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.     

Design of flexible coaxial cores for ac superconducting cables

The phase (inner) and neutral (outer) tubular conductors are each formed from two layers of niobium-clad copper strips laid in helices of opposite sense. The dielectric is lapped high density polyethylene tape impregnated with helium. The lay angles of the conductor strips are chosen to meet the requirements of thermal contraction and magnetic pressure from large fault currents.The main advantage of using twin, instead of single conductor layers is that there is no axial magnetic flux within the core and hence the associated problems are avoided. A further advantage is that when the thickness of the two copper layers is optimized there is a useful reduction in power dissipation when the superconductor is normal. This improves both the cable's ability to carry fault currents and the stability of the conductor against reversion to the normal state when carrying the rated current. Since the optimum thickness of each layer (～0.5 mm) is about half that in a single layer cable, the double layer would not be significantly more expensive.     

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.     

Electrical tests of short models of polyethylene tape dielectric for a superconducting ac cable

Electrical tests on short models of the dielectric for a lapped polyethylene insulated superconducting cable are described. Impulse breakdown strengths and 50 Hz discharge inception fields are reported. The impulse strengths were encouraging but the discharge inception fields showed that further development is required.     

ac loss measurements on a superconducting transformer for a 25 kA superconducting rectifier

ac loss measurements have been performed on a superconducting transformer. The transformer is a part of a 25 kA thermally switched superconducting rectifier operating at a frequency of 0.1 Hz. The loss measurements have been automatized by means of a microcomputer sampling four relevant signals and processing these data to reliable loss numbers. Results were obtained at amplitudes of the secondary current between 5 and 15 kA at various current rates between 5 and 30 kA s^?1.     

A flexible super conducting ac cable: radial thermal contraction and the x-ray examination of a sample length of cold core

The temperature reduction which a superconducting cable core will have to undergo following its manufacture and installation is nearly 300 K before it can be used. The satisfactory accommodation of the corresponding significant amount of thermal contraction of its component parts is therefore of major importance.This paper is concerned with such thermal contraction upon cooling of a flexible superconducting ac cable core comprising helically laid strip conductors of niobium clad copper and a polyethylene tape dielectric with electrostatic screens and bedding layers.A method is described of designing, for a controlled amount of radial contraction, a core held at near constant length. A report is also given of the x-ray examination of a sample core used for voltage tests. The relevance of the results to some other designs of core is discussed.     

Cryogenic design and test results of 30-m flexible hybrid energy transfer line with liquid hydrogen and superconducting MgB2 cable

In this paper we present the development of a new hybrid energy transfer line with 30 m length. The line is essentially a flexible 30 m hydrogen cryostat that has three sections with different types of thermal insulation in each section: simple vacuum superinsulation, vacuum superinsulation with liquid nitrogen precooling and active evaporating cryostatting (AEC) system. We performed thermo-hydraulic tests of the cryostat to compare three thermo-insulating methods. The tests were made at temperatures from 20 to 26 K, hydrogen flow from 70 to 450 g/s and pressure from 0.25 to 0.5 MPa. It was found that AEC thermal insulation was the most effective in reducing heat transfer from room temperature to liquid hydrogen in ∼10 m section of the cryostat, indicating that it can be used for long superconducting power cables. High voltage current leads were developed as well. The current leads and superconducting MgB₂ cable passed high voltage DC test up to 50 kV DC. Critical current of the cable at ∼21 K was 3500 A. It means that the 30 m hybrid energy system developed is able to deliver ∼50–60 MW of chemical power and ∼50–75 MW of electrical power, i.e. up to ∼135 MW in total.     

Electromagnetic field and ac power loss enhancements at the edges of adjacent strips in a superconducting cable

In most flexible superconducting cables the current is carried by a series of strips laid along the length of the cable with small gaps between their edges to accommodate bending. One consequence of dividing the conductor into strips is an enhancement of the electromagnetic fields in the vicinity of the gaps. The ac losses in the superconductor are thereby also enhanced. Data are presented which enable the effects of these field and loss enhancements on cable performance to be assessed. In particular it is found that there is an optimum corner radius for the strips to achieve a minimum increase in loss compared to a smooth tube and that this increase need not be large for a suitably rounded corner. A non-uniform distribution of the strips around the cross-section of the cable can be deleterious while any radial displacements are likely to increase the losses considerably.     

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.     

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)     

Temperature rise in a superconducting cable during overload

Approximate calculations are presented for the temperature rise and temperature fluctuation in an ac superconducting cable during current overload. The calculations are for a cable, which it is intended will remain superconducting throughout the overload. These show that because of the low value of T_c and low J_c niobium-titanium is not a suitable material to use. Niobium-zirconium with a 1 mm copper backing will carry continuous overloads of up to × 9 and a first cycle overload of up to × 11 provided it is fully stabilized against flux jumps. (Normal surface current rating = 4 × 10⁴ A m^?1). Niobium-tin can in principle carry very large overloads, but the limiting factors are its poor mechanical properties, the difficulty in producing thick layers of the material, and stabilization.     

Refrigeration for rotating superconducting windings of large ac electric machines

The rotating superconducting windings of large ac machines must be supplied with enough refrigeration at an adequate temperature. The problems associated with bringing liquid helium to the rotating winding structure are analysed. Existing designs as well as newly proposed systems are used to draw the general guidelines for the design concepts of cooling systems for rotating superconducting windings.     

Current redistribution and stability of superconducting triplex cable without electrical insulation carrying non-uniform current

When a small normal zone is produced in a strand in a superconducting cable by a local disturbance, current redistribution occurs and can help the recovery of its superconducting state. This effect of the current redistribution depends on the initial current distribution. In this paper, the initial current distribution in a triplex cable is controlled artificially to study its influence on the stability against local disturbances. A heat pulse is applied to a strand by a carbon paste heater to initiate the quench. Transient current distributions during quench or recovery process are measured by sets of Hall sensors placed at several locations along the cable axis. When the transport current is less than a threshold value, the stability is improved by the current redistribution from the heated strand to the others, even if the initial current distribution is not uniform. This threshold is related to the current margin of the heated strand, rather than to the current margin of the whole cable. Above this threshold current, the MQE against local disturbances is smaller in the cable with the non-uniform current distribution than in the cable with the uniform current distribution, when the total transport current is identical. If the current of the heated strand is identical and above the threshold, the MQE against local disturbances does not depend on the current distribution and agrees with the MQE of the single isolated strand. It means that the quench of one strand leads to the quench of the cable, and that the stability of one strand in the cable is not influenced by interaction with the others.     

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.     

Laying method for superconducting feeder cable along railway line

The superconducting feeder cable shrinks in cooling process from the room temperature to the temperature of liquid nitrogen. When a long superconducting feeder cable is constructed in a railway line, it is needed to consider countermeasures against the cable shrink due to cooling force. We report on a suitable method of laying the cable in 300-m class on the test track. We also report that the method is suitable for this cable because no buckling and rupture points were observed in the X-ray radiograph taken over the whole length of the cable after its installation.     

A review of dielectrics for flexible superconducting cables

The tape materials for the dielectric of a flexible superconducting cable are discussed. The tape is wrapped around the superconductor and is impregnated with supercritical helium. Based on mechanical and electrical performance the most promising materials are Valeron, high density polyethylene, polypropylene, and nylon 11. A final assessment is made based on life expectancy and mechanism of electrical failure both of which are a function of discharge activity in the tapes.     

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.