In order to ensure the control cable in the event of insulation breakdown, mechanical damage or fire, reduce the scope of the protection: dual protection of the current, voltage and DC power supply and trip control loop and other two systems that need to enhance reliability, should be independent Control Cable.

After the control cable in operation, between the different cores of the same cable, there are problems of interference between the electrical cables laid close to parallel, major cause electrical interference are:

(1) Electrostatic interference generated by the externally applied voltage on-line capacitive coupling between cores;

(2) Electromagnetic induction interference due to energization current. In general, when there is a high-voltage, high-current interference source nearby, the electrical interference is more serious. Because the distance between the cores of the same cable is small, the degree of interference is much greater than that of the parallel cable. For example, the control circuit of a phase-division operated circuit breaker of an ultra-high voltage substation, a three-phase combined cable, has had such an accident, the pulse of the phase separation operation causes the thyristors of other phases to be triggered, and the three-phase linkage is caused by mistake. With separate cables, no accidents occurred. Another example is the computer monitoring system of a power plant. Since a four-core cable is used to mix the analog low-level signal line with the power line of the transmitter, it has caused a 70V interference voltage on the signal line, which is in millivolts. The low level signal loop will obviously affect normal operation.

There are three main aspects to prevent or mitigate electrical interference.

1. One spare core of the control cable is grounded

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

2. A circuit that has serious consequences for electrical interference, without a control cable

These include: (1) weak electric signal control loop and strong electric signal control loop; (2) low-level signal and high-level signal loop; (3) alternating current circuit breaker phase-division phase weak current control loop, none The same control cable should be used. However, for each pair of reciprocating wires of the weak current circuit, if they belong to a control cable that is not the same, it may form an annular arrangement during laying, and an electric potential may be induced under the electromagnetic line of the similar power source, and the value may be for the weak electric circuit. Low-level parameter interference has a large impact, so it is advisable to use a control cable for the round-trip wire.

3, metal shield and shield grounding

Metal shielding is an important measure to reduce and prevent electrical interference, including total shielding of the core, sub-shield and double-layer general shielding. The selection of the metal shielding type of the control cable shall be included in the comprehensive suppression of interference according to the influence of possible electrical interference, to meet the requirements of reducing interference and overvoltage. The higher the requirements for the anti-interference effect, the greater the corresponding investment. When the steel belt armor and steel wire braided shield are used, the price of the cable increases by about 10% to 20%.

The control interference in the strong electric circuit, because of its strong signal, can be selected without the metal shielding control cable except that it is located in the ultra-high voltage power distribution device or in parallel with the high-voltage cable. The control cable used in the weak current signal control loop should be equipped with a metal shielded control cable when it is located in an environment with interference effects and does not have effective anti-interference measures to prevent electrical interference from malfunctioning on the low-level signal loop. Or cause insulation breakdown and other effects. If the control cable of the weak current circuit can be pulled away from the power cable a sufficient distance or laid in the steel pipe, the external electrical interference may be reduced to the allowable limit.

For the control cable of the computer monitoring system signal loop, the principle of shielding type selection is:

(1) Switching signal, available for general shielding;

(2) High-level analog signal, it is advisable to use the total shielding of the core, and if necessary, the shield of the core can be used;

(3) Low-level analog signal or pulse quantity signal, it is advisable to use the partial shielding of the wire core, and if necessary, the composite total shielding with the wire core shielding.

Regarding the grounding method of the shielding layer, attention should be paid to the following points:

(1) The control cable shielding layer of the analog signal loop of the computer control system should be grounded at a centralized point. The reason is based on the requirement to ensure the normal operation of the computer monitoring system, because even the interference voltage of only about 1V may cause the logic judgment to be delayed, and the grounding circulation can be avoided by concentrating a little grounding;

(2) Except that the control cable shield of the computer monitoring system is only allowed to be grounded at one point, the other control cable shielding layer should be grounded at two points when the electromagnetic induction interference is large, and the interference caused by static induction is large. Then use a little grounding;

(3) The inner shield of the double shield or composite shield should be grounded at one point, and the outer shield can be grounded at two points;

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