JP3349747B2 - Insulation composition and power cable - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an insulating composition capable of maintaining excellent characteristics even under a high temperature and a high electric field, and a power cable using the same as an insulator.

[0002]

2. Description of the Related Art Crosslinked polyethylene (XLPE) obtained by crosslinking low-density polyethylene is currently widely used as an insulator for power cables. A power cable using XLPE as an insulator includes a conductor, a shielding layer made of a conductive resin composition,
And three layers of insulating layers consisting of a composition obtained by blending a crosslinking agent and an antioxidant with low-density polyethylene, are simultaneously extruded at a low temperature at which crosslinking does not occur, and then continuously,
It is produced by passing through a high temperature, high pressure crosslinking zone and a low temperature, high pressure cooling zone.

[0003] The power cable thus obtained is:
Because of its excellent characteristics, the applied voltage has been increased, and in recent years, it has been applied to long-distance power transmission of 275 kV, and to drawing out of a short-range 500 kV power plant.

[0004]

However, in recent years, there has been a demand for reducing the number of cable connection parts which are apt to be a weak point in insulation performance, reducing the number of underground manholes for storing the connection parts, and reducing the construction cost. Therefore, it has been strongly demanded that the connection interval be lengthened. A concrete measure for increasing the length of the connection interval is, in short, the development of a cable having a thin insulating layer. For example, the conventional short distance 50
The thickness of the insulating layer of the 0 kV cable is 32 mm, whereas the thickness of the insulating layer is required to be 27 mm, which is reduced by about 17% in the case of a long-distance 500 kV cable that requires a connection portion. . Although there are many development problems caused by the reduction in the thickness of the insulating layer, the essence thereof is caused by an increase in the electric field applied per unit thickness of the insulating layer. Therefore, the dielectric strength is further improved, and the dielectric properties, particularly, the dielectric loss tangent (ta)
nδ) It is required to reduce the characteristic value. An object of the present invention is to provide an insulating composition and a power cable that can maintain excellent dielectric loss tangent (tan δ) characteristics even when the thickness of an insulating layer is reduced.

[0005] The dielectric loss Wd due to the dielectric properties of the insulating layer of the power cable can be expressed by the following equation. ^{Wd = (V 2/3)} ωC 0 εrtanδ V: transmission grid voltage omega: transmission power of the angular frequency (ω = 2πf, f: frequency / H
z) C ₀ : geometrical capacitance determined by the structure of the cable εr: relative dielectric constant of the insulating layer material tan δ: dielectric tangent of the insulating layer material As can be seen from the above equation, the dielectric loss is the transmission system voltage V Since the geometric capacitance C ₀ that is proportional to the square and includes the structural factors of the cable can be expressed by the following equation, it increases in proportion to the length of the cable, and the reduction in the insulation thickness also reduces the dielectric loss. It is a factor that makes it bigger. The relationship between the dielectric properties and the dielectric loss of the insulating layer material is proportional to the product εrtanδ of the relative dielectric constant and the dielectric loss tangent, as can be seen from the above equation. C ₀ = 2πε ₀ L / ln (R / r) ε ₀ : Dielectric constant of vacuum L: Length of cable R and r: Outside diameter and inside diameter of insulating layer

[0006] The dielectric loss of the power cable becomes a real problem in the case of a cable using XLPE as an insulating layer when the power transmission system voltage is 275 kV or more in a long distance transmission line. As described above, among the various factors of dielectric loss, V and C
_{Since 0} is a constant, the examination of the characteristic improvement is performed on the dielectric characteristic of the insulating material. Among the dielectric properties of the insulating material, the relative permittivity εr is a material constant and cannot be improved, and the improvement of the tan δ characteristic value is required exclusively.

As a specific example of the dielectric loss of a power cable using XLPE as an insulating layer, a case of a long-distance 500 kV cable is considered.
7 mm and the normal maximum temperature of 90 ° C., a desirable tan δ characteristic value is 0.05% or less at a temperature of 90 ° C. and a maximum stress of the insulating layer of 25 kV / mm.

When the materials of the prior art were evaluated based on the above situation, the tan δ at 90 ° C. and 20 kV / mm was evaluated.
Was over 0.1% and was unable to respond to requests.
Looking at the details of the tan δ characteristics of the material according to the prior art, at a temperature of about 60 ° C. or less, the measured stress was 5 to 3 mm.
In the range of 0 kV / mm, tan δ is 0.05% or less, which satisfies the desirable property of dielectric loss. However, 7
In the temperature range exceeding 0 ° C., the tan δ characteristic value increases as the electric field stress increases. At 90 ° C., at 5 kV / mm, tan δ is a small value of 0.02% or less, but 20 kV / mm. In mm, tan δ is 0.
It is over 1%.

As described above, the problem of the prior art is that the tan δ characteristic value sharply increases due to an increase in stress on the high temperature side. The present inventors have conducted intensive studies on this phenomenon, and as a result, it has been found that the phenomenon is caused by the presence of an electrically conductive impurity in XLPE made of a conventional material. Further, it has been found that since the amount of the electrically conductive impurity is very small, it is at a concentration level that is difficult to identify by ordinary analysis means.

The above contents are described in more detail as follows. XPLE is an insulating layer material for power cables
The tan δ characteristic is mainly classified into two types: a dipole factor caused by impurities having a polar chemical structure in a material, and a conduction factor caused by dissociated ions of ion-dissociative impurities in a material. be able to. When a low-density PE that does not contain a polar comonomer component is used as a base material of XLPE, impurities in the polar chemical structure in the material that contribute to the tan δ of the dipole factor are those in the base low-density PE. And those related to the cross-linking agent and anti-aging agent to be incorporated, and those caused by contamination during the production process. In the current state of the art of XLPE materials, the main cause of the tan δ of the dipole factor is acetophenone and cumyl alcohol, which are the residue of the cross-linking reaction of dicumyl peroxide (DCP) used as a cross-linking agent. Is a property that is large on the low temperature side, becomes smaller as the temperature becomes higher, and does not change even if the electric field stress increases. In the case of a normal power cable, the tan δ value due to this factor does not exceed 0.02% at the maximum, and is not at a level that causes a problem in terms of dielectric loss.

The substance causing tan δ, which is an electrical conduction factor, has an effect even at a very small concentration. Therefore, chemical species have not been identified. All the dissociative impurities in the cross-linking agent and antioxidant to be used and those resulting from contamination during the manufacturing process are all considered, and it is considered that the total concentration of the ion dissociable impurities is related to the characteristic value. Usually, the tan δ of the conduction factor is small on the low temperature side, rapidly increases as the temperature increases, and further increases exponentially with an increase in the electric field stress.
The problem with the dielectric loss of a power cable using XLPE as an insulating layer is a problem due to tan δ, which is a conductive factor caused by dissociated ions of the ion-dissociative impurities. , Making the problem even more serious.

SUMMARY OF THE INVENTION The present invention has been made in view of such circumstances, and by using an insulating composition having a small total amount of electrically conductive impurities and using this insulating composition as a material for an insulating layer, high temperature and high temperature can be attained. It is an object of the present invention to provide a power cable having excellent tan δ characteristics even under a voltage.

[0013]

Means for Solving the Problems In order to solve such problems, the insulating composition of the present invention comprises low-density polyethylene 100
1 to 5 parts by weight of a cross-linking agent and 0.05 of an anti-aging agent
~ 0.5 parts by weight of a crosslinked resin composition.
The resin composition is subjected to an extraction treatment of 10 g of the resin composition with 10 ml of a non-polar solvent, and the value obtained by multiplying the volume resistivity of the extract by the viscosity of the extract is 5 × 10 ¹⁰ ΩPas / c.
It is characterized by having a value of m or more.

A power cable according to a second aspect is characterized in that the insulating composition according to the first aspect is used as an insulator.

[0015]

The source of the electrically conductive impurities contained in the insulating composition and brought into the insulating layer of the power cable is, as described above, the base low-density PE which is a constituent material of the composition, a cross-linking agent, and aging. There are two routes: one that is mixed as an impurity of the inhibitor and one that is mixed during the manufacturing process of the composition. Then, regardless of which route the electrically conductive impurities are mixed in, the total sum thereof is related to the tan δ characteristic. Therefore, it can be said that the method of selecting according to the total amount of the electrically conductive impurities contained in the insulating composition is most effective. The basic requirement of the present invention is to grasp the total amount of electrically conductive impurities by “volume resistivity of an extract obtained by treating 10 g of the composition obtained by crosslinking the composition with 10 ml of a polar solvent”, and to determine “volume resistivity”. Is multiplied by the viscosity of the extract to 5 × 10 ¹⁰ Ωpas / m or more ”.

The reason for using the “cross-linked composition” as a sample to determine the total amount of electrically conductive impurities is as follows:
This is because the various conditions of the cross-linking reaction that the insulating layer of the actual power cable receives are reflected in the evaluation characteristic value, that is, the volume resistivity of the extract. The cross-linking reaction itself is a so-called radical reaction, and various changes in ion-dissociative impurities caused by a high temperature of approximately 160 ° C. or more as the surrounding conditions of the complex reaction or the cross-linking reaction are extracted characteristics. It is necessary to reflect on the volume resistivity of the liquid. By performing such an evaluation, the tan δ characteristic of a power cable having an insulating layer formed by crosslinking the composition,
In particular, excellent tan δ characteristics can be ensured even under high temperature and high electric field.

In order to grasp the total amount of ion-dissociative impurities present in the insulating layer, the evaluation is made based on “the volume resistivity of the extract obtained by treating the composition obtained by crosslinking the composition with a non-polar solvent”. This is a characteristic means of the present invention. As previously mentioned,
Since the concentration of ion-dissociative impurities present in the insulating layer is extremely small, it is difficult to identify the substance. The volume resistivity of the liquid from which the ionic dissociative impurities present in the insulating layer are extracted is the ionic dissociative impurity concentration, the ionic dissociation degree of each impurity that is also related to the dielectric constant of the liquid used for extraction,
It is determined by the mobility of each dissociated ion. In general, the ionic dissociation degree is higher for a dielectric solvent having a higher dielectric constant, that is, for a polar solvent. However, XLPE, which is an insulating layer material of a power cable, is non-polar, and its relative dielectric constant is
It is about 2.1 to 2.3 in the operating temperature range of the power cable. Therefore, ta is characteristic in XLPE.
Evaluation of the total amount of ion dissociable impurities that deteriorate the nδ characteristic should be performed using an extract with a nonpolar solvent having a relative dielectric constant equivalent to that of XLPE. From this viewpoint, a nonpolar solvent is used as a solvent used for extracting ion-dissociable impurities in XLPE obtained by crosslinking the composition. Examples of the nonpolar solvent include hydrocarbons having a boiling point of about 50 to 100 ° C., for example, hexane, heptane, octane, benzene, toluene, xylene and the like.

The purpose of "extracting 10 g of the crosslinked composition with 10 ml of a non-polar solvent" is to increase the concentration of ion dissociable impurities, which are extremely low, in the extract as high as possible. is there. If the concentration is extremely low, the volume resistivity of the extract is inversely proportional to the concentration, so even if the composition ratio during the extraction process changes from the specified ratio, the difference in the composition ratio Can be converted to a value of a predetermined composition ratio by correcting.

The volume resistivity of the extract obtained by sufficiently treating the composition obtained by crosslinking the composition with a specified extraction solvent is determined by the conductivity of the nonpolar solvent used for the extraction and the crosslinked composition. It can be regarded as the reciprocal of the sum of the electric conductivity caused by the dissociated ions of the ion dissociable impurities present therein.
The volume resistivity of the extract obtained by sufficiently treating the crosslinked composition is lower than the volume resistivity of the highly purified solvent used for extraction to at least a fraction of a value, This shows characteristic values dominated by extracted ion dissociable impurities. Therefore, the “volume resistivity of the extract” is a value related to the amount of ion-dissociative impurities, which exist in the crosslinked composition and adversely affect tanδ of XLPE.

The volume resistivity of the extract from which the ion-dissociative impurities present in the crosslinked composition are extracted depends on the ion concentration in the extract, the degree of dissociation into ions, and the mobility of ions. Since the measurement temperature of the volume resistivity affects both the dissociation degree and the mobility, it is necessary to keep the volume resistivity evaluation temperature constant. In the present invention, the temperature is usually 50 ° C. In addition, the mobility of ions depends on the type of the solvent because it depends on the viscosity of the extract. In general, the relationship of (mobility × viscosity = constant) holds, so the influence of the viscosity of the solvent affects the ion mobility. Is converted into a characteristic value of (volume resistivity × viscosity of the solvent). Also,
For the nonpolar solvent used for extraction, the boiling point is 50 to 100 ° C.
One reason for this is that a large decrease in volume resistivity can be expected even at a very low concentration by using a solvent having a small viscosity.

As a device for measuring the volume resistivity of the extract,
An ordinary electrode for measuring the volume resistivity of the insulating oil and an insulation resistance measuring device can be used.

The definition that “the value obtained by multiplying the volume resistivity of the extract by the viscosity of the extract is 5 × 10 ¹⁰ ΩPas / m or more” is as follows.
The tan δ under the condition of 90 ° C. and 25 kV / mm is 0.2.
This is a rule necessary to ensure that it is not more than 05%, and is a value determined as a result of numerous studies by the present inventors. X obtained by crosslinking a composition having a characteristic value exceeding this specification
The tan δ of the LPE, when evaluated at 90 ° C., exceeds 0.05% even under the condition that the stress does not exceed 25 kV / mm.

The conditions for extracting the cross-linked composition with a non-polar solvent are not basic requirements. What is needed is to ensure conditions that allow extraction of almost all of the ion-dissociative impurities present in the crosslinked composition. A common condition for this is to make the crosslinked composition as thin as possible and contact the solvent. In order to accelerate the extraction, it is also possible to use ultrasonic waves, soxhlet extraction using a heated solvent, or the like.

[0024]

EXAMPLES Hereinafter, the effects of the present invention will be clarified by showing examples. (Example 1) (a) was prepared as a base low-density PE. This PE
(A) has an MFR of about 1 and cleans a PE (b) synthesis line which has been conventionally used for a crosslinkable insulating composition for power cables, and particularly prevents the incorporation of ion-dissociative impurities. It was synthesized with the above in mind. As the cross-linking agent DCP, (f) obtained by purifying (e) by recrystallization was prepared. 4,4'-thiobis- (6-tert-butyl-3-
(H) prepared by recrystallization of (methylphenol) (g).

To 100 parts by weight of the base low-density PE (a), 3 parts by weight of the crosslinking agent (f) and 0.1 part of the antioxidant (h) are added.
A composition was prepared in which 3 parts by weight were blended. The outline of the composition manufacturing process is as follows. The antioxidant is kneaded by a kneading extruder at 180 ° C., and the anti-aging agent is tied out, cut in cold water to form a pellet, and after drying, the crosslinking agent is contacted with the pellet at about 80 ° C. This is a method of dissolving. However, the manufacturing process was cleaned as follows. The outline is that the cold water used for pelletization is changed to ultrapure water used in the electronics industry, etc.
(A) was divided into two batches to absorb and remove impurities.

The obtained composition was pressed under a pressing condition of 120 ° C. to a thickness of about 0.2 mm and a thickness of about 10 mm and 10 × 10 c
A m-shaped sample is preformed, and the temperature is 180 ° C.
XLPE was obtained by pressure molding under the condition of a pressure of 20 kgf / cm ² for 20 minutes.

For a sample having a thickness of about 0.2 mm, 9
At 0 ° C., the tan δ stress change was measured and
Has considered a stress exceeding 0.05%.

A sample having a thickness of about 10 mm is formed into a sheet of about 0.2 mm using a clean slicer.
20k with the combination of 30ml of high purity n-hexane
Extraction was performed for 2 hours under ultrasonic irradiation at 1 Hz / cm ² at 1 Hz. The n-hexane used for the extraction was obtained by subjecting a special-grade reagent to a non-boiling distillation and subjecting it to adsorption treatment with activated alumina. The volume resistivity at 50 ° C. was 2 × 10 ¹⁴ Ω / m, and
Was 92.1 × 10 ⁻² Pas.

Example 2 A sample was prepared in the same manner as in Example 1 except that the production process of the composition was not cleaned.

(Comparative Examples 1 to 6) PEs (b), (c), and (d), which have been conventionally used in a crosslinkable insulating composition for power cables, were directly used as low-density PEs. . As the cross-linking agent DCP, in addition to the one purified by recrystallization (f), the one not subjected to recrystallization treatment (e) was prepared. Antioxidants (4,4'-thiobis-
(6-tert-butyl-3-methylphenol))
In addition to the product purified by recrystallization (h), a product not subjected to the recrystallization treatment (g) was prepared. According to the compounding amounts shown in Table 2, a low-density PE was kneaded with a crosslinking agent and an antioxidant, respectively, to prepare compositions. No cleaning was performed in any of the kneading steps. Thereafter, samples of Comparative Examples 1 to 6 were produced in the same manner as in the above-mentioned Example.

Using the samples of the examples (2 examples) and the comparative examples (6 examples), the stress at 90 ° C. with a tan δ exceeding 0.05% and the evaluation of the volume resistivity and the viscosity of the extract were evaluated. went. The results are shown in Tables 1 and 2, respectively.

The volume resistivity of the extract is smaller than that of n-hexane used for the extraction, and the viscosity of the extract is n
It was larger than that of hexane. It is considered that the decrease in volume resistivity is due to extraction of ion dissociable impurities in the crosslinked composition, and the increase in viscosity is mainly due to extraction of the reaction residue of the crosslinker. .

As can be seen from Table 2, the samples of the comparative examples were:
All the stresses at which tan δ exceeds 0.05% at 90 ° C. were considerably lower than 25 kV / mm, and could not meet the demand for improvement. Also, in Comparative Examples 4 to 6 in which measures for reducing ion dissociable impurities were taken, “the value obtained by multiplying the viscosity of the extract by the volume resistivity of the extract obtained by crosslinking the composition” was 5 × 10 ¹⁰ These stresses, which are smaller than ΩPas / m and whose tan δ exceeds 0.05% at 90 ° C., were all lower than 25 kV / mm, and could not meet the demand for the characteristic improvement. On the other hand, as shown in Table 1, in the samples of Examples 1 and 2, the value obtained by multiplying the viscosity of the extract by the volume resistivity of the extract obtained by cross-linking the composition was 5%. × 10 ¹⁰ ΩPas / m
And tan at 90 ° C. and 25 kV / mm.
δ was 0.05% or less in all cases, indicating excellent properties meeting the demand.

[0034]

[Table 1]

[0035]

[Table 2]

[0036]

As described above , according to the present invention,
Insulating composition with low dielectric loss even under high temperature and high electric field
One criterion for obtaining the low density po
These raw materials as ethylene, cross-linking agent, anti-aging agent
The total amount of electrical impurities contained in
To adjust the temperature and power
Obtaining an insulating composition with low dielectric loss even in the field
Power cable with low dielectric loss
be able to.

──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Toru Nakashi 1-5-1, Kiba, Koto-ku, Tokyo Inside Fujikura Co., Ltd. (72) Inventor Kenji Matsui 1-5-1, Kiba, Koto-ku, Tokyo Co., Ltd. Inside Fujikura (72) Inventor Satoshi Takahashi 1-1-1, Kiba, Koto-ku, Tokyo, Japan Inside Fujikura Co., Ltd. (72) Inventor Mitsutaka 1-5-1, Kiba, Koto-ku, Tokyo, Japan Inside Fujikura Co., Ltd. (72) Invention Person Kazuhiko Goto 1-5-1, Kiba, Koto-ku, Tokyo Inside Fujikura Co., Ltd. (72) Inventor Toshio Niwa 1-5-1, Kiba, Koba-ku, Tokyo Fujikura Co., Ltd. (56) References JP-A 55-55 66804 (JP, A) JP-A-2-185535 (JP, A) JP-A-1-243306 (JP, A) JP-A-1-243307 (JP, A) JP-A-1-243308 ( P, A) JP-A-1-225003 (JP, A) JP-A-2-121207 (JP, A) JP-A-2-121208 (JP, A) JP-A-2-121209 (JP, A) JP 60-28108 (JP, A) (58) Field surveyed (Int. Cl. ⁷ , DB name) H01B 3/16-3/56

Claims (2)

(57) [Claims] A 1. A low-density polyethylene 100 parts by weight of a crosslinking agent blended with 5 parts by weight of antioxidant 0.05 to 0.5 parts by weight of the resin composition was allowed crosslinked, the resin composition is , that the resin composition 10g were extracted with a nonpolar solvent 10 ml, a value obtained by multiplying the viscosity of the extract to a volume resistivity of the extract is one having a 5 × ¹⁰ 10 ΩPas / cm or more values An insulating composition comprising: 2. A power cable using the insulating composition according to claim 1 as an insulator.