According to the reasons for the formation of "branches" and their insulation damage, can be divided into "electric branches" and "water branches" two, the previous article introduced the "Electric Branch", the following introduction to the "Water branch" that is, heat shrinkable tube insulated Water Branch aging.

For the last 15 years, the Water branch (Water Tree) has been considered to be an important cause of insulation aging in high-pressure heat shrinkable tubes. Water branches may be found more or less in the insulation of heat shrinkable tubes. Two forms of water branches can be found in the heat shrinkable tube insulation: Tubular water branches (vented tree) and butterfly water branches (bow-tie tree).

Tubular water branches are generally emitted from the interface of internal semiconductors, shielding layers and insulating layers. When the semiconductor layer and the insulating layer are extruded at the same time, the electric field concentration on the semiconductor interface is greatly reduced, and the tubular water branches are rare in the normal production of the heat shrinkable tube insulation. The butterfly Water Branch, which is common in heat shrinkable tube insulation, develops from one impurity point or other electric field concentration point to both sides into a bow shape. The length and quantity of butterfly water tree are different depending on the moisture content in the thermal shrinkage tube. The longest butterfly water tree can reach 600~800 um, with a maximum of twenty or thirty per cubic millimeter. When insulation such as heat shrinkable tubes contain moisture (not necessarily saturated), and under the action of a low electric field intensity (as in a Work field), the water branches described above are produced in its key parts.

Water branches consist of Micro-cavitis or micropores (micro-voids), which do not appear to be connectivity to each other. The size of the micropores and gaps is almost equal, about 1~2µm, but in different positions in the water branches, the number of holes and gaps in the unit volume is changed. The density of microporous is closely related to the intensity of electric field, and the larger the electric field, the greater the density. Regardless of the mechanism of the microporous and Gap, the production of water branches is always related to the local electric field strength and the insulating water saturation. The first microporous (GAP) appears, that is, the beginning of the water branch, always where the electric field is the largest on the boundary between insulation and moisture contact.

Once the water branch occurs, it will develop gradually under certain conditions. According to the thermodynamic point of view, it can be proved that when the saturation ratio of water in heat shrinkable tube insulation exceeds a certain range, the water branches will continue to develop and grow after generation. The saturation ratio that contributes to the production of water branches is at least 0.4.

People put forward a lot of theories on the mechanism of the emergence and development of water branches, but there is no consistent statement. The main theories can be divided into two categories, chemical action theory and mechanical action. The chemical theory holds that the formation of water branches is due to the injection of electrons, which causes chemical changes or chemical reactions, resulting in local chemical damage to the insulators. The view of mechanical action theory is that the water branch is caused by the mechanical super stress (Mechanical overstressing) part of the insulator. There is also reason to believe that there is a link between the two, mechanical force can strengthen the chemical action, and chemical aging will also reduce the mechanical strength of the polymer.

The water branch does not directly cause insulation breakdown, it has to have an intermediate process that breeds electric branches. But electric branches are not necessarily bred, and even if the water branches develop to penetrate the insulation, the insulators can remain at operating voltages for many days without being penetrated. Water branches in the development process even if the length is no longer increased, the internal structure is also changing, brewing power to guide the branches, so as to penetrate the insulation of the heat shrinkable casing. Whether the water branch can directly lead to insulation breakdown, it will always reduce the insulation strength, play a long role in insulating aging.

For more information, please contact to

lily@gzhelectric.com

热缩管绝缘水树枝老化

按“树枝”形成的原因及其所起的绝缘破坏作用，可分为“电树枝”及“水树枝”两种，上一篇介绍了“电树枝”，以下就介绍“水树枝”即热缩管绝缘水树枝老化。

最近15年来，水树枝(Watet Tree)被认为是导致高压热缩管绝缘老化的重要原因。在热缩管的绝缘中都可能或多或少地找到水树枝。热缩管绝缘中可以找到两种水树枝形式:管状水树枝(Vented Tree)及蝶状水树枝(Bow-tie Tree)。

管状水树枝一般是从内半导体、屏蔽层与绝缘层的界面上导发出来的。当采用半导体层与绝缘层同时挤出工艺以后，半导体界面上产生电场集中的情况大为减小，正常生产的热缩管绝缘中，管状水树枝己不多见。在热缩管绝缘中常见的蝶状水树枝，它从一个杂质点或其他电场集中点向两边发展成一蝴蝶结形状。视热缩管中含水量不同，蝶形水树的长度和数量有所不同。最长的蝶形水树可达600~800 um，每立方毫米最多可有二三十个。热缩管等绝缘含有水分时(不一定要饱和时)，并在不高的电场强度(如在工作场)作用下，就会在它的关键部位产生如上所述的水树枝。

水树枝由微隙(Micro-cavitis)或微孔(Micro-voids)所组成，此等微隙、微孔看来未必互相通连。微孔、隙的大小几乎是相等的，约为1~2µm，但在水树枝不同的地位，单位体积内的孔、隙数是有变化的。微孔的密度与电场强度有密切关系，电场愈大，密度愈大。不管产生这种微孔、隙的机理如何，水树枝的产生总是与局部的电场强度大小和绝缘含水饱和度有关。第一微孔(隙)出现，即水树枝的起始，总是在绝缘与水分接触的分界上的电场最大的地方。

水树枝一经发生，在一定条件下会逐步发展。根据热动力学的观点，可以证明当水在热缩管绝缘中的饱和比超过一定范围时，水树枝在生成后会继续发展和增长。促成水树枝产生的饱和比至少要超过0 .4。

人们对水树枝的产生和发展的机理提出不少理论，但尚无一致的说法。主要理论可分为化学作用说和机械作用说两大类。化学论的观点认为水树枝的生成是由于注射进了电子从而引起了化学变化或化学反应，导致了绝缘物的局部化学损伤。机械作用论的观点认为水树枝是由于绝缘体局部受到了机械超应力(Mechanical Overstressing)作用所致。也有理由认为二者之间有一定联系，机械力可以加强化学作用，而化学老化也会降低聚合物的机械强度。

水树枝不会直接导致绝缘击穿，它要有一个孕育电树枝的中间过程。但电树枝并不一定会孕育出来，即使水树枝发展到穿透绝缘，绝缘体也能在工作电压下保持好多天不被击穿。水树枝在发展过程中即使长度不再增加，内部结构也在变化，酝酿着导发电树枝，以至击穿热缩套管的绝缘。不管水树枝能否直接导致绝缘击穿，它总会降低绝缘强度，起着漫长绝缘老化作用。

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