The impurities in the heat shrinkable tube insulator and the spines in the semi-conductive layer can concentrate the electric field intensity and produce corona discharge. Our anatomy of the thermal shrinkage casing after a period of use has found that the source of the growth of some branches often originates from impurity particles in the insulation, which is due to the difference between the dielectric constant of the impurity particles and the dielectric constant of the heat shrinkable tube insulator, which causes the electric field to form distortion on the impurity surface, where the distorted electric field may be much higher So that in the small local breakdown, the formation of the tip, in a series of cases of cyclic development after the formation of 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.

Insulation heat shrinkable casing containing water is a topic that has been paid more and more attention in the international and domestic countries in recent years. We have known that water in the insulation causes water branches to form in the insulator, causing insulation damage. Water branches are discharge pathways consisting of many microscopic water-filled voids less than a few microns in diameter, and the combined action of electric fields and water forms water trees. The flame retardant heat shrinkable tube we produce in order to reduce the water content in the insulation, through the transformation of the crosslinking process, that is, by the wet crosslinking change into the current dry cross-linked, so that the water content in the insulation dropped almost two orders of magnitude. The water content of crosslinked insulated cable produced by wet method can reach more than 2000x10^-6 secondary square.

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热缩管绝缘性能的影响因素

热缩管绝缘体中的杂质和半导电层中的突刺可使电场强度集中而产生电晕放电。我们对使用一段时间后的热缩套管解剖发现，有些树枝生长的源头往往发源于绝缘中杂质颗粒，这是由于杂质颗粒的介电常数和热缩管绝缘体的介电常数相差甚远，使电场在杂质表面形成畸变，这些地方的畸变电场可能远远高于正常绝缘中的电场，使得在微小局部击穿，形成尖端，在一系例循环发展后形成树枝状放电。

另外，我们知道热缩管内外屏蔽层上的半球形突起物，可使电场强度提高到6倍，长而尖的突刺区，可使电场强度提高80倍。这一问题己被国外技术部门所重视，特别规定了半导电层突刺大小。例如，日本高压电缆试验分委员会规定杂质或突刺的径向尺寸应不大于250 u m，对66~77 k V电缆要求不大于100 µm；美国联合爱迪生照明公司规定杂质或突刺尺寸应不大于178µm，且在每一个16 .4 cm³(即lin³)中杂质大小为51~178 um，且不应超过10个(69~138 kV)和5个(5~69 kV)；我国暂定为对于10~35 kV热缩管，杂质或半导电层突刺小于250um，对于63~110 kV电缆突刺小于100um。

绝缘热缩套管含水是近年来国际国内比较重视的一个题目。我们己经知道绝缘中含水会引发绝缘体中形成水树枝，造成绝缘破坏。水树枝是直径小于几个微米的许多微观充水空隙所组成的放电通路，电场和水的共同作用形成水树。我们生产的阻燃热缩管为了降低绝缘中含水量，通过对交联工艺的改造，即由湿法交联改变成现在的干法交联，使得绝缘中含水量下降了几乎两个数量级。使用湿法生产的交联绝缘电缆中有的含水可达2000×10^-6次方以上。