One of the most important parameters in the mechanism of insulating materials is the voids and micropores in the insulation. In our procurement of heat shrinkable pipe should also pay attention to this parameter, specific these parameters have any impact, please see the following analysis.

The voids (Voids) and Micropores (Micro-Voids) in the insulation are holes filled with liquids or gases. The diameter of the general voids is above 0.05 mm. In the polyethylene insulation of the chemical intersection chain, the main causes of the voids are:

(1) The dispersion of the crosslinking agent or antiaging agent is not uniform;

(2) Excessive humidity in the insulator, on the conductor or in the semiconductor shielding material.

(3) The polymer contains too much low molecular weight fractions.

Cable insulation that works at low field strength allows the size of the voids to vary depending on the starting or extinguishing voltage of the partial discharge. Through the long-term aging test of the XIPE cable sample, it is considered that the voids in the diameter of 0.08mm and below, even at high field strength, do not lead to a partial discharge that seriously endangers the life of the XLPE cable. In general, at 2.5 E0, 0.5 mm diameter voids can cause 10PC discharge, 0.125mm diameter voids will cause 0. 1PC discharge. When the ultra-high pressure XLPE cable is designed, the average field strength of the general stope exceeds the insulation of 5-6 Kv/mm and no voids are allowed to exist.

A microporous is a very fine group of holes in an insulating layer or between an insulating and shielding layer. When the XLPE insulating wire core is in the steam crosslinking, the insulator is under high pressure and high temperature, and the steam will seep into the insulating layer. When the cable insulating core cools, the steam condenses to form a group of micropores. The microporous is filled with liquid or gas, but it does not directly cause partial discharge, even at high field strength. In the slices of the insulating layer, the cloudy ring, which is the Microporous group, is often visible to the naked eye. The diameter of the microporous is about 1~2 um. Although micropores do not directly lead to partial discharge, they are the root of the phenomenon of water branch, which affects cable life. In the production of high voltage XLPE cable, dry cross-chain method is required to prevent the generation of a large number of micropores.

Crosslinked polyethylene (XLPE cable) n≈6~9, latest report: XLPE cable, n≥10, gas gap XLPE Cable (air gap discharge aging), n= 8.0~9.0; 0.01 cable containing air gap and impurities below xlpe mm, n≥10. Table 2-21 lists the requirements and production levels of insulation gap dimensions for manufacturers in several countries. Because the n value is different, the life curve slope is different, in a certain field strength, the larger the N value, the longer the life. According to the study, the air gap size and: The value is related, the air gap is large, then: The value decreases, at the same time the breakdown field strength is also reduced, so that the insulation life decreases.

Because the breakdown strength of the cable insulating layer decreases with the increase of the radius of the wire core, the average electric field strength is used to determine the thickness of the insulating layer of the plastic cable.

We also know that Em (maximum working field strength)

绝缘材料中的空隙及微孔

在绝缘材料的机构中有一个很重要的参数，就是绝缘中的空隙及微孔。在我们采购热缩管的时候也要注意这个参数，具体这些参数有什么影响请看以下分析。

绝缘中的空隙(Voids)及微孔(Micro-Voids)都是充满液体或气体的孔洞。一般空隙的直径在0 .05 mm以上。在化学交链的聚乙烯绝缘中，造成空隙的主要原因为:

(1)交链剂或防老剂的分散不均匀;

(2)绝缘体中、导体上或半导体屏蔽料中过分潮湿。

(3)聚合物中含有低分子量馏分过多。

工作在低场强下的电缆绝缘允许空隙的大小可根据局部放电的起始或熄灭电压而定。通过XIPE电缆样品的长期老化试验，认为直径在0 .08mm及以下的空隙，即使在高场强下，也不会导致发生严重危害XLPE电缆寿命的局部放电。一般，在2 .5 E0时，0.5 mm直径的空隙会导致10PC放电，0 .125mm直径的空隙会导致0. 1PC放电。设计超高压XLPE电缆时，一般采田的平均场强超过5-6 kV/ mm的绝缘中不允许有空隙存在。

微孔是在绝缘层内或绝缘与屏蔽层之间的极微细的孔洞群。当XLPE绝缘线芯在蒸汽交联中，绝缘物处于高压高温之下，蒸汽会渗入绝缘层中。当电缆绝缘芯冷却时，蒸汽凝结，形成一群微孔。微孔中充满着液体或气体，但它即使在高场强下，也不会直接导致局部放电。在绝缘层的切片中，往往肉眼就可看到云雾状圆环，这就是微孔群。微孔的直径约1~2 um。微孔虽不直接导致局部放电，但它们是诱发水树枝现象(Water Tree)的根源，影响电缆寿命。在高压XLPE电缆生产中，要求用干式交链法以防止产生大量微孔。

交联聚乙烯(XLPE电缆)n≈  6~9，最新报导:XLPE电缆，n≥10，有气隙的XLPE电缆(气隙放电老化)，n= 8 .0~9 .0;含有0.01 mm以下气隙和杂质的XLPE电缆，n≥10。表2-21列出了几个国家厂商对绝缘中气隙尺寸要求及生产水平。由于n值的不同，寿命曲线斜率不同，在一定场强时，n值越大，寿命越长。根据研究，气隙尺寸大小和，:值有关，气隙大，则，:值减小，同时击穿场强也降低，使绝缘寿命下降。

由于电缆绝缘层击穿强度随线芯半径的增加而降低，因此，目前对塑料电缆绝缘层厚度的计算，采用平均电场强度来确定。

我们又知道，Em(最大工作场强)<Eв’(工频击穿强度)，为使电缆在试验电压下不击穿，再对试验电压取一定裕度120%~200%。