Control cable KW22

The core of control cable KW22 is copper, with a nominal cross-sectional area of 2.5mm2 or less, and 2 to 61 cores;

; 4～6mm2， 2-14 cores; 10mm2， 2-10 cores. The working temperature of the control cable is divided into two levels: rubber insulation at 65 ° C and polyvinyl chloride insulation at 70 ° C and 105 ° C. The control cables used in computer systems are generally insulated with polyvinyl chloride, polyethylene, cross-linked polyethylene, and fluoroplastic.

The principle for selecting the shielding type of the control cable for the signal circuit of the computer monitoring system is:

(1) Switching signal, can be shielded with a total shield;

(2) For high-level analog signals, it is advisable to use overall shielding of the wire core or separate shielding of the wire core;

(3) Low level analog signals or pulse signals should be shielded separately from the wire core, or composite total shielding containing separate shielding from the wire core can be used.

Regarding the grounding method of the shielding layer, the following points should be noted:

(1) The shielding layer of the control cable for the analog signal circuit of the computer control system should be grounded at a centralized point. The reason for this is based on the requirement for normal operation of computer monitoring systems, as even interference voltages of only about 1V may cause logical errors in judgment. Concentrating a grounding point can avoid the occurrence of grounding circulation;

(2) Except for the case where the control cable shielding layer of the computer monitoring system only allows centralized grounding at one point, for other control cable shielding layers, two-point grounding should be used when electromagnetic induction interference is significant, and one point grounding should be used when electrostatic induction interference is significant;

(3) The inner shielding layer of dual shielding or composite total shielding should be grounded at one point, while the outer shielding layer can be grounded at two points;

(4) When selecting two-point grounding, it should also be considered that the shielding layer will not be burned out under the action of transient current.

Installation measures:

The rated voltage of the control cable should not be lower than the working voltage of the circuit, and should meet the requirements of transient and power frequency overvoltage that may be experienced. In order to reduce the range of impact in the event of insulation breakdown, mechanical damage, or fire of control cables, the standard GB50217-91 "Code for Design of Cables in Power Engineering" stipulates that for two systems that require enhanced current, voltage, DC power supply, and trip control circuit with dual protection, independent control cables should be used.

After the control cable is put into operation, there is a problem of electrical interference between different cores of the same cable and between adjacent parallel cables. The main causes of electrical interference are:

(1) Due to the electrostatic interference generated by the coupling of capacitors between cores caused by external voltage;

(2) Due to electromagnetic induction interference caused by the current passing through. Overall, when there are high voltage and high current interference sources nearby, electrical interference is severe. Due to the small distance between the cores of the same cable, the degree of interference is much greater than that of adjacent cables laid in parallel. For example, in the control circuit of a phase separated operation circuit breaker in a certain voltage substation, the three phases share one cable. Such an accident once occurred, where the pulse of the phase separated operation triggered the thyristors of other phases, causing the three phases to interlock. Later, separate independent cables were used, and no further misoperation incidents occurred. For example, in the computer monitoring system of a certain power plant, the use of a four core cable to share the low-level analog signal line with the power line of the transmitter caused interference voltage of 70V on the signal line, which obviously affected the normal operation of the low-level signal circuit measured in millivolts.

The measures to prevent or mitigate electrical interference mainly include the following three aspects.

1. One spare core of the control cable is grounded

Practice has shown that when a spare core is grounded in a control cable, the amplitude of the interference voltage can be reduced to 25% to 50%, and the implementation is simple, with little increase in cable cost.

2. Do not use a single control cable for circuits that can cause serious consequences when subjected to electrical interference

This includes: (1) weak current signal control circuit and strong current signal control circuit; (2) The circuit of low-level signal and high-level signal; (3) The weak current control circuits for each phase of the AC circuit breaker phase separation operation should not use the same control cable. However, if each pair of round-trip wires in the weak current circuit belongs to a control cable that is not the same, it may form a circular arrangement during installation, and an electric potential may be induced under the electromagnetic wire interconnection of similar power sources. Its value may have a significant impact on the low-level parameter interference of the weak current circuit. Therefore, it is still advisable to use one control cable for the round-trip wires.

3. Metal shielding and shielding layer grounding

Metal shielding is an important measure to reduce and prevent electrical interference, including total shielding, sub shielding, and double-layer total shielding of wire cores. The selection of metal shielding types for control cables should be based on the strength of potential electrical interference effects and take into account comprehensive interference measures to meet the requirements of reducing interference and overvoltage. The higher the requirement for anti-interference effect, the greater the corresponding investment. When using steel tape armor and steel wire braided total shielding, the price of the cable increases by about 10% to 20%.

Control interference in high-voltage circuits, due to their strong signals, can be controlled by cables without metal shielding, except for those located in high-voltage distribution equipment or parallel to high-voltage cables for a long time. When the control cable used in the weak current signal control circuit is located in an environment with interference effects and does not have anti-interference measures, it is advisable to choose a control cable with metal shielding to prevent electrical interference from causing misoperation or insulation breakdown of the low-level signal circuit. If the control cable of the weak current circuit can be sufficiently separated from the power cable or laid in steel pipes, it may reduce external electrical interference to the allowable limit.