Power cable is a kind of cable that transmits and distributes electric energy. It has many applications. So what are the basic structures of power cables? What are the main classifications of power cables? Let me talk to my friends about these issues in detail below.
power cable
Power cables are cables used to transmit and distribute electrical energy. Power cables are commonly used in urban underground power grids, power station lead lines, internal power supply in industrial and mining enterprises, and underwater transmission lines across rivers and seas.
In power lines, the proportion of cables is gradually increasing. Power cables are cable products used to transmit and distribute high-power electric energy in the main lines of the power system, including power cables of various voltage levels and insulation from 1-500KV and above.
Basic structure of power cable
The basic structure of power cables consists of four parts: core (conductor), insulation layer, shielding layer and protective layer.
Core
The core is the conductive part of the power cable and is used to transmit electrical energy. It is the main part of the power cable.
Insulation
The insulation layer electrically isolates the wire core from the earth and wire cores of different phases to ensure the transmission of electric energy. It is an indispensable component of the power cable structure.
Shield
Power cables of 15KV and above generally have conductor shielding and insulating shielding.
The protective layer
The function of the protective layer is to protect the power cable from the intrusion of external impurities and moisture, and to prevent direct damage to the power cable by external forces.
Main classification of power cables
According to insulation material
1. Oil-impregnated paper insulated power cable A power cable insulated with oil-impregnated paper. It has the longest application history. It is safe and reliable, has a long service life and is cheap. The main disadvantage is that the laying is limited by the height difference. Since the development of non-drip paper-impregnated insulation, the problem of drop limit has been solved, allowing oil-impregnated paper insulated cables to continue to be widely used.
2. Plastic insulated power cables are power cables whose insulation layer is extruded plastic. Commonly used plastics include polyvinyl chloride, polyethylene, and cross-linked polyethylene. Plastic cables have a simple structure, are easy to manufacture and process, are light in weight, are easy to lay and install, and are not limited by the laying height. Therefore, it is widely used as medium and low voltage cables, and has a tendency to replace sticky oil-impregnated paper cables. Its biggest disadvantage is the phenomenon of dendrite breakdown, which limits its use at higher voltages.
3. The insulation layer of rubber insulated power cable is made of rubber plus various compounding agents. After being fully mixed, it is extruded and wrapped on the conductive core, and is heated and vulcanized. It is soft and elastic, suitable for occasions with frequent movement and small laying bending radius.
Commonly used rubber materials for insulation include natural rubber-styrene-butadiene rubber mixture, ethylene-propylene rubber, butyl rubber, etc.
According to voltage level
1. Low-voltage cable: suitable for fixed laying on transmission and distribution lines with AC 50Hz and rated voltage 3kv and below for transmitting electric energy.
2. Medium and low voltage cables: (generally refers to 35KV and below): PVC insulated cables, polyethylene insulated cables, cross-linked polyethylene insulated cables, etc.
3. High-voltage cables: (generally 110KV and above): polyethylene cables and cross-linked polyethylene insulated cables, etc.
4. Ultra-high voltage cable: (275~800 kV).
5. Ultra-high voltage cables: (1000 kV and above).
Common matters needing attention in the construction of power cables
1. The distance between cables and thermal pipes should be kept at 2m when installed in parallel, and at 0.5m when they cross.
2. Keep a distance of 0.5m between cables and other pipelines when installed parallel or crosswise.
3. When installing cables directly buried, the depth of direct burial of 1-35kV cables should not be less than 0.7m.
4. When cables of 10kV and below are installed in parallel, the clear distance between each other shall not be less than 0.1m, for 10-35kV cables shall not be less than 0.25m, and when they cross, the distance shall not be less than 0.5m.
5. The minimum bending radius of the cable shall not be less than 15D for multi-core cables and 20D for single-core cables (D is the outer diameter of the cable).
6. Cable joints for 6kV and above.
a. When installing the cable terminal, the semi-conductive shielding layer must be stripped off. The insulation must not be damaged during operation. Knife marks and unevenness should be avoided. If necessary, it should be smoothed with sandpaper; the shielding end should be flat and the graphite should be removed. layer (carbon particles) is removed.
b. The copper shielding and steel armor at the ends of plastic insulated cables must be well grounded. This principle should also be followed for short lines to avoid induced electrical dynamics at the ends of the steel armor during three-phase unbalanced operation, and even “ignition” and burning protection. set of accidents. The grounding lead wire should be made of tinned braided copper and the cable copper tape should be soldered with a soldering iron. It is not advisable to use a blowtorch to seal the soldering to avoid burning the insulation.
c. The three-phase copper shielding should be connected to the ground wire respectively. Note that the shielded ground wire and the steel armored ground wire should be drawn out separately and insulated from each other. The position of the welding ground wire should be as low as possible.
7. Basic requirements for cable terminals and intermediate joints: a. Good conductor connection; b. Reliable insulation. It is recommended to use radiation cross-linked heat shrinkable silicone rubber insulation material; c. Good sealing; d. Sufficient mechanical strength. Can adapt to various operating conditions.
8. The cable end must be waterproof and protected from corrosion by other corrosive materials to prevent breakdown due to aging of the insulation layer caused by water tree.
9. Cranes and forklifts must be used for loading and unloading of cables. Horizontal transportation and horizontal laying are prohibited. A cable car must be used when installing large cables to prevent the cable from being damaged by external forces or scratching the sheath and insulation layer due to manual dragging. The cable is not installed in a tray and is strictly prohibited to be pulled manually. This may cause the conductor to bend and damage the insulation layer, resulting in a short circuit.
10. If the cable cannot be laid in time for some reason, it should be stored in a dry place to prevent exposure to sunlight and water ingress at the cable end.
Through the above content, we have already understood the basic characteristics and classification of power cables. When constructing power cables, you need to strictly follow the test tube procedures to ensure safety.