Medium voltage cables are usually stranded by several or groups of wires into a rope-like cable, each group of wires is insulated from each other and often twisted around a center, and the entire outer shell is covered with a highly insulating covering.
Medium-voltage power cables are 35 kV and below power cables.
Medium-voltage power cables use a single-core structure, which helps to increase the cable's load capacity and enhance the system load.
It is also able to reduce the occurrence of phase-to-phase short circuits in the event of cable insulation faults, while also reducing the weight of a single cable and increasing the length of the loading tray, facilitating transportation and construction.
It greatly reduces the number of joints in the line and changes three-phase joints to single-phase joints, making joint sealing simpler and more reliable. However, the design is easy to ignore the correct selection and determination of the structure and cross-section of the metal shield of the medium voltage single-core power cable, resulting in the potential for serious operating accidents in the power system.
The metal shield is an important structure of medium-voltage power cable, in normal operation the cable will pass capacitive current, short circuit, and as a short-circuit current path.
At the same time, the electromagnetic field output when the cable is energized and shielded in the insulated core reduces electromagnetic interference to the outside world.
GB/T 12706-2008 standard for medium voltage power cable metal shielding structure form is specified, that is, by one or more metal tape, metal braid, a concentric layer of metal wire, or the combination of metal wire and metal tape structure, can also be a metal jacket or metal armor layer.
Copper tape shielding consists of a layer of overlapping wrapped soft copper tape, but also can use a double layer of copper tape gap wrapping.
The nominal thickness of copper tape for single-core cable shielding is not less than 0.12mm.
Copper wire shielding consists of sparsely wrapped soft copper wire, the surface of which is wrapped with reverse copper wire or copper tape tied.
The average gap between adjacent copper wires should be no more than 4mm.
In the whole cable manufacturing process, the metal shielding process is not complicated, but it is in direct contact with the insulation core. Without attention to detail, it is easy to make the metal shielding lose function, and this will cause serious cable quality problems.
Copper tape wrapping shielding is the traditional shielding process, but also in the form of single-layer overlap lap wrapping.
Power cable using this method has the advantages of a smaller outside diameter, lighter weight, and lower cost.
Requirements for the use of copper tape for annealed soft copper tape, the copper tape is not allowed to have broken, rolled edges, and other defects.
Wrapping to adjust the tension of the appropriate package tape and the closing tray to be padded with foam pearl cotton to avoid core lacerations.
Copper tape wrapping should be tight and flat, there should be no oxidation, curling, bulging, copper cracking, or wrinkling and revealing the phenomenon of a semi-conductive layer inside.
Copper tape joints are strictly prohibited to use tape lap or tin welding, must use electric welding, and lap length of not less than 20mm, welding should be flat and firm, and the joint should be sandpaper for flattening and polishing.
However, the soft copper tape used for shielding is mostly recycled copper, and its electrical properties are difficult to meet the requirements of GB/T 11091-2005 standard.
After the cable is put into operation in this way, the contact surface of the copper belt is easily oxidized.
And due to bending and hot and cold deformation of copper strip contact surface reduction, resulting in increased contact resistance, but also easy to make the copper strip arch embedded in the insulation shield layer and damage the core.
The copper wire and copper tape sparse winding shielding method can structurally improve the disadvantages of the copper tape wrapping shielding method.
It does not have the oxidation layer generated by the copper tape lap, nor strong bending and hot and cold deformation, the sparsely wound copper tape is less likely to arch embedded in the insulation shield.
However, the cable produced in this way has a larger outer diameter, heavier weight, and higher cost.
In particular, the sparsely wound copper wire is easy to strangle into the insulation shield in production and use, affecting the appearance of the insulation core. This causes an increase in partial discharge of the cable or even exceeds the standard, and can seriously cause the cable to break through.
To avoid damage to the insulation core of copper wire sparse winding, the design can be used in two ways:
① In the insulation core outside the wrapping 1 to 2 layers of thickness of about 0.2mm semi-conductive nylon tape.
② in the three-layer co-extrusion process increase the thickness of the insulation shield layer, the thickness from the usual 0.6mm to 1.0 ~ 1.2mm.
The cross-section of the metal shield directly affects the cable's ability to withstand short-circuit current during a fault.
If the cross-sectional area is too small, it will overheat or burn off when the short-circuit current passes through and damage the insulation.
Therefore, the cross-section of the metal shield needs to be determined according to the fault current capacity.
Outdoor low-voltage lines have several distinct characteristics:
① Basically, they are supported by concrete poles, and the concrete poles are short, generally not exceeding 5 meters.
② the same thickness of the wire, and the number of multiples of 4, which is because the low-voltage wire generally uses a three-phase four-wire system. If these characteristics can determine the line voltage of the wire is 380 volts, the phase voltage of 220 volts.
Identify high-voltage lines
① If the wire thickness is the same, the number of multiples of 3, this is because the transmission line is generally three-phase transmission, and these characteristics can determine the wire for 10,000 volts.
② If the wire thickness is not the same, the number of thick lines is a multiple of 3, the fine line is only 2, and the assumption is that the highest in the.
This is because the thin line is not used for power transmission, but for lightning protection, also called lightning line.
If these characteristics are available then the wire can be identified as a high-voltage line.
To improve transmission capacity, high-voltage lines generally use split wires.
In layman's terms is the original one phase of a line, now with several wire bundles in place of the original one line.
Knowing this will be easy to identify the voltage level of the wire.
①One phase of a wire is 110,000 volts.
②One-phase two-wire is 220,000 volts.
③One phase of four lines is 500,000 volts.
Now many tenders require cable companies to design the shielding structure and determine the shielding cross-section according to the laying conditions and short-circuit current.
Therefore, the design of medium-voltage single-core power cable should fully consider the ability of the metal shield to carry short-circuit current, through the calculation and selection, to provide users with qualified and assured products.
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