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Development of the Current Bypassing Methods into the Transverse Direction in Non-insulation HTS Coils
Available online at www.sciencedirect.com
ScienceDirect Physics Procedia 65 (2015) 229 – 232
27th International Symposium on Superconductivity, ISS 2014
Development of the current bypassing methods into the transverse direction in non-insulation HTS coils K. Tanaka, S.B. Kim*, H. Ikoma, D. Kanemoto Graduate School of Natural Science and Technology, Okayama University, 3-1-1, Tsushima Naka, Kita-ku, Okayama 700-8530, Japan
Abstract In the case of motors and generators, the benefits of using high temperature superconducting (HTS) coils can be represented by the reduction of 50% in both losses and sizes compared to conventional machines. However, it is hard to establish quench detection and protection devices for the HTS coils applied to the rotors of motors and generators. So, the stability of the coils is lower than for the quiescent coils applied to NMR, MRI and so on. Therefore, it is important to improve the self-protection ability of HTS coils. We have studied the methods to improve the self-protection ability of HTS coils by removing the layer-to-layer insulation and inserting metal tape instead of the electrical insulation. The operating current in the non-insulated HTS coil was bypassed into the transverse direction by the generated normal region because of their electrical contact among the winding. In this study, we examined the method to control the current bypassing on layer-to-layer for controlling the inductance of the non-insulated HTS coil.The current bypassing properties on non-insulated HTS coil wound with 2G wires will be discussed. © Elsevier B.V. This is an open access article under the CC BY-NC-ND license © 2015 2015The TheAuthors. Authors.Published Publishedbyby Elsevier B.V. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the ISS 2014 Program Committee. Peer-review under responsibility of the ISS 2014 Program Committee Keywords: HTS coil ; MQE ; Non-insulation ; current sharing ; thermal properties of 2G wire ; control bypassing current
1. Introduction High temperature superconducting (HTS) coils can generate a high power density, it is expected to advantages such as smaller and lighter than the conventional coils. When the HTS coil is applied to rotating machine, it is difficult to install the quench detector and protection device to the rotating machine. So, we have been developing the new self -protection methods, removing insulation between layer-to-layer winding and inserting various metal tapes (Cu, Ni, Stainless and/or Brass) instead of the insulation [1-3]. When operate current to the coil, voltage is generated by the sweep rate of the current (di/dt) and the inductance of the coil. Since non-insulated HTS coil which removing insulation between layer-to-layer winding is electrical contact, current is bypassing into the transverse direction. In this paper, we examined the method to control the current bypassing on layer-to-layer for controlling the inductance of the non-insulated HTS coil. In addition, we also examined the thermal properties of the longitudinal direction and the transverse direction about 2G wires.
* Corresponding author. Tel.: +81-86-251-8116; fax: +81-86-251-8110. E-mail address: [email protected]
1875-3892 © 2015 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the ISS 2014 Program Committee doi:10.1016/j.phpro.2015.05.129
230
K. Tanaka et al. / Physics Procedia 65 (2015) 229 – 232
2. Experiment and setup 2.1 Characteristics of current bypassing in non-insulated HTS coil Fig. 1 shows the schematic drawing of the single pancake non-insulated test coil wound with GdBCO wires and 5 turns. In order to obtain the quench properties in the test coils and the characteristics of current bypassing into the transverse direction in non-insulated HTS coil during the normal transition by heaters, seven voltage taps are installed in each layer and two strain gauges as a heater are attached in the test coil, and Kapton tape is inserted around current lead OA as an insulation to control bypassing current. To detect the self-magnetic field of the test coil, a Hall sensor is installed at the center position of the coil. The test coils are epoxy impregnated and the external magnetic fields (0-1 T) are used as a parameter in liquid nitrogen. The specifications of GdBCO wire used in this study are listed in Table 1. Heater 1
Cu
Material of substrate
Hastelloy
Width (mm)
4.1
Thickness of wire (μm)
180
Thickness of stabilizer (μm) Thickness of GdBCO layer (μm)
75 1.3
Critical current (A)
184
Superconductor
Current lead A
Table .1. Specifications of GdBCO Wire. Parameters Material of stabilizer
Heater 2
I
Current lead B
-
7
Bakelite Mandrel 6
I5
Hall sensor 1
Kapton tape
4
I+
3 2
Current lead O
Fig. 1. Schematic drawing of the single pancake non-insulated HTS coil wound with GdBCO wire and 5 turns.
2.2 Thermal properties in transverse direction and longitudinal direction In our research, the normal zone is generated by heater in the non-insulated HTS coil in order to bypass the current into the layer-to-layer direction. Therefore, the understanding of the thermal property about 2G wires used in non-insulation HTS coils is very important. The specifications of GdBCO wires are listed in Table. 1, and the experimental setup to measure the thermal properties of stacked GdBCO wires is shown in Fig. 2. The strain gauge with 1 k: was used as a heater and they were attached to the front (superconducting layer side) of the sample at center region. Six thermocouples are attached on the front side with interval as shown in Fig. 2, and sample wires including heater and thermocouples were impregnated with epoxy. T1 T2 T5 T6 T4
T3 3.5 cm
䞉䞉䞉Heater
1 cm 1 cm 1.5 cm
䞉䞉䞉thermocouple
Fig. 2. Schematics cross sectional view of stacked GdBCO wires to measure the thermal diffusion properties.
3. Results and discussion 3.1. MQE properties Fig. 3 shows the measured MQE of non-insulated GdBCO test coil wound with GdBCO wire and 5 turns as shown in Fig. 1. MQE was reduced with increasing the operating current and magnetic field. MQE of non-insulated HTS coil with current lead OA pattern is greater than those of current lead OB pattern because operating current can be bypassed into other layer well.
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K. Tanaka et al. / Physics Procedia 65 (2015) 229 – 232 5.0
5.0
4.5
3.5
0T 0.1 T 0.2 T 0.3 T 0.4 T
4.0
MQE (J)
MQE (J)
4.5
0T 0.1 T 0.2 T 0.3 T
4.0
3.0 2.5
3.5 3.0 2.5
2.0 1.5
2.0
(a)
1.0 75
80
85
90
95
Operating current (A)
100
(b)
1.5 60
105
70
80
90
Operating current (A)
100
110
Fig. 3. Measured MQE of non-insulated test coil (a) Current lead OA and (b) Current lead OB.
3.2. Characteristics of current bypassing in non-insulated HTS coil The characteristics of current bypassing into the transverse direction in non-insulated HTS coil wound with GdBCO wire and 5 turns is studied using test single pancake coil as shown in Fig. 1. Fig. 4 shows the generated voltages along the each layer (V1-2-V6-7) and end-to-end (Vall), and the 1/10 scaled heater input and generated magnetic field measured by Hall sensor located at center position of test coil. The number of transported turns is calculated by Biot-Savart’s law using measured self-magnetic field due to current bypassing. In test coil with current lead OA pattern, the calculated numbers of turns are 3.6, 4 and 1.9 respectively, when the thermal input by heater 1, heater 2 and both heaters are conducted at applied field of 0 T and transport current of 50 A. On the other hand, 4.2, 4.4 and 2.4 transported turns are obtained in test coil with current lead OB pattern when the applied field is 0 T and transport current is 50 A. The amount of bypassing current in current lead OA pattern is more than that of current lead OB pattern. After thermal disturbance input from the heater, the voltage is not generated in V5-6 of current lead OA pattern. However, the voltage is generated in V6-7 of current lead OB pattern. Because the bypassing routes of current lead OA pattern different from the routes of current lead OB pattern after thermal disturbance is inputted from heater. Fig. 5 shows bypassing current routes after applying thermal disturbance by both heaters. Since the current is bypassed into outer layer of current in lead OA pattern, MQE of non-insulated HTS coil with current lead OA pattern is greater than that of current lead OB pattern.
heater(1/10 scale)
30
0.000
20
-0.002
10 16
6
8
10
12
Time (s)
14
0.004 heater(1/10 scale)
0.002 0.000
20 10 16
6
heater(1/10 scale)
50 40 30 20
0.000 12
12
14
14
10 16
0.012
0.012 0.008
70 60 V1-2 V2-3 V3-4 V4-5 V5-6 V6-7 Vall
0.008 0.006 0.004 0.002
heater(1/10 scale)
-0.002
50 40 30 20
0.000 6
8
10
12
Time (s)
14
heater(1/10 scale)
0.004 0.000 -0.004
6
8
10
12
Time (s)
14
40 20 0 -20 18
16
(c) both heaters (current lead OA)
Generated magnetic field
0.010
Voltage (V)
Voltage (V)
0.004
Time (s)
10
V1-2 V2-3 V3-4 V4-5 V5-6 V6-7 Vall
10 16
80
0.040 Generated magnetic field
0.035
60
0.030 V1-2 V2-3 V3-4 V4-5 V5-6 V6-7 Vall
0.025 0.020 0.015 0.010 heater(1/10 scale)
0.005
40 20 0
0.000 -0.005
6
8
10
12
14
16
-20 18
Magnetic flux density (Gauss)
V1-2 V2-3 V3-4 V4-5 V5-6 V6-7 Vall
0.006
10
8
Magnetic flux density (Gauss)
60
Magnetic flux density (Gauss)
70 Generated magnetic field
8
30
60
0.016
(b) heater 2 (current lead OA)
0.008
6
40
80 Generated magnetic field
0.020
Time (s)
0.010
-0.002
50
-0.002
(a) heater 1 (current lead OA)
0.002
V1-2 V2-3 V3-4 V4-5 V5-6 V6-7 Vall
0.006
0.024
Voltage (V)
0.002
40
60
Voltage (V)
0.004
50
70
Generated magnetic field
0.008
Voltage (V)
Voltage (V)
V1-2 V2-3 V3-4 V4-5 V5-6 V6-7 Vall
0.006
0.010
Magnetic flux density (Gauss)
60
Magnetic flux density (Gauss)
70 Generated magnetic field
0.008
Magnetic flux density (Gauss)
0.010
Time (s)
(d) heater 1 (current lead OB) (e) heater 2 (current lead OB) (f) both heaters (current lead OB) Fig. 4. Generated the voltages along the each layer (V1-2-V6-7) and end-to-end (Vall), and the 1/10 scaled heater input and generated magnetic field measured by Hall sensor located at center position in test coils.
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K. Tanaka et al. / Physics Procedia 65 (2015) 229 – 232 Superconductor
Superconductor
Current lead A
Current lead B
I-
7
7
Bakelite Mandrel
Bakelite Mandrel
6
6 5
Hall sensor
4
I+
1
Kapton tape
I-
5
Hall sensor 3
1
Kapton tape
2
Current lead O
4
I+
3 2
Current lead O
(a)
(b)
Fig. 5. Bypassing current routes after applying thermal disturbance by both heaters (a) current lead OA and (b) current lead OB.
3.3. Thermal properties of transverse direction and longitudinal direction
86
86
85
85
T1 T2 T3 T4 T5 T6
84 83 82 81 80 79
84 83 82 81 80 79
0
5
10
15
20
25
30
77
35
Time (s)
(a) temperature profiles of each layer (2.5 J)
79
78
T2
T1
T5
77
T6
(b) transverse direction (2.5 J)
96
88 86 84 82 80
90 87 84 81
78
78
76
75
0
5
10
15
20
25
Time (s)
30
35
40
12 s 14 s 16 s 18 s 20 s 25 s
84
Temperature (K)
90
Temperature (K)
T1 T2 T3 T4 T5 T6
T4
85 12 s 14 s 16 s 18 s 20 s 25 s
93
92
T3
(c) longitudinal direction (2.5 J)
96
94
Temperature (K)
12 s 14 s 16 s 18 s 20 s
80
78
78 77
81 12 s 14 s 16 s 18 s 20 s
Temperature (K)
87
Temperature (K)
Temperature (K)
Fig. 6 shows the measured temperature profiles by T 1, T2, T3, T4, T5 and T6 with various thermal inputs. The heat capacity from 2.5 J to 5 J was applied to sample wires, and the temperature of sample wires was rises rapidly at the same time after applying thermal disturbance by heater except for T4. Because T4 is 2 cm far from the position of the heater, heat is transferred in the order of heater, T3, T4. The peak value of the temperature at T5 is higher than that of T2. Each thermocouple is attached at the surface of sample wire, and these thermocouples are insulated by Kapton tape. Therefore, the thermal conduction property along the transverse direction from the heater is little different, and the maximum temperature of T5 is higher than that of T2. In insulated HTS pancake coil, the thermal conduction property into the transverse direction is poor compare with longitudinal direction due to electrical insulator which have very low thermal conductivity. However, the improved thermal conduction property into the transverse direction was obtained by removed the insulator as shown in Fig 6.
83 82 81 80 79 78 77
T1
T2
T5
T6
T3
T4
(d) temperature profiles of each layer (5 J) (e) transverse direction (5 J) (f) longitudinal direction (5 J) Fig. 6. Measured temperature profiles during normal transition at each layer in non-insulated HTS coils wound with GdBCO wires against various thermal inputs.
4. Conclusion We studied the method for current bypassing on layer-to-layer in order to control the inductance of the non-insulated HTS coil. The characteristics of current bypassing by inserting insulation around the current lead and thermal diffusion in noninsulated HTS coil wound with 2G wires were examined experimentally. We confirmed that the non-insulated HTS coils inserted insulation around the current lead were possible to control current bypassing. References [1] S.B. Kim. T. Kaneko, H. Kajikawa, J.H. Joo, J.M. Jo, Y.J. Han and H.S. Jeong, “The transient stability of HTS coils with and without the insulation and with the insulation being replaced by brass tape” Physica C, vol. 484, pp.310-315, 2013. [2] S.B. Kim, H. Kajikawa, H. Ikoma, J.H. Joo, J.M. Jo, Y.J. Han and H.S. Jeong, “Study on the electrical contact resistance properties with various winding torques for noninsulated HTS coils,” IEEE Trans. on Appl. Supercond., vol.24, 4600405, 2014. [3] S. Hahn, D. K. Park, J. Bascuñán, and Y. Iwasa, “HTS Pancake Coils Without Layer-to-layer Insulation,” IEEE Trans. Appl. Supercond., vol. 21, no. 3, pp. 1592-1595, 2011.
ScienceDirect Physics Procedia 65 (2015) 229 – 232
27th International Symposium on Superconductivity, ISS 2014
Development of the current bypassing methods into the transverse direction in non-insulation HTS coils K. Tanaka, S.B. Kim*, H. Ikoma, D. Kanemoto Graduate School of Natural Science and Technology, Okayama University, 3-1-1, Tsushima Naka, Kita-ku, Okayama 700-8530, Japan
Abstract In the case of motors and generators, the benefits of using high temperature superconducting (HTS) coils can be represented by the reduction of 50% in both losses and sizes compared to conventional machines. However, it is hard to establish quench detection and protection devices for the HTS coils applied to the rotors of motors and generators. So, the stability of the coils is lower than for the quiescent coils applied to NMR, MRI and so on. Therefore, it is important to improve the self-protection ability of HTS coils. We have studied the methods to improve the self-protection ability of HTS coils by removing the layer-to-layer insulation and inserting metal tape instead of the electrical insulation. The operating current in the non-insulated HTS coil was bypassed into the transverse direction by the generated normal region because of their electrical contact among the winding. In this study, we examined the method to control the current bypassing on layer-to-layer for controlling the inductance of the non-insulated HTS coil.The current bypassing properties on non-insulated HTS coil wound with 2G wires will be discussed. © Elsevier B.V. This is an open access article under the CC BY-NC-ND license © 2015 2015The TheAuthors. Authors.Published Publishedbyby Elsevier B.V. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the ISS 2014 Program Committee. Peer-review under responsibility of the ISS 2014 Program Committee Keywords: HTS coil ; MQE ; Non-insulation ; current sharing ; thermal properties of 2G wire ; control bypassing current
1. Introduction High temperature superconducting (HTS) coils can generate a high power density, it is expected to advantages such as smaller and lighter than the conventional coils. When the HTS coil is applied to rotating machine, it is difficult to install the quench detector and protection device to the rotating machine. So, we have been developing the new self -protection methods, removing insulation between layer-to-layer winding and inserting various metal tapes (Cu, Ni, Stainless and/or Brass) instead of the insulation [1-3]. When operate current to the coil, voltage is generated by the sweep rate of the current (di/dt) and the inductance of the coil. Since non-insulated HTS coil which removing insulation between layer-to-layer winding is electrical contact, current is bypassing into the transverse direction. In this paper, we examined the method to control the current bypassing on layer-to-layer for controlling the inductance of the non-insulated HTS coil. In addition, we also examined the thermal properties of the longitudinal direction and the transverse direction about 2G wires.
* Corresponding author. Tel.: +81-86-251-8116; fax: +81-86-251-8110. E-mail address: [email protected]
1875-3892 © 2015 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the ISS 2014 Program Committee doi:10.1016/j.phpro.2015.05.129
230
K. Tanaka et al. / Physics Procedia 65 (2015) 229 – 232
2. Experiment and setup 2.1 Characteristics of current bypassing in non-insulated HTS coil Fig. 1 shows the schematic drawing of the single pancake non-insulated test coil wound with GdBCO wires and 5 turns. In order to obtain the quench properties in the test coils and the characteristics of current bypassing into the transverse direction in non-insulated HTS coil during the normal transition by heaters, seven voltage taps are installed in each layer and two strain gauges as a heater are attached in the test coil, and Kapton tape is inserted around current lead OA as an insulation to control bypassing current. To detect the self-magnetic field of the test coil, a Hall sensor is installed at the center position of the coil. The test coils are epoxy impregnated and the external magnetic fields (0-1 T) are used as a parameter in liquid nitrogen. The specifications of GdBCO wire used in this study are listed in Table 1. Heater 1
Cu
Material of substrate
Hastelloy
Width (mm)
4.1
Thickness of wire (μm)
180
Thickness of stabilizer (μm) Thickness of GdBCO layer (μm)
75 1.3
Critical current (A)
184
Superconductor
Current lead A
Table .1. Specifications of GdBCO Wire. Parameters Material of stabilizer
Heater 2
I
Current lead B
-
7
Bakelite Mandrel 6
I5
Hall sensor 1
Kapton tape
4
I+
3 2
Current lead O
Fig. 1. Schematic drawing of the single pancake non-insulated HTS coil wound with GdBCO wire and 5 turns.
2.2 Thermal properties in transverse direction and longitudinal direction In our research, the normal zone is generated by heater in the non-insulated HTS coil in order to bypass the current into the layer-to-layer direction. Therefore, the understanding of the thermal property about 2G wires used in non-insulation HTS coils is very important. The specifications of GdBCO wires are listed in Table. 1, and the experimental setup to measure the thermal properties of stacked GdBCO wires is shown in Fig. 2. The strain gauge with 1 k: was used as a heater and they were attached to the front (superconducting layer side) of the sample at center region. Six thermocouples are attached on the front side with interval as shown in Fig. 2, and sample wires including heater and thermocouples were impregnated with epoxy. T1 T2 T5 T6 T4
T3 3.5 cm
䞉䞉䞉Heater
1 cm 1 cm 1.5 cm
䞉䞉䞉thermocouple
Fig. 2. Schematics cross sectional view of stacked GdBCO wires to measure the thermal diffusion properties.
3. Results and discussion 3.1. MQE properties Fig. 3 shows the measured MQE of non-insulated GdBCO test coil wound with GdBCO wire and 5 turns as shown in Fig. 1. MQE was reduced with increasing the operating current and magnetic field. MQE of non-insulated HTS coil with current lead OA pattern is greater than those of current lead OB pattern because operating current can be bypassed into other layer well.
231
K. Tanaka et al. / Physics Procedia 65 (2015) 229 – 232 5.0
5.0
4.5
3.5
0T 0.1 T 0.2 T 0.3 T 0.4 T
4.0
MQE (J)
MQE (J)
4.5
0T 0.1 T 0.2 T 0.3 T
4.0
3.0 2.5
3.5 3.0 2.5
2.0 1.5
2.0
(a)
1.0 75
80
85
90
95
Operating current (A)
100
(b)
1.5 60
105
70
80
90
Operating current (A)
100
110
Fig. 3. Measured MQE of non-insulated test coil (a) Current lead OA and (b) Current lead OB.
3.2. Characteristics of current bypassing in non-insulated HTS coil The characteristics of current bypassing into the transverse direction in non-insulated HTS coil wound with GdBCO wire and 5 turns is studied using test single pancake coil as shown in Fig. 1. Fig. 4 shows the generated voltages along the each layer (V1-2-V6-7) and end-to-end (Vall), and the 1/10 scaled heater input and generated magnetic field measured by Hall sensor located at center position of test coil. The number of transported turns is calculated by Biot-Savart’s law using measured self-magnetic field due to current bypassing. In test coil with current lead OA pattern, the calculated numbers of turns are 3.6, 4 and 1.9 respectively, when the thermal input by heater 1, heater 2 and both heaters are conducted at applied field of 0 T and transport current of 50 A. On the other hand, 4.2, 4.4 and 2.4 transported turns are obtained in test coil with current lead OB pattern when the applied field is 0 T and transport current is 50 A. The amount of bypassing current in current lead OA pattern is more than that of current lead OB pattern. After thermal disturbance input from the heater, the voltage is not generated in V5-6 of current lead OA pattern. However, the voltage is generated in V6-7 of current lead OB pattern. Because the bypassing routes of current lead OA pattern different from the routes of current lead OB pattern after thermal disturbance is inputted from heater. Fig. 5 shows bypassing current routes after applying thermal disturbance by both heaters. Since the current is bypassed into outer layer of current in lead OA pattern, MQE of non-insulated HTS coil with current lead OA pattern is greater than that of current lead OB pattern.
heater(1/10 scale)
30
0.000
20
-0.002
10 16
6
8
10
12
Time (s)
14
0.004 heater(1/10 scale)
0.002 0.000
20 10 16
6
heater(1/10 scale)
50 40 30 20
0.000 12
12
14
14
10 16
0.012
0.012 0.008
70 60 V1-2 V2-3 V3-4 V4-5 V5-6 V6-7 Vall
0.008 0.006 0.004 0.002
heater(1/10 scale)
-0.002
50 40 30 20
0.000 6
8
10
12
Time (s)
14
heater(1/10 scale)
0.004 0.000 -0.004
6
8
10
12
Time (s)
14
40 20 0 -20 18
16
(c) both heaters (current lead OA)
Generated magnetic field
0.010
Voltage (V)
Voltage (V)
0.004
Time (s)
10
V1-2 V2-3 V3-4 V4-5 V5-6 V6-7 Vall
10 16
80
0.040 Generated magnetic field
0.035
60
0.030 V1-2 V2-3 V3-4 V4-5 V5-6 V6-7 Vall
0.025 0.020 0.015 0.010 heater(1/10 scale)
0.005
40 20 0
0.000 -0.005
6
8
10
12
14
16
-20 18
Magnetic flux density (Gauss)
V1-2 V2-3 V3-4 V4-5 V5-6 V6-7 Vall
0.006
10
8
Magnetic flux density (Gauss)
60
Magnetic flux density (Gauss)
70 Generated magnetic field
8
30
60
0.016
(b) heater 2 (current lead OA)
0.008
6
40
80 Generated magnetic field
0.020
Time (s)
0.010
-0.002
50
-0.002
(a) heater 1 (current lead OA)
0.002
V1-2 V2-3 V3-4 V4-5 V5-6 V6-7 Vall
0.006
0.024
Voltage (V)
0.002
40
60
Voltage (V)
0.004
50
70
Generated magnetic field
0.008
Voltage (V)
Voltage (V)
V1-2 V2-3 V3-4 V4-5 V5-6 V6-7 Vall
0.006
0.010
Magnetic flux density (Gauss)
60
Magnetic flux density (Gauss)
70 Generated magnetic field
0.008
Magnetic flux density (Gauss)
0.010
Time (s)
(d) heater 1 (current lead OB) (e) heater 2 (current lead OB) (f) both heaters (current lead OB) Fig. 4. Generated the voltages along the each layer (V1-2-V6-7) and end-to-end (Vall), and the 1/10 scaled heater input and generated magnetic field measured by Hall sensor located at center position in test coils.
232
K. Tanaka et al. / Physics Procedia 65 (2015) 229 – 232 Superconductor
Superconductor
Current lead A
Current lead B
I-
7
7
Bakelite Mandrel
Bakelite Mandrel
6
6 5
Hall sensor
4
I+
1
Kapton tape
I-
5
Hall sensor 3
1
Kapton tape
2
Current lead O
4
I+
3 2
Current lead O
(a)
(b)
Fig. 5. Bypassing current routes after applying thermal disturbance by both heaters (a) current lead OA and (b) current lead OB.
3.3. Thermal properties of transverse direction and longitudinal direction
86
86
85
85
T1 T2 T3 T4 T5 T6
84 83 82 81 80 79
84 83 82 81 80 79
0
5
10
15
20
25
30
77
35
Time (s)
(a) temperature profiles of each layer (2.5 J)
79
78
T2
T1
T5
77
T6
(b) transverse direction (2.5 J)
96
88 86 84 82 80
90 87 84 81
78
78
76
75
0
5
10
15
20
25
Time (s)
30
35
40
12 s 14 s 16 s 18 s 20 s 25 s
84
Temperature (K)
90
Temperature (K)
T1 T2 T3 T4 T5 T6
T4
85 12 s 14 s 16 s 18 s 20 s 25 s
93
92
T3
(c) longitudinal direction (2.5 J)
96
94
Temperature (K)
12 s 14 s 16 s 18 s 20 s
80
78
78 77
81 12 s 14 s 16 s 18 s 20 s
Temperature (K)
87
Temperature (K)
Temperature (K)
Fig. 6 shows the measured temperature profiles by T 1, T2, T3, T4, T5 and T6 with various thermal inputs. The heat capacity from 2.5 J to 5 J was applied to sample wires, and the temperature of sample wires was rises rapidly at the same time after applying thermal disturbance by heater except for T4. Because T4 is 2 cm far from the position of the heater, heat is transferred in the order of heater, T3, T4. The peak value of the temperature at T5 is higher than that of T2. Each thermocouple is attached at the surface of sample wire, and these thermocouples are insulated by Kapton tape. Therefore, the thermal conduction property along the transverse direction from the heater is little different, and the maximum temperature of T5 is higher than that of T2. In insulated HTS pancake coil, the thermal conduction property into the transverse direction is poor compare with longitudinal direction due to electrical insulator which have very low thermal conductivity. However, the improved thermal conduction property into the transverse direction was obtained by removed the insulator as shown in Fig 6.
83 82 81 80 79 78 77
T1
T2
T5
T6
T3
T4
(d) temperature profiles of each layer (5 J) (e) transverse direction (5 J) (f) longitudinal direction (5 J) Fig. 6. Measured temperature profiles during normal transition at each layer in non-insulated HTS coils wound with GdBCO wires against various thermal inputs.
4. Conclusion We studied the method for current bypassing on layer-to-layer in order to control the inductance of the non-insulated HTS coil. The characteristics of current bypassing by inserting insulation around the current lead and thermal diffusion in noninsulated HTS coil wound with 2G wires were examined experimentally. We confirmed that the non-insulated HTS coils inserted insulation around the current lead were possible to control current bypassing. References [1] S.B. Kim. T. Kaneko, H. Kajikawa, J.H. Joo, J.M. Jo, Y.J. Han and H.S. Jeong, “The transient stability of HTS coils with and without the insulation and with the insulation being replaced by brass tape” Physica C, vol. 484, pp.310-315, 2013. [2] S.B. Kim, H. Kajikawa, H. Ikoma, J.H. Joo, J.M. Jo, Y.J. Han and H.S. Jeong, “Study on the electrical contact resistance properties with various winding torques for noninsulated HTS coils,” IEEE Trans. on Appl. Supercond., vol.24, 4600405, 2014. [3] S. Hahn, D. K. Park, J. Bascuñán, and Y. Iwasa, “HTS Pancake Coils Without Layer-to-layer Insulation,” IEEE Trans. Appl. Supercond., vol. 21, no. 3, pp. 1592-1595, 2011.