JP3050316B1 - Submarine solid cable - Google Patents

Abstract

A submarine solid cable capable of transmitting a large amount of power by using a high voltage and a high temperature, and a transmission line and a method of operating the cable are provided. SOLUTION: An insulating layer 3 is provided on the outer periphery of a conductor 1, and the insulating layer 3 is impregnated with insulating oil. This insulating oil contains 6
It is a medium viscosity insulating oil with a viscosity at 0 ° C of 10 cst or more and less than 500 cst. The insulating layer is impregnated with the medium viscosity insulating oil directly, and it is not necessary to impregnate the insulating layer 3 with the low viscosity insulating oil in advance. And
Of the cable length immersed in seawater, the length of the cable laid at both ends within 50m depth is 5km or less, and the other part is laid in seawater deeper than 50m depth.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a power cable suitable for long-distance, large-capacity transportation, particularly a submarine solid cable suitable for DC submarine power transmission, a line using the cable, and a method of operating the cable. is there.

[0002]

2. Description of the Related Art Conventionally, a long-distance, large-capacity DC cable has been manufactured by using a kraft paper as an insulating tape material and a solid cable having an insulating layer impregnated with high-viscosity insulating oil (for example, a kinematic viscosity of about 1200 cst at 60 ° C.). Insulating solid (M) impregnated with insulating oil (Tazek “T2015”)
ass-Impregnated) cables are used. In order to increase the capacity of the solid cable, a higher voltage and a higher current may be realized. Among them, to increase the current, it is only necessary to use a conductor having a cross-sectional area as large as possible or to raise the maximum operating temperature of the conductor as much as possible. On the other hand, high voltage and high temperature use are related to the performance of the insulator and cannot be achieved unless new technology is developed.

In recent years, in order to perform large-capacity power transmission that was difficult or impossible with conventional kraft paper-insulated solid cables, solid cables using a polyolefin resin film as an insulating material have been proposed. It has become. For example, a cable that can be used even at a high voltage of DC 500 kV or more or a conductor maximum temperature of 60 ° C. or more (eg, about 80 ° C.) is being studied.

[0004] However, even in this case, the insulating oil used is a high-viscosity insulating oil used for a conventional solid cable. This is because in order to prevent the electrical characteristics from deteriorating under any conditions, it is necessary to prevent the insulating oil of the cable impregnated at the factory from being unevenly distributed or depleted due to the flow phenomenon. That is, especially in the case of a long-distance submarine solid cable, since the cable line is too long to supply or absorb the insulating oil at both ends, no flow phenomenon occurs even at the maximum operating temperature of the cable conductor (usually 55 ° C or less). This is because only high-viscosity oils have been used.

[0005]

However, in the conventional solid cable, in order to increase the capacity by using a higher voltage and a higher temperature, the following problems have become remarkable obstacles.

When the load is turned off after the conductor reaches the maximum temperature when the load is turned on, the insulating oil contracts even though the temperature near the conductor rapidly drops. High viscosity oil has high flow resistance (oil flow resistance) due to its high viscosity and cannot move rapidly from the outside of the insulation layer to the inside, so the insulation oil is exhausted near the conductor where the temperature drops and the insulation oil shrinks most. And voids may be generated to significantly lower the electrical performance.

That is, as the conductor operating temperature is increased, the amount of insulating oil that expands increases, and the temperature drop at the time of load rejection becomes even steeper, causing severe depletion and the generation of large voids. There is a problem that it cannot be easily applied to the cable insulator.

Further, a polyolefin resin film,
Alternatively, a composite insulating tape made of kraft paper has been attempted, but compared to kraft paper made of porous natural wood pulp fiber, the polyolefin resin film has holes through which liquid can flow through. And cannot pass high-viscosity insulating oil.

Therefore, when a cable core is impregnated with insulating oil in a factory, the thicker the insulating layer, the higher the viscosity of the insulating oil, the more the impregnation becomes impossible or impossible or extremely difficult. As a result, it has been almost difficult to increase industrial productivity or to increase the ratio of the polyolefin resin film in order to obtain an initial purpose.

[0010] When the ratio of the polyolefin resin film in the insulating layer increases, the oil flow resistance when the insulating oil moves in the radial direction through the insulating layer increases, and the oil flow resistance increases due to the high viscosity oil. In addition, it becomes increasingly difficult to prevent the generation of voids due to contraction of insulating oil near the conductor when the load is turned off.

Accordingly, an object of the present invention is to provide a submarine solid cable capable of transmitting a large amount of power by using a high voltage and a high temperature, and a transmission line of the cable and a method of operating the cable.

[0012]

The present invention achieves the above object by using medium-viscosity oil as the insulating oil and by limiting the cable laying conditions. That is, the submarine solid cable of the present invention includes an insulating layer on the outer periphery of the conductor, and the insulating layer is impregnated with insulating oil. This insulating oil is 60 ℃
Is a medium viscosity insulating oil with a viscosity of 10 cst or more and less than 500 cst. The insulating layer is impregnated with the medium viscosity insulating oil directly, and it is not necessary to previously impregnate the insulating layer with the low viscosity insulating oil. And, of the cable length immersed in seawater, the length of the part laid within 50 m at both ends is 5 km or less, and the other parts are laid in seawater deeper than 50 m. And

[0013] Here, the installation condition of both ends of the cable in the seawater is such that the length of the installation portion within a water depth of 70 m is 5 km.
Is more preferable, and more preferably the water depth
The length laid within 70m or 50m is less than 1km. Under such laying conditions, the inside of the cable is always maintained at a positive pressure by the water pressure, and the generation of voids due to the negative pressure at the time of load rejection is suppressed.

The first means for suppressing the generation of a negative pressure at the time of load interruption is to reduce the amount of insulating oil. The insulating layer may be a conventional kraft paper, but it is desirable to use an insulating tape containing a polyolefin-based resin film in at least a part, more preferably, a main part of the insulating layer. This allows
By reducing the amount of impregnating oil, it is possible to suppress discharge deterioration due to generation of voids due to negative pressure. That is, the polyolefin resin film has no gap through which the insulating oil penetrates, and no void is generated at this portion. Therefore, the amount of insulating oil can be reduced by the amount of the polyolefin-based resin film, and the proportion of the kraft paper layer that is a source of voids can be reduced.

The DC stress distribution of the insulator is determined by the insulation resistance of the insulator. The insulation resistance of insulating oil is 1 × 10 ¹⁵ or more
1 × 10 ¹⁶ Ω · cm, insulation resistance of kraft paper is 4 × 10 ¹⁶ ~
1 × 10 ¹⁷ Ωcm, insulation resistance of polypropylene is 1 × 10
^{It is 17} to 1 × 10 ¹⁸ Ω · cm. Therefore, even if voids are generated in the oil gap or kraft paper, the stress applied to these parts is low because the insulation resistance of the insulating oil is small, and most of the stress is applied to the polyolefin resin film layer where voids do not occur. It does not start discharge.

As the polyolefin resin film,
Examples include polyethylene, polypropylene, ethylene propylene copolymer, and polybutene. In particular, polypropylene is preferred because it has excellent electrical properties, has a high melting point and can be used up to high temperatures. Accordingly, the maximum operating temperature of the conductor can be improved to 60 ° C. or more, particularly to about 80 ° C.

The insulating tape containing a polyolefin resin film includes a composite tape obtained by laminating kraft paper on one or both sides of the polyolefin resin film, in addition to the insulating tape of the polyolefin resin film alone.

In the case of a composite tape in which kraft paper is laminated on both sides of a polypropylene film, the ratio of the thickness of the polypropylene film to the total thickness of the composite tape
(PP ratio) is preferably 80% or more and less than 90%. 80
%, The proportion of kraft paper increases and voids tend to occur in the kraft paper layer. Conversely, if it is 90% or more, it is difficult to produce a composite tape, and a practical tape cannot be obtained.

Further, by alternately winding a composite tape in which kraft paper is laminated on both sides of a polypropylene film and an insulating tape made of a polypropylene film alone to form at least a part of the insulating layer, the amount of insulating oil can be further reduced. It is possible to improve the electric performance by reducing. For example, PP ratio
If the composite tape of 80% and the insulating tape of polypropylene alone are alternately wound, the ratio of the total thickness of the polypropylene film to the total thickness of the insulator can be easily set to 90%.

It is preferable that the thickness of the kraft paper to be compounded is thinner because the thickness of the oil gap is smaller. However, it is difficult to produce a thinner kraft paper from the viewpoint of production, and a thickness of 50 to 200 μm is practical.

Further, when the polypropylene laminated paper layer used as the insulating layer is subjected to ρ crading by combining, if necessary, composite tapes having different PP ratios, the DC stress distribution becomes mild and effective.

The insulating oil has a viscosity at 60 ° C. of 10 cst or more and 500 cs
Medium viscosity insulating oil less than t. This range is preferable because the oil pressure of the insulating oil can sufficiently propagate in the longitudinal direction of the cable. If it is less than 10 cst, the movement of oil in the cable increases, and the insulating oil leaks to the outside when the cable is cut, which is not preferable. If it is 500 cst or more, the hydraulic pressure transmission at the time of load interruption is insufficient, which is not preferable. Furthermore, if it is in the range of 10 cst or more and less than 500 cst, it is not necessary to impregnate the insulating layer with a low-viscosity insulating oil in advance. It can be manufactured by impregnating oil.

Specific examples of the insulating oil having the above viscosity include polystyrene-based insulating oil, polybutene, mineral oil, synthetic oil mainly composed of alkylbenzene, heavy alkylate, and a mixture containing at least one of these. Particularly, polybutene is preferred. It is preferable that the insulating oil be an oil containing solid rubber as a part of the insulating oil, because the adhesiveness between the insulating papers can be improved, and separation between paper layers, which is a site where voids are generated, can be prevented.

The second means for suppressing the generation of voids is to increase the oil pressure by providing a reinforcing layer which suppresses the expansion of the insulating oil as much as possible. Usually, a solid cable is provided with a metal sheath (usually a lead sheath) and an anticorrosion layer (such as polyethylene) sequentially on the outer periphery of an insulating layer.
A reinforcing layer is formed on the outer periphery of the metal sheath. The reinforcing layer plays a role of sharing and reinforcing the hoop stress applied to the metal sheath (stress caused by hydraulic pressure to break the metal sheath inside the metal sheath). That is, even if the cable is used at a high temperature, the expansion of the metal sheath accompanying the expansion of the insulating oil is suppressed, and the generation of a negative pressure at the time of load rejection is suppressed by increasing the oil pressure. Therefore, it is desirable that the material of the reinforcing layer has a high tensile strength, and examples thereof include metal tapes such as stainless steel and aramid fiber tapes. More specifically, those having a tensile strength of 40 kg / mm ² or more are preferable. The reinforcing layer is provided just above the metal sheath and inside the anticorrosion layer. Thereby, the reinforcement layer is isolated from seawater to prevent corrosion of the reinforcement layer.

Further, the transmission line according to the present invention is provided on the seabed, and the submarine cable according to any one of claims 1 to 5,
A land-based cable connected to both terminals of the submarine cable via an oil stop junction box, and a medium-viscosity insulating oil is pressurized and supplied to the submarine cable from both terminals of the submarine cable. It is characterized by. It is also effective to install a check valve in the direction of the pressurized supply pipe so that the insulating oil does not flow out from the submarine cable through the pipe. This prevents the oil that has been reduced in viscosity due to use at high temperatures from expanding and damaging the terminal, and suppresses a decrease in oil pressure of the cable in the shallow sea area.

Further, in the method of operating the cable according to the present invention, a load current is applied to the submarine solid cable at a sufficiently low voltage before applying a regular voltage to the cable.
After repeating a heat cycle several times, a regular voltage is applied. "Sufficiently low voltage" means the voltage necessary to supply the rated current to the submarine cable, and adds a slight margin to the conductor resistance voltage drop obtained by multiplying the submarine cable's conductor resistance by the rated current. Voltage. The number of heat cycles may be one or more, preferably about 3 to 5 times. With such an operation method, the insulating oil can be sufficiently applied to the insulating layer, and the effect of the external water pressure can be easily transmitted uniformly in the longitudinal direction of the cable.

(Operation) In a conventional solid cable, a high-viscosity oil is used as an insulating oil, and there is no pressurization of oil from a cable end. And the electrical performance was restricted. The present inventors use a medium-viscosity oil that can transmit insulating oil pressure, and when used as a submarine cable to which water pressure is applied from the outside, can increase the internal pressure of the cable and can reduce the length of both ends of the cable at a shallow depth in the sea. It was confirmed that the negative pressure was not generated without using the pressurizing mechanism of the insulating oil by specifying the pressure.

Further, in the case of a conventional solid cable using a polyolefin resin film, with a high-viscosity insulating oil similar to a solid cable using kraft paper, the thicker the insulating layer is, the more impregnation is insufficient or impossible or extremely difficult. Was.
Also in this regard, it was confirmed that the use of the medium viscosity insulating oil can sufficiently impregnate the insulating layer even if the insulating layer becomes thick. Thus, a submarine solid cable that is easy to manufacture and can be used at high voltage and high temperature is obtained.

[0029]

Embodiments of the present invention will be described below.

FIG. 1 is a sectional view of the cable of the present invention. In order from the center, a conductor 1, an inner semiconductive layer 2, an insulating layer 3, an outer semiconductive layer 4, a metal sheath 5, a reinforcing layer 6, and an anticorrosion layer 7 are provided. The insulating layer 3 was formed by alternately winding a composite tape having a PP ratio of 80% and an insulating tape made of polypropylene alone. The metal sheath 5 uses lead. The reinforcing layer 6 was formed by winding a SUS304 tape. The anticorrosion layer 7 was a polyethylene sheath. The insulating layer 3 is impregnated with a medium-viscosity insulating oil having a viscosity at 60 ° C. of 10 cst or more and less than 500 cst. When impregnating the insulating layer 3 with the medium-viscosity insulating oil, it is not necessary to impregnate the insulating layer 3 with the low-viscosity insulating oil in advance. With a medium-viscosity insulating oil, impregnation can be performed in a shorter time than with a conventional high-viscosity insulating oil.

When such a cable is laid in the sea, the length of the portion laid at a water depth of 50 m or less at both ends is 5 km or less, and the other portions can be laid in seawater deeper than 50 m. If it is an environment, it is possible to prevent the generation of a negative pressure at the time of load shedding without any special additional mechanism.

Since the specific gravity of seawater is ≒ 1.0, the water depth (L)
When expressed in m, a water pressure of (L / 10 (kg / cm ² )) is generated. This is because, for solid cables (Fig. 1), the specific gravity of the insulating oil is usually less than 0.9 and less than the specific gravity of seawater, so the lead sheath is compressed from the outside, the insulating layer inside the lead sheath is compressed, and mainly the insulating oil Apply external pressure.

This external water pressure is positively utilized, and a constant oil pressure in the insulator when there is no load or in a constant state is set as a positive pressure. Furthermore, if the pressure reduction due to the contraction of the insulating oil immediately above the conductor when the load is off can be compensated for by quickly transmitting the insulating oil pressure outside the cable insulation layer to the inside, the solid cable can be “load off → negative pressure near the conductor → void generation →
The fundamental drawback of "electrical performance degradation" can be eliminated.

In order to confirm this, the propagation effect of the external water pressure was investigated by changing the constitution of the insulating layer and the viscosity of the insulating oil using the cable hydraulic pressure change measuring device shown in FIG.

In the apparatus shown in FIG. 2, a pair of steel pipes 10 are
The test cable 13 is disposed in a watertight case having flanges 12 provided at both ends thereof. Conductors 14 are exposed from both ends of the test cable 13 and penetrate the flange 12. An insulating material 15 is interposed between the conductor 14 and the through hole of the flange 12. The layers other than the conductors at both ends of the cable are provided with a watertight seal 16. The steel pipe 10 is provided with a connector 17 in which the pump Pu
And the water pressure gauge PG1 are connected. Seawater can be supplied into the watertight case from the pump Pu. Further, current-carrying devices 18 are connected to both ends of the conductor 14, so that a predetermined current can be passed through the conductor 14. Then, the oil pressure of the conductor 14 through the gap between the conductors 14, that is, the oil pressure immediately above the conductor, which is the most severe when the load is off and the oil pressure decreases, is determined by the hydraulic gauge PG
Measure according to 2.

In this apparatus, the external water pressure of the cable is changed (when the external water pressure is expressed as (PG1) in (kg / cm ² ), the relationship with the depth (L) is (L / 10 = PG1)). Then, a current was applied to the cable by an energizing device, and it was examined whether or not a negative pressure was observed in the hydraulic gauge PG2. This test was performed on various cables having an insulation thickness of up to 25 mm. The cable structure is the same as that shown in FIG. Table 1 shows the test results.

[0037]

[Table 1]

In Table 1, "after laying" represents the no-load state, and the other represents the pressure of the hydraulic gauge PG2 when the heat cycle from load on to off is applied, particularly when the load is off. The load of the heat cycle was a direct current rated current or an alternating current having an effective current corresponding to the rated current, and a current density of 2 A / mm ^{2 with} respect to the conductor size was applied as the rated current. "△" is PG1 is 7 kg / cm ² and out, i.e. that the condition occurs which can avoid barely negative pressure at a depth of 70m,
"O" indicates that PG1 can avoid negative pressure in the inside and outside of 5 kg / cm ² , that is, at a depth of 50 m, and almost certainly obtains a considerable positive pressure when it exceeds 70 m. “×” indicates that even if PG1 is 100 kg / cm ² , that is, 1000 m, negative pressure cannot be avoided.

When a medium viscosity oil is used, no negative pressure can be produced with kraft paper if it is deeper than 70 m. When using a polyolefin resin film, if a medium viscosity oil is used, negative pressure can be avoided even at 50 m depending on the design of the insulating layer.
It was found that the length was preferably 0 m or more, and that a considerable positive pressure was surely obtained when the heat cycle was repeated 1 to 3 cycles.

On the other hand, when a high viscosity oil was used, a negative pressure could not be avoided even if the external water pressure was increased. Note that “△ to ×” indicates that there is a case where the negative pressure can be avoided when the PG1 reaches several tens of kg / cm ² .

From the results shown in Table 1, it is found that it is extremely effective to apply a load current to the cable by applying a load current to the cable at a voltage as low as not to cause dielectric breakdown and to apply a heat cycle to the cable for 3 to 5 cycles or more. With this operation, after laying the cable, the insulating oil can be sufficiently applied to the insulating layer, and the influence of the external water pressure can be applied uniformly and sufficiently inward in the longitudinal direction of the cable. The positive pressure can be surely brought.

In a shallow sea area of 50 to 70 m or less, a negative pressure may be generated at the time of load rejection. However, by using a polyolefin resin for the insulating layer, the impregnation oil is small.
By using the reinforcing layer to reinforce the hoop stress applied to the metal sheath, it was confirmed that negative pressure can be prevented by increasing the oil pressure in the insulator in the steady state in advance, especially at the maximum load before the load is turned off. Was.

Next, since the submarine cable always goes up on land at both ends, it passes through the shallow sea. Therefore, such the cable passing through the shallow portion thought to prevent the pressurizing negative pressure insulating oil from the landing point side terminal e.g. 3~20kg / cm ^2, high-viscosity oil and medium-viscosity oil The propagation characteristics in the longitudinal direction of the cable were investigated.

As a result, it was difficult for the conventional high-viscosity oil to propagate over 1 km at room temperature after laying, even if it took a sufficient time. In addition, it is difficult to compensate for hydraulic pressure propagation when the load exceeds several tens of meters with respect to a transient change in oil pressure when the load is off.

On the other hand, it was found that when medium-viscosity oil was used, compensation could be made for at least several kilometers (5 km or more) at room temperature after the installation, and up to about 1 km even in the transient state where the load was turned off.

From the above, it has been found that the application of the medium viscosity insulating oil is extremely effective for eliminating a negative pressure even for a submarine cable having a shallow sea portion at both ends.

As described above, the cable of the present invention can sufficiently suppress the generation of a negative pressure without replenishing insulating oil from both ends of the cable. Is preferably supplied to the submarine solid cable under pressure. FIG. 3 shows a configuration of a transmission line that pressurizes medium-viscosity insulating oil from both ends of a submarine solid cable.
(However, the check valve is not shown.)

A land-based cable 22 is connected to both terminals of a submarine cable 20 having the same configuration as that shown in FIG. At both ends of the submarine cable 20, water depth 50m
The part to be laid within 5km or less and other parts are laid in seawater deeper than 50m. The location of the oil stop junction box 21 is a beach part that separates the land and the sea. The land-based cable 22 may be the same cable as the submarine cable 20, or may be an OF cable. The oil stop connection box 21 can prevent the terminal from being damaged due to the propagation of the oil pressure in the longitudinal direction of the cable when the insulation oil expands and the oil pressure rises when the load is applied to the submarine cable 20 and the submarine cable 20 is turned on. In addition, oil stop junction box 21
To the oil tank 24 via the oil pipe 23
It is configured so that medium viscosity insulating oil can be supplied under pressure to 20. Even if a negative pressure is likely to occur in the shallow sea area of the submarine cable 20, the pressurized supply can increase the oil pressure in the insulating layer and can reliably suppress the generation of the negative pressure. If a check valve for preventing the insulating oil from flowing out of the cable is installed in the oil supply pipe 23, the reverse flow of the cable oil to the oil supply tank 24 can be stopped when the load is turned on. This is preferable because it is possible to prevent the oil pressure from lowering.

[0049]

As described above, according to the submarine solid cable of the present invention, it can be used at a high voltage and a high temperature, and the generation of a negative pressure can be suppressed without pressurizing the insulating oil. Further, according to the power transmission line of the present invention, damage to the terminal can be suppressed, and generation of negative pressure can be more reliably prevented. Furthermore, according to the method of operating the cable of the present invention, the medium viscosity insulating oil can be uniformly applied to the insulating layer, and the transmission of the insulating hydraulic pressure can be smoothly performed. Therefore, long-distance large-capacity power transmission can be realized.

[Brief description of the drawings]

FIG. 1 is a sectional view showing an example of a submarine solid cable according to the present invention.

FIG. 2 shows an apparatus for measuring a change in oil pressure in a cable.

FIG. 3 is a schematic configuration diagram of a cable line of the present invention.

[Explanation of symbols]

 1 conductor 2 inner semiconductive layer 3 insulating layer 4 outer semiconductive layer 5 metal sheath 6 reinforcing layer 7 anticorrosion layer 10 steel pipe 11 insulated tubing 12 flange 13 test cable 14 conductor 15 insulating material 16 watertight seal 17 connector 18 energizing device 20 sea bottom Cable 21 Oil stop connection box 22 Onshore cable 23 Oil supply pipe 24 Oil tank

──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Hiroshi Takigawa 1-3-1 Shimaya, Konohana-ku, Osaka City Inside the Osaka Works, Sumitomo Electric Industries, Ltd. (72) Inventor Jun Jun Yoda 1-3-1 Shimaya, Konohana-ku, Osaka-shi No. Sumitomo Electric Industries, Ltd. Osaka Works (56) References JP-A-10-255562 (JP, A) JP-A-10-21761 (JP, A) JP-A-10-97813 (JP, A) 10-228820 (JP, A) (58) Field surveyed (Int. Cl. ⁷ , DB name) H01B 9/06 H01B 7/14

Claims (8)

(57) [Claims] 1. An undersea solid cable having an insulating layer on the outer periphery of a conductor and impregnated with insulating oil in the insulating layer, wherein the insulating oil is a medium-viscosity insulating oil having a viscosity at 60 ° C. of 10 cst or more and less than 500 cst. Yes, without impregnating the insulating layer with the low-viscosity insulating oil in advance, it is manufactured by directly impregnating the medium-viscosity insulating oil into the insulating layer. A submarine solid cable characterized in that the length of the place where it is laid is 5 km or less, and the other part is laid in seawater deeper than 50 m. 2. The submarine solid cable according to claim 1, wherein an insulating tape containing a polyolefin resin film is used for at least a part of the insulating layer. 3. The insulating tape is a composite tape in which kraft paper is laminated on at least one side of a polypropylene film, and a ratio of a thickness of the polypropylene film to a total thickness of the composite tape is 80% or more and less than 90%. The submarine solid cable according to claim 2, characterized in that: 4. An insulating layer formed by alternately winding a composite tape in which kraft paper is laminated on both sides of a polypropylene film and an insulating tape made of a polypropylene-based film alone. Submarine solid cable as described. 5. A metal sheath and an anticorrosion layer are provided on the outer periphery of the insulating layer, and a reinforcing layer is formed immediately above the metal sheath and inside the anticorrosion layer to share and reinforce hoop stress applied to the metal sheath. The submarine solid cable according to any one of claims 1 to 4, wherein 6. The submarine solid cable according to claim 1, wherein a maximum operating temperature of the conductor is 60 ° C. or higher. 7. A submarine cable according to any one of claims 1 to 5, laid on the seabed, and a land-based cable connected to both ends of the submarine cable via an oil stop junction box. A transmission line, wherein medium-viscosity insulating oil is supplied from both ends of the submarine cable to the submarine cable under pressure. 8. The submarine solid cable according to claim 1, wherein a load current is applied to the cable at a sufficiently low voltage before applying a regular voltage, and a heat cycle is performed several times. A method of operating a solid cable, comprising applying a regular voltage after repeating.