AN OVERVIEW OF WATER-RESISTANT CABLE DESIGNS

Manufacturers and users have widely recognized water treeing as a long -term failure mechanism of medium and high voltage (15 kV and above) c able. There are many cable constructions and materials which combat tr eeing. It is important to keep abreast of new constructions and materi als. They have different applications, they work differently, some are more available than others, and they are tested differently. The purp ose of this paper is to supply the end user with an overview of the mo st current information on several different constructions and material s that help combat water treeing. Since the mid-1970's manufacturers a nd users have known dielectric treeing as a failure mechanism. Since t hen, many people have studied treeing. The result has been a wide rang e of materials and constructions to combat this problem. Treeing is th e breakdown of the insulation. This breakdown resembles a tree or bush , when examined by a microscope. When treeing starts on the semiconduc ting conductor shield or insulation shield, the ''trunk'' of the tree is on the shielded material and the ''branches'' fan away. When the tr eeing occurs within the dielectric, the tree starts at a small point a nd fan away in each direction, making a ''Bow Tie'' tree. Treeing is a long-term failure mechanism for extruded dielectrics. Sometimes it ta kes decades to fail. In addition, the formation of trees does not mean the dielectric will fail. Often, trees will begin formation, slow dow n and then stop formation, so the dielectric will remain operative for an indefinite period of time. Voltage stress and exposure to water ac celerate the tendency of the dielectric to tree. Trees forming due onl y to high electrical stress are electrical trees. Trees forming due to exposure to water are electro-chemical or water tress. On exposure, w ater molecules penetrate the dielectric material. These molecules cong regate in amorphus (non-crystalline) areas in the dielectric and event ually cause deterioration of the insulation material. To; evaluate the performance of different methods of coping with treeing, one can do a n Accelerated Cable Life Test.