The control interference in the strong current circuit, due to its own strong signal, except for the ultra-high voltage power distribution device or the close parallel to the high-voltage cable, the control cable without metal shield can be used. The control cable used in the weak current signal control loop, when it is located in an environment affected by interference and does not have effective anti-interference measures, a metal shielded control cable should be used to prevent electrical interference from causing misoperations on the low-level signal loop Or cause insulation breakdown and other influences. If the control cable of the weak current circuit can be separated from the power cable by a sufficient distance or laid in a steel pipe, the external electrical interference may be reduced to the allowable limit.
Electromagnetic induction interference due to energized current. Generally speaking, when there are high-voltage, high-current interference sources nearby, the electrical interference is more serious. Because the distance between the cores of the same cable is small, the interference degree is much greater than that of the adjacent cables laid in parallel. For example, in the control circuit of an ultra-high voltage substation for split-phase operation circuit breakers, the three-phase combination uses a single cable. Such an accident has occurred. The pulse of the split-phase operation caused the thyristors of other phases to trigger, which caused the three-phase linkage by mistake. With separate cables, no misoperation accidents occurred again. Another example is the computer monitoring system of a certain power plant. Due to the use of a four-core cable with the analog low-level signal line and the transmitter's power line, it has caused an interference voltage of 70V on the signal line, which is measured in millivolts. The low-level signal loop will obviously affect normal operation.
The metal shield of the control cable manufacturer is an important measure to reduce and prevent electrical interference, including the overall shielding, sub-shielding and double-layer overall shielding of the core. The selection of the metal shielding type of the control cable should be included in the comprehensive interference suppression measures according to the strength of the electrical interference that may occur, so as to meet the requirements of reducing interference and overvoltage. The higher the requirements for the anti-interference effect, the greater the corresponding investment. When steel tape armor and steel wire braided general shielding are used, the price of the cable will increase by about 10% to 20%.
