Effect of Internal Impurities in Thermal Shrinkage Pipe Insulator on the Insulation Performance of Thermal Shrinkage Pipe
The impurities in the insulator and the spikes in the semi-conductive layer can concentrate the electric field strength and produce corona discharge. We found that the growth of some branches often originated from the impurity particles in the insulation after a period of time. This is because the dielectric constant of the impurity particles and the dielectric constant of the heat shrinkable tube insulator are far from each other, which causes the electric field to distort on the surface of the impurity. The distorted electric field in these places may be much higher than that in the normal insulation, making the electric field in the tiny part. Breakdown, the formation of a tip, in a series of cases after the development of the cycle to form a dendritic discharge.
In addition, we know that the hemispherical protrusions on the shielding layer inside and outside the thermal shrinkable tube can increase the electric field intensity to six times, and the long and sharp protrusion area can increase the electric field intensity 80 times. This problem has been paid attention to by foreign technical departments, especially the size of semiconducting layer spikes. For example, the Japanese High Voltage Cable Test Subcommittee stipulates that the radial dimension of impurities or spikes should be no more than 250 um and no more than 100 um for 66-77 kV cables; the United Edison Lighting Company of the United States stipulates that the size of impurities or spikes should be no more than 178 um, and the size of impurities in each 16.4 cm³ (lin³) should be 51-178 um, and should not exceed 10 (69-138 kV) and 5 (5-69 kV); For 10-35 kV heat shrinkable tubes, impurity or semi-conductive layer spikes are less than 250 um, and for 63-110 kV cables, spikes are less than 100 um.
Water cut in insulating heat shrinkable sleeve is an important topic in recent years both at home and abroad. It has been known that water in insulation can cause water tree formation in the insulator and cause insulation damage. Water tree branches are discharge paths consisting of many micro-filling voids smaller than a few microns in diameter. Water tree is formed by the interaction of electric field and water. In order to reduce the moisture content in insulation, the flame-retardant heat-shrinkable pipes produced by us have been transformed from wet cross-linking to dry cross-linking, which reduces the moisture content in insulation by almost two orders of magnitude. The moisture content of cross-linked insulated cables manufactured by wet method can reach more than 2000 *10-6 power.
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