A critical review of water treeing mechanisms

Water trees increase the susceptibility of insulating materials to breakdown. Water-treeing appears to be an intrinsic degradation mechanism for polymers under AC stress and under humid conditions. It is potentially very harmful as, unlike many breakdown and degradation mechanisms, it occurs under the moderate conditions typical of service for many polymer-insulated cables. Whilst no single theory of water tree inception and growth is generally accepted, a greater consensus on many facets of the mechanisms involved has emerged over the last ten years. Tree growth is obviously complicated and is likely to involve many different physical, chemical, and electrical mechanisms. To further complicate the scenario, some of these mechanisms may not be operative under all conditions. Whilst water tree growth depends upon a wide range of parameters (including electric field; power frequency; electrolyte type, concentration and pH; temperature; dielectric composition, structure and additives and the nature of the electrolyte-dielectric interface) it is clear however that an alternating electric field and a water electrolyte are its pre-requisites