JP2523236B2 - Filler for DC cable insulation - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an improvement of a DC power cable suitable for use in a high voltage DC transmission line such as a submarine cable.

[0002]

2. Description of the Related Art Currently, a power cable using cross-linked polyethylene as an insulator, a so-called CV cable, has many advantages in terms of excellent insulation characteristics, ease of maintenance and disaster prevention as a power cable for AC transmission. Is widely used, and in recent years, it has come into practical use as a 500 kV cable in combination with remarkable progress in manufacturing technology.

As described above, an AC cable is a CV cable that has many excellent features and a track record, but when it is used for high-voltage DC transmission, a problem peculiar to DC insulation appears conspicuously and domestically. From a global perspective, there are still no examples of application to real lines. Further, the CV cable is made of not only polyethylene but also D.I. for improving the heat resistance of the insulator and improving the allowable current of the cable. C.
P. Cross-linked polyethylene cross-linked with a cross-linking agent such as (di-cumyl peroxide) is used as an insulator. Therefore, during cable production, decomposition products such as methane and acetophenone associated with the crosslinking reaction are generated in the insulator.In general, after cable extrusion, dry the cable in a drying room for a certain period of The decomposition products remaining in the body at the time of crosslinking are kept below an allowable level (a level at which generated gas does not adversely affect the cable sheath etc.).

[0004]

As one of the problems in using a CV cable as a DC cable, it is generally the presence of space charge formed in the insulator when a DC voltage is applied to the cable. It is known.

For example, it is known that when a negative DC voltage is applied to a cable, negative space charges are formed near the conductor side, and conversely, positive space charges are formed near the shield side. In such a case, the electric field on the conductor electrode and on the shield side electrode is relaxed, but a local high electric field is generated inside the insulator and the effective insulation thickness of the cable is reduced. This is also known. Furthermore, if a lightning impulse voltage with a polarity opposite to that of the direct current (positive in this case) enters such a state, or if the polarity of the direct current voltage is abruptly reversed, the electric field directly above the conductor electrode, which was relaxed by the space charge, is generated. This causes a remarkable increase and causes an unexpected decrease in breakdown voltage.

Therefore, in order to apply a cable using polyethylene or cross-linked polyethylene as an insulator for direct current,
It is a necessary condition to suppress the above-mentioned formation of space charge as much as possible, and various proposals have been made so far as measures for suppressing it.

For example, as disclosed in Japanese Examined Patent Publication (Kokoku) No. 57-21805, 20 to 80 parts by weight of a polar non-flat inorganic insulating powder having a particle size of 50 microns or less, that is, aluminum silicate is added to polyethylene. An example is a cable having an insulator crosslinked by mixing calcium silicate, calcium carbonate, magnesium oxide, etc. and a water-blocking layer provided on the outer periphery of the insulator, and a polar inorganic insulator, so-called,
The addition of a filler prevents the deterioration of DC dielectric strength due to the accumulation of space charges.

However, when such a polar inorganic insulator is added to polyethylene or crosslinked polyethylene, the DC breakdown characteristics can be improved, but the added polar inorganic insulator acts as a foreign substance, and the excellent alternating current of crosslinked polyethylene is excellent. There is a problem that the characteristic, for example, the lightning impulse destruction characteristic is deteriorated. Needless to say, if the powder of the polar inorganic insulating material is agglomerated during the manufacturing process, it is not preferable for electrical insulation.

The above is the problem of direct current insulation related to the behavior of space charge. In addition to this, the electric potential in the cable insulator becomes a resistance share when the direct current is applied, and the electric field distribution inside becomes a resistance distribution in the radial direction of the cable. It depends a lot.
In particular, in the crosslinked polyethylene insulator, the decomposition products at the time of crosslinking remaining in the cable insulator greatly affect the resistance of the insulator, which makes the internal electric field more complicated.

FIG. 1 is a graph showing the distribution of the specific insulation resistance (ρ) in the radial direction of the insulator and the amount of residual decomposition products at the time of crosslinking in an ordinary crosslinked polyethylene cable. After extruding the cable, the temperature is about 7
Insulation thickness 1 after drying for 2 weeks at 0 ℃
It is a 3 mm cross-linked polyethylene insulated cable. A sample of an insulator sheet sliced thinly (about 0.1 mm) in the radial direction was sampled with a lathe using a special bite from this cable, and the specific insulation resistance ρ and the amount β of decomposition products during crosslinking of each sample were measured. . The specific insulation resistance ρ was calculated from the leak current at that time when DC of 4 kV was applied to the sample at room temperature. In order to minimize the influence of volatilization of the decomposition products at the time of crosslinking during the measurement, it is calculated for convenience based on the leakage current value 10 minutes after the voltage application. Similarly, the decomposition products were calculated by vacuum-drying a sampled sheet and changing the weight at that time.

As is apparent from FIG. 1, the decomposition residue amount β in a normal crosslinked polyethylene cable has a convex distribution having a peak in the center of the insulator. It is considered that the decomposition residue is uniformly generated in the insulator immediately after the cable is manufactured, but it spontaneously volatilizes to the outside (outer side of the cable or the side of the conductor) in the subsequent drying process or the process of being left in the air. Therefore, it is easily inferred that the above-mentioned convex decomposition residual amount distribution is obtained. On the other hand, the distribution of the specific insulation resistance ρ in the radial direction of the cable has a concave distribution, which is opposite to the distribution of the amount of decomposition residue β, that is high inside the cable and on the outer conductive layer side and low in the center of the insulator. From this, it is understood that the decomposition residue in the insulator reduces the specific insulation resistance ρ.

For example, D.I. C. P. Typical examples of the decomposition residue component of cross-linked polyethylene using as a cross-linking agent are methane, acetophenone, α-methylstyrene and cumyl alcohol, all of which are low molecular weight gases or liquids and are unique to each of them. As insulation resistance ρ value, acetophenone: 4 × 10 ⁷ Ω · cm, cumyl alcohol: 5 × 10 ⁹ Ω · cm, α-methylstyrene: 7
× 10 ¹² Ω · cm has been confirmed. It can be seen that this is an order of magnitude lower specific insulation resistance than polyethylene. Therefore, the characteristic insulation resistance ρ of the insulator is low where a large amount of the decomposition residue remains, and the characteristic insulation resistance ρ is relatively high on the outer conductive layer side while the decomposition residue easily volatilizes. Can be explained.

When a DC voltage is applied to a cable having a distribution of the specific insulation resistance ρ as shown in FIG. 1, the specific insulation resistance ρ
The voltage sharing becomes large in the high electric field portion, and as a result, a local high electric field portion is formed in that portion. As a matter of course, the decomposition residue is less likely to volatilize as the cable insulation thickness is larger, so this tendency is further promoted in cables with thick insulation thickness, and this is remarkable for the DC breakdown voltage recognized in crosslinked polyethylene cables. This is considered to be one of the causes of the dependence on the insulation thickness. Further, the decrease in the specific insulation resistance ρ of the insulator inevitably increases the leakage current when a direct current is applied, and the heat generated by the insulator (Joule heat: P
= I ² R), which causes thermal instability, and it is needless to say that it is desirable to increase the specific insulation resistance ρ of the insulator as much as possible in order to obtain a stable cable insulator.

By the drying treatment after the cable extrusion as described above, the amount β of the remaining decomposition residue is decreased and the specific insulation resistance ρ can be expected to be improved to some extent, but the decomposition residue is discharged to the outside by the concentration diffusion. Therefore, the relative magnitude relationship of the specific insulation resistance ρ basically does not change, and the above-mentioned nonuniformity of the distribution of the specific insulation resistance ρ cannot be eliminated. Furthermore, the thicker the insulation thickness, the longer the time required for drying, and there is the problem that it is difficult to handle in the actual cable manufacturing process.

An object of the present invention is to solve the above-mentioned problems, and in a filler applied to a crosslinked polyethylene insulation cable, it has an effect of suppressing space charge formation and an impulse breakdown characteristic. In addition to being able to improve the DC voltage, it is possible to maintain a high specific insulation resistance without the need for special drying treatment, and to make the distribution of the specific insulation resistance in the radial direction of the cable uniform Another object of the present invention is to provide a filler capable of obtaining a DC cable having excellent impulse breakdown characteristics, thermal reliability and economy.

[0016]

DISCLOSURE OF THE INVENTION The present invention provides a filler to be added to a polymer-insulated DC power cable insulator such as polyethylene or cross-linked polyethylene, wherein the filler has a particle size of 50 μm or less and has a BET ratio. It is characterized by being magnesium oxide having a surface area in the range of ²⁰ to 80 m ² / g.

Here, the reason why the additive is magnesium oxide will be described. Of the numerous polar inorganic insulating powders that are effective in suppressing the space charge accumulation of the insulator, the reason why it is limited to magnesium oxide is as follows.

(A) The deterioration of impulse breakdown characteristics of polyethylene or crosslinked polyethylene containing magnesium oxide is smaller than that of other polar inorganic insulating powder additives. That is, in comparison with various polar inorganic insulating powder-filled polyethylenes that are considered to have the effect of suppressing space charge, the magnesium oxide additive has the highest impulse breakdown strength.

(B) Since polar inorganic insulating powders such as talc and clay which are widely used industrially as fillers are natural minerals, they contain many impurities harmful to electrical insulation such as iron oxide, and their removal. Is industrially limited. On the other hand, magnesium oxide can be obtained from natural minerals such as magnesite as a resource, but magnesium carbonate and hydroxide produced industrially from solutions containing magnesium ions such as seawater, canned water, and bitter juice. It has the advantage that it can be manufactured using an artificially synthesized magnesium compound such as magnesium as a raw material, and that it is possible to supply materials with much higher purity and stable quality and physical properties compared to the aforementioned natural minerals. are doing. It is presumed that the presence of such impurities causes a decrease in the specific insulation resistance ρ.

Next, the reason why the BET specific surface area of magnesium oxide is limited to the range of 20 to 80 m ² / g will be described. The BET specific surface area is a measure of the activity of the particles, and when it is less than 20 m ² / g, the activity is too small to obtain an efficient decomposition residue adsorption action. Also, 80
If it exceeds m ² / g, on the contrary, the activity will be too large, and the particles will easily aggregate, so that a uniform dispersion cannot be obtained.

Here, the BET specific surface area is the surface area per unit weight, and the surface area is measured by the BET method. The BET method is a so-called adsorption method, which is a method for measuring the surface area of solids (particularly powder),
S. Brunauer, P .; H. Emmett, E .; T
It is named BET because it is based on a series of related equations for gas adsorption on solids proposed by eller. That is, gas molecules are adsorbed on the surface of the solid (powder) placed in the gas. Since the value of the cross-sectional area of one gas molecule is known, the total surface area of the powder is obtained by measuring the amount of gas molecules adsorbed on the powder (single molecule).

Next, the reason why the addition amount of magnesium oxide is limited to the range of 1 to 40 parts by weight will be described. 1 to 40 parts by weight is preferable for 100 parts by weight of polyethylene. That is, when the amount is less than 1 part by weight, the effect of preventing the accumulation of space charges, the effect of improving the specific insulation resistance and the homogenizing effect accompanying it are weakened, and when the amount of addition exceeds 40 parts by weight, the specific insulation resistance is decreased. .

[0023]

EXAMPLES A direct current cable to which the filler of the present invention is applied will be described in detail below. First, cross-linked polyethylene sheets to which magnesium oxide having different BET specific surface areas was added were prepared, and the influence of the BET specific surface area on the lightning impulse breakdown characteristics and the specific insulation resistance ρ and the generation state of aggregated particles were investigated. Table 1 shows the results.

The samples consisted of low density polyethylene, cross-linking agent (D.C.P.) and anti-aging agent and different BE.
A mixture of magnesium oxide having a T specific surface area was kneaded with a roll, and pressed and pressed to form a crosslinked product.
It is a 1 mm cross-linked polyethylene sheet. The amount of magnesium oxide added was polyethylene 100 for all samples.
It was unified to 5 parts by weight of magnesium oxide with respect to parts by weight.
Note that Sample I in the table was carried out for comparison, and is data of a sheet of ordinary cross-linked polyethylene which is not filled with magnesium oxide among the above-mentioned formulations.

The lightning impulse breakdown characteristic is selected in the evaluation of the breakdown characteristic because it is more sensitively reflected to defects such as agglomerated particles than the DC breakdown characteristic.
In addition, the aggregated particles generated in the crosslinked polyethylene were evaluated based on the observation result with an optical microscope.

[0026]

[Table 1]

As is clear from Table 1, by changing the BET specific surface area of magnesium oxide, the impulse breakdown strength, the specific insulation resistance ρ and the size of the maximum aggregated particles are changed. That is, the impulse rupture strength had a peak at a BET specific surface area of 50 m ² / g and tended to decrease at higher or lower levels. Here, as seen in Samples G and H, the BET specific surface area is 100 m ² /
If it exceeds g, the size of the largest agglomerated particles also increases accordingly, and it is presumed that the presence of the coarse agglomerated particles is a factor of deterioration of the characteristics. It is interpreted that this is because the activity of the particles is too high and the particles easily aggregate. On the contrary, even when the BET specific surface area is very small (Samples A, B, C), the impulse breakdown strength is low. In this case, since the activity is low, the maximum agglomerated particles tend to be improved, and it is speculated that the adhesion between the particles and the polymer itself is impaired. By the way, although the impulse fracture strength of sample E is not as high as that of the uncrosslinked polyethylene of sample I,
Good characteristics of 435 kV / mm were confirmed.

On the other hand, from the viewpoint of the specific insulation resistance ρ of the insulator, all of the samples A to G containing magnesium oxide are about two orders of magnitude higher than the sample I containing no magnesium oxide.

This is because the test sample is a cross-linked polyethylene and is an undried product, so that the specific insulation resistance ρ largely depends on the decomposition residue generated during crosslinking, and the added magnesium oxide adsorbs the decomposition residue. It is thought to be due to having. Further, the specific insulation resistance ρ increased as the BET specific surface area of magnesium oxide increased, and it tended to be saturated at a BET specific surface area of 50 m ² / g or more. This is probably because the BET specific surface area is a measure of the activity, and the larger the value, the higher the activity. Therefore, the larger the BET specific surface area, the more the adsorption action of the decomposition residue is promoted. The reason why the specific insulation resistance ρ is saturated when the BET specific surface area is a certain value or more is that the amount of decomposition residue β remaining in polyethylene is constant and the BET specific surface area of 50 m
It is considered that the adsorption is already in equilibrium at about ² / g.

From the above results, it was determined that the range of the BET specific surface area of magnesium oxide that can exhibit good characteristics in both the lightning impulse breakdown characteristic and the specific insulation resistance characteristic of the insulator is 20 to 80 m ² / g. .

FIG. 2 is a sectional view of an embodiment of the DC power cable according to the present invention. In the figure, 1 is 800 mm
^2, a conductor made of stranded copper wire, 2 an inner semiconductive layer, 4 an outer semiconductive layer, 5 a metal shielding layer, and 3 added magnesium oxide having a BET specific surface area of 50 m ² / g according to the present invention. It is an insulating layer made of crosslinked polyethylene and having an insulating thickness of 20 mm. The insulating layer 3 is a cross-linked polyethylene in which 10 parts by weight of magnesium oxide, 100 parts by weight of polyethylene, an antioxidant and a cross-linking agent (D.C.P.) are mixed in appropriate amounts.

In order to clarify the effect on the DC breakdown characteristics, a DC breakdown test was performed on the same cable immediately after extrusion (undried product). Table 2 shows the results. Samples C (Comparative Example 1) and Sample G shown in Table 1 which have the same dimensional structure and insulator blend as Comparative Examples.
The data of the cable added with the magnesium oxide used in (Comparative Example 2) and the normal additive-free crosslinked polyethylene insulated cable (Comparative Example 3) are shown under the same conditions. All test temperatures are 90 ° C.

[0033]

[Table 2]

As can be seen from Table 2, the DC breakdown voltage of the undried crosslinked polyethylene cable of Comparative Example 3 was low,
It confirms the conventional data that sufficient performance is not obtained for undried products. On the other hand, the example according to the present invention did not break even at a voltage twice as high as the voltage confirmed in the comparative example, even though it was an undried product, and showed very good DC breakdown characteristics.

Although the DC breakdown voltages of Comparative Examples 1 and 2 are improved in comparison with Comparative Example 3, they are not as high as those of Examples according to the present invention. The cause is Comparative Example 1
Regarding the instability of the specific insulation resistance ρ, Comparative Example 2
Is considered to be due to the generation of aggregated particles. In particular, Comparative Example 2 has the lowest impulse breakdown property, and when considered together with the results in Table 1, it is presumed that the generation of aggregated particles is large and the performance deterioration is affected.

Although all of the tests are carried out at a temperature of 90 ° C., the examples according to the present invention show a thermally very stable characteristic even when exposed to the above-mentioned high electric field. As a result, the effect of improving the specific insulation resistance ρ in the example is remarkably recognized. Further, although the impulse breakdown characteristic was slightly less than that of the comparative example, which is a cross-linked polyethylene cable with no additive, good characteristics comparable to it were confirmed, and the effectiveness of the cable according to the present invention was confirmed again.

[0037]

As described above, according to the DC power cable of the present invention, magnesium oxide having a BET specific surface area of 20 to 80 m ² / g is used as the filler as the insulator formed on the outer periphery of the core conductor. Since cross-linked polyethylene is adopted, the influence of decomposition residues at the time of cross-linking, which adversely affects the DC insulation characteristics, that is, the decrease in the specific insulation resistance ρ of the insulator and its non-uniformity are eliminated, and good DC breakdown characteristics In addition to the above, we have realized a DC cable that minimizes the drop in impulse breakdown voltage, which is a major problem when such inorganic substances are added.

In addition, the fact that a stable DC breakdown characteristic can be maintained even in a practical dry treatment level is combined with these features, which enables a rational insulation design, and reduces the size and size of the cable. As well as being lightweight,
It has also made it possible to reduce costs in cable manufacturing.

[Brief description of drawings]

FIG. 1 is a graph of the measurement results of the specific insulation resistance ρ and the amount of decomposition residue β of a normal crosslinked polyethylene insulated cable in the cable radial direction,

FIG. 2 is a cross-sectional view showing an example of a DC power cable to which a filler according to the present invention is applied.

[Explanation of symbols]

 1 conductor 2 inner semiconductive layer 3 insulating layer 4 outer semiconductive layer 5 metal shielding layer

Continuation of the front page (51) Int.Cl. ⁶ Identification number Reference number within the agency FI Technical display location C08L 23/00 KEC C08L 23/00 KEC (72) Inventor Akifumi Katagai 5-1, Hidakacho, Hitachi City No. 1 "Inside the Hidden Cable Co., Ltd. Electric Wire Research Laboratory" (72) Inventor Hiroaki Sato 5-1-1 Hidaka-cho, Hitachi City, Ibaraki Prefecture "Inside the Hidaka Factory Hidaka Electric Cable Co., Ltd." Ichibaguchi, Ube City, Obeshi Kogushi, 1985, Ube Chemical Industry Co., Ltd. (72) Inventor, Satoshi Sano, Ube City, Yamaguchi Prefecture, Kozagushi, 1985, Ube Chemical Industry Co. Ltd. 51-41885 (JP, A) JP 62-177805 (JP, A) JP 61-253711 (JP, A) JP 63-128504 (JP, A)

Claims (1)

(57) [Claims] 1. A filler to be added to an insulator for polymer-insulated DC power cables, such as polyethylene or cross-linked polyethylene, wherein the filler has a particle size of 50 μm or less and a specific surface area by BET method of ²⁰ to 80 m ² / g. A filler for a DC cable insulator, characterized by being magnesium oxide in the range of.