In order to ensure that the control cable is in the event of insulation breakdown, mechanical damage or fire, to reduce the scope of spread, the national standard GB50217-91 "Power engineering cable design specification" provides: dual protection of the current, voltage and DC power supply and trip control circuit, etc. Two systems requiring enhanced reliability should use separate control cables.
After the control cables are put into operation, there are electrical interferences between the different cores of the same cable and the cables laid in close proximity to each other. The main causes of electrical interference are:
(1) Electrostatic interference due to the applied capacitive coupling of the applied voltage between the cores;
(2) Electromagnetic induction interference due to energized 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 closely-arranged parallel cables. For example, in the control loop of the phase-breaking operation circuit breaker of an EHV substation, a three-phase cable is used together. Such an accident occurred. The thyristors of other phases are triggered by the pulse of the phase-separation operation. With separate cables, no accidental misoperation occurred. Another example is a computer monitoring system in a power plant. Since a signal cable with an analog low level is combined with the power line of the transmitter with a four-conductor cable, an interference voltage of 70 V is generated on the signal line, which is measured in millivolts. The low-level signal loop obviously will affect normal operation.
Measures to prevent or reduce electrical interference mainly include the following three aspects: 1. The practice of a spare core grounding of the control cable proves 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 minimal.
2. Circuits with serious consequences for electrical interference do not use a control cable which includes: (1) weak signal control loop and strong signal control loop; (2) low-level signal and high-level signal loop (3) Each phase weak circuit control loop of the AC circuit breaker shall not use the same control cable. However, for each pair of return conductors of the weak current loop belonging to control cables that are not of the same root, it may form a ring arrangement when laid, and the potential may be induced under the electromagnet wire of a similar power source, and the value may be for weak current loops. Low-level parameter interference has a greater impact, so it is advisable to use a single control cable for the return conductors.
3, metal shielding and shielding layer grounding Metal shielding is an important measure to weaken and prevent electrical interference, including the total shielding of the core, sub-shielding and double-layer total shielding. The selection of the metal shield type of the control cable shall be based on the possible influence of the electrical interference, and shall include measures for comprehensive suppression of interference to meet the requirements for reducing interference and overvoltage. The higher the requirement for anti-jamming effect, the greater the corresponding investment. When using steel tape armoring and steel wire braiding total shielding, the price of the cable is increased by about 10%~20%.
The control interference in the strong electric circuit, due to its strong signal, can be controlled by a control cable without metal shield, except in the UHV power distribution device or in parallel with the high voltage cable. The control cable used in the weak current signal control loop should be controlled by a metal shielded cable when it is located in an environment where interference exists and there is no effective anti-jamming measure, so as to prevent electrical interference from malfunctioning the low-level signal circuit. Or make insulation breakdown, etc. If the control cable of the weak current circuit can be pulled away from the power cable for a sufficient distance or laid in a steel pipe, the external electrical interference may be reduced to an allowable limit.
For the control cable of the computer monitoring system signal loop, the principle of shielding type selection is:
(1) Switch signal, total shield available;
(2) High-level analog signals, it is appropriate to use the total shield of the core, if necessary, can also be used for the sub-shielding of the core;
(3) For low-level analog signals or pulse signals, it is advisable to use separate shields for the core, and if necessary, a composite total shield with separate shields for the cores.
With regard to the grounding of the shielding layer, attention should be paid to the following points:
(1) The control cable shield of the analog signal circuit of the computer control system should be grounded with a centralized point. The reason for this is based on the requirement to ensure the normal operation of the computer monitoring system, because even if the interference voltage is only about 1V, it may cause logic judgment errors. Concentrated one point grounding can avoid ground loop current;
(2) Except for the case where the control cable shield of the computer monitoring system only allows one point to be grounded, the other control cable shields should be grounded at two points when the electromagnetic induction interference is large, and the electrostatic induction interference is large. Then use a little grounding;
(3) Double shield or composite shield The inner 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 shield layer will not be burned.
Control cable installation and wiring considerations:
1. Under the roof. Cables can only be used if they are not directly exposed to sunlight or ultra-high temperatures. Standard LAN cables can be used. Pipes are recommended. Ultraviolet (UV) - Do not apply UV-free cables to direct sunlight.
2. The outer wall. Avoid direct sunlight on the wall and human damage. Heat - The temperature of the cable in the metal pipe or raceway is high, and many polymeric materials will reduce the service life at this temperature.
3. In the pipeline (plastic or metal). If in the pipe, pay attention to the damage of the plastic pipe and the heat conduction of the metal pipe. Mechanical damage (repair costs) - Repair of fiber optic cable is very expensive, requiring at least two terminations at each discontinuity.
4. Hang the application/overhead cable. Consider the sagging and stress of the cable. Whether the cable is directly exposed to sunlight.
5. Laying directly in underground cable trenches, this environment is the least controlled. The installation of cable trenches should be regularly checked for dryness or moisture level. Grounding - If the shield of the control cable needs to be grounded, the corresponding standards must be observed.
6. Underground pipelines. In order to facilitate future upgrades, cable replacement and isolation from the surface pressure and surrounding environment, auxiliary pipelines are isolated, and auxiliary piping is a better method. But don't expect the pipeline to stay dry forever, which will affect the choice of cable types. Water - The moisture in the LAN twisted pair cable increases the capacitance of the cable, which reduces the impedance and causes near-end crosstalk problems.
After the control cables are put into operation, there are electrical interferences between the different cores of the same cable and the cables laid in close proximity to each other. The main causes of electrical interference are:
(1) Electrostatic interference due to the applied capacitive coupling of the applied voltage between the cores;
(2) Electromagnetic induction interference due to energized 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 closely-arranged parallel cables. For example, in the control loop of the phase-breaking operation circuit breaker of an EHV substation, a three-phase cable is used together. Such an accident occurred. The thyristors of other phases are triggered by the pulse of the phase-separation operation. With separate cables, no accidental misoperation occurred. Another example is a computer monitoring system in a power plant. Since a signal cable with an analog low level is combined with the power line of the transmitter with a four-conductor cable, an interference voltage of 70 V is generated on the signal line, which is measured in millivolts. The low-level signal loop obviously will affect normal operation.
Measures to prevent or reduce electrical interference mainly include the following three aspects: 1. The practice of a spare core grounding of the control cable proves 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 minimal.
2. Circuits with serious consequences for electrical interference do not use a control cable which includes: (1) weak signal control loop and strong signal control loop; (2) low-level signal and high-level signal loop (3) Each phase weak circuit control loop of the AC circuit breaker shall not use the same control cable. However, for each pair of return conductors of the weak current loop belonging to control cables that are not of the same root, it may form a ring arrangement when laid, and the potential may be induced under the electromagnet wire of a similar power source, and the value may be for weak current loops. Low-level parameter interference has a greater impact, so it is advisable to use a single control cable for the return conductors.
3, metal shielding and shielding layer grounding Metal shielding is an important measure to weaken and prevent electrical interference, including the total shielding of the core, sub-shielding and double-layer total shielding. The selection of the metal shield type of the control cable shall be based on the possible influence of the electrical interference, and shall include measures for comprehensive suppression of interference to meet the requirements for reducing interference and overvoltage. The higher the requirement for anti-jamming effect, the greater the corresponding investment. When using steel tape armoring and steel wire braiding total shielding, the price of the cable is increased by about 10%~20%.
The control interference in the strong electric circuit, due to its strong signal, can be controlled by a control cable without metal shield, except in the UHV power distribution device or in parallel with the high voltage cable. The control cable used in the weak current signal control loop should be controlled by a metal shielded cable when it is located in an environment where interference exists and there is no effective anti-jamming measure, so as to prevent electrical interference from malfunctioning the low-level signal circuit. Or make insulation breakdown, etc. If the control cable of the weak current circuit can be pulled away from the power cable for a sufficient distance or laid in a steel pipe, the external electrical interference may be reduced to an allowable limit.
For the control cable of the computer monitoring system signal loop, the principle of shielding type selection is:
(1) Switch signal, total shield available;
(2) High-level analog signals, it is appropriate to use the total shield of the core, if necessary, can also be used for the sub-shielding of the core;
(3) For low-level analog signals or pulse signals, it is advisable to use separate shields for the core, and if necessary, a composite total shield with separate shields for the cores.
With regard to the grounding of the shielding layer, attention should be paid to the following points:
(1) The control cable shield of the analog signal circuit of the computer control system should be grounded with a centralized point. The reason for this is based on the requirement to ensure the normal operation of the computer monitoring system, because even if the interference voltage is only about 1V, it may cause logic judgment errors. Concentrated one point grounding can avoid ground loop current;
(2) Except for the case where the control cable shield of the computer monitoring system only allows one point to be grounded, the other control cable shields should be grounded at two points when the electromagnetic induction interference is large, and the electrostatic induction interference is large. Then use a little grounding;
(3) Double shield or composite shield The inner 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 shield layer will not be burned.
Control cable installation and wiring considerations:
1. Under the roof. Cables can only be used if they are not directly exposed to sunlight or ultra-high temperatures. Standard LAN cables can be used. Pipes are recommended. Ultraviolet (UV) - Do not apply UV-free cables to direct sunlight.
2. The outer wall. Avoid direct sunlight on the wall and human damage. Heat - The temperature of the cable in the metal pipe or raceway is high, and many polymeric materials will reduce the service life at this temperature.
3. In the pipeline (plastic or metal). If in the pipe, pay attention to the damage of the plastic pipe and the heat conduction of the metal pipe. Mechanical damage (repair costs) - Repair of fiber optic cable is very expensive, requiring at least two terminations at each discontinuity.
4. Hang the application/overhead cable. Consider the sagging and stress of the cable. Whether the cable is directly exposed to sunlight.
5. Laying directly in underground cable trenches, this environment is the least controlled. The installation of cable trenches should be regularly checked for dryness or moisture level. Grounding - If the shield of the control cable needs to be grounded, the corresponding standards must be observed.
6. Underground pipelines. In order to facilitate future upgrades, cable replacement and isolation from the surface pressure and surrounding environment, auxiliary pipelines are isolated, and auxiliary piping is a better method. But don't expect the pipeline to stay dry forever, which will affect the choice of cable types. Water - The moisture in the LAN twisted pair cable increases the capacitance of the cable, which reduces the impedance and causes near-end crosstalk problems.
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