This chapter describes the benefits and associated technical challenges of closed-loop cryogenic helium gas circulation for cooling high-temperature superconducting cable systems. The many potential applications of gaseous helium-cooled high-temperature superconducting cables are discussed. The major technical challenges specific to the development of gaseous helium-cooled superconducting power cables are also described. The need for the development of new cryogenic dielectric material systems for realizing medium-voltage power cables is emphasized. Available technology for producing and circulating cryogenic helium gas at sufficiently large mass flow rates needed for power cables is discussed. The chapter concludes with a brief description of ongoing research and development efforts on helium gas-cooled high-temperature superconducting cables.

High-temperature superconducting (HTS) power cables cooled by helium gas

Cryogenic helium gas circulation system for advanced characterization of superconducting cables and other devices

This study investigates the dielectric properties of various mixtures of potential gaseous cryogens containing helium (He), hydrogen (H2), neon (Ne), and nitrogen (N2) under extended temperature and pressure ranges for high-temperature superconducting applications. We present the results of the Boltzmann analysis on a variety of binary and ternary gas mixtures in terms of the electron energy distribution function and the coefficients that represent the electron kinetic process, including the density-reduced ionization coefficient ( α / N), the density-reduced attachment coefficient ( η / N), the density-reduced effective ionization coefficient ( ( α − η ) / N), and the density-reduced critical electric field ( ( E / N ) c r). The study provides insights into the important characteristics and correlations that lead to the enhanced dielectric strength of gas mixtures and predicts further enhancements in the dielectric strengths of He-H2 mixtures by introducing the ternary mixtures of He-H2-N2. The study res...

The dielectric properties of gaseous cryogen mixtures of He, H2, Ne, and N2 in a temperature range of 50–80 K at pressures up to 2.0 MPa

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 (H 2 ) balanced with GHe. An 80% enhancement of the dielectric strength, both in dc and ac, is observed for the mixture with 4 mol% H 2 . The observed linear dependence of dielectric strength from varying the H 2 mol% suggests a significant potential for further enhancements by using mixtures with higher H 2 mol%. Partial discharge (PD) measurements were performed on a 1 m model cable to demonstrate the potential practical applications of GHe-H 2 mixtures for superconducting devices. The results of the PD measurements for model cable showed a 20-25% increase when the 4 mol% H 2 mixture was used. The demonstrated enhancements in the dielectric strength will provide additional design options to HTS power devices cooled with GHe mixtures.

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

We report on a 2x2 array of radio-frequency atomic magnetometers in magnetic induction tomography configuration. Active detection, localization, and real-time tracking of conductive, non-magnetic targets are demonstrated in air and saline water. Penetration in different media and detection are achieved thanks to the sensitivity and tunability of the sensors, and to the active nature of magnetic induction probing. We obtained a 100% success rate for automatic detection and 93% success rate for automatic localization in air and water, up to 190 mm away from the sensors' plane (100 mm underwater). We anticipate magnetic induction tomography with arrays of atomic magnetometers finding applications in civil engineering and maintenance, oil&gas industry, geological surveys, marine science, archeology, search and rescue, and security and surveillance.

Active Underwater Detection with an Array of Atomic Magnetometers

The need for increased magnetic signature control on Navy ships has lead to the development and adoption of a three axis advanced degaussing system. While this system is effective in reducing the ship magnetic signature, it requires significantly more copper cable than the legacy two axis systems. Degaussing only requires DC currents for field manipulation. Since DC applications is where HTS use excels, a feasibility study was conducted in FY04 to determine the benefits of HTS when used in an advanced degaussing system. Results showed reduced system size and weight, while remaining cost neutral. A series of lab based demonstrations were conducted proving out key aspects of an HTS DG system, most notably cooling a long length of flexible cryostat with gaseous helium. This led to an at sea demonstration of a single HTS DG loop aboard the USS Higgins. This was the first HTS system installed on an active combatant, and it made a successful magnetic range run in April 2009 demonstrating its capability to perform in a naval environment. This paper details the development of the HTS DG system from the initial feasibility study through the successful demonstration onboard the USS HIGGINS.

High Temperature Superconducting Degaussing From Feasibility Study to Fleet Adoption

High-temperature superconducting cables have application in the Navy for power cables as well as degaussing cables. The current interest is in using a gaseous cryogen circulated by means of a cryogenic fan as the cooling media for the HTS conductor. Flow through this style of cable is through an eccentrically situated corrugated annular region. Correlations do not currently exist for this arrangement since it is not a simple configuration. Experiments were carried out to measure pressure drop with varied flow rate of air and nitrogen through 10-m sections of tubing. A variety of tubing configurations was used including smooth tube, eccentric smooth annulus, and eccentric corrugated annulus with HTS wire bundle forming the inner filling material. A method was developed for comparing the relative flow resistance of tubing configuration, which included determining an effective hydraulic diameter based on observed data set fit to the Darcy-Weisbach equation and the Colebrook equation. The effective hydraulic diameter allowed for easy comparison of flow impedance for all configurations regardless of specific flow conditions.

Flow Resistance Experiments in HTS Cable Form

The use of high temperature superconducting (HTS) tapes in a flexible cryostat has been shown to provide great opportunities for the US Navy. The HTS tapes need to be fixed at the ends to provide an electrical connection. Since the ends are rigid but the tapes are contained in a changing environment there is a possibility that the tapes may become twisted and therefore damaged. Twisting is defined in this paper as the rotation of the tape along the length of the tape while the end has no attack-angle. The twisting length and angle of first generation (1G) BSCCO tapes is studied by measuring the voltage drop across the tapes in a liquid nitrogen bath when current is applied.

P. Ferrara

Papers

High Temperature Superconducting Degaussing From Feasibility Study to Fleet Adoption

Flow Resistance Experiments in HTS Cable Form

Experimental Study of Twisting First Generation BSCCO Tapes