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Flashover Voltage of Non-ceramic Insulators under Contaminated Conditions

FLASHOVER under contaminated conditions is a well known weakness of outdoor ceramic insulators (porcelain andglass). There are various types of contaminants that settle onthe insulators. Insulators located near coastal regions encounter sodium chloride (NaCl), those located in inland areas are contaminated by industrial pollutants such as paper pulp, fly-ash, cement dust etc. Insulators in cold climates are contaminated with salts used for deicing the streets (brine, calcium chloride). These contaminants can include a mixture of soluble and insoluble materials. The term ESDD (Equivalent Salt Deposit Density) is used to indicate the contamination severity and is expressed in terms of mg of NaCl per unit surface area (cm2) of the insulator. Contamination is of little significance under dry conditions. However in the presence of light rain, freezing rain, fog or dew the contaminants dissolve to form a conductive layer on the insulator surface initiating leakage current and partial arcs (dry band arcing) which can ultimately lead to flashover. The creation and propagation of the dry band arcing is aided by the hydrophilic or wettable nature of ceramic materials.
Users have handled this weakness of ceramic insulators by adopting conservative design rules that have been derived from field experience and laboratory tests. For new construction, relevant field experience may not be available and laboratory experiments are time consuming and expensive. A good theoretical model for simulating the flashover process is an asset as it helps to minimize experimental efforts. Theoretical models for calculating the flashover voltage of ceramic insulators under contaminated conditions were introduced in the 1950s and have undergone significant Manuscript received on 1 August 2005, in final form 9 November 2005. development. These models have helped manufacturers to optimize design and users to specify the right level of insulation required.
Nonceramic insulators (NCI) made from polymeric materials were introduced in the 1970s. This has helped users significantly as the flashover performance under contaminated conditions of these insulators is significantly better than the ceramic insulators. NCI are different from their ceramic counterparts in several respects. The housing materials are known to possess water repellent characteristics, commonly referred to as hydrophobicity, and is chiefly responsible for the improved performance under contaminated conditions. However the longevity of hydrophobicity is highly variable. It is short for some polymers such as ethylene propylene family of polymers and can be long for the silicone rubber family of polymers. Both types of materials are used presently for transmission and distribution lines.
Hydrophobicity is affected by surface contamination and aging of the material that occurs under various stresses (electrical, mechanical, chemical, environmental) encountered in service. NCI also differ significantly from ceramic insulators in the constructional aspect. For example, suspension NCI have no intermediate hardware (cap and pin), and this alters the electric field distribution significantly from ceramic insulators [1]. These differences are known to affect the flashover process. The phenomenon has not been studied adequately and hence modeling of the flashover process for NCI is still in its infancy.

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