In the general heat shrinkable pipe production process, the selection of resistivity of semi-conductive shielding layer is based on the theory and operating temperature, processing process and other factors, the general resistivity of ρν=10 4 square ωcm, this is because the shielding layer resistivity ρν value of the size of the cable Tan Delta has a great impact. In this connection, we can analyze the effect of resistivity ρν on cable Tan Delta according to the equivalent circuit. Under the action of external application voltage U, the cable-to-ground capacitance current is L, so the voltage ratio on the insulation and shielding layer is thus.

In fact, the equivalent circuit above does not consider the capacitance effect of the shielding layer. If the resistivity of the shielding layer is large, the capacitance value of the shunt capacitance should be considered, so the loss of the shielding layer will affect the loss of the whole measuring medium and increase the loss of the measuring medium. The relationship between shielding layer resistivity ρν and cable dielectric loss Tan Delta is deduced theoretically.

According to the data report, the theoretical calculation and the measured curve shape is exactly the same, but to a slight difference in amplitude. This difference mainly comes from the neglect of the original cable structure by the equivalent circuit. As can be seen from the figure, with the increase of shielding resistivity, the loss of cable media Tan Delta increased, in the ρν=10 of the 9 times around the maximum value, and when the shielding resistivity ≤10³Ωcm, cable loss Tan Delta almost no longer decline, and tend to X LPE media loss, it is considered that there is no additional loss of shielding layer at this time.

When pressurized operation, from the voltage distribution and capacitance divider point of view, in the semi-conductive shielding layer, should also bear the voltage, but because the epsilon is <ε half, so U, n U2, shielding layer on the distribution of the pressure drop is very small, when the cable size is certain, U1/u2∞ε2/ε1. At the same time, through the study of long-term power frequency, short-term power frequency and impact voltage failure characteristics of semi-conductive shielding cable, it is found that the semi-conductive shielding layer has obvious effect on reducing the long-term damage of AC, which is to prevent the occurrence of corona in long-term communication.

热缩管屏蔽层的电气特性

一般热缩管生产过程中，半导电屏蔽层电阻率的选择是基于理论和运行温度、加工过程等因素的，一般取电阻率ρν=10的4次方 Ω  cm，这是因为屏蔽层电阻率ρν值的大小对电缆tanδ的影响很大。关于这一点，我们可根据等值电路来分析电阻率ρν对电缆tanδ的影响。在外施电压U作用下，电缆对地电容电流为l，由此可知绝缘和屏蔽层上电压比。

实际上，上述等值电路没有考虑屏蔽层的电容效应。如果屏蔽层电阻率较大，就应考虑并联电容的电容值，故此就会出现屏蔽层介质损耗影响整个测量介质损耗，使测量介质损耗增加。下而对屏蔽层电阻率ρν与电缆介质损耗tanδ之间的关系作理论上推算。

根据资料报道，理论计算和实测曲线形状完全一样，只是要幅值上略有差别。这种差别主要来源于等值电路对原电缆结构上的忽略。从图中可知，随着屏蔽电阻率升高，电缆介质损耗tanδ增加，在ρν=10的9次方左右出现极大值，而当屏蔽电阻率≤10³ Ω cm时，电缆损耗tanδ几乎不再下降，而趋于X LPE介质损耗，即可认为此时已不存在屏蔽层附加损耗。

当加压运行时，从电压分配和电容分压角度看，在半导电屏蔽层上，也应承受电压，但由于ε绝<ε半，所以U, n U2，屏蔽层上分配的压降很小，当电缆尺寸一定时，U1/ U2 ∞ ε2/ ε1。同时通过对具有半导电屏蔽电缆的长期工频、短期工频和冲击电压破坏特性的研究，发现半导电屏蔽层有明显减小交流长期破坏的作用，这种作用在于长期交流防止了电晕的发生。