Heat Shrink Tube Insulated Water Tree Aging

- Dec 19, 2018-

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.

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