Partial discharge. V. PD in polymer-type line insulators

For pt.IV see ibid., vol.7, no.1, p.20 (1991). Some of the problems related to corona and partial discharge damage to polymer insulators are outlined. The basic construction of polymer insulators for overhead lines, which consist of a core, weathersheds, and metal end fittings, is described. Two types in common use are suspension/dead-end type, which is used where line loads subject the insulator core to tension forces, and the post type, which is used where line loads subject the core to appreciable bending forces in addition to tension forces. The partial discharge performance of polymer insulators, which depends on the choice of material, design, and construction of the insulators, is discussed. A factor common to all designs is poor partial discharge detection sensitivity, in the sense that a partial-discharge-producing defect along the insulator has very small capacitive coupling to the insulator terminals as a result of the large dimensions of the insulator and the small size of the terminals. Sources of cavity discharges are discussed. These can occur in the fiberglass core or in the weathersheds. External discharges, which can take place on the weathersheds or the hardware ends, are also examined.<>