Alvin R Ellis

Electrical properties of epoxy resin based nano-composites

We investigate the electrical properties of composite materials prepared as nano- and sub-micron-scale metal-oxide particles embedded in a commercial resin. The filler particles are barium titanate and calcium copper titanate. The physical and structural characteristics of the constituents and the fabricated composites are reported. Electrical characterization of the composite samples is performed using time- and frequency-domain dielectric spectroscopy techniques. The electrical breakdown strength of samples with nano- and sub-micron-sized particles have better electrical insulation properties than the unfilled resin.

Enhancement of dielectric strength in nanocomposites

In this paper, we report the dielectric breakdown properties of a nanocomposite, a potential electrical insulation material for cryogenic high voltage applications. The material is composed of a high molecular weight polyvinyl alcohol and nanosized in situ synthesized titanate particles. The dielectric breakdown strengths of the filled material samples, measured in liquid nitrogen, indicate a significant increase in their strengths as compared to unfilled polyvinyl alcohol. We conclude that nanometre-sized particles can be adopted as a voltage stabilization additive.

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.

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.

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.

Breakdown in liquid nitrogen in the presence of thermally generated bubbles for different electrode geometries

Liquid nitrogen is used as the cryogen and dielectric for many high temperature superconducting high voltage applications. When a quench in the superconductor occurs, bubbles are generated which can affect the dielectric properties of the liquid nitrogen. An experiment has been set up to generate bubbles in liquid nitrogen. Bubbles were generated using a kapton heater. Three different electrode geometries were applied: plane-plane, sphere-sphere, and sphere-plane. Breakdown measurements were made in both open bath liquid nitrogen and in a pressurized dewar at pressures up to 200 kPa absolute. The voltage applied was 60 Hz AC. For sphere-plane it was observed that the breakdown did not always occur at the minimum gap and this was likely due to the actual location of the bubble when breakdown was initiated. For plane-plane geometry where bubbles were generated in the plane electrode, breakdown voltages dropped at a certain heater power and remained low thereafter. The heater power at which the drop occurred increased with pressure. Breakdown data for subcooled liquid nitrogen will also be presented.