HV composite polymeric insulators are being accepted increasingly for use in outdoor installations by the traditionally cautious electric power utilities worldwide. They currently represent -60 to 70% of newly installed HV insulators in Nortb America. The tremendous growth in the applications of non-ceramic composite insulators is due to their advantages over the traditional ceramic and glass insulators. These include light weight, higher mechanical strength to weight ratio, resistance to vandalism, better performance in the presence of heavy pollution in wet conditions, and comparable or better withstand voltage than porcelain or glass insulators. However, because polymeric insulators are relatively new, the expected lifetime and their long-term reliability are not known and therefore are of concern to users. Additionally they might suffer from erosion and tracking in the presence of severe contamination and sustained moisture. This leads to the development of dry band arcing that under certain circumstances could lead to failure of polymer insulators. In this paper a review is presented of the recent performance experience of HV composite polymeric insulators in outdoor service, testing methods, aging, the ranking of the materials, the role of fillers, the role of low molecular weight components present in the insulators, the mechanisms responsible for the loss and recovery of hydrophobicity, one of the most important properties of polymers, the mechanisms of failure, detection of faults, type and quantity of natural contaminants, effects of exposure to rain, hydrocarbons, stationary air and wind, various methods to optimize the electrical performance and a relatively new method for evaluating the performance status of polymeric insulators in the field.

Outdoor HV composite polymeric insulators

This paper proposes to survey a good part of the research work accomplished to date on the atmospheric icing of conductors and insulators in the presence of high voltage, with emphasis laid on the studies carried out at the University of Quebec in Chicoutimi. The review covers laboratory testing and mathematical modelling. The role of several electrical parameters, such as electric field strength and polarity, corona discharge, water droplet charge and ionic wind velocity, on the structure and amount of ice accretion on high–voltage conductors, is discussed. Concerning the icing of insulators, the initiation of electrical discharge on the ice surface, the formation of local arcs along the air gaps and their development to a flashover arc along the insulators are discussed. Basic experiments on the role of several major parameters relating to ice accretion, insulator characteristics and voltage type and polarity, on the maximum withstand voltage of short insulators, are also discussed. Finally, several measures for improving the withstand voltage of insulators are briefly recalled.

Ice accretions on high–voltage conductors and insulators and related phenomena

Test methods for evaluating flashover voltage of ceramic and nonceramic insulators under ice, snow, and cold-fog conditions are recommended. The paper describes the procedure to be followed at various stages of an evaluation test including preparation of the test object, insulator precontamination, ice, snow, and cold-fog deposits as well as voltage applications and flashover voltage evaluation. Some relevant parameters for ice accretion, flashover voltage evaluation, artificial pollution, and power source requirements are taken from IEC 60129, IEC 60507, and IEEE Std.-4-1995 on standard techniques for high-voltage testing.

Insulator icing test methods and procedures: a position paper prepared by the IEEE task force on insulator icing test methods

This paper first presents two methods for testing insulators under icing conditions. These methods were based on and developed from those described in the standard of IEC 507, which gives instructions for testing insulators under pollution. Then, the effects of various factors, including wind velocity, ice thickness, dry arcing distance of insulators, and conductivity of freezing water, on the flashover performance of a short string of IEEE standard insulators, were investigated. The experiments were carried out in a cold room. Moreover, the repercussions of ice on actual insulators were evaluated by examining the maximum withstand stress of the insulators employed on Hydro-Quebec networks and, by comparison, the effects of ice with salt contamination.

Flashover performance of IEEE standard insulators under ice conditions

Based on the results of experimental investigations performed on the polluted and iced IEC standard suspension insulator strings of 3~21 units, the ac flashover performance and discharge process are discussed in this paper. It is put forward that the icing stress product (ISP), defined as the product of the ice-layer conductivity and the ice layer weight per centimeter of dry arc distance can be taken as a characteristic parameter to indicate the effect of ice accretions and contaminants on the flashover voltage. The flashover stress of an ice-covered insulator string can be expressed as a power function of ISP and has a linear relationship with the length of the insulator string. The partial arc of an iced insulator string is initiated by the breakdown of the air gaps between the tips of icicles and iced insulator surface and the persistence is much longer than that of a polluted insulator string

Study on AC Flashover Performance and Discharge Process of Polluted and Iced IEC Standard Suspension Insulator String

The electrical performance of outdoor insulation degrades severely during combinations of factors that include surface contamination (C), ice (I), fog (F) and an ambient temperature that rises through 0 C (T{sub 0{degree}}). Failures at operating voltage on 115-kV, 230-kV and 500-kV systems occur with increasing probability under these conditions. A new CFT{sub 0{degree}} or cold-fog test method has been developed to reproduce the flashovers at all three voltage levels. Three options are identified for improving CFT{sub 0{degree}} performance: use of semi-conductive glazes, substitution of silicone for porcelain and use of silicone coatings on existing porcelain insulators.

The Cold Fog Test

The electrical performance of outdoor insulation degrades severely during combinations of factors that include surface contamination, ice, fog and an ambient temperature that rises through 0/spl deg/C (T(0/spl deg/)). Failures at operating voltage on 115 kV, 230 kV and 500 kV power systems occur with increasing probability under those conditions. A new CFT(0/spl deg/) or "cold-fog" test method has been developed to reproduce the flashovers at all three voltage levels. Three options are identified for improving CFT(0/spl deg/) performance: use of semi-conductive glazes; substitution of silicone for porcelain; and use of silicone coatings on existing porcelain insulators.

Y. Tam

Papers

The Cold Fog Test

The cold-fog test [for outdoor insulators]