Composite insulators are considered mature products and a broad range of insulator types is nowadays available, including line insulators, hollow-core insulators and apparatus insulators, for applications ranging from distribution voltages to the highest transmission voltages. This development has created a need to elaborate diagnostic methods and tools for determining the quality of composite insulators. A review on recognized insulator faults and methods for their detection is provided.

Diagnostic Methods for Outdoor Polymeric Insulators

This paper provides an overview of the electric field (E-field) distribution on transmission line composite insulators applied in alternating current applications. Factors that affect the E-field distribution are discussed as well as the influence of the E-field distribution on the short and long term performance. Modeling and measurement methods are reported and examples of calculated E-field magnitudes determined are presented together with corona ring application information. This paper was developed by the IEEE Task force on electric fields and composite insulators.

Electric Fields on AC Composite Transmission Line Insulators

This paper surveys the properties which give silicone rubber its distinctive and highly desirable performance characteristics for use as outdoor insulation. The methods of assessing these properties arediscussed and this is followed byadetailedsurveyof whatis knownof agingaud recovery mechanisms. Corona is identified as the principle electrical aging agent and evidence is presented for irreversible loss of polymer as a result of aging. However, the surface recov. ery mechanisms are robust and, provided there is an adequate resting period, the hydrophobic properties recover, This supports the view, from service experience, that there is no substantial evidence to show that normal aging, unless it results in the exposure of the glass fiber core, detracts from the performance of insulators.

Review of aging and recovery of silicone rubber insulation for outdoor use

The influence of long-term corona on the degradation of non-ceramic composite insulator (NCI) housing materials was studied. Polymer samples were obtained from full-scale NCI, employing three different housing materials, namely silicone rubber, ethylene propylene diene monomer rubber and a blend of EPDM with silicone rubber. The effect of relative humidity and externally applied mechanical stress as acceleration factors was assessed. Several modes of degradation were observed, namely, cracking, roughening, and discoloration. In general the severity of degradation increased with the relative humidity level and application of mechanical stress. Periodic monitoring of corona discharge magnitude and pulse repetition rate allowed for a simplistic computation of the energy needed for initiation of surface changes. A comparison of these calculated values with expected conditions in the field was made, It is postulated that even under severe operating conditions over a period of many years, degradation of the housing materials evaluated due to water droplet corona is highly unlikely.

Effect of long-term corona on non-ceramic outdoor insulator housing materials

This paper presents measurements of partial discharge (PD) from water droplets on a silicone rubber insulating surface in an ac field. Tests are done by placing droplets on the surface using a micro-pipette, and by condensing water in a controlled humidity chamber. The field enhancement factor due to droplets placed on the insulation surface is experimentally and numerically determined. Values in the range of 3.8 to 6.3 are obtained depending on the size and number of droplets. The time resolved analysis of PD from water condensation indicates a threshold field, between 3.0 and 3.5 kV/cm, above which the PD activity increases with time, and brings the insulation to a surface breakdown. The corresponding field enhancement factor obtained for the condensation experiments is between 5.8 and 6.8. The results therefore suggest that the PD activity on the hydrophobic surface can be used effectively to learn about the insulation surface conditions.

A study of partial discharges from water droplets on a silicone rubber insulating surface

The fundamental principles of water drop corona as an aging mechanism for nonceramic insulators are presented. It is demonstrated that water drops in the sheath regions enhance the electric field and may cause corona which can play an important role in long-term performance. Electric field enhancement caused by water drops in different locations on the shed and sheath are demonstrated by means of electric field calculations and small scale experiments. The threshold magnitude of the surface electric field for corona from water drops is presented for two silicone rubber surfaces having different hydrophobic properties. The effect of water drop corona activity on the properties of the surface material is shown using small scale aging experiments.

Aging of nonceramic insulators due to corona from water drops

It is shown that certain regions of commercially available 500 kV non-ceramic insulators have surface electric fields that are above the threshold for water drop corona. Observations made in a full scale accelerated aging chamber, and in service, demonstrate that water drop corona occurs on such insulators. A correlation between the magnitude of the surface electric fields and the condition of the silicone rubber non-ceramic insulator surfaces due to water drop corona is illustrated for two different insulator types in an accelerated aging chamber. Other examples of degradation of nonceramic insulators due to water drop corona are illustrated.

A.J. Phillips

Papers

Aging of nonceramic insulators due to corona from water drops

Water drop corona effects on full-scale 500 kV non-ceramic insulators