount of nitrogen may be 0.20 - 0.25.

According to alternatives the amount of manganese may be 2 -6.

According to alternatives Silica may be 0.1 - 1.5. ln particular the supporting steel wire may have a composition of: C: 0,03; N: 0.22; Ni1.49; Cr 21.6; Si: 0,71; i\/lo 0.15; i\/ln: 4.8, i\/lo 0.15.

BRIEF DESCRIPTION OF DRAWINGS Figure 1: a schematic drawing of a cross-section of an overhead electric power cableaccording to the invention.

Figure 2: a schematic drawing of a side view of a portion of an overhead electric powercable according to the invention.

DETAILED DESCRIPTION OF THE INVENTION Figure 1 shows the inventive overhead power cable 1 in cross-section. Figure 2 shows aside view of a portion of the inventive power cable. The general design of the cable isknown in the art and its construction will therefore on be briefly described here. Thepower cable, which could be of any length and diameter, comprises several aluminaconductors 2, i.e. alumina wires. The cable could comprise any number of aluminaconductors for example 3-10, at least 20 at least 50, in figure 1 the cable comprises 30alumina conductors. The alumina conductors are surrounds a supporting wire 3 which isarranged in the center of the power cable. The supporting wire 3 may be one single wireor 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 mayhave any suitable diameter for example 1 - 5 mm, typically 1.78 - 4.75mm _ The entirepower cable is manufacture by winding and twining alumina wires around the centralsupporting wire.

The supporting wire consists of, i.e. is entierly manufactured from, a ferritic-austeniticsteel alloy which consists of the following alloying elements: Carbon (C) strongly promotes the formation of austenite. Carbon further increases themechanical 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 thecorrosion resistance of the steel alloy. The amount of carbon should therefore be limitedto 5 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 resistanceof the steel alloy against pitting corrosion. Nitrogen further increases the mechanicalstrength of the steel alloy by precipitation of nitrides. However, too high amounts ofnitrogen may lead to the precipitation of a brittle phase of chromium nitrides. Higheramounts of nitrogen also increase the risk of exceeding the solubility limit for nitrogen inthe solid phase, giving rise to gas phase (bubbles) in the steel. The content of nitrogenin the steel should therefore be 0.1 - 0.6 wt%, alternatively 0.1 - 0.3 wt%, alternatively0.20 - 0.25 wt%.

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

Chromium (Cr) is an important element of the stainless steel alloy since it providescorrosion resistance by the formation of a chromium-oxide layer on the surface of thesteel alloy.Chromium is further a ferrite stabilizing alloy element and is thereforeimportant for ensuring a duplex phase of both austenite and ferrite in the steel of thesupporting wire. The content of chromium should be 18 - 30, alternatively 18-25. l\/langanese (l\/ln) stabilizes the austenite phase and is therefore an important elementas a replacement for nickel, in order to control the amount of ferrite phase formed in thesteel alloy. The cost of manganese is lower than the cost of nickel and therefore the totalcost of the alloy may be reduced by replacing nickel with manganese. Another positiveeffect of manganese is that it promotes the solubility of nitrogen in the solid phase, andby that also indirectly increases the stability of the austenitic microstructure. l\/langanese will however increase the deformation hardening of the steel alloy, which increases thethe mechanical strength and lowers the ductility, causing an enlarged risk of formation ofcracks in the steel alloy during cold working. High amounts of manganese also reducethe corrosion resistance of the steel alloy, especially the resistance against pittingcorrosion. The amount of manganese in the steel alloy should therefore be limited to arange from 1 - 10 wt%, alternatively 2 - 8 wt%, alternatively 2 -6 wt%.

Silicon (Si) is necessary for removing oxygen from the steel melt during manufacturingof the steel alloy and therefore the steel alloy will contain some amounts of silicon.However, silicon increases the tendency for precipitation of intermetallic phases whichmakes the material brittle and therefore has a negative impact on the torsion ductility ofthe material. High torsion ductility is an important property of the supporting wire sincethe wire is twisted several turns during manufacture of the overhead power cable and itis therefore important to keep the content of silicon as low as possible in the supportingsteel wire. Consequently, the amount of silicon in the supporting steel wire houldtherefore 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. Athigh temperatures, a too high amount of copper sharply reduces the hot workability ofthe steel.The amount of copper in the steel alloy should therefore be limited to amaximum of 3.0 wt%.

Cobalt (Co) can be used to replace some Ni as an austenite-stabilizing elementhowever, cobalt is an expensive element so it should be limited to a maximum of 3.0wt%. l\/lolybdenum (l\/lo) greatly improves the corrosion resistance in most environments. ltalso has a strong stabilizing effect on the ferrite phase. However, molybdenum is anexpensive alloying element and therefore, the amount of molybdenum in the steel alloyshould be limited to a maximum of 2.0 wt% The supporting steel wire of the inventive overhead power cable should have a so calledduplex structure. By duplex structure is meant that the structure of the supportive steelwire 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, incomparison with austenite, ferrite is soft and has low ductility. The austenitic phase hashigh thermal expansion and is tough and has high strength. ln a duplex steel, thepresence of a mixture of ferrite and austenite yields good ductility. ln the supportingsteel wire of the inventive overhead power cable the amounts of the alloy elementsdescribed above are balanced so that the amount of measurable ferritic phase is 30 to70 vol% and the amount of austenitic phase is 30 - 70 vol%. The amount of ferrite mayfor example be measured by microscope, i.e. ocular.

EXAI\/IPLE Following is a concrete example showing the unexpected high thermal expansioncoefficient that was measured in the supporting wire of inventive overhead electricpower cable.

A heat of a steel alloy having the following chemical composition was prepared byconventional steel making processes. The steel alloy had the following composition: Wt% C Si l\/ln P S Cr Ni l\/lo Co0.03 0.71 4.8 0.021 <0.001 21.6 1.49 0.15 0.048V Cu Nb Ta N W B Sn Fe0.12 0.37 <0.01 <0.005 0.22 0.01 0.0022 <0.005 rest 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 sample were determined as Critical Pitting Temperature(CPT) 45 -60 °C (0,1 % NaCl +300mV).

The mechanical properties of the samples were determined _ lt was found that thesamples had the following mechanical properties: Tensile strength, Fšm = 1877 l\/lPa.Yield strenght, Rp = 1416 l\/lPa, torsion ductility of 24 - 28 turns over a length of wirewhich is equal to the wire diameter x 100.

The CTE data (Coefficient of Thermal Expansion) was thereafter determined on two wiresamples (AL 167 and AL 168) in the temperature interval from a relation temperature RTof 30°C to a final temperature of 400°C by a dilatometer (Bähr DlL 802). A measuringsystem of fused silica was used which was calibrated against a reference sample ofplatinum.

The relative thermal expansion was determined to be below 0,4% for both samples overthe temperature range.

Table 2 shows the results from the measurements, of the Coefficient of ThermalExpansion [CTE].

From table 2 it can be concluded that for both samples the CTE was in the interval 9.1-11.5 ><1o'6/°c [um/(m°c)].

However, at temperatures from 150°C and above, the thermal expansion was below 9.8x10'6/°C, where the lowest values, with an average around 9.2 x10'6/°C, were seen at200°C, followed by a slight increase with increasing temperatures. At 250°C the averagewas around 9.4 x10'6/°C and at 300 °C the average was around 9.6 x10'6/°C Thehighest values were found initially, but decreased rapidly with temperature.

Temperature Thermal Expansion Coefficient x 9.2 x10'6/°CAL 167 AL168 Average 50°C 11,32 11,54 11,43100°C 10,73 11,02 10,88150 °C 9,28 9,54 9,41160 °C 9,18 9,43 9,30170 °C 9,10 9,35 9,23180 °C 9,06 9,29 9,18190 °C 9,04 9,26 9,15200 °C 9,05 9,25 9,15210°C 9,08 9,26 9,17250 °C 9,36 9,50 9,43300 °C 9,59 9,70 9,64350 °C 9,68 9,79 9,73400 °C 9,75 9,85 9,80Table 3: CTE-Values for inventive samples ln 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: C Si Mn P S Cr Ni Mo W CoCom1 0,019 0,47 0,74 0,019 0,0007 22,22 5,19 3,33 <0,01 0,059Com2 0,016 0,49 0,81 0,021 0,0006 22,14 5,16 3,12 <0,01 0,046 V Ti Cu Al As Sn Nb Ta B NCom1 0,053 <0,005 0,18 0,009 0,004 <0,005 0,01 <0,005 0,0023 0,180Com2 0,044 <0,005 0,13 0,009 0,003 <0,005 0,01 <0,005 0,0027 0,168 Table 4: Chemical composition of test samples The following results were recorded on the comparative samples Com 1 and Com 2 Temperature Thermal expansion x10'6/°C100 °C 12,78200 °C 11,52300 °C 11,45400 °C 11,77 Table 5: CTE-values on comparative samples The above measurements show that the comparative samples Com 1 and Com 2 hassubstantially higher CTE-values, 11.52 and 11.45 x10'6/°C than the CTE values thatwere measured on the samples of the steel of the inventive supporting wire.

Claims (11)

11 CLA||\/IS

1. An overhead electric power cable (1) comprising conducting wires (2) and asupporting 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%): CI < 0.1 N: 0.1 -0.5 Ni: 0.1 - 3 Cr: 18 - 30 l\/ln: 1 - 10 Si: 5 2 Cu: 5 3 C02 5 3 IVIOI 5 2 the balance Fe and normally occurring impurities.

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

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

4. The overhead electric power cable according to any of claims 1 - 3, wherein the steelalloy has a nitrogen content of 1 - 3.

5. The overhead electric power cable according to any of claims 1 - 4, wherein the steelalloy has a nitrogen content of 0.1 - 0.3. 12

6. The overhead electric power cable according to any of claims 1 - 5, wherein the steelalloy has a manganse content of 2 - 8.

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

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

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

10. The overhead electric power cable according to claim 1, wherein the steel alloy hasthe following composition in percent by weight (wt%): C: 0.01 -0.07; N: 0.1- 0.3; Ni 1 - 3; Cr: 18 - 25; l\/ln: 2 -8; Si: 0.1 -1.5; l\/lo: <1.0.

11. The overhead electric power cable according to claim 1, wherein the steel alloy hasthe following composition in percent by weight (wt%): C: 0.03; N: 0.22; Ni 1.49; Cr 21.6;Si: 0.71; i\/lo 0.15; i\/ln: 4.8, i\/lo 0.15.