Cables connected from the control center to various systems to transmit signals or control operational functions are collectively referred to as control cables. The early functions of the control cable are relatively simple, including: indicator light display, instrument indication, relay and switchgear operation, alarm interlock system.
The main products of today's control cables are: polyvinyl chloride insulated control cables, natural styrene butadiene rubber insulated control cables and polyethylene insulated control cables. In addition, there are crosslinked polyethylene insulation and ethylene propylene rubber insulation products.
Control cable core is copper core wire, nominal section 2.5mm and below, 2~61 core; 4~6mm, 2~14 cores; 10mm, 2~10 cores. Control cable operating temperature: rubber insulation 65℃, PVC insulation 70℃ and 105℃. The control cables used in the computer system are generally PVC, polyethylene, crosslinked polyethylene and fluoroplastic insulation products.
In order to ensure that the control cable can reduce the impact range in the event of insulation breakdown, mechanical damage or fire, the national standard GB50217-91 "Code for the Design of Electric Power Engineering cables" stipulates: current, voltage, DC power supply and trip control loop. Separate control cables should be used for the two systems that require increased reliability.
After the control cable is put into operation, there is electrical interference between different cores of the same cable and between cables laid in parallel. The main causes of electrical interference are:
(1) Electrostatic interference caused by capacitive coupling between iron cores due to applied voltage;
(2) electromagnetic induction interference caused by current. In general, when there is a high-voltage, high-current interference source nearby, electrical interference is more serious. Because the core distance of the same cable is small, the interference degree is much greater than that of adjacent parallel cables.
For example, in the control circuit of the phase-operated circuit breaker in an ultra-high voltage substation, the three phases share a single cable. There was such an accident. The pulses running in separate phases cause the thyristors of other phases to be triggered, resulting in three-phase misalignment. Use separate cables, no more misoperation accidents.
The measures to prevent or reduce electrical interference are mainly in the following three aspects
1.A spare core of the control cable is grounded
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.
- the circuit that will have serious consequences for electrical interference is not suitable for a control cable, including:
(1) weak signal control loop and strong signal control loop;
(2) The loop between the low-level signal and the high-level signal;
(3) The AC circuit breaker phase operation of each phase of the weak current control circuit should not use the same control cable. However, for each pair of round-trip wires in a weak current circuit, if they belong to different control cables, they may form a ring arrangement during the laying process, and a potential will be induced under the cross-linking of the electromagnetic lines. Similar to the power supply, the value may affect the weak current circuit. Low level parameters have a large interference effect, so it is recommended to use the control cable as a round-trip wire.
3.The metal shield is grounded with the shield layer
Metal shielding is an important measure to reduce and prevent electrical interference, including the overall shielding of the iron core, sub-shielding and double-layer overall shielding. The selection of metal shielding type of control cable should be based on the strength of electrical interference that may be generated, and comprehensive anti-interference measures should be taken to meet the requirements of reducing interference and overvoltage. The higher the requirements for countering interference effects, the greater the corresponding investment. When the use of steel tape armor and steel wire braided overall shielding, the cable price will increase by about 10% to 20%.
Post time: 2024-01-22 14:26:27