Temperature state of electrical insulation of a superconducting DC cable

The purpose of the study was to build a nonlinear mathematical model governing the steady-state one-dimensional temperature distribution in the electrical insulation layer made in the form of a long hollow circular cylinder whose surfaces are given a constant electric field potential difference. By means of this model, integral ratios, which connecting the parameters of the temperature state of this layer, the heat transfer conditions on its surfaces and the temperature-dependent heat conduction coefficient and the electrical resistivity of the electrical insulating material with a given electrical potential difference, were built. A quantitative assay of the integral ratios is carried out with regard to the electrical insulation layer of a superconducting cable cooled by liquid nitrogen.     

超导电缆循环过冷液氮温度场模拟

建立了循环过冷液氮在恒温器内迫流冷却超导电缆的模型,利用Fluent软件模拟了30 m电缆系统内液氮温度场在不同流量、终端漏热、交流损耗等工况条件下的变化情况。仿真结果表明,在终端漏热和交流损耗一定时,液氮出口断面的最高温度随流量的增大而降低;在流量和交流损耗一定时,液氮出口断面的最高温度随终端漏热的增大而升高;在流量和终端漏热一定时,液氮出口断面的最高温度随交流损耗的变化不明显。     

On conduction-cooling of a high-temperature superconducting cable

Current generation high-temperature superconducting (HTS) power transmission cables use liquid nitrogen as a coolant that circulates along the cable. In this work, the use of axial conduction-cooling in attaining HTS temperatures in transmission lines is proposed. Liquid coolant use is envisioned only at periodic length intervals along the transmission lines, in combination with insulation and copper. The proposed concept is feasible due to the high thermal conductivity of pure copper at cryogenic temperatures. A basic design for the insulated cable is proposed and a detailed numerical simulation of heat transfer in such a cable is carried out for various case studies considering the superconducting materials MgB₂ and BSCCO-2223.     

Temperature rise in a viscoplastic material during dynamic crack growth

Dynamic steady-state crack growth has been analyzed under mode I plane stress, small-scale yielding conditions using a finite element procedure. A Perzyna type viscoplastic constitutive equation has been employed in this analysis. The viscoplastic work rate is converted into heat input and the temperature distribution is determined by solving the governing conduction/convection equation also by a finite element method. The Stream-line Upwinding Petrov-Galerkin formulation has been employed for this purpose because of the high Péclet number that results in such a type of analysis. The effect of strain rate sensitivity and crack speed on the temperature distribution near the crack tip is examined.     

Electrical characteristics of a dc superconducting cable

A dc superconducting cable is ideal for transmitting large blocks of electrical power over a long distance. However, it must be designed to operate reliably within the constraints of the electrical system. Therefore, system analysis must be performed for each application. The conductor losses caused by the harmonics on the dc must be within the design goals; a system fault should not drive the cable normal with eventual damage to the cable and interruption of power flow; and, the dielectric system of the cable must be designed to be compatible with the expected transient voltages by proper insulation coordination. Transient overvoltages are of concern to electrical power systems; these are especially critical to cryogenic cables because of the susceptibility of the cryogenic enclosures to these transients. This paper discusses the electrical system constraints which are particularly applicable to a dc superconducting cable and shows how such a cable can be designed to be compatible with the electrical system.This paper also summarizes the work on low temperature dielectrics performed at Los Alamos. It shows the variation of break down voltage of dielectric materials, in sheet form and cable configuration, with temperature and pressure under dc and impulse voltage. The surface flashover characteristics with large creepage distance as well as electrical conductivity of dielectric materials at cryogenic temperatures are discussed. These studies are essential for the design of high voltage apparatus operating in cryogenic environments.     

Natural frequency spectrum of a flat superconducting cable

A matrix method is used to investigate the current damping process in a flat superconducting cable. A discrete spectrum of natural frequencies is obtained, each determining the rate of exponential damping of the corresponding induced current. Although the number of natural frequencies increases as the size of the cable increases, their spectrum remains finite because the maximum and minimum frequencies tend to finite limits. An analysis is made of the induced currents for the limiting frequencies. It is shown that in the range of minimum natural frequencies the induced currents are long-lived long current loops. At high frequencies the distribution of the induced currents in cable layers is sinusoidal. Pis’ma Zh. Tekh. Fiz. 25, 42–47 (July 12, 1999)     

Investigation of superconducting cable cooldown

An analytical expression for a longitudinal wall temperature profile emerging with the cooldown of a superconducting cable has been obtained. The method used assumes a zone of steady temperature profile travelling along the line at a constant velocity. A comparison with experiment is made.     

Temperature rise during the tensile test in superfluid helium

Measurements were made on the temperature rise of tensile test specimens undergoing plastic deformation and discontinuous flow in superfluid helium. Compared with the results in normal liquid helium, the frequency of load drop increased, and the temperature rise was suppressed.     

Current distribution calculation for a superconducting multi-layered power cable

The calculation method for the current distributions within a coaxial multi-layered power cable consisting of helically wound long superconducting tape conductors have been studied. The analytical expressions for the self- and mutual inductances of the coaxial helical long thin arch-shaped tapes are analytically and numerically compared with straight arch-shaped and flat tapes. It is confirmed that these expressions are consistent with each other. The current distributions within a coaxial 3-layered cable are calculated, using the self- and mutual inductances obtained due to the thin approximation. The effect of the non-uniform gaps between tapes in a coaxial multi-layered power cable is studied.     

dc and ac behaviour of a normal joint in a superconducting cable

The application of joints which can be disassembled is studied within the framework of the development of high current (25 kA), high power (0.5 kW) cryogenic current supplies (flux pumps).^1,2 The behaviour of a scale model with currents up to 5 kA will be presented here.Several means of diminishing the joint resistance have been researched with special attention being paid to detection methods to measure the resistance, and their accuracies.The role of the solder has been investigated. A simple model gives good qualitative understanding of the matter.The presence of ac currents in a cryogenic current supply causes considerably higher losses in the joint because of the non-homogeneous distribution of the current. A good impression of this distribution can be obtained by measuring the field along the joint. The measured results are in good agreement with calculated ones. A useful length of the joint under ac conditions can be defined and is helpful for design purposes.     

Influence of a transverse conductance on current sharing in a two-layer superconducting cable

A simplified model is used to study the current sharing in a two-layer superconducting cable. Since the matrix in the case of the composites, and the copper oxide in the case of the cables are conductors, they can couple wires together electrically so that the resulting current redistribution no longer depends only on the mutual and self-inductance coefficients. The boundary conditions assigned by the series resistance at the input terminals then lead to a particular sharing which propagates along the conductor according to a diffusion equation. Several typical examples are calculated which point out the absolute necessity of using fully transposed cables, to achieve equal sharing for pulsed currents. The transverse conductance associated with series resistance can help distribution for very slowly rising currents in small coils. Also it can help to carry the current when a wire happens to return to the normal state or to be broken in a pulsed magnet.     

Coolant flow rate control in a superconducting cable under changing current load

This paper deals with coolant flow rate G(t) control in an ac superconducting cable under a highly changing current load l(t). The advantages of a cable with a flow rate that changes according to an optimum law G_optl(t) over a cable with a steady flow rate G, equal to time average value G_optl(t), are evaluated on a short cable. These advantages lie in the reduction of the time average heat in-leak by 15–100% due to hysteretic losses and of the required superconductor safety factor.     

Experimental simulation of helium pressure rise during a quench of a superconducting coil cooled by a superfluid helium bath

Experimental and numerical studies have been conducted with the aim of modeling pressure rises which occur in the helium, during quenches of the 11.7-T superconducting magnet named Iseult. Iseult is based on a double-pancake winding internally cooled by superfluid helium channels opening into a pressurized He II bath at 1.8 K. A scale mock-up has been built of 10 copper equivalent pancake slices and 7 helium channels per pancake. The heat produced by a quench of the Iseult magnet is simulated by electrical heaters put inside each copper plate. Cryogenic pressure and temperature sensors have been fitted in the helium channels and in the bath. Bath pressure measurements are given for various heating powers, various numbers of heated plates and various bath volumes. Comparisons with a simple numerical model permit to identify the main physical mechanisms which drive the pressure rise during a quench.     

Comparison of efficiency of superconducting cable envelopes

By comparison of power consumptions, the energy parameters and optimum operating temperatures were calculated for different type of cryogenic envelopes for superconducting power cables.     

Research and development of superconducting cable in Super-ACE project

Research and development of fundamental technologies for superconducting AC power equipments (called Super-ACE) project started as a national project in 2000 fiscal year by Ministry of Economy, Trade and Industry and New Energy and Industrial Technology Development Organization. This project is to research and develop the basic technology of high temperature superconducting cable (HTS cable), fault current limiter, and transformer. Main subjects of the cable in this project are to study the 3 kA superconducting cable conductor, cooling technology of 500 m HTS cable, and power system analysis of cable and AC equipments. The present paper describes the background of this project, target of each research subject, schedule of research and development, and major results of research activity in 2001.     

Fabrication of sections of a Nb3Sn-based rigid superconducting cable

Using Nb₃Sn layers deposited onto the outer and inner surfaces of copper tubes 30 and 50 mm in diameter, we have fabricated sections of a rigid superconducting coaxial cable up to 1 m in length. The highest current-carrying capacity of the cable at 4.2 K was 800–850 A/mm, which corresponded to a critical current density of (5.0–5.5) × 10¹⁰ A/m² in the Nb₃Sn layer.     

Temperature stabilization conditions for a cryogenic cable

Conditions for temperature stabilization of a cryogenic cable are analyzed as a function of redistribution of coolant flow over axial and peripheral channels.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 33, No. A, pp. 656–661, October, 1977.     

The impulse voltage flashover of dielectric spacers in a helium-insulated superconducting cable model

Phototypes of a helium-insulated superconducting cables, with conductors and dielectric of realistic geometry and construction, have been subjected to impulse voltage tests in a superconducting cable test facility. Five metre lengths of a co-axial pair of tubular conductors, separated by insulating spokes, were evaluated in helium at 5.5 K and 4 bar pressure. A pressure vessel was designed and built for this purpose and was accommodated in a horizontal cryostat, 8 m in length and 305 mm bore. Cooldown of the cable was achieved by a flow of cold helium circulated from an external refrigerator.Previous work had shown that dielectric spacers made from either polypropylene or a filled epoxy resin (Stycast 2850 FT) might perform well. First flashover voltages were 96 kVp for polypropylene spacers and 94 kVp for Stycast spacers. Flashover took place across spacers in the lower region of the cable; it is suggested that particulate contamination governed flashover. Polypropylene spacers deformed during test, but Stycast spacers only broke up at flashover sites and are preferred.