Control Cable Installation
Control cableThe rated voltage 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 spread of the control cable in the event of insulation breakdown, mechanical damage or fire, current, voltage, DC power supply, trip control circuit and other dual protection systems that require enhanced reliability, each independent control cable shall be used. After the control cable is put into operation, there is electrical interference between different cores of the same cable and between adjacent parallel cables. The main causes of electrical interference are as follows:
Electrostatic interference caused by capacitive coupling between cores due to applied voltage; electromagnetic induction interference caused by current. Generally speaking, when there is a high-voltage and 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 adjacent cables laid in parallel. For example, the control circuit of an ultra-high voltage substation, three-phase sharing a cable, such an accident occurred. The pulse of split-phase operation triggers the thyristors of other phases, resulting in three-phase misconnection. After switching to separate cables, misoperation accidents did not happen again. Another example is a computer monitoring system for a power plant. Since the analog low-level signal line and the transmitter power line share a four-core cable, the interference voltage of 70V in the signal line has been caused, which obviously affects the normal operation of the low-level signal circuit, and the unit is millivolts.
Measures to prevent or mitigate electrical interference mainly include the following three aspects.Yanggu Cable FactoryExplain for everyone.
A spare core of the control cable is grounded: practice has proved that when a spare core wire of the control cable is grounded, the amplitude of the interference voltage can be reduced to 25% ~ 50%, the implementation is simple, but the cable cost increases little.
When electrical interference occurs, the control cable is not suitable for circuits with serious consequences
Including: weak current signal control circuit and strong signal control circuit; low level signal and high level signal loop; in split phase operation, each phase weak current control circuit of AC circuit breaker shall not use the same control cable. However, if each pair of round-trip conductors in the weak current line belong to different control cables, they may form a ring arrangement during laying, and the potential will be induced under the cross-linking of the electromagnetic wires of the same power supply, and its value may have a great influence on the low-level parameter interference of the weak current line, so the round-trip conductors should share a control cable.
Metal shielding and shielding layer grounding: metal shielding is an important measure to reduce and prevent electrical interference, including core full shielding, partial shielding and double-layer full shielding. The type of metal shielding of the control cable shall be selected according to the influence of possible electrical interference, and integrated interference suppression measures shall be included 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 braid general shielding, the cable price will increase by about 10% to 20%.
The control interference in the high-voltage circuit can choose the control cable without metal shielding because of its strong signal, except for those located in the ultra-high voltage power distribution device or close to the high-voltage cable for a long time and parallel to it. When the control cable used in the weak current signal control circuit is located in an interference environment and there is no effective anti-interference measures, the control cable with metal shielding should be selected to prevent electrical interference from causing misoperation or insulation breakdown of the low-level signal circuit. If the control cable of the weak current line can be fully separated from the power cable, or laid in the steel pipe, the external electrical interference can be reduced to the allowable limit. Want to know moreLow voltage power cable, Yanggu cableThe content, can pay attention to us.
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