- Email: [email protected]
Investigation of electrical characteristics of no-insulation coil wound with surface-processed HTS tape
Physica C: Superconductivity and its applications 539 (2017) 25–29
Contents lists available at ScienceDirect
Physica C: Superconductivity and its applications journal homepage: www.elsevier.com/locate/physc
Investigation of electrical characteristics of no-insulation coil wound with surface-processed HTS tape Haeryong Jeon a, Woo Seung Lee a, Jinsub Kim a, Geonwoo Baek a, Sangsu Jeon b, Yong Soo Yoon c, Tae Kuk Ko a,∗ a b c
School of Electrical and Electronic Engineering, Yonsei University, Seoul 120-749, South Korea Department of Electrical Engineering, Korea National University of Transportation, Chungju 380-702, South Korea Department of Electrical Engineering, Shin Ansan University, Ansan-si 425-792, South Korea
a r t i c l e
i n f o
Article history: Received 10 March 2017 Revised 13 June 2017 Accepted 21 June 2017 Available online 22 June 2017 MSC: 00-01 99-00 Keywords: Characteristic resistance Contact resistance No-insulation coil Surface roughness
a b s t r a c t This paper deals with the electrical characteristics of no-insulation coil wound with surface-processed HTS tape. The bypassing current path through turn-to-turn contacts within a coil is formed in the noinsulation coil, and this bypassing current path determines two characteristics: 1) self-protection and 2) charge-discharge delay. The amplitude of bypassing current is determined by contact resistance between the turn-to-turn contacts of the no-insulation coil. The surface roughness of the HTS tape is one of the parameters to change the contact resistance. The HTS tapes were processed to roughen by bead blast and abrasive paper, and the no-insulation coil is fabricated using processed HTS tape. We have studied the charge-discharge delay and self-protecting characteristic of each no-insulation coil by 1) sudden discharge tests and 2) overcurrent tests. The FEM simulations of contact resistance of no-insulation coil were carried out. The contact surface resistance of a case processed by abrasive paper has almost three times larger than that of reference no-insulation coil, and a case processed by bead blast presents almost same contact surface resistance with reference no-insulation coil.
1. Introduction The normal-zone propagation (NZP) velocity of the lowtemperature superconducting (LTS) magnet tends to be relatively fast, and this NZP velocity facilitates self-protection against overheating by quench. In the HTS magnet, NZP velocity is normally much slower than that of LTS magnet, so the HTS magnet must rely on active protection system to protect itself [1]. The one of the method to own the self-protecting characteristic to the HTS magnet is the no-insulation winding method [2]. The radial current path through turn-to-turn contacts within a coil is formed in the no-insulation coil, and this radial current path determines two characteristics: 1) self-protection and 2) chargedischarge delay. When overcurrent flows to the no-insulation coil, the resistance of the superconductor is increased more than the stabilizer resistance, so current is bypassed to the radial current path which depend on turn-to-turn contact resistance [3]. This characteristic protects the HTS magnet against overheating by quench, however the charge-discharge delay is occurred. The turnto-turn contacts make radial current path when current starts to
∗
Corresponding author. E-mail address: [email protected] (T.K. Ko).
http://dx.doi.org/10.1016/j.physc.2017.06.003 0921-4534/© 2017 Elsevier B.V. All rights reserved.
© 2017 Elsevier B.V. All rights reserved.
flow in the no-insulation coil [2]. The delay is occurred until all the current flows to the spiral current path of superconducting coil, instead of the radial current path. The self-protecting characteristic and the charge-discharge delay are key factors to determine a stability margin and charging time of the no-insulation coil [4]. Several research to reduce the charge-discharge delay have been studied [5–7]. The typical parameters to change the characteristic of the no-insulation coil are insulation area, co-wound material and winding tension mentioned above the researches. In this paper, the self-protecting characteristic and the charge-discharge delay of the no-insulation coil are studied in relation to surface roughness of the HTS tape. The advantage of this method is that the coil can be wound easily and densely compared with other methods. The surface of the HTS tape is processed to change the turnto-turn contact resistance using bead blast and abrasive paper. The turn-to-turn contact area is reduced when the rough surface HTS tape is used to fabricate the no-insulation coil, and also the self-protection characteristic is in inverse proportion to the charge-discharge characteristic. The contact resistance of each no-insulation coil is calculated, and the generated power when overcurrent flows to the no-insulation coil is obtained to verify the self-protecting characteristic of each coil.
26
H. Jeon et al. / Physica C: Superconductivity and its applications 539 (2017) 25–29
Fig. 1. Equivalent circuit model for a no-insulation coil. Table 1 Specifications of fabricated HTS coils. Parameters HTS tape Size of HTS tape Winding structure Turns Inner diameter Critical current Processing method Inductance Tape length Winding tension
Case 1
Case 2
Fig. 2. Processed surfaces of the HTS tapes, viewed here at 250 times magnification. Case 3
SuNAM’s brass laminated GdBCO tape Width: 4.2 mm, Thickness: 0.22 mm Single pancake 40 65 mm 100 A 99 A 100 A Bead blast (120 μ m) Sand paper (80 grit) 192 μ H 9.1 m 40 N
2. Test and simulation setup Fig. 1 shows the equivalent circuit of the no-insulation coil, where Lcoil , Rsc , Rstab , and Rc are the inductance, superconducting layer resistance, stabilizer resistance, and characteristic resistance of the no-insulation coil, respectively. The characteristic resistance determines characteristics of no-insulation coil in the equivalent circuit. Sudden discharge test is carried out to measure the characteristic resistance of the each case, and the contact resistance is calculated using Eq. (2) and FEM simulation. To calculate the generated power when overcurrent flows to the no-insulation coil, voltage and current are measured from the overcurrent tests. The hall sensor is mounted at the center of the HTS coil to measure the magnetic field. Table 1 lists the specification of the fabricated HTS coils in our tests. The SuNAM’s GdBCO tape is used to wind the no-insulation coils. The case 1 is the reference sample. The case 2 and the case 3 are processed to reduce the charge-discharge delay by bead blast and abrasive paper, respectively. Single side is processed in the case 2 and case 3. The no-insulation coils are fabricated with the winding tension of 40 N, and each critical current of the coil is 100 A, 99 A, and 100 A, respectively.
2.1. Surface roughness of the no-insulation coil The surface of the HTS tape is roughened to reduce the chargedischarge delay. The bead blast and sandpaper are used to make the rough surface of the HTS tape. Bead blast is the operation of forcibly propelling a stream of small glass bead against a surface of the HTS tape. The glass beads heat the surface of the HTS tape under high pressure, so that surface is broken by the glass bead. The 120 μ m diameter of the glass bead is used to process surface of the HTS tape. We tried to use sand ball and steel ball to fabricate
more rough surface HTS tape, but this method damaged to HTS layer. In the case 3, the HTS tape and sand paper were clamped together by the vise grip. Force of 930 N/m2 is consistently applied to the HTS tape until all the HTS tape is pulled out from the vise grip. The surface of the HTS tape is then ground to the length direction by the sandpaper. The 80 grit sandpaper is used to roughen the surface in the case 3. Fig. 2 shows the processed surface of the HTS tape that is magnified 250 times. The whole surface area of the HTS tape is roughened by the bead blast in case 2, however local scratched line is made by the sandpaper in case 3. Fig. 3. Shows the surface roughness profile of the HTS tape that was measured by the surface profiler. The surface roughness is measured in the width direction of HTS tape. The center line average height roughness (Ra ) of the each case is 82.85, 791.55, and 204.89 nm, respectively. However, it doesn’t mean the contact area of the each turn-to-turn of the no-insulation coil is proportional with surface roughness. 2.2. Sudden discharge test The characteristic resistance is calculated by sudden discharge test [1]. The power supply is switched off after the magnet is charged to 20 A by 1 A/s ramping rate. The loop 1 in the Fig. 1 is the current path when the power supply is switched off. All the energy is consumed at the Rc and exponential field decay is came out from the discharge test and the time constant is calculated from the graph. The characteristic resistance in the no-insulation coil is as follows:
Rc =
Lcoil
(1)
τ
where Lcoil , Rc and τ are inductance of the HTS magnet, characteristic resistance and time constant value, respectively. 2.3. Simulation of contact resistance The contact surface resistance in the turn-to-turn contacts is calculated by as follows:
Rc =
Nt i=1
Ri =
Nt i=1
Rct 2π ri wd
(2)
where Nt , Ri , Rct , ri and wd are the total number of turns, turn-toturn contact resistance, contact surface resistance, radius of the ith turn and width of the HTS tape [8]. The problem is assumed that
H. Jeon et al. / Physica C: Superconductivity and its applications 539 (2017) 25–29
27
Fig. 5. Normalized axial center field of all test cases measured from sudden discharge test at 20 A.
coil is set as the ground. The discharge current flows from terminal to ground in Fig. 4. In the FEM simulation, the iteration sequence to find the contact surface resistance is operated until the time constant value is matched with the test results of characteristic resistance using parametric sweep in function of the COMSOL Multiphysics. 2.4. Overcurrent test
Fig. 3. Surface roughness profiles of the HTS tape measured using a surface profiler.
The overcurrent tests are carried out to verify the selfprotecting characteristic. The procedure of the overcurrent test is described as follow: 1) the current charged to the coil with 2 A/s ramping rate. 2) the current is maintained for over 30 s. 3) the current discharge with 2 A/s ramping rate. The each test is carried out from 112 A to 130 A that the current is increased an interval of 6 A. The current and voltage is measured, and the generated power is calculated from the test. 3. Results and discussion 3.1. Characteristic resistance
Fig. 4. Schematic view of the no-insulation coil, in the simulation model, it is assumed that the contact surface resistance is the same at each of the turn-to-turn contacts. The discharge current flows from terminal to ground.
the contact surface resistance is the same in the every turn-to-turn contacts also characteristic resistance comes mostly from the turnto-turn contacts in this equation. To compare the results of the Eq. (2), the contact resistance is calculated by FEM simulation using by COMSOL Multiphysics 5.0. Fig. 4 shows the schematic view of the 2D asymmetric model to calculate the characteristic resistance. The outside surface of the coil is set as input current terminal, and the inside surface of the
Fig. 5 shows the magnetic field of the sudden discharge tests. The magnetic field is normalized to compare the delay time. Each time constant value is 1.95, 1.85, and 0.54, respectively. The characteristic resistance is calculated from the time constant, and calculated characteristic resistances are 98.45, 103.78, and 355.55 μ , respectively. The characteristic resistance of the case 1 and case 2 is almost same. The characteristic resistance of the case 3, sandpaper processing, is almost three times larger than reference case. The sandpaper processing is more effective to increase the characteristic resistance. In Fig. 3, the Ra of case 2 is more bigger than the case 3, but the characteristic resistance is small. It means the contact resistance of no-insulation coil is not just determined by surface roughness of the HTS tape, also the processing method must be considered to increase the contact resistance. We speculate that the reason of this results is winding tension of NI coil to each turn-to-turn contacts. In the case 2, overall surface of HTS tape is processed by bead blast. When the winding tension is applied to the each HTS tape, the stabilizers are pressed and the contact area is widened. However, in the case 3, if the winding tension is applied to the HTS tape, the scratched line is not contacted because this area is caved in. The roughness shape in Fig. 2 is changed with respect to the processing method, and it influences the contact resistance. As a result, discharge time of
28
H. Jeon et al. / Physica C: Superconductivity and its applications 539 (2017) 25–29
Fig. 6. FEM simulation results of contact surface resistance: case 1, case 2 and case 3 is 2.37, 2.5, and 8.53 n · m2 , respectively.
each case is 2.07, 1.98, and 0.58 s, respectively. To verify the results, the same test with 20 turns HTS coil is carried out and the equal results are obtained from the tests. 3.2. Contact surface resistance The contact surface resistance is calculated by Eq. (2), and the results are 2.55, 2.76, and 9.70 n · m2 , respectively. Fig. 6 shows the FEM simulation results of contact resistance. The contact resistances are 2.37, 2.5, and 8.53 n · m2 , respectively. It is in good agreement with results of Eq. (2). The contact surface resistance of case 3 is the almost three times larger than that of case 1, and case 2 is same with the results of the characteristic resistance. 3.3. Self-protection Fig. 7 shows the voltage of the HTS magnet in the overcurrent tests. As the high overcurrent flows to the HTS magnet, the voltage is fluctuated and the average of the generated voltage at 130 A is 9.67, 9.91, and 11.90 mV, respectively. Assuming that the all overcurrent flows to the Rc in Fig. 1, and the generated power at 130 A are 290.1, 307.21, and 368.9 mW, respectively. In each test case, the no-insulation coil remains undamaged until the level of current flowing through the coil exceeds 130%. The case 1 has the better self-protecting characteristic than the case 3 in terms of the generated power. In general, however, a stability margin of 30% or more is enough to safeguard a HTS magnet against overheating by quench.
Fig. 7. Test results of induced voltage with respect to overcurrent. (a),(b) and (c) are case 1, case 2, and case 3, respectively.
4. Conclusion
the enough stability margin over 30% to protect itself. In conclusion, the contact resistance of a no-insulation coil is not determined by HTS-tape surface roughness alone; rather, the method used to process the surface of a HTS tape needs also to be considered with regard to increasing contact resistance.
We studied the effect of the surface roughness of the HTS tape to reduce the charge-discharge delay of the no-insulation coil. The summary of test results are listed in Table 2. The calculated contact surface resistances by FEM simulation is in good agreement with analytical results of equation. The scratched HTS tape using sandpaper is effective to reduce the charge-discharge delay, and is reduced from 2.07 to 0.58 s, but the reference no-insulation coil have the better self-protecting characteristic than the other coils in terms of the generated power. However, the other coils also have
Table 2 Summary of test results. Parameters
Case 1 Case 2 Case 3
Time constant Discharge time (s) Characteristic resistance (μ ) Contact surface resistance by Eq. (2) (n · m2 ) Contact surface resistance by FEM simulation (n · m2 ) Average of generated voltage at 130 A (mV) PGenerated power at 130 A (mW)
1.96 2.07 97.92 2.55 2.37 9.67 290.1
1.85 1.98 103.68 2.76 2.5 9.91 307.21
0.54 0.58 353.28 9.70 8.53 11.90 368.9
Acknowledgments This work was supported in part by the Human Resources Program in Energy Technology of Korea Institute of Energy Technology Evaluation and Planning (KETEP) granted financial
H. Jeon et al. / Physica C: Superconductivity and its applications 539 (2017) 25–29
resource from the Ministry of Trade, Industry & Energy, Republic of Korea (No. 20164030201100) and by National R&D Program through the National Research Foundation of Korea(NRF) funded by the Ministry of Science, ICT and Future Planning (NRF2015M1A7A1A02050725). References [1] Y. Iwasa, Case Studies in Superconducting Magnets, Springer, New York, 2009, pp. 484–538. [2] S. Hahn, D.K. Park, J. Bascun, Y. Iwasa, HTS pancake coils without turn-to-turn insulation, IEEE Trans. Appl. Supercond. 21 (2011) 15921595. [3] S.B. Kim, a. Saitou, J.H. Joo, T. Kadota, The normal-zone propagation properties of the non-insulated HTS coil in cryocooled operation, Phys. C Supercond. Appl. 471 (2011) 14281431.
29
[4] S. Choi, H.C. Jo, Y.J. Hwang, S. Hahn, T.K. Ko, A study on the no insulation winding method of the HTS coil, IEEE Trans. Appl. Supercond. 22 (2012) 03. [5] Y.H. Choi, S. Hahn, J.B. Song, D.G. Yang, H.G. Lee, Partial insulation of gdBCO single pancake coils for protection-free HTS power applications, Supercond. Sci. Technol. 24 (2011) 125013. [6] T.S. Lee, Y.J. Hwang, J. Lee, W.S. Lee, J. Kim, S.H. Song, M.C. Ahn, T.K. Ko, The effects of co-wound kapton, stainless steel and copper, in comparison with no insulation, on the time constant and stability of gdBCO pancake coils, Supercond. Sci. Technol. 27 (2014) 65018. [7] K.L. Kim, S. Hahn, Y. Kim, D.G. Yang, J.-B. Song, J. Bascunan, H. Lee, Y. Iwasa, Effect of winding tension on electrical behaviors of a no-insulation reBCO pancake coil., IEEE Trans. Appl. Supercond. 24 (2014) 15. [8] X. Wang, S. Hahn, Y. Kim, J. Bascun, J. Voccio, H. Lee, Y. Iwasa, Turn-to-turn contact characteristics for an equivalent circuit model of no-insulation reBCO pancake coil, Supercond. Sci. Technol. 26 (2013) 35012.
Contents lists available at ScienceDirect
Physica C: Superconductivity and its applications journal homepage: www.elsevier.com/locate/physc
Investigation of electrical characteristics of no-insulation coil wound with surface-processed HTS tape Haeryong Jeon a, Woo Seung Lee a, Jinsub Kim a, Geonwoo Baek a, Sangsu Jeon b, Yong Soo Yoon c, Tae Kuk Ko a,∗ a b c
School of Electrical and Electronic Engineering, Yonsei University, Seoul 120-749, South Korea Department of Electrical Engineering, Korea National University of Transportation, Chungju 380-702, South Korea Department of Electrical Engineering, Shin Ansan University, Ansan-si 425-792, South Korea
a r t i c l e
i n f o
Article history: Received 10 March 2017 Revised 13 June 2017 Accepted 21 June 2017 Available online 22 June 2017 MSC: 00-01 99-00 Keywords: Characteristic resistance Contact resistance No-insulation coil Surface roughness
a b s t r a c t This paper deals with the electrical characteristics of no-insulation coil wound with surface-processed HTS tape. The bypassing current path through turn-to-turn contacts within a coil is formed in the noinsulation coil, and this bypassing current path determines two characteristics: 1) self-protection and 2) charge-discharge delay. The amplitude of bypassing current is determined by contact resistance between the turn-to-turn contacts of the no-insulation coil. The surface roughness of the HTS tape is one of the parameters to change the contact resistance. The HTS tapes were processed to roughen by bead blast and abrasive paper, and the no-insulation coil is fabricated using processed HTS tape. We have studied the charge-discharge delay and self-protecting characteristic of each no-insulation coil by 1) sudden discharge tests and 2) overcurrent tests. The FEM simulations of contact resistance of no-insulation coil were carried out. The contact surface resistance of a case processed by abrasive paper has almost three times larger than that of reference no-insulation coil, and a case processed by bead blast presents almost same contact surface resistance with reference no-insulation coil.
1. Introduction The normal-zone propagation (NZP) velocity of the lowtemperature superconducting (LTS) magnet tends to be relatively fast, and this NZP velocity facilitates self-protection against overheating by quench. In the HTS magnet, NZP velocity is normally much slower than that of LTS magnet, so the HTS magnet must rely on active protection system to protect itself [1]. The one of the method to own the self-protecting characteristic to the HTS magnet is the no-insulation winding method [2]. The radial current path through turn-to-turn contacts within a coil is formed in the no-insulation coil, and this radial current path determines two characteristics: 1) self-protection and 2) chargedischarge delay. When overcurrent flows to the no-insulation coil, the resistance of the superconductor is increased more than the stabilizer resistance, so current is bypassed to the radial current path which depend on turn-to-turn contact resistance [3]. This characteristic protects the HTS magnet against overheating by quench, however the charge-discharge delay is occurred. The turnto-turn contacts make radial current path when current starts to
∗
Corresponding author. E-mail address: [email protected] (T.K. Ko).
http://dx.doi.org/10.1016/j.physc.2017.06.003 0921-4534/© 2017 Elsevier B.V. All rights reserved.
© 2017 Elsevier B.V. All rights reserved.
flow in the no-insulation coil [2]. The delay is occurred until all the current flows to the spiral current path of superconducting coil, instead of the radial current path. The self-protecting characteristic and the charge-discharge delay are key factors to determine a stability margin and charging time of the no-insulation coil [4]. Several research to reduce the charge-discharge delay have been studied [5–7]. The typical parameters to change the characteristic of the no-insulation coil are insulation area, co-wound material and winding tension mentioned above the researches. In this paper, the self-protecting characteristic and the charge-discharge delay of the no-insulation coil are studied in relation to surface roughness of the HTS tape. The advantage of this method is that the coil can be wound easily and densely compared with other methods. The surface of the HTS tape is processed to change the turnto-turn contact resistance using bead blast and abrasive paper. The turn-to-turn contact area is reduced when the rough surface HTS tape is used to fabricate the no-insulation coil, and also the self-protection characteristic is in inverse proportion to the charge-discharge characteristic. The contact resistance of each no-insulation coil is calculated, and the generated power when overcurrent flows to the no-insulation coil is obtained to verify the self-protecting characteristic of each coil.
26
H. Jeon et al. / Physica C: Superconductivity and its applications 539 (2017) 25–29
Fig. 1. Equivalent circuit model for a no-insulation coil. Table 1 Specifications of fabricated HTS coils. Parameters HTS tape Size of HTS tape Winding structure Turns Inner diameter Critical current Processing method Inductance Tape length Winding tension
Case 1
Case 2
Fig. 2. Processed surfaces of the HTS tapes, viewed here at 250 times magnification. Case 3
SuNAM’s brass laminated GdBCO tape Width: 4.2 mm, Thickness: 0.22 mm Single pancake 40 65 mm 100 A 99 A 100 A Bead blast (120 μ m) Sand paper (80 grit) 192 μ H 9.1 m 40 N
2. Test and simulation setup Fig. 1 shows the equivalent circuit of the no-insulation coil, where Lcoil , Rsc , Rstab , and Rc are the inductance, superconducting layer resistance, stabilizer resistance, and characteristic resistance of the no-insulation coil, respectively. The characteristic resistance determines characteristics of no-insulation coil in the equivalent circuit. Sudden discharge test is carried out to measure the characteristic resistance of the each case, and the contact resistance is calculated using Eq. (2) and FEM simulation. To calculate the generated power when overcurrent flows to the no-insulation coil, voltage and current are measured from the overcurrent tests. The hall sensor is mounted at the center of the HTS coil to measure the magnetic field. Table 1 lists the specification of the fabricated HTS coils in our tests. The SuNAM’s GdBCO tape is used to wind the no-insulation coils. The case 1 is the reference sample. The case 2 and the case 3 are processed to reduce the charge-discharge delay by bead blast and abrasive paper, respectively. Single side is processed in the case 2 and case 3. The no-insulation coils are fabricated with the winding tension of 40 N, and each critical current of the coil is 100 A, 99 A, and 100 A, respectively.
2.1. Surface roughness of the no-insulation coil The surface of the HTS tape is roughened to reduce the chargedischarge delay. The bead blast and sandpaper are used to make the rough surface of the HTS tape. Bead blast is the operation of forcibly propelling a stream of small glass bead against a surface of the HTS tape. The glass beads heat the surface of the HTS tape under high pressure, so that surface is broken by the glass bead. The 120 μ m diameter of the glass bead is used to process surface of the HTS tape. We tried to use sand ball and steel ball to fabricate
more rough surface HTS tape, but this method damaged to HTS layer. In the case 3, the HTS tape and sand paper were clamped together by the vise grip. Force of 930 N/m2 is consistently applied to the HTS tape until all the HTS tape is pulled out from the vise grip. The surface of the HTS tape is then ground to the length direction by the sandpaper. The 80 grit sandpaper is used to roughen the surface in the case 3. Fig. 2 shows the processed surface of the HTS tape that is magnified 250 times. The whole surface area of the HTS tape is roughened by the bead blast in case 2, however local scratched line is made by the sandpaper in case 3. Fig. 3. Shows the surface roughness profile of the HTS tape that was measured by the surface profiler. The surface roughness is measured in the width direction of HTS tape. The center line average height roughness (Ra ) of the each case is 82.85, 791.55, and 204.89 nm, respectively. However, it doesn’t mean the contact area of the each turn-to-turn of the no-insulation coil is proportional with surface roughness. 2.2. Sudden discharge test The characteristic resistance is calculated by sudden discharge test [1]. The power supply is switched off after the magnet is charged to 20 A by 1 A/s ramping rate. The loop 1 in the Fig. 1 is the current path when the power supply is switched off. All the energy is consumed at the Rc and exponential field decay is came out from the discharge test and the time constant is calculated from the graph. The characteristic resistance in the no-insulation coil is as follows:
Rc =
Lcoil
(1)
τ
where Lcoil , Rc and τ are inductance of the HTS magnet, characteristic resistance and time constant value, respectively. 2.3. Simulation of contact resistance The contact surface resistance in the turn-to-turn contacts is calculated by as follows:
Rc =
Nt i=1
Ri =
Nt i=1
Rct 2π ri wd
(2)
where Nt , Ri , Rct , ri and wd are the total number of turns, turn-toturn contact resistance, contact surface resistance, radius of the ith turn and width of the HTS tape [8]. The problem is assumed that
H. Jeon et al. / Physica C: Superconductivity and its applications 539 (2017) 25–29
27
Fig. 5. Normalized axial center field of all test cases measured from sudden discharge test at 20 A.
coil is set as the ground. The discharge current flows from terminal to ground in Fig. 4. In the FEM simulation, the iteration sequence to find the contact surface resistance is operated until the time constant value is matched with the test results of characteristic resistance using parametric sweep in function of the COMSOL Multiphysics. 2.4. Overcurrent test
Fig. 3. Surface roughness profiles of the HTS tape measured using a surface profiler.
The overcurrent tests are carried out to verify the selfprotecting characteristic. The procedure of the overcurrent test is described as follow: 1) the current charged to the coil with 2 A/s ramping rate. 2) the current is maintained for over 30 s. 3) the current discharge with 2 A/s ramping rate. The each test is carried out from 112 A to 130 A that the current is increased an interval of 6 A. The current and voltage is measured, and the generated power is calculated from the test. 3. Results and discussion 3.1. Characteristic resistance
Fig. 4. Schematic view of the no-insulation coil, in the simulation model, it is assumed that the contact surface resistance is the same at each of the turn-to-turn contacts. The discharge current flows from terminal to ground.
the contact surface resistance is the same in the every turn-to-turn contacts also characteristic resistance comes mostly from the turnto-turn contacts in this equation. To compare the results of the Eq. (2), the contact resistance is calculated by FEM simulation using by COMSOL Multiphysics 5.0. Fig. 4 shows the schematic view of the 2D asymmetric model to calculate the characteristic resistance. The outside surface of the coil is set as input current terminal, and the inside surface of the
Fig. 5 shows the magnetic field of the sudden discharge tests. The magnetic field is normalized to compare the delay time. Each time constant value is 1.95, 1.85, and 0.54, respectively. The characteristic resistance is calculated from the time constant, and calculated characteristic resistances are 98.45, 103.78, and 355.55 μ , respectively. The characteristic resistance of the case 1 and case 2 is almost same. The characteristic resistance of the case 3, sandpaper processing, is almost three times larger than reference case. The sandpaper processing is more effective to increase the characteristic resistance. In Fig. 3, the Ra of case 2 is more bigger than the case 3, but the characteristic resistance is small. It means the contact resistance of no-insulation coil is not just determined by surface roughness of the HTS tape, also the processing method must be considered to increase the contact resistance. We speculate that the reason of this results is winding tension of NI coil to each turn-to-turn contacts. In the case 2, overall surface of HTS tape is processed by bead blast. When the winding tension is applied to the each HTS tape, the stabilizers are pressed and the contact area is widened. However, in the case 3, if the winding tension is applied to the HTS tape, the scratched line is not contacted because this area is caved in. The roughness shape in Fig. 2 is changed with respect to the processing method, and it influences the contact resistance. As a result, discharge time of
28
H. Jeon et al. / Physica C: Superconductivity and its applications 539 (2017) 25–29
Fig. 6. FEM simulation results of contact surface resistance: case 1, case 2 and case 3 is 2.37, 2.5, and 8.53 n · m2 , respectively.
each case is 2.07, 1.98, and 0.58 s, respectively. To verify the results, the same test with 20 turns HTS coil is carried out and the equal results are obtained from the tests. 3.2. Contact surface resistance The contact surface resistance is calculated by Eq. (2), and the results are 2.55, 2.76, and 9.70 n · m2 , respectively. Fig. 6 shows the FEM simulation results of contact resistance. The contact resistances are 2.37, 2.5, and 8.53 n · m2 , respectively. It is in good agreement with results of Eq. (2). The contact surface resistance of case 3 is the almost three times larger than that of case 1, and case 2 is same with the results of the characteristic resistance. 3.3. Self-protection Fig. 7 shows the voltage of the HTS magnet in the overcurrent tests. As the high overcurrent flows to the HTS magnet, the voltage is fluctuated and the average of the generated voltage at 130 A is 9.67, 9.91, and 11.90 mV, respectively. Assuming that the all overcurrent flows to the Rc in Fig. 1, and the generated power at 130 A are 290.1, 307.21, and 368.9 mW, respectively. In each test case, the no-insulation coil remains undamaged until the level of current flowing through the coil exceeds 130%. The case 1 has the better self-protecting characteristic than the case 3 in terms of the generated power. In general, however, a stability margin of 30% or more is enough to safeguard a HTS magnet against overheating by quench.
Fig. 7. Test results of induced voltage with respect to overcurrent. (a),(b) and (c) are case 1, case 2, and case 3, respectively.
4. Conclusion
the enough stability margin over 30% to protect itself. In conclusion, the contact resistance of a no-insulation coil is not determined by HTS-tape surface roughness alone; rather, the method used to process the surface of a HTS tape needs also to be considered with regard to increasing contact resistance.
We studied the effect of the surface roughness of the HTS tape to reduce the charge-discharge delay of the no-insulation coil. The summary of test results are listed in Table 2. The calculated contact surface resistances by FEM simulation is in good agreement with analytical results of equation. The scratched HTS tape using sandpaper is effective to reduce the charge-discharge delay, and is reduced from 2.07 to 0.58 s, but the reference no-insulation coil have the better self-protecting characteristic than the other coils in terms of the generated power. However, the other coils also have
Table 2 Summary of test results. Parameters
Case 1 Case 2 Case 3
Time constant Discharge time (s) Characteristic resistance (μ ) Contact surface resistance by Eq. (2) (n · m2 ) Contact surface resistance by FEM simulation (n · m2 ) Average of generated voltage at 130 A (mV) PGenerated power at 130 A (mW)
1.96 2.07 97.92 2.55 2.37 9.67 290.1
1.85 1.98 103.68 2.76 2.5 9.91 307.21
0.54 0.58 353.28 9.70 8.53 11.90 368.9
Acknowledgments This work was supported in part by the Human Resources Program in Energy Technology of Korea Institute of Energy Technology Evaluation and Planning (KETEP) granted financial
H. Jeon et al. / Physica C: Superconductivity and its applications 539 (2017) 25–29
resource from the Ministry of Trade, Industry & Energy, Republic of Korea (No. 20164030201100) and by National R&D Program through the National Research Foundation of Korea(NRF) funded by the Ministry of Science, ICT and Future Planning (NRF2015M1A7A1A02050725). References [1] Y. Iwasa, Case Studies in Superconducting Magnets, Springer, New York, 2009, pp. 484–538. [2] S. Hahn, D.K. Park, J. Bascun, Y. Iwasa, HTS pancake coils without turn-to-turn insulation, IEEE Trans. Appl. Supercond. 21 (2011) 15921595. [3] S.B. Kim, a. Saitou, J.H. Joo, T. Kadota, The normal-zone propagation properties of the non-insulated HTS coil in cryocooled operation, Phys. C Supercond. Appl. 471 (2011) 14281431.
29
[4] S. Choi, H.C. Jo, Y.J. Hwang, S. Hahn, T.K. Ko, A study on the no insulation winding method of the HTS coil, IEEE Trans. Appl. Supercond. 22 (2012) 03. [5] Y.H. Choi, S. Hahn, J.B. Song, D.G. Yang, H.G. Lee, Partial insulation of gdBCO single pancake coils for protection-free HTS power applications, Supercond. Sci. Technol. 24 (2011) 125013. [6] T.S. Lee, Y.J. Hwang, J. Lee, W.S. Lee, J. Kim, S.H. Song, M.C. Ahn, T.K. Ko, The effects of co-wound kapton, stainless steel and copper, in comparison with no insulation, on the time constant and stability of gdBCO pancake coils, Supercond. Sci. Technol. 27 (2014) 65018. [7] K.L. Kim, S. Hahn, Y. Kim, D.G. Yang, J.-B. Song, J. Bascunan, H. Lee, Y. Iwasa, Effect of winding tension on electrical behaviors of a no-insulation reBCO pancake coil., IEEE Trans. Appl. Supercond. 24 (2014) 15. [8] X. Wang, S. Hahn, Y. Kim, J. Bascun, J. Voccio, H. Lee, Y. Iwasa, Turn-to-turn contact characteristics for an equivalent circuit model of no-insulation reBCO pancake coil, Supercond. Sci. Technol. 26 (2013) 35012.