The rated voltage of the control cable shall not be lower than the working voltage of the circuit, and shall meet the requirements of transient and power frequency overvoltages that may be experienced. [3] In order to ensure that the control cable reduces the scope of the impact when insulation breakdown, mechanical damage or fire occurs, the national standard GB50217-91 "Code for Design of Electric Power Engineering Cables" stipulates: double protection of current, voltage, DC power and trip Two systems that require enhanced reliability, such as control loops, should use separate control cables.
After the control cable is put into operation, there is a problem of electrical interference between different cores of the same cable and between cables laid in parallel. The main causes of electrical interference are:
(1) The electrostatic interference caused by the capacitive coupling between the cores due to the applied voltage;
(2) Electromagnetic induction interference caused by energized current. In general, 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 degree of interference is also much greater than the adjacent cables laid in parallel. For example, the control circuit of a phase-break circuit breaker of an ultra-high-voltage substation, which uses one cable for three phases, has occurred in such an accident. The pulses of phase separation operation triggered the thyristors of other phases, which caused the three-phase linkage by mistake. With separate cables, no more accidents occur. Another example is the computer monitoring system of a power plant. Because the analog low-level signal line and the transmitter's power line are combined with a four-core cable, it has caused a 70V interference voltage on the signal line. The low-level signal loop will obviously affect normal operation.
There are three main measures to prevent or reduce electrical interference.
1. Ground a spare core of the control cable
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% to 50%, and the implementation is simple, and the cost of the cable is only slightly increased.
2. Do not use a control cable for circuits that have serious consequences when electrical interference occurs.
These include: (1) weak current signal control loop and strong current signal control loop; (2) low level signal and high level signal loop; (3) each phase weak current control loop of the AC circuit breaker split-phase operation Use the same control cable. However, if each pair of back-and-forth wires of the weak current circuit belong to a control cable that is not the same, it may form a ring arrangement when laying, and the electric potential will be induced under the interlinking of the electromagnetic wires of the similar power supply. Low-level parameter interference has a greater impact, so it is advisable to use a control cable for the round-trip conductor.
3.Ground metal shield and shield
Metal shielding is an important measure to reduce and prevent electrical interference, including overall shielding of cores, sub-shielding, and double-layer overall shielding. The selection of the metal shielding type of the control cable should be based on the comprehensive interference suppression measures according to the strength of the possible electrical interference effects to meet the requirements of reducing interference and overvoltage. The higher the requirements for anti-interference effects, the greater the corresponding investment. When steel tape armor and steel wire braiding are used, the price of the cable increases by about 10% to 20%.
