JPH05120917A - Filler for direct current cable insulator - Google Patents

Abstract

PURPOSE:To improve filler for direct current power cable insulator used for a high voltage direct current transmission line, such as a submarine cable. CONSTITUTION:In filler, to be added to insulator for a high polymer insulation direct current cable, such as polyethylene or cross linked polyethylene, the filler is made of magnesium oxide, whose particle diameter is 50mum or less, and specific surface, by means of BET method, stays in a range of 20-80m<2>/g.

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 At present, 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 management, and disaster prevention as a power cable for AC power transmission. Is widely used and, in recent years, 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, the problems peculiar to DC insulation appear conspicuously and domestically. From an international point of view, 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 the cable has been extruded, the cable is dried in a drying room for a certain period of time to insulate it. The decomposition products remaining in the body at the time of cross-linking are kept below an allowable level (a level at which the generated gas does not adversely affect the cable sheath etc.).

[0004]

One of the problems in using a CV cable as a DC cable is that the presence of space charges formed in the insulator when a DC voltage is applied to the cable is generally present. 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 DC (positive in this case) enters such a state, or if the polarity of the DC voltage is abruptly reversed, the electric field directly above the conductor electrode, which has been relaxed by space charge, is generated. It will rise remarkably, causing 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 formation of the above-mentioned 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 Patent Publication 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 cross-linked with calcium silicate, calcium carbonate, magnesium oxide, etc. and a water-impervious layer provided around the insulator, and a polar inorganic insulator, so-called,
The addition of a filler prevents a decrease in DC dielectric strength due to the accumulation of space charges.

However, when such a polar inorganic insulating material is added to polyethylene or crosslinked polyethylene, the DC breakdown characteristics can be improved, but the added polar inorganic insulating material 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 internal electric field distribution becomes a resistance distribution in the radial direction of the cable. It depends a lot.
In particular, in the case of a crosslinked polyethylene insulation, the decomposition products remaining in the cable insulation during crosslinking greatly affect the resistance of the insulation, making the internal electric field more complicated.

FIG. 1 is a graph showing the measured 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 thinly sliced (about 0.1 mm) in the radial direction was sampled on a lathe using a special bite from this cable, and the specific insulation resistance ρ and the amount β of decomposition products at the time of crosslinking of each sample were measured. .. The specific insulation resistance ρ was calculated from the leakage current when DC of 4 kV was applied to the sample at room temperature. In order to minimize the effect of volatilization of decomposition products during crosslinking during the measurement, it is calculated for convenience based on the leakage current value 10 minutes after the voltage application. In addition, the decomposition products were similarly dried by vacuum-drying the sampled sheets and calculating from the weight change 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-described convex decomposition residue 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 high in the inner and outer conductive layers of the cable and low in the center of the insulator, contrary to the distribution of the decomposition residue amount β. 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 crosslinked polyethylene using as a crosslinking agent are methane, acetophenone, α-methylstyrene, and cumyl alcohol, all of which are low molecular weight gases or liquids and are unique to each other. 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 decomposition residue remains, and the characteristic insulation resistance ρ is relatively high on the outer conductive layer side while the decomposition residue is easily volatilized. 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. Naturally, the decomposition residue is less likely to volatilize as the cable insulation thickness is larger, and this tendency is further promoted in cables with thick insulation thickness. 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), it 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 reduced 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 relation 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, the longer the time required for drying, which is difficult to handle in the actual cable manufacturing process.

The 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 enabling improvement, 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 so that the DC 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 relates to 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 many 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 a space charge suppressing effect, the magnesium oxide-added product 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, there are many impurities harmful to electrical insulation such as iron oxide, and their removal. Is industrially limited. On the other hand, although magnesium oxide can be obtained from natural minerals such as magnesite as a resource, 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. is doing. Note that 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 if 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, whereby 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 of measuring the surface area of a solid (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 desirable with respect to 100 parts by weight of polyethylene. That is, when the amount is 1 part by weight or less, the effect of preventing the accumulation of space charges, the effect of improving the specific insulation resistance and the accompanying homogenization effect 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, crosslinked polyethylene sheets having different BET specific surface areas to which magnesium oxide 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. The results are shown in Table 1.

The samples were low density polyethylene, a cross-linking agent (D.C.P.) and an 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 in the above-mentioned composition.

The lightning impulse breakdown characteristic was selected in the evaluation of the breakdown characteristic because it is more sensitively reflected to defects such as agglomerated particles than the direct current breakdown characteristic.
In addition, the aggregated particles generated in the crosslinked polyethylene were evaluated based on the results of observation 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 breaking strength had a peak at a BET specific surface area of 50 m ² / g, and there was a tendency for it to decrease above and below. 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, 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 conjectured that the adhesiveness 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 the cross-linking, and the added magnesium oxide adsorbs the decomposition residue. It is thought to be due to having. Further, the specific insulation resistance ρ increases as the BET specific surface area of magnesium oxide increases, and it tends 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 is 50 m because both are crosslinked under the same conditions.
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 BET specific surface area of magnesium oxide that can exhibit good characteristics in both lightning impulse breakdown characteristics and specific insulation resistance characteristics of the insulator is 20 to 80 m ² / g. ..

FIG. 2 is a sectional view of an embodiment of a 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 cross-linked 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.

To clarify the effect on the DC breakdown characteristics, a DC breakdown test was performed on the same cable immediately after extrusion (undried product). The results are shown in Table 2. Samples C (Comparative Example 1) and Sample G shown in Table 1 which have the same dimensional structure and insulating composition as Comparative Examples.
Data obtained by conducting tests under the same conditions are shown for the cable added with magnesium oxide used in (Comparative Example 2) and a normal non-added crosslinked polyethylene insulated cable (Comparative Example 3). 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 is low,
It supports the conventional data that sufficient performance is not obtained in the undried product. On the other hand, the examples according to the present invention did not break even at twice the voltage confirmed in the comparative examples, even though they were undried products, 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 of this is Comparative Example 1
Regarding the instability of the specific insulation resistance ρ,
Is considered to be due to the generation of aggregated particles. In particular, Comparative Example 2 has the lowest impulse breakdown characteristic, and in consideration of the results shown in Table 1, it is presumed that the generation of aggregated particles greatly affects the performance deterioration.

Although all of the tests were 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. Furthermore, although the impulse breakdown characteristics were slightly less than those of the comparative example, which was a cross-linked polyethylene cable with no additive, good characteristics comparable to those 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 peripheral portion of the core conductor. Adopting cross-linked polyethylene eliminates the effects 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, 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 with a practical dry treatment is combined with these features, which enables a rational insulation design, and enables the miniaturization of cables and As well as weight reduction,
It also enables cost reduction 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 in the radial direction of a normal crosslinked polyethylene insulated cable,

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. ⁵ Identification number Reference number within the agency FI Technical display location C08L 23/00 KEC 7107-4J (72) Inventor Akifumi Katagai 5-1, Hidaka-cho, Hitachi, Ibaraki No. 1 "Inside the Hitachi Cable Co., Ltd. Electric Wire Research Laboratory" (72) Inventor Hiroaki Sato 5-1-1 Hidakacho, Hitachi City, Ibaraki Prefecture "Inside the Hitachi Cable Co., Ltd. Hidaka Plant" (72) Inventor Shinichi Yamamoto `` Ube Chemical Industry Co., Ltd., '' 1985, Ogushi, Ube City, Yamaguchi Prefecture (72) Inventor Satoshi Sano 1985, Ube Chemical Industry Co., Ltd., Obeshi, Ube City, Yamaguchi Prefecture

Claims (1)

[Claims] 1. 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 a BET specific surface area of 20 to 80 m ² / g. A filler for a DC cable insulator, characterized by being magnesium oxide in the range of.