阅读说明：本技术 超高压直流电力电缆 (Ultra-high voltage direct current power cable ) 是由 郑贤贞 南振镐 柳政锡 梁艺瑟 赵珉祥 许星益 于 2017-12-04 设计创作，主要内容包括：本发明涉及一种超高压直流电力电缆。具体而言,本发明涉及一种超高压直流电力电缆,所述超高压直流电力电缆能够同时防止或最小化因绝缘体内的空间电荷(space charge)积累而引起的电场畸变和直流介电强度的降低以及脉冲破坏强度的降低。(The invention relates to an ultrahigh voltage direct current power cable. More particularly, the present invention relates to an ultra-high voltage direct current power cable capable of simultaneously preventing or minimizing electric field distortion and a decrease in direct current dielectric strength and a decrease in pulse destruction strength due to space charge (space charge) accumulation in an insulator.)

1. An extra-high voltage direct current power cable, comprising:

a conductor formed by twisting a plurality of conductive wires;

an inner semiconductive layer covering the conductor;

an insulating layer covering the inner semiconductive layer; and

an outer semiconductive layer covering the insulating layer,

the insulating layer is formed of an insulating composition comprising a polyolefin resin and a crosslinking agent,

the thickness of the insulating layer is divided into three equal parts into an inner layer, a middle layer and an outer layer,

the average value of the total content of alpha-cumyl alcohol, acetophenone, and alpha-methylstyrene contained in the crosslinking by-products of the inner layer, the total content of alpha-cumyl alcohol, acetophenone, and alpha-methylstyrene contained in the crosslinking by-products of the intermediate layer, and the total content of alpha-cumyl alcohol, acetophenone, and alpha-methylstyrene contained in the outer layer is 4000ppm or less.

2. The extra-high voltage direct current power cable according to claim 1,

in the insulating layer, a total content of three specific crosslinking by-products contained in the inner layer is 3990ppm or less.

3. The extra-high voltage direct current power cable according to claim 1 or 2,

the electric field enhancement factor FEF defined by the following equation 1 is 140% or less,

[ mathematical formula 1]

FEF ═ (electric field added to the insulation sample maximum/electric field applied to the insulation sample) × 100

In the above-mentioned mathematical formula 1,

the insulation sample was a sample produced by crosslinking of the insulation composition forming the insulation layer and had a thickness of 120 μm,

the electric field applied to the insulation sample was a direct current electric field applied to electrodes respectively connected to surfaces facing each other in the insulation sample and was 50kV/mm,

the electric field that increased most in the insulation sample was the maximum of the electric fields that increased during 1 hour of applying a direct current electric field of 50kV/mm to the insulation sample.

4. The extra-high voltage direct current power cable according to claim 1 or 2,

the polyolefin resin comprises a polyethylene resin.

5. The extra-high voltage direct current power cable according to claim 1 or 2,

the crosslinking agent is a peroxide crosslinking agent.

6. The ultra high voltage direct current power cable of claim 5,

the peroxide-based crosslinking agent contains at least one selected from the group consisting of dicumyl peroxide, benzoyl peroxide, lauroyl peroxide, t-butylperoxycumene, di (t-butylperoxyisopropyl) benzene, 2, 5-dimethyl-2, 5-di (t-butylperoxy) hexane and di-t-butyl peroxide.

7. The extra-high voltage direct current power cable according to claim 1 or 2,

the insulating composition further comprises one or more additives selected from the group consisting of an antioxidant, an extrudability improver, and a crosslinking assistant.

8. The extra-high voltage direct current power cable according to claim 1 or 2,

in the semiconductive composition forming the inner semiconductive layer and the outer semiconductive layer, the content of the crosslinking agent is 0.1 to 5 parts by weight based on 100 parts by weight of the matrix resin of the semiconductive composition.

9. The ultra high voltage direct current power cable of claim 8,

the matrix resin includes one or more selected from the group consisting of ethylene vinyl acetate EVA, ethylene methyl acrylate EMA, ethylene methyl methacrylate EMMA, ethylene ethyl acrylate EEA, ethylene ethyl methacrylate EEMA, ethylene (iso) propyl acrylate EPA, ethylene (iso) propyl methacrylate EPMA, ethylene butyl acrylate EBA, and ethylene butyl methacrylate EBMA.

Technical Field

The invention relates to an ultrahigh voltage direct current power cable. In particular, the present invention relates to an ultra high voltage direct current power cable capable of simultaneously preventing or minimizing electric field distortion and a reduction in direct current dielectric strength and a reduction in pulse destruction strength due to accumulation of space charge (space charge) in an insulator.

Background

In general, in a large-scale power system requiring large-capacity and long-distance power transmission, high-voltage power transmission in which a transmission voltage is increased is required from the viewpoints of reduction in power loss, a problem in construction sites, increase in transmission capacity, and the like.

The power transmission system can be roughly classified into an ac power transmission system and a dc power transmission system, in which electric energy is transmitted by a dc power transmission system. Specifically, the dc transmission method is a method in which ac power on the transmission side is first changed to an appropriate voltage and converted into dc power by a rectifier device, then the dc power is transmitted to the power reception side by the transmission line, and the dc power is converted into ac power again by an inverter device on the power reception side.

In particular, the dc transmission system has advantages of being advantageous for long-distance transmission of large-capacity electric power and being capable of asynchronous power system interconnection, and is currently used in many cases because dc transmission has less power loss and higher stability than ac transmission in long-distance transmission.

The insulator of the (ultra) high voltage direct current transmission cable used in the direct current transmission system may be formed of an insulating paper impregnated with an insulating oil or an insulating composition containing a polyolefin resin as a matrix resin, but in recent years, an insulator formed of an insulating composition containing a polyolefin resin, which enables the cable to be operated at a relatively high temperature, thereby enabling an increase in transmission capacity without fear of leakage of the insulating oil, has been frequently used.

However, the polyolefin resin has a linear molecular chain structure to improve mechanical and thermal characteristics through a crosslinking process for application to a cable insulation layer, and there is a problem in that space charges are accumulated in the cable insulation layer due to the influence of crosslinking byproducts inevitably generated by decomposition of a crosslinking agent during the crosslinking process, and the space charges may distort an electric field in a (ultra) high voltage direct current power transmission cable insulation, resulting in dielectric breakdown at a voltage lower than an originally designed dielectric breakdown voltage.

In the case of a cable used in current type direct current transmission (LCC) requiring polarity inversion for switching the transmission direction, in order to solve the above-described problems, an inorganic additive such as magnesium oxide is uniformly dispersed in the cable insulation layer, and the inorganic additive is polarized and traps (trap) space charges under a direct current electric field, so that electric field distortion due to the accumulation of space charges can be minimized. However, in the case of voltage type direct current transmission (VSC), without polarity reversal, insulation compositions with added organic additives are used to optimize the electrical stress to which the cable insulation is subjected, and therefore the space charge content in the insulation layer needs to be precisely controlled.

Therefore, under the present circumstances, there is an urgent need for an ultra-high voltage dc power cable which can simultaneously prevent or minimize electric field distortion and a decrease in dc dielectric strength and a decrease in pulse breakdown strength due to accumulation of space charge (space charge) in an insulator, and is particularly suitable for voltage source dc transmission (VSC).

Disclosure of Invention

Problems to be solved by the invention

An object of the present invention is to provide an ultra-high voltage dc power cable capable of simultaneously preventing or minimizing electric field distortion and a decrease in dc dielectric strength and a decrease in pulse breakdown strength due to accumulation of space charge (space charge) in an insulator.

Technical scheme for solving problems

In order to solve the above-mentioned problems, the present invention,

provided is an extra-high voltage direct current power cable, characterized by comprising: a conductor formed by twisting a plurality of conductive wires; an inner semiconductive layer covering the conductor; an insulating layer covering the inner semiconductive layer; and an outer semiconductive layer covering the insulating layer, the insulating layer being formed of an insulating composition containing a polyolefin resin and a crosslinking agent, wherein when the insulating layer is divided into an inner layer, an intermediate layer, and an outer layer by trisecting the thickness of the insulating layer, the average value of the total content of three specific crosslinking by-products, namely, alpha-cumyl alcohol (alpha-CA), Acetophenone (AP), and alpha-methylstyrene (alpha-MS), contained in the crosslinking by-products of the respective layers is 3890ppm or less.

There is provided an extra-high voltage direct current power cable characterized in that the total content of three specific crosslinking by-products contained in the inner layer in the insulating layer is 3990ppm or less.

Also provided is an extra-high voltage direct current power cable, characterized in that the Field Enhancement Factor (FEF) defined by the following equation 1 is 140% or less.

[ mathematical formula 1]

FEF ═ (electric field added to the insulation sample maximum/electric field applied to the insulation sample) × 100

In the above-mentioned mathematical formula 1,

the insulation sample was a sample produced by crosslinking of the insulation composition forming the insulation layer and had a thickness of 120 μm,

the electric field applied to the insulation sample was a direct current electric field applied to electrodes respectively connected to surfaces facing each other in the insulation sample and was 50kV/mm,

the electric field that increased most in the insulation sample was the maximum of the electric fields that increased during 1 hour of applying a direct current electric field of 50kV/mm to the insulation sample.

Further, there is provided an extra-high voltage direct current power cable, characterized in that the polyolefin resin comprises a polyethylene resin.

Also, provided is an ultra-high voltage direct current power cable, characterized in that the crosslinking agent is a peroxide crosslinking agent.

The peroxide-based crosslinking agent includes at least one selected from the group consisting of dicumyl peroxide, benzoyl peroxide, lauroyl peroxide, t-butylperoxycumene, di (t-butylperoxyisopropyl) benzene, 2, 5-dimethyl-2, 5-di (t-butylperoxy) hexane, and di-t-butyl peroxide.

Further, the present invention provides an extra-high voltage direct current power cable, wherein the insulation composition further comprises one or more additives selected from the group consisting of an antioxidant, an extrudability improver, and a crosslinking assistant.

On the other hand, there is provided an extra-high voltage direct current power cable, wherein a content of a crosslinking agent in a semiconductive composition forming the inner semiconductive layer and the outer semiconductive layer is 0.1 to 5 parts by weight based on 100 parts by weight of a matrix resin of the semiconductive composition.

The ultrahigh-voltage direct-current power cable is characterized in that the matrix resin contains one or more selected from the group consisting of Ethylene Vinyl Acetate (EVA), Ethylene Methyl Acrylate (EMA), Ethylene Methyl Methacrylate (EMMA), Ethylene Ethyl Acrylate (EEA), Ethylene Ethyl Methacrylate (EEMA), ethylene isopropyl acrylate (EPA), ethylene isopropyl methacrylate (EPMA), Ethylene Butyl Acrylate (EBA), and Ethylene Butyl Methacrylate (EBMA).

Effects of the invention

The ultra-high voltage direct current power cable according to the present invention precisely controls the content of a specific crosslinking byproduct generated at the time of crosslinking by adjusting the content of a crosslinking agent added to an insulation composition forming an insulation layer and the crosslinking degree based on appropriate modification of a matrix resin, thereby exhibiting an excellent effect of simultaneously preventing or minimizing electric field distortion and a decrease in direct current dielectric strength and a decrease in pulse rupture strength due to accumulation of space charges in the insulation.

Drawings

Fig. 1 schematically shows a longitudinal cross-section of a high voltage direct current power cable.

Fig. 2 is a graph showing the results of measuring the electric Field Enhancement Factor (FEF) in the examples.

Detailed Description

Hereinafter, preferred embodiments of the present invention will be described in detail. However, the present invention is not limited to the embodiments described herein, and may be implemented in other ways. Rather, the embodiments described herein are provided so that this disclosure will be thorough and complete, and will fully convey the concept of the invention to those skilled in the art. Throughout the specification, the same reference numerals denote the same constituent elements.

Fig. 1 schematically shows a longitudinal cross-section of an extra high voltage direct current power cable according to the present invention.

Referring to fig. 1, the power cable 200 includes: a conductor 210 formed by twisting a plurality of wires; an inner semiconductive layer 212 covering the conductor; an insulating layer 214 covering the inner semiconductive layer 212; and an outer semiconductive layer 216 that covers the insulating layer 214 and includes a cable core that transmits power only in the cable longitudinal direction along the conductor 210 and prevents current from leaking in the cable radial direction.

The conductor 210 serves as a path through which current flows for transmitting power, and may be formed of a material, such as copper or aluminum, which is excellent in conductivity and has strength and flexibility suitable for the manufacture and use of a cable, so as to be able to minimize power loss.

The conductor 210 may be a circular compressed conductor in which a plurality of circular conductive wires are twisted and compressed into a circular shape, or may be a flat conductor having a circular cross section as a whole and including a flat conductive wire layer 210C composed of a circular center conductive wire 210A and a flat conductive wire 210B twisted so as to cover the circular center conductive wire 210A.

However, the conductor 210 is formed by twisting a plurality of wires, and thus its surface is not smooth, so that the electric field may be non-uniform and corona discharge is easily generated locally. Further, if a gap is formed between the surface of the conductor 210 and an insulating layer 214 described later, the insulating performance may be degraded. In order to solve the above-described problems, the inner semiconductive layer 212 is formed outside the conductor 210.

The inner semiconductive layer 212 has semiconductivity by adding conductive particles such as carbon black, carbon nanotubes, carbon nanoplates, and graphite to an insulating material, and functions to stabilize insulating performance by preventing a rapid change in electric field from occurring between the conductor 210 and an insulating layer 214 described later. Further, the suppression of the uneven charge distribution on the conductor surface makes the electric field uniform, prevents the formation of a gap between the conductor 210 and the insulating layer 214, and also serves to suppress corona discharge, dielectric breakdown, and the like.

An insulating layer 214 is provided outside the inner semiconductive layer 212 so as to be electrically insulated from the outside, thereby preventing current flowing along the conductor 210 from leaking to the outside. In general, the insulating layer 214 is required to have a high breakdown voltage and to be able to stably maintain insulating properties for a long period of time. Further, it is required to have heat resistance characteristics such as heat resistance with less dielectric loss. Therefore, polyolefin resin such as polyethylene and polypropylene can be used for the insulating layer 214, and polyethylene resin is more preferable. Here, the polyethylene resin may be composed of a crosslinked resin.

An outer semiconducting layer 216 is provided outside the insulating layer 214. The outer semiconductive layer 216 is formed of a material having semiconductivity by adding conductive particles, such as carbon black, carbon nanotubes, carbon nanoplates, graphite, and the like, to an insulating material, as in the inner semiconductive layer 212, and stabilizes the insulating performance by suppressing non-uniform charge distribution between the insulating layer 214 and a metal sheath layer 218 described later. In addition, in the cable, the outer semiconductive layer 216 serves to smooth the surface of the insulating layer 214, to alleviate electric field concentration, to prevent corona discharge, and to physically protect the insulating layer 214.

The cable core, and particularly the inner semiconductive layer 212, the insulating layer 214, and the outer semiconductive layer 216, are the portions most likely to cause electric field distortion due to the generation, accumulation, or injection of the space charge described above, and thus dielectric breakdown, as will be described in detail later.

The core may further be provided with a moisture absorbing layer for preventing moisture from penetrating into the cable. The moisture absorption layer may be formed between the twisted wires and/or outside the conductor 210, and may be formed in the form of powder, tape, coating, or film including Super Absorbent Polymer (SAP) having a high speed of absorbing moisture permeated into the cable and an excellent ability of maintaining a water-absorbed state, thereby playing a role of preventing moisture from permeating in the longitudinal direction of the cable. In addition, the moisture absorption layer may have semiconductivity to prevent a sharp electric field change.

The power cable laid in an environment where the seabed is exposed to much moisture is further provided with an external packaging part. The protective sheath portion and the outer package portion protect the cable core from various environmental factors such as moisture permeation, mechanical external damage, corrosion, and the like, which may affect the power transmission performance of the cable.

The protective jacket portion includes a metal jacket layer 218 and an inner jacket 220 to protect the cable core from fault currents, external forces, or other external environmental factors.

The metal sheath layer 218 is grounded at an end of the power cable, and functions as a passage through which a fault current flows when a fault such as grounding or short-circuiting occurs, thereby protecting the cable from an external impact and preventing an electric field from being discharged to the outside of the cable. In addition, in the case of a cable laid in an environment such as the sea bottom, the metal sheath layer 218 is formed to seal the core part, so that it is possible to prevent impurities such as moisture from intruding to degrade the insulation performance. For example, forming a continuous outer surface having no seam by extruding molten metal to the outside of the core can make the water-shielding property excellent. Lead (Lead) or aluminum is used as the metal, and particularly in the case of an ocean cable, Lead having excellent corrosion resistance against seawater is preferably used, and a Lead alloy (Lead alloy) to which a metal element is added is more preferably used in order to enhance mechanical properties.

In addition, in order to further improve corrosion resistance, water shielding property, etc. of the cable and to improve adhesion with the inner sheath 220, an anti-corrosion compound, for example, blown asphalt, etc., may be applied to the surface of the metal sheath layer 218. Furthermore, a copper woven tape (not shown) or a moisture absorption layer may be further disposed between the metal sheath layer 218 and the core. The Copper wire woven tape is composed of a Copper wire (Copper wire), a non-woven fabric tape, or the like, and plays a role of smoothing electrical contact between the outer semi-conductive layer 216 and the metal sheath layer 218, and the moisture absorption layer is composed of a powder, a tape, a coating, a film, or the like, including a Super Absorbent Polymer (SAP) having a high speed of absorbing moisture permeated into the cable and an excellent ability of maintaining a water absorption state, and plays a role of preventing moisture from permeating in a cable longitudinal direction. In order to prevent a rapid change in electric field in the moisture absorbing layer, the moisture absorbing layer may include a copper wire.

An inner sheath 220 made of resin such as polyvinyl chloride (PVC) or polyethylene (polyethylene) is formed outside the metal sheath layer 218, so that corrosion resistance, water resistance, and the like of the cable are improved, and the cable can be protected from mechanical external damage, and other external environmental factors such as heat, ultraviolet rays, and the like. In particular, in the case of a power cable laid on the seabed, a polyethylene resin having excellent water-shielding properties is preferably used, and a polyvinyl chloride resin is preferably used in an environment where flame retardancy is required.

The protective cover unit further includes: a metal reinforcing layer composed of a semiconductive non-woven tape or the like, thereby buffering external force applied to the power cable; and an outer sheath made of a resin such as polyvinyl chloride or polyethylene, so that the corrosion resistance, water-shielding property, and the like of the power cable are further improved, and the cable can be further protected from mechanical external damage and other external environmental factors such as heat, ultraviolet rays, and the like.

In addition, since the power cable laid on the seabed is easily damaged by the anchor of the ship, etc., and is also damaged by the bending force generated by the current, the wave, etc., the frictional force with the sea floor, etc., an external packing portion may be formed outside the protective cover portion in order to prevent this.

The outer wrap may include an armor layer and a glue layer. The armor layer may be formed by winding at least one layer of wire made of steel, galvanized steel, copper, brass, bronze, or the like and having a circular or flat cross-sectional shape. The armor layer not only functions to reinforce the mechanical properties and performance of the cable, but also further protects the cable from external forces. The subbing layer made of polypropylene yarn or the like is formed in one or more layers on the upper and/or lower portion of the armor layer to protect the cable, and the subbing layer formed on the outermost portion is made of two or more materials having different colors, thereby ensuring visibility of the cable laid on the seabed.

The inner semiconductive layer 212 and the outer semiconductive layer 216 are formed by extrusion of a semiconductive composition in which conductive particles of carbon black, carbon nanotubes, carbon nanoplates, graphite, and the like are dispersed in a matrix resin and further a crosslinking agent, an antioxidant, a scorch inhibitor, and the like are added.

Here, for interlayer adhesion of the semiconductive layers 212, 216 and the insulating layer 214, the matrix resin preferably uses an olefin resin of a similar series to that of the matrix resin of the insulating composition forming the insulating layer 214, and more preferably, an olefin and a polar monomer, such as Ethylene Vinyl Acetate (EVA), Ethylene Methyl Acrylate (EMA), Ethylene Methyl Methacrylate (EMMA), Ethylene Ethyl Acrylate (EEA), Ethylene Ethyl Methacrylate (EEMA), ethylene (iso) propyl acrylate (EPA), ethylene (iso) propyl methacrylate (EPMA), Ethylene Butyl Acrylate (EBA), Ethylene Butyl Methacrylate (EBMA), and the like, are preferably used in view of compatibility with the conductive particles.

In addition, the crosslinking agent may be a silane-based crosslinking agent depending on the crosslinking mode of the matrix resin contained in the semiconductive layers 212 and 216; or organic peroxide-based crosslinking agents such as dicumyl peroxide, benzoyl peroxide, lauroyl peroxide, t-butylperoxyisopropyl benzene, di (t-butylperoxyisopropyl) benzene, 2, 5-dimethyl-2, 5-di (t-butylperoxy) hexane, and di-t-butylperoxide.

The semiconductive composition forming the inner semiconductive layer 212 and the outer semiconductive layer 216 may contain conductive particles such as carbon black in an amount of 45 to 70 parts by weight based on 100 parts by weight of the matrix resin. When the content of the conductive particles is less than 45 parts by weight, sufficient semiconductive characteristics cannot be achieved, and on the contrary, when it exceeds 70 parts by weight, extrudability of the inner semiconductive layer 212 and the outer semiconductive layer 216 is lowered to deteriorate surface characteristics or productivity of the cable.

In addition, in the semiconductive composition forming the inner semiconductive layer 212 and the outer semiconductive layer 216, the content of the crosslinking agent may be accurately adjusted to 0.1 to 5 parts by weight, preferably 0.1 to 1.5 parts by weight, based on 100 parts by weight of the matrix resin thereof.

Here, in the case where the content of the crosslinking agent exceeds 5 parts by weight, the content of crosslinking by-products inevitably generated upon crosslinking of the matrix resin included in the semiconductive composition is excessive, such crosslinking by-products move to the inside of the insulating layer 214 through the interface between the semiconductive layers 212 and 216 and the insulating layer 214 to accumulate heterogeneous charges (heterocharge), thereby increasing distortion of an electric field, and a problem of lowering a dielectric breakdown voltage of the insulating layer 214 may be caused, and on the contrary, when less than 0.1 part by weight, the degree of crosslinking is insufficient, and thus mechanical characteristics, heat resistance, and the like of the semiconductive layers 212 and 216 may be insufficient.

For example, the base resin of the insulating layer 214 may be a polyolefin resin such as polyethylene or polypropylene, and the insulating layer 214 may be preferably formed by extrusion of an insulating composition containing a polyethylene resin.

The polyethylene resin may be Ultra Low Density Polyethylene (ULDPE), Low Density Polyethylene (LDPE), Linear Low Density Polyethylene (LLDPE), Medium Density Polyethylene (MDPE), High Density Polyethylene (HDPE), or a combination thereof. The polyethylene resin may be a homopolymer, a random or block copolymer of ethylene and an α -olefin such as propylene, 1-butene, 1-pentene, 1-hexene, 1-octene, or a combination thereof.

In addition, the insulation composition forming the insulation layer 214 comprises a cross-linking agent, whereby the insulation layer 214 may be constituted as a cross-linked polyolefin (XLPO), preferably cross-linked polyethylene (XLPE), by a separate cross-linking process at or after extrusion. In addition, the insulation composition may further include other additives such as an antioxidant, an extrudability improver, and a crosslinking assistant.

The crosslinking agent contained in the insulating composition may be the same as the crosslinking agent contained in the semiconductive composition, and may be, for example, a silane-based crosslinking agent depending on the manner of crosslinking the polyolefin; or organic peroxide-based crosslinking agents such as dicumyl peroxide, benzoyl peroxide, lauroyl peroxide, t-butylperoxyisopropyl benzene, di (t-butylperoxyisopropyl) benzene, 2, 5-dimethyl-2, 5-di (t-butylperoxy) hexane, and di-t-butyl peroxide. Here, the crosslinking agent included in the insulation composition may be included in a content of less than 1 wt%, for example, 0.1 wt% or more and less than 1 wt%, based on the total weight of the insulation composition.

The present inventors confirmed through experiments that specific cross-linking byproducts inducing space charge generation among cross-linking byproducts inevitably generated upon cross-linking of the insulating layer 214 are α -cumyl alcohol (α -CA), Acetophenone (AP), and α -methylstyrene (α -MS), and confirmed through experiments that the following contents are produced, thereby completing the present invention: the content of the crosslinking agent contained in the insulating composition forming the insulating layer 214 is limited to less than 1% by weight, and after the crosslinking of the insulating layer 214, the content of the specific crosslinking by-product can be limited by degassing (degassing), particularly the content of the specific crosslinking by-product can be limited by location over the thickness of the insulating layer, and the space charge generation and the electric field distortion can be significantly reduced by limiting the content of such specific crosslinking by-product, with the result that the reduction of the direct current dielectric strength and the pulse destruction strength of the insulating layer 214 can be prevented or minimized at the same time.

In addition, the present inventors confirmed the following through experiments, thereby completing the present invention: the content of the crosslinking agent is limited to less than 1 wt%, so that the degree of crosslinking of the insulating layer 214 is reduced, and as a result, the problem that the mechanical and thermal characteristics of the insulating layer 214 may be reduced can be solved by increasing the vinyl content of the base resin included in the insulating composition forming the insulating layer 214 to a degree of crosslinking of 60% or more, for example, 60% to 70%.

Specifically, when the thickness of the insulating layer 214 is trisected into a bottom layer disposed directly above the conductor 210, that is, an inner layer, an intermediate layer disposed on the inner layer, and an outer layer disposed on the intermediate layer, the average value of the total content of the three specific crosslinking by-products contained in each layer is adjusted to 3890ppm or less, thereby suppressing the generation of space charges in the insulating layer 214, and as a result, the Field Enhancement Factor (FEF) of the following equation 1, which indicates the degree of electric Field distortion in the insulating layer 214, is adjusted to about 140% or less, and as a result, it is possible to simultaneously prevent or minimize the reduction in the direct current dielectric strength and the reduction in the pulse breakdown strength of the insulating layer 214.

[ mathematical formula 1]

FEF ═ (electric field added to the insulation sample maximum/electric field applied to the insulation sample) × 100

In the above-mentioned mathematical formula 1,

the insulation sample was a sample made by cross-linking the insulation composition forming the insulation layer 214 and had a thickness of 120 μm,

the electric field applied to the insulation sample was a direct current electric field applied to electrodes respectively connected to surfaces facing each other in the insulation sample and was 50kV/mm,

the electric field that was the largest in the increase of the insulation sample was the largest among the electric fields that were increased during 1 hour of applying a direct current electric field of 50kV/mm to the insulation sample.

In the insulating layer 214, the inner layer is disposed directly above the conductor 210 to form a hetero interface with the inner semiconductive layer 212, and is a portion that is weak in insulation when a relatively high electric field is applied, and therefore, it is more preferable to adjust the total content of the three specific crosslinking by-products contained in the inner layer to 3990ppm or less.