Technical Progress
in Applications Development


INTERMAGNETICS GENERAL/ORNL/ WAUKESHA ELECTRIC SYSTEMS/ROCHESTER GAS AND ELECTRIC COMPANY SUPERCONDUCTING TRANSFORMER PROJECT

This report covers only the nonproprietary ORNL contributions to the HTS Transformer Project for the FY 1996. Other than ORNL, the organizations participating in the project are the Intermagnetics General Corporation (IGC), Waukesha Electric Systems of General Signal Corporation (WES), and the Rochester Gas and Electric Company (RG&E). Rensselaer Polytechnic Institute provides consulting under subcontract to IGC. The objectives of the project are (1) to develop a low-cost, wind-after-react HTS suitable for use in transformers and other ac apparatus; (2) to determine the markets, technical and economic feasibility, and benefits to society of HTS power transformers of medium (30 MVA) to large rating; and (3) to design, build, and test an ~1-MVA single-phase HTS demonstration transformer. Significant progress in achieving the first two objectives has resulted in the decision to continue into the demonstration phase, which, together with the completion of key experiments, has been the major focus of efforts this year.

In FY 1996, the team

Design and Testing of Subsystems

Core, Tank, and Bushing Subsystem

It is highly desirable to use as many components from existing transformer technology as possible. With this in mind, we decided to determine whether existing transformer tank technology could be applied to the HTS transformer. The resulting design for the 1-MVA demonstration is externally very similar to a full-scale commercial HTS tank and resembles a conventional tank without a heat exchanger. As a part of the development process, a sub-sized tank was built at WES and was successfully tested for weld integrity at ORNL. The core section for the demonstration unit is similar in size to a single phase of the 30-MVA design. This choice was made to allow the use of the core for prototype testing of several coil systems, to gain experience with full-size components, and to allow use of existing core technology at WES. A small core cross section is presently undergoing tests at ORNL for vacuum outgassing performance. In addition, a full-scale bushing is undergoing tests at ORNL.

Cooling and Cold Mass Support Subsystem

ORNL has lead responsibility for the 80 K cold mass, liquid nitrogen, and cooling loop subsystems. Initial design has been completed. Final testing and integration awaits completion of coil and core fabrication.

AC Loss Measurements in Coil Prototypes

Two sets of ac loss tests were performed in the ORNL variable-temperature cryostat on sub-sized transformer coil pairs provided by IGC. The coils were fabricated from IGC’s surface-coated BSCCO-2212 tape using winding techniques based upon IGC’s experience with homopolar motor coils being developed for a related Naval Research Laboratory program. The ac loss measurements were made by measuring the calibrated temperature rise in He cooling gas (15­45 K) that resulted from ac current pulses. Fault-current performance was also successfully completed on one-third height near-design diameter coils at IGC.



TESTS OF THREE PROTOTYPE HIGH TEMPERATURE SUPERCONDUCTING TRANSMISSION CABLES: SOUTHWIRE CRADA

During the initial phase of the CRADA between ORNL and Southwire Co. to develop HTS underground transmission cable, two 500-A-class and one 2000-A-class prototype cables were constructed. The cables and short samples of the Bi-2223/Ag HTS tapes were tested systematically at ORNL.

The cables were tested with both dc and ac currents in liquid nitrogen. Both cables achieved design currents; however, substantial degradation in comparison with the short-sample critical currents (Ics) was observed. A simple calorimetric technique was used to measure the ac losses of the cables. A method of utilizing the broad resistive transition of the HTS cable was devised to calibrate the ac loss. Different ac-loss behaviors were observed on the insulated and uninsulated cables.

Short Sample Testing

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     Fig. 2.1. Short sample of Ic along the length of the tape for use in cable 2.

A series of short sample tests were performed on the Bi-2223/Ag HTS tapes acquired by Southwire Co. Seventy eight samples for the winding of the first cable and 11 samples for the winding of the second cable were measured. These 1-in.-long samples were tested in liquid nitrogen with up to 0.5-T magnetic field parallel and perpendicular to the wide face of the tape. Figure 2.1 shows the measured zero-field short sample Ics (at the 1-µV/cm criterion) along the length of the spool used to wind the second cable. Critical current varies significantly (by a factor of two) along the length of the tape. A mean Ic value of 20 A was measured (the end-to-end value was 17 A). Similarly, a mean Ic value of 19 A was measured for the tapes used to wind the first cable (the end-to-end value was 12 A). Apparently, damaged spots on a large spool were apt to be skipped when short samples (about 1-in. long) were taken.

Magnetic fields degrade the Bi-2223/Ag HTS tapes significantly at liquid nitrogen temperatures. At a background field value of 0.01 T, the present tapes showed an average of 10% degradation in Ic with field parallel to the wide face and 50 % degradation with field perpendicular to the wide face of the tape.

Bending tests were performed on selected samples of the HTS tapes. In a series of tests, I-V curves of 3-in.-long samples were measured before and after being wrapped side-by-side around a 1-in.-diam former. The samples from the lower-Ic spool showed an average degradation of 30%; those from the higher-Ic spool showed an average degradation of 53%.

Bending tests were also performed with samples about 30-cm long by wrapping them with lay angles of up to 30. Critical current degradation between 40 and 50% of the 1-in.-long short sample values was observed.

Prototype Cables

Two prototype 500-A-class transmission cables were fabricated by Southwire using the 3.5 × 0.22 mm HTS tested tapes. The 1.2-m-long cables were made by spirally winding the tapes on a 22-mm (7/8-in.) copper former with lay angles of about 15.

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     Fig. 2.2. Cable 1 assembled for both dc and ac current tests.

For the first cable, no insulation was used to electrically separate the tapes. The ends of the tapes on the first layer were soldered onto the former. Successive layers were wound with alternating twist angles, and the ends were soldered to the previous layer. Seventy three tapes were wound in four layers in the first cable. Figure 2.2 shows a picture of the cable assembled and ready to be lowered into the test dewar. The main body of the cable was enclosed in a micarta pipe filled with wax to establish adiabatic conditions to measure the temperature rise (and thus the ac loss) of the cable.

The second cable was fabricated in a way similar to the first cable, except that Kapton tape was used between layers for insulation. Sixty six HTS tapes was used in the second cable.

The third Southwire cable was wound from similar HTS tapes as were used in cables 1 and 2. The tapes were wound in 10 layers on a 1-in.-diam stainless steel former. Similar to layers in cable 2, the successive layers were insulated from each other with Kapton tape. Two hundred HTS tapes were used in winding this cable.

DC Current Measurements

The electrical tests of the cables were carried out in liquid nitrogen with the HTS cable held upright in a 1.6-m-deep dewar.

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     Fig. 2.3. CD I-V curves of cable 1 at the first cooldown.

DC I-V of Cable 1

Four voltage taps were placed on the cable, separated from each other by about 30 cm, and were labeled as V1 to V4. Figure 2.3 shows the I-V curves of the different sections of the cable and of the whole cable (VTot). Gradual resistive voltage rise was seen for currents starting at about 400 A. All resistive voltage of the cable came from the midsection (V23 ) at currents up to 650 A, caused by visible damage near the middle of the cable. Nevertheless, the overall Ic of 670 A at the 1-µV/cm criterion is higher than the design value of 500 A.

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     Fig. 2.4. I-V curves of outermost layer of cable 2 at three different bath temperatures.

DC I-V of Cable 2

The layers of cable 2 were insulated from each other with Kapton tape, and separate current leads were brought out for each layer. Thus the cable could be tested as a whole or separately on individual layers. When the cable as a whole was tested, an Ic of 560 A was measured. Notice also that because of the broad resistive transition, both cable 1 and 2 can be operated stably at more than 1 kA.

During the test of the outermost layer of cable 2, the liquid nitrogen bath was pumped to lower temperatures. Figure 2.4 shows the I-V curves of this layer at three different temperatures of the liquid nitrogen bath. The Ic o-f this layer increased from 149 to 186 A when the bath temperature was lowered from 77 to 69 K. Thus an increase of about 25% in current-carrying capability can be achieved in the cable by operating with subcooled liquid nitrogen (at about 69 K).

DC Test of Cable 3

DC current test of the cable was performed with a 2-kA power supply. Broad and smooth resistive transition of the cable, similar to those of the previous two cables, was observed. A critical current of 1630 A was measured at the 1-µmV/cm criterion. At the power supply limit of 2 kA, the cable produced an average resistive voltage of 2.2 µV/cm.

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     Fig. 2.5. U-E cyrves if cable 1 on successive thermal cycles.

Thermal Cycle of Cable 1

After a few cycles of cooling down and warming up of cable 1 for dc and ac current measurements, a series of continuous thermal cycle tests was performed. An I-V curve was measured, and the cable was pulled out of the liquid nitrogen bath. After it was warmed up to room temperature in air, the sample was lowered back down to the liquid nitrogen bath. Another I-V curve was measured. Figure 2.5 shows a series of these I-V curves at different thermal cycles. Significant degradation was observed on thermal cycling; however, the degradation seems to level off after the fifth cycle. Critical current of the cable decreased from 670 to 460 A after 10 thermal cycles (a 30 % degradation). Power law fitting of the I-V curves between 0.2 to 2 µV/cm also shows a decrease of n-value from 3.5 to 2.6.


Comparison of Short Sample and Cable Ic

The measured Ic per tape of cables 1 and 2 averages about 8.8 A. This is significantly lower than the average short sample value of 19.5 A measured on 1-in.-long short samples. As is described in the series of short sample measurements, several mechanisms can contribute to the degradation of the cable Ic. Short sample Ic measurements can be misleading because they can skip bad spots in the long lengths of the tape. Mechanical strain similar to that applied in winding the cable can degrade the Ic by about 50%. This can come from just handling the long lengths of the tape and from the bending applied in the cabling. The magnetic-field degradation by the cable self-field is well known. In addition, thermal-cycling degradation was also observed.

AC Current Measurements

Cables 1 and 2 were tested with 60-Hz ac currents up to 600 A rms. Steady rms voltages were observed at all test currents. A calorimetric technique was adopted to measure the ac loss of the cable at the applied ac currents. As is shown in Fig. 2.2, a micarta pipe filled with wax was used to thermally isolate the cable from the liquid nitrogen bath. A Cromel-Constantan thermocouple was attached to the middle of the cable to measure its temperature rise against liquid nitrogen at the same depth of the bath; temperature rise (T) of up to 0.3 K was observed.

To calibrate T against the power-loss rate, we used the broad resistive transition feature of the HTS cable itself. The cable was charged and held at a dc current above its Ic, where a resistive voltage can be measured. The T of the cable was also measured under this dc current. The dc EI product gave the average power loss for the measured T. This technique was found to be more responsive than the heater wires tried on cable 1. Because Ic is not uniform along the length of the cable, power generation is not uniform, but this is true for both dc power and ac loss. Therefore, the calibration technique is a good simulation of the ac loss. In addition, Joule heating at the cable ends did not contribute to the measured temperature rise because the ends of the cable were immersed in liquid nitrogen. This was verified by the observation that in the dc current calibration runs no temperature rise was observed until the current greatly exceeded Ic.
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     Fig. 2.6. The ac losses of both cable 1 and 2 in reference to their DE I-E curve. For the ac current, rms currents is plotted.

Figure 2.6 shows the measured ac losses of the two cables as a function of the rms current. Also shown in Fig. 2.6 for reference respective dc I-E curves for two cables. Cable 1, which was not insulated, behaved like a cryoresistive conductor, showing power loss at all ac currents. Similar behavior was reported by Gannon et al.1 Cable 2 which was insulated, showed no measurable ac loss until about 300 A rms, where the cable also started to show measurable dc resistive voltage. An average ac loss of about 0.2 W/m was measured at 400 A rms. Analysis of the loss data indicated that the measured loss is governed by the power law behavior of the HTS tape in the resistive transition.

Cable 3 was also subjected to ac current tests. Currents measuring up to 2.2 kA rms were charged and held for about 10 min. Steady voltage (mostly inductive voltage) was observed over the cable in each case.

Summary

Two prototype 500-A-class HTS cables have been designed, constructed, and tested. Both cables achieved dc critical currents greater than the design value of 500 A. Furthermore, because of the broad resistive transition, they can be operated stably at more than 1 kA. A third cable was tested successfully to >2000 A ac current. Comparison of the cable Ic and the short sample values indicated a degradation of about 55%. Several mechanisms were identified as the probable cause of the degradation. Mechanical strain from handling the long lengths of the tape and from bending applied in winding the cable is thought to be the biggest source of degradation.

A calorimetric technique was used to measure ac losses of the cables. A scheme of utilizing the broad resistive transition of the HTS cable was successfully used to calibrate the loss rate. Loss measurements made on the cables with 60-Hz ac currents showed that insulation between the tapes is effective in reducing the ac loss of the cable.

Reference

  1. J. J. Gannon, Jr., et al., “Performance Summary of a 4,000-A High Temperature Superconducting Cable Conductor Prototype,” IEEE Trans. Appl. Supercond. 5 (2), 953–56 (1995).


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