HK1115223B - Superconductive cable and dc transmission system incoporating the superconductive cable - Google Patents

Description

Superconducting cable and DC transmission system including the same

Technical Field

The present invention relates to a superconducting cable formed by twisting a plurality of cable cores together and a DC transmission system including the superconducting cable. More particularly, the present invention relates to a superconducting cable that can easily form a twisted structure.

Background

As an AC superconducting cable, a three-core twisted type cable is known which is formed by twisting three cable cores together. Fig. 7 is a cross-sectional view of a three-core twisted type cable for three-phase AC. The superconducting cable 100 has a structure in which three cable cores 102 are twisted together and housed in a thermal insulation pipe 101. The heat insulating pipe 101 has a double pipe structure composed of an outer pipe 101a and an inner pipe 101b, with a heat insulating material (not shown) interposed therebetween. The space between the outer tube 101a and the inner tube 101b is evacuated. An anti-corrosion coating 104 is provided on the outer periphery of the heat insulating pipe 101. Each cable core 102 includes, from the center, a former 200, a superconducting conductor layer 201, an insulating layer 202, a superconducting shield layer 203, and a protective layer 204 in this order. The void 103 surrounded by the inner tube 101b and the cable core 102 forms a passage for a coolant such as liquid nitrogen.

When AC transmission is performed using the above superconducting cable, not only AC loss due to inductance but also current at the time of short circuit is large, so that temperature may excessively rise due to loss at that time. The DC transmission using the superconducting cable not only eliminates AC loss but also reduces short-circuit current, compared to AC transmission. As a DC superconducting cable, patent document 1 has proposed a superconducting cable formed by twisting three cable cores, each having a superconducting conductor and an insulating layer, together. In the superconducting cable, each core includes a superconducting conductor, an insulating layer provided on an outer periphery of the conductor, and a return conductor composed of a superconducting wire provided on an outer periphery of the insulating layer. The unipolar transmission is performed by using the superconducting conductor as a go line and the return conductor as a return line.

Patent document 1: published Japanese patent application Tokukai 2003-249130.

Disclosure of Invention

Technical problem to be solved by the invention

In the superconducting cable disclosed in the aforementioned patent document 1, DC transmission, such as unipolar transmission and bipolar transmission, can be performed using one cable. Since the superconducting cable has a structure formed by twisting a plurality of cable cores together, the cable may have a margin for shrinkage when the cable is cooled. However, the cable has cable cores each provided with a superconducting conductor and a return conductor both made of a superconducting material. Therefore, each core is manufactured by using a large amount of superconducting material. This increases the tendency of the flexural rigidity of the core to increase. As a result, it is quite difficult to twist the three cable cores together. Therefore, improvement in the performance is required. In addition, in the case of the AC superconducting cable shown in fig. 7, also because the cable has cable cores each having a superconducting conductor and a superconducting shield layer each made of a superconducting material, it is considerably difficult to twist the cores together.

In view of the above circumstances, a main object of the present invention is to provide a superconducting cable that can easily form a twisted structure even when a plurality of cable cores are used. It is another object of the present invention to provide a superconducting cable suitable for DC transmission. Still another object is to provide a DC transmission system including the superconducting cable described above.

Means for solving the problems

The present invention achieves the above-mentioned objects by reducing the amount of superconducting material used for the entire cable.

(type formed by twisting together cable cores having two different types in a structure: type 1)

As a method of reducing the amount of superconducting material used for the entire cable, the present invention first proposes to reduce the amount of superconducting material used for each cable core.

The present invention proposes a superconducting cable formed by twisting a plurality of cable cores each having a superconducting layer and an insulating layer together. The cable has the feature that it comprises the following core having the following structure:

(a) a first core having a first superconducting layer; and

(b) a second core having a second superconducting layer with an inner diameter larger than an outer diameter of the first superconducting layer.

The present invention proposes a DC transmission system comprising a superconducting cable of type 1 comprising the first and second cores described above. The transmission system is described below.

(monopole transmission)

The first superconducting layer provided in the first core is used as a go line, and the second superconducting layer provided in the second core is used as a return line.

(Bipolar transmission)

A plurality of first cores are provided. The first superconducting layer provided in the at least one first core is used for transmission for one pole, which is either a positive pole or a negative pole. The first superconducting layer provided in the remaining at least one first core is used for transmission for the other pole. The second superconducting layer provided in the second core is used as a neutral line.

The invention also proposes another embodiment of a different type than the one with the two different types of cable cores described above. The newly proposed embodiment has a structure formed by twisting together a core or cores each having a superconducting layer made of a superconducting material and a member having no superconducting layer.

(type provided with a coolant circulation pipe: type 2)

The present invention proposes another superconducting cable formed by twisting a plurality of cable cores together. The cable has a feature in which the cable is formed by twisting together a coolant circulation tube having the same diameter as a cable core and two cable cores each having a structure including:

(a) a superconducting conductor layer;

(b) an insulating layer provided on an outer periphery of the superconducting conductor layer; and

(c) an outer superconducting layer provided on an outer periphery of the insulating layer.

The present invention also proposes another DC transmission system including a superconducting cable of type 2 formed by twisting the above two cores and one coolant circulation pipe together. The transmission system is described below.

(monopole transmission)

The superconducting conductor layers provided in both cores are used as go lines, and the outer superconducting layers provided in both cores are used as return lines.

(Bipolar transmission)

The superconducting conductor layer provided in one of the cores is used for transmission for one pole, which is either a positive pole or a negative pole. A superconducting conductor layer provided in the other core is used for transmission for the other pole. The outer superconducting layer provided in both cores is used as a neutral line.

In the AC superconducting cable shown in fig. 7 and the DC superconducting cable described in patent document 1 described above, a three-core twisted structure is used to secure a margin for shrinkage at the time of cable cooling. In addition, patent document 1 has proposed performing unipolar transmission by using a core including a superconducting conductor and a return conductor each formed of a superconducting material. However, when a large amount of superconducting materials are used for the cable cores of the superconducting cable, these cores increase their flexural rigidity. As a result, it becomes considerably difficult to perform three-core stranding. In view of this problem, the present invention proposes to reduce the amount of superconducting material for each cable core so that the stranding operation is easy. More specifically, the following two types of cores are used:

(a) a core (corresponding to a first core) having no outer superconducting layer and only a superconducting conductor layer as a superconducting layer; and

(b) another core (corresponding to the second core) has only an outer superconducting layer as a superconducting layer, and has no superconducting conductor layer (the core has a structure inverted from the former core).

Here, the outer superconducting layer is a layer used as a return conductor in DC transmission and as a shield in AC transmission, and the superconducting conductor layer is a layer used as an outer conductor in DC transmission and as a conductor in AC transmission. Alternatively, the present invention proposes to facilitate the stranding operation by using a structure in which one of the three cores is formed by a member having no superconducting material at all. More specifically, a coolant circulation tube is used instead of one cable core. The present invention is explained in more detail below.

(type 1)

The superconducting cable of the present invention is formed by stranding at least one first core and at least one second core, each of which includes a superconducting layer and an insulating layer, together. The or each first core (hereinafter referred to as first core) has a first superconducting layer composed of a superconducting material, and does not have other superconducting layers made of a superconducting material. The or each second core (hereinafter referred to simply as second core) has a second superconducting layer composed of a superconducting material, and does not have other superconducting layers made of a superconducting material. More specifically, the first core has a superconducting layer at a central portion side of the core, and does not have a superconducting layer at an outer peripheral side of the core. The second core has a superconducting layer at an outer peripheral side of the core, and no superconducting layer at a central portion side of the core. The second superconducting layer of the second core is formed to have an inner diameter larger than an outer diameter of the first superconducting layer.

It is proposed that the superconducting layers of the first core and the second core are formed by spirally winding a strip-shaped wire having a structure in which a plurality of filaments made of, for example, a Bi-2223-based superconducting material are disposed in a matrix such as a silver sheath. The superconducting layer may be a single layer or may be composed of a plurality of layers. When a multilayer structure is used, an insulating layer may be provided between constituent superconducting layers. The insulating layer between the constituent superconducting layers may be provided by, for example, spirally winding an insulating paper such as kraft paper or a semisynthetic insulating paper such as PPLP (registered trademark, produced by Sumitomo Electric Industries, ltd.) (PPLP is an abbreviation of polypropylene laminated paper).

The first superconducting layer is formed by spirally winding the aforementioned wire made of a superconducting material on the outer periphery of the former. The former may be a solid or hollow body formed by using a metallic material such as copper or aluminum. For example, it may have a structure in which a plurality of copper wires are stranded. As the copper wires, wires each having an insulating coating layer may be used. The former is used as a member for maintaining the shape of the first superconducting layer. A buffer layer may be provided between the former and the first superconducting layer. The buffer layer prevents direct metallic contact between the former and the superconducting wire to prevent damage to the superconducting wire. In particular, when the former is formed of stranded wires, the buffer layer may also serve to further smooth the surface of the former. As a special material for the cushion layer, insulating paper or carbon paper can be suitably used.

The second superconducting layer is formed by spirally winding a wire made of the aforementioned superconducting material on the outer periphery of the core member. It is proposed that the core member is formed by using a material which does not increase the rigidity of the second core as compared with the cable core including the superconducting conductor layer and the outer superconducting layer. The core member may be formed of an insulating material or a conductor material (other than a superconducting material). For example, it may be formed by any one of the following methods:

(a) an insulating material similar to an insulating layer formation material explained below is used;

(b) using a plastic material;

(c) stranded metal wires such as copper wires; and

(d) an insulating material is spirally wound on an outer circumference of the plastic material or the stranded metal wire.

As the above core member, a coolant circulation pipe may be used. In this case, it is desirable to use the following cable structure:

(a) using a space surrounded by the first and second cores and a heat insulating tube described below as an external passage (coolant passage) of the coolant; and

(b) the coolant circulation pipe for the core member is used as a return passage of the coolant.

When the second core has the coolant circulation tube at the central portion thereof, the external passage and the return passage of the coolant may be provided in the thermal insulation tube without allowing the presence of the coolant circulation tube to reduce the space surrounded by the first and second cores and the thermal insulation tube. As a result, in the present structure in which the coolant circulation tube is provided in the second core, the space surrounded by the first and second cores and the thermal insulation can be sufficiently secured, compared to the structure in which the coolant circulation tube for the return flow channel is provided in the thermal insulation tube separately from the cores. In other words, by sufficiently securing the external coolant passage, the coolant can be sufficiently circulated within the external coolant passage. The coolant circulation line formed by the external passage and the return passage is equipped with a refrigerator for cooling the coolant, a pump for forcibly feeding the coolant, and the like. These machines determine the length of the circulation line (cooling section) in such a manner that the coolant can circulate at an appropriate temperature. As described above, with a structure in which the coolant can sufficiently circulate in the external cooling passage, a large flow rate of the coolant can further reduce the temperature rise of the coolant due to the heat of invasion and other reasons. Thus, coolant under appropriate temperature conditions can be transported over long distances. As a result, one cooling portion can be extended. Also, as described above, a sufficient space can be secured to circulate the coolant. Therefore, the circulation pressure of the coolant can be reduced, and the pressure loss can be reduced. This may, for example, reduce the electrical power used to drive the pump.

It is desirable that the above-described coolant circulation tube be made of a metal material having not only excellent strength even at the coolant temperature but also flexibility to some extent so that it can be twisted with other components. Types of the coolant circulation pipe include, for example, a metal pipe, a spiral steel strip, and a hollow body formed by spirally winding a metal wire such as a copper wire on the spiral steel strip. Corrugated metal tubing is desirable because it not only has excellent flexibility making it easier to twist with other components, but also tends to shrink as the cable cools. When the coolant circulation tube made of a metal material is used, an insulating layer is formed on the outer periphery of the coolant circulation tube by using an insulating material. Then, a second superconducting layer is provided on the insulating layer. In particular, when a corrugated tube is used, it is desirable to provide an insulating layer on the corrugated tube so that the surface on which the second superconducting layer is formed can be smoothed.

In the first core, an insulating layer is provided on an outer periphery of the first superconducting layer. In the second core, an insulating layer is provided on the outer periphery of the second superconducting layer. These insulating layers may be formed by spirally winding a semisynthetic insulating paper such as PPLP (registered trademark) or an insulating paper such as kraft paper. The insulating layer provided in the first core is provided on the first superconducting layer so that the first superconducting layer can have an insulation strength required for insulation against a voltage to ground. An insulating layer provided in the second core is provided on the second superconducting layer so that the second superconducting layer can have an insulation strength required for insulating against a voltage to ground.

When the superconducting cable of the present invention is used for DC transmission, the above-described insulating layer may be configured to have ρ (resistivity) grading to flatten the radial (thickness direction) distribution of the DC electric field. The ρ grading is performed such that the resistivity decreases as the radial position moves towards the innermost portion of the insulating layer and increases as the radial position moves towards the outermost portion. The resistivity in the thickness direction of the insulating layer is changed stepwise by performing ρ grading. The ρ grading can flatten the distribution of the DC electric field in the thickness direction throughout the insulating layer. As a result, the insulation thickness can be reduced. Each layer has a different resistivity, and the number of layers is not particularly limited. However, in practice, about two or three layers are used. In particular, when the thickness of each layer is equal, the DC electric field distribution can be effectively flattened.

For the purpose of ρ grading, it is proposed to use insulating materials with different resistivities (ρ). For example, when using an insulating paper such as kraft paper, the resistivity may be varied, for example, by changing the density of the kraft paper or by adding cyanoguanidine to the kraft paper. When a composite paper composed of an insulating paper and a plastic film, such as PPLP (registered trademark), is used, the resistivity can be varied by changing the ratio k of the thickness tp of the plastic film of the composite paper to the total thickness T (ratio k, expressed as (tp/T) × 100) or by changing the density, mass, additives, etc. of the insulating paper. It is desirable that the value of the ratio k is in the range of 40% to 90% or so, for example. In general, as the ratio k increases, the resistivity ρ increases.

In addition, when the insulating layer has a high ∈ (dielectric constant) layer in the vicinity of the superconducting layer, the high ∈ (dielectric constant) layer has a higher dielectric constant than other portions, and not only the performance of withstanding DC voltage but also the performance of withstanding pulse voltage can be improved. The values of the dielectric constant ε (at 20 ℃) are summarized as follows:

(a) common kraft papers: about 3.2-4.5

(b) Composite paper with a ratio k of 40%: about 2.8

(c) Composite paper with a ratio k of 60%: about 2.6

(d) Composite paper with a ratio k of 80%: about 2.4.

In particular, it is desirable to form the insulating layer by using composite paper having a high ratio k and including kraft paper having a considerably high air tightness because such a structure is excellent in withstanding both DC and impulse voltage.

In addition to the above-described ρ grading, the insulating layer may be structured such that the dielectric constant ∈ increases as the radial position thereof moves toward the innermost portion, and decreases as the radial position moves toward the outermost portion. The epsilon grading can also be formed radially in the insulating layer everywhere. As described above, by performing the ρ grading, the superconducting cable of the present invention becomes a cable having excellent DC performance, making itself suitable for DC transmission. On the other hand, most transmission lines are now constructed as AC systems. In view of the transition of a future transmission system from AC to DC, it is conceivable that there are also cases where AC transmission is performed by temporarily using the cable of the present invention before transition to DC transmission. For example, there may be a case where, although a part of the cable in the transmission line has been replaced with the superconducting cable of the present invention, the remaining part is composed of an AC transmission cable. Another conceivable case is one in which, although the AC transmission cable in the transmission line is replaced by the superconducting cable of the present invention, the power transmission device connected to the cable is still used for AC use. In this case, first, AC transmission is temporarily performed using the cable of the present invention, and further, finally, transition to DC transmission is performed. Therefore, it is desirable that the cable of the present invention not only have excellent DC performance but also be designed by considering AC performance. Also, when AC performance is considered, a cable having excellent resistance to a pulse voltage such as a surge voltage can be constructed by using an insulating layer whose dielectric constant ∈ increases as the radial position moves toward the innermost portion and decreases as the radial position moves toward the outermost portion. Later, when the DC transmission is started at the end of the aforementioned transition period, the cable of the present invention used in the transition period can be used as a DC cable without any change. In other words, the cable of the present invention constructed not only by ρ grading but also by ε grading is also suitable for use as an AC/DC cable.

In general, the above-mentioned PPLP (registered trademark) has such properties that the resistivity ρ increases and the dielectric constant ∈ decreases as the ratio k increases. Therefore, when the insulating layer is configured in such a manner that the resistivity ρ increases while the dielectric constant ∈ decreases as the radial position moves toward the outermost portion, using PPLP (registered trademark) having a higher ratio k, the insulating layer may have such a property that the resistivity ρ increases while the dielectric constant ∈ decreases as the radial position moves toward the outermost portion.

On the other hand, kraft paper generally has such properties that, as the airtightness increases, the resistivity ρ increases and the dielectric constant ∈ also increases. Therefore, when only kraft paper is used, it is difficult to configure the insulating layer in such a manner that the resistivity ρ increases while the dielectric constant ∈ decreases as the radial position moves toward the outermost portion. Therefore, when kraft paper is used, it is desirable to construct the insulating layer by bonding composite paper. For example, it is suggested that a kraft layer is formed at the innermost portion of the insulating layer, and a PPLP layer is formed at the outer side of the kraft layer. In this case, the PPLP layer has a higher resistivity ρ than the kraft layer, while the PPLP layer has a lower dielectric constant ∈ than the kraft layer.

In addition, a semiconductor layer may be formed between the first superconducting layer and the insulating layer and between the insulating layer and the second superconducting layer. When the semiconductor layer is formed in the above manner, the superconducting layer increases contact with the insulating layer, so that destruction accompanying partial discharge or the like will be suppressed.

The superconducting cable of the present invention is a multi-core cable formed by twisting at least one first core and at least one second core together, each core having the above-described structure. The number of first cores and the number of second cores may be the same or different. However, the number of the first and second cores is adjusted so that the amount of the superconducting material used in the first superconducting layer of the first core is the same as the amount of the superconducting material used in the second superconducting layer of the second core. For example, when the number of first cores used is larger than the number of second cores used, it is recommended to adjust the amount of superconducting material used for each second core to be larger than the amount of superconducting material used for each first core. As described earlier, the cable of the present invention has the first and second superconducting layers such that the inner diameter of the second superconducting layer is larger than the outer diameter of the first superconducting layer. Therefore, even when the number of the first cores is predetermined to be larger than the number of the second cores so that the first cores use the same amount of superconducting material as the total amount of the superconducting material of the at least one second core in total, the at least one second core does not need to have the second superconducting layer with an excessively increased thickness. Therefore, the flexural rigidity of the second core is not excessively increased. In the case of performing the unipolar transmission, at least one first core and at least one second core are prepared and twisted together. Thereby, a superconducting cable having at least one first core and at least one second core is produced for transmission. In addition to the unipolar transmission, in the case of performing the bipolar transmission, at least two first cores and one second core are prepared and twisted together. Thereby, a multi-core superconducting cable having at least two first cores and one second core is manufactured for transmission. In this case, it is proposed that the first core be used for the transmission of the individual poles, while the second core is used as a neutral line. It is desirable that these first and second cores have the same diameter to ease the twisting operation.

It is proposed that the superconducting cable of type 1 including the aforementioned first and second cores be constructed such that the twisted first and second cores are accommodated in the thermal insulation pipe. The heat insulating pipe may have a structure in which, for example, a double pipe structure is composed of an outer pipe and an inner pipe, a heat insulating material is disposed between the two pipes, and a space between the outer pipe and the inner pipe is evacuated. In the inner tube, a space surrounded by the outer surfaces of the first and second cores and the inner surface of the inner tube is filled with a coolant, such as liquid nitrogen, for cooling the first and second cores. By using a resin such as polyvinyl chloride, an anticorrosive coating can be provided on the outer periphery of the heat insulating pipe. The contents regarding the thermal insulation pipe are also applied to the following type 2 superconducting cable.

The superconducting cable of the above-described type 1 has a structure in which a plurality of cores are twisted together. Therefore, the cable has a margin for shrinkage upon cooling of the cable, as in the conventional superconducting cable having a three-core twisted structure. To provide a margin for shrinkage, for example, the cores may be twisted together by providing the cores with slack. These slack portions may be provided, for example, by: the core and the separator disposed between the adjacent cores are twisted together, and the separator is removed while the twisted core is received in a previously formed thermal insulation tube or while the thermal insulation tube is formed on the outer circumference of the twisted core. The spacer may be formed from a sheet of felt, for example, about 5mm thick. It is recommended to appropriately vary the thickness of the spacer according to the diameter of the cable core. The contents regarding the margin for shrinkage are also applied to the superconducting cable of the following type 2.

The superconducting cable of type 1 having the above-described structure can be used for unipolar transmission by using the first superconducting layer of the first core as a go line and the second superconducting layer of the second core as a return line.

Also, the superconducting cable of type 1 having the above-described structure may be used for bipolar transmission by: providing a plurality of first cores; a first superconducting layer provided in at least one first core is used for transmission for one pole; the anode is a positive electrode or a negative electrode; a first superconducting layer provided in the remaining at least one first core is used for transmission for the other pole; the second superconducting layer provided in the second core is used as a neutral line. In addition, in a process of performing bipolar transmission, for example, in the first superconducting layer for one pole or in a DC-AC converter connected to a cable, the pole may be subjected to an abnormal situation. In this case, when the pole needs to stop power transmission due to the abnormality, the normal first and second cores for the other pole may be used for unipolar transmission. More specifically, the first superconducting layer of the first core may be used as a fronthaul line, and the second superconducting layer of the second core may be used as a return line.

In either one of the transmission systems of the unipolar transmission or the bipolar transmission, the second superconducting layer is set to a ground potential. In bipolar transmission, the positive electrode current and the negative electrode current are generally almost equal in magnitude and cancel each other out. Therefore, the second superconducting layer serving as a neutral line has almost no applied voltage. However, when unbalance occurs between the positive electrode and the negative electrode, an unbalanced current flows through the second superconducting layer. In addition, due to an abnormal condition in one pole, when the unipolar transmission is switched to the bipolar transmission, since the second superconducting layer is used as a return line for the unipolar transmission, a current equivalent to the transmission current will flow through the second superconducting layer. In view of these circumstances, in the present invention, the second superconducting layer is set at ground potential.

By providing the insulating layer configured with epsilon grading as described above, the superconducting cable of the present invention having the first and second cores is suitable to be used not only for DC transmission but also for AC transmission. The first and second cores have no conductor portion serving as a shield when AC transmission is performed. Therefore, if the superconducting cable is used for high-voltage transmission, the leakage electric field becomes large. Therefore, when the superconducting cable is used for AC transmission, it is desirable to use the cable for low-voltage transmission. In addition, when single-phase AC transmission is performed, it is proposed to use a superconducting cable having one first core and one second core twisted together. In this case, the superconducting layers of the two cores may be used for power transmission of the phases. Alternatively, the superconducting layer of one of the cores may be used for phase power transmission, while the remaining core is used as a spare core. When the superconducting cable is used for DC transmission after being used for single-phase AC transmission, the cable can be used for unipolar transmission. On the other hand, three-phase AC transmission is performed by using a superconducting cable having at least three cores, which are formed by joining first and second cores. If more than three cores are used, the excess cores may be used as spares. When the superconducting cable is used for DC transmission after being used for three-phase AC transmission, the cable may be used for unipolar transmission or bipolar transmission. Alternatively, three-phase AC transmission may be performed by using two or three superconducting cables each having one first core and one second core twisted together so as to have at least three cores in total. In this case, when two cables are used, the number of cores is four in total. Thus, one core may be used as a spare core. When three cables are used, it is recommended that each cable be used to transmit power for each phase. In other words, it is proposed that two cores be used to transmit power for one phase.

(type 2)

The superconducting cable of type 2 of the present invention has a three-core twisted structure in which two cable cores and one coolant circulation pipe are twisted together. Each cable core has the following components in this order from the center:

(a) a superconducting conductor layer constructed using a superconducting material;

(b) an insulating layer constructed using an insulating material; and

(c) an outer superconducting layer constructed using a superconducting material.

The superconducting conductor layer may be formed by spirally winding a wire made of a Bi-2223-based superconducting material, like the aforementioned first superconducting layer of the first core and the second superconducting layer of the second core. In addition, the superconducting layer may be a single layer or composed of multiple layers, as in the first superconducting layer of the first core and the second superconducting layer of the second core described above. The superconducting conductor layer is formed on the outer periphery of the former like the first superconducting layer of the first core.

As in the insulating layer of the first core, the insulating layer may be formed by spirally winding semisynthetic insulating paper, kraft paper, or the like on the superconducting conductor layer. The insulating layer is designed to have an insulation strength required for insulation between the superconducting conductor layer and the ground. In addition, like the insulating layers of the first and second cores described above, the insulating layer may be configured to have ρ grading to flatten the distribution in the thickness direction of the DC electric field throughout the insulating layer. The ρ grading is performed such that the resistivity decreases as the radial position moves toward the innermost portion and increases as the radial position moves toward the outermost portion. Further, like the insulating layers of the first and second cores described above, the insulating layer may have a high epsilon layer in the vicinity of the superconducting conductor layer, the layer having a higher dielectric constant than other portions. By having the aforementioned ρ grading and high ∈ layer, the cable can be a superconducting cable more suitable for DC transmission. In addition, like the insulating layers of the first and second cores described above, the insulating layers may be configured in such a manner that, in addition to the ρ grading, the dielectric constant ∈ increases as the radial position thereof moves toward the innermost portion, and decreases as the radial position moves toward the outermost portion. When such a structure is used, the cable may be a superconducting cable more suitable for DC and AC transmission.

Like the superconducting conductor layer, the outer superconducting layer is formed on the outer periphery of the insulating layer by using a superconducting material. The outer superconducting layer may be formed by using a material similar to that used for forming the superconducting conductor layer. The outer superconducting layer is set at ground potential. When bipolar transmission is performed using the superconducting cable of type 2, generally, the positive electrode current and the negative electrode current have almost the same magnitude and cancel each other out. Therefore, the outer superconducting layer serving as a neutral line has almost no applied voltage. However, when unbalance occurs between the positive electrode and the negative electrode, unbalanced current flows through the outer superconducting layer. In addition, due to an abnormal situation in one pole, when bipolar transmission is switched to unipolar transmission, since the outer superconducting layer is used as a return line for unipolar transmission, a current equivalent to the transmission current will flow through the outer superconducting layer. In view of these circumstances, in the present invention, the outer superconducting layer is formed using a superconducting material. It is desirable that a protective layer, which also serves as an insulating layer, be provided on the outer periphery of the outer superconducting layer.

In addition, the semiconductor layer may be formed on the inner periphery, the outer periphery, or both of the insulating layer. More specifically, it may be formed between the superconducting conductor layer and the insulating layer, between the insulating layer and the outer superconducting layer, or between both. When the former, that is, the inner semiconductor layer or the latter, that is, the outer semiconductor layer is formed, the contact of the superconducting conductor layer or the outer superconducting layer with the insulating layer is increased. As a result, destruction accompanying partial discharge or the like is suppressed.

The coolant circulation tube twisted together with the aforementioned two cable cores is used as a return channel of the coolant, while the space surrounded by the two cores, the coolant circulation tube, and the thermal insulation tube is used as an external channel (coolant channel) of the coolant. It is desirable that the coolant circulation tube, as provided in the aforementioned second wick, is made of a metal material that not only has excellent strength even at the coolant temperature but also has flexibility to some extent so that it can be twisted together with other components. In particular, it is desirable that the coolant circulation pipe have a shape excellent in flexibility. More specifically, it is desirable to use corrugated metal tubing. Since the corrugated tube can expand and contract with less difficulty, even if a slack portion (which is used as a margin for contraction upon cooling of the cable) is not provided when the two cable cores are twisted together, it can absorb the amount of thermal contraction by its own expansion and contraction performance. In other words, in the case where the corrugated tube is used as a coolant circulation tube, even if the corrugated tube is twisted together with the core without providing the above-described slack portion, the corrugated tube can sufficiently react to shrinkage when the cable is cooled. Also, in the present invention, the coolant circulation tube has the same outer diameter as the cores, so that not only can the coolant circulation tube secure a sufficient size as a return passage, but also a twisted structure having the two cable cores can be stably formed. The superconducting cable of type 2 is constructed by accommodating a body formed by twisting the two cable cores and the coolant circulation pipe together in the above-described heat circulation pipe.

In the case where a metal pipe is used as the aforementioned coolant circulation pipe, when a combined body formed by twisting the two cable cores and the coolant circulation pipe together is received in a thermally insulating pipe (inner pipe), the thermally insulating pipe or the coolant circulation pipe may be damaged when the coolant circulation pipe is in physical contact with the thermally insulating pipe. Moreover, metal powder may also be produced at the same time. The metal powder can be carried to the termination of the cable by circulation of the coolant, where electrical problems arise. To solve the above problem, a protective layer may be provided on the outer periphery of the coolant circulation pipe to protect it from contact with the heat insulating pipe, so that problems caused by contact with the heat insulating pipe can be avoided. The protective layer may be formed by spirally winding kraft paper, for example.

The superconducting cable of type 2 having the above-described structure can be used for monopole transmission by using the following arrangement:

(a) the superconducting conductor layer provided in both cores is used as a go route; and

(b) the outer superconducting layer provided in both cores is used as a return line.

In addition, the cable can also be used for bipolar transmission by using the following arrangement:

(a) a superconducting conductor layer provided in one of the cores is used for transmission for one pole, which is either a positive pole or a negative pole;

(b) a superconducting conductor layer provided in the other core is used for transmission for the other pole; and

(c) the outer superconducting layers provided in the respective cores are used as neutral lines.

Furthermore, during bipolar transmission, one pole may be subjected to abnormal conditions in the superconducting conductor layer for that pole or, for example, in a DC-AC converter connected to the cable. In this case, when the pole needs to stop power transmission due to the abnormality, cores for other good poles may be used for unipolar transmission. More specifically, the superconducting conductor layer of the core for the perfect pole may be used as a go line, and the outer superconducting layer of the same core is used as a return line. In either of the transmission systems of unipolar or bipolar transmission, the outer superconducting layers of the two cores are set at ground potential.

The superconducting cable of type 2 of the present invention is suitable for not only DC transmission but also AC transmission by providing the insulating layer constructed by the above-described epsilon grading. When single-phase AC transmission is performed, one type 2 superconducting cable may be used. In this case, the superconducting conductor layer of each core may be used for power transmission of the phase, while the outer superconducting layer of each core is used as a shielding layer. Alternatively, the superconducting conductor layer of one of the cores may be used for power transmission of the phase, while the outer superconducting layer of the same core may be used as a shielding layer, and the remaining core may be used as a spare core. On the other hand, in performing three-phase AC transmission, two or three types 2 of superconducting cables are prepared so that the total amount of cores becomes at least three. When two cables are used, the total number of cores becomes four. Therefore, it is suggested that one core is used as a spare core, the superconducting conductor layers of the remaining three cores are used for transmission of the respective phases, and the outer superconducting layer provided at the outer side of the superconducting conductor layers is used as a shielding layer. When three cables are used, it is suggested that the superconducting conductor layers of the respective cables are used for transmission of the respective phases, and the outer superconducting layer provided at the outer side of these superconducting conductor layers is used as a shielding layer. In other words, it is proposed that two cores be used for transmission of one phase.

Effects of the invention

The superconducting cable of the present invention having the above-described structure achieves a special effect of easily forming a twisted structure. This effect is obtained by reducing the amount of superconducting material used in each cable core to reduce flexural rigidity, and by using a formed member, which is a coolant circulation tube, without using superconducting material to facilitate the stranding operation.

In particular, in the superconducting cable of type 1 formed by twisting two different types of cores together, when the second core including the coolant circulation pipe is used, the return passage of the coolant can be formed by sufficiently securing the space surrounded by the core and the thermal insulation pipe. On the other hand, in the superconducting cable of type 2 formed by twisting two cores and the coolant circulation pipe together, by providing the coolant circulation pipe instead of one core, a passage having the largest cross-sectional area can be secured as a return passage of the coolant. Also, by providing a protective layer on the outer periphery of the coolant circulation pipe, the coolant circulation pipe can be protected from contact with the heat insulating pipe. As a result, it is possible to suppress the breakage of the coolant circulation pipe and the heat insulation pipe, the generation of metal powder, and other problems caused by the contact.

In addition, in the core provided in the superconducting cable of the present invention, by performing ρ grading in the insulating layer, distribution in the thickness direction of the DC electric field can be made flat at various places of the insulating layer. As a result, the performance of withstanding DC voltage is improved, and therefore, the thickness of the insulating layer can be reduced. By providing an insulating layer having a high ∈ in the vicinity of a superconducting layer used as a conductor in addition to the ρ grading, in addition to the improvement in the DC voltage withstanding performance described above, the pulse voltage withstanding performance can be improved. Specifically, by constructing the insulating layer in such a manner that epsilon increases as the radial position thereof moves toward the innermost portion and epsilon decreases as the radial position moves toward the outermost portion, the superconducting cable of the present invention can be a cable having excellent AC electrical properties. Therefore, the superconducting cable of the present invention is suitable not only for DC transmission and AC transmission but also for use during transition of a transmission system between AC and DC.

Best mode for carrying out the invention

The following explains embodiments of the present invention. First, an explanation is given of a type 1 superconducting cable of the present invention formed by twisting two types of cores having different structures together.

Example 1

Fig. 1 is a schematic view showing a state in which a DC transmission line for monopole transmission is constructed by using a superconducting cable of the present invention. In the following drawings, like reference numerals denote like parts. The superconducting cable 1 is formed by twisting two types of cores (two first cores 2 and one second core 3) having different structures together and accommodating the twisted cores in the thermal insulation tube 7. More specifically, each first core 2 has a first superconducting layer 2a composed of a superconducting material at the inner circumferential side of the insulating layer 4, and has no layer composed of a superconducting material at the outer circumferential side of the insulating layer 4. The second core 3 is provided with a core member 5b at the central portion side, a second superconducting layer 3a composed of a superconducting material at the outer peripheral side of the core member 5b, and a layer composed of a superconducting material is not provided at the central portion side of the core member 5 b. The second superconducting layer 3a is formed so as to have an inner diameter larger than the outer diameter of the first superconducting layer 2 a.

(first core 2)

In this embodiment, the first superconducting layer 2a is formed by constructing using a Bi-2223-based superconducting tape wire (Ag-Mn coated tape wire) and by spirally winding a plurality of layers of tape wires on the outer periphery of the former 5 a. The former 5a is formed by twisting a plurality of copper wires. A buffer layer (not shown) made of insulating paper is formed between the former 5a and the first superconducting layer 2 a. The insulating layer 4 is formed on the outer periphery of the first superconducting layer 2 a. The insulating layer 4 is constituted by spirally winding semisynthetic insulating paper (PPLP: registered trademark, produced by Sumitomo Electric Industries, ltd.) so that it has an insulating strength required for insulation between the first superconducting layer 2a and the ground. In this embodiment, two first cores 2 as described above are prepared. In addition, the two first cores 2 are designed to have the same diameter.

(second core 3)

In the present embodiment, one second core 3 is used. The second core 3 is formed so as to have the same diameter as the aforementioned first core 2. First, the core member 5b is formed. In the present example, the core member 5b is constituted by spirally winding semisynthetic insulating paper (PPLP: registered trademark, produced by Sumitomo Electric Industries, ltd.) on the outer periphery of an inner core member (not shown) formed of a stranded copper wire. The second superconducting layer 3a is provided on the outer periphery of the core member 5 b. The second superconducting layer 3a is formed by using the same superconducting material (Bi-2223-based superconducting tape wire (Ag-Mn coated tape wire)) as the first superconducting layer 2a of the aforementioned first core 2. Like the first superconducting layer 2a, the second superconducting layer 3a is formed by spirally winding a strip-shaped wire in a plurality of layers. The amount of the strip-shaped wires for forming the second superconducting layers 3a is predetermined to be the same as the total amount of the strip-shaped wires for forming the first superconducting layers 2a of the aforementioned two first cores 2. In the second core 3, the core member 5b is sized so that the second superconducting layer 3a of the second core 3 has an inner diameter (which is equal to the outer diameter of the core member 5 b) larger than the outer diameter (the aforementioned outer diameter is equal to the inner diameter of the insulating layer 4) of the first superconducting layer 2a of the first core 2. This makes it possible to form the second superconducting layer 3a without excessively increasing the number of windings (hence, the number of winding layers) of the superconducting tape-shaped wire. As a result, the flexural rigidity of the second core 3 is not excessively increased. An insulating layer 6 is provided on the outer periphery of the second superconducting layer 3 a. The insulating layer 6 is formed by spirally winding kraft paper so that it has an insulating strength necessary for insulation between the second superconducting layer 3a and the ground.

(superconducting Cable 1)

The superconducting cable 1 is formed by twisting two first cores 3 and one second core 3 together and accommodating the twisted cores in the thermal insulation tube 7. Here, each first core 2 has only the first superconducting layer 2a as a layer composed of a superconducting material, and the second core 3 has only the second superconducting layer 3a as a layer composed of a superconducting material. In this embodiment, the three cores composed of two cores 2 and one core 3 are twisted together to have a slack portion so that they have a margin for shrinkage when thermally shrunk due to cooling of the coolant. More specifically, the stranding operation is performed by disposing spacers (not shown) between two first cores 2, between the first core 2 and the second core 3, and between the second core 3 and another first core 2. The separator is removed when the strand is accommodated in the heat insulating tube 7 (or when the heat insulating tube 7 is formed on the strand body). Therefore, the twisted body is accommodated in the heat insulating pipe 7 in a state of having a slack portion. In this embodiment, the spacer is formed from a piece of felt having a thickness of 5mm and a rectangular cross-section. In addition, in this embodiment, the heat insulating pipe 7 is formed of a corrugated stainless steel pipe. As in the conventional superconducting cable shown in fig. 7, the heat-insulating pipe 7 has a double-pipe structure composed of an outer pipe 7a and an inner pipe 7b, and a heat-insulating material (not shown) is provided in multiple layers between the two pipes. The space between the double tubes is evacuated. Therefore, the thermal insulation tube 7 has an evacuated multi-layer thermal insulation structure. The space 8 enclosed by the inner tube 7b and the three cores consisting of two cores 2 and one core 3 forms a passage for a coolant such as liquid nitrogen. An anti-corrosion coating made of polyvinyl chloride is formed on the outer periphery of the heat insulating pipe 7.

The superconducting cable 1 of the present invention having the above-described structure may be used for DC transmission, and more particularly, may be used for bipolar transmission or unipolar transmission. First, a case where the unipolar transmission is performed is explained. For unipolar transmission, it is proposed to construct the transmission line as shown in fig. 1. More specifically, one end of the first superconducting layer 2a provided in one of the first cores 2 is connected to a DC-AC converter 10a, which is connected to an AC system (not shown), through a lead wire 20 and a lead wire 21. The other end of the same first superconducting layer 2a is connected to a DC-AC converter 10b, which is connected to an AC system (not shown), through a lead 22. Likewise, one end of the first superconducting layer 2a provided in the other first core 2 is connected to the DC-AC converter 10a through a lead 23 and a lead 21. The other end of the same first superconducting layer 2a is connected to the DC-AC converter 10b through a lead 22. On the other hand, one end of the second superconducting layer 3a provided in the second core 3 is connected to the DC-AC converter 10a through a lead wire 24. The other end of the second superconducting layer 3a is connected to the DC-AC converter 10b through a lead wire 25. The lead 24 is grounded. The grounding sets the second superconducting layer 3a to ground potential. In this example, a single-ended ground is used. However, it is also possible to use a both-end ground by grounding the lead 25. Leads 20 to 25 electrically connect the superconducting layers 2a and 3a with the DC-AC converters 10a and 10 b.

The DC transmission line provided with the foregoing configuration can perform unipolar transmission by using the first superconducting layers 2a provided in the two first cores 2 as outgoing lines carrying unipolar currents and the second superconducting layers 3a provided in the second cores 3 as return lines carrying return currents. In addition, the superconducting cable 1 is formed by twisting the three cores and the provided slack portion together. Thus, the slack portion can absorb the amount of thermal contraction of the core upon cooling. Also, a small amount of superconducting material is used in each core of the superconducting cable 1, compared to a conventional superconducting cable formed by twisting three cable cores together, each provided with two layers (a superconducting conductor and an outer superconducting layer) composed of superconducting material. Therefore, the core of the present invention has a small flexural rigidity, and thus easily forms a twisted structure.

Example 2

Next, a case where bipolar transmission is performed is explained. Fig. 2(a) is a schematic configuration diagram showing a state where a DC transmission line for bipolar transmission is constructed by using the superconducting cable of the present invention. Fig. 2(B) is a schematic diagram showing a configuration of a state where a DC transmission line for unipolar transmission is constituted by using one first core and a second core of two first cores. The superconducting cable 1 used in example 1 can also be used for bipolar transmission. For bipolar transmission, it is proposed to construct a transmission line as shown in fig. 2 (a). More specifically, one end of the first superconducting layer 2a provided in one of the two first cores 2 (in fig. 2(a), the top first core 2) is connected to a DC-AC converter 11a connected to an AC system (not shown) through a lead 30. The other end of the same first superconducting layer 2a is connected to a DC-AC converter 11b, which is connected to an AC system (not shown), through a lead 31. Similarly, one end of the first superconducting layer 2a provided in the other first core 2 (in fig. 2(a), the first core 2 on the left) is connected to a DC-AC converter 12a connected to an AC system (not shown) through a lead wire 32. The other end of the same first superconducting layer 2a is connected to a DC-AC converter 12b, which is connected to an AC system (not shown), through a lead 33. On the other hand, one end of the second superconducting layer 3a provided in the second core 3 is connected to the DC-AC converters 11a and 12a through a lead wire 34. The other end of the second superconducting layer 3a is connected to the DC-AC converters 11b and 12b through a lead 35. The lead 34 is grounded. This grounding places the second superconducting layer 3a at ground potential. In this example, single-ended grounding is used by grounding only lead 34. However, a both-end ground may also be used by grounding the lead 35. Leads 30 to 35 electrically connect DC-AC converters 11a, 11b, 12a, and 12b and superconducting layers 2a and 3 a.

The above structure constitutes a positive electrode line in the forward direction, which includes the DC-AC converter 11b, the lead wire 31, the first superconducting layer 2a of the first core 2 on top of fig. 2(a), the lead wire 30, the DC-AC converter 11a, the lead wire 34, the second superconducting layer 3a of the second core 3, and the lead wire 35. On the other hand, the structure also constitutes a negative wiring in the forward direction, which includes the DC-AC converter 12b, the lead wire 33, the first superconducting layer 2a of the first core 2 on the left in fig. 2(a), the lead wire 32, the DC-AC converter 12a, the lead wire 34, the second superconducting layer 3a of the second core 3, and the lead wire 35. The positive and negative lines enable bipolar transmission. In this structure, the second superconducting layer 3a of the second core 3 is used not only as a neutral line but also to circulate an unbalanced current or an abnormal current between the positive and negative electrodes. In this example, in fig. 2(a), the first core 2 at the top is used for the positive electrode, and the first core 2 at the left is used for the negative electrode. Of course, however, this use may be reversed.

Even when one of the poles stops power transmission due to an abnormality in the first superconducting layer for the pole or the DC-AC converter, unipolar transmission can be performed by using the first superconducting layer for the intact pole. For example, in fig. 2(a), when an abnormal situation occurs in the first core 2 on the left, the DC-AC converters 12a and 12b, and the like, that is, when an abnormal situation occurs in the negative electrode, the transmission using the first core 2 on the left in fig. 2(a) is stopped. In this case, as shown in fig. 2(B), by using another first core 2 (the top first core 2 in fig. 2 (a)), a transmission line for unipolar transmission can be formed. More specifically, by using the first superconducting layer 2a of the first core 2 as a forward path and the second superconducting layer 3a of the second core 3 as a return path, unipolar transmission can be performed. In this example, a case where abnormality occurs in the negative electrode is explained. However, similar measures may be taken when an abnormality occurs in the positive electrode. In this case, by using the first superconducting layer 2a of the other first core 2 (the first core 2 on the left in fig. 2 (a)) as a forward path and the second superconducting layer 3a of the second core 3 as a return path, unipolar transmission can be performed.

As explained above, the superconducting cable of the present invention can be used for both bipolar transmission and unipolar transmission.

As described previously, for DC transmission, when the insulating layer 4 of the first core 1 and the insulating layer 6 of the second core 2 are constituted by ρ grading so that the resistivity decreases as the radial position moves toward the innermost portion of the insulating layers and increases as the radial position moves toward the outermost portion, the distribution of the DC electric field in the insulating layers can become flat in the thickness direction. The resistivity can be varied by using different sets of PPLP (registered trade mark), each having a different ratio k. As the ratio k increases, the resistivity tends to increase. In addition, when the insulating layer 4 is provided with a high ∈ layer in the vicinity of the first superconducting layer 2a, the performance against a pulse voltage can be improved in addition to the performance against a DC voltage. The high-e layer can be formed by using PPLP (registered trademark) having a low ratio k, for example. In this case, the high e layer also becomes a low p layer. Also, in addition to the above ρ grading, when the insulating layers 4 and 6 are formed such that the dielectric constant ∈ increases as the radial position moves toward the innermost portion and decreases as the radial position moves toward the outermost portion, the insulating layers also have excellent AC performance. Therefore, the superconducting cable 1 is also suitable for AC transmission. For example, by using different sets of PPLP (registered trademark), each having a different ratio k as follows, the insulating layer can be formed to have three different levels of resistivity and dielectric constant. It is proposed that the following three layers are arranged from inside to outside in the following order (each X and Y represents a constant):

low ρ layer: ratio (k): 60%, resistivity (. rho.) (at 20 ℃): x Ω · cm, dielectric constant (∈): y;

a middle rho layer: ratio (k): 70%, resistivity (. rho.) (at 20 ℃): about 1.2 X.OMEGA.cm, dielectric constant (. epsilon.): about 0.95Y; and

high ρ layer: ratio (k): 80%, resistivity (. rho.) (at 20 ℃): about 1.4 X.OMEGA.cm, dielectric constant (. epsilon.): about 0.9Y.

When using the superconducting cable 1 for three-phase AC transmission, it is proposed to use the superconducting layers 2a and 3a in each of the cores 2 and 3 for transmission for the respective phases. When single-phase AC transmission is performed using the superconducting cable 1, it is recommended to perform transmission using the superconducting layers 2a and 3a in each of the cores 2 and 3 as the same phase. Each of the cores 2 and 3 does not have a superconducting layer serving as a shield. Therefore, when the superconducting cable 1 is used for AC transmission, it is suggested to be used for low-voltage transmission.

The superconducting cable 1 can be used for DC transmission such as the above-described unipolar transmission and bipolar transmission after being used for the above-described AC transmission. As described above, the superconducting cable of the present invention having the insulating layer composed of the ρ grading and the ε grading is suitable for use as a DC/AC cable. The contents relating to ρ grading and ε grading are also applied in example 3 below.

Example 3

In the above-described embodiments 1 and 2, an explanation is given of the structure of the core member in which the stranded copper wire is used as the second core. However, a coolant circulation pipe may be used as the core member. Fig. 3 is a schematic cross-sectional view showing a superconducting cable of the present invention having a coolant circulation pipe inside the second superconducting layer of the second core. The second core 3 shown in this example has the same basic structure as that shown in examples 1 and 2. The only difference is that the coolant circulation pipe 9a is provided as an inner core member of the core member 5 b. The explanation is given below by focusing on this point.

In this example, the coolant circulation pipe 9a is formed using a corrugated stainless steel pipe. An insulating layer 9b is formed on the outer periphery of the coolant circulation pipe 9a by spirally winding a semisynthetic insulating layer. In this example, in particular, the semisynthetic insulating layer is wound so as to cover the shape of the corrugated tube formed of the peaks and valleys, so that the layer can have a uniform insulating thickness with respect to the second superconducting layer 3 a. Therefore, the insulating layer 9b is provided so as to have a smooth outer peripheral surface. As in example 1, the second superconducting layer 3a was provided on the outer periphery of the insulating layer 9b, and then the insulating layer 6 was provided on the outer periphery of the second superconducting layer 3 a. Thereby, the second core 3 is formed so as to have the same diameter as the first core 2.

By using the second core 3 having the aforementioned coolant circulation pipe 9a, the space 8 surrounded by the inner pipe 7b and the three cores composed of two cores 2 and one core 3 can be used as an external passage for coolant such as liquid nitrogen, and the coolant circulation pipe 9a can be used as a return passage for the coolant. In particular, since the coolant circulation pipe 9a is provided in the second core 3, a return passage of the coolant can be provided, and the cross-sectional area of the space 8 is not reduced as compared with the case where the coolant circulation pipe 9a is provided in the space 8 located outside the second core 3. In addition, this example uses a corrugated tube excellent in flexural rigidity as the coolant circulation tube 9 a. This not only makes the operation of twisting together the first core 2 easy, but also makes it easy for the coolant circulation tube 9a itself to contract when the cable is cooled. As described above, the core member 5b provided at the inner side of the second superconducting layer 3a may be formed of different materials (in this example, the coolant circulation pipe 9a and the insulating pipe 9 b).

Next, an explanation is given of a superconducting cable of type 2 of the present invention, which is formed by twisting two cable cores and one coolant circulation pipe together.

Example 4

Fig. 4 is a schematic configuration diagram showing a state where a DC transmission line for monopole transmission is constructed by using the superconducting cable of the present invention. In fig. 4 and fig. 5(a) and 5(B) described below, the coolant circulation line is omitted. The superconducting cable 40 is a cable formed by stranding two cable cores 41 each provided with a superconducting conductor layer 44 made of a superconducting material and an outer superconducting layer 46 coaxially disposed, and one coolant circulation pipe 42 together and then accommodating the stranded body in the thermal insulation pipe 7. Each cable core 41 is provided with a former 43, a superconducting conductor layer 44, an insulating layer 45, an outer superconducting layer 46, and a protective layer 47 in this order from the center.

(Cable core 41)

In this embodiment, the superconducting conductor layer 44 and the outer superconducting layer 46 are formed by using a Bi-2223-based superconducting tape wire (Ag-Mn coated tape wire). The superconducting conductor layer 44 is formed by spirally winding a plurality of layers of the aforementioned superconducting tape-shaped wire around the outer circumference of the former 43. An outer superconducting layer 46 is formed on the insulating layer 45 in the same manner as above. The former 43 is formed by twisting a plurality of copper wires. A buffer layer (not shown) made of insulating paper is formed between the former 43 and the superconducting conductor layer 44. The insulating layer 45 is formed by spirally winding semisynthetic insulating paper (PPLP: registered trademark, by Sumitomo electric industries, Ltd.) on the outer periphery of the superconducting conductor layer 44. The insulating layer 45 is provided so as to have an insulation strength required for insulation between the superconducting conductor layer 44 and the ground. A protective layer 47 is provided on the outer periphery of the outer superconducting layer 46 by spirally winding insulating paper. Two cable cores 41 as described above were prepared. In addition, the two cable cores 41 have the same diameter.

(Coolant circulation pipe 42)

In this example, the coolant circulation tube 42 twisted together with the two cable cores 41 described above is formed of a corrugated stainless steel tube having the same diameter as the cores 41.

(superconducting cable 40)

The superconducting cable 40 is formed by stranding two cable cores 41, each provided with the aforementioned superconducting conductor layer 44 and outer superconducting layer 46, a coolant circulation pipe 42 together and accommodating the stranded body in the thermal insulation pipe 7. The two cores 41 are twisted together to have a slack portion so that they have a margin for shrinkage required for thermal shrinkage when cooled by the coolant. As in example 1, the slack portion was formed by providing a spacer (a piece of felt having a thickness of 5 mm) between the cores 41 at the time of the stranding operation, and removing the spacer while accommodating the stranded body in the heat insulating pipe 7. Since the coolant circulation pipe 42 is formed of an expandable and contractible bellows, it can secure a sufficient margin for contraction even when twisted together with the core 41 without providing a slack portion. In this example, the thermal insulation tube 7 is formed of a corrugated stainless steel tube. As in the conventional superconducting cable shown in fig. 7, the heat-insulating pipe 7 has a double-pipe structure composed of an outer pipe 7a and an inner pipe 7b, with a plurality of layers of heat-insulating materials (not shown) interposed therebetween. The space between the double tubes is evacuated. Thus, the thermal insulation tube 7 has an evacuated multi-layer thermal insulation structure. The space 8 surrounded by the inner pipe 7b, the two cable cores 41, and the coolant circulation pipe 42 forms an external passage for a coolant such as liquid nitrogen. The coolant circulation pipe 42 serves as a return passage for the coolant. An anti-corrosion cover (not shown) made of polyvinyl chloride is formed on the outer periphery of the heat insulating pipe 7.

The superconducting cable 40 of the present invention having the above-described structure may be used for DC transmission, more specifically, for bipolar transmission or unipolar transmission. First, a case where the unipolar transmission is performed is explained. For unipolar transmission, it is proposed to construct the transmission line as shown in fig. 4. More specifically, one end of the superconducting conductor layer 44 provided in the core 41 on the right side of fig. 4 is connected to the DC-AC converter 13a, which is connected to an AC system (not shown), through a lead 50 and a lead 51. The other end of the same superconducting conductor layer 44 is connected to a DC-AC converter 13b, which is connected to an AC system (not shown), through a lead 52. Similarly, one end of the superconducting conductor layer 44 provided in the core 41 on the left side of fig. 4 is connected to the DC-AC converter 13a through a lead 53 and a lead 51. The other end of the same superconducting conductor layer 44 is connected to the DC-AC converter 13b through a lead 52. On the other hand, the outer superconducting layers 46 of the two cores 41 are connected to the DC-AC converter 13a through leads 54, 55, and 56 and to the DC-AC converter 13b through leads 57. In this example, the lead 56 is grounded. The grounding places the outer superconducting layer 46 at ground potential. In this example, a single-ended ground is used. However, a both-end ground may also be used by grounding the lead 57. The leads 50 to 57 electrically connect the superconducting conductor layer 44 and the outer superconducting layer 46 using the DC-AC converters 13a and 13 b.

The DC transmission line provided with the foregoing configuration can perform unipolar transmission by using the superconducting conductor layers 44 provided in the two cores 41 as outgoing lines carrying unipolar currents, and the outer superconducting layers 46 provided in the two cores 41 as return lines carrying return currents. In addition, the superconducting cable 40 is formed by twisting together two cable cores 41 having a slack portion and a coolant circulation tube 42 made of an expandable and contractible bellows. Thus, the slack portion and the expandable and contractible function can absorb the amount of thermal contraction upon cooling. Also, the superconducting cable 40 has a structure in which a coolant circulation tube 42 is provided instead of one core, as compared with a structure in which three cable cores each provided with a superconducting conductor and an outer superconducting layer are each made of a superconducting material by twisting together. Therefore, the return passage of the coolant can be provided without reducing the cross-sectional area of the space 8. In particular, since the coolant circulation tube 42 has the same diameter as the cable core 41, the cable 40 can have a maximum cross-sectional area for a return passage of the coolant. In addition, the cable 40 may have the same diameter as the cable of the three-core twisted structure. Therefore, the cable diameter is not increased.

Example 5

Next, a case where bipolar transmission is performed is explained. Fig. 5(a) is a schematic configuration diagram showing a state where a DC transmission line for bipolar transmission is constructed by using the superconducting cable of the present invention. Fig. 5(B) is a schematic structural diagram showing a state where a DC transmission line for unipolar transmission is constructed by using a superconducting conductor layer and an outer superconducting layer of one of the cores. The superconducting cable 40 used in example 4 can also be used for bipolar transmission. For bipolar transmission, it is proposed to construct a transmission line as shown in fig. 5 (a). More specifically, one end of the superconducting conductor layer 44 provided in one of the cores 41 (the right-side core 41 in fig. 5 (a)) is connected to the DC-AC converter 14a, which is connected to an AC system (not shown), through a lead 60. The other end of the same superconducting conductor layer 44 is connected to a DC-AC converter 14b, which is connected to an AC system (not shown), through a lead 61. Similarly, one end of the outer superconducting layer 46 provided in the same core 41 is connected to the DC-AC converter 14a through a lead 62 and a lead 63. The other end of the same outer superconducting layer 46 is connected to the DC-AC converter 14b through a lead 64. On the other hand, one end of the superconducting conductor layer 44 provided in the other core 41 (the left core 41 in fig. 5 (a)) is connected to a DC-AC converter 15a, which is connected to an AC system (not shown), through a lead wire 65. The other end of the same superconducting layer 44 is connected by a lead 66 to a DC-AC converter 15b, which is connected to an AC system (not shown). Similarly, one end of the outer superconducting layer 46 provided in the same core 41 is connected to the DC-AC converter 15a through a lead 67 and a lead 63. The other end of the same outer superconducting layer 46 is connected to the DC-AC converter 15b through a lead 64. The lead 63 is grounded. The grounding sets the outer superconducting layers 46 of the two cores 41 at ground potential. In this example, single-ended grounding is used by grounding only lead 63. However, a two-terminal ground may also be used by grounding the lead 64. The leads 60 to 67 electrically connect the superconducting conductor layer 44 and the outer superconducting layer 46 using the DC-AC converters 14a, 14b, 15a, and 15 b.

The above-described structure constitutes a positive electrode line in the forward direction, which includes the DC-AC converter 14b, the lead wire 61, the superconducting conductor layer 44 of the right-hand core 41 in fig. 5(a), the lead wire 60, the DC-AC converter 14a, the lead wire 63, the lead wire 62, the outer superconducting layer 46 of the right-hand core 41, and the lead wire 64. On the other hand, this structure also constitutes a negative electrode line in the forward direction, which includes the DC-AC converter 15b, the lead 66, the superconducting conductor layer 44 of the core 41 on the left side of fig. 5(a), the lead 65, the DC-AC converter 15a, the lead 63, the lead 67, the outer superconducting layer 46 of the core 41 on the left side, and the lead 64. The positive and negative lines shown above in the forward direction enable bipolar transmission. In this structure, the outer superconducting layers 46 of the two cores 41 are used not only as a neutral line but also to circulate an unbalanced current or an abnormal current between the positive and negative electrodes. In this example, in fig. 5(a), the core on the right side is used for the positive electrode, and the core on the left side is used for the negative electrode. Of course, however, this use may be reversed.

Even when power transmission of one of the poles using its superconducting conductor layer is terminated due to an abnormality in the superconducting conductor layer for that pole or the DC-AC converter, unipolar transmission can be performed by using the superconducting conductor layer for the intact pole and the outer superconducting layer. For example, in fig. 5(a), when an abnormal situation occurs in the left core 41, the DC-AC converters 15a and 15b, and the like, that is, when an abnormal situation occurs in the negative electrode, the transmission using the left core 41 of fig. 5(a) is stopped. In this case, as shown in fig. 5(B), a transmission line for unipolar transmission is formed by using another core 41 (the core 41 on the right side in fig. 5 (a)). More specifically, by using the superconducting conductor layer 44 of the core 41 as a go line and the outer superconducting layer 46 of the same core as a return line, unipolar transmission can be performed. In this example, a case where abnormality is generated in the negative electrode is explained. However, similar measures are also taken when an abnormality occurs in the positive electrode. In this case, by using the superconducting conductor layer 44 of the other core 41 (the core 41 on the left side in fig. 5 (a)) as a go line and the outer superconducting layer 46 of the same core as a return line, unipolar transmission can be performed.

As explained above, the superconducting cable of the present invention can be used for bipolar transmission and unipolar transmission. In particular, the cable is designed with two cable cores and one coolant circulation tube. Therefore, the amount of superconducting material used for the entire cable can be reduced and the formation of a stranded structure is easier compared to a structure having three cable cores.

As described previously, for DC transmission, the insulating layer 45 of the core 41 may be configured to have ρ grading to flatten the DC electric field distribution in the thickness direction in the insulating layer, as in the above example 2. In addition, the insulating layer 45 may be provided with a high-e layer in the vicinity of the superconducting conductor layer 44 to improve DC voltage resistance and pulse voltage resistance. Also, in addition to the above ρ grading, the insulating layer 45 may be configured to have an ∈ grading to have excellent AC performance, as in the above example 2. In obtaining the above improvement, the superconducting cable 40 is suitable for not only DC transmission but also AC transmission. For example, by using different sets of PPLP (registered trademark), each having a different ratio k, insulating layers having three different levels of resistivity and dielectric constant can be formed as follows. It is proposed that the following three layers are arranged from inside to outside in the following order (each X and Y represents a constant):

low ρ layer: ratio (k): 60%, resistivity (. rho.) (20 ℃): x Ω · cm, dielectric constant (∈): y;

a middle rho layer: ratio (k): 70%, resistivity (. rho.) (20 ℃): about 1.2 X.OMEGA.cm, dielectric constant (. epsilon.): about 0.95Y; and

high ρ layer: ratio (k): 80%, resistivity (. rho.) (20 ℃): about 1.4 X.OMEGA.cm, dielectric constant (. epsilon.): about 0.9Y.

When using the superconducting cable 40 for three-phase AC transmission, it is recommended to use two or three superconducting cables 40. When two cables 40 are used, it is recommended that one core 41 of the four cores 41 of the two cables 40 be used as a spare core, the superconducting conductor layers 44 of the remaining three cores 41 be used for transmission of the respective phases, and the outer superconducting layers 46 of the three cores 41 be used as shielding layers. When three cables 40 are used, each cable 40 is used for transmission of each phase. More specifically, two cores 41 provided in each cable 40 are used for transmission for one phase. In this case, the superconducting conductor layers 44 of the two cores 41 provided in each cable 40 are used for transmission for the respective phases, and the outer superconducting layers 46 provided at the outer sides of these superconducting conductor layers 44 are used as shielding layers. When the superconducting cable 40 is used for single-phase AC transmission, it is proposed to prepare one superconducting cable 40, use the superconducting conductor layers 44 of the respective cores 41 for transmission of the same phase, and use the outer superconducting layers 46 provided at the outer sides of these superconducting conductor layers 44 as shielding layers.

The superconducting cable 40 may also be used for DC transmission, such as the above-described unipolar transmission or bipolar transmission, after the above-described AC transmission. As described above, the superconducting cable of the present invention having the insulating layer configured by the ρ grading and the ∈ grading can be suitably used as a DC/AC cable. The contents regarding ρ grading and ε grading are also applied to example 6 below.

Next, an explanation is given of another structure of the superconducting cable of the present invention formed by twisting two cable cores and one coolant circulation pipe together. Fig. 6 is a schematic sectional view showing an example in which a protective layer is provided on the outer periphery of the coolant circulation pipe.

Example 6

In the structures shown in embodiments 4 and 5, when a combined body formed by twisting together two cable cores 41 and a coolant circulation tube 42 is inserted into a heat insulating tube (see fig. 4, fig. 5(a) and (B)), the outer peripheral surface of the coolant circulation tube 42 may come into physical contact with the inner peripheral surface of the heat insulating tube (inner tube). When this occurs, metal powder may be generated, or the coolant circulation pipe 42 or the heat insulating pipe may be damaged. To solve this problem, as shown in fig. 6, a protective layer 42b may be provided on the outer circumference of the coolant circulation pipe 42a to prevent it from coming into contact with the heat insulating pipe. In this example, the protective layer 42b is formed by spirally winding kraft paper. In addition, in this example, the diameter of the coolant circulation tube 42a is selected so that the protective layer 42b is provided to have the same diameter as the cable core 41. This structure eliminates contact between the coolant circulation pipe 42a and the heat insulating pipe, thereby preventing problems due to the contact.

Industrial applicability

The superconducting cable of the present invention is suitable for use in a power line for power transmission. In particular, the superconducting cable of the present invention is suitable not only for an apparatus for transmitting DC power but also for transmitting AC power during a transition period in which a transmission system is switched from AC to DC. Further, the DC transmission system of the present invention is suitably used when DC transmission is performed by using the above superconducting cable of the present invention.

Brief description of the drawings

Fig. 1 is a schematic view showing a state where a DC transmission line for unipolar transmission is constructed by using a superconducting cable of the present invention formed by twisting a first core and a second core together.

Fig. 2(a) is a schematic view showing a state where a DC transmission line for bipolar transmission is constructed by using the superconducting cable of the present invention formed by twisting the first core and the second core together, and fig. 2(B) is a schematic view showing a state where a DC transmission line for unipolar transmission is constructed by using the first superconducting layer in one first core and the second superconducting layer of the second core in the same superconducting cable described above.

Fig. 3 is a schematic cross-sectional view showing an example of a superconducting cable of the present invention formed by stranding a first core and a second core provided with a coolant circulation tube at an inner side of a second superconducting layer together.

Fig. 4 is a schematic view showing a state where a DC transmission line for unipolar transmission is constructed by using a superconducting cable of the present invention formed by twisting two cores and one coolant circulation pipe together.

Fig. 5(a) is a schematic view showing a state where a DC transmission line for bipolar transmission is constructed by using a superconducting cable of the present invention formed by twisting two cores and one coolant circulation pipe together, and fig. 5(B) is a schematic view showing a state where a DC transmission line for unipolar transmission is constructed by using a superconducting conductor layer and an outer superconducting layer of one core in the same superconducting cable described above.

Fig. 6 is a schematic cross-sectional view showing another structure of the superconducting cable of the present invention formed by stranding two cores and one coolant circulation pipe together, the structure being formed by providing a protective layer on the outer circumference of the coolant circulation pipe.

Fig. 7 is a sectional view of a three-core twisted type superconducting cable for three-phase AC.

Explanation of reference numerals

1 and 40: a superconducting cable; 2: a first core; 2 a: a first superconducting layer; 3: a second core; 3 a: a second superconducting layer; 4. 6 and 9 b: an insulating layer; 5 a: a former; 5 b: a core member; 7: a thermally insulating tube; 7 a: an outer tube; 7 b: an inner tube; 8: a space; 9 a: a coolant circulation pipe; 10a, 10b, 11a, 11b, 12a, 12b, 13a, 13b, 14a, 14b, 15a, and 15 b: a DC-AC converter; 20-25, 30-35, 50-57 and 60-67: a lead wire; 41: a cable core; 42 and 42 a: a coolant circulation pipe; 42 b: a protective layer; 43: a former; 44: a superconducting conductor layer; 45: an insulating tube; 46: an outer superconducting layer; 47: a protective layer; 100: superconducting cables for three-phase AC applications; 101: a thermally insulating tube; 101 a: an outer tube; 101 b: an inner tube; 102: a cable core; 103: a space; 104: corrosion protection coating 200: a former; 201: a superconducting conductor layer; 202: an insulating layer; 203: superconducting shield layer 204: and a protective layer.

Claims (13)

1. A superconducting cable formed by twisting a plurality of cable cores together, each cable core having a superconducting layer and an insulating layer;

the superconducting cable includes:

(a) a first core having a first superconducting layer; and

(b) a second core having a second superconducting layer having an inner diameter larger than an outer diameter of the first superconducting layer,

wherein the first superconducting layer and the second superconducting layer each comprise a spirally wound strip-shaped wire having a structure in which a plurality of filaments of a superconducting material are disposed in a matrix,

wherein the first core has no other superconducting layer made of a superconducting material other than the first superconducting layer, and,

wherein the second core has no other superconducting layer made of a superconducting material other than the second superconducting layer,

wherein the number of the first and second cores is adjusted such that the amount of the superconducting material used in the first superconducting layer of the first core is the same as the amount of the superconducting material used in the second superconducting layer of the second core.

2. The superconducting cable of claim 1, wherein the second core has a coolant circulation tube inside the second superconducting layer.

3. The superconducting cable of claim 2, wherein the coolant circulation tube is a metal tube.

4. The superconducting cable of claim 2, wherein the coolant circulation tube is a spiral steel strip or a corrugated metal tube.

5. The superconducting cable of claim 1, which is formed by twisting two first cores and one second core together.

6. The superconducting cable of claim 1, wherein the first core has the same diameter as the second core.

7. The superconducting cable of claim 1, having a core-twisted structure with a margin for shrinkage upon cooling of the cable.

8. The superconducting cable of claim 1, wherein, to flatten the radial distribution of the DC electric field in the insulating layer, the insulating layer is constructed by using ρ grading such that the resistivity decreases as the radial position moves toward the innermost portion of the insulating layer and increases as the radial position moves toward the outermost portion.

9. The superconducting cable of claim 8, wherein the insulating layer has a high-e layer having a higher dielectric constant than other portions in the vicinity of the superconducting layer.

10. The superconducting cable of claim 8, wherein the insulating layer is configured such that the dielectric constant ∈ increases as its radial position moves toward the innermost portion, and decreases as the radial position moves toward the outermost portion.

11. The superconducting cable of claim 1, wherein the matrix is a silver sheath.

12. A DC transmission system comprising a superconducting cable according to any one of claims 1-11, which DC transmission system performs monopole transmission by:

(a) using a first superconducting layer provided in a first core as a fronthaul line; and

(b) the second superconducting layer provided in the second core is used as a return line.

13. A DC transmission system comprising a superconducting cable according to any one of claims 1-11, the DC transmission system performing bipolar transmission by the following specifications:

(a) providing a plurality of first cores to the superconducting cable;

(b) transmitting for one pole selected from the group consisting of the positive pole and the negative pole using a first superconducting layer provided in at least one first core;

(c) transmitting for the other pole using a first superconducting layer provided in the remaining at least one first core; and

(d) the second superconducting layer provided in the second core is used as a neutral line.