Claire Vincent

Electrostatic-Field Optimization of the Profile of Epoxy Spacers for compressed SF6-Insulated Cables

Electrostatic-field optimization of the profile of the gas-dielectric interface was studied as a means of improving the dielectric performance of epoxy spacers. An optimum disc-shaped spacer was defined with a dielectric cone angle of 750, assuming a dielectric constant of the epoxy resin of 5 or higher. The dielectric performance of the optimum disc-shaped spacer is found to be limited, however, to about 85% of that of the conductor system without spacer.

Statistical Significance of Test Methods for Low Probability Breakdown and Withstand Voltages

A systematic evaluation of the statistical significance of different test methods currently used for the determination of the breakdown and withstand voltages was made. The breakdown probability of the test voltages was evaluated by both direct calculation and computer simulation of the test methods, assuming a known intrinsic breakdown probability in a single voltage application (or one cycle ac). The latter was assumed to be of either normal, smallest values or Weibull type, and to be unaffected by the event occurring in the pre- ceding voltage applications (or ac cycles).

Comparative evaluation of parameters of the dielectric breakdown test on transformer oil

The standard AC dielectric breakdown test result is generally used as one of the acceptance criteria for insulating oil and a maintenance tool for high-voltage power transformers in service. Using the standard electrode parameters and voltage application procedure, a small-oil-volume breakdown is obtained. Unfortunately however, these test parameters are far from field conditions. Therefore, oil breakdown results were systematically evaluated and compared using ASTM and IEC standard procedures and varying many of the test parameters such as the shape and dimensions of the electrodes, the oil circulation, the voltage application procedure, etc. Both clean and contaminated transformer oils were tested. The results reflect the surface and volume effects, giving lower breakdown voltages when a larger coaxial electrode arrangement replaces the standard shape. A 1-min step-by-step voltage application is also found to give lower breakdown voltages. Finally, the effect of water and solid particle contamination is evaluated and compared for different test methods.

Effect of impurity particles on transformer oil under normal operating conditions

A systematic survey was conducted of transformer oil samples taken from operating transformers and reactors used in Hydro-Quebecs transmission system. It was found that the particle content in the oil follows a distribution of the power function type and that it can be related to the dielectric strength of the oil and constitutes a good indication of the overall performance of the apparatus. Filtering the oil reduces the particle content and improves its dielectric strength.

Bundled-Conduuctors for EHV Transmission Systems with Compressed -- SF 6 Insulation

This report presents the results of a theoretical study to evaluate the practicability of bundled conductors in SF6-insulated transmission systems operating at EHV levels. The bundled conductors are evaluated according to their ability to maintain the electric-field intensity at the conductor surface below the corona-on-(or breakdown) level of the conductor, and are compared with practical coaxial-conductor systems. The results obtained show that the concept of bundled conductors can be adapted for use in SF6-insulated systems. The proposed bundle configuration consists of a central conductor surrounded by n small subconductors forming an electrostatically protective screen, the conductor bundle itself enclosed within an outer envelope. Compared with conventional coaxial configurations, it presents several advantages, namely, flexibility of design to meet the systems high-current, mechanical and insulation requirements separately, smaller electrode area exposed to high field intensities, and less stringent specifications for the supporting epoxy spacers.

Electrode area and stressed volume: Two apparent effects of large-volume oil insulation

Analysis of the dielectric degradation observed in large-volume oil insulation revealed that the phenomenon can be interpreted in terms of an apparent effect of either the electrode area or the stressed volume. A semi-empirical method was developed for quantitative evaluation of the dielectric degradation of large-oil-volume insulation. The method applies the weak-link theory to an oil gap, assuming an apparent effect with a unit element of known breakdown probability distribution determined from experimental data.

CRITICAL AVALANCHE SIZE FOR IMPULSE BREAKDOWN IN QUASI-UNIFORM FIELD SF6 GAPS

ABSTRACT A method of calculation has been developed for determining the average voltage-time (V-T) characteristics of quasi-uniform field gaps subjected to high impulse voltages. The method consists of evaluating the test voltage which will produce a critical electron avalanche in the gas gap, the size of which is a function of the time to breakdown. An empirical relationship determined from experimental data obtained at IREQ defines the critical avalanche size which would cause breakdown of the gap at a time Tf, the measured time to flashover. Comparison of the calculated average V-T curves with experimental data for various gap configurations shows reasonable agreement. The error margin was found to be about 10–15% over the range of times to breakdown from 0.2 to 2.0 μs and for gas pressures up to 1.065 MPa.

Low-temperature behavior of synthetic oils

Synthetic oils are increasingly used in the insulation of power equipment, particularly in power capacitors where the use of polymer films has practically excluded mineral oils for compatibility reasons. However, little data are available on their behavior at low temperatures, -10 to -50/spl deg/C, representative of the climate in northern Quebec, Canada. This paper presents the results of a study comparing the behavior of typical synthetic oils with Volt-Esso 35 mineral oil over this range of temperatures.

Dielectric performance of CGI cables for EHV transmission at 550 and 800 kV

This paper presents the results of dielectric and power tests on practical CGI cables operating at 550 and 800 kV. These results indicate that typical CGI cable designs fulfill or even exceed the test specifications at operating pressure. They also suggest that optimization of the spacer performance could help to reduce the overall cable dimensions.