Peter Cheetham

Dielectric Properties of Cryogenic Gas Mixtures Containing Helium, Neon, and Hydrogen

Past efforts of cooling high temperature superconducting (HTS) power cables by gaseous cryogens focused exclusively on helium. The limited dielectric strength of helium gas necessitated alternatives that could be used in the temperature range suitable for HTS power applications. This paper presents the benefits of gas mixtures containing helium with small concentrations of hydrogen or neon to mitigate the limited dielectric strength of pure helium gas. The expectation was that such gas mixtures could improve dielectric characteristics while maintaining the thermal, non-flammable and non-corrosive properties of pure helium gas. The AC breakdown voltage of helium gas mixtures containing 4 mol% neon or 4 mol% hydrogen respectively have been measured and compared to those of pure helium and pure neon. All measurements were performed at 77 K at gas pressure levels between 0.5 and 2.0 MPa. While the 4 mol% neon mixture did not result in any improvement over pure helium, the 4 mol% hydrogen mixture resulted in 80% higher breakdown strength. This is expected to enable higher operating voltages for gas cooled HTS power devices.

Use of partial discharge inception voltage measurements to design a gaseous helium cooled high temperature superconducting power cable

The occurrence of partial discharge is one of the challenges that limit the design options and voltage ratings of gaseous helium cooled high temperature superconducting (HTS) cables. The measurements of partial discharge inception voltage (PDIV) at cryogenic temperatures on several model cables were used in the optimization of the dielectric design of a 30 m long HTS power cable, which was successfully fabricated and tested at DC currents up to 6 kA and voltage of 3.5 kV RMS. Details of the PDIV measurements are described.

Enhancement of Dielectric Strength of Cryogenic Gaseous Helium by Addition of Small Mol% Hydrogen

A valuable solution to the voltage rating limitation of gaseous helium (GHe) cooled HTS power devices is demonstrated through the use of cryogenic gaseous mixtures of small mol% of gaseous hydrogen (H₂) balanced with GHe. An 80% enhancement of the dielectric strength, both in dc and ac, is observed for the mixture with 4 mol% H₂. The observed linear dependence of dielectric strength from varying the H₂ mol% suggests a significant potential for further enhancements by using mixtures with higher H₂ mol%. Partial discharge (PD) measurements were performed on a 1 m model cable to demonstrate the potential practical applications of GHe–H₂ mixtures for superconducting devices. The results of the PD measurements for model cable showed a 20–25% increase when the 4 mol% H₂ mixture was used. The demonstrated enhancements in the dielectric strength will provide additional design options to HTS power devices cooled with GHe mixtures.

Practical considerations for the design of a superconducting gas-insulated transmission line for shipboard applications

Superconducting gas-insulated transmission lines (S-GIL) are introduced for shipboard power applications as a lightweight and energy efficient alternative to conventional power distribution cables. Requirements for naval ship power distribution and the corresponding design options for S-GIL are presented. Needs for further developments for meeting the power density requirements for US Navy all-electric ship and design requirements for its operational environment are discussed.

A versatile model for estimating breakdown voltage and its application for cryogenic gas mixtures

An improved model for the estimation of the breakdown voltages of gas mixtures is developed. The application and utility of the model is demonstrated by using it in estimating the breakdown voltages of gas mixtures containing helium and hydrogen that are being developed as an alternative cryogenic insulation and cooling media for certain high temperature superconducting (HTS) power devices. The concept of the developed model, which incorporates a generalization factor m, is presented. The first step of the model performs a nonlinear least square regression on the generalized formula of the pressure normalized ionization coefficient α/p for the parameter approximation. The second step introduces the parameter m that minimizes the root mean square error (RMSE) of the nonlinear least square regression to derive the versatile model. The validity of the model is demonstrated by comparing its estimated values with the results of the experimental breakdown measurements of several gas mixtures. Compared to the conventional breakdown voltage models, the new model showed improved agreement with the experimental results of the breakdown voltages.

Exploration of Additive Manufacturing for HTS Cable Components for Electric Aircrafts

High temperature Superconducting (HTS) technology has the potential to deliver the required power densities necessary for electric aircrafts. A promising HTS cable design for electric aircraft applications that we have been investigating is the Superconducting Gas-Insulated Transmission Line (S-GIL). The S-GIL requires insulator spacers that are compatible with cryogenic temperatures as well as the high electrical and mechanical stresses. One of the major mechanical stresses currently limiting the design options is the compatibility of insulation materials and designs for cryogenic temperatures and associated thermal shocks. This paper discusses the investigation of using various 3D printed materials to fabricate prototype insulator spacer designs for the S-GIL and evaluate their compatibility with the required cryogenic operating conditions. The studies demonstrated that several materials and designs selected show suitability to operate at cryogenic temperature and we plan to proceed with further mechanical and electrical characterization of the 3D printed components.

Effect of high voltage electrical breakdown on critical current of High Temperature Superconducting tapes

Effects of high voltage electrical impulse events on the critical current of 2G HTS tapes are studied experimentally to develop an understanding on the potential damage that could occur if superconducting cables undergo electrical breakdown. The need for understanding the degradation is discussed in the context of the superconducting gas insulated lines (S-GIL), a novel design concept for superconducting cables. Since the S-GIL uses the gaseous cryogen as the sole insulation system which does not degrade in an electrical breakdown event, it is essential that the superconducting tapes themselves stay undamaged for the HTS cable system as a whole to be operational after electrical breakdown. Critical current of 2G HTS tapes were measured before and after subjecting them to electrical impulses at various voltage levels. It is shown that the extent of degradation depends on the number and voltage level of the impulses. The results of degradation of critical current are discussed in terms of the total energy released in an impulse event.

Comparative Study of Cryogenic Dielectric and Mechanical Properties of Insulation Materials for Helium Gas Cooled HTS Power Devices

A comparative study of cryogenic dielectric and mechanical characteristics of electrical insulation materials is described. Breakdown strengths and tensile strengths of Cryoflex, Kapton, PPLP, and Teflon were measured at 77 K in liquid nitrogen. Breakdown strengths of Cryoflex, Kapton, PPLP were also measured in gaseous helium at 2 MPa and 77 K. The relative breakdown strengths of Cryoflex and Kapton vary between liquid nitrogen and gaseous helium. Kapton performed better than Cryoflex both for dielectric and mechanical properties. Among the materials studied, Teflon exhibited the least breakdown strength and mechanical properties.