Niklas Magnusson

Commercial Induction Heaters With High-Temperature Superconductor Coils

The induction heater for aluminum, copper and brass extrusion billets is an application where high-temperature superconductors (HTSs) provide clear benefits compared to conventional technology. With an energy efficiency of just some 50% for conventional technology and limitations in processing speed, the potential for improvements is evident. A novel heater design based on superconductors, and a fast product development have led to an industrial breakthrough. With one unit in operation since August 2008 and four more sold, the superconducting induction heater has become the first true commercial HTS product. Radical efficiency increases, improvements in process speed, quality and cost have been demonstrated in a rough industrial environment. In this paper we explain how to fully utilize the superconductor in this application by altering the established concepts of induction heating. The electromagnetic realization as well as cryogenic integration based on off-the-shelf cryo-coolers are described. Furthermore, operational experience from heating up 10,000 tons of aluminum (350,000 billets) in the first commercial installation is presented.

Losses in a BSCCO/Ag tape carrying AC transport currents in AC magnetic fields applied in different orientations

In most prospective electric power applications of high-temperature superconductors (HTSs), the conductor is wound in a coil configuration. For a coil of finite length, the magnetic field orientation is axial in the middle of the coil, while the field contains a substantial radial component at the coil ends. In a superconducting BSCCO/Ag tape, the AC losses depend strongly on the orientation of the magnetic field. In this study, we present experimental results of the AC losses in a multifilamentary silver-sheathed Bi-2223 HTSs tape, carrying alternating transport currents in externally applied alternating magnetic fields at different orientations, with respect to the face of the tape and perpendicular to the transport current. The AC losses were measured calorimetrically, at fixed temperature and frequency. The results are compared to semi-empirical models of the AC losses in HTSs tape. We present a more general model of the angular dependence of the AC losses.

Design, building and testing of a 10 kW superconducting induction heater

Conventional 50/60 Hz induction heaters for aluminum billets have very large losses. By replacing the copper windings with windings of high-temperature superconducting (HTS) tapes, there is a substantial potential for efficiency improvements, especially if low AC loss HTS tapes become available. To examine the feasibility of using HTS in induction heaters, a first, small-scale working model has been designed and built. The induction coil is made of 24 double pancake coils of Bi-2223/Ag tapes. In the initial test, a workpiece of aluminum situated in the warm bore of the coil was heated up to 300/spl deg/C.

Design of an MgB2 race track coil for a wind generator pole demonstration

An MgB2 race track coil intended for demonstrating a down scaled pole of a 10 MW direct drive wind turbine generator has been designed. The coil consists of 10 double pancake coils stacked into a race track coil with a cross section of 84 mm x 80 mm. The length of the straight section is 0.5 m and the diameter of the end sections is 0.3 m. Expanded to a straight section of 3.1 m it will produce about 1.5 T magnetic flux density in the air gap of the 10 MW 32 pole generator and about 3.0 T at the edge of the superconducting coil with an operation current density of the coil of 70 A/mm 2 .

Efficiency analysis of a high-temperature superconducting induction heater

In an induction heater for heating of aluminum billets, the main reason for using high-temperature superconducting (HTS) tapes instead of conventional copper conductors is to reduce the large power losses in the induction coil. In this work we present how to calculate the different loss contributions of an induction heater based on HTS tapes. Calculations of these losses are used in the design of an HTS induction heater with a rated power of about 10 kW operating at liquid nitrogen temperature. The calculations predict an optimal current that yields the highest efficiency of the induction heater.

Ripple Field AC Losses in 10-MW Wind Turbine Generators With a MgB 2 Superconducting Field Winding

Superconducting (SC) synchronous generators are proposed as a promising candidate for 10-20-MW direct-drive wind turbines because they can have low weights and small sizes. A common way of designing an SC machine is to use SC wires with high current-carrying capability in the dc field winding and the ac armature winding is made with copper conductors. In such generators, the dc field winding is exposed to ac magnetic field ripples due to space harmonics from the armature. In generator design phases, the ac loss caused by these ripple fields needs to be evaluated to avoid local overheating and an excessive cooling budget. To determine the applicability of different design solutions in terms of ac losses, this paper estimates the ac loss level of 10-MW wind generator designs employing a MgB2 SC field winding. The effects on ac losses are compared between nonmagnetic and ferromagnetic teeth with different numbers of slots per pole per phase. The necessity of an electromagnetic shield is then discussed based on the obtained loss levels. The results show that the total ac loss is so small that ferromagnetic teeth can be applied in the generator design without using an electromagnetic shield.

Bolted Connectors for Stranded Aluminum Power Conductors

Sixteen different commercial bolted connectors for 50 and 240 mm stranded aluminum cable conductors have been examined in order to identify and clarify the correlations between the quality of a connector and its design characteristics, material usage, assembly procedures, bolt numbers and configurations and other relevant parameters. The quality is assessed with basis in resistance measurements after short circuit testing and during thermal cycling as specified by the IEC 61238-1 standard. The tightening torque of the bolts was measured during connector assembly, and the resulting axial compressive forces were calculated. A very strong correlation is observed between this compressive force and the connector quality; large forces give connections with low and stables resistances. The various other design parameters considered are found to have significantly less or in some cases hardly any notable influence on the electrical test results.

AC Loss Measurements on Multi-Filamentary

The MgB2 superconductor is an attractive choice for many high-current dc applications, particularly for its low cost compared with, e.g., YBCO, and the higher operating temperature than the low-temperature superconductors. To extend the operating range to include also ac applications, the ac losses need to be determined and a path towards a low-loss conductor needs to be established and verified. To accomplish this development, systematic and reliable measurements of the losses are essential. In this paper, we present ac loss measurement results on nontwisted MgB2 wires with titanium and copper-nickel matrices. 50 hertz ac losses were measured calorimetrically in applied magnetic fields, with transport currents, and with combinations of both. The temperature range was between 25 K and Tc, the magnetic field was varied up to 300 mT, and the transport current up to 113 A. The measurement results are used as a basis to discuss the necessary ac loss reduction steps in MgB2 superconductors.

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This article proposes how thermal and pressure gradients cause cavities to form in the insulation system of mass-impregnated nondraining HVDC subsea cable insulation, during high load and load changes.

Wires With Non-Magnetic Sheath Materials

The MgB2 superconductor is an attractive choice for many high-current dc applications, particularly for its low cost compared with, e.g., YBCO, and the higher operating temperature than the low-temperature superconductors. To extend the operating range to include also ac applications, the ac losses need to be determined and a path towards a low-loss conductor needs to be established and verified. To accomplish this development, systematic and reliable measurements of the losses are essential. In this paper, we present ac loss measurement results on nontwisted MgB2 wires with titanium and copper-nickel matrices. 50 hertz ac losses were measured calorimetrically in applied magnetic fields, with transport currents, and with combinations of both. The temperature range was between 25 K and Tc, the magnetic field was varied up to 300 mT, and the transport current up to 113 A. The measurement results are used as a basis to discuss the necessary ac loss reduction steps in MgB2 superconductors.