J.R. Laghari

Models for insulation aging under electrical and thermal multistress

A review of the methods and models used in lifetime studies of solid insulators under single- and multiple-stress conditions is presented. The statistical methods used with this type of data are described briefly, including the two-parameter Weibull distribution and the log-normal distribution, which are the two distributions most frequently used in aging studies. Lifetime models under multiple stresses are discussed. In particular, several models under combined electrical and thermal stresses are presented and their applicability is analyzed and discussed. These include multistress models proposed by L. Simoni (1981, 1984) and T.S. Ramu (1985, 1987), both employing the inverse power law for electrical aging, the exponential model by B. Fallou (1979), the probabilistic model by G.C. Montanari (1989), and the physical model by J.-P. Crine (1989). Trends in the development of these models are discussed. >

A brief survey of radiation effects on polymer dielectrics

Future space power needs are extrapolated to be at least three to four orders of magnitude more than is currently available. This long-term reliable power will be required on missions such as the Space Station, Pathfinder, Space Plane, and high-powered satellites, and for national defense. Electrical insulation and dielectrics are the key electrical materials needed to support these power systems, where a single-point system failure could prove catastrophic or even fatal for the whole mission. Therefore, the impact of radiation, an environmental stress, on the properties and performance of insulation and dielectrics must be understood. The influence of radiation on polymer dielectrics, the insulating materials most commonly used for power transmission and storage, is reviewed. The effects of the type of radiation, dose, rate, and total exposure on the key electrical, mechanical, and physical properties of polymer dielectrics are described and explained. >

Dependence of the Electric Strength on Thickness Area and Volume of Polypropylene

A dependence of the electric strength on thickness, area and volume of dielectric is demonstrated for polypropylene film. It is found that the electric strength of theoly polymer film decreases with volume. It is shown that theunc function of electric strength is not simply dependent on the volume as obtained from theory of extreme values, but that an increase of the volume by increasing the thickness of the dielectric is much more significant than the same increase of the dielectric volume caused by an increase in area.

A short method of estimating lifetime of polypropylene film using step-stress tests

Polypropylene films were tested for lifetimes under high electric fields, using both constant and step-stress methods. Various time intervals for the step-stress test and voltages for the constant stress test were selected. The inverse power law was used in the analysis of the experimental data, with the value of the power exponent determined from the step-stress test. This exponent was found similar to that obtained from the constant stress test. The lifetimes of polypropylene were then estimated from the step-stress test and were found to show good agreement with the experimental results of the constant stress test. The Weibull distribution was employed in the analysis of the experimental data. >

Lifetimes of polypropylene films under combined high electric fields and thermal stresses

The lifetimes of polypropylene films under high AC and DC field stresses were determined at 23, 70, and 90 degrees C. A two-parameter Weibull distribution was used in the evaluation of the data, with the shape and scale parameters estimated using graphical and maximum-likelihood methods. For DC aging, the results indicate that a power-law type of relation may hold true, with the parameters changing with temperature. However, for AC aging more anomalous results were obtained. These anomalies were more noticeable in the intermediate voltage region where the 23 degrees C lifecurve fell below the lifecurves at the other two temperatures. This is possibly due to the partial discharges which are mainly responsible for the breakdown in this range of voltage and the progressive impregnation of the dielectric films with transformer oil. >

Effects of multistress aging (radiation, thermal, electrical) on polypropylene

Capacitor-grade polypropylene films were aged under multiple stresses (electrical, thermal, and radiation). The aging experiments were performed for both singular and simultaneous combined stresses. The polypropylene was exposed to combined neutron-gamma radiation with a total dose of 1.6*10/sup 6/ rad, electrical stress at 40 V/sub rms// mu m, and thermal stress at 90 degrees C. Post-aging diagnostics consisting of electrical, mechanical, physical and chemical characterization were carried out to identify degradation mechanisms for polypropylene films under multifactor stress aging. The most pronounced changes were observed in the mechanical properties of the film. Significant decrease in elongation at break and tensile strength proved deterioration of the polypropylene under combined neutron-gamma radiation. This decrease was caused by chain-scission of the polypropylene molecules. The temperature stress had an opposite effect, causing an increase in the above-mentioned properties and offsetting, therefore, the negative effect of radiation. Although changes were observed in the electrical properties, they were not as significant as those for the mechanical characteristics. It can be concluded, that the failure mechanism of the electrical insulation under multistress aging conditions could be a mechanical failure of the material, rather than direct homogeneous decay in the dielectric strength or thermal breakdown of the polymer. >

High temperature dielectric properties of Apical, Kapton, PEEK, Teflon AF, and Upilex polymers

Apical, Upilex, Kapton, Teflon AF, and PEEK polymers are characterized for AC and DC dielectric breakdown in air and in silicone oil at temperatures up to 250 degrees C. The materials are also tested in terms of their dielectric constant and dissipation factor at high temperatures with an electrical stress of 60 Hz, 200 V/mil present. The effects of thermal aging on the properties of the films are determined after 15 h of exposure at 200 and 250 degrees C. The preliminary data indicate that most of the tested films remain relatively stable when exposed to temperature as high as 250 degrees C. The PEEK film however tends to display some wrinkling and exhibit discoloration when exposed to temperatures of about 200 degrees C and higher. All other films displayed good dielectric and physical properties even up to 250 degrees C.<<ETX>>

Effects of fast neutron radiation on polypropylene

Capacitor-grade polypropylene films were irradiated in a 2-MW thermal nuclear reactor and exposed to fast neutron radiation at a flux rate of 2.6*10/sup 12/ neutron/cm/sup 2/ s and gamma radiation at a level of 10/sup 7/ rad/h. The postirradiation effects on changes in the electrical and chemical properties of the films were studied for irradiation times up to 10 h. The electrical properties were DC and AC breakdown voltages, life under pulsed voltage stress, dielectric permittivity, dielectric losses, and volume resistivity. Chemical analysis was performed using the infrared spectroscopy technique. Small changes were detected in the dielectric strength, dielectric properties, and volume resistivity of the film. These changes are believed to be caused by oxidation of the polypropylene film, as was evidenced by the infrared spectra showing an increase in the carbonyl absorption peak at 1720 cm/sup -1/. >

Characterization of electron-irradiated biaxially-oriented polypropylene films

Post-radiation changes in the electrical and mechanical properties of capacitor-grade isotactic polypropylene films exposed to electron radiation were previously reported by the authors (ibid., vol.NS-34, no.6, p.1822-6, Dec. 1987). Based on the data obtained, it was suggested that crosslinking, chain scission and oxidative degradation were responsible for the radiation-induced changes in the film. In the present investigations, additional electrical characterizations that included the DC breakdown voltage and AC conductivity measurements were performed. Effects of the electron radiation on the physical and chemical properties were also evaluated to identify the actual degradation mechanisms. These studies included scanning electron microscopy, X-ray diffraction, infrared spectroscopy and sol-gel measurements. The results obtained confirm that crosslinking and chain scission of the polymer are responsible for the changes in the lower dose range, whereas oxidative degradation becomes predominant at higher dose levels. >

Testing and Evaluation of Insulating Films Part I: Description of Test Facilities at the University at Buffalo

This first part in a two-part series examines the test facilities available at the University at Buffalo, Buffalo, New York. The facilities have been set up to study-the effect of electrical, thermal, and radiation, prestressing upon the physical properties of dielectric materials proposed for use in high voltage energy storage and transfer systems.