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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