Advances in High Temperature Superconductors (HTS) are enabling a new class of synchronous rotatingmachines (SuperMotors and SuperGenerators) that can generically be categorized as SuperMachines. Compared toconventional machines of equivalent rating, these SuperMachines are expected to be less expensive, lighter, morecompact, efficient, and provide significantly superior stable operation in a power system. The field windings aremade with HTS conductor material (BSCCO, or Bi-2223) which operates at 35-40 K and can be cooled withinexpensive, off-the-shelf cryocoolers available from a number of manufacturers throughout the world. As will bediscussed, these advanced SuperMachines are attractive for use in industrial as well as naval and commercialmaritime industry applications. This paper discusses recent SuperMachine work at AMSC and other companies.HTS rotating machine technology is maturing rapidly, and electricity producers as well as the end-users willundoubtedly benefit enormously from these advancements.
The advent of high-temperature superconductivity hascreated the opportunity for a quantum leap in thetechnology of large electric machines. HTS-basedmotors and generators will be smaller, lighter, moreefficient, and less expensive to manufacture andoperate than conventional machines. The potentiallysignificant cost, size, weight and efficiency benefits ofsuperconducting machines will change the dynamics ofthe electrical machinery industry. This unique situationleads to reduced manufacturing costs.The initial use for HTS motors will likely be intransportation applications, particularly naval andcommercial ship (marine) electric propulsion, wherecritical size and weight savings will provide a keybenefit by increasing ship design flexibility. Electricdrive has already penetrated the cruise ship segment ofthe market because of its marked advantages overcompeting mechanical systems. The increased powerdensity and operating efficiency as well as otherbenefits of HTS based marine propulsion systems willsignificantly further expand the attractiveness ofelectric propulsion systems. HTS motors are ideal foruse in pumps, fans, compressors, blowers, and beltdrives deployed by utility and industrial customers,particularly those requiring continuous operation.These motors will be suitable for large processindustries such as steel milling, pulp and paperprocessing, chemical, oil and gas refining, mining,offshore drilling, and other heavy-duty applications.Superconducting wire in its Low TemperatureSuperconductor (LTS) form has been in widespreaduse now for over 30 years, and commercialapplications today range from high-powered particleaccelerators to sensitive resonance imaging systemsutilized for medical diagnostics. General Electric andWestinghouse independently conducted largesuperconducting generator design studies during the1970’s; both approaches were based on LTS wire.General Electric also built and tested a 20 MVAsuperconducting generator in the 1970’s, and aJapanese consortium built and tested a 70 MWgenerator during the 1990’s. These machinesemployed LTS wire made up of a niobium-titanium(NbTi) alloy. The high current density achievable insuperconducting electromagnets makes it possible tocreate very compact and power-dense rotatingmachinery. However, even at such large ratings, thecomplexity and cost of the refrigeration equipment, andthe challenging nature of thermal isolation systems thatare necessary for allowing LTS materials to operate atan ultra-low 4K, have made any conceivablecommercialization of this early superconductingtechnology in rotating machine applications aprohibitive concept.However, rapid advances in the development of HTSwire over the past 13 years have resulted insuperconducting electromagnets that can operate atsubstantially higher temperatures than those made ofLTS materials, and which as a consequence can utilizerelatively simpler, less costly, and more efficientrefrigeration systems. These factors make HTS wiretechnically suitable and economically feasible for usein the development and commercialization of motorand generator applications at power ratings much lowerthan could be considered with LTS wire.American Superconductor Corporation (AMSC) hasbuilt and tested a 5000 hp, 1800-RPM motor forindustrial market. They have also developed a designfor a 5 MW HTS (“model”) model motor for shippropulsion; this motor demonstrates technologies to beemployed in a full-size 25 MW, 120 RPM HTS motor.The model motor is scheduled for completion andtesting by the end of 2002. AMSC has also developeda conceptual design for a 50 MW, 3600-RPM HTSgenerator. Other companies are also developingSuperMachines. A 1000 hp, 1800-RPM motor fundedunder the SPI program, and built by a team consistingof Rockwell Automation, AMSC, and others, wassuccessfully operated in May 2000. Siemensdemonstrated a 550 hp, 1800-RPM motor in thesummer of 2001. GE has just won a DoE-SPI contractfor the design and development of a 100 MVA HTSgenerator.The discussion in this paper is centered on applicationsof HTS SuperMachines to an All-Electric Ship. Figure1 shows a single line diagram for a ship electricalsystem employing superconducting generators,propulsion motors and general-purpose industrialmotors.
II. HTSWIRE STATUS
Over the past ten years, the performance ofmultifilamentary composite HTS wire has continuallyimproved. Currently, AMSC is producing this wire at arate of about 500km/year and the end of 2002 willproduce 10,000 km/year in its new factory. This Bi-2223 high current density wire is available forindustrial applications and prototypes. Bi-2223 highstrength reinforced wire is able to withstand close to300 MPa tensile stress and 0.4% tensile strain at 77K.Reinforced wires provide a mechanically robust andreliable product, which are suitable for making highperformance prototype propulsion motors andgenerators.
III. SUPERMACHINE TOPOLOGY
The major components of a rotating machineemploying HTS winding is shown in Figure 2. Onlythe field winding employs HTS cooled with acryocooler subsystem to about 35-40K. The cryocoolermodules are located in a stationary frame and a gas,such as helium, is employed to cool components on therotor. The stator winding employs conventional copperwinding but with a few differences. The stator windingis not housed in conventional iron core teeth becausethey saturate due to high magnetic field imposed by theHTS winding.
Compared to conventional generators, HTS generatorsare expected to be less expensive, lighter, morecompact, efficient and reliable, and significantlysuperior at maintaining power system stability. Theyalso exhibit higher efficiency under partial loadconditions and could operate as virtual condenser todeliver its rated current.The SuperGenerator1 shown in Figure 3 has threemajor subsystems; 1) rotor, 2) rotor cooling and 3)stator. Physically, this generator is expected to beabout half (1/2) the length and two-thirds (2/3) thediameter of a conventional machine. This generator hasa low synchronous reactance of 0.28 pu but thetransient and sub-transients reactances are similar tothose of conventional machines. The overall efficiencyof the generator is 98.6% which is retained down to1/3rd of the rated load. The majority of losses (65%) arein the conventional copper armature winding. Thecryogenic cooling system power consumption ismerely 2% of the total losses in the machine.SuperGenerator produces nearly clean AC voltage inthe stator winding. Both rotor and stator windingsgenerate minimal harmonics. The field windingproduces 2% of 5th harmonic voltage in statorwinding; all other harmonics are negligible.
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