Basic knowledge of high voltage cable

High voltage cable is a kind of power cable, which is used to transmit power cable between 10kV and 35kV (1kV = 1000V). It is mainly used in the main road of power transmission.

The main types of high voltage cables are YJV cable, VV cable, yjlv cable and VLV cable.

YJV cable full name: XLPE insulated PVC sheathed power cable (copper core)

Full name of VV cable PVC insulated PVC sheathed power cable (copper core)

Yjlv cable full name: XLPE insulated PVC sheathed aluminum core power cable

VLV cable is PVC insulated PVC sheathed aluminum core power cable

Due to the excellent conductivity of copper conductor, more and more projects use copper core power cable as the main road of power supply system, while the application of aluminum core power cable is less, especially in the higher voltage power system, the more copper core cable is selected.

Structure of high voltage cable

The components of high voltage cable from inside to outside include: conductor, insulation, inner protective layer, filler (armor) and outer insulation. Of course, the armored high-voltage cable is mainly used for ground burial, which can resist the high-strength compression on the ground and prevent other external force damage.

Na-yjv, nb-yjv, XLPE insulated PVC sheathed class A (b) fire-resistant power cables can be laid in indoor, tunnel and pipeline with fire resistance requirements.

Na-yjv22, nb-yjv22, XLPE insulated steel strip armored PVC sheathed class A (b) fire-resistant power cables are suitable for buried laying when fire resistance is required, but not suitable for laying in pipelines.

Na-vv, nb-vv, PVC insulated and PVC sheathed class A (b) fire-resistant power cables can be laid in indoor, tunnel and pipeline with fire resistance requirements.

Na-vv22, nb-vv22, PVC insulated steel strip armored PVC sheathed class A (b) fire-resistant power cables are suitable for buried laying when fire resistance is required, but not suitable for laying in pipelines.

Wdna-yjy23, wdnb-yjy23, XLPE insulated steel strip armored polyolefin sheathed a (b) class halogen-free low smoke fire-resistant power cables are suitable for laying underground when there are requirements for halogen-free low smoke and fire resistance, but not suitable for laying in pipelines.

Za-yjv, za-yjlv, zb-yjv, zb-yjlv, zc-yjv, zc-yjlv, XLPE insulated PVC sheathed class A (B, c) flame-retardant power cables can be laid in indoor, tunnel and pipeline with flame retardant requirements.

Za-yjv22, za-yjlv22, zb-yjv22, zb-yjlv22, zc-yjv22, zc-yjlv22, XLPE insulated steel strip armored PVC sheathed class A (B, c) flame-retardant power cables are suitable for buried laying when flame retardant requirements are required, not suitable for laying in pipelines. Za-vv, za-vlv, zb-vv, zb-vlv, zc-vv, zc-vlv, PVC insulated and PVC sheathed class A (B, c) flame-retardant power cables can be laid in indoor, tunnel and pipeline with requirements for flame retardant.

Za-vv22, za-vlv22, zb-vv22, zb-vlv22, zc-vv22, zc-vlv22, PVC insulated steel strip armored PVC sheathed class A (B, c) flame retardant power cables are suitable for buried laying when flame retardant requirements are required, not suitable for laying in pipelines. Wdza-yjy, wdza-yjly, wdzb-yjy, wdzb-yjly, wdzc-yjy, wdzc-yjly, XLPE insulated polyolefin sheathed class A (B, c) flame-retardant power cables can be laid in indoor, tunnel and pipeline with flame retardant and halogen-free and low smoke requirements. WDZA-YJY23,WDZA-YJLY23,WDZB-YJY23,WDZB-YJLY23 ,WDZC-YJY23,WDZC-YJLY23,

Class A (B, c) flame retardant power cables with cross-linked polyethylene insulation and steel strip armored polyolefin sheath are suitable for buried laying when flame retardant and halogen-free low smoke are required, and are not suitable for laying in pipelines.

VV, VLV, copper (aluminum) core PVC insulated and PVC sheathed power cables are laid indoors, in tunnels and pipelines or in outdoor brackets without bearing pressure and external mechanical forces

Vy, Vly, copper (aluminum) core PVC insulated PE sheathed power cable

Vv22, vlv22, copper (aluminum) core PVC insulated steel strip armored PVC sheathed power cables are laid in indoor, tunnel, cable trench and directly buried soil. The cable can withstand pressure and other external forces

Vv23, vlv23, copper (aluminum) core PVC insulated steel strip armored polyethylene sheathed power cable

Service characteristics of high voltage cable

The product is suitable for power supply, transmission and distribution with AC rated voltage of 35kV and below, and can be used for fixed lines. The maximum long-term working temperature of cable conductor is 90 ℃, and the maximum temperature of cable conductor is not more than 250 ℃ in case of short circuit (the longest time is less than 5S).

UHV cable
1kV and below are low voltage cables; 1kV ~ 10kV are medium voltage cables; 10kV ~ 35kV are high voltage cables; 35 ~ 220kV are UHV cables;

UHV cable is a kind of power cable with the continuous development of cable technology. As the central link of large-scale transmission system, UHV cable is a kind of high-voltage cable with high technology content, which is mainly used for long-distance power transmission.

Raw Materials for Wire and Cable

There are thousands of wire and cable products, which are used in various industries. There are two general uses for them, one is to transmit current and the other is to transmit signals. The main technical performance index of the transmission current cable is the time-voltage performance of the conductor resistance; the main technical performance index of the transmission signal cable is the transmission performance-characteristic impedance, attenuation and crosstalk. Of course, the transmission signal mainly relies on electric current (electromagnetic wave) as the carrier. Now with the development of science and technology, light waves can be used as the carrier for transmission.

Main Materials of Wire and Cable:
1. The copper wire uses electrolytic copper as the raw material, and the copper wire made by the continuous casting and rolling process is called low-oxygen copper wire; the copper wire made by the up-drawing method is called oxygen-free copper wire. The oxygen content of the low-oxygen copper wire is 100-250ppm, the copper content is 99.9-9.95%, and the conductivity is 100-101%. The oxygen content of oxygen-free copper wire is 4–20p

pm, the copper content is 996-999%, and the conductivity is 102%. The specific gravity of copper is 889g/cm3.

2. The aluminum wire used as the wire must be annealed and softened. Aluminum wires used for cables generally do not need to be softened. The electrical resistivity of aluminum for wire and cable is required to reach 0.0282649mm2/m, and the specific gravity of aluminum is 2.703g/cm3.

3. Polyvinyl chloride (PVC): Polyvinyl chloride plastic is based on polyvinyl chloride resin and mixed with various compounding agents, such as antioxidants, antioxidants, fillers, brighteners, flame retardants, etc. The characteristics of PVC materials with a density of about 1.38~1.46g/m3: superior mechanical properties, chemical corrosion resistance, non-flammability, good weather resistance, good electrical insulation properties, and easy processing. The disadvantages of polyvinyl chloride materials: a large amount of toxic smoke is emitted during combustion; poor thermal aging performance, polyvinyl chloride has insulating materials and sheath materials.such as pvc insualted power cable. 

4. Polyethylene (PE): Polyethylene is polymerized by refined ethylene. According to the density, it can be divided into low density polyethylene (LDPE), medium density polyethylene (MDPE), and high density polyethylene (HDPE). The density of low-density polyethylene is generally 0.91-0.925g/m3; the density of medium-density polyethylene is generally 0.925-0.94g/cm3; the density of high-density polyethylene is generally 0.94~0.97g/cm3.PE cable become a good choice because it’s more economic.

The advantages of polyethylene materials: high insulation resistance and withstand voltage strength; in a wide frequency band, the dielectric constant and the dielectric loss tangent value are small; rich in flexibility, good wear resistance; heat aging performance, low temperature Good performance and chemical resistance; good water resistance and low moisture absorption; cables made with it are light in weight and easy to use and lay. Disadvantages of polyethylene materials: easy to burn when exposed to flame; low softening temperature.

5. Cross-linked polyethylene (XLPE): There are two main types of cross-linked polyethylene currently used in the cable industry. This is a kind of silane cross-linking agent called silane cross-linking material, which is mainly used in the insulation layer of low-voltage wires and cables. The other is a cross-linked polyethylene material with dicumyl peroxide (DCP) as a cross-linking agent, which is mainly composed of polyethylene, a cross-linking agent and an antioxidant. It is mainly used on the insulation layer of medium and high voltage cables. The higher the insulation withstand voltage level, the higher the purity required. The cross-linked polyethylene material used as a cross-linking agent is mainly composed of polyethylene, a cross-linking agent and an antioxidant. It is mainly used on the insulation layer of medium and high voltage cables. The higher the insulation withstand voltage level, the higher the purity required.such as 25-400m2 PVC XLPE insulated power cable .

DCP is a kind of peroxide, th

e higher the temperature, the faster the decomposition, after its decomposition, it will catalyze the cross-linking reaction of polyethylene. DCP will also decompose at room temperature, but the decomposition rate is very slow, but it will quickly decompose when heated to a certain temperature (the temperature exceeds 125°C). The silane crosslinking agent also has the same performance, but it also needs water as an auxiliary agent to achieve a complete grafting reaction. There are currently two types of silane cross-linked polyethylene in the market, one is called the one-step method, and the other is called the two-step method (this is different from the academic silane cross-linking method). The insulating material of the one-step method is premixed with the crosslinking agent and the catalyst. It is also called the copolymer method. The two-step method is divided into material A and material B. Mix the materials A and B before production. The storage time of the one-step method is very short, and the main material of the two-step method is separated from the catalyst material, so the storage time is longer, but the mixed material must be used immediately. In addition, carbon black masterbatch can also be added to the silane cross-linking material to play a role in weather resistance and used in the insulated overhead cable series. The advantages of cross-linked polyethylene materials: electrical properties are better than polyethylene and its mechanical properties are better than polyethylene, so it is more widely used than polyethylene. The softening temperature is higher than that of PⅤC, and the normal operating temperature of the cable can reach 90℃. The disadvantage is that it is difficult to process and easy to burn.
The advantages of cross-linked polyethylene materials: electrical properties are better than polyethylene; its mechanical properties are better than polyethylene, so it is more widely used than polyethylene. The softening temperature is higher than that of PⅤC, and the normal operating temperature of the cable can reach 90℃. The disadvantage is that it is difficult to process and easy to burn.

How to Prevent Wires and Cables from Catching Fire ?

How to prevent wires and cables from catching fire due to overload of wires?

During the operation of the wire and cable, it will generate heat due to the existence of resistance. The resistance of the wire is generally very small, and its heating power can be expressed by the formula q=I^2R. q=I^2R indicates: for a piece of wire in actual use (R is basically constant), the larger the current passing through the wire, the greater the heating power; if the current is constant, the heating power of the wire is also constant . The heat released during operation will be absorbed by the wire itself and cause the temperature of the wire to rise. Although the wire is constantly absorbing the heat released by current work, its temperature will not rise indefinitely.

Because the wire is constantly radiating heat to the outside while absorbing heat, the facts show that the temperature gradually rises after the wire is energized, and finally the temperature is constant at a certain point. At this constant point, the wire has the same heat absorption and heat dissipation power, and the wire is in a thermal equilibrium state. The ability of the wire to withstand higher temperature operation is limited, and operation exceeding a certain maximum temperature will be dangerous. This maximum temperature naturally corresponds to a certain maximum current, and running a wire that exceeds this maximum current is an overload. The overload of the wire directly causes the temperature of the wire itself and nearby objects to increase. Temperature rise is the most direct cause of this type of fire.

Overload damages the insulation layer between the twin wires and causes a short circuit, which burns down the equipment and causes a fire. The double-stranded wires are separated by the insulating layer between them, and the overload will soften and destroy the insulating layer, which will cause the two-stranded wires to directly contact and cause a short circuit and burn the equipment. At the same time, the high temperature generated by the large current at the moment of short-circuit causes the line to catch fire and fuse, and the resulting molten beads fall to combustible materials to cause a fire. Overload temperature rise can also directly ignite nearby combustibles. The heat transfer of the overloaded wire increases the temperature of nearby combustibles. For nearby combustibles with low ignition point, it is possible to ignite them and cause a fire. This danger is particularly prominent in warehouses for storing flammable materials and buildings that are easy to use and combustible decorations.

Overload also causes the connections in the circuit to be overheated, which accelerates the oxidation process. Oxidation produces a thin layer of oxide film that is not easy to conduct electricity at the connection points. The oxide film increases the resistance between the contact points, resulting in ignition and other phenomena, causing a fire.
So, how to prevent fire due to wire and cable overload?

1. During the circuit design process, the capacity of the site should be accurately verified, and the possibility of adding capacity in the future should be fully considered, and the appropriate type of conductor should be selected. For large capacity, thicker wires should be selected. Circuit design and reasonable selection are key steps to prevent overload. If the design is not properly selected, it will leave inherent hidden dangers that are difficult to rectify. Some small projects and places are not carefully designed and selected. It is very dangerous to choose and lay lines at will. New electrical appliances and electrical equipment should fully consider the capacity of the original lines. If the original line does not meet the requirements, it should be redesigned and modified.

2. The line should be constructed and laid by qualified electricians in accordance with relevant specifications. The wiring conditions directly affect the heat dissipation of the wires. Generally speaking, the wiring should not pass through flammable and combustible materials and stacks, which will cause poor heat dissipation of the wires, heat accumulation, and the possibility of igniting the surrounding combustible materials, which increases the risk of fire under overload conditions; Lines laid in the decoration ceiling of public entertainment places should be protected by steel pipes to separate the ceiling from the lines. Even if there are molten beads in overload, short circuit, etc., they will not fall to avoid fire.

 

3. Strengthen power management, avoid random wiring and wiring, and use mobile sockets with caution. Random wiring, random wiring, and use of mobile sockets are actually adding electrical equipment to a certain section of the line, which increases the amount of current and may cause overload. There are obviously more mobile socket jacks than fixed wall sockets. If too many electrical equipment are used on the mobile sockets, the original wiring must be unbearable. For larger power equipment and electrical appliances, separate lines should be provided, and mobile sockets should not be used as wiring sources.

4. Speed ​​up the renewal and transformation of old lines to eliminate fire hazards. Old enterprises, old residential areas and other units have been used for a long time, and many lines have been aging and have exceeded the service life. Even if the current-carrying capacity of some lines is not large, the aging circuit can hardly bear such current-carrying capacity, and it also has the danger of overload. Especially in old residential areas, the lines have long been aging, but with the improvement of people’s living standards and the increase of household appliances, their electricity consumption is still rising year by year, which is really worse. For old lines, timely supervision and coordination should be carried out, and rectification should be promoted as soon as possible to eliminate fire hazards and ensure safety.