Water Treeing in Polymer Cable Insulations

摘要

The behavior of water trees in extruded cables, especially in medium-voltagecables, is inventoried. The phenomenology of water treeing is reviewed, the growth mechanisms of vented trees are evaluated, methods to determine the rate of cable degradation by water treeing are described, as accelerated-aging procedures necessary to arrive at a discrimination between cable insulation with a low or a high water-tree susceptibility. Finally, measures to reduce water-tree growth are reviewed. The effects of electric stress, voltage frequency, temperature, mechanical stress, relative humidity, chemistry and morphological parameters of the insulation on the growth of water trees are also studied. One of the main conclusions reached is that a vented tree can be considered as insulating material. Electrochemical degradation was found to be the cause of vented-tree growth. The effects of electrochemical degradation are consistent with the phenomenology observed. Measures to suppress the growth of water trees are discussed. A test procedure to establish the level of degradation of a cable was developed and its use to investigate a medium-voltage extruded-cable is presented. The rate of degradation and its relation to construction and aging parameters are discussed. The development of a procedure to obtain accelerated aging is described.