Influence of humidity and pollution on the dielectric strength of components used in Isolated LPS

The paper presents the evaluation of dielectric strength of components for insulated LPS under the influence of humidity and pollution. The two main insulation technologies are presently used in insulated LPS. These are “Insulating standoffs” supporting conventional down conductors and “Insulating down conductors”. Comprehensive laboratory tests have shown that the isolation capability in both technologies is significantly influenced by the impulse duration which is within the relevant range for real lightning strokes [1]-[4]. Newly arranged lab tests show additional effects caused by the surface conditions due to humidity and pollution. Both effects have to be considered in parallel which is the main aspect of this paper. The results of the lab tests are presented by a set of voltage / time characteristics that allow the evaluation of the combined influence of impulse duration and surface condition on the insulation behavior. The theoretical analysis made is based on the constant-area-criterion (CAC) that is also used for the determination of the k;factor in the IEC standards [5, 6]. In [5] a rod-rod gap in air is used for reference. Up to now this reference has only been treated with dry ideal conditions. The impact of humidity on this reference is a new aspect described in this paper. In order to resolve the quality of an insulated LPS with respect to insulation capability it is necessary to examine the results of lab tests linked with real lightning threat. For the characterization of real lightning threat the statistical distributions of lightning parameters described in IEC 62305-1 171 are used. To make the description of insulating capability comparably independent of the insulation technology the concept of the total probability of failure [10] is applied. As a first example how insulation capability can be improved in the presence of humidity and pollution first results gained with coated insulating stand-offs are presented. Finally a compilation of the degradation effects of humidity and pollution on the insulation capability of components of insulated LPS is given.