US3594492A

US3594492A - Pipe-type cable systems with reduced ac losses

Info

Publication number: US3594492A
Application number: US862353A
Authority: US
Inventors: George Bahder; Carlos Katz
Original assignee: General Cable Corp
Current assignee: General Cable Corp
Priority date: 1969-09-30
Filing date: 1969-09-30
Publication date: 1971-07-20
Anticipated expiration: 1988-07-20

Abstract

A magnetic, low-loss liner in a metal pipe reduces the AC loss of high-voltage electrical cable enclosed within the pipe; or the cable in the pipe can be wrapped with a sheet or tapes of the magnetic low-loss material. Tapes used for the purpose can be plastic with suitable metal, such as ferromagnetic material of high permeability distributed through the plastic.

Description

United States Patent [72] Inventors George Bahder 2x B6 MB 14 5 un BM 41 7 1 2,992,425 7/1961 3,080,446 3/1963 V01k......... 3,160,702 12/1964 Lapsley 3,179,904 4/1965 Paulsen 3,339,007 8/ 1967 Blodgett.... 3,413,405 11/1968 Myers........... 3,484,532 12/1969 Anderson.....................

FOREIGN PATENTS 66 mm UN .m m PM U M m e .n C mm 8. cl .I-T GF M an h ww mm WMmm a m 38 ME 55 t M mm m 541 PIPE-TYPE CABLE SYSTEMS wrru REDUCED AC LOSSES 16 Claims, 12 Drawing Figs.

Attorney-Sandoe Hopgood & Calimafde ed3d .h imwmm w m c s m mmdw m mewn ilpTs, a., 33 m m Ufmm mmm uw b sum. 1% i mmm m c mmw .wfi an mmm m m eoe M0 8 .m r mmm Ac..o m 63mm D. m um mr m em w am mn erm Mmmmw 5 3 m m n us l/ 3 04.0. H7Ir. m mn M m N W m n .u m MM; N m 3 U m 6 L12 M .mF um m mm 5 174125 the plastic.

2,665,328 1/1954 Atkinson e1 rents.

may break through, causing I 1 shield-and subsequently, the cable insulation.

ssckosouuoanpsuhmssvF THE INVENTION This invention relates to high-pressure pipe-type cables in-' stalled in'metallict'ype 'pipes, such as'a steel pipe. These ca bles operate at" pressure oil or gas and are used for tu l-v I dergroundand submarine "transmission of electric power at highand extra high voltages. In particular, the present inven tion relates to pipc typcbable systems with reduced AC losses.-

The of steel'pipesas an enclosure for'the three-phase cablesystem increasesand distorts the magnetic field inthe ages For this cables installed in the pipe must be surrounded with alayer of 'special'magnetic material. The sur- "rounding layer should cover at' least three-quarters, but

preferably more, of the,cable assembly circumference and-' must have the followingproperti esz conductors, causing a relativel'y large increase'in the conductors AC resistance over-the corresponding' valuesfor the cables placed in airzln addition to theincrease inconductor AC resistance, big'losses. are originated. in the pipe duefto eddy currents and hysteresis.. -For instance, ina three-phase, 345

kV, HPOF cable system having segmental copper conductors with c2000 M'cM cross section, the total losses in pipe,..with the conductors'arranged in-a triangular formation, are in creased about'3'5 percent with respect to the losses measured in air and-withthe conductors arranged" in; cradle formation this increue is about SOpercentofthe corresponding values inair. J t I. v

' Severalffltempts have been 'made in the past to reduce the I increase in AC resistance due to the use of steelpipes in highpressurecable-s'ystems. following methods were proposed to reduce the AC losses in pipe-type cables:

I. Theuse f pipes made of a different material '2. The use of magnetic pipesprovided with asection of non magneticmaterial,- .or a' section having ilongitudinal slits u. s, Pati 1Ne'. 2 ,7 1s,s42- or wvidedjjwith circumferential slits (Us; Pat. NO; 2,787.65 1 i a I v Considerations; madekinthe past at 60 f I Hz could not be'redueed over those attainable with steel pipes,

by using made of any availablenonmagnetic metal of reasonable low'aprice. Tests using n'onferrous materials such as Everdur pipe (6.5percentconductivity of copper) and aluminum pipe.(l percent conductivity of copper) and aluminum pipe(6'l percent conductivitybf'copper) gave AC re-' sistance of the same order of magnitudeasobtained with steel u. -s. Pat. ue. 2.713.542 Electric Cable; .Systems" describes. the use ofa magneticpipe having a small longitudinalseetionof nonmagneticmate'rial and/or suggests the use of a pipemade of magnetic-material provided with numerous longitudinal slits. u.s.' Pat.; No. 2,787,651, .Electric Cable Systems suggests the of a pipe made of magnetic material provided'with numerous'circumferential, or a helical slit.

These methods provide only a very slight reduction in the AC 1 than steel. v

i Theme er {pipe "lined with a tmagnetic field trap (U.S.'

l. The reluctance of the layer along the circumferenceshould be lower than 5X l0 ill-l per foot ofcable.

: ZgThe 4 resistance in the longitudinal-direction should be a higher than 10" 'ohmsper foot ofeable.v a

3. The electrical resistance in theradial direction should be i than I ohmper foot ofcable.

With this invention at least v23 when transmitting AC can be' saved; or the line-current-ca'rry- I ing capacity can be increased accordingly.

' Otherobjects features and advanta'gesof the invention will appear or be pointed out'asthe description proceeds.

BRIEF osscmvrlou 0F DRA ING ln'the drawing, forming a part hereof, in which like reference characters indicate corresponding parts in all. the viewszf 1 1 FIG. I .is a diagrammatic view showing electric cables enclosediwithin a pipe and provided with shield, in accordance with this invention; FIG. 2 isa modification of the construction shown in FIG. 1

with the'special magnetic shield made a part of the cable;

'FIG. 3 is a diagrammatic view showing material from which the special magnetic shield can be made; I

FIG; 4 is a'greatly enlarged sectional view'of the line 4-4 of FIGS;

FIG. 5 a a view similar m Ficl's'but showing a-modified of magnetic shield material;

- forthe specialmagnetic shieldf I 1 a FIG. 7 h a sectional view taken on the line 1-1 of FIG. 6;

I G. 8 -is a view similar to FIG. 6 bu t showing still another 7 with thisinvention;

' Dsscs r'nou or PREFERRED smooth/mars FIG. 1- ehewe electrical'cables 10 0f conventional construction andeach of which comprises a center conductor 12, a

' conductor shield. insulation 15, an insulation shield 16,

losses, much below the valuewhich might be interesting from the practicalpointof view. This isdue to the'fact-that the the proposed methods giveonlya small reduction of these curus. PathNo. 13,466,702, f Alternating Current Pipe CAble System with Magnetic Field Trap,'-discloses-the use of a pipe lining material'tnade o fa dielectric resin and having dispersed- .in its magneticinateriai. Such pipe iiningcreates an insulation layer between the cables and the pipe and consequently the cable isinsulate'd from-the pipe. The use ofsuch a pipe apovervoltages. Under overvoltage conditions the lining material arcing which destroy the cable 'With the invention described in this specification, the AC 4 major portion of the-losses are caused by eddy currents and overlying moisture protection tapes l8 and skid wires 20. These cables 10 carry three-phase power 'and they are enclosed in a metal pipe 22, usually a steel pipe.

In order to reduce the alternating current power losses, the

metal pipe-22 has a special magnetic liner 24. This liner provides a magnetic shield inside the pipe 22. The liner 24 is" 'made'of. materialhaving high permeability and low loss. The

magnetic shield-can be a metalalloy, as will be explained in connection with the other figures. 1

When used an liner for the-pipe 22, the magnetic shield 24 preferably covers the entire inside surface of the pipe; but this isnot essential; For good results, however, the shield 24 should extend around at least'three-quarters of the circumference of the pipe. The shield 24 reduces the magnetic flux in the pipe 22 and thereby decreases the AC losses. On the other pears to be'dangerousfor the cable system in case oflightning, and switching surges, and in case ofground faults which cause losses in pipe-type cablesystems can be substantially reduced without increasing thesusceptibility of the syst'em'to overvolt-.

hand, the high longitudinal resistivityof the shield 24 prevents high losses within the shield.

In describing the special magnetic shields of this invention as having high 'permeability and low'loss, these terms are to be understood as designating the relation between themagnetic flux density at a pointin a material to'the'magnetic'intensity at the same point, and the reluctance and resist'ance'characteristics described above. lnidescribing the magnetic shield as percent of'the power loss,

a special magnetic FIG. 6 is'asectional view anotherfform of material r the cable.

I being made of a metal alloy, the term alloy" isu'sed in a spe cial broad sense to include not only truealloys of metalsbutf "also metal in powdered'form dispersed throughout a plastic matrix or metal encapsulated in plastic tapes or sheets, as will be described in connection with FIGS. 5- 1 l.

' With the construction shown' in FIG. I, this invention requires no change in the design of the cables. In the construction which will be described in connection with FIG. 2, special 24 intothepipebecaus'e the liner is made a part of the cable itself and is, therefore; conveniently pulled into the pipe 'with FIG. 3 shows a section of the metal sheet before it is inserted into the pipe 22.0'f FIG. 1. The sheet is preferably a strip having a width substantially equal to the inside circumference ofthe pipein which it is to be used. This particular sheet is a low-loss, grain-oriented silicon steel, as indicated by the cross section shown inFIG. 4., l

The low-loss,high-permeabilitymetal can also be manufactured into a pipewith an outside diameter'equivalent to the inside diameter of-the pipe 22 and factory installed in the pipe Ifthe materialusedfor the special magnetic 'shield is sulfi- V ciently flexible it can be wrapped as a sheet around the group" of cablesibut to make this more practical with cables that have to be pulled into. long lengths of pipe 22, a cable of speconductors to form acompositefcable. Skid wires 36 are I wound over the special magnetic shield 34.

" Ifthe resistivity of atape 38 used for the magnetic shield 34 would otherwiseb'e too high, it is intercalated with another tape 40 which maybe nonmagnetic or magnetic material but which has good electrical and thermal conduction properties.

.Copperor aluminum are'appropriate for the tape 40. With this intercalated construction, fault currents or overvoltages beeasily dissipated tog'round- I If the special cable construction shown in FIG. 2 is not commercially available, the wrapping of the

tapes

38 and 40 and the application of the skid wires 36 canbe done in the field at the time the cables are to be pulled into the pipe 22.

FIGS. l'and2 show the'conductors 12 and 12, respectively, in a triangular configuration. The invention can beused with cables in a cradle configuration or in any other configuration but because the triangular configuration in a cable system has lower losses, it is more efficient for. use with this invention.

FIG. 5 shows a cross section of a tape made of plastic material having ferromagnetic material dispersed through the plastic. The ferromagnetic material can be metal or synthetic; and it should have a very fine powdery appearance before being mixed. Magnetic materials such as polycrystalline ceramics used in the manufacture of ferrit'es, containing iron,

1 oxygen and one or more of the following metals: copper,

nickeL'zinc, cadmium, magnesium, manganese, or others providing the characteristics 'of magnetic ferrites, are appropriate for this invention.

FIGS. 6 and 7 show. a special construction in whicha lowloss magnetic material in the form of rods 50 are iinb'edded or encapsulated in a plastic tape 52. These rods 50 are disposed in two rows with the rods in one row staggered in relation to cable is used which does not require the insertion of the liner Ibination or FIGS. 8 and9 show' a'modified construction in which plates 1 tape 58.

FIGS. I0 and 11 show anotherconstruction in which a sheet or tape of metal 60 is of continuous extent throughout the length and breadth of a tape 62; This metal 60 is a low-loss magnetic metal, as in thecase of the rods and plates of FIGS. 6- -9, and its flexibility is increased by having it corrugated with the corrugations extending transversely of the lengthof the tape, and preferably alright angles to the length of the tape. The metal 60 is coatedon both sides with plastic 64. v

The magnetic low-loss metal alloysand the'flexible comcornposite magnetic. materials previously described, can be used in the form of pipes, sheets, tapes, strips or any other form suitable for theproposed application. The materials can be used in single or multiple layers, they can be plain or embossed, they can be coated or not coated. In the case where they are used in the form of tapes, they may be applied butt and if the thickness allows, overlapped. The magnetic shield materials to be used in the described systems have to be able to: withstand the mechanical stresses, which may be developed during installation or service life of the cables without breaks or tears; they have to be compatible with the oils (natural or synthetic) used in' cable installations; and they have to be able to withstand the cable service temperatures. Some high polymer materials, such as polytetrafluorotheylene, hexafluropropylene, polyethylene terephtalate, polypropylene, polycarbonate and others are appropriate for use as encapsulating materials; however, any other, materials providing the above characteristics could be used for this application- The combination or composite material can be irradiated to improve thememory and thermal characteristics of the plastic. 1

In cases where the tape used for the special magnetic shield has electrical resistance which. is otherwise too high,-the insulation shield can be-connected, to the metallic pipe by acontinuous strip of highly conductive material applied intercalated with the magnetic shield material. It should be understood that the low-loss magnetic materials used for the special magnetic shield of this invention can be applied in either single or multiple layers; and that the material can be plain or embossed and coated or not coated, as conditions warrant.

FIG. 12'is a graph showing new results obtained with this invention, as compared with the results obtained by the prior art.

FIG. 12 allows the comparison of two sets of AC/DC resistance ratios measured at various currents on an assembly of three cables in a triangular configuration. In both cases, the same ID inches ID steel pipe was used. The upper curve indicates AC/DC resistance ratios for the'cables installed in the pipe without a magnetic shield (present practice). The lower .curve indicates A'C/DCresistance ratios for the same cables installed using the high-permeability,relatively high-resistivity screen in accordance with one of the variations of the disclosure (wrapped as illustrated in FIG. 2). The pipe used for these tests is similar to that used by an electric utility for the installation of 345 kV, HPOF cables. The high-permeability material used in this particular case consisted of a grainoriented silicon steel tape, 2 inches wide, l4 mil thick, insulated with an inorganic coating. This material had a relative magnetic permeability s) of about 6000 at a flux density of 50 gausses and a resistivity of 50 microhm=centimeter.

FIG. 12 gives the AC/DC resistance ratios; which are practically independent of the current. For any given current the AC/DG ratio is equal to the ratio of watts loss when carrying alternating-current to the watts loss when carrying direct-current. I

T Further tests indicate that by using other variations of these methods, considerable savings in electric power or increase in Cable Assembly: AC/DC Resistance Ratio:

1. In a steel pipe as presently used 2. In a steel pipe lined with a high permeability siliconsteel, except for a longitudinal strip l inch wide 3. In a steel pipe lined with a high permeability silicon lteel lOO percent coverage) 4. In a steel pipe with the three cables wrapped in a sheet of high' permeability silicon steel It was found that with the cables and conditions used during these tests, an increase in the thickness of the silicon steel above the 14 mils used would not produce any further significant decrease in the AC/DC resistance ratios of this cable system.

The preferred embodiments of the invention have been illustrated and described and they are defined in the appended claims.

We claim 1. An electrical cable system for transmitting alternatingcurrent when the cable is enclosed in a pipe, including a group of individually'insulated conductors extending side by side, and a special magnetic shield surrounding at least three-quarters of the circumference of the group, the shield being made of material havinghigh permeability and low loss, the magnetic shield having a reluctance along its circumference lower than 5Xl0" l/H per foot of the cable system, an electrical resistance in the direction of the length of the cable system higher than ohms per foot of the cable system, and an electrical resistance in the direction of the radius of the cable system greater than 1 ohm per foot of the cable system.

2. The electrical cable system described in claim 1 characterized by the magnetic shield having a reluctance along its circumference lower than 5X10 l/l-l per foot of the cable system, an electrical resistance in the direction of the length of the cable system highe r than 10" ohms per foot of the cable system, and an electrical resistance in the direction of the radius of the cable system greater than 1 ohm per foot of the cable system.

3. The electrical cable system described in claim 1 characterized by the special magnetic shield being wrapped around the outside of the group of insulated conductors, and a skid wire over the outside of the shield.

4. The electrical cable system described in claim 1 characterized by the individual conductors being wrapped together by the special magnetic shield.

5. The electrical cable system described in claim 1 characterized by there being three conductors in the group and each of which has a conductor shield, an insulating layer over the conductor shield, an insulation shield covering the outside of the insulation, moisture protection means outside of the insulation shield, the special magnetic shield extending around the full circumference of the group of conductors outside of the moisture protection means, and a skid wire over the outside of the special magnetic shield.

6. The electrical cable system described in claim 5 characterized by the special magnetic shield comprising tapes made of plastic material with metal distributed through the plastic, the metal distribution including metal that is oriented so that .the projections thereof on the inner surface-of the tape cover the entire inner surface thereof.

'7. The electrical cable system described in claim 4 characterized by the special magnetic shield being a helically wound layer of flexible tape made of plastic material having two rows of flexible rods therein with the rods extending lengthwise of the tape and with the rods of one row in staggered relation to the rods of the other row, all of the rods being generally parallel to one another and the rods of each row being spaced from one another transversely of the tape, and the rods of one row being of a diameter greater than the spacing of the rods of the other row.

8. The electrical cable system described in claim 4 characterized by the special magnetic shield being a helically wound layer of flexible tape made of plastic material having two rows of plates therein with the platescxtending lengthwise of the Q tape and with the plates of one row in staggered relation to the plates of the other row, all of the plates being generally parallel to one another and the plates of each row being spaced from one another transversely of the tape, the plates of one row being of a width greater than the spacing of the plates of the other row. I

9. The electrical cable system described in claim 4 characterized by the special magnetic shield being a helically wound layer of flexible tape made of plastic material having ferromagnetic material dispersed throughout the plastic.

10. The electrical cable system described in claim 4 characterized by the special magnetic shield being made'of flexible magnetic tape.

11. The electrical cable system described in claim 10 characterized by the magnetic tape being intercalated with metal tape.

12. The electrical cable system described in claim 4 characterized by the special magnetic shield being a helically wound layer of flexible metal tape with the metal corrugated and the corrugations extending transversely of the tape, and with layers of plastic bonded to the surfaces of the corrugated metal.

13. The electrical cable system described in claim 4 characterized by the special magnetic shield being made of flexible magnetic tape, a metal pipe in which the cable is enclosed, and

a continuous strip of highly conductive metal connecting the shield to the pipe in which the cable is enclosed.

14. The electrical cable system described in claim 1 characterized by a metal pipe in which the cable is enclosed, the special magnetic shield being a liner in the pipe in contact with the inside surface of the pipe and extending around substantially the entire circumferential extent of said surface.

15. An electrical cable system enclosure including a metal pipe, and a liner in the pipe made of metal having higher permeability and lower loss than the metal pipe the liner having a reluctance along its circumference lower than 5X10 l/H per foot of the cable system, an electrical resistance in the direction of the length of the cable system higher than l0 ohms per foot of the cable system, and an electrical resistance in the direction of the radius of the cable system greater than 1 ohm per foot of the cable system.

16. The electrical cable system enclosure described in claim 15 characterized by the liner being an inner pipe enclosed in the first pipe.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3 594 492 Dated JulV 2O 1971 Inventor) George Bahder et al It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 1, lines 44 and 45, cancel "an aluminum pipe (61% conductivity of copper)"; Column 2, line 8, "4" should read electrical C lumn 2, line 11, Column 5, lines -37 and 44 and Column 6, line 57,

"10 each occurrence, should read 10' Signed and sealed this 1st day of August 1972.

(SEAL) Attest:

EDWARD M. FLETCHER,JR.

Commissioner of Patents FORM PO-105O (10-69) USCOMM-DC 60376-P69 n u 5. GOVERNMENT Pmmme OFFICE l9" O'!66-Cl!l

Claims

1. An electrical cable system for transmitting alternatingcurrent when the cable is enclosed in a pipe, including a group of individually insulated conductors extending side by side, and a special magnetic shield surrounding at least three-quarters of the circumference of the group, the shield being made of material having high permeability and low loss, the magnetic shield having a reluctance along its circumference lower than 5 X 107 1/H per foot of the cable system, an electrical resistance in the direction of the length of the cable system higher than 10 4 ohms per foot of the cable system, and an electrical resistance in the direction of the radius of the cable system greater than 1 ohm per foot of the cable system.

2. The electrical cable system described in claim 1 characterized by the magnetic shield having a reluctance along its circumference lower than 5 X 107 1/H per foot of the cable system, an electrical resistance in the direction of the length of the cable system highe r than 10 4 ohms per foOt of the cable system, and an electrical resistance in the direction of the radius of the cable system greater than 1 ohm per foot of the cable system.

6. The electrical cable system described in claim 5 characterized by the special magnetic shield comprising tapes made of plastic material with metal distributed through the plastic, the metal distribution including metal that is oriented so that the projections thereof on the inner surface of the tape cover the entire inner surface thereof.

7. The electrical cable system described in claim 4 characterized by the special magnetic shield being a helically wound layer of flexible tape made of plastic material having two rows of flexible rods therein with the rods extending lengthwise of the tape and with the rods of one row in staggered relation to the rods of the other row, all of the rods being generally parallel to one another and the rods of each row being spaced from one another transversely of the tape, and the rods of one row being of a diameter greater than the spacing of the rods of the other row.

8. The electrical cable system described in claim 4 characterized by the special magnetic shield being a helically wound layer of flexible tape made of plastic material having two rows of plates therein with the plates extending lengthwise of the tape and with the plates of one row in staggered relation to the plates of the other row, all of the plates being generally parallel to one another and the plates of each row being spaced from one another transversely of the tape, the plates of one row being of a width greater than the spacing of the plates of the other row.

10. The electrical cable system described in claim 4 characterized by the special magnetic shield being made of flexible magnetic tape.

13. The electrical cable system described in claim 4 characterized by the special magnetic shield being made of flexible magnetic tape, a metal pipe in which the cable is enclosed, and a continuous strip of highly conductive metal connecting the shield to the pipe in which the cable is enclosed.

15. An electrical cable system enclosure including a metal pipe, And a liner in the pipe made of metal having higher permeability and lower loss than the metal pipe the liner having a reluctance along its circumference lower than 5 X 107 1/H per foot of the cable system, an electrical resistance in the direction of the length of the cable system higher than 10 4 ohms per foot of the cable system, and an electrical resistance in the direction of the radius of the cable system greater than 1 ohm per foot of the cable system.