The analysis of shielding layer of heat shrinkable tube insulated cable is as follows:
The performance of semi-conductive shielding layer inside and outside of heat shrinkable tube insulated cables plays an extremely important role in the performance of XLPE insulated cables. At the same time, its development and transformation are also symbols of continuous improvement and development of XLPE insulated cables. Research on it will play a key role in understanding and understanding the whole structure of XLPE insulated cables.
The insulating shielding layer of heat shrinkable bushing is composed of semiconductor materials. It is squeezed between the metal core and the insulating layer or between the insulating layer and the metal shielding layer. It is called the internal shielding and the external shielding of insulation respectively. Their function is to prevent partial discharge by uniform electric field. If the semiconductor shielding layer itself can not guarantee smooth electrical properties (rough surface, even sharp protrusion), there are four uneven pits or cracks, and the contact between the fracture and insulation is not good, it will be difficult to play the role of uniform electric field, and may even cause serious electric field concentration, leading to partial discharge or insulation breakdown.
The inner and outer semiconductor layers are composed of polar polymers such as PE, EPM or EVA and selected high conductivity carbon black. The average particle size of the most commonly used high conductivity carbon black is (200-400)*10-9 mm. X LPE power cables below 20 kV can be shielded by a strippable semiconductor mixture containing polar materials, while cables of 24 kV and above 1L grade should be shielded by crosslinked semiconductors (see Semiconductor Shielding Design below).
The contact surface between the inner shield and the insulating layer is the place where the electric field strength of the cable is the highest. Any defect in the contact can easily cause the electric field to be highly concentrated. The American standard AELCNo S stipulates that cables that are protruding from inner shielded cylinders or whose unevenness is greater than 0.25 mm are not qualified. The main reasons for the prominent internal shielding are that the semiconductor "secretion" attached to the extrusion die escapes from the surface of the inner shielding layer, i.e. DieBleed, and the carbon black caking on the surface of the semiconductor. In normal extrusion, the inner diameter of the die sleeve is generally larger than the outer diameter of the inner shielding layer (0.5-0.9 M m), so the carbon black caking of 0.25 m is easy to pass through the die hole. Surface roughness or pitting can also lead to electric field concentration. Carbon black particles are often embedded in insulators on the surface of rough semiconductor layers and become concealed protrusions.
The high field intensity caused by the protrusion or embedding of semiconductors will also induce the cold emission of electrons from carbon black particles, i.e. the electrons of moving A escape from the conductor and enter the medium at high field intensity. In this way, it will lead to more serious partial discharge.