GB1571460A - Superconductive cable - Google Patents

Description

(54) IMPROVED SUPERCONDUCTIVE CABLE (71) We, GOSUDARSTVENNY NAUCHNO-ISSLEDOVATELSKY ENERGETICHESKY INSTITUT IMENI G.M. KRZHIZHANOVSKOGO, of Leninsky prospekt, 19, Moscow, USSR., a USSR corporate body, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- The present invention relates to electrical power transmission cables and, more particularly, to multisection superconducting cables for carrying alternating current and can find its practical utilization, for example, in superconductive power transmission lines.

It is an object of the present invention to provide a cable of small size, a cable which may lose its superconductivity under a fault current condition and which insures uninterrupted transmission of power to the users.

A further object of the invention is to simplify the cable production techniques.

Still another object of the invention is to improve cable stabilization under the rated current conditions.

Yet another object of the invention is to reduce the amount of metal used in cable production and to enhance the stabilizing effect of a conductor core made of a normal metal.

Still another object of the invention is to improve the effectiveness of the thermally insulating envelope.

A further object of the invention is to enhance the reliability of cable operation under overload current conditions.

According to the present invention there is provided a multisection superconductive cable for carrying alternating current, each section of which comprises a heat insulation envelope surrounding a superconductive shield having a stabilizing substrate and which surrounds an electrical portion for carrying an electrical current and including at least one conductor assembly of two coaxial cores, the outer of said cores being made tubular and including a superconductive layer disposed upon a stabilizing substrate, and the inner of said cores being made of a normal metal, each said stabilizing substrate being formed by segments of a stabilizing material separated by a material of a lower electrical conductivity than that of the stabilizing segments and the effective resistance of each said stabilizing substrate within any cable section being in excess of the impedance of said electrical portion of said section when both cores thereof are electrically connected together at least at the ends thereof while the radial thickness of the stabilizing segments material is not in excess of the effective depth of penetration of alternating current.

It is preferred to make the stabilizing substrate of the shield similar to the stabilizing substrate of the conductor assembly.

In a modification, a dielectric can be used as the material separating the segments of the stabilizing material.

Where the cable shield is used as a return wire it is expedient to supply the shield with a normal metal core that surrounds the shield and is separated from it with a layer of thermal insulation, the shield and the core being electrically connected together at least at the cable ends. To simplify the cable production techniques, a metal or alloy may be used as a separating material for the stabilizing material of the substrate.

For better stabilization of the cable operation under the rated current conditions, it is preferred to reinforce the segments of the stabilizing material with a superconducting material.

Where a metal or an alloy is used as a separating material, the exterior surface of the separating material of the shield and the interior surface of the separating material of the conductor may be covered with a layer of a superconductive material similar to that of the basic superconductive layer.

It is also practicable to reduce the amount of metal utilized in the cable and to enhance the stabilizing action of the normal metal core of the conductor operating under the rated conditions by separating the normal metal core from the superconductive core of the conductor with a layer of thermal insulation.

It is also preferable in order to obtain better thermal insulation to use superconductive jumpers for electrical connection between the or each respective normal metal core and the superconductive core.

It is also expedient for more reliable operation of the cable under overload current conditions to place, at least at the cable ends, superconductive inserts made of type II superconductor and satisfying the condition.

H2 I2 (2) H1 Ir where: H, and H2 are the first and second critical fields of the superconductive material of the insert, respectively; and I, and I2 are the rated and overload currents of the cable, respectively.

The exact nature of the invention will be readily apparent from the following specification of the superconductive cable with due reference to the accompanying drawings in which: Fig. 1 is a transverse sectional elevation of the single-phase superconductive cable with tubular coaxial conductors; Fig. 2 is a longitudinal sectional view of the cable shown in Fig. 1; Fig. 3 is showing construction of the stabilizing substrate of the superconductive core; Fig. 4 is a section through line IV-IV of Fig. 3; Fig. 5 is showing electrical connection of the superconductive and normal metal cores for both the forward and return wires of a single-phase cable; Fig. 6 is a transverse sectional elevation of a three-phase superconductive cable with ribbon-type conductors; Fig. 7 is a variant of the single-phase cable with thermal insulation;; Fig. 8 is a variant of construction of a single-phase cable wherein the stabilizing material of the substrate is reinforced with a superconductive material; Fig. 9 is a longitudinal sectional view of a single-phase cable with inserts placed between individual lengths thereof.

The embodiment of a cable according to the invention shown in Figures 1 and 2 comprises a thermally insulating envelope 1 surrounding an electrically conductive portion 2 including a core 3 which is made tubular and is provided with a superconductive layer 4 which is stabilized by means of a substrate 5, and a normal metal core 6 running inside the core 3, as well as a superconductive shield 7.

The shield 7, which may also serve as a return conductor and will hereinafter be referred to as a return wire, is supplied with a substrate 8 similar to the substrate 5 of the electrical portion 2 while the return wire 7 is surrounded with a core 9 made from a normal metal. The stabilizing substrates 5 and 8 are formed by locally situated segments 10 (see Figs 3--4) of a stabilizing material, which segments are separated from one another by a material 11 of lower electrical conductivity than that of the segments. The core 6 is electrically connected to core 3 and the core 9 is connected to the return wire 7 at junction points 12 (see Fig. 5) designed for connection of the cable to the terminal devices 13, and the cores may also be connected together at junction points 14 between two adjoining lengths 15 of the cable. Cooling of the cable is effected by helium, fed into a space 16 (see Figs. 1 and 2), which, in the single-phase cables with tubular conductors, is used as the main electrical insulating member. With three-phase cables incorporating ribbon-type conductors, electrical insulation is effected by means of a hard ribbon-form dielectric 17 (see Fig. 6).

Positioning of the forward and return wires in a single-phase cable is effected with the use of dielectric spacers 18 (Figs. 1 and 2) supplied with electrodes 19.

Positioning of the inner core 6 within the tubular core is accomplished with the use of supports 20, while positioning within the thermally insulating sheath of the entire electrical portion of the cable is effected with the use of supports 21.

A three-phase cable may, as shown in Figure 6, comprise three electrical portions 2, 22 and 23, (Fig. 6), the two last-mentioned electrical portions being made similar to the electrical portion 2 whose sectional view has been shown.

Fig. 6 shows a support 24 for the superconductive core 3 of the cable with ribbon conductors.

Advantages of the proposed construction of the cable will be further explained with reference to Figures 1 and 2, showing a single-phase coaxial cable including the electrical portion 2 and tubular shield which is used in this case as a return wire.

An analytical expression of redistribution of current between the superconductive core and the normal metal core for a single-phase coaxial cable may be written as

where: I4=total current at cable input; I3=current in superconductive core; R,=effective resistance of normal metal core; R2=rms (equivalent) resistance of a superconductive core taking into account the substrate resistance; wAL=difference between impedances of superconductive core and normal metal core.

Superconductive cables which use high purity metals satisfy the following condition: AL > R, (4) When the input current I4 does not exceed the critical value,

However, as the input current grows in excess of the critical value, a sharp increase in the superconductor resistance is produced due to movement of the magnetic lines of force. Since the superconductive core is situated in a cryogen with a finite value of heat transfer coefficient, the superconductor starts heating up thus bringing about an avalanche-like process of destruction of superconductivity, i.e., a growth of resistance R2 up to the value corresponding to that of.a superconductor that has changed to a normal nonsuperconducting state and is shunted by the substrate.

As R2 increases, a current redistribution takes place between the superconductive core and the normal metal core.

Thus, for example, at R2= AL,

For a cable with a substrate of conventional design but supplied with a normal metal core, the resistance of the superconductive core will be controlled by the resistance of the substrate, which, even if it equals the impedance wAL; which is inconsistent with the requirements for stabilization of a superconductor under normal rated conditions, will result in a similar redistribution of current::

In the proposed construction, the values of R2 exceed wAL due to a substantial increase in resistance offered by the substrate to the current carried, which results in a current redistribution that may be written

Consequently, a greater portion of the current will be flowing in the normal metal core constructed, for example, of transposed conductors wherein heat evolution is no longer governed by the perimeter of the superconducting core but by the section surrounded thereby.In situations like this, a decrease in heat evolution in such a core is related to heat evolution in the substrate of the cable of conventional design by W, D =K, (6) W2 8 where W,=heat evolution in substrate of conventional cable; W2=heat evolution in normal metal core of proposed cable; K=coefficient of filling of conductor section with normal metal core; D=diameter of conductor; 8=skin-layer of substrate stabilizing material.

Elimination of pronounced influence of eddy current losses in a normal metal core upon the total heat evolution in said core can be easily accomplished by selecting a suitable diameter for individual strands of the core.

The requirements for resistance of the superconductive core may be related to the following considerations.

Heat evolution in the substrate of a conventional cable equals W=I2R, (7) where: I=overload current (input current of cable); R=resistance of substrate.

The substrate resistance will be given by 1 R=p , (8) D6 where: p=specific electrical resistance of stabilizing material of substrate; I=cable length.

On the other hand, heat evolution in a superconducting core (with the same cable length) will be given by (wAL)2 W=I2 (9) R2 So, resistance of a superconductive core will be in excess of (wAL)2 (10) R If the above condition (10) is satisfied, the total heat evolution in a normal metal core and a superconductive core proves to be far less than the losses in the substrate of a conventional cable of the same diameter.

However, if practically all of the fault current is passed through the normal metal core, the substrate of the superconducting core becomes situated within an alternating magnetic field with the result that eddy currents are induced in the substrate stabilizing material. Selecting the thickness of the substrate stabilizing material less than its skin layer thickness results in reduction of the eddy current losses with respect to the heat evolution in the substrate of a conventional cable by an amount given by N=0.5( )3, (11) A where: N is a number which indicates the number of times the eddy current losses are less than the losses that result from passage of a transport current in the substrate of a conventional cable.

A substantial decrease in thickness of the stabilizing material layer of the substrate is limited by the requirements to stabilize the superconductive core under the rated current conditions.

Hence, the maximum decrease in heat evolution and, therefore, in size of the cable can be provided in a cable which employs superconductors whose resistance is greater than the normal conditions and whose stabilizing substrate offers maximum resistance to a transport current, the thickness of the substrate stabilizing material layer being less than its skin-layer thickness.

As is apparent from the above estimates, in the proposed cable the biggest contribution to the total heat evolution is made by the eddy current losses in the substrate.

Shunting the cable sections by a normal metal core makes it possible to protect the superconductor of the cable from burnouts in the event that the normal zone continues to spread along the superconductor despite the stabilizing action of its substrate under the rated conditions. Such an arrangement enhances reliability of the cable under the normal rated conditions.

Fig. 7 shows an embodiment which is characterized in that in a single-phase cable with tubular coaxial conductors there is provided an effective thermal insulation of, for example, vacuum type, which insulation separates the normal metal core 6 from the superconductive core 3 and also separates the return wire 7 from the normal core 9 surrounding said return wire.

The superconductive core 3 and the return wire 7 are each supplied with a vacuum-tight sheath 25 made, for example, of steel and are separated from the respective cores 6 and 9 of normal metal, made as tubular conductors, by means of a thermal insulation medium 26, said medium being vacuum in this case. Electrical connection of the cores 3 and 6 to each other and of the return wire 7 to the core 9 is accomplished with the aid of jumpers 27 made of a superconductive material.

Proper spacing of the cores 6 and 9 is achieved through supports 28 of low heat conductivity.

In the illustrated embodiment, the normal metal core acts as a heat accumulator for the period of a short circuit, and absorption of heat by the refrigerant can be made commensurable with the rate of flow of the refrigerant.

Introduction of thermal insulation into construction of the cable makes it possible to simplify the construction of the normal metal core, to make it, for example, tubular, to reduce metal content in the core, to select a cheaper though less pure material for the conductor, to enhance the stabilizing effect of the core upon the superconductor under the rated conditions. For electrical connection of the superconductive core 3 and the normal metal core 6 (similarly, the return wire 7 and its core 9) there can be employed superconductive jumpers acting as heat dissipating elements. Besides, incorporation of a thermal insulation member makes cooling of the normal metal core 6 optional, said cooling being so arranged that it may differ from the cooling of the superconductive core 3 of the cable in that it can be advantageously accomplished, for example, through the medium of such a refrigerant as liquid hydrogen.

As stated above, the eddy current losses in the stabilizing material of the substrate are largely responsible for heat evolution occurring under a fault current condition, the magnitude of said losses being governed by the thickness of the substrate and specific resistance of the stabilizing material thereof.

However, the thickness of the stabilizing layer can be reduced and conductivity of the stabilizing material worsened only within the scope determined by the requirements for stabilization of the superconductive cable under the rated conditions of its operation.

On the other hand, employment of such superconductors as say, niobium-tin (Nb2Sn) whose production techniques dictate the presence of a niobium underlayer used to shunt the superconductor and the substrate places less stringent requirements for the electrical conductivity of the separating material of the substrate thus making it possible to use as a separating material not necessarily a dielectric but, for example, niobium or its alloy, as is the case under discussion.

Accordingly, an embodiment will now be described with reference to Fig. 8, showing a single-phase coaxial cable with tubular conductors, the techniques of production of which cable being simplified and stabilizing properties of the substrate considerably improved.

In the cable according to the invention, the local segments 10 of the stabilizing material of the substrates 5 and 8 are reinforced with superconductor 29, while the exterior surface of the separating material layer 11 of the substrate 8 of the shield 7 and the interior surface of the separating material layer 11 of the substrate 5 of the superconductive core 3 are covered with a layer 30 of a superconductive material like Nb3Sn (niobium-tin), the basic layers 11 of the separating material being made, for example, of niobium alloyed with zirconium.

The superconductive materials used in the substrates 5 and 8 do not exceed in their current carrying capacity the basic superconductive layer and do not create single superconducting paths for transport of electrical current, but only cause certain localized growth of conductivity thus reducing the heat evolution during stabilization of the basic superconductor and making it possible to either reduce the thickness of the substrate or to worsen the purity of the normal metal, which results in decreasing the eddy current losses.

Such an embodiment paves the way to utilization of production wastes of certain industries engaged in production of superconducting busbars for various magnetic systems and electrical machines and, besides, is based on the existing techniques of application of superconductive layers.

The most up-to-date techniques used for producing blanks for current carrying cores, for example, of coaxial tubular conductors is based on the metallurgical method which can be considerably simplified if such materials as niobium with various additives or its alloys are used for substrate manufacture.

As noted above, very attractive is the embodiment which allows to preserve superconductivity under a fault current condition.

As is apparent from the preceding discussion, redistribution of current between the superconductive core 3 and the normal metal core 6 is the better, the greater the resistance of the superconductive core, which resistance reaches its maximum when the cable becomes non-superconducting. The loss of superconductivity in this case is brought about by a thermal shut off of the superconductor, whereas a similar effect can be derived from the action of a magnetic field upon the superconductor.

However, the superconductors utilized in the cable posssess high critical parameters so that the shut off fields by far exceed the fault current fields.

All this allows to carry out the invention into an embodiment assuring a selective shut off of the inserts placed between individual sections of the cable, said inserts being made of a II-type superconductive material which satisfies the condition H2 I2 (12) H1 I1 where: H1 and H2 are the first and second critical fields of superconductive material of insert, respectively; I1 and I2 are the rated and overload currents of cable, respectively.

Said inserts may be situated at each junction of the individual cable lengths or form localized sections or one common section disposed anywhere along the cable.

The total length of the inserts should build up a resistance assuring such a redistribution of current which, if taken with regard for other heat evolutions in the superconductive core viz. eddy current losses, will preserve the superconductive properties of the core with the result that this core will carry the residual current without losses.

The insert will be shut off with a fault current and will accumulate all the major heat evolutions, while cooling of these known areas will be organized in a simpler way and may differ from the way the entire cable is cooled. For example, the cooling of an insert may be exercised separately from the cooling of the entire cable, and these different cooling arrangements within the cable may be separated by means of thermal insulation. Under overload current conditions, and particularly, after the overload conditions cease, the insert may be forced cooled.

Unlike the prior art controlled superconducting inserts which exercise a current limiting effect, the proposed insert is required only for the purpose of current redistribution, which, in the first place, calls for a much lower ohmic resistance in the insert when the latter loses its superconductivity than would be necessary for limitation of the current at some particular location of the superconductive cable within a power transmission system, i.e., saves the expenses of a superconducting material, while, in the second place, the proposed insert does not require controlling arrangements thus enhancing the reliablity of the cable operation.

Referring now specifically to Fig. 9, the proposed cable is supplied with inserts 31 that use a superconductor 32 which satisfies the condition mentioned above.

The construction of the substrate of the superconductor of the insert 31 is similar to that of the substrates of the tubular core and shield.

It should be emphasized that the superconductor of the insert may be stabilized by another method, e.g., the enthalpy method, which will allow making the substrate of a heat-absorbing material possessing high electrical resistance.

Besides, it is also possible, for example, to make the entire cable of such a superconductor which will allow redistribution of current between the superconductive core and the normal metal core within a practically "cold" current carrying system.

It should be noted also that employment of the inserts 31 allows to increase the content of the stabilizing material in the substrate of the superconductive cores of the individual cable lengths.

The size of the insert, with possibility of other cooling arrangements taken into account, may not differ from the sizes of individual cable lengths. In the proposed embodiment, the normal metal cores are not provided with heat insulation.

The proposed modifications of the invention relating the the multisection superconducting cable for carrying alternating current make it possible to make this cable so that it practically suits the rated parameters of a power transmission system irrespective of the parameters of the fault current conditions and locations of the cable within- said system.

WHAT WE CLAIM IS: 1. A multisection superconductive cable for carrying alternating current, each section of which comprises a heat insulation envelope, surrounding a superconductive shield having a stabilizing substrate which surrounds an electrical portion for carrying an electrical current and including at least one conductor assembly of two coaxial cores, the outer of said cores being made tubular and including a superconductive layer disposed upon a stabilizing substrate, and the inner of said cores being made of a normal metal, each said stabilizing substrate being formed by segments of a stabilizing material separated by a material of a lower electrical conductivity than that of the stabilizing segments and the effective resistance of each said stabilizing substrate within any cable section being in excess of the impedance of said electrical portion of said section when both cores thereof are electrically connected together at least at the ends thereof while the radial thickness of the stabilizing segments is not in excess of the effective depth of penetration of alternating current.

2. A cable as claimed in Claim 1, wherein a dielectric is used as a material separating the local segments of the stabilizing substrates.

3. A cable as claimed in Claims 1 and 2, wherein the shield, when used as a return wire, is provided with a normal metal core which surrounds the shield while being separated from it by means of a layer of thermal insulation, said shield and core being electrically connected at least at the cable ends.

4. A cable as claimed in Claims 1 and 3, wherein a metal or an alloy is used as the material separating the local segments of the stabilizing substrates.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (10)

**WARNING** start of CLMS field may overlap end of DESC **. in the superconductive core viz. eddy current losses, will preserve the superconductive properties of the core with the result that this core will carry the residual current without losses. The insert will be shut off with a fault current and will accumulate all the major heat evolutions, while cooling of these known areas will be organized in a simpler way and may differ from the way the entire cable is cooled. For example, the cooling of an insert may be exercised separately from the cooling of the entire cable, and these different cooling arrangements within the cable may be separated by means of thermal insulation. Under overload current conditions, and particularly, after the overload conditions cease, the insert may be forced cooled. Unlike the prior art controlled superconducting inserts which exercise a current limiting effect, the proposed insert is required only for the purpose of current redistribution, which, in the first place, calls for a much lower ohmic resistance in the insert when the latter loses its superconductivity than would be necessary for limitation of the current at some particular location of the superconductive cable within a power transmission system, i.e., saves the expenses of a superconducting material, while, in the second place, the proposed insert does not require controlling arrangements thus enhancing the reliablity of the cable operation. Referring now specifically to Fig. 9, the proposed cable is supplied with inserts 31 that use a superconductor 32 which satisfies the condition mentioned above. The construction of the substrate of the superconductor of the insert 31 is similar to that of the substrates of the tubular core and shield. It should be emphasized that the superconductor of the insert may be stabilized by another method, e.g., the enthalpy method, which will allow making the substrate of a heat-absorbing material possessing high electrical resistance. Besides, it is also possible, for example, to make the entire cable of such a superconductor which will allow redistribution of current between the superconductive core and the normal metal core within a practically "cold" current carrying system. It should be noted also that employment of the inserts 31 allows to increase the content of the stabilizing material in the substrate of the superconductive cores of the individual cable lengths. The size of the insert, with possibility of other cooling arrangements taken into account, may not differ from the sizes of individual cable lengths. In the proposed embodiment, the normal metal cores are not provided with heat insulation. The proposed modifications of the invention relating the the multisection superconducting cable for carrying alternating current make it possible to make this cable so that it practically suits the rated parameters of a power transmission system irrespective of the parameters of the fault current conditions and locations of the cable within- said system. WHAT WE CLAIM IS:

1. A multisection superconductive cable for carrying alternating current, each section of which comprises a heat insulation envelope, surrounding a superconductive shield having a stabilizing substrate which surrounds an electrical portion for carrying an electrical current and including at least one conductor assembly of two coaxial cores, the outer of said cores being made tubular and including a superconductive layer disposed upon a stabilizing substrate, and the inner of said cores being made of a normal metal, each said stabilizing substrate being formed by segments of a stabilizing material separated by a material of a lower electrical conductivity than that of the stabilizing segments and the effective resistance of each said stabilizing substrate within any cable section being in excess of the impedance of said electrical portion of said section when both cores thereof are electrically connected together at least at the ends thereof while the radial thickness of the stabilizing segments is not in excess of the effective depth of penetration of alternating current.

2. A cable as claimed in Claim 1, wherein a dielectric is used as a material separating the local segments of the stabilizing substrates.

3. A cable as claimed in Claims 1 and 2, wherein the shield, when used as a return wire, is provided with a normal metal core which surrounds the shield while being separated from it by means of a layer of thermal insulation, said shield and core being electrically connected at least at the cable ends.

4. A cable as claimed in Claims 1 and 3, wherein a metal or an alloy is used as the material separating the local segments of the stabilizing substrates.

5. A cable as claimed in Claims 1, 2, 3 and 4, wherein the local segments of the

stabilizing material of the substrates are reinforced with a superconductive material.

6. A cable as claimed in Claims 1, 3, 4 and 5, wherein the outer surface of the separating material of the shield and the inner surface of the layer of the separating material of the conductor assembly are covered with a layer of a superconductive material if a metal or an alloy is used as said separating material.

7. A cable as claimed in any one of the preceding claims wherein the inner core of normal metal and the superconductive core of the conductor assembly are separated by a layer of thermal insulation.

8. A cable as claimed in Claim 7, wherein the or each said normal metal core is connected to the superconductive core of the respective conductor with the aid of superconductive jumpers.

9. A cable as claimed in Claims 1, 2, 3, 4, 5, 6, 7 and 8, wherein superconductive inserts made of the type II material are placed at least at the cable ends, the superconductive material of the inserts satisfying the condition: H2 I2 H1 It where: H, and H2=the first and second critical fields of the superconductive material of the insert, respectively; Il and I2=the rated and overload currents of the cable, respectively.

10. A cable as claimed in any one of the preceding claims, and substantially as herein described with reference to and as illustrated in the accompanying drawings.