Swarn S. Kalsi

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.

The performance of a 5 MW high temperature superconductor ship propulsion motor

A 5 MW, 230 RPM, 6-pole high temperature superconductor (HTS) ship propulsion motor is presently under test at the Center for Advance Power Systems (CAPS). This paper provides a summary of the key design features of the motor, predicted performance, factory test results and extended test results to date at CAPS. This motor was designed and built under the U.S. Navys Office of Naval Research (ONR) funding (Contract #N00014-02-C-0190) to address the next generation of electric ship propulsion systems. HTS motors are characterized by high power density, quiet operation and high efficiency. HTS air-core motors have unique electrical characteristics and therefore require dynamic testing to validate all modes of operation. The test program at CAPS is designed to address dynamic performance and simulation of this class of propulsion motor. The motor has been operated at 5 MW load for over 3 hours at CAPS.

The status of HTS ship propulsion motor developments

The development of ship propulsion synchronous motors with high temperature superconductor (HTS) field windings for Naval electric ship applications has progressed to the point where a full scale motor is now under construction. A 5 MW, 230-rpm prototype ship propulsion motor was built and tested by the Center for Advanced Power Systems (CAPS) on behalf of U.S. Office of Naval Research (ONR). It met or exceeded all its design goals. Currently, a 36.5 MW, 120-rpm ship propulsion motor is being built for delivery to ONR at the end of 2006. This paper presents test results of the 5 MW motor and the status of the 36.5 MW motor

Power applications of high-temperature superconductors: status and perspectives

The potential of superconductors to have a revolutionary impact on how electric power is generated, delivered and used has long been recognized. The first superconducting power-grid application to achieve full commercial status is superconducting magnetic energy storage (SMES); the magnets of these systems have so far been fabricated primarily with metallic low-temperature superconductors (LTS). Although LTS prototypes have been demonstrated for motors, generators, power cables, transformers and current limiters, high-temperature superconducting (HTS) systems offer striking economic and system reliability advantages and are now seen as the central vehicle for broad commercialization of superconductivity in the power grid. Operating at temperatures from 30 to 80 K, they open the door to highly simplified cryogenics and increased stability, which result in economic systems not feasible with LTS. HTS prototypes at commercial power levels have already been demonstrated, particularly power transmission cables and motors. Key merits as well as remaining open technical challenges for such HTS applications are reviewed in this paper.

The status of HTS motors

The status of high temperature superconducting (HTS) motor development is presented. HTS synchronous machines have been under development for over 12 years around the world. The unique characteristics for selected applications such as ship propulsion are discussed. The beneficial characteristics of air core HTS motors for ship propulsion include high power density, high efficiency and low noise production. This paper also addresses recent developments including a 5,000 HP 1800 RPM 4 pole prototype and the ongoing construction of a 5 MW 230 RPM motor. The 1800 RPM motor is a prototype constructed to validate technologies for industrial motors and generators and the 230 RPM motor is being constructed to validate technologies for ship propulsion motors in the range of 25 MW and 120 RPM.

HTS generator topologies

A variety of topologies have been studied for superconducting generators. The superconducting generators constructed over the past 40 years using low temperature superconductors were built with iron located only in the stator core. The performance characteristics of HTS has led to the consideration of topologies with iron in the center of the rotor inside the field winding, iron in the rotor pole faces and iron in the armature teeth. As the reluctance of the flux path is decreased, the amount of superconductor is minimized and the forces on the rotor and stator coils are reduced. The impact of this addition of iron on other machine parameters including transient stability following a system fault is addressed in this paper

Superconductor synchronous condenser for reactive power support in an electric grid

High Temperature Superconductor (HTS) SuperVAR dynamic synchronous condensers (DSC) developed by American Superconductor have a small foot print, are readily transportable, and are expected to be an economic option for providing peak and dynamic reactive compensation to a power system. HTS DSC machines are also inherently stable to close in faults and can provide up to twice their nominal rating for about one minute (peak rating) during depressed voltage events. Last, but not least, HTS DSC machines use less than half of the energy of a conventional synchronous condenser and about the same amount of energy as a modern Flexible AC Transmission System (FACTS) device consumes. It is expected to be highly reliable. The first HTS DSC machine is being operated at an arc furnace where it is being tested for its ability to mitigate flicker and provide dynamic power factor compensation. This location also exposes the machine to a large number of transients providing an excellent accelerated age test of the device. This paper describes features and test results of the HTS DSC.

Superconducting dynamic synchronous condenser for improved grid voltage support

Synchronous condensers are an attractive source of dynamic VARs (both inductive and capacitive) to improve system stability and maintain voltages under varying load conditions and contingencies. A synchronous condenser is a rotating machine that runs synchronized with the grid. Its field is controlled with a voltage regulator to either generate or absorb reactive power as needed by the power system. American Superconductor Corporations (AMSC) proprietary SuperVAR/spl trade/ dynamic superconducting condenser (DSC) machines upgrade existing technology by using a conventional armature mated with a field winding made from high temperature superconducting (HTS) wires. The result is a DSC that is both more efficient and has lower maintenance than conventional machines. It can provide up to 8 pu current for short periods to support transient VAR requirements. The Tennessee Valley Authority (TVA) is sponsoring the development and field testing of an 8 MVAR prototype unit to be followed by five 10 MVAR production units. Larger units are planned for the future. This paper describes features of the DSC and its performance in grid applications.

Long length calorimetric measurement of AC losses of Bi-2223 external field oriented perpendicular to the tape width

AC applications are projected to be a significant market for HTS conductor. The AC magnetic fields perpendicular to the wide face of Bi-2223 conductor (or parallel to the crystallographic c-axis of the oxide superconductor) are responsible for a significant portion of the total loss in AC coils. ASC has developed a calorimetric apparatus which can measure long lengths of conductor in a uniform continuous perpendicular field. This apparatus provides Accurate measurement of interfilament or interstrand coupling losses. Calorimetric measurements from long lengths of twisted multifilamentary conductor are presented.

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.