N. Giao Trinh

Electrode Design for Testing in Uniform Field Gaps

The efficiency of producing uniform fields in the test gap was evaluated for practical electrode configurations corresponding to the plane, the Rogowski and the Bruce profiles. The field intensity was calculated using the known charge simulation method and the calculated distributions of the field intensity at the surface of different electrode configurations were compared. The results obtained show a pronounced end effect at plane-profile electrodes. With the Rogowski profile, the effective uniform-field region is dependent on the gap spacing. The Bruce profile was found to be a good compromise, with a negligible end effect and a nearly constant uniform field region. Practical considerations are also presented together with the limitations of various profile configurations in the design of uniform field electrodes.

A Study of the Corona Performance of Hydro-Qu??bec's 735-kV Lines

The corona (in particular RI and AN) performance of two basic configurations of Hydro- Québecs 735-kV transmission lines was recorded over a two-year period between 1976 and 1978. Statistical analysis of the data shows that both the RI and AN performances of the lines compare well with previously recorded data and are well within accepted norms.

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).

A Study of Corona Discharges at Water Drops Over the Freezing Temperature Range

Foul weather corona have been observed to behave differently at temperatures above and below the freezing point1.2. Generally speaking, the discharge activities are much more intensive at temperatures above than below the freezing point. Such behaviour of the discharge is so far associated with the differences in the physical properties of water in its liquid and solid phases. No detailed study has, however, been made to correlate the discharges characteristics with the macroscopic changes in the water drop over the freezing temperatures range. In the present study, the development of corona discharge at a watet drop in a cylindrical field was investigated during its transition from the liquid to solid phase. This was done by correlating the discharge current with variations of the water drop temperature. The results obtained show that the noise generation and energy losses are related to both the corona discharges in the surrounding air and to the continuously changing conditions at the water drop during its transition into ice (and vice versa).

High Voltage AC Transmission Lines: Reduction of Corona Under Foul Weather

Can improved conductor designs lower the corona levels (audible noise, electromagnetic interference, power losses) of HVAC transmission lines under foul weather conditions? Three types of improved conductors are considered: hydrophilic, hydrophobic and ion-shielded.

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.

Prevention of Recovery Overvoltages on High Voltage Testing Transformers

During flashover tests with ac voltages, both the transformer and the test object may be subjected to excessively high overvoltages. If the recovery overvoltages occur during the negative half cycle, as is generally the case, external flashovers may not necessarily take place and the resultant high stress can damage the internal insulation of the transformer or the test object. The danger of the recovery overvoltage is even more pronounced in extra high voltage tests where several transformer units are cascaded to provide the required test voltage levels.

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.

Considerations on the effect of a temperature gradient on the dielectric performance of a CGI bus

When a compressed-gas-insulated (CGI) bus section is subjected to a temperature gradient, the gas molecules tend to concentrate in the cold portion of the bus, creating a local reduction in the gas density in the warm portion, and thus affecting the dielectric strength of the overall bus, The effect is even greater when the outdoor temperature reaches the point of gas liquefaction: a) it causes the gas liquefaction in the cold portion to occur at a slightly higher temperature; b) it intensifies the reduction of the gas density in the warm portion with a corresponding deterioration of the dielectric strength of the bus.