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The electric characteristics of HTS double-pancake coil with AC pulse over currents
Cryogenics 52 (2012) 45–50
Contents lists available at SciVerse ScienceDirect
Cryogenics journal homepage: www.elsevier.com/locate/cryogenics
The electric characteristics of HTS double-pancake coil with AC pulse over currents Jingye Zhang ⇑, Shaotao Dai, Dong Zhang, Guomin Zhang, Zikai Wang, Zhiqin Zhu, Fengyuan Zhang, Liangzhen Lin, Liye Xiao Key Lab of Applied Superconductivity, Chinese Academy of Sciences, Beijing 100190, China Institute of Electrical Engineering, Chinese Academy of Sciences, Beijing 100190, China
a r t i c l e
i n f o
Article history: Received 7 December 2009 Received in revised form 27 October 2011 Accepted 1 November 2011 Available online 11 November 2011 Keywords: F. Double-pancake C. Over-current C. Quench characteristics C. Burned out
a b s t r a c t A double pancake coil was designed and manufactured with a 36-m long Bi-2223/Ag tape. The tape was insulated by 25 lm thick Kapton tapes, which can stand a voltage of 400 Vrms in liquid nitrogen. The whole double pancake was impregnated with epoxy resin. AC over-current experiments of the coil were performed by applying constant AC voltages to the two terminals of the coil and lasted for 3 s. The experiment began first at a lower voltage of 33.6 Vrms, and then the voltage stepped up till the coil was burned out at the pulse voltage of 202.7 Vrms. All of the experiments were carried out with the coil dipped in liquid nitrogen. The current waveforms were measured. The impedance and resistance characters of the HTS coil with its over pulse currents were analyzed from the experiment results. At the end of this paper, some conclusions derived from the experiment results and their analyses are given, which are helpful for the safety operating of the HTS coils in power applications. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction High temperature superconductor (HTS) is developed for applications in HTS power devices such as transformers, superconducting fault current limiter (SFCL), superconducting magnetic energy storage (SMES), motor, and generators. These HTS power devices applied in power systems might undergo a larger over current, such as over load, fault conditions, and unbalanced current. So, the electric properties of the HTS coils with alternating over currents are very important for the design and operation of HTS power devices [1]. In the case of devices using low temperature superconducting coils, the current exceeding the critical current (Ic) means immediate quench. However, in the case of devices using HTS coils, it is not true for its lower N values and high thermal stabilities. Actually, HTS tapes and coils can carry currents higher than their Ic and without thermal runaway. If so, the loss stays in a reasonable level, it is the practical available current for some special cases. Prof. H. Matsuura called it as ‘‘tolerance current’’ [2]. When the currents of HTS tapes or coils exceed their Ic, quenching will occur and the temperatures will increase. The thermal quench properties induced by temperature have been studied by many groups [2–6]. In this paper, a HTS double pancake coil was fabricated with a 36 m Bi-2223 tape insulated by Kapton film. The electric properties of the double pancake with ac pulse over currents, including the impedance and the resistance, have been investigated by experiment. As a result, we could find the optimal
operating condition considering the protection of the HTS tape against the alternating over-current. 2. Experiment 2.1. The HTS double pancake coil In this work, an experimental double pancake coil was manufactured with a 36 m Bi-2223/Ag stainless steel reinforced tape. The tape was insulated with 25 lm thick Kapton film, which could stand a voltage of 400 Vrms when dipped in liquid nitrogen. The specifications of the reinforced HTS tape and the double pancake coil are shown in Tables 1 and 2, respectively. When the double pancake coil transports 100 A DC current, the magnetic field contour lines in the winding analyzed by FEA are shown in Fig. 1. We can see from Fig. 1b that the maximal radial magnetic field Br (T) is located at the middle turns of each pancake coil. The lowest Ic of the coil is also located there due to the strong anisotropy of the Bi-2223/Ag tape [7,8]. In order to diminish the eddy current loss in the copper bobbin of the double pancake, a 2 mm wide notch was cut and a Fiberglass-Reinforced Plastics (FRP) of same size was embedded in it. The whole coil was impregnated by epoxy resin. To get high thermal conductivity and little deformation, silicon dioxide was doped with epoxy resin. 2.2. Experiment setup
⇑ Corresponding author at: Institute of Electrical Engineering, Chinese Academy of Sciences, Beijing 100190, China. Fax: +86 10 82547137. E-mail address: [email protected] (J. Zhang). 0011-2275/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.cryogenics.2011.11.002
The experimental setup was shown in Fig. 2. The Bi-2223 double pancake coil was dipped in liquid nitrogen. A DC power supply was
46
J. Zhang et al. / Cryogenics 52 (2012) 45–50
Table 1 Specifications of the Bi-2223/Ag stainless steel reinforced tape. Item
Specifications
Width Total thickness Thickness of stainless steel tapes on both sides Ratio of Ag and superconductors Filaments Minimum Ic
4.8 mm 0.31 mm 0.15 mm 7:3 55 115 A*
*
At 77 K, self filed, 1 lV/cm.
Table 2 Specifications of the double pancake coil. Item
Specifications
Inner diameter Outer diameter Number of turns Total length of Bi-2223 tape Inductance Impedance Zl (f = 50 Hz)
80 mm 132 mm 108 36 m 1.532 mH 0.4813 X
shown in Fig. 3. The critical current Ic of the coil was 60 A and the N value was 12.6. Then constant AC voltages which lasted for 3 s were applied to the two terminals of the coil and its currents were measured. The value of the AC voltage stepped up till the double pancake coil was burned out. All the AC voltages with frequencies of 50 Hz were measured in rms. After 15 min of each AC current pulse, the Ic of the coil was measured to check if there was any damage during the current pulse. The interval between every two test was 20 min so as to cool down the coil completely. After the pulse test with the AC voltage of 175.7 Vrms, the Ic of the coil decreased to 52 A and the N value decreased from 12.6 to 8.04 as shown in Fig. 3. It indicated that some damage has been done to the coil during the current pulse. When the pulse voltage increased to 202.7 Vrms, the coil was burned out and the pictures were shown in Fig. 4. You can see from the photos that the burned-out place located at the middle turns of one pancake, where the radial field Br is the highest and the local Ic is the lowest. 3. Experimental results and analysis
Fig. 1. The magnetic field distributions on the winding when the double pancake coil transports 100 A DC current. (a) The distributions of the overall magnetic field B (T); (b) the distributions of radial magnetic field Br (T).
used to measure the critical current Ic of the double pancake coil. A 250 kV A voltage regulator with frequency of 50 Hz worked as the AC power supply. It has an output voltage from 0 to 420 Vrms. Each of the AC pulse currents in all of the experiments lasted for 3 s. The duration of the pulse current were achieved by a silicon-controlled switch controlled by DSP. The DC and AC power supply could interchange by the switch 1 and switch 2 during the experiment, as shown in Fig. 2. The currents passed through the double pancake coil and the voltages between the two terminals were measured. 2.3. Measurement First of all, the critical current Ic of the double pancake coil was measured by the DC power supply by the criterion of 1 lV/cm as
As mentioned previously, the current of the coil was measured while a constant 3 s AC voltage pulse was applied to the two terminals of the coil. When a lower AC voltage was applied to the coil, the current was almost constant as shown in Fig. 5a, except the first 2 or 3 cycles as shown in Fig. 5b. But when higher AC voltage pulse was applied to the coil, the current was no longer constant. It decreased gradually during the pulse time, as shown in Fig. 6. For all of the tests, the first 2 or 3 cycles were aberrance for some unidentified reasons, for example, were caused by the electrocircuit or its equipments. In this study, they were left out of account. For convenience of expression, three points during the pulse time were chosen to facilitate the analysis of experiment results. As shown in Figs. 5b and 6, the first point 01 was at the fourth cycle of the pulse current. The second point 02 and the last point 03 were in the middle and the end of the pulse time, namely at 1.5 s and 3 s, respectively, as shown in Fig. 6. The variations of the currents (I01, I02 and I03) with the corresponding pulse voltages for the three chosen points were studied. The curves of the currents I01, I02 and I03 varying with their corresponding pulse voltages were shown in Fig. 7. We can see that I01 is almost linear with increasing voltage; I01, I02 and I03 are coincided when the voltage was below 60 Vrms; when the voltages were between 60 and 190.4 Vrms, I02 and I03 are almost unchanged as the voltage grows; there is no degeneration of the Ic of coil when voltage below 169.2 Vrms; when the voltage goes up to 202.7 Vrms, the coil was burned out, then I03 and Ic of the coil fall to zero. From the voltages and the currents (I01, I02 and I03), the curves of their impedances (Z01, Z02 and Z03) were obtained, which were shown in Fig. 8. We can see that the impedances (Z01, Z02 and Z03) behave disparately with their corresponding currents I01, I02 and I03. Z01 is almost unchanged as the voltage grows up, i.e. the impedance of the coil almost unchanged at the first several cycles with the voltage pulse increasing. Z01, Z02 and Z03 are coincided when the voltage below 60 Vrms. It indicates that the impedance of the coil Z changes little with pulse time when the voltage below 60 Vrms. Z02 and Z03 are almost linear with the voltage from 60 Vrms to 169.2 Vrms, indicating that the impedance of the coil Z02 and Z03 change rapidly with the voltage increasing. When the voltage pulses go over 169.2 Vrms, the Ic of the coil degrades, indicating that some damages occurs to the coil. For HTS double pancake coil, its impedance Z can be described by the following formulae.
~ Z ¼ R þ~jZ l
ð1Þ
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J. Zhang et al. / Cryogenics 52 (2012) 45–50
Fig. 2. Experiment setup.
11
Before any pulsed test N value: 12.6 (0.1-1.0 μV/cm) After pulsed test with voltage:175.7 Vrms N value: 8.04 (0.1-1.0 μV/cm)
10 9
Voltage (μV/cm)
8 7 6 5 4 3 2 1 0 -1 0
10
20
30
40
50
60
70
Current (A) Fig. 3. The DC current–voltage curves of the Bi-2223/Ag double pancake coil measured at liquid nitrogen temperature.
Fig. 4b. The picture of the burned out part of the double pancake coil after experiment.
160 140
Constant voltage: 38.66 Vrms
120 100
Current (A)
80 60 40 20 0 -20 -40 -60 -80 -100 0.0 Fig. 4a. The panoramic picture of the burned out double pancake coil after experiment.
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi R ¼ Z 2 Z 2l
0.3
0.6
0.9
1.2
1.5
1.8
2.1
2.4
2.7
3.0
3.3
3.6
Time (s) Fig. 5a. The AC current characters of the double pancake coil at the constant AC voltage of 38.66 Vrms.
ð2Þ
where Zl = 0.4813 X is calculated from 2pfL and shown in Table 2. Here, we assumed that the inductance of the HTS double pancake unchanged in all experiments. For the current mainly change from superconductor filaments to Ag matrix when the current over than
Ic and the size of the Ag matrix and the superconductor filaments are almost same. R is the equivalent resistance of the double pancake coil. When the amplitude of the current is smaller than the Ic of the coil, R is mainly caused by the AC losses of the coil; when the amplitude
J. Zhang et al. / Cryogenics 52 (2012) 45–50
2.2
66
2.0
60 54
Z 01
1.6
48
Z 02
1.4 1.2
Z 03
1.0
IC
42 36 30
0.8
24
0.6
18
0.4
12
0.2
6
0.0
0 20
40
60
80
Constant voltage: 100.3 Vrms
01
250
02
03
150
Current (A)
160
180
200
2.2
66
2.0
60 54
1.8
200
100 50 0 -50 -100 -150
1.6
IC
48
1.4
R01
42
1.2
R02
36
1.0
R03
30
0.8
24
0.6
18
0.4
12
0.2
6
0.0
0 20
-200
40
60
80
100
120
140
160
180
200
Voltage (Vrms) 0.0
0.4
0.8
1.2
1.6
2.0
2.4
2.8
3.2
Time (s) Fig. 6. The AC current characters of the double pancake coil at the constant AC voltage of 100.3 Vrms.
70
280
I 01
200
50
I 02 160
I 03
40
120
IC
30
80
20
40
10
DC Current (A)
60
240
AC Current (Arms)
140
Fig. 8. The curves of the impedances (Z01, Z02 and Z03) of the HTS double pancake coil with their pulse rms AC voltages.
Resistance (Ω)
300
120
Voltage (Vrms)
Time (s) Fig. 5b. The first several cycles of the AC current characters of the double pancake coil at the constant AC voltage of 38.66 Vrms.
100
Current (A)
1.8
DC Current (A)
160 Constant voltage: 38.66 Vrms 01 140 120 100 80 60 40 20 0 -20 -40 -60 -80 -100 -120 0.45 0.48 0.51 0.54 0.57 0.60 0.63 0.66 0.69 0.72 0.75
Impedance (Ω)
Current (A)
48
0
0 20
40
60
80
100
120
140
160
180
200
Voltage (Vrms) Fig. 7. The curves of the AC currents (I01, I02 and I03) of the HTS double pancake coil with their pulse AC voltages.
of pulse current is above Ic, it caused by many intricate reasons due to over currents. According to formula (2), the resistance R could be derived from the impedance Z of the coil. Then, converting Figs. 8 and 9, we can
Fig. 9. The curves of the resistances (R01, R02 and R03) of the HTS double pancake coil with its pulse rms AC voltages.
see that the resistances R01, R02 and R03 behave similarly with their corresponding impedances Z01, Z02 and Z03. It indicates that the differences among the impedances Z01, Z02 and Z03, and the tendency of variation with the voltages of them, are mainly dominated by the resistances R01, R02 and R03. It is well known that the energy dissipation of the HTS coil due to over currents can be expressed as I2R and the energy can be released as heat to the coil. When the resistance R appears and become larger, the HTS coil will be heated. Whether the loss of the HTS coil stays in a reasonable level or leads to thermal runaway, it can be judged from Fig. 9. (a) When the voltage below 60 Vrms, R01, R02 and R03 are coincided. It indicates that the resistance of the coil R changes little with the pulse time, and the pulse currents can last any long without damage to the coil. (b) The resistance of the coil (R01) is almost unchanged at the first several cycles with the pulse voltage increasing. It indicates that if the pulse time is short enough, the coil can stand a very high AC voltage pulse, namely, can stand a very large pulse current and without thermal runaway. (c) R02 and R03 are almost linear when the voltage is above 60 Vrms. This indicates that the resistances of the coil R02 and R03 change rapidly with the voltage increasing. If the current lasts long enough, it will result in a thermal runaway and the coil may be damaged. Based on the above results and analysis, we can see that the pulse voltages from 60 Vrms to 169.2 Vrms play a key role in the over currents pulse process. Thus, the characters of the coil for the pulse voltage of 60 Vrms, 169.2 Vrms and 181.4 Vrms were studied respectively. Fig. 10 gives their characters of the rms currents with
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J. Zhang et al. / Cryogenics 52 (2012) 45–50
260
I60V
1.8
R 60V
240
I169.2V
1.6
R 169.2V
I181.4V
1.4
R 181.4V
Resistance (Ω)
I (Arms)
220 200 180 160 140
1.2 1.0 0.8 0.6 0.4
120
0.2
100 0.0
0.3
0.6
0.9
1.2
1.5
1.8
2.1
2.4
2.7
0.0
3.0
0.0
Time (S)
0.9
1.2
1.5
1.8
2.1
2.4
2.7
3.0
Fig. 12. The curves of the resistances of the HTS double pancake coil in its pulse time corresponding to Fig. 10.
Z 60V
400
Z 169.2 V
300
Z 181.4 V
200
Current (A)
Impedance (Ω)
1.6
0.6
Time (S)
Fig. 10. The curves of the rms currents of the HTS double pancake coil vary with its pulse time. The currents I60V, I169.2V, I181.4V; their corresponding voltages pulse were 60 Vrms, 169.2 Vrms and 181.4 Vrms individually.
1.8
0.3
1.4 1.2 1.0
Constant voltage: 202.7 Vrms
100 0 -100 -200 -300
0.8
-400 0.6 0.0 0.4 0.0
0.3
0.6
0.9
1.2
1.5
1.8
2.1
2.4
2.7
3.0
Time (s)
Impedance (Ω)
Z60V
0.595 0.590 0.585 0.580 0.0
0.3
0.6
0.9
1.2
1.5
0.8
1.2
1.6
2.0
2.4
2.8
3.2
Time (s) Fig. 13a. The AC current characters while the double pancake coil was burned out at the AC voltage pulse of 202.7 Vrms.
Fig. 11a. The curves of the impedances of the HTS double pancake coil vary with its pulse time. The impedances Z60V, Z169.2V, Z181.4V, are corresponding to voltages pulse of 60 Vrms, 169.2 Vrms and 181.4 Vrms individually.
0.600
0.4
1.8
2.1
2.4
2.7
3.0
Time (s) Fig. 11b. The partial enlarged view of the impedances Z60V curve with the pulse time.
the pulse time at the three key voltages, 60 Vrms, 169.2 Vrms and 181.4 Vrms, respectively. We can see that for the pulse voltage 60 Vrms, the current changes little with pulse time. As the voltage increases up to 169.2 Vrms and 181.4 Vrms, their currents decrease with the pulse time. Beside, there is an intersection of the two curves, this shows that the current for 181.4 Vrms decreases more rapidly than that for 169.2 Vrms. Using the voltages (60 V, 169.2 V and 181.4 V in rms) and the currents (I60V, I169.2V and I181.4V), we can get the curves of their impedances (Z60V, Z169.2V and Z181.4V) with the pulse time as shown in Fig. 11. We can see from it that the impedances Z60V is almost
horizontal, indicating that the impedance of the coil changes little as the pulse time goes on. Z169.2V and Z181.4V almost linearly increase with pulse time goes on. According to formula (2), the resistance R60V, R169.2V and R181.4V are calculated and shown in Fig. 12. We can see from it that, as the pulse time goes on, the resistances R60V, R169.2V and R181.4V behave similarly to their corresponding impedances Z60V, Z169.2V and Z181.4V. Similarly, the differences among the impedances Z60V, Z169.2V and Z181.4V, and the changes of them with the pulse time, are mainly dominated by the resistances R60V, R169.2V and R181.4V due to the over pulse currents. Namely, we can see from Fig. 12 that: (a) The resistance of the coil (R60V) is almost unchanged with the pulse time going on when the voltage below 60 Vrms. It indicates that if the voltages pulse below 60 Vrms, the pulse time and the pulse currents can last any long and has no damage to the coil. (b) The resistance of the coil R169.2V and R181.4V increase almost linearly with the pulse time. If the current last long enough, it will result in a thermal runaway and the coil may be damaged, or burned. Fig. 13 shows the current characters of the burned-out process of the HTS coil. When the voltages pulse over 169.2 Vrms, some damages have been done to the coil. Figs. 3 and 9 show that when the voltages pulse up to 175.7 Vrms, the Ic has degraded 13%. When up to 202.7 Vrms, the coil burned out. We can see from Fig. 13a that
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J. Zhang et al. / Cryogenics 52 (2012) 45–50
500
Constant voltage: 202.7 Vrms
400
Current (A)
300 200
performed by applying constant AC voltages pulse to the two terminals of the coil in liquid nitrogen bath. The currents, impedance and resistance characters were analyzed from the measured current waveforms. Some useful conclusions derived from the experimental results and their analyses are given as following:
100 0 -100 -200 -300 -400 -500 2.83 2.84 2.85 2.86 2.87 2.88 2.89 2.90 2.91 2.92 2.93
Time (s) Fig. 13b. The enlarged view of the several cycles in the red box of (a). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
the maximum current at the first several cycles was higher than 400 A; as the pulse time went on, the current decreased smoothly and at the time of 2.18 s the current arrived its lowest value (185 A). Then, the current increased rapidly and till the pulse time of 2.85 s the current become zero fitfully. This can be seen from the overall process of Fig. 13a and the part enlarged view in Fig. 13b. We know that, from the pulse time of 2.18 s to 2.85 s, the HTS coil burned out. When the coil was burned out, the damage caused many turns to short-circuit at the burned place. So the impedance of the coil decreasing rapidly and this resulted in the current increased rapidly after the pulse time 2.18 s. When the burnout lasted a little time (here is about 0.67 s) the coil became open-circuit fitfully as seen from Fig. 13b. We can see from Fig. 4 that the burnout position located at the middle turns of the coil where the radial magnetic field Br is maximal (shown in Fig. 1). 4. Conclusions A double pancake coil was manufactured with a 36 m long Bi-2223/Ag tape. The AC over-current experiments of the coil were
(1) The impedance and the resistance of the coil have little changed at the first several cycles as the pulse voltages increasing. It indicates that if the pulse time is short enough, the coil can stand a very high AC pulse voltage, or a very large pulse current. (2) When the pulse voltages are low values (for the model coil lower than 60 Vrms), the coil is in safety region. The pulse time could last long enough and there no damage to the coil. (3) When the pulse voltages at a high value (for the model coil higher than 60 Vrms), the impedances and the resistances of the coil change rapidly with the voltages pulse and the pulse time. If the pulse time lasted long enough, over current pulse will cause the performances degenerated of the coil, or even burned out of it. (4) The most severely destroyed place is located in the middle turns of the HTS double pancake coil where the radial magnetic field Br is maximal and the local Ic of the tapes in the coil is the lowest.
References [1] Iwasa Y. Case studies in superconductivity magnets. New York: Plenum Press; 1994. [2] Matsuura H, Ninomiya A, Ishigohka T. IEEE Trans. Appl. Supercond. 2007;17:3109–12. [3] Lee J et al. IEEE Trans. Appl. Supercond. 2007;17:1603–6. [4] Lee J, Kim Ho Min, et al. IEEE Trans. Appl. Supercond. 2008;18:1267–70. [5] Kim Min Jae, Ahn Min Cheol, et al. IEEE Trans. Appl. Supercond. 2008;18:660–3. [6] Zhang J, Dai S, et al. IEEE Trans. Appl. Supercond. 2010;20:1135–8. [7] Kobabyashi Shin-ichi, Kaneko Tetsuyuki, Kato Takeshi, Fujikami Jun, Sato Kenichi. Physica C 1996;258:336–40. [8] Larbalestier David, Gurevich Alex, Matthew Feldmann D, Polyanskii Anatoly. Nature 2001;414:368–77.
Contents lists available at SciVerse ScienceDirect
Cryogenics journal homepage: www.elsevier.com/locate/cryogenics
The electric characteristics of HTS double-pancake coil with AC pulse over currents Jingye Zhang ⇑, Shaotao Dai, Dong Zhang, Guomin Zhang, Zikai Wang, Zhiqin Zhu, Fengyuan Zhang, Liangzhen Lin, Liye Xiao Key Lab of Applied Superconductivity, Chinese Academy of Sciences, Beijing 100190, China Institute of Electrical Engineering, Chinese Academy of Sciences, Beijing 100190, China
a r t i c l e
i n f o
Article history: Received 7 December 2009 Received in revised form 27 October 2011 Accepted 1 November 2011 Available online 11 November 2011 Keywords: F. Double-pancake C. Over-current C. Quench characteristics C. Burned out
a b s t r a c t A double pancake coil was designed and manufactured with a 36-m long Bi-2223/Ag tape. The tape was insulated by 25 lm thick Kapton tapes, which can stand a voltage of 400 Vrms in liquid nitrogen. The whole double pancake was impregnated with epoxy resin. AC over-current experiments of the coil were performed by applying constant AC voltages to the two terminals of the coil and lasted for 3 s. The experiment began first at a lower voltage of 33.6 Vrms, and then the voltage stepped up till the coil was burned out at the pulse voltage of 202.7 Vrms. All of the experiments were carried out with the coil dipped in liquid nitrogen. The current waveforms were measured. The impedance and resistance characters of the HTS coil with its over pulse currents were analyzed from the experiment results. At the end of this paper, some conclusions derived from the experiment results and their analyses are given, which are helpful for the safety operating of the HTS coils in power applications. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction High temperature superconductor (HTS) is developed for applications in HTS power devices such as transformers, superconducting fault current limiter (SFCL), superconducting magnetic energy storage (SMES), motor, and generators. These HTS power devices applied in power systems might undergo a larger over current, such as over load, fault conditions, and unbalanced current. So, the electric properties of the HTS coils with alternating over currents are very important for the design and operation of HTS power devices [1]. In the case of devices using low temperature superconducting coils, the current exceeding the critical current (Ic) means immediate quench. However, in the case of devices using HTS coils, it is not true for its lower N values and high thermal stabilities. Actually, HTS tapes and coils can carry currents higher than their Ic and without thermal runaway. If so, the loss stays in a reasonable level, it is the practical available current for some special cases. Prof. H. Matsuura called it as ‘‘tolerance current’’ [2]. When the currents of HTS tapes or coils exceed their Ic, quenching will occur and the temperatures will increase. The thermal quench properties induced by temperature have been studied by many groups [2–6]. In this paper, a HTS double pancake coil was fabricated with a 36 m Bi-2223 tape insulated by Kapton film. The electric properties of the double pancake with ac pulse over currents, including the impedance and the resistance, have been investigated by experiment. As a result, we could find the optimal
operating condition considering the protection of the HTS tape against the alternating over-current. 2. Experiment 2.1. The HTS double pancake coil In this work, an experimental double pancake coil was manufactured with a 36 m Bi-2223/Ag stainless steel reinforced tape. The tape was insulated with 25 lm thick Kapton film, which could stand a voltage of 400 Vrms when dipped in liquid nitrogen. The specifications of the reinforced HTS tape and the double pancake coil are shown in Tables 1 and 2, respectively. When the double pancake coil transports 100 A DC current, the magnetic field contour lines in the winding analyzed by FEA are shown in Fig. 1. We can see from Fig. 1b that the maximal radial magnetic field Br (T) is located at the middle turns of each pancake coil. The lowest Ic of the coil is also located there due to the strong anisotropy of the Bi-2223/Ag tape [7,8]. In order to diminish the eddy current loss in the copper bobbin of the double pancake, a 2 mm wide notch was cut and a Fiberglass-Reinforced Plastics (FRP) of same size was embedded in it. The whole coil was impregnated by epoxy resin. To get high thermal conductivity and little deformation, silicon dioxide was doped with epoxy resin. 2.2. Experiment setup
⇑ Corresponding author at: Institute of Electrical Engineering, Chinese Academy of Sciences, Beijing 100190, China. Fax: +86 10 82547137. E-mail address: [email protected] (J. Zhang). 0011-2275/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.cryogenics.2011.11.002
The experimental setup was shown in Fig. 2. The Bi-2223 double pancake coil was dipped in liquid nitrogen. A DC power supply was
46
J. Zhang et al. / Cryogenics 52 (2012) 45–50
Table 1 Specifications of the Bi-2223/Ag stainless steel reinforced tape. Item
Specifications
Width Total thickness Thickness of stainless steel tapes on both sides Ratio of Ag and superconductors Filaments Minimum Ic
4.8 mm 0.31 mm 0.15 mm 7:3 55 115 A*
*
At 77 K, self filed, 1 lV/cm.
Table 2 Specifications of the double pancake coil. Item
Specifications
Inner diameter Outer diameter Number of turns Total length of Bi-2223 tape Inductance Impedance Zl (f = 50 Hz)
80 mm 132 mm 108 36 m 1.532 mH 0.4813 X
shown in Fig. 3. The critical current Ic of the coil was 60 A and the N value was 12.6. Then constant AC voltages which lasted for 3 s were applied to the two terminals of the coil and its currents were measured. The value of the AC voltage stepped up till the double pancake coil was burned out. All the AC voltages with frequencies of 50 Hz were measured in rms. After 15 min of each AC current pulse, the Ic of the coil was measured to check if there was any damage during the current pulse. The interval between every two test was 20 min so as to cool down the coil completely. After the pulse test with the AC voltage of 175.7 Vrms, the Ic of the coil decreased to 52 A and the N value decreased from 12.6 to 8.04 as shown in Fig. 3. It indicated that some damage has been done to the coil during the current pulse. When the pulse voltage increased to 202.7 Vrms, the coil was burned out and the pictures were shown in Fig. 4. You can see from the photos that the burned-out place located at the middle turns of one pancake, where the radial field Br is the highest and the local Ic is the lowest. 3. Experimental results and analysis
Fig. 1. The magnetic field distributions on the winding when the double pancake coil transports 100 A DC current. (a) The distributions of the overall magnetic field B (T); (b) the distributions of radial magnetic field Br (T).
used to measure the critical current Ic of the double pancake coil. A 250 kV A voltage regulator with frequency of 50 Hz worked as the AC power supply. It has an output voltage from 0 to 420 Vrms. Each of the AC pulse currents in all of the experiments lasted for 3 s. The duration of the pulse current were achieved by a silicon-controlled switch controlled by DSP. The DC and AC power supply could interchange by the switch 1 and switch 2 during the experiment, as shown in Fig. 2. The currents passed through the double pancake coil and the voltages between the two terminals were measured. 2.3. Measurement First of all, the critical current Ic of the double pancake coil was measured by the DC power supply by the criterion of 1 lV/cm as
As mentioned previously, the current of the coil was measured while a constant 3 s AC voltage pulse was applied to the two terminals of the coil. When a lower AC voltage was applied to the coil, the current was almost constant as shown in Fig. 5a, except the first 2 or 3 cycles as shown in Fig. 5b. But when higher AC voltage pulse was applied to the coil, the current was no longer constant. It decreased gradually during the pulse time, as shown in Fig. 6. For all of the tests, the first 2 or 3 cycles were aberrance for some unidentified reasons, for example, were caused by the electrocircuit or its equipments. In this study, they were left out of account. For convenience of expression, three points during the pulse time were chosen to facilitate the analysis of experiment results. As shown in Figs. 5b and 6, the first point 01 was at the fourth cycle of the pulse current. The second point 02 and the last point 03 were in the middle and the end of the pulse time, namely at 1.5 s and 3 s, respectively, as shown in Fig. 6. The variations of the currents (I01, I02 and I03) with the corresponding pulse voltages for the three chosen points were studied. The curves of the currents I01, I02 and I03 varying with their corresponding pulse voltages were shown in Fig. 7. We can see that I01 is almost linear with increasing voltage; I01, I02 and I03 are coincided when the voltage was below 60 Vrms; when the voltages were between 60 and 190.4 Vrms, I02 and I03 are almost unchanged as the voltage grows; there is no degeneration of the Ic of coil when voltage below 169.2 Vrms; when the voltage goes up to 202.7 Vrms, the coil was burned out, then I03 and Ic of the coil fall to zero. From the voltages and the currents (I01, I02 and I03), the curves of their impedances (Z01, Z02 and Z03) were obtained, which were shown in Fig. 8. We can see that the impedances (Z01, Z02 and Z03) behave disparately with their corresponding currents I01, I02 and I03. Z01 is almost unchanged as the voltage grows up, i.e. the impedance of the coil almost unchanged at the first several cycles with the voltage pulse increasing. Z01, Z02 and Z03 are coincided when the voltage below 60 Vrms. It indicates that the impedance of the coil Z changes little with pulse time when the voltage below 60 Vrms. Z02 and Z03 are almost linear with the voltage from 60 Vrms to 169.2 Vrms, indicating that the impedance of the coil Z02 and Z03 change rapidly with the voltage increasing. When the voltage pulses go over 169.2 Vrms, the Ic of the coil degrades, indicating that some damages occurs to the coil. For HTS double pancake coil, its impedance Z can be described by the following formulae.
~ Z ¼ R þ~jZ l
ð1Þ
47
J. Zhang et al. / Cryogenics 52 (2012) 45–50
Fig. 2. Experiment setup.
11
Before any pulsed test N value: 12.6 (0.1-1.0 μV/cm) After pulsed test with voltage:175.7 Vrms N value: 8.04 (0.1-1.0 μV/cm)
10 9
Voltage (μV/cm)
8 7 6 5 4 3 2 1 0 -1 0
10
20
30
40
50
60
70
Current (A) Fig. 3. The DC current–voltage curves of the Bi-2223/Ag double pancake coil measured at liquid nitrogen temperature.
Fig. 4b. The picture of the burned out part of the double pancake coil after experiment.
160 140
Constant voltage: 38.66 Vrms
120 100
Current (A)
80 60 40 20 0 -20 -40 -60 -80 -100 0.0 Fig. 4a. The panoramic picture of the burned out double pancake coil after experiment.
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi R ¼ Z 2 Z 2l
0.3
0.6
0.9
1.2
1.5
1.8
2.1
2.4
2.7
3.0
3.3
3.6
Time (s) Fig. 5a. The AC current characters of the double pancake coil at the constant AC voltage of 38.66 Vrms.
ð2Þ
where Zl = 0.4813 X is calculated from 2pfL and shown in Table 2. Here, we assumed that the inductance of the HTS double pancake unchanged in all experiments. For the current mainly change from superconductor filaments to Ag matrix when the current over than
Ic and the size of the Ag matrix and the superconductor filaments are almost same. R is the equivalent resistance of the double pancake coil. When the amplitude of the current is smaller than the Ic of the coil, R is mainly caused by the AC losses of the coil; when the amplitude
J. Zhang et al. / Cryogenics 52 (2012) 45–50
2.2
66
2.0
60 54
Z 01
1.6
48
Z 02
1.4 1.2
Z 03
1.0
IC
42 36 30
0.8
24
0.6
18
0.4
12
0.2
6
0.0
0 20
40
60
80
Constant voltage: 100.3 Vrms
01
250
02
03
150
Current (A)
160
180
200
2.2
66
2.0
60 54
1.8
200
100 50 0 -50 -100 -150
1.6
IC
48
1.4
R01
42
1.2
R02
36
1.0
R03
30
0.8
24
0.6
18
0.4
12
0.2
6
0.0
0 20
-200
40
60
80
100
120
140
160
180
200
Voltage (Vrms) 0.0
0.4
0.8
1.2
1.6
2.0
2.4
2.8
3.2
Time (s) Fig. 6. The AC current characters of the double pancake coil at the constant AC voltage of 100.3 Vrms.
70
280
I 01
200
50
I 02 160
I 03
40
120
IC
30
80
20
40
10
DC Current (A)
60
240
AC Current (Arms)
140
Fig. 8. The curves of the impedances (Z01, Z02 and Z03) of the HTS double pancake coil with their pulse rms AC voltages.
Resistance (Ω)
300
120
Voltage (Vrms)
Time (s) Fig. 5b. The first several cycles of the AC current characters of the double pancake coil at the constant AC voltage of 38.66 Vrms.
100
Current (A)
1.8
DC Current (A)
160 Constant voltage: 38.66 Vrms 01 140 120 100 80 60 40 20 0 -20 -40 -60 -80 -100 -120 0.45 0.48 0.51 0.54 0.57 0.60 0.63 0.66 0.69 0.72 0.75
Impedance (Ω)
Current (A)
48
0
0 20
40
60
80
100
120
140
160
180
200
Voltage (Vrms) Fig. 7. The curves of the AC currents (I01, I02 and I03) of the HTS double pancake coil with their pulse AC voltages.
of pulse current is above Ic, it caused by many intricate reasons due to over currents. According to formula (2), the resistance R could be derived from the impedance Z of the coil. Then, converting Figs. 8 and 9, we can
Fig. 9. The curves of the resistances (R01, R02 and R03) of the HTS double pancake coil with its pulse rms AC voltages.
see that the resistances R01, R02 and R03 behave similarly with their corresponding impedances Z01, Z02 and Z03. It indicates that the differences among the impedances Z01, Z02 and Z03, and the tendency of variation with the voltages of them, are mainly dominated by the resistances R01, R02 and R03. It is well known that the energy dissipation of the HTS coil due to over currents can be expressed as I2R and the energy can be released as heat to the coil. When the resistance R appears and become larger, the HTS coil will be heated. Whether the loss of the HTS coil stays in a reasonable level or leads to thermal runaway, it can be judged from Fig. 9. (a) When the voltage below 60 Vrms, R01, R02 and R03 are coincided. It indicates that the resistance of the coil R changes little with the pulse time, and the pulse currents can last any long without damage to the coil. (b) The resistance of the coil (R01) is almost unchanged at the first several cycles with the pulse voltage increasing. It indicates that if the pulse time is short enough, the coil can stand a very high AC voltage pulse, namely, can stand a very large pulse current and without thermal runaway. (c) R02 and R03 are almost linear when the voltage is above 60 Vrms. This indicates that the resistances of the coil R02 and R03 change rapidly with the voltage increasing. If the current lasts long enough, it will result in a thermal runaway and the coil may be damaged. Based on the above results and analysis, we can see that the pulse voltages from 60 Vrms to 169.2 Vrms play a key role in the over currents pulse process. Thus, the characters of the coil for the pulse voltage of 60 Vrms, 169.2 Vrms and 181.4 Vrms were studied respectively. Fig. 10 gives their characters of the rms currents with
49
J. Zhang et al. / Cryogenics 52 (2012) 45–50
260
I60V
1.8
R 60V
240
I169.2V
1.6
R 169.2V
I181.4V
1.4
R 181.4V
Resistance (Ω)
I (Arms)
220 200 180 160 140
1.2 1.0 0.8 0.6 0.4
120
0.2
100 0.0
0.3
0.6
0.9
1.2
1.5
1.8
2.1
2.4
2.7
0.0
3.0
0.0
Time (S)
0.9
1.2
1.5
1.8
2.1
2.4
2.7
3.0
Fig. 12. The curves of the resistances of the HTS double pancake coil in its pulse time corresponding to Fig. 10.
Z 60V
400
Z 169.2 V
300
Z 181.4 V
200
Current (A)
Impedance (Ω)
1.6
0.6
Time (S)
Fig. 10. The curves of the rms currents of the HTS double pancake coil vary with its pulse time. The currents I60V, I169.2V, I181.4V; their corresponding voltages pulse were 60 Vrms, 169.2 Vrms and 181.4 Vrms individually.
1.8
0.3
1.4 1.2 1.0
Constant voltage: 202.7 Vrms
100 0 -100 -200 -300
0.8
-400 0.6 0.0 0.4 0.0
0.3
0.6
0.9
1.2
1.5
1.8
2.1
2.4
2.7
3.0
Time (s)
Impedance (Ω)
Z60V
0.595 0.590 0.585 0.580 0.0
0.3
0.6
0.9
1.2
1.5
0.8
1.2
1.6
2.0
2.4
2.8
3.2
Time (s) Fig. 13a. The AC current characters while the double pancake coil was burned out at the AC voltage pulse of 202.7 Vrms.
Fig. 11a. The curves of the impedances of the HTS double pancake coil vary with its pulse time. The impedances Z60V, Z169.2V, Z181.4V, are corresponding to voltages pulse of 60 Vrms, 169.2 Vrms and 181.4 Vrms individually.
0.600
0.4
1.8
2.1
2.4
2.7
3.0
Time (s) Fig. 11b. The partial enlarged view of the impedances Z60V curve with the pulse time.
the pulse time at the three key voltages, 60 Vrms, 169.2 Vrms and 181.4 Vrms, respectively. We can see that for the pulse voltage 60 Vrms, the current changes little with pulse time. As the voltage increases up to 169.2 Vrms and 181.4 Vrms, their currents decrease with the pulse time. Beside, there is an intersection of the two curves, this shows that the current for 181.4 Vrms decreases more rapidly than that for 169.2 Vrms. Using the voltages (60 V, 169.2 V and 181.4 V in rms) and the currents (I60V, I169.2V and I181.4V), we can get the curves of their impedances (Z60V, Z169.2V and Z181.4V) with the pulse time as shown in Fig. 11. We can see from it that the impedances Z60V is almost
horizontal, indicating that the impedance of the coil changes little as the pulse time goes on. Z169.2V and Z181.4V almost linearly increase with pulse time goes on. According to formula (2), the resistance R60V, R169.2V and R181.4V are calculated and shown in Fig. 12. We can see from it that, as the pulse time goes on, the resistances R60V, R169.2V and R181.4V behave similarly to their corresponding impedances Z60V, Z169.2V and Z181.4V. Similarly, the differences among the impedances Z60V, Z169.2V and Z181.4V, and the changes of them with the pulse time, are mainly dominated by the resistances R60V, R169.2V and R181.4V due to the over pulse currents. Namely, we can see from Fig. 12 that: (a) The resistance of the coil (R60V) is almost unchanged with the pulse time going on when the voltage below 60 Vrms. It indicates that if the voltages pulse below 60 Vrms, the pulse time and the pulse currents can last any long and has no damage to the coil. (b) The resistance of the coil R169.2V and R181.4V increase almost linearly with the pulse time. If the current last long enough, it will result in a thermal runaway and the coil may be damaged, or burned. Fig. 13 shows the current characters of the burned-out process of the HTS coil. When the voltages pulse over 169.2 Vrms, some damages have been done to the coil. Figs. 3 and 9 show that when the voltages pulse up to 175.7 Vrms, the Ic has degraded 13%. When up to 202.7 Vrms, the coil burned out. We can see from Fig. 13a that
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J. Zhang et al. / Cryogenics 52 (2012) 45–50
500
Constant voltage: 202.7 Vrms
400
Current (A)
300 200
performed by applying constant AC voltages pulse to the two terminals of the coil in liquid nitrogen bath. The currents, impedance and resistance characters were analyzed from the measured current waveforms. Some useful conclusions derived from the experimental results and their analyses are given as following:
100 0 -100 -200 -300 -400 -500 2.83 2.84 2.85 2.86 2.87 2.88 2.89 2.90 2.91 2.92 2.93
Time (s) Fig. 13b. The enlarged view of the several cycles in the red box of (a). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
the maximum current at the first several cycles was higher than 400 A; as the pulse time went on, the current decreased smoothly and at the time of 2.18 s the current arrived its lowest value (185 A). Then, the current increased rapidly and till the pulse time of 2.85 s the current become zero fitfully. This can be seen from the overall process of Fig. 13a and the part enlarged view in Fig. 13b. We know that, from the pulse time of 2.18 s to 2.85 s, the HTS coil burned out. When the coil was burned out, the damage caused many turns to short-circuit at the burned place. So the impedance of the coil decreasing rapidly and this resulted in the current increased rapidly after the pulse time 2.18 s. When the burnout lasted a little time (here is about 0.67 s) the coil became open-circuit fitfully as seen from Fig. 13b. We can see from Fig. 4 that the burnout position located at the middle turns of the coil where the radial magnetic field Br is maximal (shown in Fig. 1). 4. Conclusions A double pancake coil was manufactured with a 36 m long Bi-2223/Ag tape. The AC over-current experiments of the coil were
(1) The impedance and the resistance of the coil have little changed at the first several cycles as the pulse voltages increasing. It indicates that if the pulse time is short enough, the coil can stand a very high AC pulse voltage, or a very large pulse current. (2) When the pulse voltages are low values (for the model coil lower than 60 Vrms), the coil is in safety region. The pulse time could last long enough and there no damage to the coil. (3) When the pulse voltages at a high value (for the model coil higher than 60 Vrms), the impedances and the resistances of the coil change rapidly with the voltages pulse and the pulse time. If the pulse time lasted long enough, over current pulse will cause the performances degenerated of the coil, or even burned out of it. (4) The most severely destroyed place is located in the middle turns of the HTS double pancake coil where the radial magnetic field Br is maximal and the local Ic of the tapes in the coil is the lowest.
References [1] Iwasa Y. Case studies in superconductivity magnets. New York: Plenum Press; 1994. [2] Matsuura H, Ninomiya A, Ishigohka T. IEEE Trans. Appl. Supercond. 2007;17:3109–12. [3] Lee J et al. IEEE Trans. Appl. Supercond. 2007;17:1603–6. [4] Lee J, Kim Ho Min, et al. IEEE Trans. Appl. Supercond. 2008;18:1267–70. [5] Kim Min Jae, Ahn Min Cheol, et al. IEEE Trans. Appl. Supercond. 2008;18:660–3. [6] Zhang J, Dai S, et al. IEEE Trans. Appl. Supercond. 2010;20:1135–8. [7] Kobabyashi Shin-ichi, Kaneko Tetsuyuki, Kato Takeshi, Fujikami Jun, Sato Kenichi. Physica C 1996;258:336–40. [8] Larbalestier David, Gurevich Alex, Matthew Feldmann D, Polyanskii Anatoly. Nature 2001;414:368–77.