Water Trees and Electrical Trees and their role in cable insulation failure

Water trees are tree-like defects, filled with water, which develop in the insulation of cables. The defects usually originate from defects, voids or contaminants. The trees can cause premature failure of the insulation. Water trees usually propagate in the direction of the electric field. They occur only in the presence of water in the insulation. They are usually invisible to the naked eye in the dry condition. Special dying techniques are available which can make them noticeable.

Water trees are found more in sections of cables which are in a state of tension such as in bends. While it is possible to identify the conditions which may cause the formation of water trees, the exact mechanism and the chemical processes involved in their development is not yet fully understood. Water trees reduce the breakdown strength of the cable.

Electrical Trees are formed in the absence of water in dry conditions. They are caused by voids, impurities and defects in the insulation. High electrostatic stress which reverses direction as in AC cables can also accelerate the phenomenon. Occasionally, water trees may evolve into electrical trees. These trees are accompanied with partial discharge which may accelerate insulation failure. Electrical trees are readily visible to the naked eye.

Trees can be classified broadly into vented and bow-tie trees. Vented trees are those which originate from an electrode and reach out to another electrode. These trees grow faster as they have access to air which aids partial discharges.

Bow-tie trees are those that originate inside the insulation. Since they originate inside the insulation they do not have access to air and hence limited partial discharge occurs. They progress slower than vented trees.

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