Challenging Defects of High Voltage Insulation Systems

Abstract

The primary function of high voltage insulators is to insulate, i.e., prevent the flow of electric current and to keep oppositely charges conductors mechanically separated during all service conditions. This means that the insulation of a power apparatus is designed to withstand any electrical, thermal, and mechanical stress likely to occur during manufacturing, testing, and the long-expected service lifetime of 30 years or more. Typical insulation systems, therefore, constitute combinations of different gasses, oils, and solids. Experience shows that important characteristic properties, as electric breakdown strength, electric conduction, dielectric loss, and long-term endurance, are limited by minute amounts of additives, voids, particles, and other impurities, embedded in the insulation or at electrode interfaces. Such composite systems are difficult to describe in terms of the characteristics of each individual component, and efforts to improve manufacturing and refine the quality of materials are therefore often based upon an empirical approach.In this paper, a more theoretical approach is taken, aiming at predicting how important properties are affected by different types of impurities and additives. In the case of defects evenly distributed within the insulation, electric breakdown strength becomes the weakest link mechanism described by extreme value statistics. In practice, electrical conduction current originates from many different moving charge carriers, i.e., moving ions, particles, and electrons. At moderate and high electric stress, these charge carriers result in a different and highly non-linear increase in current versus applied voltage. The value of permittivity and dielectric loss may increase, even at power frequencies, due to interface charging and increased conductivity of the materials. Voids and particle inclusions act as starting points for partial discharge and electrical treeing, resulting in a premature electric breakdown. Absorption and condensation of water lead to slow water tree degradation and reduced serviceability of polymer insulated cables.It is concluded that in order to advance the science of high voltage insulation engineering, a fundamental approach is needed to understand and utilize the complex physical mechanisms governing and limiting the properties.The term insulating material has no meaning unless its molecular microstructure, crystallinity, content of byproduct from the manufacturing process and other additives are clearly defined.