J. W. Lue

Measurements of temperature dependence of the stability and quench propagation of a 20-cm-long RABiTS Y-Ba-Cu-O tape

Thermal stability and quench propagation in a composite tape made of YBa/sub 2/Cu/sub 3/O/sub x/ (YBCO) superconductor were studied experimentally. Quench propagation in each test was initiated by applying a sequence of a short overcurrent pulse followed by a longer pulse at a typical operating current for the tape. The resulting change in resistivity due to internal heating was measured through voltage taps across different zones of the tape. Measurements were performed as a function of both initial overcurrent and operating current for several operating temperatures between 45 and 80 K. These experimental results provided the thermal stability margin, the minimum propagation current, and the quench propagation velocity for the tape. Experimentally obtained temperature dependence of normal zone propagation velocity was compared with the adiabatic theory taking into account minimum propagation current. It was noted that the measured normal zone propagation velocity compared favorably with the theory at each operating temperature.

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.

Tests of tri-axial HTS cables

The Ultera/ORNL team have built and tested 3-m and 5-m triaxial cables rated at 3 and 1.3 kA-rms, respectively. The three concentric superconducting phases are made of BSCCO-2223 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. Tests of these cables were performed at temperatures ranging from 70 to 84 K. AC loss data reconfirmed the previous result on a 1.5-m prototype cable that the total 3-phase ac loss is about the sum of the calculated ac losses of the three concentric phases. These and other test results of the 1.3 and 3 kA cables will be used to construct a second 5-m triaxial cable rated at 3 kA-rms, 15 kV. Preliminary test results supporting this new cable and the associated termination are summarized.

Substrate and stabilization effects on the transport AC losses in YBCO coated conductors

In support of second generation HTS conductor development for ac applications, transport ac loss measurements were conducted on a series of RABiTS-processed YBa/sub 2/Cu/sub 3/O/sub x/ (YBCO) coated conductors with different nickel alloy substrates and copper stabilization at 77 K. Each 1-cm wide sample had a critical current density between 1.0 and 2.0 MA/cm/sup 2/ and had either a 75 /spl mu/m Ni-5at%W substrate or a 75 /spl mu/m Ni-10%Cr-2%W substrate with 2-/spl mu/m nickel overlayer. Samples with copper stabilization had a 50-/spl mu/m strip of 1 cm wide copper laminated to a 3-/spl mu/m thick silver coated YBCO sample. Using thermal and electrical measurement techniques, the ac losses were measured as a function of the peak current ratio at 60 Hz. Experimental results were compared to the Norris thin strip and elliptical models to determine the influence of the ferromagnetic loss of the substrate and the copper lamination on the total ac loss.

Measurements of the performance of BSCCO HTS tape under magnetic fields with a cryocooled test rig

The use of high-temperature superconducting (HTS) materials for electric power applications is being realized in prototype systems. A test rig was designed and fabricated that uses a 6-T cryocooled magnet with an 20.3 cm warm bore. Inserted in the bore is a stainless steel vacuum vessel that has a Cryomech GB37 cryocooler to conductively cool the sample. Critical current measurements were made on BSCCO-2223 tapes under externally applied perpendicular and parallel magnetic fields. A description of the test rig design and results from a series of measurements will be presented.

The role of nickel substrates in the quench dynamics of silver coated YBCO tapes

A pair of silver coated YBCO tapes with varying degrees of electrical contact between the silver and the YBCO nickel substrate were studied to examine the impact of nickel on normal zone formation and stability. The YBCO tapes were fabricated using the rolling assisted bi-axially textured (RABiTS) method and 15-cm-long samples with 2 /spl mu/m of silver were prepared. The samples were place in a conduction cooling environment at 45 K to study quench or recovery when a series of dc transient over-current pulses were applied. We used a series of voltage taps on both the silver and nickel to characterize the nature of the contact between the silver and the nickel through the measured voltages. In as-manufactured samples and those sample with continuous contact between the nickel and silver, we found that the silver and nickel can not be treated as conductors in parallel when a normal zone is present because of current present in the nickel even while sections of the sample remains superconducting. This nonparallel contact makes stability characterization difficult although the samples with intentional contact were able to withstand larger current pulses. In addition, the samples with intentional electrical contact between the silver and nickel exhibited normal zone propagation in both the silver and the nickel with speeds between 4 mm/s and 8 mm/s.

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.

Over-current testing of HTS tapes

High-temperature superconducting (HTS) transmission cables are subjected to short-circuit fault currents 10 to 30 times the normal operating current and lasting up to 15 cycles. These over-currents will drive the HTS conductor normal and generate heat during the fault. A concern is whether the fault current will either electromechanically or thermally damage the HTS conductor and degrade it or burn-out the tape altogether. Electromechanical and thermal limitations of over-current pulses were measured on BSCCO and YBCO tapes in a liquid nitrogen bath. With pulse lengths as short as 35 ms, it is found that single BSCCO and YBCO tapes can be pulsed to at least 1 to 1.2 kA without being damaged electromechanically. Longer pulses at moderate (450-750 A) over-currents indicated that HTS tapes can be heated transiently to over 400 K without suffering degradation. Thus, it is likely that other considerations of the cable rather than the HTS tape itself would set the limit for short-circuit fault protection.

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.