J.C. Tolbert

Design, analysis, and fabrication of a tri-axial cable system

Encouraged by the positive test results of a /spl sim/1.5-m long prototype tri-axial cable, the Southwire Company/Oak Ridge National Laboratory (ORNL) team has conceived, designed, and built a 5-m tri-axial cable with three-phase terminations. The three concentric superconducting phases are made of BSCCO-2223 high-temperature superconducting (HTS) tapes, separated by layers of cold-dielectric (CD) tape. A copper braid is added as the grounding shield. The completed tri-axial 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 problem 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 performance (strength and contraction on cool down). The resulting three-phase, CD cable and termination design is nearly as compact as the single-phase, co-axial design developed previously by Southwire/ORNL and represents the highest cable current density achievable in an electric alternating-current power cable.

AC losses of prototype HTS transmission cables

Since 1995 Southwire Company and Oak Ridge National Laboratory (ORNL) have jointly designed, built, and tested nine, 1 m long, high temperature superconducting (HTS) transmission cable prototypes. This paper summarizes the AC loss measurements of five of the cables not reported elsewhere, and compares the losses with each other and with theory developed by Dresner. Losses were measured with both a calorimetric and an electrical technique. Because of the broad resistive transition of the HTS tapes, the cables can be operated stably beyond their critical currents. The AC losses were measured in this region as well as below critical currents. Dresners theory takes into account the broad resistive transition of the HTS tapes and calculates the AC losses both below and above the critical current. The two sets of AC loss data agree with each other and with the theory quite well. In particular, at low currents of incomplete penetration, the loss data agree with the theoretical prediction of hysteresis loss based on only the outer two layers carrying the total current.

High-temperature superconducting tri-axial power cable

Abstract Encouraged by the positive test results of a 1.5-m long prototype tri-axial cable, the Southwire/ORNL team has conceived, designed and built a 5-m tri-axial cable with three-phase terminations. The three concentric superconducting phases are made of BSCCO-2223 HTS tapes, separated by layers of cold-dielectric tape. A copper braid is added as the grounding shield. The completed tri-axial 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. The terminations used in the cable tests are cooled by a separate liquid nitrogen stream. The resulting three-phase, cold dielectric, cable and termination design is nearly as compact as the single-phase, co-axial design developed previously by Southwire/ORNL and represents the highest known cable current density achievable in an electric AC power cable. DC testing of the 5-m cable includes V–I curves for each of the concentric HTS phases, cable heat loads at varying DC currents, liquid nitrogen flow-pressure measurements, and over-current tests. AC testing of the cable includes ac loss measurements, induced-current in the Cu-shield measurements and operation at the line voltage test. The ac losses are measured calorimetrically by measuring the temperature differential of the coolant across the cable length due to the ac loss in the superconductors. Both balanced and un-balanced currents among the three phases are used in ac loss and induced current measurements.

Operating experience with the southwire 30-meter high-temperature superconducting power cable

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.

5-M Single-Phase HTS Transmission Cable Tests

Two 5-m, single-phase, High-Temperature Superconducting (HTS) transmission cables have been built and tested at Oak Ridge National Laboratory (ORNL) in a joint program between Southwire Company and ORNL. The active conductor of the cable consists of layers of Bi-2223/Ag tapes helically wound on a flexible former, followed by layers of cryogenic dielectric tapes, and then HTS shield layers. Subcooled liquid nitrogen at temperatures of 70–80 K and pressures of 3–7 bar flows down the inner pipe, turns around at a termination, and returns through the annulus between the cable and the vacuum-jacketed outer pipe to cool the cable. The DC V-I curves of the cables have been mapped as a function of temperature and the critical currents measured. The cables have exceeded the performance goal of 1250-A AC at 7.2 kV line-ground voltage. The AC loss measurements showed a loss of about 1 W/m for one and 0.7 W/m for the second cable at 76 K. High voltage measurements showed partial discharge at only background level for voltage up to 18 kV AC (2.5 times design value). Stable cable temperature was maintained at 1250-A rms at temperatures up to 80.5 K, where the operating current is almost twice the cable’s critical current. These test results provide confidence in the design approach and the extrapolation to the next phase: development of a 30-m, three-phase cable.

Testing of an HTS Power Cable Made From YBCO Tapes

Oak Ridge National Laboratory (ORNL) has designed, built, and tested a 1.25-m-long, prototype high temperature superconducting (HTS) power cable made from second-generation YBa2Cu3Ox (YBCO)-coated conductor tapes. Electrical tests of this cable were performed in liquid nitrogen at 77 K. DC testing of the HTS cable included determination of the V-I curve with a critical current of about 2100 A, which was consistent with the critical currents of the two layers of 4.4-mm wide YBCO tapes. AC testing of the cable was conducted at currents up to about 1500 Arms. The ac losses were determined electrically by use of a Rogowski coil with a lock-in amplifier. Over-current testing was conducted at peak current values up to 4.9 kA for pulse lengths of 0.3-0.5 s. Test results are compared to earlier data from a 1.25-m-long power cable made from 1-cm-wide YBCO tapes and also comparable BSCCO cables. This commercial-grade HTS cable demonstrated the feasibility of second-generation YBCO tapes in an ac cable application.

Testing of a 1.5-m single-phase short-sample cable made with copper laminated HTS tapes at ORNL

The use of high temperature superconducting (HTS) materials for power transmission cable applications is being realized in several utility demonstration projects. Tape testing on short-sample cables is conducted to determine the suitability of HTS tapes for use in different cable designs. Testing includes determining the DC critical current, ac loss and overcurrent behavior in a wound cable configuration. The short-sample cable configuration is similar in physical respects, such as winding diameter, winding pitch, application of dielectric, to a practical length cable. With more attention being paid to the short-circuit fault protection, the over-current pulse behavior of the cable will be tested thoroughly. The temperature history of the cable in the radial direction and the re-cooling of the cable will be monitored and compared with a model calculation. This paper describes the testing and results from one such series of tests of a short-sample cable made with copper laminated HTS tapes for use in future projects.

Testing of 3-Meter Prototype Fault Current Limiting Cables

Two 3-m long, single-phase cables have been fabricated by Ultera from second generation (2G) superconductor wire supplied by American Superconductor. The first cable was made with two layers of 2G tape conductor and had a critical current of 5,750 A while the second cable had four layers and a critical current of 8,500 A. AC loss was measured for both cables at ac currents of up to 4 kArms. Ultera performed initial fault current studies of both cables in Denmark with limited currents in the range from 9.1 to 44 kA. Results from these tests will provide a basis for a 25-m long, three-phase, prototype cable to be tested at ORNL early next year and a 300-m long, fault current limiting, superconducting cable to be installed in a Consolidated Edison substation in New York City.

Performance tests of an HTS power transmission cable splice

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.

Test Results for Different High Temperature Superconducting Transmission Cable Prototypes

Development has begun on high temperature superconductor (HTS) cable systems for power transmission. Many design factors affect the performance of a HTS transmission cable. Typical constructions will have multiple layers of superconductor. It is known that the current distribution among the layers has an effect on the performance of the cable. Measurements on two different prototype cable constructions have been performed. The prototypes are approximately 1 m long and were fabricated by helically winding Ag sheathed Bi-2223 high temperature superconductor tape on a stainless steel tube former. Both prototypes had four HTS layers consisting of 2 pairs of oppositely wound tapes. Comparisons of the measured performance of the two prototypes and the measured current distributions are compared and discussed.