D. R. James

On the role of electron attachment in the breakdown strength of gaseous dielectrics

Experimental data on electron attachment and breakdown voltages Vs for two groups of strongly electron attaching gases-perfluorocarbons and freons-and some of their mixtures with electron slowing-down gases have been used in an effort to quantify the role of electron attachment on the breakdown strength of gases. The perfluorocarbons (PFC) considered have positive electron affinities (EAS) and capture electrons at thermal and near-thermal energies principally non-dissociatively. The freons capture slow electrons dissociatively producing mainly Cl-. Vs is strongly influenced by the magnitude and energy dependence of the attachment cross-section, sigma a( epsilon ). The increase in Vs with increasing attachment, however, seems to saturate, indicating an upper limit in Vs of approximately 2.5 times that of SF6. Electron detachment seems to be indicated for PFC and SF6.

Triaxial HTS Cable for the AEP Bixby Project

Ultera has installed a single 200-meter long high temperature superconducting (HTS) 3-phase triaxial design cable at the American electric power (AEP) Bixby substation in Columbus, Ohio. The cable connects a 138/13.2 kV transformer to the distribution switchgear serving seven outgoing circuits. It was designed to carry 3000 Arms. Testing of 3- to 5-meter length prototype cables, including a 5-meter prototype with full scale terminations tested at ORNL was conducted prior to the manufacture and installation of the AEP triaxial cable. These prototypes were used to demonstrate the crucial operating conditions including steady state operation at the 3000 Arms design current, high voltage operation, high voltage withstand and 110 kV impulse, and overcurrent fault capability. A summary of the results from the thermal analysis and testing conducted by Ultera and ORNL will be presented. Some analysis of the cable thermal-hydraulic response based on the testing that were used to determine some of the cable cryogenic system requirements are also presented.

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.

Tests of tri-axial HTS cables

The Ultera/ORNL team have built and tested 3-m and 5-m triaxial cables rated at 3 and 1.3 kA-rms, respectively. The three concentric superconducting phases are made of BSCCO-2223 HTS tapes, separated by layers of cold-dielectric tapes. A copper braid is added as the grounding shield on the outside of the three active phases. Tests of these cables were performed at temperatures ranging from 70 to 84 K. AC loss data reconfirmed the previous result on a 1.5-m prototype cable that the total 3-phase ac loss is about the sum of the calculated ac losses of the three concentric phases. These and other test results of the 1.3 and 3 kA cables will be used to construct a second 5-m triaxial cable rated at 3 kA-rms, 15 kV. Preliminary test results supporting this new cable and the associated termination are summarized.

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.

Electrical Insulation Characteristics of Glass Fiber Reinforced Resins

Non-metallic structural materials that act as an electrical insulation are needed for cryogenic power applications. One of the extensively utilized materials is glass fiber reinforced resins (GFRR) and may also be known as GFRP and FRP. They are created from glass fiber cloth that are impregnated with an epoxy resin under pressure and heat. Although the materials based on GFRR have been employed extensively, reports about their dielectric properties at cryogenic temperatures and larger thicknesses are generally lacking in the literature. Therefore to guide electrical apparatus designers for cryogenic applications, GFRR samples with different thicknesses are tested in a liquid nitrogen bath. Scaling relation between the dielectric breakdown strength and the GFFR thickness is established. Their loss tangents are also reported at various frequencies.

Nanodielectric system for cryogenic applications: Barium titanate filled polyvinyl alcohol

In the current study we focus on dielectric properties (as a function of frequency and temperature) of a polymeric composite system composed of polyvinyl alcohol and barium titanate nano powder. In the investigations, the temperature range is between 50-295 K, and the frequency range is between 20 Hz-1 MHz. Polarization and conduction processes are investigated in the linear regime. Dielectric breakdown strengths of samples are also reported. The materials presented have potential to be implemented in cryogenic capacitor or field grading applications.

Dielectric gas mixtures with polar components

Dielectric gas mixtures comprising mainly one electron-attaching component and one dipolar component have been investigated. It has been found that polar electron-slowing-down components effect a sharp increase in the breakdown voltage, Vs, with small percentages of electron-attaching additives. The effect of electron-dipole scattering on Vs for multicomponent gas dielectrics is assessed, especially in combination with indirect electron scattering via negative ion states. The results demonstrate further the beneficial effect of large electron scattering cross-sections at subexcitation energies on Vs and suggest that a careful combination of gases slowing down electrons via dipole scattering and via negative ion states can effect large Vs values. This and earlier studies suggest that a number of dielectric gas mixtures containing one or two electron-attaching components from c-C4F8, 2-C4F8, SF6 and a dipolar component from CHF3, CH2F2 or 1,1,1-CH3CF3 with or without N2 are excellent candidates for large-scale testing for possible eventual industrial adoption.

Anomalous electron attachment properties of perfluoropropylene (1-C3F6) and their effect on the breakdown strength of this gas

Electron attachment to perfluoropropylene (1-C3F6) in 1-C3F6/buffer gas mixtures has been found to be anomalously dependent on both the 1-C3F6 and buffer gas pressures, as well as on the gas temperature. The authors have found also that the uniform field breakdown strength of pure 1-C3F6 is dependent on gas pressure but not on the gas temperature. A mass spectrometer study of the negative ions formed in a moderately high pressure corona discharge has been made, and the principal negative ions formed by electron attachment to 1-C3F6 have been identified. Based on these studies, a reaction scheme for electron attachment to 1-C3F6 is proposed which results in stable negative ion clusters but does not involve pre-existing dimers. Further, the authors propose that the observed pressure dependence in the breakdown strength of this gas is due to the large pressure dependence of the electron attachment process they observed.

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.