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AC losses in a composite tubular superconductor for power transmission
AC losses have been measured as a function o f current and temperature in a 63.5 mm diameter niobium/copper composite tube, forming the inner conductor of a co-axial superconducting transmission line. The conductor has a 2.6 mm thick substrate o f highconductivity copper with a 50 #m thick niobium surface layer bonded by coextrusion and cold-drawing. Closed-cycle refrigeration allowed losses to be measured as a continuous function of temperature between 4. 4 and 8.0 K for surface current densities between 23 and 89 A m m "1. Losses were less than O. 1 W m "2 at 5 K for surface current densities less than 50 A mm" 1, and obeyed approximately the empirical relation: loss oc h 7 where h is the ratio o f surface current density to Hc2 for the superconductor.
AC losses in a composite tubular superconductor for power transmission J. A. Baylis, K. G. Lewis, J. C. Male, and J. A. No~
Recent design studies 1,1 for ac superconducting power transmission lines have recommended various arrangements of co-axial tubular conductors to carry the phase and neutral currents. Suitable tubular conductors, with a thin niobium surface layer bonded to a substrate of high-conductivity copper, have been developed by CERL in collaboration with Imperial Metal Industries and preliminary assessments of ac losses at 4.2 K have been made on small samples of the niobium layer etched away from the substrate. 3, 4 These measurements used induced currents, flowing on both surfaces of the sample, and may not be representative of the losses to be expected in the composite conductor under transport current conditions. Losses may depend on surface quality and on the current direction relative to anisotropic surface or bulk structure of the superconductor, s It is therefore desirable that such loss measurements made on small, isolated samples should be confirmed under circumstances as close as possible to the design operating conditions of the proposed power transmission line. Meyerhoff and Beall 6 have reported loss measurements on short transmission lines based on co-axial arrangements of niobium and niobium/copper tubes. They used liquid helium as the coolant and varied the conductor temperature between 4.2 and 5.0 K. The present measurements were made with a co-axial arrangement of niobium/copper composite conductors, cooled by helium gas at pressures of 1.1 to 1.2 atm and temperatures between 4.4 and 8.0 K.
production of these conductors has been outlined previously. 3,4 The conductors were mounted in an 8 m long horizontal cryostat (see Fig.l) and cooled by helium gas supplies from a 150 W refrigerator operating in a closed cycle. The conductor temperatures could be continuously varied above 4.4 K by adjustments to a heat-exchanger by-pass valve on the refrigerator. 7 The current leads connecting the conductors to the roomtemperature water-cooled co-axial bus-bars were 1.08 m long copper tubes, which were cooled independently by liquid helium. 50 Hz power supplies could provide up to 10 kA continuously or 16 kA for 30 minutes. A fuller description of the test facility with details of the method of joining current leads to conductors is given elsewhere. 8 The method of ac loss measurement was basically similar to that described by Carter et al. s The electric field parallel to the current direction at the conductor surface was detected by pick-up loops or contacting probes of 100/Jm diameter copper wire spanning the central 2 m of the tube.
Loss measurements were made on the inner conductor of a co-axial pair of length 5.5 m. This conductor was 63.5 mm in diameter and of wall thickness 2.6 ram, with a 50/2m thick niobium layer on both inner and outer surfaces. The substrate was annealed copper with a resistivity ratio (R 300 K :R4.2 K) of 300:1. The outer conductor was of similar composite structure, with diameter 102 mm, wall thickness 1.8 ram, and substrate resistivity ratio 70 : 1. The
Multiple loops were obtained by threading bundles of ten wires simultaneously through 1 mm diameter holes drilled through the conductor wall and passing tile free ends out along the bore of the tube via multiway plugs and sockets for series connexion outside the cryostat. Contacts were made by indium-soldering single wires to 100 ~tm thick circumferential bands of copper, bonded to the conductor surface during the production process and left in place at the final etching stage. All the wires were enamel-insulated and held in contact with the conductor surface by 50/am thick adhesive Mylar tape. Where the wires passed through the conductor wall, the drilled holes were first lined with Stycast 2850 GT epoxy resin and the wires were subsequently secured in place with the same material so that a gas-tight seal was obtained.
The authors are with the Central Electricity Research Laboratories, Kelvin Avenue, Leatherhead, Surrey, UK. Received 12 July 1974.
A signal proportional to the conductor current was obtained by integrating the output from a toroidally-wound pick-up coil in the annular space between the inner and outer
Experimental
CRYOGENICS. OCTOBER 1974
553
Co-axial current leads Co-axial busbars
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the conductor became normal in the course of a run. This indicates the absence of serious flux-trapping problems. 9 The electric field waveforms also showed no evidence of flux-trapping and were similar, regardless of whether contacts or loops were employed. Sample waveforms are given in Fig.3. The results are plotted in Fig.4 alternatively as loss versus temperature, with surface current density as parameter, or as loss versus surface current density, with temperature as parameter. The latter plot also includes the results at
_
b Loss x I 0 0 /
Fig.1 a -- Schematic illustration of the arrangement of cryostat, co-axial conductors, bus-bars and current leads (not to scale); b -- detail of test-section showing pick-up loops and contacting probes
conductors. The value of the current was determined by measuring the voltage across a 7.5 mm thick, 150 mm long section of the inner co-axial bus-bar. This shunt was previously calibrated against a high-precision current transformer and the estimated accuracy of current measurements is -+ 2%. The normal experimental procedure was to record the ac loss continuously as a function of temperature from about 4.4 K up to the point at which some predetermined loss was reached or the conductor became normal. Loss was recorded with temperature falling as well as rising in order that any effects of temperature hysteresis could be allowed for. Sample X Y recordings are shown in Fig.2. Temperature uniformity along the test section was checked by comparing the indications of eight carbon resistance thermometers held in spring-loaded copper blocks against the inside surface of the conductor. At low currents these generally showed uniformity within + 2% (that is, + 0.1 K at 5 K) but at surface current densities greater than 65 A mm "1 vibration of their mounting blocks caused the thermometers to reach individual temperatures appreciably above that of the conductor. A single thermometer held in position by varnish (see Fig.lb) was not affected by vibration and was normally used to record conductor temperatures during experimental runs at high currents. Results
X Y recorder traces of loss versus temperature were generally reproducible and free from significant hysteresis, even when
554
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Fig.2 X Y recorder traces of loss versus temperature for a surface current density of 34 A mm "1
a
b Fig.3 Electric field waveforms derived from contacting probes and pick-up loops a -- contacts at 65 A mm -] and 4.8 K; b -- 8 loops in series at 28 A mm -1 and 7.8 K
CRYOGENICS.
OCTOBER 1974
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The present tube has a performance which is adequate for a rated current of about 50 A mm q at 5 K and shows that the co-processing route by which it was manufactured is satisfactory. The measured losses at 4.5 K are similar in their magnitude and dependence on surface current density to losses measured at 4.2 K in a 15 mm diameter niobium/ copper composite tube produced via the same extrusion route (Carter et al, s sample 1). When current was induced circumferentially, this latter tube had considerably lower losses and a dependence of loss on surface current density which resembled the general behaviour reported by Penczynski 10 for circumferential currents in composite tubes. In particular, over the range measured, the loss was close to that of his 'seamlessly-drawn' tube (sample 6). As pointed out by Carter et al, s it may be important to base estimates of ac losses in proposed composite conductors for power transmission lines only on measurements carried out with the appropriate current direction relative to the superconducting surface.
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The present conductor had been heavily cold-worked and had a low effective value of first critical field (Hcl). Values of Hcl and Hc2 obtained from magnetization data (D. A. Ward, private communication) are plotted against temperature in Fig.5, together with the surface current densities required to give various constant levels of ac loss. The 101 W m -2 curve involves some extrapolation of the experimental data.
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Fig,4 a -- ac loss as a f u n c t i o n o f t e m p e r a t u r e f o r v a r i o u s surface c u r r e n t densities; b -- ac loss as a f u n c t i o n o f surface c u r r e n t d e n s i t y for various temperatures
C R Y O G E N I C S . OCTOBER 1974
Discussion Careful design of the cryogenic envelope of a superconducting cable 2 can reduce the heat inleak through the heliumcooled surface to about 0.1 W m "2 . Since refrigeration is a major element in the total cost of a cable, it is desirable that the ac losses in the conductors should not dominate the heat inleak, and hence should be < 0.1 W m "2. On this basis the present inner conductor is suitable for operation at current densities up to 52 A mm q at 5 K and 44 A mm q at 6 K. (The choice of current density and temperature depends on overall cost optimization, variation of dielectric strength with temperature, and considerations of behaviour during short-circuit currents.) In their designs, Rogers et al 2 assume a current density of 36 A m m q with a loss of 0.02 W m -2 at 5 K, while Eigenbrod et al 1 assume 34 or 56 A mm "l , corresponding to losses of about 0.03 and 0.2 W m -2 at 4.2 K respectively in the 10 mm diameter niobium/copper conductor of Meyerhoff and Beall. 6
id 2
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4.2 K noted above for a sample of the niobium surface layer of the conductor, measured as a foil with current in the original axial direction. 4 [Note that the losses were incorrectly plotted in that paper and :should be reduced by 50% (B. J. Maddock, private communication)]. Although these results are averaged over both surfaces of the foil, it can be seen that there is reasonable agreement with the results obtained from the transport current measurements.
The indistinctness of H q in the magnetization data indicates a high critical current density at low field and also makes any correlation with Hc~ untrustworthy. However, the results may be correlated quite well with Hc2. In Fig.6 all the data from Fig.4a are plotted in the form of loss versus h, where h = H/H c_ and H is the peak value of the surface current density. T~e straight line represents losses proportional to tl. 7
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The authors would like to acknowledge the work of all their colleagues at CERL and MEL who assisted in the preparation and execution of the experiments described. The work was carried out at the Central Electricity Research Laboratories and the paper is published by permission of the Central Electricity Generating Board.
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1 2 3
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Eigenbrod, L. K., Long, H. M., Notaro, J. IEEE Trans PAS-89 (1970) 1995 Rogers,E. C., Slaughter, R. J., Swift, D.A. Proc lEE 118 (1971) 1493 Graeme-Barber,C., Maddock, B. J., Popley, R. A., Smedley, P. N. Cryogenics 12 (1972) 317
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4 5
References
t
0.2
6 7 8 9 10
Graeme-Barber,C., Maddock, B. J. Proc Applied Superconductivity Conference, Annapolis, IEEE Pub No 72CHO6825-TABSC (1972) 211 Carter, C. N., Male, J. C., Graeme-Barber, C. Cryogenics 14 (1974) 332 Meyerhoff,R. W., Beall, W. T. J A p p l Phys 42 (1971) 147 Carter, C. N., Lewis, K. G., Maddock, B. J., No~, J. A. Annexe 1969 - 1 Bulletin IIR (1969) 331 Baylis,J. A., Lewis, K. G., Meats, R. J., No~, J. A. Proc ICEC5, Kyoto, Japan, 1974 (IPC, forthcoming 1974) Male, J. C. Cryogenics 10 (1970) 381 Penczynski, P. Siemens Forsch u Entwickl Ber 2 (1973) 296
C R Y O G E N I C S . OCTOBER 1974
AC losses in a composite tubular superconductor for power transmission J. A. Baylis, K. G. Lewis, J. C. Male, and J. A. No~
Recent design studies 1,1 for ac superconducting power transmission lines have recommended various arrangements of co-axial tubular conductors to carry the phase and neutral currents. Suitable tubular conductors, with a thin niobium surface layer bonded to a substrate of high-conductivity copper, have been developed by CERL in collaboration with Imperial Metal Industries and preliminary assessments of ac losses at 4.2 K have been made on small samples of the niobium layer etched away from the substrate. 3, 4 These measurements used induced currents, flowing on both surfaces of the sample, and may not be representative of the losses to be expected in the composite conductor under transport current conditions. Losses may depend on surface quality and on the current direction relative to anisotropic surface or bulk structure of the superconductor, s It is therefore desirable that such loss measurements made on small, isolated samples should be confirmed under circumstances as close as possible to the design operating conditions of the proposed power transmission line. Meyerhoff and Beall 6 have reported loss measurements on short transmission lines based on co-axial arrangements of niobium and niobium/copper tubes. They used liquid helium as the coolant and varied the conductor temperature between 4.2 and 5.0 K. The present measurements were made with a co-axial arrangement of niobium/copper composite conductors, cooled by helium gas at pressures of 1.1 to 1.2 atm and temperatures between 4.4 and 8.0 K.
production of these conductors has been outlined previously. 3,4 The conductors were mounted in an 8 m long horizontal cryostat (see Fig.l) and cooled by helium gas supplies from a 150 W refrigerator operating in a closed cycle. The conductor temperatures could be continuously varied above 4.4 K by adjustments to a heat-exchanger by-pass valve on the refrigerator. 7 The current leads connecting the conductors to the roomtemperature water-cooled co-axial bus-bars were 1.08 m long copper tubes, which were cooled independently by liquid helium. 50 Hz power supplies could provide up to 10 kA continuously or 16 kA for 30 minutes. A fuller description of the test facility with details of the method of joining current leads to conductors is given elsewhere. 8 The method of ac loss measurement was basically similar to that described by Carter et al. s The electric field parallel to the current direction at the conductor surface was detected by pick-up loops or contacting probes of 100/Jm diameter copper wire spanning the central 2 m of the tube.
Loss measurements were made on the inner conductor of a co-axial pair of length 5.5 m. This conductor was 63.5 mm in diameter and of wall thickness 2.6 ram, with a 50/2m thick niobium layer on both inner and outer surfaces. The substrate was annealed copper with a resistivity ratio (R 300 K :R4.2 K) of 300:1. The outer conductor was of similar composite structure, with diameter 102 mm, wall thickness 1.8 ram, and substrate resistivity ratio 70 : 1. The
Multiple loops were obtained by threading bundles of ten wires simultaneously through 1 mm diameter holes drilled through the conductor wall and passing tile free ends out along the bore of the tube via multiway plugs and sockets for series connexion outside the cryostat. Contacts were made by indium-soldering single wires to 100 ~tm thick circumferential bands of copper, bonded to the conductor surface during the production process and left in place at the final etching stage. All the wires were enamel-insulated and held in contact with the conductor surface by 50/am thick adhesive Mylar tape. Where the wires passed through the conductor wall, the drilled holes were first lined with Stycast 2850 GT epoxy resin and the wires were subsequently secured in place with the same material so that a gas-tight seal was obtained.
The authors are with the Central Electricity Research Laboratories, Kelvin Avenue, Leatherhead, Surrey, UK. Received 12 July 1974.
A signal proportional to the conductor current was obtained by integrating the output from a toroidally-wound pick-up coil in the annular space between the inner and outer
Experimental
CRYOGENICS. OCTOBER 1974
553
Co-axial current leads Co-axial busbars
Helium gas in
I t_ _ _
I
I
/
,
_ t._i__
Helium
He um gas flow
Ar o ~ yc,s t a t
_ . . (.onauctors
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• Spring- loaded thermometers
a
x Varnished thermometer
Outer conductor x1,, Helium gas flow
~,
I
--
2m Gas-tight epoxy seal
C o n t a c t soldered to IOOJJm thick copper band
Resistance thermometer varnisl~d in position
Copper shorting flange
J
2 m test
Parallel plate ~ busbars
Test conductor
gas pipe
J
~
Pick-up loops
/
~
Evacuated
Liquid nitrogencooled shield
the conductor became normal in the course of a run. This indicates the absence of serious flux-trapping problems. 9 The electric field waveforms also showed no evidence of flux-trapping and were similar, regardless of whether contacts or loops were employed. Sample waveforms are given in Fig.3. The results are plotted in Fig.4 alternatively as loss versus temperature, with surface current density as parameter, or as loss versus surface current density, with temperature as parameter. The latter plot also includes the results at
_
b Loss x I 0 0 /
Fig.1 a -- Schematic illustration of the arrangement of cryostat, co-axial conductors, bus-bars and current leads (not to scale); b -- detail of test-section showing pick-up loops and contacting probes
conductors. The value of the current was determined by measuring the voltage across a 7.5 mm thick, 150 mm long section of the inner co-axial bus-bar. This shunt was previously calibrated against a high-precision current transformer and the estimated accuracy of current measurements is -+ 2%. The normal experimental procedure was to record the ac loss continuously as a function of temperature from about 4.4 K up to the point at which some predetermined loss was reached or the conductor became normal. Loss was recorded with temperature falling as well as rising in order that any effects of temperature hysteresis could be allowed for. Sample X Y recordings are shown in Fig.2. Temperature uniformity along the test section was checked by comparing the indications of eight carbon resistance thermometers held in spring-loaded copper blocks against the inside surface of the conductor. At low currents these generally showed uniformity within + 2% (that is, + 0.1 K at 5 K) but at surface current densities greater than 65 A mm "1 vibration of their mounting blocks caused the thermometers to reach individual temperatures appreciably above that of the conductor. A single thermometer held in position by varnish (see Fig.lb) was not affected by vibration and was normally used to record conductor temperatures during experimental runs at high currents. Results
X Y recorder traces of loss versus temperature were generally reproducible and free from significant hysteresis, even when
554
Loss x I 0
2
,
t
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I
I
4,5
5.0
I
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Fig.2 X Y recorder traces of loss versus temperature for a surface current density of 34 A mm "1
a
b Fig.3 Electric field waveforms derived from contacting probes and pick-up loops a -- contacts at 65 A mm -] and 4.8 K; b -- 8 loops in series at 28 A mm -1 and 7.8 K
CRYOGENICS.
OCTOBER 1974
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The present tube has a performance which is adequate for a rated current of about 50 A mm q at 5 K and shows that the co-processing route by which it was manufactured is satisfactory. The measured losses at 4.5 K are similar in their magnitude and dependence on surface current density to losses measured at 4.2 K in a 15 mm diameter niobium/ copper composite tube produced via the same extrusion route (Carter et al, s sample 1). When current was induced circumferentially, this latter tube had considerably lower losses and a dependence of loss on surface current density which resembled the general behaviour reported by Penczynski 10 for circumferential currents in composite tubes. In particular, over the range measured, the loss was close to that of his 'seamlessly-drawn' tube (sample 6). As pointed out by Carter et al, s it may be important to base estimates of ac losses in proposed composite conductors for power transmission lines only on measurements carried out with the appropriate current direction relative to the superconducting surface.
6K
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I II I-
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The present conductor had been heavily cold-worked and had a low effective value of first critical field (Hcl). Values of Hcl and Hc2 obtained from magnetization data (D. A. Ward, private communication) are plotted against temperature in Fig.5, together with the surface current densities required to give various constant levels of ac loss. The 101 W m -2 curve involves some extrapolation of the experimental data.
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b
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I
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Surface current density, AtomI, rms
Fig,4 a -- ac loss as a f u n c t i o n o f t e m p e r a t u r e f o r v a r i o u s surface c u r r e n t densities; b -- ac loss as a f u n c t i o n o f surface c u r r e n t d e n s i t y for various temperatures
C R Y O G E N I C S . OCTOBER 1974
Discussion Careful design of the cryogenic envelope of a superconducting cable 2 can reduce the heat inleak through the heliumcooled surface to about 0.1 W m "2 . Since refrigeration is a major element in the total cost of a cable, it is desirable that the ac losses in the conductors should not dominate the heat inleak, and hence should be < 0.1 W m "2. On this basis the present inner conductor is suitable for operation at current densities up to 52 A mm q at 5 K and 44 A mm q at 6 K. (The choice of current density and temperature depends on overall cost optimization, variation of dielectric strength with temperature, and considerations of behaviour during short-circuit currents.) In their designs, Rogers et al 2 assume a current density of 36 A m m q with a loss of 0.02 W m -2 at 5 K, while Eigenbrod et al 1 assume 34 or 56 A mm "l , corresponding to losses of about 0.03 and 0.2 W m -2 at 4.2 K respectively in the 10 mm diameter niobium/copper conductor of Meyerhoff and Beall. 6
id 2
[ t
4.2 K noted above for a sample of the niobium surface layer of the conductor, measured as a foil with current in the original axial direction. 4 [Note that the losses were incorrectly plotted in that paper and :should be reduced by 50% (B. J. Maddock, private communication)]. Although these results are averaged over both surfaces of the foil, it can be seen that there is reasonable agreement with the results obtained from the transport current measurements.
The indistinctness of H q in the magnetization data indicates a high critical current density at low field and also makes any correlation with Hc~ untrustworthy. However, the results may be correlated quite well with Hc2. In Fig.6 all the data from Fig.4a are plotted in the form of loss versus h, where h = H/H c_ and H is the peak value of the surface current density. T~e straight line represents losses proportional to tl. 7
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Fig.5 rms surface current density required to achieve various fixed levels of loss as a function of temperature. Also plotted are the rms surface current densities giving peak values equivalent to Hcl and Hc2 , as determined from magnetization data
The authors would like to acknowledge the work of all their colleagues at CERL and MEL who assisted in the preparation and execution of the experiments described. The work was carried out at the Central Electricity Research Laboratories and the paper is published by permission of the Central Electricity Generating Board.
i(~l
1 2 3
556
Eigenbrod, L. K., Long, H. M., Notaro, J. IEEE Trans PAS-89 (1970) 1995 Rogers,E. C., Slaughter, R. J., Swift, D.A. Proc lEE 118 (1971) 1493 Graeme-Barber,C., Maddock, B. J., Popley, R. A., Smedley, P. N. Cryogenics 12 (1972) 317
I
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__
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__
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Fig.6 ac loss plotted as a f u n c t i o n of the ratio of peak surface field to H c_ f o r the range of temperatures studied. The solid line represents I~oss cc h 7
4 5
References
t
0.2
6 7 8 9 10
Graeme-Barber,C., Maddock, B. J. Proc Applied Superconductivity Conference, Annapolis, IEEE Pub No 72CHO6825-TABSC (1972) 211 Carter, C. N., Male, J. C., Graeme-Barber, C. Cryogenics 14 (1974) 332 Meyerhoff,R. W., Beall, W. T. J A p p l Phys 42 (1971) 147 Carter, C. N., Lewis, K. G., Maddock, B. J., No~, J. A. Annexe 1969 - 1 Bulletin IIR (1969) 331 Baylis,J. A., Lewis, K. G., Meats, R. J., No~, J. A. Proc ICEC5, Kyoto, Japan, 1974 (IPC, forthcoming 1974) Male, J. C. Cryogenics 10 (1970) 381 Penczynski, P. Siemens Forsch u Entwickl Ber 2 (1973) 296
C R Y O G E N I C S . OCTOBER 1974