J.P. Stovall

Installation and operation of the Southwire 30-meter high-temperature superconducting power cable

Southwire Company has installed, tested and is operating the first real-world application of a high-temperature superconducting cable system at its headquarters in Carrollton, GA, USA. The cable is powering three Southwire manufacturing plants, marking the first time a company has successfully made the difficult transition front laboratory to practical field application of an HTS cable. The cables are rated at 12.4-kV, 1250-A, 60 Hz and are cooled with pressurized liquid nitrogen at temperatures from 70-80 K. Before placing the cables into service, extensive offline electrical testing was performed including voltage withstand, measurement of DC critical current, extended load current testing, rated voltage testing and partial discharge measurement. The cables were energized on Jan. 5, 2000 for online testing and operation, and by the end of August 2000, had provided 100% of the customer load for 2164 hours.

Practical AC loss and thermal considerations for HTS power transmission cable systems

The use of high-temperature superconducting materials for power-transmission cable applications is being realized in prototype situations. It is well known that AC loss decreases as the temperature of the conductor decreases. Also, thermal losses are higher at lower temperatures, owing to the increased temperature difference between ambient and cryogenic operating conditions. Both counterflow and parallel-flow cooling arrangements have been proposed in the literature and significantly affect temperature distribution along the cable. In this investigation, the counteracting AC loss and thermal losses are analyzed for both cooling configurations to determine the benefits and limits of each. The thermal-insulation performance levels of materials versus those of typical systems in operation are presented. Widespread application of long-length flexible cable systems, from the refrigeration point of view, will depend on an energy-efficient cryogenic system that is economical to manufacture and operate. While the counterflow arrangement will typically have a lower heat load, it has a length limit arising from the large pressure drop associated with the configuration.

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.

Fault current tests of a 5-m HTS cable

The first industrial demonstration of a three-phase, HTS power transmission cable at the Southwire manufacturing complex is in progress. One crucial issue during operation of the 30-m HTS cables is whether they can survive the fault current (which can be over an order of magnitude higher than the operating current) in the event of a short circuit fault and how HTS cables and the cryogenic system would respond. Simulated fault-current tests were performed at ORNL on a 5-m cable. This single-phase cable was constructed in the same way as the 30-m cables and is also rated for 1250 A at 7.2 kV AC line-to-ground voltage. Tests were performed with fault-current pulses of up to 15 kA (for 0.5 s) with pulse lengths of up to 5 s (at 6.8 kA). Although a large voltage drop was produced across the HTS cable during the fault-current pulse, no significant changes in the coolant temperature, pressure, or joint resistance were observed. The cable survived all 15 simulated fault-current shots without any degradation in its V-I characteristics.

Development and testing of HTS cables and terminations at ORNL

The Oak Ridge National Laboratory (ORNL) and the Southwire Company have used the ORNL 5 m cable test facility to develop high-temperature superconducting (HTS) cables and terminations to support the first industrial demonstration of an HTS cable at the Southwire manufacturing complex. Two 5 m, cold dielectric cables have been tested for direct current (DC) voltage, alternating current (AC) losses, AC withstand at 18 kV, thermal-hydraulic performance, heat load, and long-term operation at rated voltage (7.2 kV) and current (1250 A). Two separate termination concepts, one operating at 10/sup -4/-10/sup -5/ mbar vacuum and the other operating with pressurized nitrogen gas at <10 bar, have been developed and tested with the 5-m cables. A 5-m cable has been removed from the facility and bent in a test rig to simulate transport in a spool. A testing program for a third 5-m cable with a splice is in progress. The test program at ORNL has validated the basic design of the cables and terminations and indicated areas for further R&D to optimize this technology for electric utility applications.

Cryogenic System for a High Temperature Superconducting Power Transmission Cable

High-temperature superconducting (HTS) cable systems for power transmission are under development that will use pressurized liquid nitrogen to provide cooling of the cable and termination hardware. Southwire Company and Oak Ridge National Laboratory have been operating a prototype HTS cable system that contains many of the typical components needed for a commercial power transmission application. It is being used to conduct research in the development of components and systems for eventual commercial deployment. The cryogenic system was built by Air Products and Chemicals, Allentown, Pennsylvania, and can circulate up to 0.35 kg/s of liquid nitrogen at temperatures as low as 67 K at pressures of 1 to 10 bars. Sufficient cooling is provided for testing a 5-m-long HTS transmission cable system that includes the terminations required for room temperature electrical connections. Testing of the 5-m HTS transmission cable has been conducted at the design ac conditions of 1250 A and 7.5 kV line to ground. This paper contains a description of the essential features of the HTS cable cryogenic system and performance results obtained during operation of the system. The salient features of the operation that are important in large commercial HTS cable applications will be discussed.

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.

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.

Acceptance test and operation of the Southwire Company 30-m high-temperature superconducting cable cryogenic system

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. The acceptance tests and performance of the system with the HTS cable on-line are discussed.