There is a grounded (copper) shielding layer outside the core, and a radial distribution electric field is formed between the conductive wire core and the shielding layer. In other words, the electric field of the normal cable only from the (copper) wire along the radius to (copper) shielding layer of the power line, there is no core spool to the electric field (power line), the electric field distribution is uniform. When making the cable head, the shielding layer is peeled off, and the original electric field distribution of the cable is changed, which will produce a tangential electric field which is extremely unfavorable to the insulation (the power line along the axial axis of the wire). The power line stripping the shielding layer core wire is concentrated at the fracture of the shielding layer. Then at the fracture of the shielding layer is the most easily penetrating part of the cable.

The cable is the easiest to penetrate the shielding layer at the fracture, we take the dispersion of this concentrated power line (electrical stress), with the dielectric constant as 20~30, volume resistivity for 108~1012Ω·cm material made of the electrical stress Control tube (referred to as the stress tube), set at the fracture of the shielding layer, to disperse the fault at the electric field stress (power line), Ensure that the cable can operate reliably. In order to make the cable run reliably, the stress tube in the cable head is very important, and the stress tube can achieve the effect of dispersing the electrical stress without destroying the main insulating layer. In the cable body, the outer surface of the core line cannot be a standard circle, the distance of the core line to the shielding layer will be not equal, according to the electric field principle, the electric field strength will also have the size, which is also unfavorable to the cable insulation. In order to make the internal electric field of the cable as uniform as possible, there is a circular semiconductor layer outside the core line, which makes the thickness of the main insulating layer basically equal and achieves the goal of uniform distribution of electric field. Outside the main insulating layer, the outer semiconductor layer in the copper shielding layer is also set up to eliminate the unevenness of the copper shielding layer and prevent the electric field from being uneven. In order to minimize the stress dispersion of the cable at the fracture of the shielding layer, the contact length of the stress tube and the copper shielding layer is not less than 20mm, the contact surface of the stress tube will be insufficient, the power line on the stress tube will be insufficient (because the length of the stress tube is certain), the electric field dispersion area (segment) will be reduced Generally around the 20~25mm. When making the intermediate joint, the main insulating layer must also be peeled off part, the core wire with copper to take over the crimping, with the filler Bao ping (circle). There are two ways to make it:

1. Heat shrinkable Casing: The main insulated casing made of heat shrinkage material is shrunk (it is recommended to choose the mother drain heat shrinkable casing), the main insulated casing shrinkage semiconductor tube, and then package the metal shielding layer, and finally the outer casing.

2. Prefabricated accessories: The materials used are generally silicone rubber or ethylene propylene rubber. As a hollow cylindrical body, the inner hole wall is a semiconductor layer, and the semiconductor layer is the main insulating material outside.

Prefabricated installation requires a high heat shrinkable casing, which is difficult. The aperture of tubular precast parts is smaller than that of the main insulating layer of cable 2~5mm. The intermediate joint precast tube shall be set at both ends outside the main insulating layer of the cable, and the length of each connection with the main insulating layer shall not be less than 10mm. There is no need to sharpen the pencil head on the main insulating head of the cable (keep the semiconductor layer as far as possible on the cable core line). Copper takeover surface to deal with smooth, coated with appropriate amount of filler.

Measures to improve the distribution of electric fields

In the power cable connectors of 35kv and below, there are several ways to improve the electric field distribution at the disconnection of the sheath.

(1) Swelling horn: In the lead bag cut at the edge of the lead bag pry up, into a horn shape, its edge should be smooth, rounded, symmetrical.

(2) reserved turnkey insulation: A packet of insulating paper is left between the lead-bag incision and the separation point of the cable core line.

(3) Resection of semi-conductive paper: the semi-conductive paper is removed below the horn.

(4) wrap around the stress cone: with insulating straps and conductive metal materials wrapped into a conical, artificially expand the shielding layer to improve the electric field distribution.

(5) Equipotential method: For dry package or crosslinked polyethylene cable head, wrap a metal band on the insulating surface of each line core profile and connect it together.

(6) Installation of stress control tubes: for 35kv and below the heat shrinkable casing cable head, first from the wire core copper shielding layer end direction through the semiconductor belt to the wire core insulation profile wrapped around 2 layers of semiconductor belt, and then the corresponding specifications of the flexural stress tube, set at the end of the copper shielding, thermal shrinkage forming.

At present, there are two main types of measures to improve the distribution of electric field in medium voltage cable accessories. One is geometry: It changes the electric field distribution by changing the geometry at the voltage concentration in the cable attachment, reducing the electric field intensity, such as the packet stress cone, the precast stress cone, the pencil-cutting head, the swelling horn and so on. The second is the parametric type: It is the stress control layer made of a certain parametric material on the insulation at the copper shielding cutting end of the cable terminal, which changes the potential distribution of the insulating layer surface and achieves the goal of improving the electric field distribution. Such as common stress control tubes, stress bands and so on.

热缩套管在解决高压电缆故障中的应用-保护电场

芯外均有一接地的（铜）屏蔽层，导电线芯与屏蔽层之间形成径向分布的电场。也就是说，正常电缆的电场只有从（铜）导线沿半径向（铜）屏蔽层的电力线，没有芯线轴向的电场（电力线），电场分布是均匀的。在做电缆头时，剥去了屏蔽层，改变了电缆原有的电场分布，将产生对绝缘极为不利的切向电场（沿导线轴向的电力线）。在剥去屏蔽层芯线的电力线向屏蔽层断口处集中。那么在屏蔽层断口处就是电缆最容易击穿的部位。

电缆最容易击穿的屏蔽层断口处，我们采取分散这集中的电力线（电应力），用介电常数为20~30，体积电阻率为108~1012Ω•cm 材料制作的电应力控制管（简称应力管），套在屏蔽层断口处，以分散断口处的电场应力（电力线），保证电缆能可靠运行。要使电缆可靠运行，电缆头制作中应力管非常重要，而应力管是在不破坏主绝缘层的基础上，才能达到分散电应力的效果。在电缆本体中，芯线外表面不可能是标准圆，芯线对屏蔽层的距离会不相等，根据电场原理，电场强度也会有大小，这对电缆绝缘也是不利的。为尽量使电缆内部电场均匀，芯线外有一外表面圆形的半导体层，使主绝缘层的厚度基本相等，达到电场均匀分布的目的。在主绝缘层外，铜屏蔽层内的外半导体层，同样也是消除铜屏蔽层不平，防止电场不均匀而设置的。为尽量使电缆在屏蔽层断口处电场应力分散，应力管与铜屏蔽层的接触长度要求不小于20mm，短了会使应力管的接触面不足，应力管上的电力线会传导不足（因为应力管长度是一定的），长了会使电场分散区（段）减小，电场分散不足。一般在20~25mm左右。在做中间接头时，必须把主绝缘层也剥去一部分，芯线用铜接管压接后，用填料包平（圆）。有二种制作方法：

1. 热缩套管: 用热缩材料制作的主绝缘套管缩住（建议选用母排热缩套管），主绝缘套管外缩半导体管，再包金属屏蔽层，最后外护套管。

2. 预制式附件: 所用材料一般为硅橡胶或乙丙橡胶。为中空的圆柱体，内孔壁是半导体层，半导体层外是主绝缘材料。

预制式安装要求比热缩套管高，难度大。管式预制件的孔径比电缆主绝缘层外径小2~5mm。中间接头预制管要两头都套在电缆的主绝缘层外，各与主绝缘层连接长度不小于10mm。电缆主绝缘头上不必削铅笔头（在电缆芯线上尽量留半导体层）。铜接管表面要处理光滑，包适量填料。  

改善电场分布的措施

在35kv及以下电力电缆接头中，改善其护套断开处电场分布的方法有几种

(1)胀喇叭口：在铅包割断处把铅包边缘撬起，成喇叭状，其边缘应光滑、圆整、对称。

（2）预留统包绝缘：在铅包切口至电缆芯线分开点之间留有一段统包绝缘纸。

（3）切除半导电纸：将半导电纸切除到喇叭口以下。

（4）包绕应力锥：用绝缘包带和导电金属材料包成锥形，人为地将屏蔽层扩大，以改善电场分布。

（5）等电位法：对于干包型或交联聚乙烯电缆头，在各线芯概况绝缘表面上包一段金属带，并将其连接在一起。

（6）装设应力控制管：对于35kv及以下热缩套管电缆头，首先从线芯铜屏蔽层末端方向经半导体带至线芯绝缘概况包绕2层半导体带，然后将相应规格折应力管，套在铜屏蔽的末端处，热缩成形。

目前中压电缆附件中改善电场分布的措施主要有两大类型。一是几何型：是通过改变电缆附件中电压集中处的几何形状来改变电场分布，降低该处的电场强度，如包应力锥、预制应力锥、削铅笔头、胀喇叭口等。二是参数型：是在电缆末端铜屏蔽切断处的绝缘上加一层一定参数材料制成的应力控制层，改变绝缘层表面的电位分布，达到改善该处电场分布的目的。如常见的应力控制管、应力带等。