Raji Sundararajan

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

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.

In-service aging and degradation of 345 kV EPDM transmission line insulators in a coastal environment

Aging and degradation of 345 kV ethylene propylene diene monomer (EPDM) transmission line insulators removed from service is presented. These suspension type insulators were installed in a New Hampshire coastal area in 1995 and were removed in 2000 after unexplained outages in that structure. The purpose of this paper is to better understand the aging and degradation mechanisms of EPDM insulators in service. The insulators showed severe chalking and discoloration and partial loss of hydrophobicity on the side facing the sun. The surface structural changes were studied in detail using advanced surface analysis techniques, such as attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR), scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). For the first time, the significant differences in surface properties between the chalked/discolored (white) and the other surfaces (dark) were studied quantitatively. The Fourier transform infrared (FTIR) absorption spectra showed a significant decomposition of the CH groups of the white surface, elucidating the effect of photo-oxidation on the EPDM polymer. The SEM micrographs showed the cracking of the surfaces. The XPS spectra showed the formation of various polar carboxyl groups and the presence of high surface energy compounds, such as silica, and silicates. This study provided valuable basic information on the changes in the surface properties of EPDM insulators during service in a coastal environment.

Insulator Icing Test Methods and Procedure

This is a position paper prepared by the IEEE Task Force on Insulator Icing Test Methods. 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 Standard 4-1995 on standard techniques for high-voltage testing.

Selection of station insulators with respect to ice and snow-part I: technical context and environmental exposure

In selecting adequate insulators for substations at distribution and transmission voltage levels, special measures may be needed in locations exposed to freezing conditions. This first part of the paper describes the environmental and insulator parameters that influence the risk of flashover on station insulators exposed to freezing conditions.

Role of non-soluble contaminants on the flashover voltage of porcelain insulators

The influence of non-soluble or inert contaminants on the flashover voltage of HV outdoor porcelain insulators is examined. A dynamic arc model, presented earlier for predicting the flashover voltage of porcelain insulators covered with water soluble contaminants, has been modified suitably. The extended model has been used to predict the flashover voltage of suspension and station post insulators of various shapes for both ac and dc voltages. Good correlation with experimental data has been shown. The model can thus be used as a computer aided tool for design and maintenance of HV porcelain insulators.

Selection of station insulators with respect to ice and snow-part II: methods of selection and options for mitigation

In locations exposed to freezing conditions over the station service life, additional measures for selecting adequate station insulators at distribution and transmission voltage levels should be taken. This second part of the paper describes the selection process and mitigation options, based on the environmental parameters, as discussed in Part I. These include insulator size, shape, surface material, surface quality, electric field improvement, and orientation.

Computer-aided design of porcelain insulators under polluted conditions

The development of a user-friendly, interactive personal computer package for designing insulators to be used in polluted conditions is presented. This is accomplished by integrating a dynamic arc model that computes the pollution flashover voltage within the Microsoft Windows application program. Suspension, station post and pin type insulators of numerous shapes have been incorporated into the package. The model is capable of handling AC and DC voltages, in addition to uniform and nonuniform pollution distributions on the insulator surface. The use of the model as a design tool to predict the insulation requirement for a given system voltage, and as a maintenance tool to determine the critical contamination level (in terms of ESDD) that will cause flashover of the insulator at the operating voltage, has been demonstrated. Wherever possible, comparisons of the model predictions with experimental data have been made, and good agreement has been obtained. >

Multistress accelerated aging of polymer housed surge arresters under simulated coastal Florida conditions

Long-term performance of polymer housed surge arresters under multistress accelerated conditions simulating coastal Florida service environment in the lab has been presented. Commercial 9 kV ethylene propylene silicone blend (EPSB) and silicone rubber (SiR) arresters were procured and used for this purpose. Multi-stress aging was used to realistically and meaningfully simulate the actual service environment that has the synergy of several environmental stresses. The various stresses applied include ultraviolet-A radiation (UVA), rain, heat, clear mist, and salt fog. Summer and winter weather cycles were designed to apply these stresses cyclically as they occur in the service environment. This method provides a comprehensive evaluation of the insulator, the hardware and its attachment. Coastal Florida conditions were chosen as the year-around warm temperatures and the high humidity challenge any insulating material, ceramic or polymeric. The evaluation was conducted for a period of 15 simulated service years of aging (15 lab-years) in the multistress accelerated aging chamber. Compared to the unaged (virgin) sample, the EPSB arresters showed chalking, reduced hydrophobicity, and reduction in hydrocarbon (CH) group molecules in the attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) measurements. The SiR arrester surfaces showed no noticeable changes in the FTIR measurements. However, scanning electron microscopy (SEM) micrographs indicated increased surface roughness and disintegration of the material for both the SiR and EPSB surfaces. Research results indicated that multistress accelerated tests are desirable since they reflect the real world.

Linear stochastic analysis of polluted insulator leakage current

The measurement and analysis of leakage current (LC) for condition-based monitoring of, and as a means of predicting flashover of, polluted insulators has attracted a lot of research in recent years. Results presented in the technical literature show that surface arcing of contaminated insulators causes significant variations in both the magnitude and shape of the insulators leakage current, and several techniques have been proposed for analysis. This paper submits the results of applying linear stochastic and statistical (probabilistic) analysis as a signature analysis tool for flashover prediction. A new twist is presented based on statistical analysis of the leakage current envelope (rather than just instantaneous peak values), in particular, level crossings, mean exceedances over high thresholds, and extreme value analysis of the envelope. The analysis was performed offline using digitized samples of leakage current obtained from a data acquisition system, which continuously scans the leakage current during flashover tests in a laboratory fog chamber.

Effect of insulator profiles on DC flashover voltage under polluted conditions. A study using a dynamic arc model

From field experience and laboratory tests, it has been observed that the contamination flashover voltage of insulators used on DC lines is more influenced by geometry (diameter and profile) than those used on AC lines. Presented in this paper are the results and analysis of a study of the effect of insulator shapes on the DC contamination flashover voltage. A dynamic arc model is used for this purpose. The salient feature of the model is that it takes into account the geometry of the insulator at every instant of the arc propagation, and thus includes the role of geometry in the contamination flashover process. A variety of porcelain cap and pin insulator geometries that are widely used in practice have been evaluated using this model. Nonuniform pollution distribution on insulators, which is more representative of field conditions, has also been considered, and its effect on the flashover voltage has been evaluated for the various geometries. Results are in good agreement with the experimental findings. Use of the model as a design tool to select type and number of insulators for HVDC transmission lines has been illustrated. >