JPH11134946A - Solid dc cable - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a solid DC cable in which electric discharge occurrence in voids is suppressed even in the case voids are formed in an insulating layer formed immediately on a conductor at the time of shutting off load. SOLUTION: A low resistance Kraft paper layer 3B whose insulating oil is put in negative pressure at the time of shutting off a load is formed immediately on a conductor by using Kraft paper having lower resistance than that of a main insulating layer 3A. Since sharing of a DC electric field is carried out in proportion to the resistance of each position of an insulating layer 3, no DC electric field is shared to the low resistance Kraft layer 3B having low resistivity and even in the case voids are formed in the layer 3B, electric discharge in the voids ca be suppressed. Further, a low resistance Kraft paper layer 3B may be formed on the outer circumference of the main insulating layer 3A.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a solid DC cable.

[0002]

2. Description of the Related Art Conventionally, an insulating tape is wound on a conductor, and a power cable having an insulating layer impregnated with a high-viscosity insulating oil (for example, by impregnating an insulating oil having a viscosity of 5,000 cSt or more at room temperature). Becomes a laminated insulating solid (Mass-I
The thickness of the insulating tape is generally about 70 to 200 μm. This is because a thin insulating tape has low mechanical strength and a large winding machine is required as the number of windings increases.

[0003] Unlike an OF cable, a solid DC cable does not supply insulating oil from both ends of the cable, so voids are generated in the insulating layer due to lack of insulating oil, and when the voids become harmful, discharge occurs. Starting point Such voids are likely to occur first in the oil gap that is inevitable when the insulating tape is spirally wound, and then in the gaps between the natural fibers in the insulating tape. The oil gap increases as the insulation tape becomes thicker,
Conventional solid DC cables have a relatively small operating voltage of, for example, 400 kV or less and a transmission current of less than 1000 A, and thus have not been regarded as particularly problematic for voids that are likely to occur in an oil gap immediately above a conductor.

[0004]

However, in recent years, plans for transmitting large amounts of electric power over long distances using solid DC cables have appeared one after another. For example, the transmission voltage is 45
Lines with a transmission current exceeding 0 kV or 500 kV and a transmission current exceeding 1000 A have been planned. When the high voltage and the high current are reached, the formation of harmful voids in the insulating layer immediately above the conductor cannot be ignored.

SUMMARY OF THE INVENTION Accordingly, it is a primary object of the present invention to provide a solid DC cable capable of suppressing discharge in a harmful void even when a void is generated when a load is interrupted.

[0006]

SUMMARY OF THE INVENTION The present invention achieves the above-mentioned object, and is characterized in that the insulating oil has a negative pressure within a range in which the insulating oil has a negative pressure when the load is cut off directly above the conductor, compared with the kraft paper of the main insulating layer. That is, we have provided low-resistance kraft paper. That is, the solid DC cable of the present invention includes an insulating layer and a metal sheath on the outer periphery of the conductor, and the insulating layer is formed of a main insulating layer made of kraft paper and kraft paper having a lower resistance than the main insulating layer. A low-resistance kraft paper layer, wherein the low-resistance kraft paper layer is provided immediately above the conductor.
This low resistance kraft paper layer is preferably formed in a range where the insulating oil has a negative pressure when the load is interrupted. Further, it is preferable to provide the low-resistance kraft paper layer directly below the metal sheath.

Hereinafter, the progress of completing the present invention will be described. The present inventors studied a mechanism of generating harmful voids at the time of load rejection. It was investigated how the pressure of the insulating oil at each position in the layer changes. FIG. 6 is a graph showing the change in the hydraulic pressure. In the figure, the curve shows the oil pressure in the insulating layer (innermost circumference) immediately above the conductor, the curve shows the oil pressure in the position of about 10 kraft papers from above the conductor, and the curve shows the oil pressure just below the metal sheath (outermost circumference).

When a load current is applied, first, the temperature of the conductor rises, and accordingly, the temperature of the insulating layer also rises from the inner circumference to the outer circumference. At that time, the insulating oil expands in proportion to the product of its volume (or unit volume), the thermal expansion coefficient, and the temperature rise. The expansion moves radially toward the outer periphery of the insulating layer and partially expands the outer metal coating (sheath), but also increases the pressure of the insulating oil itself. Immediately after energization, the temperature of the outer periphery is lower, so the viscosity of the insulating oil in that part is higher,
High viscous drag. Therefore, the movement of the insulating oil is difficult, and the expanded inner circumferential insulating oil cannot immediately move to the outer circumferential side, and the oil pressure in the insulating layer on the inner circumferential side rises more steeply. afterwards,
As the time passes, the insulating oil moves to the outer peripheral side, so that the oil pressure in the insulating layer immediately above the conductor also decreases, and the distribution of the oil pressure in the radial direction of the insulating layer becomes uniform.

When the load current is cut off in this state, the temperature suddenly drops from the conductor, so that the temperature decreases sharply on the conductor side in the insulating layer and slowly on the sheath side. Then, the insulating oil starts to contract, but since the viscosity of the insulating oil is relatively high, the insulating oil does not return from the outer peripheral side to the inner peripheral side as the steep contraction on the conductor side is followed. As a result, a negative pressure is temporarily generated in the insulating layer immediately above the conductor, especially in the oil gap, and a void is generated in that portion. If the time further elapses, the outer peripheral side of the insulating layer is positive pressure, so that the insulating oil on the outer peripheral side returns to the inner peripheral side, and both the void and the negative pressure are eliminated.

Generally, the voltage of the transmission line is imposed regardless of whether the load current is on or off. Therefore, when the load is cut off, a negative pressure is generated in the insulating layer immediately above the conductor, voids are generated, and when the magnitude exceeds a certain value and the DC electric stress applied thereto exceeds a certain value, discharge occurs, This is not a good thing for solid cables.

As described above, voids generated when the load is interrupted tend to be generated immediately above the conductor. Therefore, even if a void is formed in the insulating oil directly above the conductor, and even if this void is more than a dangerous size, the insulating oil becomes negative pressure when the load is cut off so as not to share the voltage in that part. A kraft paper having a lower resistance than the kraft paper of the main insulating layer was wound within the range. The range in which the low-resistance kraft paper is wound may be all or a part of the range in which the insulating oil has a negative pressure when the load is interrupted.

The role of the low-resistance kraft paper here is that it does not share a harmful amount of DC stress, but has substantially the same thermal resistance as the insulating tape with respect to the conductor temperature. To produce a temperature gradient. Therefore, as can be understood from FIG. 6, a sudden change in the conductor temperature when the load is cut off is also greatly reduced by the low-resistance kraft paper layer.
In the insulating layer on the outer periphery of the low-resistance kraft paper layer, a steep temperature change hardly occurs, and the shrinkage of the insulating oil decreases, so that voids hardly occur in the insulating layer. Also, even if voids are generated, their locations are concentrated in the low-resistance kraft paper layer close to the conductor.

It is also conceivable to wind a material to which no electric field (electric stress) is applied, for example, a copper tape, instead of the low-resistance kraft paper. However, in this case, the thermal resistance of the copper tape is too small, so that no temperature gradient occurs in the copper tape layer. Therefore, in the end, the steep temperature change and the steep contraction of the insulating oil similar to those of the conventional cable begin in the insulating tape layer immediately around the copper tape, so that it is easily understood that the effect of the present invention cannot be obtained. .

The resistivity (ρ ₁ ) of the low-resistance kraft paper used in a range where a negative pressure is generated directly above the conductor is 0.1 ρ ₀ ≦ ρ with respect to the volume resistivity (ρ ₀ ) of the main insulating kraft paper. ₁
Those satisfying ≦ 0.7ρ ₀ are preferable. Thereby, the low-resistance kraft paper does not share a harmful DC electric field, which is effective in suppressing the discharge in the void.

The resistivity (ρ ₁ ) of the low-resistance kraft paper is 0.
When it is larger than 7ρ _0, the volume resistivity (ρ ₀ ) of the main insulated kraft paper is so close that there is no difference in the DC electric field generated in proportion to the resistance. D of the changing part (the part where voids are likely to occur when the load is cut off)
C electric field cannot be relaxed. On the other hand, when it is smaller than 0.1ρ ₀ , almost all DC stress is shared by the main insulating layer, and the original role of sharing electric stress as an insulating layer of the low-resistance kraft paper layer is not fulfilled at all. In addition, an impulsive abnormal wave and D
Since the withstand voltage starts to decrease with respect to the C voltage itself, it is not preferable.

Low-resistance kraft paper having a resistivity of 0.1 ρ ₀ ≦ ρ ₁ ≦ 0.7ρ ₀ with respect to the kraft paper of the main insulating layer may be obtained by adding a certain additive to ordinary kraft paper. Can be obtained by using certain derivative kraft paper. This makes it possible to obtain kraft paper having a desired resistivity over the entire temperature range when the cable is used, and having a DC and impulse with a destruction value as low as that of conventional kraft paper. Specifically, an amine may be added to kraft paper, or the kraft paper may be composed of cyanoethyl paper. Various properties of the low-resistance kraft paper and the conventional kraft paper are shown in Table 1 in comparison.

[0017]

[Table 1]

As described above, the low-resistance kraft paper has a resistivity satisfying 0.1ρ ₀ ≦ ρ ₁ ≦ 0.7ρ ₀ over the entire temperature range (usually about 20 to 60 ° C.) when the cable is used. Recognize. Therefore, by using such a low-resistance kraft paper, it becomes possible to configure an insulating layer that does not share an electric field even when a void is generated, and it is possible to suppress discharge in the void.

The range in which the negative pressure is generated in the insulating layer and what percentage of the range is constituted by the low-resistance kraft paper from the conductor side can be calculated or determined after the use conditions, dimensions and structure of the cable are determined, or an experiment on a prototype cable. It can be obtained by
Generally, it is desirable that the thickness of the wrapped low resistance kraft paper is 0.5 mm or more. Experiments have shown that if it is smaller than this, the above-mentioned sudden change in the conductor temperature at the time of load interruption cannot be absorbed in low-resistance kraft paper. In general, in order to sufficiently absorb and mitigate the effects of the sudden drop in temperature when the load is rejected, a study such as that shown in FIG.
It is more preferable to wind up to. Increasing the low-resistance kraft paper layer further increases the DC that the low-resistance kraft paper shares.
When the voltage is too low, the number of turns of the entire insulating layer including the low-resistance kraft paper layer and the insulating tape layer serving as the main insulating layer increases, and the thickness thereof also increases. When the number of windings is increased, the size of the tape winding machine becomes too large or the working efficiency is reduced during cable production, and the resulting cable becomes large and uneconomical.

Further, the thickness of the low-resistance kraft paper used is preferably about 50 to 150 μm. If the thickness is less than 50 μm, the material strength of the low-resistance kraft paper decreases, and
If it exceeds μm, the oil gap of the low-resistance kraft paper layer is undesirably large.

The low-resistance kraft paper layer may be provided not only on the inner peripheral side but also on the outer peripheral side of the main insulating layer. The DC stress is higher on the inner circumference side at normal temperature and higher on the outer circumference side at high temperature. Without the use of low-resistance kraft paper, electrical breakdown occurs at high stress in the insulating layer, that is, from the innermost layer of the insulating layer (at no load or low load) or the outermost layer (at high load). The interface, which is the weakest point of a normal cable such as the layer and the outer peripheral surface of the conductor or the insulating layer and the inner peripheral surface of the metal sheath, becomes the maximum stress and easily causes electric breakdown. By applying kraft paper with low resistivity to this high stress area, the stress at the inner and outer interfaces of the insulating layer, which is likely to be the weakest point, is reduced, and the maximum point of stress is essentially uniform with a strong fracture strength. As described above, it is possible to reduce the electric stress on the innermost peripheral side of the insulating layer in which harmful voids are likely to be generated at the time of load rejection, as described above. Therefore, applying a low resistance kraft paper layer on both the inside and outside of the insulating layer is a reliable solid DC
It is effective in realizing a cable.

[0022]

Embodiments of the present invention will be described below. FIG. 1 is a sectional view of the solid DC cable of the present invention. This cable is composed of a conductor 1, an inner semiconductive layer 2, an insulating layer 3, an outer semiconductive layer 4, a metal shielding layer 5, and a sheath 6 in this order from the inner circumference. The insulating layer 3 is a main insulating layer on the outer peripheral side.
3A and an inner peripheral low resistance kraft paper layer 3B.
The main insulating layer 3A is formed by winding kraft paper, and the low-resistance kraft paper layer 3B is formed by winding kraft paper having a lower resistivity than the kraft paper of the main insulating layer 3A. Note that a low-resistance kraft paper layer may be provided between the main insulating layer 3A and the external semiconductive layer 4.

(Test Example 1) A cable having an insulating layer having a low-resistance kraft paper layer formed on both the inner and outer circumferences of the main insulating layer (Example) and a cable having an insulating layer having no low-resistance kraft paper layer (Example) Comparative Example) and
C destruction characteristics were examined. The conductor size of the cable is 800mm
^{In 2} , the thickness of the kraft paper and the low-resistance kraft paper of the main insulating layer is 130 μm. The test conditions are the starting voltage:
-500 kV, step-up condition: -100 kV / 3 days, duty cycle: energizing (70 ° C.) for 8 hours and cooling (RT) for 16 hours. Table 1 shows the cable configuration and test results.

[0024]

[Table 2]

As shown in Table 2, Examples 1 and 2 are Comparative Examples 1
Thus, it can be estimated that the discharge is suppressed even if voids are generated immediately above the conductor. In particular, Example 2 in which the thickness of the low-resistance kraft paper layer was 1.5 mm was more effective in improving the DC breakdown value.

Test Example 2 Next, using the same cable as in Test Example 1, the relationship between the difference in the thickness of the low-resistance kraft paper layer and the DC electric field in the insulating layer was examined. Here, a low-resistance kraft paper layer is provided on both the inner peripheral side (conductor side) and the outer peripheral side (sheath side) of the main insulating layer, and the thickness of each low-resistance kraft paper layer is 0.5 mm or 1.5 mm. I did it. Test conditions were: applied voltage / 350 kV DC, conductor size: 800
mm ² , and the thickness of the insulating layer is 14.0 mm. For comparison, a similar test was performed on a cable without a low-resistance kraft paper layer. FIG. 2 shows the test results when the temperature is constant at 25 ° C., and FIG. 3 shows the test results when the conductor temperature is 55 ° C.

As shown in both figures, when the temperature is low (FIG. 2), the DC electric field intensity is higher on the inner peripheral side of the insulating layer, and conversely, when the temperature is high (FIG. 3), the DC electric field intensity is higher on the outer peripheral side. In each case, the DC electric field is reduced by the low-resistance kraft paper layer. In particular, it can be seen that providing a low-resistance kraft paper layer on the outer peripheral side of the main insulating layer is also effective in alleviating the electric field at the interface between the insulating layer and the metal sheath, which is a weak point at high temperatures.

Test Example 3 Further, using the same cable as in Test Example 1, the relationship between the difference in resistivity of the low-resistance kraft paper layer and the DC electric field in the insulating layer was examined. The resistivity of the low-resistance kraft paper here is 0.1, 0.3, 0.5, 0.
A 7-fold one was used. The low-resistance kraft paper layer is provided on both the inner and outer peripheral sides of the main insulating layer, and each has a thickness of 1.5 mm. Further, the test conditions and the comparative example having no low-resistance kraft paper layer were also tested, as in Test Example 2. FIG. 4 shows the test results when the temperature is constant at 25 ° C., and FIG. 5 shows the test results when the conductor temperature is 55 ° C.

Also in this test, at low temperatures (FIG. 4), the DC electric field intensity is higher on the inner peripheral side of the insulating layer, and conversely at higher temperatures (FIG. 5), the DC electric field intensity is higher on the outer peripheral side. Then, it can be seen that any of the resistivity values in the tested range is effective in alleviating the DC electric field at the interface between the insulating layer and the conductor or the metal sheath.

[0030]

As described above, according to the solid DC cable of the present invention, the discharge can be suppressed even if a harmful void is generated due to the generation of a negative pressure in the insulating layer when the load is interrupted, and the electrical weakness of the cable is reduced. Since the electric field at the interface between the insulating layer and the conductor and the interface between the insulating layer and the metal sheath can be reduced, the electric cable having high electric breakdown strength and suitable for large power and long-distance power transmission can be configured.

[Brief description of the drawings]

FIG. 1 is a sectional view of a solid DC cable of the present invention.

FIG. 2 is a graph showing a relationship between a difference in thickness of a low-resistance kraft paper layer at a temperature of 25 ° C. and a DC electric field in an insulating layer.

FIG. 3 is a graph showing a relationship between a difference in thickness of a low-resistance kraft paper layer and a DC electric field in an insulating layer when a conductor temperature is 55 ° C.

FIG. 4 is a graph showing a relationship between a difference in resistivity of a low-resistance kraft paper layer at a temperature of 25 ° C. and a DC electric field in an insulating layer.

FIG. 5 is a graph showing a relationship between a difference in resistivity of a low-resistance kraft paper layer at a conductor temperature of 55 ° C. and a DC electric field in an insulating layer.

FIG. 6 is a graph showing a change in oil pressure in an insulating layer immediately above a conductor when power application is stopped from the start of power application.

[Explanation of symbols]

 Reference Signs List 1 conductor 2 inner semiconductive layer 3 insulating layer 3A low resistance kraft paper layer 3B main insulating layer 4 outer semiconductive layer 5 metal shielding layer 6 metal sheath

Claims (7)

[Claims] 1. A solid D having an insulating layer on the outer periphery of a conductor
In the C cable, the insulating layer has a main insulating layer made of kraft paper and a low-resistance kraft paper layer made of kraft paper having a lower resistance than the kraft paper of the main insulating layer. Solid DC characterized in that a kraft paper layer is disposed directly above a conductor or directly above a conductor and directly below a metal sheath.
cable. 2. The solid DC cable according to claim 1, wherein the low resistance kraft paper layer is provided in a range where the insulating oil has a negative pressure when the load is interrupted. 3. The resistivity (ρ ₁ ) of the low-resistance kraft paper is _{0. 0 to} the resistivity (ρ ₀ ) of the kraft paper of the main insulating layer.
Solid DC cable according to claim 1, wherein the 1ρ is ₀ ≦ _{ρ 1} ≦ _{0.7ρ 0.} 4. The thickness of the low-resistance kraft paper layer is 0.5 mm.
2. The solid D according to claim 1, wherein
C cable. 5. A low-resistance kraft paper having an insulating layer thickness of 1
The solid DC cable according to claim 1, wherein the cable is wound to 0%. 6. The solid DC cable according to claim 1, wherein the low-resistance kraft paper is an amine-added paper. 7. The solid DC cable according to claim 1, wherein the low-resistance kraft paper is cyanoethyl paper.