Requirements of Power Cable Cross Section

Power cable section
The selection of power cable conductor cross-section should meet the following requirements:
1 The temperature of the cable bare conductor under the action of the maximum operating current shall not exceed the allowable value of the service life of the cable. Cable conductor work of continuous working loop
The temperature should meet the requirements of Appendix A of this code.
2 The temperature of the cable conductor under the action of the maximum short-circuit current and short-circuit time shall comply with the provisions of Appendix A of this code.
3 The voltage drop of the connected circuit under the maximum operating current shall not exceed the allowable value of the circuit.
4 In addition to meeting the requirements of paragraphs 1 to 3 above, the cross-section of power cables of 10kV and below should be based on the initial investment of the cable and the operation during its service life.
The principle selection of comprehensive cost economy. The selection method of economical current cross-section for power cables of 10kV and below should comply with the regulations in Appendix B of this code.
5 The minimum cross-section of multi-core power cable conductors, copper conductors should not be less than 2.5mm2, and all aluminum conductors should not be less than 4mm2.
6 For cables laid underwater, when the conductor is required to withstand the tensile force and is reasonable, the cross section can be selected according to the tensile requirements.

Commonly used cables of 10kV and below shall determine the allowable minimum cross-section of the cable conductor at 100% continuous working current, which should meet the requirements of Appendix C and Appendix D of this code. The current carrying capacity shall be greater than The working current of the loop.
1 Difference in ambient temperature.
2 The difference in soil thermal resistance coefficient when directly buried.
3 The influence of multiple parallel cables.
4 The influence of sunlight on outdoor overhead laying without shading.

Except for the conditions specified in Article 3.7.2 of this code, when the cable determines the minimum allowable cross-section of the cable conductor at 100% continuous working current, it shall be verified by calculation or test, and the calculation content or parameter selection shall meet the following requirements:
1 Non-coaxial cables used in power supply circuit cables with higher harmonic loads or intermediate frequency load circuits should be included in the skin effect and the increase in proximity effect
And other additional heating effects.
2 For single-core high-voltage cables that are cross-connected and grounded, when the three sections in the unit system are of unequal length, the effect of the additional loss and heating of the metal layer should be included.
3 The cables laid in the protective tube shall be included in the influence of thermal resistance; the cables with different holes in the pipe shall also be included in the influence of mutual heating factors.
4 Cables laid in closed, semi-enclosed or ventilated refractory trough boxes should be included in the heat resistance of the type of material and the thickness and size of the box.
The impact of increased resistance.
5 When the thickness of the fireproof coating, tape and other covering layer applied on the cable is greater than 1.5mm, the thermal resistance should be included.
6 When the cable in the trench is buried with sand and there is no regular water supplement, a thermal resistance coefficient greater than 2.0K·m/W should be selected according to the sand quality to include the influence on the increase of the thermal resistance of the cable.

When calculating the continuous allowable current-carrying capacity of the cable whose conductor working temperature is greater than 70℃, the following requirements should be met:
1 When a large number of such cables are laid in tunnels and shafts without mechanical ventilation, the impact on environmental temperature rise should be included.
2 Cables are directly buried in dry or moist soil. Except for the implementation of soil replacement treatment to avoid water migration, the soil thermal resistance coefficient is not
It should be less than 2.0K·m/W.

Control Cable and Its Metal Shield

1 The current and voltage of dual protection, as well as the two systems that need to enhance reliability, such as DC power supply and trip control loop, should use separate control cables.

2 In the following cases, the same control cable should not be used together:
1 Weak current signal, control circuit and strong current signal, control circuit.
2 Low-level signal and high-level signal loop.
3 Weak current control circuit of each phase of AC circuit breaker split-phase operation.
3 Each pair of round-trip wires of the weak current circuit should belong to the same control cable.
4 The phase wire and neutral wire of each group of secondary windings of current transformer and voltage transformer should be arranged in the same cable.
5 The control cables of strong current loops may not contain metal shields, except for those located in high-voltage power distribution devices or close to and parallel to high-voltage power cables and need to suppress interference.
6 The control cables of weak current signals and control circuits should have metal shields when they are located in an environment affected by interference and do not have effective anti-interference measures.7 The selection of the metal shielding type of the control cable should be included in the comprehensive interference suppression measures based on the possible electrical interference impact, and should meet the requirements for reducing interference or overvoltage, and should meet the following requirements:1 For the weak current control cables of power distribution devices above 110kV, general shielding or double-layer general shielding should be used.
2 The control cable used for the current, voltage and signal contacts of integrated circuit and microcomputer protection should be shielded.
3 The shield selection of the signal loop control cable of the computer monitoring system shall meet the following requirements:
1) Switch signal, general shield can be used.
2) For high-level analog signals, a pair of twisted cores should be used for general shielding, and a pair of twisted cores for separate shielding can also be used when necessary.
3) For low-level analog signals or pulse signals, it is advisable to use a pair of twisted cores and separate shields, and if necessary, a pair of twisted cores and a composite general shield can also be used.

4 In other cases, the appropriate shielding type should be selected according to factors such as electromagnetic induction, electrostatic induction and ground potential rise.
5 When the cable has steel armor or metal sheath,such as SWA cable , its shielding function should be fully utilized.
8 For control cables that need to reduce electrical interference, a grounded spare core can be added, and it should be grounded at one point on the side of the control room.
9 The grounding method of the metal shield of the control cable shall meet the following requirements:
1 The analog signal loop control cable shielding layer of the computer monitoring system shall not form two or more points of grounding, and shall be grounded at one point in a centralized manner.
2 The cable shielding layer of the current, voltage and signal of the integrated circuit and microcomputer protection shall be grounded at the same time as the switch placement place and the control room.

3 For the shielding layer of control cables other than the above, when the electromagnetic induction interference is large, two points should be grounded; when the electrostatic induction interference is large,
One point grounding can be used.
For double shielding or composite general shielding, it is advisable to use one or two grounding points for the inner and outer shields respectively.
4 In the choice of two-point grounding, the shielding layer should not be melted under the action of transient current.
10 The conductor cross-section of the strong current control loop should not be less than 1.5mm2, and the weak current control loop should not be less than 0.5mm2.

Cable Insulation Type

The choice of cable insulation type should meet the following requirements:
1 Under the operating voltage, operating current and its characteristics and environmental conditions, the cable insulation characteristics should not be less than the conventional expected service life.
2 The selection should be based on factors such as operational reliability, ease of construction and maintenance, and comprehensive economy of allowable maximum operating temperature and cost.
3 It shall meet the requirements of fire prevention places and shall be conducive to safety.
4 When it is clear that it is necessary to coordinate with environmental protection, an environmentally-friendly cable insulation type should be selected.

The selection of the insulation type of commonly used cables should meet the following requirements:
1 In addition to the selection of insulation type for medium and low-voltage cables, it shall comply with the provisions of Article 3.4.3~3.4.7 of this code.
Linked polyethylene type extruded insulation type, medium voltage cable should use cross-linked polyethylene (xlpe) insulation type.such as 600V XLPE ACWU90 AC90 cable.

When it is clearly necessary to coordinate with environmental protection, PVC insulated cables shall not be used.
2 The cable line in the high-voltage AC system should be of XLPE insulation type. In areas with more operating experience, self-contained oil-filled can be used
3 For high voltage DC transmission cables, non-drip impregnated paper insulation and self-contained oil-filled types can be selected. When it is necessary to increase the transmission capacity, half
The type of synthetic paper material construction.
Ordinary XLPE cables should not be used for DC transmission systems.
Cables such as rubber insulation should be used for circuits that are frequently bent and moved or require higher flexibility, such as mobile electrical equipment.

In the place where radiation is applied, cables with radiation resistance strength such as cross-linked polyethylene or ethylene-propylene rubber insulation should be selected according to the requirements of the insulation type.
Heat-resistant polyvinyl chloride, cross-linked polyethylene or ethylene-propylene rubber insulated cables should be selected for heat-resistant polyvinyl chloride, cross-linked polyethylene or ethylene-propylene rubber insulated cables according to the requirements of high temperature and duration and insulation type for high temperature above 60℃; minerals should be selected for high temperature above 100℃ Insulated cables.
Ordinary PVC insulated cables should not be used in high temperature locations.
For low temperature environments below 15°C, cross-linked polyethylene, polyethylene insulated, and cold-resistant rubber insulated cables should be selected according to the low temperature conditions and insulation type requirements.
PVC insulated cables are not suitable for low temperature environments.
In crowded public facilities and places with low-toxicity, flame-retardant and fire-proof requirements, cross-linked polyethylene or ethylene-propylene rubber and other non-halogen insulated cables can be used.
When fire protection has low toxicity requirements, PVC insulated cables should not be used.

Except for the cases specified in 3.4.5~3.4.7 of this code, PVC insulated cables can be used for circuits below 6kV.
For 6kV important circuits or cross-linked polyethylene cables above 6kV, the three-layer co-extrusion process characteristics of inner and outer semi-conductive and insulating layers should be used
The style.

Basic knowledge of bare conductor 

The processing and production of  bare conductor are carried out in accordance with the national standards. The latest domestic standard is: GB/T1179-2008. The relevant national standards used before are no longer used. Since the previous standards are relatively simple and clear, refer to It is very convenient to use. Except for the change in the representation method, the other conductor structure, performance and technical parameters have not changed. Therefore, many current power design, bidding and procurement units still use the 83 standard (ie: GB/1179-83).

1. bare conductor-a material used to transmit current, made of multiple insulated single wires twisted together.
2. Concentric stranded wire —— a wire composed of one or more layers of single wires is spirally twisted around a central core, and the adjacent layers are twisted in opposite directions.
3. Twisting direction-the twisting direction of a layer of single wires, that is, the direction away from the observer. The right direction is clockwise, and the left direction is counterclockwise.
4. Pitch —— A single wire in the twisted wire forms the axial length of a complete spiral.
5. Pitch diameter ratio —— the ratio of the pitch of the single wire in the stranded wire to the outer diameter of the layer.
6. Steel ratio-the ratio of steel cross-sectional area to aluminum cross-sectional area expressed in percentage.
7. Cross-sectional area-the total cross-sectional area of ​​all aluminum or aluminum alloy single wires that make up the wire.
8. Linear density——mass per unit length.

The General Type of bare conductors including AAC(All Aluminum Conductor), AAAC (All Aluminum Alloy Conducor),ACSR(Aluminum Conductor Steel Reinforced Cable),ACAR (Aluminum Conductor Aluminum Reinforced Cable) and etc .

The quality of the workmanship and stranding of the wire 
1.  Process quality requirements 
a. The surface of the round aluminum wire should be smooth and clean
b.  Any round aluminum wire in the 7-strand aluminum wire is not allowed to have a joint.
The aluminum stranded wire with more than 7 strands and the round aluminum wire in the steel core aluminum stranded wire are allowed to have joints, but the distance between the two joints on the finished stranded wire should not be less than 15 meters.
c.  The joints should be smooth and round, usually cold-pressed butt welding joints.
d.  Galvanized steel wire does not allow joints.

2. Stranding of wires
A. Single wire twisting should be even and tight;
b. All single wires should be twisted concentrically;
c. The twisting direction of the adjacent layers should be opposite, if not specified by the ordering party, the twisting direction of the outermost layer is “right”;
d. Each layer of single wires should be evenly and tightly twisted on the lower core or inner twisted layer;
E. For stranded wires with multiple layers, the pitch diameter ratio of any layer should not be greater than the pitch diameter ratio of the adjacent inner layer;


 3. Technical requirements for finished wires 
a. The finished wire should meet the technical conditions required by the bidding unit and meet the requirements of the GB/T 1179-2008 standard.
b. There should be no visible defects on the wire surface, such as obvious scratches, indentations, etc., and there should be no defects that are not commensurate with good products, and the surface should be smooth and clean without corrosion, blackening, and graying.
c. The pitch-to-diameter ratio of the wires is within the limits specified in GB/T 1179-2008. Once the stranding starts, the same stranding parameters are maintained for all the entire batch of wires shipped to a given destination. The steel core aluminum stranded wire provided is a product formed by one-time stranding.
d. The finished wire is a uniform cylindrical surface, and can withstand normal loading and unloading during transportation and installation without causing deformation that increases corona loss and radio interference.
e. The steel core aluminum stranded wire has no excessive drawing lubricants, metal particles and powders, and no defects that are inconsistent with the industrial products and the engineering process quality requirements of the bidding party. Factory products no longer require design measures to limit the occurrence of corona and radio interference.
F. The steel core of the wire is manufactured by a manufacturer.
g. When stranding, the steel core or partially stranded semi-finished products and strands to be stranded are stored in the factory for a long enough time to ensure that the steel core or partially stranded semi-finished products and the strands to be stranded The strands are at the same temperature and will maintain the same temperature throughout the stranding process.
h. Once the stranding and storage process has started, in order to make all the wires have the same stranding law as possible, follow the corresponding process for the insufficient part of the wire at the given destination.
I. The wire is suitable for tension stringing and also suitable for loose stringing. During normal winding, any lantern, loose strands, jumpers, broken strands and influences on the winding construction will be regarded as unqualified products.
J. The manufacturing of the wire and the twisting of the strands should ensure that there is no obvious twisting or loose strands after the wire is cut. The steel core is formed so that after the wire is cut and the aluminum single wire is peeled from the steel core according to the requirements of the joint, the steel core can be reassembled without difficulty, and it is easy to use one hand to sleeve the splice tube from the cut end of the wire into the steel core . Similarly, the molding of aluminum single wire also allows the aluminum wire to be easily reassembled, and it is convenient to use one hand to put the splice tube into the aluminum wire at the cut end of the wire, so that the twisting is tight and uniform, no strands are missing, and no jumps. Lines and continuous strands must not be biased.


Comparison of 3 Cross-Linking Processes for Cross-Linked Cables

Immediately in the immediate future of electrical and industrial production
Noble gas cross-linking, melting gas cross-linking, cross-linking cross-linking, domestic leveling type 2 instant-drying chemical cross-linking; class 2 cross-linking chemical cross-linking; class 3 cross-linking cross-linking. Type 1 chemical cross-linking, cross-linking cross-linking function mother particle application method
Immediately, cross-linking power matrix particles for cross-linking, existing ordinary cross-linking power particles, small amount of this power-carrying power particles, and immediate power-linking power.

1. Chemical crosslinking
The DCP cross-linking production line adopts catenary or tower equipment, and the equipment and plant cost are high, and it needs to consume a lot of water, electricity, gas, etc., so the investment is relatively large; because the production line is longer, it is only suitable for the production of large length cables; High environmental requirements require specialized production workshops.
2. Silane crosslinking
The silane crosslinking production line only needs ordinary extruder and heating device (hot pool or steam chamber), low investment, low environmental requirements, flexible production, suitable for the production of long or short cables, and can be manufactured in different lengths according to customer needs power Cables.
3. Irradiation cross-linking, irradiation cross-linking is to use the high-energy electron beam generated by the electron accelerator to bombard the insulating layer and the sheath, and break the polymer chain of the linear molecular structure of polyethylene (PE) material cable . Every breakpoint becomes a free radical. Free radicals are unstable and must be recombined with each other. After recombination, the original chain molecular structure becomes a three-dimensional network molecular structure to form crosslinks. In addition, the radiation dose applied in the production process of radiation cross-linking leaves a large safety margin from the destruction dose. The radiation damage dose of polyethylene is 1000KGY, and the processing dose is about 200KGY. In addition to the improvement of the special formula, it is still cross-linked by radiation in a relatively wide range, so its performance will be affected by radiation during a long period of early use.

1. Chemical cross-linking
Cross-linking method: 100 US conduit or standing tower high pressure system, high temperature, water-cooled cross-linking
Crosslink: High
Favorable area: Suitable ultra-high or high pressure electric power 10-110Kv
Main advantages: 1. Cross-linking, cross-linking, cross-linking, cross-linking, cross-linking;
2. Production equipment and production craft maturity
Main problem: Incompatible small cross section, thin insulation;
Chemical residue;
2. Spokes
Crosslink method: Collection / release line, electronic bundle physical crosslink
Crosslink: Low
Fluorine area: Medium-low pressure system, special equipment line, flame retardant line, cross-linking PVC insualted cable , PP, cross-linked product, etc.
Main advantages: 1. Production and engineering crafts, available original equipment, cross-linking available
2. High-performance materials such as various types of processed materials, temperature resistance, and flame retardancy
Main problem: Incompatibility 10kV or more Electric wire
Partial large-line irradiation device Existence problem, irradiation equality difference;

3. Crosslink
Cross-linking method: Cross-linking, warm water immersion or steam fumigation 8 hours or more cross-linking;
Crosslink: Crosslink
Favorable area: Medium-low pressure system, building line, ordinary waiting, etc.
Main advantages: Industrial and industrial crafts, available original equipment;
Single production production low;
Main problems: production and production control, product quality instability
Low production efficiency

Difference between Fire-Resistant and Flame-Retardant Cables

1 The difference in principle
Fire-resistant cables have different principles from flame-retardant cables. The flame-retardant principle of halogen-containing cable is based on the flame-retardant effect of halogen, and the flame-retardant principle of halogen-free cable is to reduce the temperature of the water to extinguish the fire. Fire-resistant cables rely on the fire-resistant and heat-resistant characteristics of the mica material in the fire-resistant layer to ensure that the cable works normally even in a fire.

2 The difference between structure and material
The structure and materials of fire-resistant cables are different from those of flame-retardant cables.
The basic structure of flame retardant cable is:
(1) The insulating layer is made of flame-retardant materials
(2) Flame-retardant materials are used for the sheath and outer sheath;
(3) The wrapping tape and filling are made of flame retardant materials.

Fire-resistant cables usually add a fire-resistant layer between the conductor and the insulating layer, so theoretically, a fire-resistant layer can be added to the structure of the flame-retardant cable to form a flame-retardant and fire-resistant cable. This is not necessary.

Because of the fire-resistant layer of fire-resistant cables, multi-layer mica tapes are usually used to wrap the wires directly. It can withstand long-term combustion, even if the polymer at the place where the flame is applied is burned, it can ensure the normal operation of the line.

3. What is the difference in the structure of flame-retardant, fire-resistant, and flame-retardant fire-resistant cables?

The structure of flame-retardant cables is basically the same as that of ordinary cables. The difference lies in its insulating layer, sheath, outer sheath and auxiliary materials (wrap tape and Filling) all or part of the flame-retardant material is used, and the fire-resistant cable usually adds a fire-resistant layer between the bare conductor and the insulating layer. Therefore, theoretically, a fire-resistant layer can be added to the structure of the flame-retardant cable to form Both flame-retardant and fire-resistant cables, but this is not actually necessary. Because of the fire-resistant layer of fire-resistant cables, multi-layer mica tapes are usually directly wrapped on the wires. It can withstand long-term burning. Even if the high polymer at the place where the flame is applied is burned, the normal operation of the line can be guaranteed.

4. What is the difference between low-smoke and halogen-free cables, low-smoke and halogen-free fire-resistant cables and low-smoke and halogen-free flame-retardant and fire-resistant cables?

As the name suggests, it is the difference in sheath materials. Low-smoke and halogen-free is environmentally friendly, that is, it does not produce a large amount of toxic gas during combustion, which ensures the safety of relevant places. Fire resistance means being able to stay in the flame without melting. Ordinary is a general sheath material.

5. The classification principles of flame retardant a, b, and C levels and what is the difference in structure are divided according to standard regulations. They are used to evaluate the ability of vertically installed bundles of wires and cables or optical cables to inhibit the vertical spread of flame under specified conditions:
Flame retardant Class A refers to: the cable is installed on the test steel ladder so that the total volume of the non-metallic materials contained in the test is 40 minutes, and the cable is burned to dry the sample. The maximum carbon circumference of the test sample shall not be higher than the bottom resistance of the blowtorch.

The flame-retardant type B cable is installed on the test steel ladder so that the test material in the total volume is the burning and drying test sample, and the maximum carbon circumference of the test sample shall not be higher than the bottom of the blowtorch
The flame-retardant type C is installed on the test steel ladder so that the total volume of the non-metallic materials in the test is 5/m, and the fire time is 20 minutes. After the cable burning stops, dry the sample, and the maximum carbonization of the sample is measured. Not higher than the bottom of the blowtorch

XLPE Cable Process Method & Characteristics

Cross-linked cable is short for cross-linked polyethylene (XLPE)  insulated cable. Cross-linked cables are suitable for transmission and distribution lines with power frequency AC voltage of 500KV and below.
At present, most of the high-voltage cables have been used for cross-linked polyethylene insulated cross-linked cables, which usually means that the insulation layer of the cable is made of cross-linked materials. The most commonly used material is cross-linked polyethylene (XLPE).
Processing engineering is the process of using polyethylene (PE) material with linear molecular structure through a specific processing method to form a cross-linked polyethylene with a body-shaped network split line structure. The long-term allowable working temperature is increased from 70°C to 90°C (or higher), and the short-circuit allowable temperature is mainly increased from 140°C to 250°C (or higher). While maintaining its original excellent electrical performance, it has greatly improved Actual performance.

At present, the process methods for producing cross-linked cables in the cable industry are divided into three categories:
The first type of peroxide chemical crosslinking includes saturated steam crosslinking, inert gas crosslinking, molten salt crosslinking, and silicone oil crosslinking;
The second type of silane chemical crosslinking;
The third type of radiation crosslinking.

1. Inert gas cross-linking: Dry chemical cross-linking adopts polyethylene insulating material added with peroxy compound cross-linking agent. After the conductor shielding layer, insulating layer, and insulating shielding layer are extruded through three layers, they are continuously and uniformly filled with high temperature , The sealed cross-linking tube of high-pressure nitrogen completes the cross-linking process. The heat transfer medium is nitrogen (inert gas), the cross-linked polyethylene has excellent electrical properties, and the production range can reach 500KV.
2. Silane chemical cross-linking: warm water cross-linking adopts polyethylene insulating material with silane cross-linking agent, and after extruding the foreign body shielding layer, insulating layer, and insulating shielding layer through the 1+2 extrusion method, the cooled and loaded plate The insulated core is immersed in 85-95℃ hot water for hydrolysis and cross-linking, because wet cross-linking will affect the water content in the insulating layer. Generally, the highest voltage level is only 10KV.
3.Irradiation cross-linking: Physical cross-linking uses modified polyethylene insulating material, and after the extrusion of the different shielding layer, insulating layer, and insulating shielding layer is completed by 1+2 extrusion, the cooled insulated core , The cross-linking process is completed evenly through the irradiation scanning window of the high-energy electron accelerator. No cross-linking agent is added to the radiation cross-linked cable material. During cross-linking, the high-energy electrons generated by the high-energy electron accelerator effectively penetrate the insulating layer, and the cross-linking reaction is generated through energy conversion, because the electrons have high energy. , And evenly pass through the insulating layer, so the formed cross-linked bond has high binding energy and good stability. The physical performance shown is that the heat resistance is better than the chemical cross-linked cable. However, it is mainly limited by the energy level of the accelerator (generally no more than 3. The effective penetration thickness of the OMev electron end is less than 10mm, considering the geometric factors, the voltage level of the production cable can only reach 10KV, and the advantage is below 6KV.


XLPE cable characteristics
The aging life of cable insulation material mainly depends on its thermal aging life. It is determined by the speed of thermal oxygen oxidation, thermal cracking, thermal oxidative cracking, polycondensation and other chemical reactions that occur in the insulating material under heat. Therefore, the insulating material The thermal aging life of the cable directly affects the service life of the cable. According to the chemical reaction kinetics derivation and the artificial accelerated thermal aging test (20-30 years), the long-term allowable working temperature of the irradiated cross-linked cable is:
1. If the power cable 0.6/1KV(LV Cable)  is deduced according to the rated working temperature of 105 degrees, its thermal aging life exceeds 60 years.
If deduced according to the rated operating temperature of 90 degrees, its thermal aging life exceeds 100 years.
2. Overhead insulated cable 10KV 122 degrees

When overhead insulated cables are laid in the open air, the environmental and radiation resistance of insulating materials is more important. Irradiation cross-linked insulating materials have to undergo irradiation processing, and they have good radiation resistance. The radiation dose applied in the cross-linking production process leaves a large safety margin from the destruction dose. The radiation damage dose of polyethylene is 1000KGY, and the processing dose is about 200KGY. In addition to the improvement of the special formula, it is still cross-linked by radiation in a relatively wide range, so its performance will be affected by radiation during a long period of early use.

In cable production recently, the most commonly used insulating plastics are polyethylene and polyvinyl chloride. Among them, polyethylene materials have better electrical properties and better crosslinking properties. Therefore, a variety of industrial crosslinking production processes have been developed. Cross-linking and radial cross-linking In the process of production and laying, the insulation layer of the currently commonly used cross-linked cables has a higher hardness and strength (at room temperature), especially more difficult to peel off than PVC insulation. Because the spoke cross-linked cable has the best cross-linking performance and the highest degree of cross-linking, relatively speaking, the peel strength is also the highest. If the peeling of the cross-linked cable insulation layer is relatively easy (similar to polyvinyl chloride), it must be insufficient or no cross-linking. Under normal circumstances, the cross-linked cable produced by the warm water cross-linking process has more cases where the degree of cross-linking is not enough. The reason is that the degree of cross-linking of such products is relatively low, and the cross-linking process is not continuous and cannot be automatically controlled. Affected by human factors, under-crosslinking easily occurs.

What is XLPE cable?

What is XLPE cable?

The cross-linked polyethylene(XLPE) insulated cable uses peroxide cross-linking method to change the polyethylene molecule from a linear molecular structure to a three-dimensional network structure, and from a thermoplastic material to a thermosetting material. The working temperature is increased from 70°C to 90°C. Improve the current carrying capacity of the cable;

What is the advantages of XLPE cable ?
1. Heat resistance: MPE with a net-like three-dimensional structure has excellent heat resistance. It will not decompose and carbonize below 300°C. The long-term working temperature can reach 90°C and the thermal life can reach 40 years.
2. Insulation performance: XLPE maintains the original good insulation properties of PE, and the insulation resistance is further increased. The tangent of the media loss angle is very small, and it is not greatly affected by temperature.
3. Mechanical properties: due to the establishment of new chemical bonds between macromolecules, the hardness, stiffness, wear resistance and impact resistance of XLPE have been improved, thus making up for the shortcomings of PE  cable being vulnerable to environmental stress and cracking;
4. Chemical resistance: XLPE has strong acid and alkali resistance and oil resistance. Its combustion products are mainly water and carbon dioxide, which are less harmful to the environment and meet the requirements of modern fire safety.

what’s the XLPE useage ?

Cross-linked polyethylene insulated cables are suitable for power distribution networks, industrial installations or other fields that require large-capacity power consumption. They are used for fixed laying on power transmission and distribution lines with AC 50Hz and rated voltages of 6kV to 35kV. The main function is to transmit electrical energy.
Cross-linked polyethylene insulated flame-retardant cables have the function of preventing fire from spreading in the cable circuit, can avoid the expansion of fire accidents and reduce losses, and are suitable for places with high cable laying density such as subways, tunnels, and high-rise buildings.

Is XLPE low smoke ?

The halogen-free low-smoke flame-retardant XLPE insulated cable( LSOH cable) has excellent flame-retardant properties, and the sheath material does not contain halogen, which can ensure that only a small amount of toxic and corrosive gases are released during combustion, and it has the characteristics of less smoke. An environmentally-friendly new product, with superior electrical properties, heat resistance, chemical resistance, environmental stress cracking resistance, aging resistance, and long service life.

Structure Analysis of 10kv XLPE Power Cable

XLPE power cable has good electrical and corrosion resistance. It is easy to install and easy to operate and maintain. It is widely used in 10 kV rural distribution networks. However, once it fails, it is difficult to repair and find. Combining the actual operation and work, focus on the analysis of the principle and structure of the cross-linked cable.

1. Analysis of insulation principle of cross-linked polyethylene material (XLPE)
Polyethylene is used as the basic insulating material, and chemical and physical cross-linking methods are used to convert the high molecular compound polyethylene from a linear molecular structure to a three-dimensional network structure of cross-linked polyethylene. It completely maintains the high electrical and physical properties of polyethylene, such as: high breakdown strength, large insulation resistance, low dielectric loss tangent value, etc. At the same time, due to the cross-linking process, it has aging resistance, heat resistance, mechanical properties, The corrosion resistance has been greatly improved.

The XLPE cables used daily are mostly chemical cross-linking method-inert gas cross-linking. The polyethylene material with peroxide (commonly used dicumyl) crosslinking agent is used. After three-layer co-extrusion, it continuously and uniformly passes through a sealed crosslinking tube filled with high temperature and high pressure nitrogen. The peroxide is thermally decomposed to produce Free radicals, free radicals can combine with hydrogen atoms in polyethylene, and polyethylene molecules that have lost hydrogen atoms unite to form cross-linked polyethylene to complete the cross-linking process.

However, XLPE as a polymer also has its inherent defects. The macromolecular solid structure of the polymer makes it easy to accumulate “space charge” inside. Space charge is also called trap charge, that is, the part of the charge that stays in the medium after being trapped. , Can also refer to the polarization charge caused by uneven polarization. The formation of traps is due to pollution in the production process, introduction during mechanical processing or generated during application, but electrode injection is considered to be the main reason for the formation of space charges. The space charge is generally distributed in the medium impurities, physical defects and between polymer molecular chains. Factors such as residual charge in the production process, high electrode injection or polarization caused by impure materials will cause the accumulation of space charge, and these factors are inevitable in actual production. Space charge is very harmful to XLPE cable insulation.

In addition, due to the network molecular structure of XLPE, it has a greater water permeability problem. When the XLPE insulator invades a trace amount of moisture, it will cause the formation of water branches in the insulator, and at the same time cause a high electric field similar to the gas free process, causing insulation damage. The diameter of the water branches is generally only a few microns, and there are many microscopic small water drop gaps. composition. When voltage is applied to the cable, under the action of a strong electric field, water branches will evolve into electrical branches to induce insulation breakdown. It can be seen that the problem of water permeability is the flaw of the XLPE material, and XLPE is the main insulation, so it is necessary to strictly prevent the intrusion of the cable during installation and use.

2. Structural analysis of XLPE power cable

The cross-linked cable is mainly composed of a core conductor, an insulating layer and a protective layer. The core conductor is located in the center, and the insulation shielding layer of the “three-layer co-extrusion” process is on the periphery, which has excellent insulation shielding and heat resistance and heat dissipation performance. The outermost periphery is a protective layer, which is composed of an inner sheath, an armor, and an outer sheath to seal the conductor and the main insulation. This simple structure of cross-linked cable is based on high technology and craftsmanship. Each layer of the structure has special functions and requirements. If a certain layer of structure has a problem, the entire cable will be scrapped. In order to clarify The structural requirements of each layer of the cable, and the principle analysis of each structural part of the cross-linked power  cable are as follows.

(1).  Analysis of core conductor structure

The bare conductor of cable plays the role of transmitting electric energy. When the alternating current passes through the conductor, the skin effect makes the charge density near the surface of the conductor high. Since the electric field intensity on the surface of a conductor is directly proportional to its surface charge density and inversely proportional to the radius of curvature of the conductor surface, the radius of curvature at the edge or tip of the conductor is the smallest, the surface charge density is the largest, the space charge is most likely to accumulate, and the electric field strength is the highest. Local strong electric field discharge is most likely to occur, and this phenomenon is called “edge effect”.

If impurities invade the inner structure of the cable, the impurities will form tips and cause partial discharges, which will eventually lead to breakdown. This is another important reason why no impurities can penetrate into the cable structure.

In order to avoid the “edge effect” harm to the cable insulation, try to make the conductor electric field uniform and reduce the insulation requirements, we should make the conductor into a geometric shape with the largest radius of curvature-round, and make the conductor surface as smooth as possible to avoid The sharp electric field is strong. For this reason, the cross-linked cable core is a multi-core compact round stranded wire. It can be seen that during cable laying and installation, it is necessary to strictly avoid the behavior of damaging the flatness of the internal structure of the cable and destroying the uniform electric field.

(2). “Three-layer co-extrusion” analysis of the inner and outer semi-conductive layer and the main insulating layer

The semiconducting layer is a polymer material with a higher dielectric constant (it is a conductor under a high electric field), which makes up for the stranded cores that cannot be completely rounded, and the uneven electric field on the surface of the cores is uniform. At the same time, it can prevent manufacturing The local high electric field caused by the accidental stab of the core and the introduction of external impurities during the process. It contains substances that quickly capture moisture, which can effectively block moisture from intruding into the insulating layer from inside and outside, prevent moisture from spreading along the core, and prevent the generation of water branches. In addition, the inherent thermal resistance of the polymer can play a role in thermal shielding of separate temperature and increase the current carrying capacity of the cable.

The “three-layer co-extrusion” process is to tightly extrude the inner and outer semiconducting layers and the insulating layer, so that the three layers are tightly combined. This process avoids the local high electric field caused by the intrusion of external impurities (air, moisture, foreign particles, etc.), makes the electric field uniform and smooth, thereby increasing the initial free discharge voltage and greatly improving the insulation strength.

(3). Copper shield and armor

Between the outer semiconducting layer and the inner lining layer, two layers of annealed copper tape are spirally covered to form a cylindrical concentric conductor layer. This is the copper shielding layer, which has good contact with the shielded semiconducting layer, and For the equipotential. During installation, both ends of the copper shielding tape are grounded, so that the outer semi-conductive layer of the cable is always at zero potential, thereby ensuring that the electric field is evenly distributed in the longitudinal direction. When the cable fails, the protective device will act quickly through the ground current or short-circuit current on the copper shielding layer, thereby protecting the non-faulty part of the cable. Note that the length of the copper shielding layer not in contact with the outer semiconducting layer shall not exceed 2 cm after calculation.

The main function of armoring is to increase the longitudinal mechanical stress of the cable, reduce the influence of mechanical force on the cable, and at the same time, it also plays a role in uniform electric field and protection through fault current.

When the cable is subjected to insulation breakdown, lightning strike, operating overvoltage or large fault current flowing in the core, the induced voltage of the metal sheath may cause the inner sheath to break down and cause arcing, and the fault current at this time is not enough When the relay protection is activated, the breakdown phenomenon will gradually increase until the metal sheath is burnt into a hole, which further increases the fault. In order to eliminate this hazard, the metal sheath must be grounded at the terminal.

Main Components of Overhead Lines

Overhead lines mainly refer to overhead cable, erected on the ground, and are transmission lines that use insulators to fix transmission wires on poles that stand upright on the ground to transmit electrical energy. Overhead lines have low cost, strong mobility, and easy maintenance. However, overhead lines hinder traffic and construction and are easily polluted by debris in the air; moreover, overhead lines may collide with or get too close to trees and other tall facilities or objects, causing electric shocks, short circuits and other accidents.

The main components of overhead transmission lines are Low-voltage cables, insulators, cross arms, low-voltage poles, high-voltage suspension insulators, clamps, high-voltage cables, high-voltage poles,Lightning line.

1.Pole and Tower
The general term for supporting structures that suspend wires through insulators. Pole and tower are the general term for electric poles and iron towers.
The purpose of the tower is to support wires, lightning protection wires and other accessories. In order to keep a certain safe distance between the wires, the wires and the avoidance, the wires and the ground, and the crossing objects.

2. Overhead Wire and Cables
Classification: It can be divided into four types: single-stranded wire, multi-stranded wire, composite multi-stranded wire and insulated wire.
(1). Single-strand Cable: Due to the manufacturing process, when the cross-section increases, the mechanical strength of the single-strand wire decreases. Therefore, the cross-section of the single-strand wire is generally below 10 square millimeters, and currently it is widely used up to 6 square millimeters.
(2). The multi-stranded cable is twisted by multiple thin wires, and the twisting direction of the adjacent layers of the multi-layered wire is opposite to prevent twisting and twisting when paying off. Its advantages are high mechanical strength, flexibility, and flexibility ; And because the surface oxidation resistivity of the strands increases, the current flows along the strands, the skin effect is small, and the resistance is slightly reduced compared with the single-stranded wire of the same cross section.The common type have all aluminum cable (AAC) and all Aluminum Alloy Cable (AAAC ) etc.

(3). Composite multi-stranded wire refers to the multi-stranded wire of two materials. The common type are steel-cored aluminum stranded wire(ACSR cable) and ACAR Cable ,The core part is twisted by steel wire, and the external stranded aluminum wire is integrated. The mechanical properties of aluminum and the electrical properties of aluminum have become the most widely used overhead cable.
(4). Overhead power lines generally use multiple strands of bare conductors, but in recent years, 10kV overhead power distribution lines in urban areas have gradually switched to overhead insulated conductors. The operation proved that it has many advantages, the line failure is obviously reduced, the contradiction between the line and the tree is solved to a certain extent, the maintenance workload is reduced, and the safety and reliability of the line are obviously improved. 10kv mainly has copper core and aluminum core cross-linked polyethylene insulated wires, and low voltage mainly includes JV and JLV.

3. Lightning wire
The lightning protection wire is installed above the wire and is directly grounded for lightning protection to reduce the chance of lightning strikes to the wire, improve the lightning resistance level of the line, reduce the lightning trip rate, and ensure the safe power transmission of the line. Overhead lines have long distances and are exposed to the wilderness, and there are more chances of being struck by lightning. According to the voltage level of the network, the importance of the load, and the number of lightning days and investment conditions in the area, it can be along the whole line or only in the Install lightning protection wires at the entry and exit lines of the substation.

4. Insulator
Insulators, also called porcelain bottles, are used to fix wires and insulate wires and poles. Therefore, the insulator should have sufficient electrical insulation strength and mechanical strength. There are two types of line insulators: high voltage and low voltage. The insulator is a kind of electrical isolation component, its purpose is to insulate between the wire and the wire and between the wire and the ground, support, suspend the wire, and fix it on the cross arm of the tower. The insulator should have good electrical and mechanical properties. . It should also have strong resistance to rain, snow, fog, wind, ice, sudden changes in temperature and the erosion of harmful substances in the atmosphere.

5. Hardware
The role of the fittings: used for suspension, fixation, protection, connection on overhead lines, connecting overhead lines or insulators, and metal devices used to connect the cables on the cable structure of the cable tower.
Basic requirements Electric power fittings are generally made of cast steel and malleable cast iron. It is required that the circuit fittings should have sufficient mechanical strength, and the part of the fittings connecting the conductors should have good electrical properties.