Cable insulation structure is not only the core of cable, but also the core of heat shrinkable tube or heat shrinkable sleeve. It is determined by manufacturing level, process level and raw material level.
At present, the thickness of insulation layer of XLPE insulated cables with 35 kV and above voltage levels is uniformly 3.6/6 grade 2.5-3.2 mm; 6/6 grade 3.4 mm; 6/10 grade 3.4 mm; 8.7/10 grade 4.5 mm; 21/35 grade 9.3 mm; 26/35 grade 10.5 mm. This is mainly because the electric field strength in the insulation of medium and low voltage cables is not high, so it is not necessary to divide the thickness of insulation layer too thin to avoid unnecessary confusion. But for high voltage XLPE cables, such as 110 kV XLPE cable insulation structure, the thickness of insulation layer of cables will change according to the change of conductor cross section.
Generally speaking, for low-voltage small cross-section cables, the thickness of insulation layer is mainly determined by the thinnest insulation allowed by technology, while for low-voltage large cross-section cables, it should be determined by the mechanical damage (bending) and non-uniformity of insulation that may be suffered during installation and production. Because the bending stress increases with the increase of the core cross-section and the non-uniformity increases with the increase of the core cross-section, the insulation thickness of this kind of cable increases with the increase of the core cross-section. When the insulation thickness meets the above requirements, it can meet the requirements of the electrical breakdown strength.
Only when the working voltage of the cable is higher than 10 kV, especially for the ultra-high voltage cable, the breakdown strength of the insulation gradually becomes the main factor determining the thickness of the insulation layer. According to the principle that the maximum electric field strength in cable insulation is equal to the breakdown electric field strength, the thickness of cable insulation layer is designed. Considering the dispersion of breakdown strength and ensuring the safety margin of cable insulation, the thickness of XLPE cable insulation layer can be determined by the following formula:
E - corresponding to U is the breakdown strength when the maximum voltage of power frequency, pulse and operation wave is used.
R - cable insulation outer diameter;
RC - Cable Insulation Internal Diameter.
The thickness of insulating layer is
Delta = R-rc = RRC [exp (mUl/Erc) 1]
For high voltage power cables, the relationship between power frequency breakdown strength E of insulating materials and pressure time T can be expressed as follows:
Eb = E _ plus A/n_t
EB - power frequency breakdown strength in the formula;
The breakdown strength of E - - voltage action for infinite time;
E life index;
A - constant;
T One life span, one year.