C. de Tourreil

Mechanism of Surface Charging of High-Voltage Insulators in Vacuum

In vacuum, the surface of insulators becomes electrically charged when subjected to high-voltage stresses. The charging mechanism is described. A model simulating the charging shows that the surface charge densities are proportional to the applied voltage and depend on the secondary electron emission of the dielectric surface. It is also time dependent. Surface charges are shown to explain many results obtained in studies of insulator surface breakdown.

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.

Surface Resistance Measurements on Nonceramic Insulators

The results and analysis of surface resistance measurements performed on nonceramic insulators (NCI) are presented in this paper. The goal of this study was to determine if the surface resistance measurement procedure can be formalized to ensure repeatable and reproducible results. The NCIs evaluated had housing materials made from one type of silicone rubber and one type of ethylene propylene diene terpolymers (EPDM) rubber. A porcelain post insulator was used as a reference. The Task Force concluded that it is possible to establish a standard for surface resistance measurement in the laboratory. Such a standard is valuable since surface resistance provides: (1) an indication of aging on NCI housing material, and (2) a more sensitive indicator of true contamination severity on insulators than provided by conventional equivalent salt deposit density (ESDD) measurements. As a first step in this direction, guidelines for measurement have been provided in this paper. Suggestions for future work needed for standardization are also included in the paper.

Failure Analysis Of Synathetic Insulators With Fiberglass Rod Submitted To Mechanical Loads

Four end fitting designs of long rod fiberglass core suspension insulators are studied and the failure mechanisms are analyzed after UTS and creep tests. The fracture occurs in the fiberglass rod due to the stresses induced by the metal part of the endfitting that hold the insulating components.

Round Robin Testing of RTV Silicone Rubber Coatings for Outdoor Insulation

This paper presents the results of round robin tests performed to evaluate the electrical performance of Room Temperature Vulcanizing (RTV) silicone rubber coatings used for improving the contamination performance of outdoor ceramic insulators. The test, performed in six different laboratories, included several salt-fog chambers and a tracking wheel. Most test facilities were equipped with an on-line data acquisition system for measurement of leakage current activity. Four generic formulations of RTV silicone rubber coatings applied on ceramic rods were evaluated. The results indicate a general consensus in the ranking of the coatings` leakage current suppression capability, despite the variation in the experimental conditions employed. This suggests that guidelines for a laboratory test for evaluating the RTV coating material`s electrical performance under contaminated conditions could be developed.

Surface Flashover Voltage of Spacers in Vacuum at Cryogenic Temperatures

An electrical insulator in low-temperature equipment is stressed not only by the electrical field but also by mechanical forces, partly due to thermal gradients. At cryogenic temperatures some electrical properties, such as the dielectric losses, may be expected to improve. However, some mechanical properties of the dielectrics may deteriorate and in turn affect their electrical performance.

Description and performance of high voltage synthetic insulators

For very high voltage transmission lines, it may be advantageous or even necessary to replace the conventional porcelain or tempered glass insulator strings by lighter non ceramic insulators. Modern composite materials such as resin bonded glass fibers and filled polymers may be used to produce high strength-to-weight ratio insulators. The electrical and mechanical characteristics of these new synthetic insulators have been found satisfactory in short term evaluation. However, the properties of polymeric materials are expected to change with time when subjected to stresses. The initial results of long term uniaxial mechanical tests and of accelerated aging tests indicate no unacceptable degradation with time. Further investigation and in service evaluation are needed to characterize the long term behaviour and/or possibly optimize the design of these high voltage synthetic insulators.

Effect of electrode profile on the breakdown voltage of polymer sheet materials

Various electrode systems are evaluated for the dielectric strength measurements of solid dielectric plates. It is found that the breakdown test results obtained with metal electrodes embedded in filled epoxy resin yield a better evaluation of the properties of the dielectric material investigated than the results obtained with bare metal electrodes directly immersed in oil.

Evaluation of epoxy resin formulations for SF 6 cable spacer materials

Research is underway at IREQ on the insulation performance of compressed SF6 insulated cables for power transmission at system voltages up to 800 kV. The work reported here is concerned with the choice of materials for the design and testing of model size and full size spacers for 138 kV and 800 kV system voltage respectively. Five epoxy resin formulations were considered together with the effect of the filler. The electrical evaluation of the materials included the study of surface properties such as dry are resistance and tracking and erosion resistance and volume properties such as dielectric losses, permittivity and dielectric strength with point to plane geometry. The mechanical impact resistance was measured. Thermal measurements consisted in the evaluation of the heat distorsion temperature and the coefficient of thermal expansion. The test results and the usefulness of the tests with respect to the judicious choice of spacer dielectric materials are discussed.

Electrical behavior of crosslinked dielectric materials at elevated temperatures

An extensive study has been carried out into the electrical behavior of crosslinkable dielectric materials at elevated temperatures under a reserach contract from the Electric Power Research Institute. This work is part of a program to establish emergency operating guidelines for extruded solid dielectric power cables. Materials studied include XLPE, filled XLPE and EPR compounds. The A. C. and impulse breakdown strengths, A. C. loss and dielectric constant, d.c. resistivity and corona degradation, were investigated for a range, of temperatures between 25°C and 170°C. The effect of the degree of crosslinking in XLPE was investigated and measurements were performed on laboratory molded specimens as well as specimen cut from full size extruded power cables. The results will be discussed in the framework of current models of conduction and breakdown in high polymers. The overall electrical performance of crosslinked dielectric materials at elevated temperature will be discussed as regards to their use in solid dielectric power cables.