Water Treeing Phenomenon
The electrical degradation process of treeing in polymer occurs in the presence of electricsl stress and moisture. This is acknowledged as a leading cause of nonmechanical insulation degradation and cable failure.
The water-treeing process may be consider to occur in three phases (figure 1)
Initiation is due to uneven stress at interface and or contamination (micron level) in insulation.
Growth continues in the presence of moisture (in ppm levels) under electrical stress , with the water tree growing along electrical field, contributing to lowering the electrical strength of the insulation/cable. When the water tree gets long enough, with the electrical strength of the insulation being quite low, any surge due to swithching or lightening can lead to ‘bridging’, formation of ‘electrical tree’, leading to rapid cable failure.