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What is the difference between a power cable and a control cable?
The power cable is used to transmit electrical energy. That is to say, the voltage connected to the cable and the current passing through it are relatively large. Generally, the insulation level is relatively high, the conductor cross section is large, and a cable contains at most 5 core wires.
As the control cable, the main transmission is the small current electric signal: such as the connection of the drive control contactor, the operation of the relay, the transmission of sound, the video signal, etc., the power transmission of the wire is small, the voltage level is low, the control cable The number of internal cores is large and it is possible that the wires will have a separate shield.
What is the difference between power cable and control cable?
The power cable is 0.6/1kv such as VV, YJV; the control cable is 450/750v such as KVV, KVVP. Generally, the power cables are all within 5 cores. Generally, they are used for high-voltage power. There are many control cable cores. Sometimes customers have different requirements. Generally, they have 37 cores, but there are more cores.
Commonly used cable classifications are divided into five categories:
(1) Wire and bare inner conductor products: Refers to conductors only, without insulation and other structures. Such as: copper, aluminum and various composite metal single wire; various structures of overhead transmission with stranded wire, flexible wiring, profiles and profiles.
(2) Electromagnetic wire: A wire that in the form of a winding cuts a magnetic field line to induce a current, or a wire that generates a magnetic field by a current is called a magnet wire. Such as enameled wire, fiber wrapped wire, inorganic insulated wire and so on.
(3) Power cable: A wire product used to transmit and distribute high-power electric energy in the main line of a power system, called a power cable. Such as: plastic cable of various voltage levels, oil-impregnated paper cable and no dripping.
(4) Communication cable: A cable for transmitting various telecommunication information above audio and audio. Such as: local telephone cable, long-distance symmetric communication cable, coaxial communication cable, cable for dian telephone equipment and data cable.
(5) Wire and cable for electrical equipment: The power is directly transmitted from the distribution point of the power system to the power cable for power connection of various electrical equipment and appliances. Such as: wire and cable for electrical connection and control signals in various industrial and agricultural equipment. These products are widely used and widely used, and most of them must be combined with the characteristics of the equipment used and the environmental conditions used to determine the structure and performance of the product.
Therefore, in addition to a large number of general-purpose products, there are many special and special products. Such as: general insulated wire, installation line, rubber sheathed flexible cable, control cable, signal cable, aircraft, automobile, tractor special cable, motor electrical lead wire, cable for rolling stock, radio device wire, mining cable, marine cable , agricultural cables, oil mines and detection cables, cables for field work, various high-voltage DC equipment wires, flexible cables, etc.
The power cable is used to transmit and distribute large functional power in the main line of the power system, and the control cable directly transfers the electric energy from the power distribution point of the power system to the power connection line of various electrical equipment appliances. The rated voltage of the power cable is generally 0.6/1kV and above, and the control cable is mainly 450/750V. In the production of power cables and control cables of the same specification, the insulation and sheath thickness of the power cables are thicker than the control cables.
First, the control cable is a cable for electrical equipment, and the power cable is two of the five major categories of cable. The standard of the control cable is 9330, and the standard of the power cable is GB12706; the color of the insulated core of the control cable is generally black and white, and the low voltage of the power cable is generally separated. The cross section of the control cable generally does not exceed 10 square meters. The power cable is mainly for transmitting electricity, and generally has a large cross section.
Due to the reasons mentioned above, the specifications of power cables can generally be as large as 500 square meters (the range that can be produced by conventional manufacturers). The manufacturers with relatively large cross-sections are relatively few, and the cross-section of control cables is generally small. Smaller, the maximum is generally no more than 10 square feet.
From the number of cable cores, the power cable is usually up to 5 cores according to the grid requirements, while the control cable transmits control signals, and the number of cores is large. According to the standard, there are 61 cores, but it can also be produced according to user requirements. .
Electrical and thermal control cable shield grounding
There are two ways to ground the cable shield, that is, two-point grounding and one-point grounding.
To prevent transient overvoltage, it is better to use two points of grounding for the shield. The two-point grounding causes the electromagnetic induction to generate an induced longitudinal current on the shield. This current produces a secondary field opposite to the main disturbance, canceling the action of the main interference field and reducing the interference voltage.
However, there are two problems with two-point grounding:
First, when a short-circuit current or a lightning strike current occurs on the grounding grid, the potential of the two points of the cable shielding layer is different, so that a current flows in the shielding layer, and the shielding layer may be burned.
Second, when current flows through the shield, interference signals are generated for each core. For relay protection and automatic devices, both the input and the output have one end in the high-voltage or ultra-high-voltage environment of the switchyard. Electromagnetic induction interference is the main contradiction, and the loop of the cable core is a strong electric circuit, so the interference signal generated by the shield current is less affected. Therefore, the relay protection and automatic device regulations stipulate that the shielding layer should be grounded at both ends; Professional cable, electromagnetic induction interference is not prominent in comparison, and the interference of the shield current generated by the two-point grounding on the core wire may cause the device to malfunction, so it is better to use a little grounding. Therefore, the two-point grounding specified by the relay protection and the automatic device does not contradict the grounding of the thermal regulation.
Ground the two ends of the control cable shield. The ability of the shielding layer to reduce the induced overvoltage is mainly based on the cancellation of the magnetic field generated by the interference current by the magnetic field generated by the shielding current. The shielding layer is grounded at both ends because the short-circuit current and the lightning current pass, because the large short-circuit current and the lightning current have a short acting time, it is not easy to burn the shielding layer. If one end of the shielding layer is grounded, there is no current loop, but its overvoltage and anti-interference ability are low, so the shielding layer can not achieve good shielding effect.
Rectification measures: First, control the cable with shielding layer, the shielding layer is grounded at the same time in the switch field and the control room, the shielding layer of the communication cable should also be connected to the ground correctly and reliably; the second is to lay a special grounding copper bar for the secondary equipment and the secondary cable. Try to eliminate the ground potential difference interference; the third is to use the special opto-isolation of all the switch input and output contacts of the substation.
The induced current flowing in the shielding layer is induced by the external electromagnetic field, and its actual function is to counteract the interference of the external electromagnetic field. Therefore, the cable shielding layer is grounded at both ends, which can effectively suppress electromagnetic induction.
The ungrounded shielding layer has no shielding effect on the electric field interference, and the shielding layer which is grounded at one end and grounded at both ends has the same shielding effect on the electric field. If the shield is well grounded, the electric field terminates in the shield being directly coupled to ground.
The metal shielding layer of the shielded cable has an electrostatic shielding function, so that the strong power line of the primary line high-voltage power supply terminates in the metal shield, and the internal electric field strength is zero, so that the core wire in the shielding layer is protected from the interference of the external strong electric field. From the point of view of electrostatic shielding, in order to make the surface of the shielding layer a fixed equipotential surface, one end of the shielding layer should be grounded.