Determination Of Thickness Of Cable Insulating Layer

- Dec 24, 2018-

Cable insulation structure is the core of the cable, but also the core of the heat shrinkable tube or heat shrinkable casing, it is determined by the manufacturing level, process level and raw material level.

At present, the thickness of insulating layer of XLPE insulated cable in the voltage grade of kv and above is unified as: 3.6/6 grade 2.5~3.2 mm; 6/6 Level 3.4 mm; 6/10 Level 3.4 mm; 8.7/10 level 4.5 mm; 21/35 level 9.3 mm; 26/35 levels 10.5 mm. This is mainly because the electric field strength in the insulation of medium and low voltage cable is not high, there is no need to be too thin on the thickness of the insulating layer, so as not to cause unnecessary confusion, but for high-voltage XLPE cable, for example, the XLPE cable insulation structure, the insulation thickness of the cable will change according to the conductor cross section.

In summary, for low-voltage small section cables, the thickness of the insulating layer is mainly determined by the thinnest insulation allowed by the process, while for low-voltage large-section cables it should be determined according to the mechanical damage (bending) and insulation inhomogeneity that may be suffered during installation and production. Because the stress of the melody increases with the increase of the cross-sectional area of the line core, the inhomogeneity increases with the increase of the line core cross section, so the thickness of the insulating layer of this kind of cable increases with the increase of the cross-sectional area of the line core, and the requirements of the electrical breakdown strength can be met when the insulating layer thickness is satisfied

Only when the operating voltage of the cable is higher than above, especially the UHV cable, the breakdown strength of the insulation gradually becomes the main factor that determines the thickness of the insulating layer. According to the principle that the maximum electric field strength in the cable insulating layer is equal to its breakdown electric field strength, the thickness of the cable insulating layer is designed, and considering the dispersion of the breakdown strength and ensuring that the cable insulation has a certain safety degree, the insulating layer thickness of the XLPE cable can be determined by the following formula:

Em>Emax=U/[rc In( R/ rc)]

E—Correspondingly, the breakdown strength of U is the maximum voltage of the working frequency, pulse and Operation Wave.

m—Insulation margin;

R—Cable insulated outer diameter;

rc—Cable insulated inner diameter.

Insulation layer thickness is

△=R一rc=rrc[exp(mUl/Erc)一1]

For high voltage power cables, the relationship curve between the frequency breakdown strength E of the insulating material and the pressurized time t can be expressed in the following formula:

Eb=E∞+A/n√t

EB—Frequency breakdown strength of the industry;

E-—The breakdown strength of the voltage acting infinitely for a long time;

E∞—Life index;

A—constant

t一Life/year.


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