M.O. Pace

On dielectric breakdown statistics

In this paper, we investigate the dielectric breakdown data of some insulating materials and focus on the applicability of the two- and three-parameter Weibull distributions. A new distribution function is also proposed. In order to assess the model distributions trustworthiness, we employ the Monte Carlo technique and, randomly selecting data-subsets from the whole dielectric breakdown data, determine whether the selected probability functions accurately describe the breakdown data. The utility and strength of the proposed expression are illustrated distinctly by the numerical procedure. The proposed expression is shown to be a valuable alternative to the Weibull ones.

Recent Advances in Gaseous Dielectrics at Oak Ridge National Laboratory

Recent developments in gaseous dielectrics are discussed with emphasis on dielectric gas mixtures containing SF6. Recent findings on the isotopic, pressure, and nonuniform field dependence of the dielectric strength of gases are also presented, and ways to enhance the corona-stabilization characteristics of gas mixtures by appropriate additives (e.g. to SF6) are outlined. In addition, results are presented on the spark decomposition products of SF6 and the alleviation of the harmful effects of conducting particles by particle coating in situ. Finally, electron attachment, electron drift velocity, and breakdown properties of gas mixtures (e.g. C2F6 or C3F8 in Ar or CH4) - comprised of electron attaching and electron conductivity enhancing components - which have promise for use in pulsed power technologies are reported.

Observation of partial discharge in hexane under high magnification

Partial discharges are observed in hexane by shadow photography under the application of DC voltages. A nonuniform field geometry is used and the growth of cavities associated with partial discharges at a point cathode was photographed at 200* magnification. The use of an image-preserving optical delay allows a record of the conditions which exist in the liquid prior to the initiation of the low-density streamer to be obtained. The cavity growth at a point cathode is anisotropic, which suggests that electrostatic forces are of primary importance in driving its expansion. The onset of instabilities in the cavity wall is suggested. The initial partial discharge current pulse precedes or is concurrent with the growth of the cavity. An upper bound of 60 ns between first current pulse and the growth of the cavity is obtained. >

High voltage studies of dielectric materials for HTS power equipment

The discovery of high temperature superconductors (HTS) has triggered renewed interest in the study of dielectric materials at cryogenic temperatures. While considerable work was done in the 1970s and 1980s on dielectrics immersed in liquid helium for low temperature superconducting applications, there remains a need for dielectric research at liquid nitrogen temperature for HTS applications, requiring experimental data oriented toward practical situations. We report on AC breakdown (puncture and/or flashover), and impulse breakdown of solid materials in either vacuum or in liquid nitrogen. Solid materials which we examined, include fiberglass reinforced plastics, epoxies with and without filler, and polymeric tape. Combinations of some of these materials have also been studied at low temperatures. Additionally we have measured permittivity and dissipation factor for materials for which these parameters are not available at 77 K. Finally, we also discuss specific applications for HTS cables including breakdown and aging studies on model cables, with lapped tape electrical insulation, immersed in liquid nitrogen.

Measurements of the performance of BSCCO HTS tape under magnetic fields with a cryocooled test rig

The use of high-temperature superconducting (HTS) materials for electric power applications is being realized in prototype systems. A test rig was designed and fabricated that uses a 6-T cryocooled magnet with an 20.3 cm warm bore. Inserted in the bore is a stainless steel vacuum vessel that has a Cryomech GB37 cryocooler to conductively cool the sample. Critical current measurements were made on BSCCO-2223 tapes under externally applied perpendicular and parallel magnetic fields. A description of the test rig design and results from a series of measurements will be presented.

Prebreakdown current pulses in n-hexane and other dielectric fluids

A point-plane electrode geometry was used to apply negative DC stresses to n-hexane, trimethylpentane, and transformer oil. The resulting prebreakdown currents and light were observed with a state-of-the-art, low-noise amplifier and a high-speed digitizer. Currents appear as bursts of fast (on the order of 100 ns) negative pulses with progressively increasing amplitudes, and a wide bandwidth (at least 10 MHz) is required to resolve individual pulses. Light pulses occur coincidentally with current pulses, but are narrower. The effect of adding particles to the sample or increasing the applied high voltage is primarily to increase the burst rate, with little or no effect on other pulse characteristics such as shape, amplitude, or rate of increase. These observations suggest a model describing the source of the current pulses as a series of discharges inside an expanding low-density region. >

Improved Unitary and Multicomponent Gaseous Insulators

Improved unitary and multicomponent gaseous insulators are systematically designed according to detailed knowledge of fundamental electron-molecule interactions. Knowledge of the electron attachment and electron slowing-down properties of dielectric gases/mixtures as functions of electron energy is shown to be especially significant. On the basis of such knowledge it is possible to improve the gaseous dielectrics breakdown strength by effectively controlling the numbers and energies of the electrons present. Several unitary (e.g., C<inf>4</inf>+F<inf>6</inf>, c-C<inf>4</inf>F6, and iso-C<inf>4</inf>F<inf>6</inf>) and multicomponent (e.g., C<inf>4</inf>F<inf>6</inf>/SF<inf>6</inf>/N<inf>2</inf> and c-C<inf>4+</inf>F<inf>8</inf>/C<inf>4+</inf>F<inf>6</inf>/SF<inf>6</inf>/N<inf>2</inf>) gaseous systems have been tested and found to have better DC breakdown strength properties than SF<inf>6</inf>. These findings are reported and discussed.

Effect of gas pressure on partial discharge in voids in epoxy

The pressure inside an enclosed void in a solid dielectric is generally determined by calculation based on equating the measured partial discharge (PD) onset voltage and the breakdown voltage from the Paschen curve for a particular gas. Here it is assumed that the gas is known or that physical properties remain approximately constant. In the present experiment, two different large voids were placed in epoxy adjacent to the ground electrode containing a small pump out port. This arrangement allowed for direct measurement and setting of the pressure for different gases filling the void. PD onset and extinction voltages were determined as a function of pressure over the range from 13.3 Pa (0.1 torr) to 101 kPa (760 torr) for nitrogen and SF/sub 6/ at room temperature. The onset voltages generally followed the Paschen curve dependence on pressure but were higher in voltage at pressures around the minimum, while the extinction voltage followed the Paschen curve more closely in both pressure dependence and voltage. Agreement with the Paschen curve is better at the higher pressures. One possible explanation is the dominant role that real surfaces play at lower pressures.

High Voltage Testing of a 5-meter Prototype Triaxial HTS Cable

High voltage tests were performed on a 5-m long prototype triaxial HTS cable (supplied by Ultera) at ORNL in preparation for installation of a 200-m HTS cable of the same design at the AEP utility substation in Columbus, Ohio. The triaxial design comprises three concentric phases and shield around a common former with the phase to phase dielectric at cryogenic temperature. Advantages of this design include increased current density, a reduced amount of HTS tape needed, and reduced heat load. The phase to phase voltage will be 13.2 kVrms (7.6 kVrms to ground). Preliminary testing was done on half-scale and full-scale terminations which successfully passed AC withstand, partial discharge, and impulse tests. High voltage tests conducted on the 5-m cable with the cable straight and after bending 90 degrees were ac withstand to 39 kVrms, partial discharge inception, and a minimum of 10 positive and 10 negative lightning waveform impulses at 110 kV. Phase to phase insulation was tested by applying high voltage to each phase one at a time with all the other phases grounded. Partial discharge data will be presented. The 5-m prototype triaxial HTS cable passed all the HV tests performed, with a PD inception voltage significantly above the required voltage.

Low-noise wide-band amplification system for acquiring prebreakdown current pulses in liquid dielectrics

A low-noise wideband amplification system consisting of a preamplifier with input protection circuit and an amplifier were developed for measuring the prebreakdown currents in liquid dielectrics. The system design considerations and experimental results are presented. The preamplifier was voltage-sensitive with a voltage gain of 154 V/V, and had a bandwidth of 56 MHz and an equivalent noise voltage spectral density of 0.93 nV/Hz/sup 1/2/. The amplifier had a voltage gain of about 12 and a bandwidth of 58 MHz and was capable of driving mod 2 mod V across two paralleled 50 Omega loads. The overall system had a gain of 92400 V/A when loaded with 50 Omega , and it had a minimum detectable current of 1.9 mu A for an overall bandwidth of about 29 MHz. >