Heinz-Werner Neumueller

Development status of rotating machines employing superconducting field windings

Superconducting rotating machines have looked promising since multifilamentary niobium-titanium (NbTi) superconductors became available in the mid-1960s. Both dc homopolar and ac synchronous machines were successfully tested from the 1970s to the 1990s. Three different 70-MW generators were recently demonstrated by the SuperGM project in Japan. However, economic considerations with respect to competitive cost combined with the requirement for liquid helium cooling did not make these machines commercially attractive. On the other hand, high-temperature superconductors (HTSs) can operate at much higher temperatures (30-40 K), providing much larger thermal margin and simpler cooling systems. This refrigeration advantage has provided new impetus to the development of such machines for commercial applications. In the last few years, a number of superconducting rotating machines with HTS field windings have been demonstrated and several projects are currently transitioning to advanced development stages. HTS machines with ratings from a few kilowatts to several megawatts have been demonstrated in the United States and Europe. Currently, large high-torque ship propulsion motors, large generator prototypes, and synchronous condensers are under development and are expected to be commercially available in the next few years. Prospects for improved life cycle cost, smaller size, less weight, and higher efficiency benefits are providing incentives for the development of these larger rating HTS machines. This paper reviews the past and recent progress on the worldwide development of industrial-grade superconducting rotating machines utilizing low-temperature superconductor and HTS field windings and provides an outlook on the benefits and opportunities of this new technology.

Investigation of YBCO Coated Conductors for Fault Current Limiter Applications

The switching behavior of second generation (2G) HTS wires with a coated conductor (CC) architecture for application in fault current limiters (FCL) has been studied. Measurements have been performed on short pieces of meter length, as well as on bifilar coils wound from 20 meters of CC. CCs with and without stainless steel stabilizers have been compared regarding the temperature reached after applying a constant voltage per length to the conductor for a certain time (typically 50 ms). Three coils from stainless steel stabilized wires have been connected in series to demonstrate uniform switching of the individual coils in a single phase FCL model with 120 kVA nominal power. The response to transient currents has been investigated by applying currents above the critical current Ic with different amplitudes and measuring the time at which an irreversible voltage increase occurs. All CCs investigated proved well suitable for FCL applications exhibiting the required uniformity and excellent current limiting performance.

Switching behavior of YBCO thin film conductors in resistive fault current limiters

The quenching process of YBCO thin film conductors designed for resistive fault current limiters has been analyzed by electrical and optical measurements. The influence of the switching voltage as well as the critical current and normal resistance of the thin film conductors on the quench propagation has been studied in detail. The experimental results show that homogeneous quenching can be achieved. The YBCO parameters can not be varied independently of the shunt layer thickness.

Test of a 1 kA superconducting fault current limiter for DC applications

A low voltage fault current limiter (FCL) having a nominal current of 1 kA has been set up using switching elements based on YBCO thin films fabricated by reactive thermal co-evaporation. The films showed critical current densities exceeding 1 MA/cm/sup 2/ @ 77 K. After patterning and contacting the films, the elements have been assembled in a closed cryostat for operation in a liquid nitrogen bath. The FCL model was successfully tested using prospective fault currents from 25 kA to 150 kA. The electric data show a peak current of 2.7 kA within 1 ms and a limitation to approximately nominal current within 5 ms. Due to this fast response of the YBCO switching elements, FCL coupled grids can be instantaneously decoupled during a fault, leaving the faultless part of the grid practically unimpaired.

HTS flux pump for cryogen-free HTS magnets

A reliable method for persistent-current mode operation of HTS magnets is still not available. As an alternative solution to external high-current power supplies we have developed a cryogen-free operating transformer-rectifier type HTS flux pump. Using this device only small ac currents have to be supplied into the cryogenic system. Both thermally triggered HTS thin film switches as well as cryogenic MOSFET switches have been used to operate a conduction-cooled 0.5 T HTS magnet coil. This paper presents results of experimental test and modeling of the flux pump operation.

Transposed-Cable Coil & Saddle Coils of HTS for Rotating Machines: Test Results at 30 K

We have manufactured and tested new HTS coil configurations in order to extend the range of rotating machines to which HTS tapes can be applied. The rotors of large utility generators require operating currents of several kA, which can be achieved by connecting several HTS tapes in parallel in a transposed cable. Design of a generator poses questions on winding technique for the coils, cooling technique at the required operating temperatures around 30 K, and V-I relation and stability of such coils. These questions were addressed by winding a test coil using 31 m of transposed cable comprising 500 m of BSCCO tape and testing it at 30 K. Smaller generators coupled to gas turbines have to work at many thousands of rpm. The high g-forces and need for compactness make it necessary to place the rotor coils as close as possible to the shaft, which requires coils with 3D-bent heads, like in accelerator dipoles. We have produced this type of coils with BSCCO tapes for the first time. After developing a winding technique using simple tooling, 700 m of BSCCO tapes were used to manufacture two 3D demonstrator coils, which were tested at 30 K as well. We describe winding methods, cooling technique and test results for both types of coils.

Economically Viable Fault Current Limiters using YBCO coated conductors

Resistive type superconducting fault current limiters (FCL) utilize a current-driven transition from the superconducting state to the normal state to limit short circuit currents in electric power grids. The FCL needs no active triggering at inception of a short-circuit and recovers automatically after the short circuit has been opened. The technical performance of superconducting fault current limiters has been demonstrated by numerous successful projects worldwide. Since the advent of commercial second generation (2G) high temperature superconductor wires (HTS) based on YBCO thin films, a cost-effective commercial design is becoming feasible. We have investigated the fault current limiting performance of 2G HTS wire stabilized with stainless steel tapes. Bifilar coils have been manufactured and tested with a typical limitation period of 50 ms under stepwise increasing voltage loads to determine the maximum temperature the wires can withstand without degradation. Several coils have been assembled into a limiter model to demonstrate uniform tripping of the individual coils and fast recovery within a few seconds. The FCL assembly planned for grid application consists of a switching module of HTS coils in parallel with an inductor made of normal conductor. It is designed to limit only a part of the total fault current. This permits use of existing circuit breakers and time dependent protection system. During normal operation, the effective impedance of the FCL assembly is essentially zero whereas during a fault, the HTS module transitions to the normal state and the fault current is limited by the inductor.