CA1286737C - Direct current electric cables having a coumpound impregnated insulation - Google Patents
Direct current electric cables having a coumpound impregnated insulationInfo
- Publication number
- CA1286737C CA1286737C CA000543527A CA543527A CA1286737C CA 1286737 C CA1286737 C CA 1286737C CA 000543527 A CA000543527 A CA 000543527A CA 543527 A CA543527 A CA 543527A CA 1286737 C CA1286737 C CA 1286737C
- Authority
- CA
- Canada
- Prior art keywords
- cable
- tapes
- reinforcing structure
- fact
- sheath
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired - Lifetime
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Classifications
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01B—CABLES; CONDUCTORS; INSULATORS; SELECTION OF MATERIALS FOR THEIR CONDUCTIVE, INSULATING OR DIELECTRIC PROPERTIES
- H01B9/00—Power cables
- H01B9/06—Gas-pressure cables; Oil-pressure cables; Cables for use in conduits under fluid pressure
- H01B9/0611—Oil-pressure cables
Landscapes
- Insulated Conductors (AREA)
Abstract
ABSTRACT OF THE DISCLOSURE
A direct current electrical cable with a conductor encircled by stratified insulation impregnated with a conventional insulating compound, a sheath around the insulation and a tensioned reinforcing structure around the sheath. The tension on the reinforcing structure is sufficient to compress the sheath and to eliminate micro-cavities in the insulation. The reinforcing structure is made of tapes with sufficient elasticity to withstand the expansion and contraction of the cable during use to thereby prevent formation of micro-cavities during use of the cable.
A direct current electrical cable with a conductor encircled by stratified insulation impregnated with a conventional insulating compound, a sheath around the insulation and a tensioned reinforcing structure around the sheath. The tension on the reinforcing structure is sufficient to compress the sheath and to eliminate micro-cavities in the insulation. The reinforcing structure is made of tapes with sufficient elasticity to withstand the expansion and contraction of the cable during use to thereby prevent formation of micro-cavities during use of the cable.
Description
DIRECT CURRENT ELECTRIC CA~LE~ HAVING A
COMPOUND-IMPREGNATED INSULATION
The present invention relates to direct current electric cables having insulation impregnated with a compound and without pressurized gas being present.
The electric cables of such type and according to the invention comprise a conductor which is surrounded by a stratiEied insulation ~ormed by the winding of tapes oE
insulating material. The stratified insulation is impregnated with a viscous type of compound. A metallic sheath surrounds the stratified insulation impregnated with said compound, and a reinforcing structure, which is practically inextensible in the radial sense and which is formed by windings of tapes of a metallic material, surrounds the sheath.
The known D.C. electric cables of the type briefly described are subjected, during use, to perforation risks due to the presence in the stratified insulation of micro-cavities containing low pressure gas which form during cable construction and which, during use, continue to change their position and their dimensions.
These micro-cavities are inevitably formed during the cable construction because it is practically impossible to have a complete impregnation of the strati~ied insulation with a compound. In fact, the maximum impregnatlon that can be realized is 99% in the known cables of the type being considered.
When a cable is functioning, the micro-cavities continue to change their position and their dimensions, or the reasons set forth hereinafter.
Dueing cable operation, the cable undergoes thermal cycles of heating and cooling.
During heating cycles, the compound impregnating the stratified insulation of the cable becomes reduced in viscosity and become subjected to a greater thermal expansion as compared to that of the other cable components. The consequent volume increase of the compound in the cable causes the micro-cavities which were formed in the cable during cable construction, to disappear.
During the cooling cycles, said micro-cavities reappear with changes in their position and their dimensions due to the volume contraction undergone by the compound.
It is known that any micro-cavities in a D.C. cable which is impregnated with a compound are dangerous since they provide places for electrical discharges which can give rise to electrical perforations in the cable. Also, it is known that greater perforation risks are to be encountered during the cooling cycles when, due to the efEect of the contraction of the compound, these micro-cavities re-form in the stratified insulation of the cable.
For solving the problem of perforation risks which exists in the cables having insulation impregnated with a compound, it has been proposed to have said insulation aided by a high pressure gas introduced into the cable and more precisely, a gas having a pressure of not less than 1~ bar.
To date, this known solution has been considered to be the best solution possible for facing the problem constituted by the risk of perforations, but this solution is quite unsatiseactory due to the Eact that it has the drawbacks mentioned hereinafter.
In the first place, the construction Oe the cable is rendered complex due to the need for the presence of tanks holding pressurized gas near the cable and for connections between these tanks and cable to provide the pressurized gas in the cable.
Another drawback of the known solution is the considerable limitation of the maximum length permissible for the cable.
~2~3~737 In fact, the known cables which have the insulation thereoe fully impregnated with a compound (known as "mass impregnated cables") and which have gas under pressure therein cannot be produced in lengths greater than 5 Km in order to keep the pressure losses of said gas within acceptable values during the movement oE the gas along the cable.
Such limitations of length represent a very serious drawback because, practically speaking, this Eact prevents the utilizing oE such cables in the submarine field where very long cable lengths are usually required.
One object of this invention is to provide D.C. cables which have insulation impregnated with a compound in which the risks of perforations occurring due to the presence of micro-cavities are eliminated, which are not subject to limitations as to the lengths of the cables and for which there is no need to provide complicated cable constructions.
In accordance with the preferred embodiment, a D.C.
electric cable comprising a~ least one conductor, a stratified insulation disposed around the conductor and impregnated with a compound, a radially outermost metallic sheath and a reinforcing structure formed by the winding oE at least one tape which completely surrounds the sheath is characterized by the e act that the said reinEorcing structure is subjected to a tension sufficient to reduce the diameter of the sheath with the tapes forming said reinorcing structure presenting mechanical hysteretical cycles closed upon themselves, the diE~erence between the maximum and minimum per cent deformation in said cycles being in the range from 0.3% to 0.5%.
Other objects and advantages oE the present invention will be apparent from the following detailed description of the presently preferred embodiments thereoe, which description should be considered in conjunction with the accompanying drawings in ~673~7 which:
Fig~ 1 is a perspective view oE a cable length, according to the invention, with parts removed for better showing its structure; and Fig. 2 is a graph of a mechanical hysteretical cycle, closed upon itself, showing the relation between tension Eorces and the per cent deformations.
In Fig. 1, there is shown a cable having a diameter in the range from 50 mm to 80 mm. The conductor 1 has a cross-section in the range from 400 to 1200 mm2, and it is ormed by a plurality of wires made, for example, of copper, layed-up together, and surrounded by a semi-conductive screen 2 which comprises at least one winding of a semi~conductive tape.
Around the semi-conductive tape 2, there is a stratified insulation 3 impregnated with an insulating compound. The said insulating compound with which the said stratified insulation is impregnated can be any known compound of the type used for impregnating cables. Said compound has a viscosity, at room temperature (20C), of over 1000 cSt. and preferably, less than 50,000 cSt.
The stratified insulation 3 is covered outwardly by a semi-conductive screen 4 having, Eor example, a structure identical to the semi-conductive screen 2.
A metallic sheath 5 made, for example, oE lead or lead alloy, adheres to the radially outer surface of the semi-conductive screen or layer 4.
A reinforcing structure 6, the characteristics oE which Eor the purpose oE the present invention will be given hereinafter, entirely surrounds the metallic sheath 5.
In the cable shown in Fig. 1, the reinforcing structure 6 is formed by helicoidally winding a single tape 7, but Eor forming said reinforcing structure, several tapes can be used, either placed i2~1~;737 adjacent to one another, or one overlapping the next. In the latter case, the tapes can also have opposite winding directions.
The characteristics of the reinEorcing structure 6 Oe a cable according to the invention, are as described hereinafter.
A Eirst characteristic is that the reinforcing structure 6 is tightened on the sheath 5 while being subjected to a tension which causes as reduction of the diameter of said sheath 5 and which exerts a compression orce upon the outermost layers of the stratified insulation with an accompanying transmission of a hydrostatic pressure to the impregnating compound. In this way, with a compound having a viscosity of over 1000 cSt., the full impregnation o the cable is had with a complete elimination of the micro-cavities existing therein and without causing any appreciable movements of the compound in the longitudinal direction of the cable.
A second characteristic of the reinforcing structure 6 of a cable according to the invention is that the component tapes must have mechanical hysteretical cycles which are closed upon themselves, like the one shown in Fig. 2 (and also defined urther on in the text), wherein the difference between the maximum a and minimum b per cent deformations is in the range from 0.3% to 0.5%. The range o~ values just given corresponds with the per cent variation o the outer circumference o~ the sheath 5 which can be caused by the thermal expansion oE the compound between the minimum and the maximum temperatures to which the D.C. cable will be subjected.
As a consequence of the second characteristic, it results that the full impregnation of the cable is maintained during the use of the cable and hence, in the thermal cycles which occur during cable operation, the ormation of any micro-cavities, is practically prevented.
In this text, by the term "closed", as it re~Eers to ~2~ ;737 "mechanical hysteretical cycles", is meant the curves drawn and representing the tractional forces ~ and the per cent deformations by means of an extensometer device, according to the Standards ASTM E/83, such as, Eor example, an extensometer INSTROM 2630036 which is used, in a manner known to those skilled in the art, on a test-sample of tape which is subjected to increasing and decreasing loads within a range of predetermined deEormations while recording the data obtained.
Moreover, in this text, by the term "closed", as it refers to "mechanical hysteretical cycles" is meant that in the graph of tension e orces and per cent deformations, a curve "closed upon itself" can be identiEied, such a graph being shown in Fig. 2 wherein both, for the forces and the deeormations, there exist a maximum point c and a minimum point d, both said points remaining substantially fixed, with the cycles passing throu~h these points, repeatedly.
Tapes which permit realizing the cables according to the invention, are nickel-chrome steel tapes, aromatic polyamide tapes, fiber glass tapes, and polyester tapes. Other tapes which are suitable are carbon steel tapes ~or tapes oE other metals, the mechanical characteristics Oe which are comparable with those oE carbon steel tapes) provided either with micro-undulations disposed in the direction transverse to the tape, with micro-drawings disposed in honeycomb Eashion or with rhombus shaped slottings which are adapted for conferring on them elastic elongations under tensile stresses which are higher than those due to the material forming the tapes themselves.
The experimental tests to be described hereinaEter prove that with cables according to this invention, any perforation 30 risks which can be attributed to micro-cavities are practically eliminated without involving the drawback of any construction complications in the cables, without the drawback Oe being subjected to a limit of the cable lengths and without there being any need for permanently feeding the cable with gas under high pressure.
Cables "A" and "B" according to the invention which were subjected to the experimental tests are described hereinafter.
CABLE "A" - ACCORDING TO THE INVENTION
This cable had a conductor formed by a copper rope having a diameter oE 39 mm and covered with a semiconductive layer and by a strati~ied insulation having a thickness of 18 mm.
The stratified insulation was Eormed by windings oE
cellulose tapes and was impregnated by a viscous compound having a viscosity (at 20C) of 18000 cSt. and constituted by 98% of mineral oil and by 2% of polyisobutylene.
The stratified insulation, covered externally by a semiconductive layer, was enclosed with a lead sheath having a thickness o 3.5 mm.
Around the lead sheath, there was a reinforcing structure formed by two overlapped layers obtained by windings of nickel-chrome steel tapes with a width of 25 mm and a thickness of 0.2 mm. In said tapes, closed mechanical hysteretical cycles are obtainable with the difEerence between the maximum and the minimum deformation being up to 0.45%.
At a temperature oE 10C, the tapes Eorming the reinforcing structure of the cable are subjected to a tensile stress oE a value of 9.6 Kg/mm2, and this value represents the tensile stress existing in the reinforcing structure oE the cable which is adapted to cause a compression e orce of 9 atm. upon the sheath and which eliminates any micro-cavity existing in the cable.
CABLE "B" - ACCORDING TO THE INVENTION
This cable differs from the above-described CABLE "A" solely by the fact that the reinEorcing structure, present around the sheath, is formed by four overlapping layers obtained by windings ~86737 of fiber-glass tapes having a width of 30 mm and a thickness of 0.20 mm. In said tapes, closed, mechanical hysteretical cycles are obtainable with the differences existing between the maximum and the minimum deEormation of up to 1%.
At a temperature of 10C, the tapes forming the eeinforcing structure of cable B are subjected to a tensile stress of 5 Kg/mm2 and this value represents the tensile stress existing in the reinEorcing structure of the cable which is adapted to cause a compression force of 9 atm. on the sheath of said cable.
The experimental tests employed were those known a the "Loading Cycle and Polarity Reversal Tests" recommended by the "Working Group 21-10, ~tudy Committee No. 21 of the CIGRE" and published in the review "ELECTRA", issue No. 72.
Following the methods established by this document, 30 meter lengths of each cable under examination were subjected to thirty thermal heating and cooling cycles, between room temperature and the maximum working temperature, which is 65C for the cables under examination as measured on the conductor, and with increasing, after every said thirty cycles, the value oE the continuous voltage applied to the cable itselE until the value of the voltage at which the performation is found.
In addition, the following four types of cables, identified as "C" - "D" - "F" - "G" which were not constructed in accordance with the invention, were subjected to the above-described experimental tests:
- A first type of cable, CABLE "C"~ having the identical structure, materials and dimensions as that of the cable "A"
according to the invention and which difEers Erom the latter only by the fact that, prior to the cable being put into service, the resistant structure which surrounds the cable sheath does not, as a practical matter, apply any compression force on the sheath.
- A second type of cable, CABLE "D", which diEfers Erom the 73'7 CABLE "C" solely by reason of the Eact that its impregnated insulation has nitrogen at a pressure of 1~ atm. applied thereto.
The second type oE cable belongs to the category of known cables which have their impregnated insulations assisted by the presence of a pressurized gas.
- A third type of cable, CABLE "F", which differs from the CABLE "B" according to the invention solely by reason o the fact that the reinforcing structure disposed around the sheath is formed by four overlapping layers o~ cotton tapes, i.e., tapes which are devoid o-f any closed, mechanical hysteretical cycles.
Said cotton tapes are applied on the sheath with a tension of 5Kg/mm2 which is adapted to cause a compression force of 9 atm.
which guarantees the full impregnation of the cable prior to its being put into service.
- A fourth type of cable, CABLE "G", which differs from the cables according to the invention solely by reason of the fact that the reinforcing structure disposed around the sheath is formed by four overlapping layers of copper tapes, i.e., tapes wherein a closed hysteretical cycle can only be obtained when the 20 difference between the maximum and the minimum deformation does not exceed 0.12%. These copper tapes are applied over the sheath, with a tension of 5 Kg/mm2 which is adapted to cause a compression force of 9 atm. which guarantees the full impregnation of the cable prior to its being put into service.
The experimental tests carried out on a plurality of cable lengths according to the invention and on the other cable lengths described hereinbefore have given the results which are set forth in the following TABLE:
~2~36~37 Increment % oE the increase in perforation voltage with respect TYPE OF CABLE to CABLE C
CABLE A according to the invention 80%
CABLE B according to the invention 80 CABLE D - impregnated insulation with nitrogen under pressure at 14 atm.
CABLE F 15%
CABLE G 20%
An examination of the results set forth in the TABLE
supports the following conclusions.
First and foremost, for the solution to the problem, it appears to be essential that the cable have not only the Eirst characteristic of the resistant structure which brings about the full impregnation oE the cable but also the second characteristic relative to the tapes with which the cable's resistant structure ~0 is formed and which allow for maintaining said ~ull impregnation of the cable during use.
Moreover, with respect to known cables having the insulations thereof impregnated with a compound and subjected to high pressure gas, i.e., cables which to date were considered capable of providing a solution sufficient-Eor overcoming problems of perforation risks caused by the presence of micro-cavities in the insulation, the cables according to the invention provide an increase of 66% in the perforation voltage values.
What is more, the cables according to the invention solve the problem in question without also involving any drawbacks. In particular, the cables according to the invention do not require any modifications to be ef~ected in the plants actually being ~2~6737 utilized for cable manufacture, do not require any introduction of complications in the structure of these cables themselves and do not impose any limi~ations for the lengths of the cables.
Although preferred embodiments of the present invention have been described and illustrated/ it will be apparent to those skilled in the art that various modifications may be made without departing from the principles of the invention.
COMPOUND-IMPREGNATED INSULATION
The present invention relates to direct current electric cables having insulation impregnated with a compound and without pressurized gas being present.
The electric cables of such type and according to the invention comprise a conductor which is surrounded by a stratiEied insulation ~ormed by the winding of tapes oE
insulating material. The stratified insulation is impregnated with a viscous type of compound. A metallic sheath surrounds the stratified insulation impregnated with said compound, and a reinforcing structure, which is practically inextensible in the radial sense and which is formed by windings of tapes of a metallic material, surrounds the sheath.
The known D.C. electric cables of the type briefly described are subjected, during use, to perforation risks due to the presence in the stratified insulation of micro-cavities containing low pressure gas which form during cable construction and which, during use, continue to change their position and their dimensions.
These micro-cavities are inevitably formed during the cable construction because it is practically impossible to have a complete impregnation of the strati~ied insulation with a compound. In fact, the maximum impregnatlon that can be realized is 99% in the known cables of the type being considered.
When a cable is functioning, the micro-cavities continue to change their position and their dimensions, or the reasons set forth hereinafter.
Dueing cable operation, the cable undergoes thermal cycles of heating and cooling.
During heating cycles, the compound impregnating the stratified insulation of the cable becomes reduced in viscosity and become subjected to a greater thermal expansion as compared to that of the other cable components. The consequent volume increase of the compound in the cable causes the micro-cavities which were formed in the cable during cable construction, to disappear.
During the cooling cycles, said micro-cavities reappear with changes in their position and their dimensions due to the volume contraction undergone by the compound.
It is known that any micro-cavities in a D.C. cable which is impregnated with a compound are dangerous since they provide places for electrical discharges which can give rise to electrical perforations in the cable. Also, it is known that greater perforation risks are to be encountered during the cooling cycles when, due to the efEect of the contraction of the compound, these micro-cavities re-form in the stratified insulation of the cable.
For solving the problem of perforation risks which exists in the cables having insulation impregnated with a compound, it has been proposed to have said insulation aided by a high pressure gas introduced into the cable and more precisely, a gas having a pressure of not less than 1~ bar.
To date, this known solution has been considered to be the best solution possible for facing the problem constituted by the risk of perforations, but this solution is quite unsatiseactory due to the Eact that it has the drawbacks mentioned hereinafter.
In the first place, the construction Oe the cable is rendered complex due to the need for the presence of tanks holding pressurized gas near the cable and for connections between these tanks and cable to provide the pressurized gas in the cable.
Another drawback of the known solution is the considerable limitation of the maximum length permissible for the cable.
~2~3~737 In fact, the known cables which have the insulation thereoe fully impregnated with a compound (known as "mass impregnated cables") and which have gas under pressure therein cannot be produced in lengths greater than 5 Km in order to keep the pressure losses of said gas within acceptable values during the movement oE the gas along the cable.
Such limitations of length represent a very serious drawback because, practically speaking, this Eact prevents the utilizing oE such cables in the submarine field where very long cable lengths are usually required.
One object of this invention is to provide D.C. cables which have insulation impregnated with a compound in which the risks of perforations occurring due to the presence of micro-cavities are eliminated, which are not subject to limitations as to the lengths of the cables and for which there is no need to provide complicated cable constructions.
In accordance with the preferred embodiment, a D.C.
electric cable comprising a~ least one conductor, a stratified insulation disposed around the conductor and impregnated with a compound, a radially outermost metallic sheath and a reinforcing structure formed by the winding oE at least one tape which completely surrounds the sheath is characterized by the e act that the said reinEorcing structure is subjected to a tension sufficient to reduce the diameter of the sheath with the tapes forming said reinorcing structure presenting mechanical hysteretical cycles closed upon themselves, the diE~erence between the maximum and minimum per cent deformation in said cycles being in the range from 0.3% to 0.5%.
Other objects and advantages oE the present invention will be apparent from the following detailed description of the presently preferred embodiments thereoe, which description should be considered in conjunction with the accompanying drawings in ~673~7 which:
Fig~ 1 is a perspective view oE a cable length, according to the invention, with parts removed for better showing its structure; and Fig. 2 is a graph of a mechanical hysteretical cycle, closed upon itself, showing the relation between tension Eorces and the per cent deformations.
In Fig. 1, there is shown a cable having a diameter in the range from 50 mm to 80 mm. The conductor 1 has a cross-section in the range from 400 to 1200 mm2, and it is ormed by a plurality of wires made, for example, of copper, layed-up together, and surrounded by a semi-conductive screen 2 which comprises at least one winding of a semi~conductive tape.
Around the semi-conductive tape 2, there is a stratified insulation 3 impregnated with an insulating compound. The said insulating compound with which the said stratified insulation is impregnated can be any known compound of the type used for impregnating cables. Said compound has a viscosity, at room temperature (20C), of over 1000 cSt. and preferably, less than 50,000 cSt.
The stratified insulation 3 is covered outwardly by a semi-conductive screen 4 having, Eor example, a structure identical to the semi-conductive screen 2.
A metallic sheath 5 made, for example, oE lead or lead alloy, adheres to the radially outer surface of the semi-conductive screen or layer 4.
A reinforcing structure 6, the characteristics oE which Eor the purpose oE the present invention will be given hereinafter, entirely surrounds the metallic sheath 5.
In the cable shown in Fig. 1, the reinforcing structure 6 is formed by helicoidally winding a single tape 7, but Eor forming said reinforcing structure, several tapes can be used, either placed i2~1~;737 adjacent to one another, or one overlapping the next. In the latter case, the tapes can also have opposite winding directions.
The characteristics of the reinEorcing structure 6 Oe a cable according to the invention, are as described hereinafter.
A Eirst characteristic is that the reinforcing structure 6 is tightened on the sheath 5 while being subjected to a tension which causes as reduction of the diameter of said sheath 5 and which exerts a compression orce upon the outermost layers of the stratified insulation with an accompanying transmission of a hydrostatic pressure to the impregnating compound. In this way, with a compound having a viscosity of over 1000 cSt., the full impregnation o the cable is had with a complete elimination of the micro-cavities existing therein and without causing any appreciable movements of the compound in the longitudinal direction of the cable.
A second characteristic of the reinforcing structure 6 of a cable according to the invention is that the component tapes must have mechanical hysteretical cycles which are closed upon themselves, like the one shown in Fig. 2 (and also defined urther on in the text), wherein the difference between the maximum a and minimum b per cent deformations is in the range from 0.3% to 0.5%. The range o~ values just given corresponds with the per cent variation o the outer circumference o~ the sheath 5 which can be caused by the thermal expansion oE the compound between the minimum and the maximum temperatures to which the D.C. cable will be subjected.
As a consequence of the second characteristic, it results that the full impregnation of the cable is maintained during the use of the cable and hence, in the thermal cycles which occur during cable operation, the ormation of any micro-cavities, is practically prevented.
In this text, by the term "closed", as it re~Eers to ~2~ ;737 "mechanical hysteretical cycles", is meant the curves drawn and representing the tractional forces ~ and the per cent deformations by means of an extensometer device, according to the Standards ASTM E/83, such as, Eor example, an extensometer INSTROM 2630036 which is used, in a manner known to those skilled in the art, on a test-sample of tape which is subjected to increasing and decreasing loads within a range of predetermined deEormations while recording the data obtained.
Moreover, in this text, by the term "closed", as it refers to "mechanical hysteretical cycles" is meant that in the graph of tension e orces and per cent deformations, a curve "closed upon itself" can be identiEied, such a graph being shown in Fig. 2 wherein both, for the forces and the deeormations, there exist a maximum point c and a minimum point d, both said points remaining substantially fixed, with the cycles passing throu~h these points, repeatedly.
Tapes which permit realizing the cables according to the invention, are nickel-chrome steel tapes, aromatic polyamide tapes, fiber glass tapes, and polyester tapes. Other tapes which are suitable are carbon steel tapes ~or tapes oE other metals, the mechanical characteristics Oe which are comparable with those oE carbon steel tapes) provided either with micro-undulations disposed in the direction transverse to the tape, with micro-drawings disposed in honeycomb Eashion or with rhombus shaped slottings which are adapted for conferring on them elastic elongations under tensile stresses which are higher than those due to the material forming the tapes themselves.
The experimental tests to be described hereinaEter prove that with cables according to this invention, any perforation 30 risks which can be attributed to micro-cavities are practically eliminated without involving the drawback of any construction complications in the cables, without the drawback Oe being subjected to a limit of the cable lengths and without there being any need for permanently feeding the cable with gas under high pressure.
Cables "A" and "B" according to the invention which were subjected to the experimental tests are described hereinafter.
CABLE "A" - ACCORDING TO THE INVENTION
This cable had a conductor formed by a copper rope having a diameter oE 39 mm and covered with a semiconductive layer and by a strati~ied insulation having a thickness of 18 mm.
The stratified insulation was Eormed by windings oE
cellulose tapes and was impregnated by a viscous compound having a viscosity (at 20C) of 18000 cSt. and constituted by 98% of mineral oil and by 2% of polyisobutylene.
The stratified insulation, covered externally by a semiconductive layer, was enclosed with a lead sheath having a thickness o 3.5 mm.
Around the lead sheath, there was a reinforcing structure formed by two overlapped layers obtained by windings of nickel-chrome steel tapes with a width of 25 mm and a thickness of 0.2 mm. In said tapes, closed mechanical hysteretical cycles are obtainable with the difEerence between the maximum and the minimum deformation being up to 0.45%.
At a temperature oE 10C, the tapes Eorming the reinforcing structure of the cable are subjected to a tensile stress oE a value of 9.6 Kg/mm2, and this value represents the tensile stress existing in the reinforcing structure oE the cable which is adapted to cause a compression e orce of 9 atm. upon the sheath and which eliminates any micro-cavity existing in the cable.
CABLE "B" - ACCORDING TO THE INVENTION
This cable differs from the above-described CABLE "A" solely by the fact that the reinEorcing structure, present around the sheath, is formed by four overlapping layers obtained by windings ~86737 of fiber-glass tapes having a width of 30 mm and a thickness of 0.20 mm. In said tapes, closed, mechanical hysteretical cycles are obtainable with the differences existing between the maximum and the minimum deEormation of up to 1%.
At a temperature of 10C, the tapes forming the eeinforcing structure of cable B are subjected to a tensile stress of 5 Kg/mm2 and this value represents the tensile stress existing in the reinEorcing structure of the cable which is adapted to cause a compression force of 9 atm. on the sheath of said cable.
The experimental tests employed were those known a the "Loading Cycle and Polarity Reversal Tests" recommended by the "Working Group 21-10, ~tudy Committee No. 21 of the CIGRE" and published in the review "ELECTRA", issue No. 72.
Following the methods established by this document, 30 meter lengths of each cable under examination were subjected to thirty thermal heating and cooling cycles, between room temperature and the maximum working temperature, which is 65C for the cables under examination as measured on the conductor, and with increasing, after every said thirty cycles, the value oE the continuous voltage applied to the cable itselE until the value of the voltage at which the performation is found.
In addition, the following four types of cables, identified as "C" - "D" - "F" - "G" which were not constructed in accordance with the invention, were subjected to the above-described experimental tests:
- A first type of cable, CABLE "C"~ having the identical structure, materials and dimensions as that of the cable "A"
according to the invention and which difEers Erom the latter only by the fact that, prior to the cable being put into service, the resistant structure which surrounds the cable sheath does not, as a practical matter, apply any compression force on the sheath.
- A second type of cable, CABLE "D", which diEfers Erom the 73'7 CABLE "C" solely by reason of the Eact that its impregnated insulation has nitrogen at a pressure of 1~ atm. applied thereto.
The second type oE cable belongs to the category of known cables which have their impregnated insulations assisted by the presence of a pressurized gas.
- A third type of cable, CABLE "F", which differs from the CABLE "B" according to the invention solely by reason o the fact that the reinforcing structure disposed around the sheath is formed by four overlapping layers o~ cotton tapes, i.e., tapes which are devoid o-f any closed, mechanical hysteretical cycles.
Said cotton tapes are applied on the sheath with a tension of 5Kg/mm2 which is adapted to cause a compression force of 9 atm.
which guarantees the full impregnation of the cable prior to its being put into service.
- A fourth type of cable, CABLE "G", which differs from the cables according to the invention solely by reason of the fact that the reinforcing structure disposed around the sheath is formed by four overlapping layers of copper tapes, i.e., tapes wherein a closed hysteretical cycle can only be obtained when the 20 difference between the maximum and the minimum deformation does not exceed 0.12%. These copper tapes are applied over the sheath, with a tension of 5 Kg/mm2 which is adapted to cause a compression force of 9 atm. which guarantees the full impregnation of the cable prior to its being put into service.
The experimental tests carried out on a plurality of cable lengths according to the invention and on the other cable lengths described hereinbefore have given the results which are set forth in the following TABLE:
~2~36~37 Increment % oE the increase in perforation voltage with respect TYPE OF CABLE to CABLE C
CABLE A according to the invention 80%
CABLE B according to the invention 80 CABLE D - impregnated insulation with nitrogen under pressure at 14 atm.
CABLE F 15%
CABLE G 20%
An examination of the results set forth in the TABLE
supports the following conclusions.
First and foremost, for the solution to the problem, it appears to be essential that the cable have not only the Eirst characteristic of the resistant structure which brings about the full impregnation oE the cable but also the second characteristic relative to the tapes with which the cable's resistant structure ~0 is formed and which allow for maintaining said ~ull impregnation of the cable during use.
Moreover, with respect to known cables having the insulations thereof impregnated with a compound and subjected to high pressure gas, i.e., cables which to date were considered capable of providing a solution sufficient-Eor overcoming problems of perforation risks caused by the presence of micro-cavities in the insulation, the cables according to the invention provide an increase of 66% in the perforation voltage values.
What is more, the cables according to the invention solve the problem in question without also involving any drawbacks. In particular, the cables according to the invention do not require any modifications to be ef~ected in the plants actually being ~2~6737 utilized for cable manufacture, do not require any introduction of complications in the structure of these cables themselves and do not impose any limi~ations for the lengths of the cables.
Although preferred embodiments of the present invention have been described and illustrated/ it will be apparent to those skilled in the art that various modifications may be made without departing from the principles of the invention.
Claims (6)
1. A direct current electrical cable comprising at least one conductor, a stratified insulation impregnated with an insulating compound disposed around the conductor, an outer metallic sheath around said insulation and a reinforcing structure formed by the winding of at least one tape which completely surrounds the sheath, characterized by the fact that said reinforcing structure is under tension sufficient to reduce the diameter of the sheath and that the tapes forming said reinforcing structure have mechanical hysteretical cycles closed upon themselves, the difference between the maximum and minimum per cent deformation in said cycles being in the range from 0.3%
to 0.5%.
to 0.5%.
2. A cable according to claim 1 characterized by the fact that the tapes forming the reinforcing structure are selected from among nickel-chrome steel tapes, aromatic polyamide tapes, fiber glass tapes and polyester tapes.
3. A cable according to claim 1 characterized by the fact that the tapes forming the reinforcing structure are steel tapes provided with micro-undulations which are disposed in substantially a direction tranverse to the length of the tape.
4. A cable according to claim 1 characterized by the fact that the tapes forming the reinforcing structure are steel tapes provided with micro-drawings disposed in honeycomb fashion.
5. A cable according to claim 1 characterized by the fact that the tapes forming the reinforcing structure are steel tapes provided with rhombus shaped slottings.
6. A cable according to claim 1 characterized by the fact that the compound impregnating the stratified insulation of the cable, has a viscosity at room temperature in the range from 1000 to 50,000 cSt.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| IT21405A/86 | 1986-08-04 | ||
| IT21405/86A IT1197064B (en) | 1986-08-04 | 1986-08-04 | DIRECT CURRENT ELECTRIC CABLES WITH MIXTURE IMPREGNATED ISOLATEN |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1286737C true CA1286737C (en) | 1991-07-23 |
Family
ID=11181281
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000543527A Expired - Lifetime CA1286737C (en) | 1986-08-04 | 1987-07-31 | Direct current electric cables having a coumpound impregnated insulation |
Country Status (4)
| Country | Link |
|---|---|
| BR (1) | BR8704536A (en) |
| CA (1) | CA1286737C (en) |
| GB (1) | GB2196781B (en) |
| IT (1) | IT1197064B (en) |
Families Citing this family (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US6395975B1 (en) | 1998-07-06 | 2002-05-28 | Pirelli Cavi E Sistemi S.P.A. | High voltage direct current electrical cable with mass-impregnated insulation |
| PT2512803E (en) * | 2009-12-16 | 2013-12-23 | Prysmian Spa | High voltage direct current cable having an impregnated stratified insulation |
-
1986
- 1986-08-04 IT IT21405/86A patent/IT1197064B/en active
-
1987
- 1987-07-31 CA CA000543527A patent/CA1286737C/en not_active Expired - Lifetime
- 1987-08-04 BR BR8704536A patent/BR8704536A/en not_active IP Right Cessation
- 1987-08-04 GB GB8718458A patent/GB2196781B/en not_active Expired - Lifetime
Also Published As
| Publication number | Publication date |
|---|---|
| IT8621405A0 (en) | 1986-08-04 |
| BR8704536A (en) | 1988-04-26 |
| GB8718458D0 (en) | 1987-09-09 |
| GB2196781A (en) | 1988-05-05 |
| IT1197064B (en) | 1988-11-25 |
| GB2196781B (en) | 1990-05-02 |
| IT8621405A1 (en) | 1988-02-04 |
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| MKEX | Expiry |