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Following the positive results of a 1.5-m long prototype cable, the Ultera and Oak Ridge National Laboratory (ORNL) team built and tested a 5-m triaxial cable rated at 1.3-kA rms, 15-kV ac. The three concentric superconducting phases are made of BSCCO-2223 high-temperature superconducting (HTS) tapes, separated by layers of cold-dielectric tapes. A copper braid is added as the grounding shield on the outside of the three active phases. To facilitate the calorimetric ac loss measurement, the cable was cooled by liquid nitrogen flowing in the annulus between the cable and the cryostat only. Tests of this cable were performed at temperatures ranging from 76 to 84 K. AC loss data reconfirmed the previous gratifying result on the 1.5-m prototype cable that the total three-phase loss is approximately the sum of the calculated ac losses of the three separate phases. No additional loss is due to possible coupling among the three phases. This and other test results of the 5-m, 1.3 kA triaxial cable are reported here. The next step in the project is to build and install a triaxial cable rated at 3-kA rms, 13-kV ac at a utility site. A single-phase 3-kA cable prototype was also built and tested for use as the most demanding inner phase of the triaxial cable. © 2004 American Institute of Physics [ABSTRACT FROM AUTHOR]

Ultera and Oak Ridge National Laboratory (ORNL) have jointly designed, built, and tested a 1.25-m-long, prototype high-temperature superconducting (HTS) power cable made from 1-cm-wide, second-generation YBa2Cu3Ox (YBCO)-coated conductor tapes. Electrical tests of this cable were performed in boiling liquid nitrogen at 77 K. DC testing of the 1.25-m cable included determination of the V-I curve, with a critical current of 4200 A. This was consistent with the properties of the 24 individual YBCO tapes. AC testing of the cable was conducted at currents up to 2500 Arms. The ac losses were measured calorimetrically by measuring the response of a calibrated temperature sensor placed on the former and electrically by use of a Rogowski coil with a lock-in amplifier. AC losses of about 2 W/m were measured at a cable ac current of 2000 Arms. Overcurrent testing was conducted at peak current values up to 12 kA for pulse lengths of 0.1–0.2 s. The cable temperature increased to 105 K for a 12 kA, 0.2 s overcurrent pulse, and the cable showed no degradation after the sequence of overcurrent testing. This commercial-grade HTS cable demonstrated the feasibility of second-generation YBCO tapes in an ac cable application. © 2004 American Institute of Physics [ABSTRACT FROM AUTHOR]

This tri-axial high-temperature superconducting (HTS) cable design uses three concentric superconducting layers for the phase conductors, separated by a cold dielectric material. The design offers an efficient HTS cable configuration by reducing the amount of superconductor needed and places all three phases in a single cryostat. The tri-axial cable cooling circuit analyzed includes heat loads at the ends for the cable terminations and cable heat loads due to ac, dielectric, and thermal losses. The HTS cable critical current and ac loss are functions of the local temperature that must be determined by the analysis. The radial heat transfer also has an influence on these parameters due to the relatively low thermal conductivity of the dielectric material separating the HTS phases. The study investigates whether the tri-axial cable must be cooled both inside the former and outside of the cable. In this study, the range of operating parameters for a tri-axial HTS cable system and refrigeration requirements are determined based on expected HTS tape performance. © 2004 American Institute of Physics [ABSTRACT FROM AUTHOR]

The Southwire/ORNL team has conceived, designed, built, and tested a 5-m, three-phase, tri-axial cable with terminations. The cable has three concentric superconducting phases made of BSCCO-2223 HTS tapes, separated by layers of cold-dielectric tape with a copper braid as the grounding shield. The cable is enclosed in a flexible cryostat. Cooling of the cable and terminations is achieved by liquid nitrogen flowing through the annulus between the cable and the cryostat. A challenging analysis and design issue was development and implementation of an insulator material between the concentric phases with high enough thermal conductivity to meet temperature gradient requirements and acceptable mechanical and electrical performance. The resulting 3-phase, cold dielectric, cable and termination design is nearly as compact as the single-phase, co-axial design developed previously by Southwire/ORNL, and it represents the highest cable current density achievable in an electric ac power cable. This paper presents results from operation of the 1.3-kA root-mean-square (rms) phase current termination and design changes required to scale to 3-kA rms phase current. © 2004 American Institute of Physics [ABSTRACT FROM AUTHOR]

A series of BSCCO-2223 and YBCO tapes were subjected to burnout tests in a liquid-nitrogen bath to observe operational stability limits when different layers of dielectric tape are added to the sample surface. In this study, the BSCCO tapes were composed of a silver/alloy sheath with nickel/copper plating, while the YBCO tapes had a 50-μm layer of copper attached to the silver surface. After attaching the tapes to a thermally insulated G-10 holder, the stability of the tapes was found by applying current greater than the critical current and holding it constant for up to 1 min. If the sample voltage increased rapidly during this period, the tape was considered unstable at this current. This was repeated at different layers of Cryoflex™, and the results were compared to a numerical simulation of the energy balance equation. This simulation was also utilized to investigate the effect of the layers on the stability limit and estimate the thermal conductivity of the Cryoflex™. © 2004 American Institute of Physics [ABSTRACT FROM AUTHOR]

The role of silver in the quench dynamics of RABiTS-processed YBCO tapes was examined. The voltage distribution along the 20 cm long YBCO tapes with silver thickness between two and eight micrometers was measured when different transport current pulses were applied. Measurements on each sample were performed in a conduction-cooling environment at approximately 50 K. After normal regions were induced in the sample by short over-current pulses, an operating current was applied to monitor the sample recovery or quench. When scaled to the lowest critical current, a thermal runaway current was identified and found to increase with increasing silver thickness. A simple one-dimensional model of the system supported this trend. [This paper is also published in Advances in Cryogenic Engineering Volume 47, AIP Conference Proceedings Volume 613, pp. 449-456. [ABSTRACT FROM AUTHOR]

A 30-m demonstration high-temperature superconducting (HTS) transmission cable system has been installed by Southwire Company in Carrollton, Georgia. The three-phase power transmission system is designed for operation at 1250 A and 12.4-kV alternating current. It supplies all the electricity for three large manufacturing facilities and has been operating under full load for more than 8000 h as of June 2001. The HTS cables are cooled by circulating subcooled liquid nitrogen. Refrigeration of the circulating liquid nitrogen is accomplished using an open subcooler system under vacuum to obtain temperatures below 77 K. The HTS cable cryogenic system is briefly described.discussed. [ABSTRACT FROM AUTHOR]

Practical applications of high-temperature superconducting (HTS) transmission cables require that cable sections be periodically spliced together. An HTS transmission cable splice with a cold dielectric construction rated at 1250-A phase current and 7.5-kV phase-to-ground voltage has been fabricated by Southwire Company and tested at Oak Ridge National Laboratory. The splice joins the HTS phase and the neutral conductors as well as the Cryoflex™ dielectric tapes between the HTS conductors. Testing has demonstrated the nominal operating capability of the HTS cable splice and consisted of direct-current characterization and alternating-current high-voltage withstand testing at 18 kV. In addition, overcurrents up to 14 kA for 2 s were applied to the cable splice repeatedly without impacting the performance. The splice generates less than 1 W of heat into the cable at rated current. The results of these tests demonstrate the feasibility of splicing HTS power transmission cables. [ABSTRACT FROM AUTHOR]

Southwire Company is operating a high-temperature superconducting (HTS) cable system at its corporate headquarters. The 30-m long, 3-phase cable system is powering three Southwire manufacturing plants and is rated at 12.4-kV, 1250-A, 60-Hz. Cooling is provided by a pressurized liquid nitrogen system operating at 70-80 K. The cables were energized on January 5, 2000 for on-line testing and operation and in April 2000 were placed into extended service. As of June 1, 2001, the HTS cables have provided 100% of the customer load for 8000 hours. The cryogenic system has been in continuous operation since November 1999. The HTS cable system has not been the cause of any power outages to the average 20 MW industrial load served by the cable. The cable has been exposed to short-circuit currents caused by load-side faults without damage. Based upon field measurements described herein, the cable critical current — a key performance parameter — remains the same and has not been affected by the hours of real-world operation, further proving the viability of this promising technology. [ABSTRACT FROM AUTHOR]

A high-temperature superconducting (HTS) transmission cable based on the cold-dielectric concept with an HTS shield makes it possible to house all three phases inside a single cryostat without causing large degradation and loss due to magnetic fields generated by the neighboring phases. A further optimization can be realized by making the three phases concentric to each other. No shielding layer is required in such a triaxial configuration. It would be more compact and require only about one-half of the HTS tapes as that of three separately shielded phases. A 1.5-m-long triaxial HTS cable prototype has been fabricated and tested. Each phase consists of two layers of BSCCO-2223 HTS tapes. Electrical tests of the cable included the direct current (dc) V-I curve and alternating current (ac) loss measurements. The ac losses were measured both electrically and calorimetrically. A finite-element thermal modeling was performed to check the calorimetric calibration. Individual phase ac losses indicate good agreement with the monoblock theory. Three-phase ac loss is close to that of the sum of the three individual phases. There is no measurable excess ac loss due to the presence of the other concentric phases. A total three-phase ac loss of about 1 W/m was measured at the design operating current of 1250 A. [ABSTRACT FROM AUTHOR]