Nearly more than 10 years of operation and research have shown that insulation damage of polyethylene, crosslinked polyethylene and some other polymers is mainly preceded by the aging process of the branches. "Branch" (treeing) is an image noun. It is a branch-like discharge or branching aggregation of heat shrinkable tube insulation in aging, affected by electric field, producing weaker parts of the medium. According to the reasons for the formation of "branches" and their insulation damage, can be divided into "electric branches" and "water branches" two kinds, "water branches" also known as "electrochemical branches."

The electric branch is caused by the partial discharge produced by various defects in the thermal shrinkage tube insulation system. It generally can be produced and developed under higher field strength. Most of the discharge of the electric branch begins on the discontinuous interface of the material (air gap, impurity, internal and external semiconductor shielding layer interface, etc.), as well as from the water branch.

Electric branches generally branch clear, branch tube continuous, no moisture inside, pipe wall has coking carbon particles traces. This kind of branch is not recoverable, develops to a certain extent, will form a conductive channel in the insulation, causes the breakdown. There is also a dendritic discharge in the voidless insulation, which is mainly caused by the space load (spacecharge). The injected electron part of the insulation is absorbed into a space load. The space load is gradually accumulated. The generation of an electric branch in a gap insulation is the result of the sudden release of this aggregate space load.

Whether it is the electric branch formed by charge release or partial discharge, it will gradually lead to local breakdown of insulation in the direction of electric field, thus forming a branch-shaped channel. The more the channel extends, the more the insulating distance between the electrodes is shortened. When the field strength is short and should be penetrated, the insulation is quickly penetrated. The formation of electric branches is a relatively slow process.

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热缩管绝缘电树枝老化

近十几年的运行和研究表明，聚乙烯、交联聚乙烯和一些其他聚合物的绝缘破坏主要先经过树枝老化过程。“树枝”( Treeing)是形象名词。它是热缩管绝缘在老化中，受电场影响，产生介质较弱部位的枝状放电或枝状结集。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，“水树枝”也有叫做“电化树枝”的。

电树枝是由热缩管绝缘体系内部种种缺陷所产生的局部放电所导致。它一般在较高场强下才能产生和发展。电树枝的放电多数从材料的非连续介面(气隙、杂质、内外半导体屏蔽层介面等)上开始的，也有从水树枝上导发的。

电树枝一般分枝清晰、枝管连续，内无水分，管壁有焦化炭粒痕迹。这种树枝是不可恢复的，发展到一定程度，会在绝缘中形成一条导电通道，造成击穿。无隙(Voidless)绝缘中也有引发枝状放电，这主要是由空间负荷(SpaceCharge)所导致。在绝缘中注入电子部分被吸收成为空间负荷。空间负荷逐渐积集。在无隙绝缘中产生电树枝就是突然释放这部积集空间负荷的结果。

无论是电荷释放或局部放电所形成的电树枝，都会逐步按电场方向导致绝缘局部击穿，从而形成一条树枝形通道。通道愈延伸，电极间绝缘距离愈缩短。短至应有击穿场强时，绝缘就被迅速击穿。电树枝的形成是比较缓慢的过程。

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