Cable shielding

1. Definition of shielding of power cables

The electric field of the cable is enclosed in an insulating layer surrounding the conductor with a conductive or semiconducting layer. The conductive or semi-conductive layer is tightly fitted to the inner and outer surfaces of the insulation. In other words, the outer shield closes the electric field between the wire and the shield. The inner shield or twisted stress relief layer is at the potential of the wire or near the potential of the conductor. The outer shield or insulation shield is designed for the transmission of the capacitor current and, in many cases, to transmit the fault current. The conductivity of the shield is determined by the cross-sectional area and resistivity of the metal strip or wire used with the semiconducting layer. The stress control layer on the inner and outer surfaces of the insulation, as it is close to the smooth surface of the insulating surface, reduces the stress concentration and minimizes the gap. In this gap, the ionization of the air may cause some insulation material to gradually damage until the last complete damage so far.

2. Insulation of the shield for a variety of purposes

The electric field is enclosed inside the cable; the voltage gradient inside the insulation is balanced to minimize the surface discharge; the induced potential is avoided to better reduce the risk of electric shock. The voltage distribution between the unshielded cable and the ground plane assumes that the air and the insulator are the same from electrical performance, and that the cable above the ground plane is in a uniform dielectric, thus allowing a simple graph Cable-related voltage distribution and electric field. In the shielded cable, the equipotential surface between the conductor and the shield is concentrically cylindrical, the voltage distribution changes according to the simple logarithmic law, and the electrostatic field is all enclosed in the insulating layer. The power lines and stresses are uniform and radiated, intersecting the equipotential surfaces at right angles, eliminating tangential or longitudinal stresses in the insulation or on the insulating surface. The equipotential surface of the unshielded system is cylindrical, but not concentric with the conductor, intersecting the cable surface with many different potentials. For the unshielded cable that runs the high system, at the point of the cable, May be in the dry place cable terminal leakage distance of the normal recommended value several times. In this case, the surface of the leakage traces, combustion and destructive discharge to the ground may occur. However, the correct design of the unshielded cable described in the National Electrical Code limits the achievable surface energy, which is derived from these effects, which may affect the proper use of the cable.

For cables operating at voltages below 1 kV, unshielded structures are generally used, and cables above 10 kV are shielded to comply with national electrical codes. In the range of 1 ~ 10kV, to allow the use of shielded cable and non-shielded cable, as long as its structure can meet the requirements of the national standard. Since the price of shielded cables is generally higher than that of non-shielded cables, and because the end of the shielded cable needs to be more careful and require more space, so in the range of 1 ~ 10kV, has been widely used non-shielded cable , Non-shielded cable is also used for 10kV voltage level. However, the use of shielded cables may be specified where buried directly or on the surface of the cable may accumulate a large amount of conductive material (salt, soot, conductive wear grease).

3. Control cable and its metal shield

The control cable should be protected from damage by insulation, mechanical damage, fire or electrical interference. Dual protection of the current, voltage and DC power supply and trip control circuit and other systems to enhance the reliability, should use their own independent control cable. The following conditions of the circuit, should not be used with each other the same control cable: weak signal, control circuit and strong signal, control loop; low signal and high signal circuit; AC circuit breaker phase of the phase of the weak control circuit The The distance between the same cable core is smaller, the coupling, the electromagnetic induction is stronger than the interference between the cables. A power plant computer monitoring system analog low-level signal line with the transmitter power cable common a four-core cable, causing the signal line to produce a common mode interference voltage of about 70V, the low-level signal loop in millivolts, obviously Affect the normal work.

Each pair of reciprocating wires of the weak circuit should belong to the same control cable. Strong electric circuit control cable, in addition to the ultra-high voltage power distribution device or with the high-voltage cable close to the parallel length, need to suppress the interference situation, can not contain metal shield. Weak control signals, control circuit control cable, when there is interference in the presence of the environment and do not have effective anti-jamming measures, should be a metal shield. Control cable The choice of metal shield type, should be affected by the possible electrical interference, included in the comprehensive suppression of interference measures to meet the need to reduce the interference or over-voltage requirements. In the 110kV or more power distribution device of the weak control cable, should have a total shielding, double-layer total shielding. Computer monitoring system signal loop control cable shielding choice, for the switch signal, available total shielding. For high-level analog signal, it is appropriate to shield the total cable shield, if necessary, can also be used to shield the twisted pair of core, and for low-level analog signal or pulse signal, it is appropriate to shield the core, Can also be used to shield the composite core shield shield. Other circumstances, should be according to electromagnetic induction, electrostatic induction and ground potential rise and other factors, the use of appropriate shielding type.

To reduce the electrical interference of the control cable, the number of working cores can be added to a grounded spare core. When the control cable is grounded, the interference voltage amplitude can be reduced to 50% ~ 25% or more, and the implementation is simple and the cable cost is very small. Control cable metal shielding grounding, for the computer monitoring system analog signal loop control cable shield, shall not constitute two or more grounding, it is appropriate to use a little ground. In addition to the need for a little grounding control cable shielding layer, when the electromagnetic induction interference is large, should adopt two points to ground; electrostatic induction interference, can be a little ground. Double shield or composite total shielding, should be inside and outside the shield with a little, two ground. Two-point grounding choice, but also should be considered under the action of transient current shielding layer will not be burned.

If the same round-trip wire is divided into two cables, the possibility of laying the ring is difficult to avoid, in the similar power supply under the intersection of the electromagnetic wire will induce potential, the order of magnitude of the weak circuit often weak interference parameters of the larger impact. Weak circuit control cable and power cable if you can pull enough distance, or laying in the steel pipe, steel closed tray, etc., may make the external interference to the allowable limit. Otherwise, the power cable adjacent to the parallel laying, or located in the high-voltage power distribution device and near the ground trunk, etc., the interference amplitude is often shielded control cable connected to the low-level signal loop, etc. will produce a malfunction or insulation Breakdown and so on. Control cable with metal shield to reduce the effect of interference, and shield structure type related. At the same time to see the higher requirements of the shield structure, the corresponding investment is also greater. There is no metal shielded control cable cost, about 10% to 20% (steel tape armor, wire braid total shield) or greater share.

The control cable of the electronic device digital signal circuit is shielded and grounded so that the voltage drop interference on the ground line is minimized. The interference voltage, which is only about 1V based on the computer, may cause a logic error, thus emphasizing the importance of calculating the monitoring system. Analog signal loop control cable to suppress interference requirements, should be implemented a little ground, and a little ground can have a variety of implementation methods, is to monitor the computer system, specify to meet the conditions to avoid the occurrence of ground loops, centralized grounding. The current distribution of the grounding grid in the distribution device has been measured by 13% of the ground current, while the 110 to 500 kV voltage stage short circuit current has reached 35 ~ 18kA.

4. How to shield the cable

Remove a cable shield at the end of the cable and leave the necessary leakage distance between the conductor and the shield to form longitudinal stresses on the exposed cable insulation surface. The combined effect of radial and longitudinal electrical stresses on the cable end terminal device results in a maximum stress present at that point. However, these stresses can be controlled and reduced to within the safe working range of the production of the terminal device material. The most common way to reduce these stresses is to gradually increase the total insulation thickness of the cable terminating device with an insulating tape to form a cone, the stress cone. China's distribution network cable mainly used in the viscous impregnated oil-paper insulated cables, PVC cables and XLPE cables and XLPE cables and other three. Viscous impregnated oil-paper insulated cable has the advantages of high operating temperature, low dielectric loss, high withstand voltage strength and long service life. The disadvantage is that the bending performance of the insulating material is poor and can not be laid at low temperature. According to the impregnation method, Drip dry oil-impregnated paper insulation and no drip dry oil-immersed paper insulation three, according to the level of cable laying height difference were used. Ordinary oil-immersed paper insulated cable laying level difference is only allowed to 5 ~ 20m, dripping dry oil-immersed paper insulation cable laying the level of height difference to allow 100 ~ 300m, no dripping oil-immersed insulation cable laying no high difference limit. PVC cable main advantages of the manufacturing process is simple, no laying height difference limit, bending performance, oil, acid and alkali corrosion, non-flames, low prices. The disadvantage is that the insulation resistance is less than the oil-insulated paper insulation cable, the dielectric loss is high, not on the important 6kV line. Cross-linked polyethylene cable with excellent performance, simple structure, light weight, large capacity, easy to lay, no height difference in the distribution network is widely used.