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Test and simulation of a textured YBCO superconductor for high current fuse
Physica C 372–376 (2002) 1610–1614 www.elsevier.com/locate/physc
Test and simulation of a textured YBCO superconductor for high current fuse J.G. Larsen
a,*
, I.D. Kledal b, J.N. Nielsen J. Sommerschield a
c,1
, J. Christiansen a,
a
b
Haldor Topsøe A/S, Nymøllevej 55, 2800 Lyngby, Denmark Ingeniørhøjskolen København, IHK, Lautrupvang 15, 2750 Ballerup, Denmark c DEFU, DTU, 2800 Lyngby, Denmark
Abstract A textured YBCO superconductor (high temperature superconductor, HTS) measuring 18 2:5 0:3 cm has been tested above the critical current in order to characterise it for high current application as a fuse element. The HTS had a Ic of 585 A (77 K) and was tested with (a) an asymmetric AC peak current, Imax , of 2.5 kA falling to 1.6 kA where the HTS survived an over current of 4Ic , and (b) an asymmetric Imax of 5.2 kA decreasing to 3 kA where the HTS heats up, quench and break after 80 ms. The rupture is followed by a periodic arc that dies out after 1.5 period as the current is passed through a shunt. In order to act as an efficient current limiting device the fuse should become normal conducting within 5 ms and this can only be obtained by a fault current of more than 8Ic . Calculations indicate that with a current of 16 kArms /cm2 or 23 kApeak /cm2 the HTS element would melt within this time interval, but the HTS element would presumably break at lower current due to the formation of a hot spot. Ó 2002 Elsevier Science B.V. All rights reserved. Keywords: YBCO; Fuse; Test; Simulations
1. Introduction High temperature superconducting fault current limiting devices have been designed, manufactured and tested by several groups [1] due to the potential benefit of such a device in power transmission network, where the fault current may exceed a level, where circuit breakers cannot be relied upon [2]. The advantage in using a high tempera-
*
Corresponding author.. Present address: NESA Hagedornsvej 4, 2820 Gentofte, Denmark. 1
ture superconductor (HTS) for this purpose is that this material automatically and fast will become normal conducting, when the critical current, Ic , is exceeded. Thus a resistance is introduced which will instantly prohibit the current to reach the Imax of a fault current. As soon as the current again has reached a level below the Ic and the HTS has regained its superconducting state the current will flow again without resistance, if the recovery time due to cooling of the quenched HTS is short. In order to reduce the fault current a normal state resistance of e.g. 0.5–1 X is needed, but with a specific resistivity of 0.2 mX cm (77 K) of a HTS a long length is needed, dependent on the Jc of the
0921-4534/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 ( 0 2 ) 0 1 0 8 4 - 5
J.G. Larsen et al. / Physica C 372–376 (2002) 1610–1614
material. The present approach is to use a bulk HTS as a sacrificial conductor in parallel with a fuse, whereby the length and cost of the HTS is strongly reduced. The idea is that when the Ic current of the HTS is exceeded, the HTS will heat up and break within a few milliseconds and divert the reduced current through a fuse or resistive shunt. The HTS should on the other hand be able to handle over-currents for some time before a quench is triggered. This type of fault current limitation, which is patent pending, is somewhat similar to the Carlor Emag fault current limiter where the main conductor is removed by an explosive charge triggered by a strong increase in the current with time [3].
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Fig. 2. The 4 point DC I–U curve of ST117 (10 cm between the electrodes).
2. Description and characterization of the test sample The tested HTS, ST117, was a textured 30% porous, polycrystalline YBCO slab with some ab-plane alignment, measuring 28 2:5 0:3 cm. Inhomogeneity of the Jc in the HTS is expected from the work in Ref. [4] where texture and Jc have been compared. The slab had 5 cm silver coated terminals in each end soldered into Cu terminals, which were fixed to a glass fiber reinforced resin support tube (Fig. 1). The contact resistance between the HTS and each of the Cu terminals was 0.025 and 0.052 lX, respectively (77 K, contact area: 25 cm2 ). The resistance of the 18 cm free length of the HTS was 4.5 mX at 95 K and 17 mX at 300 K as measured on similar samples. The I–U curve is shown in Fig. 2 based on a pulsed DC from a battery. The critical current was 585 A at a voltage of 1 lV/cm. At a current of 1600 A the U was only 0.40 mV or 0.04 mV/cm compared to 350 mV/cm in the normal state at 77 K.
Fig. 3. Diagram of the experimental set up. Note that Ch9 measured the voltage across the whole free superconductor.
3. Experimental set up The experiments were performed at IHK Short Circuit Laboratory. The test arrangement is shown in Fig. 3. The HTS was shunted by a cable with a resistance of 109 mX. The current was measured by Rogowski coils and the voltage by Isobe 3000 fiber Optic isolated probes. Signals were recorded with a frequency of 40 kHz and data processing was performed by the TeamPro version 2 program. The short circuit switch was closed at random and thus generating asymmetrical currents. The fault current was left standing for 1 s before the switch was opened.
4. Results
Fig. 1. The superconductor ST117 with terminals, support tube and electrodes.
The measurements are shown in Fig. 4a and b. In the first test of the HTS (Fig. 4a) an Imax of 2.5 kA decreased over six periods to the symmetric current of 1.6 kA (Ipeak ) or 1.1 kArms . The transformer open-circuit voltage was 235 V dropping to 3.7 Vrms when closed. A voltage of 80 mVrms over the HTS was noise as seen from comparison of
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J.G. Larsen et al. / Physica C 372–376 (2002) 1610–1614
current shows that only part of the HTS was driven into the normal state. The rupture of the HTS has caused a U jump of 24 V. After the rupture a periodic arc was ignited over the crack at a voltage of 100 V. During the build-up of this voltage the current was flowing through the shunt. When the arc was formed the voltage dropped to 24 V. Over the next two half periods the ignition and arc voltages increased until all the current of the HTS was commuted into the shunt at 160 V. The current was reduced from 2.5 to 1.7 kArms . Inspection showed that the superconductor had failed 1–2 cm from one of the terminals and that a gap of 5–10 mm had formed due to melting and evaporation of the HTS by the arc. The tests show that the textured HTS can allow an over-current of at least 4Ic for more than 1 s and up to 8Ic for 1–2 periods before a thermal run-away breaks the superconductor. Fault currents reach much higher levels and should therefore cause a rupture much faster. In order to evaluate how fast a thermal run away may occur, some rough simulation calculations have been performed.
5. Simulation of test results Fig. 4. (a) Current test of superconductor ST117 up to Ipeak ¼ 2:59 kA. Upp is the peak to peak voltage, covering noise at I ¼ 0 and noise during the current load. (b) Current test of superconductor ST117 up to 5.16 kA covering its rupture at 210 ms. Note the changed voltage scale close to 200 ms. From 240 to 350 ms the peak voltage was outside the range of the meter (Ip ¼ Ipeak ).
measurements before and during the test (Ch9 of Fig. 4a). The detection limit of the instrument was 30 mV. The second test (Fig. 4b) was run at a level of 2.5 kA, but due to the asymmetric start current Imin reached )5.2 kA and the voltage )0.1 V below the noise level. The voltage over the HTS increased especially during the third period, and the HTS broke after 80 ms where the voltage had reached 1.9 Vpeak (a false DC component of 3.7 V has been subtracted) and the Ipeak current had reached 3.2 kApeak . The relatively low voltage (1.9 V compared to 6.4 V for a wholly quenched HTS) at this peak
The transition behaviour of the HTS from the superconducting state to the normal state is in general described by complicated equations. The transition sequence for the HTS fuse is initiated when the current is exceeding a critical value in the superconductor’s HJT-space, so that it is brought into a resistive highly non-linear flux flow state. In this state the HTS is subjected to a power dissipation causes a temperature rise and thus a further reduction of the Ic and Hc . If the heating of the HTS continues the Tc will be exceeded and the normal state resistivity will be reached. In a fuse application of the HTS, it is assumed that the critical HJT-values are heavily exceeded within few milliseconds (I1st peak =Ic > 8) such that the normal state resistivity is a sufficient approximation to the superconductor’s transition behaviour. Based on the measured values of the test sample, the normal state resistivity, qn , is described by a linear function of the temperature,
J.G. Larsen et al. / Physica C 372–376 (2002) 1610–1614
qn ðT Þ ¼ aT þ b
ð1Þ
where the constants a and b are 2:5 108 X m/K and )5 107 X m. The applied asymmetric test current is approximately described by the equation: p t iðtÞ ¼ 3300 sin xt 1827 exp : 2 0:053 ð2Þ Further, the temperature rise due to joule heating is assumed adiabatic using the equation: dT ¼ J 2 qn ðT Þc1 p dt
ð3Þ
where cp (for YBCO) is taken as a constant ¼ 1 106 J m3 K1 (although cp increase with temperature), T is temperature in Kelvin and t is time in seconds. Based on these rough assumptions the simulated transient heating temperature of three HTS-slabs with thickness between 2.1 and 3 mm is shown in Fig. 5. A thickness of 2.1 represents a 100% dense equivalent to the HTS-slab, ST117. The melting temperature of the YBCO material is 1300 K. According to the curve T2.1 mm the melting temperature will be reached within 50 ms. In the experiment, however, the low voltage over the HTS fuse just before its rupture after 80 ms indicates, that only a part of the HTS was in the normal state at that time. Furthermore melting
1613
had only occurred where the arc was formed. Thus, the rupture was possibly due to thermal stress from a hot spot caused by a local quench. Since both magnetic forces and thermal energy are directly proportional to the square of the current, it is important to limit the Ic to a value as small as possible. Therefore, the requirement to a peak limiting HTS-fuse is a breaking time of less than 1/4 current cycle i.e. 4–5 ms. With the assumptions that the HTS switches to the normal state immediately and the destructive heating is adiabatic, the relation between current density and the heating temperature is given by: sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi cp Tm a þ b J¼ ln ð4Þ Ts a þ b at where cp is the volumetric heat capacity, t is the time, Tm is the melting temperature and Ts is the initial temperature of the fuse. By insertion of typical values of the YBCO: cp ¼ 1 106 J m3 K1 , t ¼ 5 ms, Tm ¼ 1300 K and Ts ¼ 77 K a current density is found to be about 16 kArms /cm2 or 23 kApeak /cm2 . The rupture will probably occur at a much lower temperature (and thus lower current) than the melting temperature of the YBCO material due to thermally induced stress either from a hot spot or between the still cold terminals and the hot free part of the superconductor. Although HTS fuse elements can be prepared from HTS-materials with relatively low Jc it is evident that with higher Jc and, thus, less cross section the fuse will break at fault currents below 8Ic .
6. Conclusion The preliminary experimental results and simulation calculations indicate, that textured YBCO material may be used as a low cost sacrificial conductor for high current application.
Acknowledgements Fig. 5. Simulated temperature of YBCO slabs with thickness 2.1, 2.5 and 3 mm.
Kim Høj Jensen, NESA, assisted during the testing at IHK.
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J.G. Larsen et al. / Physica C 372–376 (2002) 1610–1614
References [1] R.F. Giese, Directory of Superconducting Device Projects Bearing upon the Electric Power Sector, IEA-report, Argonne National Lab., 1997, pp. 2.1–2.16. [2] R.F. Giese, M. Runde, Fault-Current Limiters, IEA-report, Argonne National Lab., 1991, pp. 1–63.
[3] C. B€ ottger, The application of Is -limiters in three phase systems, Calor Emag leaflet 1000/8.67, 1967. [4] B.H. Larsen et al., IEEE Trans. Appl. Supercond. 11 (2001) 3513–3516.
Test and simulation of a textured YBCO superconductor for high current fuse J.G. Larsen
a,*
, I.D. Kledal b, J.N. Nielsen J. Sommerschield a
c,1
, J. Christiansen a,
a
b
Haldor Topsøe A/S, Nymøllevej 55, 2800 Lyngby, Denmark Ingeniørhøjskolen København, IHK, Lautrupvang 15, 2750 Ballerup, Denmark c DEFU, DTU, 2800 Lyngby, Denmark
Abstract A textured YBCO superconductor (high temperature superconductor, HTS) measuring 18 2:5 0:3 cm has been tested above the critical current in order to characterise it for high current application as a fuse element. The HTS had a Ic of 585 A (77 K) and was tested with (a) an asymmetric AC peak current, Imax , of 2.5 kA falling to 1.6 kA where the HTS survived an over current of 4Ic , and (b) an asymmetric Imax of 5.2 kA decreasing to 3 kA where the HTS heats up, quench and break after 80 ms. The rupture is followed by a periodic arc that dies out after 1.5 period as the current is passed through a shunt. In order to act as an efficient current limiting device the fuse should become normal conducting within 5 ms and this can only be obtained by a fault current of more than 8Ic . Calculations indicate that with a current of 16 kArms /cm2 or 23 kApeak /cm2 the HTS element would melt within this time interval, but the HTS element would presumably break at lower current due to the formation of a hot spot. Ó 2002 Elsevier Science B.V. All rights reserved. Keywords: YBCO; Fuse; Test; Simulations
1. Introduction High temperature superconducting fault current limiting devices have been designed, manufactured and tested by several groups [1] due to the potential benefit of such a device in power transmission network, where the fault current may exceed a level, where circuit breakers cannot be relied upon [2]. The advantage in using a high tempera-
*
Corresponding author.. Present address: NESA Hagedornsvej 4, 2820 Gentofte, Denmark. 1
ture superconductor (HTS) for this purpose is that this material automatically and fast will become normal conducting, when the critical current, Ic , is exceeded. Thus a resistance is introduced which will instantly prohibit the current to reach the Imax of a fault current. As soon as the current again has reached a level below the Ic and the HTS has regained its superconducting state the current will flow again without resistance, if the recovery time due to cooling of the quenched HTS is short. In order to reduce the fault current a normal state resistance of e.g. 0.5–1 X is needed, but with a specific resistivity of 0.2 mX cm (77 K) of a HTS a long length is needed, dependent on the Jc of the
0921-4534/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 ( 0 2 ) 0 1 0 8 4 - 5
J.G. Larsen et al. / Physica C 372–376 (2002) 1610–1614
material. The present approach is to use a bulk HTS as a sacrificial conductor in parallel with a fuse, whereby the length and cost of the HTS is strongly reduced. The idea is that when the Ic current of the HTS is exceeded, the HTS will heat up and break within a few milliseconds and divert the reduced current through a fuse or resistive shunt. The HTS should on the other hand be able to handle over-currents for some time before a quench is triggered. This type of fault current limitation, which is patent pending, is somewhat similar to the Carlor Emag fault current limiter where the main conductor is removed by an explosive charge triggered by a strong increase in the current with time [3].
1611
Fig. 2. The 4 point DC I–U curve of ST117 (10 cm between the electrodes).
2. Description and characterization of the test sample The tested HTS, ST117, was a textured 30% porous, polycrystalline YBCO slab with some ab-plane alignment, measuring 28 2:5 0:3 cm. Inhomogeneity of the Jc in the HTS is expected from the work in Ref. [4] where texture and Jc have been compared. The slab had 5 cm silver coated terminals in each end soldered into Cu terminals, which were fixed to a glass fiber reinforced resin support tube (Fig. 1). The contact resistance between the HTS and each of the Cu terminals was 0.025 and 0.052 lX, respectively (77 K, contact area: 25 cm2 ). The resistance of the 18 cm free length of the HTS was 4.5 mX at 95 K and 17 mX at 300 K as measured on similar samples. The I–U curve is shown in Fig. 2 based on a pulsed DC from a battery. The critical current was 585 A at a voltage of 1 lV/cm. At a current of 1600 A the U was only 0.40 mV or 0.04 mV/cm compared to 350 mV/cm in the normal state at 77 K.
Fig. 3. Diagram of the experimental set up. Note that Ch9 measured the voltage across the whole free superconductor.
3. Experimental set up The experiments were performed at IHK Short Circuit Laboratory. The test arrangement is shown in Fig. 3. The HTS was shunted by a cable with a resistance of 109 mX. The current was measured by Rogowski coils and the voltage by Isobe 3000 fiber Optic isolated probes. Signals were recorded with a frequency of 40 kHz and data processing was performed by the TeamPro version 2 program. The short circuit switch was closed at random and thus generating asymmetrical currents. The fault current was left standing for 1 s before the switch was opened.
4. Results
Fig. 1. The superconductor ST117 with terminals, support tube and electrodes.
The measurements are shown in Fig. 4a and b. In the first test of the HTS (Fig. 4a) an Imax of 2.5 kA decreased over six periods to the symmetric current of 1.6 kA (Ipeak ) or 1.1 kArms . The transformer open-circuit voltage was 235 V dropping to 3.7 Vrms when closed. A voltage of 80 mVrms over the HTS was noise as seen from comparison of
1612
J.G. Larsen et al. / Physica C 372–376 (2002) 1610–1614
current shows that only part of the HTS was driven into the normal state. The rupture of the HTS has caused a U jump of 24 V. After the rupture a periodic arc was ignited over the crack at a voltage of 100 V. During the build-up of this voltage the current was flowing through the shunt. When the arc was formed the voltage dropped to 24 V. Over the next two half periods the ignition and arc voltages increased until all the current of the HTS was commuted into the shunt at 160 V. The current was reduced from 2.5 to 1.7 kArms . Inspection showed that the superconductor had failed 1–2 cm from one of the terminals and that a gap of 5–10 mm had formed due to melting and evaporation of the HTS by the arc. The tests show that the textured HTS can allow an over-current of at least 4Ic for more than 1 s and up to 8Ic for 1–2 periods before a thermal run-away breaks the superconductor. Fault currents reach much higher levels and should therefore cause a rupture much faster. In order to evaluate how fast a thermal run away may occur, some rough simulation calculations have been performed.
5. Simulation of test results Fig. 4. (a) Current test of superconductor ST117 up to Ipeak ¼ 2:59 kA. Upp is the peak to peak voltage, covering noise at I ¼ 0 and noise during the current load. (b) Current test of superconductor ST117 up to 5.16 kA covering its rupture at 210 ms. Note the changed voltage scale close to 200 ms. From 240 to 350 ms the peak voltage was outside the range of the meter (Ip ¼ Ipeak ).
measurements before and during the test (Ch9 of Fig. 4a). The detection limit of the instrument was 30 mV. The second test (Fig. 4b) was run at a level of 2.5 kA, but due to the asymmetric start current Imin reached )5.2 kA and the voltage )0.1 V below the noise level. The voltage over the HTS increased especially during the third period, and the HTS broke after 80 ms where the voltage had reached 1.9 Vpeak (a false DC component of 3.7 V has been subtracted) and the Ipeak current had reached 3.2 kApeak . The relatively low voltage (1.9 V compared to 6.4 V for a wholly quenched HTS) at this peak
The transition behaviour of the HTS from the superconducting state to the normal state is in general described by complicated equations. The transition sequence for the HTS fuse is initiated when the current is exceeding a critical value in the superconductor’s HJT-space, so that it is brought into a resistive highly non-linear flux flow state. In this state the HTS is subjected to a power dissipation causes a temperature rise and thus a further reduction of the Ic and Hc . If the heating of the HTS continues the Tc will be exceeded and the normal state resistivity will be reached. In a fuse application of the HTS, it is assumed that the critical HJT-values are heavily exceeded within few milliseconds (I1st peak =Ic > 8) such that the normal state resistivity is a sufficient approximation to the superconductor’s transition behaviour. Based on the measured values of the test sample, the normal state resistivity, qn , is described by a linear function of the temperature,
J.G. Larsen et al. / Physica C 372–376 (2002) 1610–1614
qn ðT Þ ¼ aT þ b
ð1Þ
where the constants a and b are 2:5 108 X m/K and )5 107 X m. The applied asymmetric test current is approximately described by the equation: p t iðtÞ ¼ 3300 sin xt 1827 exp : 2 0:053 ð2Þ Further, the temperature rise due to joule heating is assumed adiabatic using the equation: dT ¼ J 2 qn ðT Þc1 p dt
ð3Þ
where cp (for YBCO) is taken as a constant ¼ 1 106 J m3 K1 (although cp increase with temperature), T is temperature in Kelvin and t is time in seconds. Based on these rough assumptions the simulated transient heating temperature of three HTS-slabs with thickness between 2.1 and 3 mm is shown in Fig. 5. A thickness of 2.1 represents a 100% dense equivalent to the HTS-slab, ST117. The melting temperature of the YBCO material is 1300 K. According to the curve T2.1 mm the melting temperature will be reached within 50 ms. In the experiment, however, the low voltage over the HTS fuse just before its rupture after 80 ms indicates, that only a part of the HTS was in the normal state at that time. Furthermore melting
1613
had only occurred where the arc was formed. Thus, the rupture was possibly due to thermal stress from a hot spot caused by a local quench. Since both magnetic forces and thermal energy are directly proportional to the square of the current, it is important to limit the Ic to a value as small as possible. Therefore, the requirement to a peak limiting HTS-fuse is a breaking time of less than 1/4 current cycle i.e. 4–5 ms. With the assumptions that the HTS switches to the normal state immediately and the destructive heating is adiabatic, the relation between current density and the heating temperature is given by: sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi cp Tm a þ b J¼ ln ð4Þ Ts a þ b at where cp is the volumetric heat capacity, t is the time, Tm is the melting temperature and Ts is the initial temperature of the fuse. By insertion of typical values of the YBCO: cp ¼ 1 106 J m3 K1 , t ¼ 5 ms, Tm ¼ 1300 K and Ts ¼ 77 K a current density is found to be about 16 kArms /cm2 or 23 kApeak /cm2 . The rupture will probably occur at a much lower temperature (and thus lower current) than the melting temperature of the YBCO material due to thermally induced stress either from a hot spot or between the still cold terminals and the hot free part of the superconductor. Although HTS fuse elements can be prepared from HTS-materials with relatively low Jc it is evident that with higher Jc and, thus, less cross section the fuse will break at fault currents below 8Ic .
6. Conclusion The preliminary experimental results and simulation calculations indicate, that textured YBCO material may be used as a low cost sacrificial conductor for high current application.
Acknowledgements Fig. 5. Simulated temperature of YBCO slabs with thickness 2.1, 2.5 and 3 mm.
Kim Høj Jensen, NESA, assisted during the testing at IHK.
1614
J.G. Larsen et al. / Physica C 372–376 (2002) 1610–1614
References [1] R.F. Giese, Directory of Superconducting Device Projects Bearing upon the Electric Power Sector, IEA-report, Argonne National Lab., 1997, pp. 2.1–2.16. [2] R.F. Giese, M. Runde, Fault-Current Limiters, IEA-report, Argonne National Lab., 1991, pp. 1–63.
[3] C. B€ ottger, The application of Is -limiters in three phase systems, Calor Emag leaflet 1000/8.67, 1967. [4] B.H. Larsen et al., IEEE Trans. Appl. Supercond. 11 (2001) 3513–3516.