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.     

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.     

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.     

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.     

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.     

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)     

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.     

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.     

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.     

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.     

Dielectric losses in a polyethylene insulated superconducting cable between 4 and 22 K

The dielectric losses in a 5 m length of a superconducting cable, insulated with lapped polyethylene tape, have been determined between 4.3 K and 22 K with an inductively coupled ratio arms bridge which gave a resolution in tan δ of about 10^?6. The cable sample was built to a specification appropriate for power transmission at 132 kV and 6 kA; it was impregnated with helium at 0.4 MPa and cooled by a flow of cold helium circulated from a refrigerator.In the temperature range 4 to 5 K, applicable to a cable with niobium conductors, the dielectric loss increased slightly with electric field, and the observed tan δ (relative to tan δ of the reference capacitor) was 20 × 10^?6 at the maximum test stress of 7 MV m^?1. It is thought that a significant part of this loss can be attributed to the bedding layers and electrostatic screens between the dielectric and conductors.Tan δ showed a minimum at 18 K, which is encouraging for future developments of superconducting cables which might use hydrogen as a coolant and dielectric impregnant. The observed variation of tan δ with temperature is consistent with the existence of a low temperature relaxation mechanism with low activation energy.     

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.     

Current test on a flexible superconducting core for a 2 GVA ac cable

A 5 m long prototype co-axial flexible superconducting cable core has been made and tested at currents up to 30kA. The inner and outer tubular conductors were both formed from helically-laid strips, and the dielectric between was lapped polyethylene tape. The dielectric was tested in separate experiments. The conductor strips contained layers of nobium, niobium-zirconium and high conductivity copper. The axial contraction of the cable core was restrained by titanium tie-rods and the lay angles of the conductor strips were chosen so that the core tightened radially on cooldown. Lead-filled termination cylinders between the ends of the cable and the current leads inhibited the formation and propagation of normal regions at high currents. Local and average ac loss measurements were made from 4.7 to 10.2 K and at current densities between 10 and 200 A mm^? with very satisfactory results.     

Solenoids of composite conductor simulate ac losses in flexible superconducting cable

The complex current distribution on the individual strands of a flexible superconducting cable influences the ac loss. Hence reliable loss predictions cannot be made from previous measurements. It is shown that the required current patterns and losses can be simulated by winding the conductor into single or double layer solenoids. Both total and localized losses can be measured with suitably arranged voltage probes. Measurements have been made on five different strip conductors wound into single layer solenoids to simulate losses on the inner conductor of a flexible cable. Measurements on niobium clad copper composite conductor showed that edge losses contributed more to the total loss than had been predicted theoretically for an idealized case. Despite this the total losses averaged only 15 mW m^?2 at 40 A mm^?1 rms, at 4.2 K. Measurements were also made on a $\text{Nb}\text{Nb-25\%}$$\text{Zr}\text{Cu}$ conductor, developed to carry fault currents in the NbZr and fabricated by soldering together Nb clad NbZr and Nb clad Cu composites. At fields below about 100 A mm^?1 rms, currents were carried by the niobium surface layer and at higher fields flux penetrated into the NbZr underlayer. Thus losses were acceptably low over the entire field range.     

Superconducting properties of Nb-25%Zr relevant to its use in a superconducting power cable

The critical current density of co-processed copper-clad Nb-25%Zr wire has been studied as a function of annealing time and temperature with reference to the use of this alloy in a superconducting power transmission cable. By a suitable choice of annealing conditions a critical current density of more than 9 kA mm⁻² was obtained at 6.5 K in a 250 μm wire in zero applied field. This exceeds the minimum likely to be necessary for the hard type II component in a cable.¹ It is suggested that the sharply defined dislocation sub-structure is more likely to control the critical current density in the specimens investigated than either the β_Zr or α_Zr precipitates.