Future transportation aircraft is expected to rely on a new propulsion system based on both gas turbines and electric motors and generators. The technology distributed turbo-electric propulsion, labled as potential technology for the N+3 aircraft, is expected to reach a TRL 5-6 by the year 2025. As part of a NASA supported project, a high fidelity sizing tool for fully superconducting machines was developed with a new AC losses scaling law for superconducting stators. The model covers all the aspects of the design (electromagentic, thermal, structural and cryogenic) and generates designs at the preliminary design details level. This paper presents the final version of the sizing tool as well as two optimized designs for a propulsion motor and turbo-generator. The objective for the machines was set in terms of specifics power (> 25 kW/kg) and overall efficiency. The paper will also include a comparison with state-of-the-art conventional permanent magnet machines.

This work was partially supported by NASA Glenn Research Center.

A novel double-layer 2G HTS winding is proposed for use as the armature winding in fully HTS machines. Our study shows that a conventional single-layer 2G HTS winding results in a large component of backward travelling magnetic field, which will cause undesirable AC loss in both the HTS excitation winding and the HTS armature winding. To design a HTS machine with low AC loss and high efficiency, we introduce the double-layer HTS armature winding concept. Our numerical analysis shows that the double-layer HTS armature winding is very effective in eliminating the backward travelling magnetic field, whilst producing a forward travelling field for the machine operation with a low harmonic content. AC loss analysis by finite element methods shows that the double-layer HTS armature winding is effective in reducing the AC loss in both excitation and armature windings.

In this work, a novel 15 kW-class fully-superconducting synchronous generator was designed and constructed, using HTS coils in both stator and rotor. It has a basic structure of eight poles and nine slots. The rated output power is 15 kW at the rotating speed of 150 rpm. There are 16 field coils and 18 armature coils, both of which were wound by using 5 mm width YBCO tapes with the critical current above 200 A at 77 K. The total tape consumption is below 1.8 km. Iron cores assembled by silicon-steel sheets were installed in both stator and rotor. The armature loading can reach 500 A/cm. Gap field of 0.8 T can be achieved with 45 A excitation current in field coils. A concentrated winding configuration was proposed for both windings to prevent interference at the ends of adjacent HTS coils. The present machine is designed to be conduction cooled by liquid nitrogen. The cryogenic structure can be divided into two parts: the rotating one for field coils, and the static one for armature coils. Both parts share the same vacuum chamber. Two thermal-insulated supporting rings (TISR) were installed at the ends of the static cooling part, which co-axially supported the main body of the HTS generator in the vacuum chamber. A thermal-insulated torque cage (TITC) was installed to link the cold rotor and the warm shaft. It was constructed by two flanges and several G10 sticks. Structural optimization for this TITC was carried out considering both stress and heat leakage. Simulations showed the total heat leakage through TISR and TITC is below 15 W and the stress level is safe. The present work is a firm step for future large HTS generators/motors using fully-superconducting configuration.

The authors own great thankfulness for the support from the National Natural Science Foundation of China (51475257), the Development Foundation of Shenzhen (JCYJ20130402145002389), and the Tribology Science Fund of State Key Laboratory of Tribology, China.

We developed a 6 kVA 3-phase model of the flux-transfer type “current-limiting” transformer. In this type of “current-limiting” transformer, the primary and the secondary coils are wound on two independent iron cores, and, then, the two iron cores are coupled with each other by using a closed loop of YBCO superconducting tapes. The possibility of applications is based on the principle of the magnetic flux constancy in the closed loop. We have tested the system and the results in the case of the 1-, 2- and 3-phase fault current with the break of one secondary coil. We compare the joule loss (I²R) in the coils to the results in the conventional transformer. The limiting current can be kept at a considerably lower value than the fault current of a traditional 3-phase transformer of the same apparent power. The short circuit current in this arrangement is less than the operational current within 3-4 periods. Since the fault current is decreased to some extent in the circuit, it is not good enough for the conventional protection. Therefore, we introduce this method to fit it for actual protection. The superconducting wire used for the transformer windings as closed loops is found to be very suitable for the effective fault current limiting. In the measurements, the fault time is found to be more than 1 s, and the superconducting wire and the device can operate continuously without any damage. The recovery time is found to be dependent on the serial or parallel superconducting wire arrangements with different coupling mechanisms between iron cores. We report the evaluation of the method in terms of the efficiency, technological novelty and feasibility for use in potential applications in the grid. These results are to be reported as new, and detailed analysis with all accounts will be presented.

This research has been financially supported by Republic of Turkey Ministry of Development (Grant No. 2010K120520).

Electrical power infrastructure in the world needs to evolve to meet the increasing demands of incorporating renewable energy sources and mitigate society’s discomfort with unintended consequences of the use of fossil fuels. There have been significant investments in developing the science and technology of superconducting power devices to provide answers to some of the challenges. Prototypes of superconducting power cables, fault current limiters, motors, generators, etc., have been successfully demonstrated in many countries, but their complexity and costs have hindered their widespread application. There is a need for innovations in systems engineering of superconducting power infrastructure to make their designs and operations simpler, rugged, and more reliable. Significant advances in cryogenic engineering and superconducting device technology are necessary to bring the promise highly power dense, efficient, and reliable superconducting power devices to the marketplace.
Superconducting power devices are complex and successful engineering of superconducting power systems require symbiosis among superconducting materials science, cryogenic thermal engineering, mechanical and structural engineering, electrical engineering, controls, and systems engineering. The emphasis in this direction at Florida State University has been simpler and versatile cryogenic cooling of superconducting power devices using closed loop cryogenic helium gas circulation (GHe). This paper will present the potential the GHe technology to simplify and broaden the design space for superconducting power applications and enhanced opportunity for system optimization based on multiple constraints such as cost, power density, variable power rating, etc. An analysis of outstanding technical challenges and current research and development on making GHe a viable option for cooling HTS devices are presented.

This work was supported by the Office of Naval Research.